<>EPA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
EPA-600/2-78-213
November 1978
Research and Development
Proton Scattering
Analysis for Light
Elements in Air
Particulate Matter
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development. U S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-213
November 1978
PROTON SCATTERING ANALYSIS FOR LIGHT ELEMENTS IN
AIR PARTICULATE MATTER
by
J. William Nelson, Principal Investigator
G. M. Hudson, H.C. Kaufmann, W. J. Courtney, I. Williams
K. R. Akselsson, D. Meinert, J. W. Winchester
Forida State University
Tallahassee, Florida 32306
Grant R - 802913
Project Officer
Thomas G. Dzubay (MD-47)
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
-------
DISCLAIMER
This report has been reviewed by the Environmental
Sciences Research Laboratory, U.S. Environmental Protection
Agency, and approved for publication. Approval does not signify
that the contents necessarily reflect the views and policies of
the U.S. Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or
recommendation for use.
-------
PREFACE
The intent of this report is to summarize our research
on proton scattering as it applies to the analysis of samples
of particulate matter found in the atmosphere. The most
important details (such as kinematics and angular distributions)
are treated in the reprints which form the appendices. While
the scattering literature of nuclear physics is vast, only
articles of direct relevance to the subject matter of this
report are included in the bibliography. Included as appendix
B is a reprint which describes the computer code REX which
serves as the model for computer code SCAT.
111
-------
ABSTRACT
Proton scattering analysis has been developed to provide
light element analyses as a complementary method to x-ray anal-
ysis. Samples of air particulate matter from both filter and
impaction devices have been analyzed and time sequence and par-
ticle size information obtained. In combination with x-ray
analysis, total mass may be determined by summation of elemental
masses; however, further investigation is needed to compare with
the gravimetric method. Like x-ray analysis, it has a broad
range, and is rapid, subject to automation, and non-destructive
of the prepared sample. Analyses of typical atmospheric samples
may be performed in five minutes or less. With further develop-
ment, the method can readily be advanced from experimental to
routine usage.
This report was submitted in fulfillment of Grant
Number R-802913 by Florida State University under the sponsor-
ship of the U.S. Environmental Protection Agency. This report
covers a period from May 1, 1974 to December 31, 1977, and
work was completed as of January 16, 1978.
IV
-------
CONTENTS
Preface. . iii
Abstract o.. iv
Figures vii
Tables ix
1. Introduction 1
2. Conclusions 5
3. Recommendations.. 6
4. Proton Scattering by Particle Analysis -
The Choice 8
5. Air Particulate Analyses 11
6. Computer Code for Data Evaluation - SCAT 15
7. Total Mass Determination 19
8. Carbon Determinations 23
References 28
Bibliography 29
Appendices .... 30
A. "Proton Scattering for Analyses of
Atmospheric Particulate Matter" by
K. R. Akselsson, J. W. Nelson and
J. W. Winchester (Reprint from NBS
SP 425, Vol. A, 1975).... 3d
B. "Rex-A Computer Programms for PIXE
Analysis" by Henry C. Kaufmann,
K. Roland Akselsson and William J.
Courtney (Reprint from NUCL. Inst.
and Meth 142, 251 (1977) 34
v
-------
C. "Light Element Analysis by Proton
Scattering" by J. W. Nelson and W. J.
Courtney (Reprint from Nucl. Inst. and
Meth 142, 127 (1977) 41
VI
-------
FIGURES
Number
1. Schematic representation of proton scattering
by 12C and 28Si,.................................. 4,
2o Proton scattering spectrum for an impactor
sample........................................... 4
3. a) Inelastic scattering y~raY spectrum for filter
aerosol sample. . , 9
b) Elastic and inelastic particle spectrum for
the same sample.............................. 9
4. PESA spectrum for an aerosol sample from St. Louis,
Missouri o .................. 11
5. Time plot of PESA data from the General Motors
Sulfate Dispersion Experiment 12
6, Combined PESA-PIXE analysis of an aerosol sample
taken with an impactor 13
7. Particle size plot from combined PESA-PIXE
analyses......................................... 14
8. Calibration spectrum using the plastic Kapton.... 17
9, Calibration spectrum from an aluminum foil....... 17
10. Scattering spectrum for an indoor aerosol
sample........................................... 2.0
VII
-------
Number Page
11. Background scattering spectrum taken a few
millimeters distant from the spectrum of
Figure 10 20
12. PESA spectra for Nuclepore and Millipore
filters 25
13. PESA spectra for possible impactor surfaces-
Nylon and Mylar 25
14. PESA spectrum for an impactor sample which allows
quantitative determination of aerosol carbon.... 26
15. Spectrum for Nuclepore filter sample of sufficient
thickness to permit carbon content
determination 27
Vlll
-------
TABLES
Number Page
1. Approximate detection limits for light element
PESA determinations on typical aerosol field
samples taken on Nuclepore filter except for
carbon which is reported for a polystyrene backed
impactor sample 2
Masses of the light elements determined by PESA
for an indoor industrial filter sample 22
IX
-------
SECTION 1
INTRODUCTION
The scattering of protons by the nuclei of elements has
been successfully applied to the quantitative analysis of the
light elements. The method is broad-range, rapid, subject to
automation and non-destructive of the prepared sample. As such,
it furnishes an ideal complement to x-ray analysis especially
since the necessary sample requirements are almost identical.
Absorption of x-rays by the sample itself places a limit on the
lightest elements measurable by detection of x-rays. For
samples thicker than about 1 mg/cm2 (or particle sizes
about 10 ym) x-ray absorption corrections become difficult for
elements of atomic number Z = 13 (aluminum) and below. A proton
scattering system was developed to observe the light elements
and overlap the lower range of the x-ray technique.
Proton scattering is essentially a form of nuclear mass
spectroscopy which may be carried out in an efficient, broad-
range fashion utilizing commercially available silicon surface
barrier detectors. Figure 1 pictoralizes the elastic scattering
12 28
of 16 MeV protons by the C and Si nuclei for three angles.
The energies of the scattered protons are predicted using the
laws of conservation of mass-energy and conservation of linear
momentum. Our earliest exploratory measurements on a size
-------
fractioned aerosol sample from Tallahassee, Florida is shown in
Figure 2. Resolution is sufficient to permit analyses up to and
including Z = 17, chlorine. Following this initial success,
efforts were directed to the design, construction and test of an
efficient system suitable for both filter and impactor aerosol
samples. This report details the results obtained on samples
from field projects, progress on creating a computer code for
evaluation of such data and results of supportive calculations
(kinematics) and measurements (angular distributions and excita-
tion functions) needed to fully evaluate and define the limits
of the system.
TABLE 1 APPROXIMATE DETECTION LIMITS FOR LIGHT ELEMENT PESA
DETERMINATION ON TYPICAL AEROSOL FIELD SAMPLES TAKEN ON
NUCLEPORE FILTER EXCEPT FOR CARBON WHICH IS REPORTED
FOR POLYSTYRENE BACKED IMPACTOR SAMPLES.
Element
C
N
0
F and Mg
Na thru Ca
3a Detection Limit
0.2
0.2
1.4
2.0
0.5
(yg/m3)
-------
Using actual aerosol field samples, typical 30 detection
limits for light elements have been calculated and are shown in
Table 1. As will be detailed in this report, these limits allow
significant measurements on atmospheric particulate matter with
time resolution as short as two hours. Carbon determinations
are treated in detail in Section 8. All spectra presented in
this report are the raw spectra obtained by multichannel analy-
sis with no background subtraction or other treatment.
-------
PROTON ELASTIC SCATTERING
12.00
I35°\
13.52
I90'
15.23
.'45°
»*
16 MeV
14.15
I35°\
14.89
190°
15.67
' 45°
16 MeV
P
28Si
12
Figure 1. Schematic representation of proton scattering by C
and 28Si.
900
400
g
§ 3°°
1 o
200
100
n
.
Ep- 15.98 MeV PROTON SCATTERING AIR PARTICULATE- TLH
6L'I35°
_ Q -4/iC 0 2^i _
I2C 1 1
\ \
-
1
1
JJ60keV
I3C *?
'
*'->Kn ' '. ..
v ' , t
_.,-,.,-.-...-.,-,- : n|^wy,«JL/*v---';
"No
i
J
If.-. V
-
.
Ji "f .
vl J
.. /i |!"J'
i s. i ^ ^j»^*y i i f i
i rwp-pf .|i£*._..^.
I2X>0 12.50 13.35
SCATTERED PROTON ENERGY (MeV)
Figure 2. Proton scattering spectrum for an impactor sample.
-------
SECTION 2
CONCLUSIONS
Proton elastic scattering analysis for light elements
was developed as the only choice for a broad-range complement
to x-ray analysis. Like x-ray analysis it is broad-range, rap-
id, subject to automation and non-destructive of the prepared
sample. Furthermore, it's sample requirements are nearly iden-
tical to those of x-ray analysis.
For air particulate matter it has been demonstrated to
be effective for both filter and impactor samples to produce
both time sequence and particle size information with excellent
resolution. In combination with x-ray analysis total mass may
be determined by summation of elemental masses. Carbon deter-
minations represent a special problem for any method, in that
most of the desirable backing materials contain carbon. This
problem appears surmountable by use of thinner filters and im-
paction surfaces.
-------
SECTION 3
RECOMMENDATIONS
In light of the reported findings proton scattering is
recommended for further development in the following aspects:
1. develop more precise determination of carbon content
through the use of Teflon filters.
2. explore use of thinner filters to facilitate deter-
mination of carbon content of the aerosol while
maintaining compatibility with x-ray analysis and
excellent time resolution.
3. pursue advanced programming methods for spectral
evaluation with emphasis on non-Gaussian peak
shapes.
4. move angle of observation from 120 to 135 degrees
to enhance sensitivity of other elements relative
to carbon.
5. continue to refine the method by application to
field samples.
6. add detectors for hydrogen determination.
7. use PESA to better understand the attenuation cor-
rections needed to x-ray analysis.
8. confirm the efficacy of the method for nitrogen
determination for aerosol samples.
-------
9. examine and define volatility limits for aerosol
samples under analysis conditions.
-------
SECTION 4
PROTON SCATTERING BY PARTICLE DETECTION - THE CHOICE
A variety of nuclear reactions may be used for quanti-
tative measurements of the light elements. Proton scattering
was chosen primarily for two reasons. Firstly, its broad-range
characteristic enables measurement of all light elements simul-
taneously in a single bombardment with a single detector.
Secondly, proton scattering cross sections are generally higher
in value than those of other nuclear reactions. Still another
desirable characteristic is the appearance of both the elastic
and one or more inelastic peaks in the spectra which provides a
definitive "signature" for each element.
Proton inelastic scattering by gamma-ray detection is
also a method which was examined. This method has been studied
in considerable detail by B. L. Cohen ejt al with samples of
about 200 mg. In order to maximize sensitivity over a range of
elements multiple detectors of different sizes should be used
depending upon the energy of the emitted gamma ray. Further
improvement in sensitivity and selectivity would necessitate
bombardments at different energies for different elements on
particular substrates. In addition, Germanium detectors for
gamma-rays are considerably more expensive than silicon detec-
tors for charged particle observation.
8
-------
a)
5000
4000-
3000
D
D
O
2000
1000
0
b)
10000 -
1000
c
3 100
10
Inelastic Scattering y-raySpectrum
Air Particulate - Nuclepore
100 200 300 400 500 600 700 800 900
Channel Number
Proton Scattering-Particle Spectrum
Air Part culate - Nuclepore
|QJ L.
100 200 300 400 500 600 700 800 900
Channel Number
Figure 3 a.
b.
Inelastic scattering j-ray spectrum for a filter
aerosol sample.
Elastic and inelastic particle spectrum for the
same sample and same integrated charge.
-------
Our goal was finding a light element measurement scheme
suitable for use on the same samples as used for x-ray analysis.
The spectra resulting from bombardment of such an air partic-
ulate sample on Nuclepore filter substrate are shown in Figure
3. The inelastic scattering gamma-ray spectrum of Figure 3A
12 32
reveals thei peaks due to scattering from the C and S first
excited states. This spectrum was measured at a proton bombard-
ing energy of 7.0 MeV an 8% efficient, 3.5 keV FWHM lithium
drifted Germanium detector. Higher energy peaks due to 0 are
excluded due to insufficient bombardment energy, however, if
included a considerable increase in background would result.
By contrast, Figure 3B shows the particle spectrum from bombard-
ment of the same sample. Carbon, nitrogen, oxygen, silicon,
sulfur and potassium-calcium peaks are observed. The potassium-
calcium (mass numbers 39-40) are not resolved but could readily
be resolved and quantitatively analyzed by the x-ray emission
analysis. The above considerations led to the choice of devel-
oping the particle scattering method as the most rapid and the
only truly broad-range method for analyzing light elements in
aerosol samples.
10
-------
SECTION 5
AIR PARTICULATE ANALYSES
An essential component in the development of a useful
analysis tool is the demonstration of the applicability to real-
istic field samples. Two examples of the application of PESA to
typical aerosol samples are given in this section. Figure 4
shows the proton scattering spectrum for an 0.3m air particulate
filter (Nuclepore) sample taken in St. Louis, Missouri. A total
of eight elements is observed in a spectrum quite suitable for
overlapping with x-ray analysis.
200
CO
I-
Z
0IOO
0
PESA
Ep = 15.990
0=120°
Target STL'WU
Streaker 0.3m
4C
. Ca
35
23
2Q0.
Si , -.
f|
U '
Na C. ;''
j_
_L
J_
12.420
12.879
13.232
14.018 14.351 I4.547J 14.796
14.665 14.824
Figure 4.
Scattered Proton Energy (MeV)
PESA spectrum for an aerosol sample from St. Louis
Missouri.
11
-------
From similar spectra, the time plots (with two hour
resolution) w,ere obtained and a,r-e shown in Figure 5,
ro
2.0
0
1.0
o
2.5
o
,* o
LJ
o
z
o
o
1.0
0
GM SULFATE EXPERIMENT PROTON
SCATTERING
Ca-K
I I
10
Si-AI
N
12 13 14 15 16 Oct.
TIME
Figure 5. Time plot of PESA data from the General Motors
Sulfate Dispersion Experiment.
12
-------
For particle size analysis, impactor collected
particulate matter may be analyzed as shown in Figure 6.
Spectra such as these may be used to construct particle size
plots as shown in Figure 7.
COUNTS
0 0
no OJ
10°
I04
10 3
t-
z
n
O 2
o 10
10°
BERMUDA AEROSOL 1-3 NOV. 73
II2-SI STAGE 4 23.1 HOURS
.ill i 1 1 1 1 I
.''2c PROTON SCATTERING (PESA) -
Ep = 16.0 MeV
9L =120°
. ' Q =33^0
. . ;'. I6o
"f -vs
.- ?. ' "< « : ""ft ? 'i
f.. -. y N . -i '
i ... >,,. ..-.--, : : r. ;r
-,...,-:. \^.. .. ..-..i... . - -. _.- -. ; /!
-. ..:.:.'/ \ -. . > \'- -.-/.X /' ,» .-.- *.-.. J P.- ^.J /
MeV 12.4277 12.8840 14.3028 14.5112 14.8
12.6735 13.2401 14.3601 14.556
SCATTERED PROTON ENERGY (MeV)
I 1 1 jl 1 I I II 1
. Si -.S /Co
.» .'',ci '' X- RAY (PIXE)
'. .''. ' r Ep=4.0 MeV
. '. " / Fe 9L = I35°
"." '-'* Ti - Q = 1.96 p-C
* ,
'"' ,"> '-p
\
* * .
"Aj^c . ' Zn
*'..;.. '
''"' "V" -_'.' '".*«.. .'v'.f.-fJ. \r'.. ".'...- -/.'.' .;'
1 1. 1 1 1 1 1 _ 1 1 I
2.62 3.69 4.51 ' 6.40
337
X-RAY ENERGY (keV)
Figure 6. Combined PESA-PIXE analysis of an aerosol sample
taken with an impactor.
.13
-------
10
ro
10
en
c
c
o
c 10
o
c
o
CJ
10
Bermuda Aerosol
1973
Oct
PESA
PIXE
4321
Impactor Stage
Figure 7. Particle size plot from combined PESA-PIXE
analyses.
These examples demonstrate the practical usefulness
of PESA technique applied to both filter and impactor samples.
14
-------
SECTION 6
COMPUTER CODE FOR DATA EVALUATION - SCAT
Central to any rapid, automated spectral analysis method
is a peak fitting and data evaluation computer code. The codes
SCAT and REX have been simultaneously developed to provide the
best possible analyses for PESA and PIXE respectively. Charac-
teristics of the programs are virtually identical with the ex-
ception of the shape of the continuum which underlies the peaks.
During this grant period more experience has been gained with
REX since it was in routine analytic use while SCAT was applied
to developmental spectra.
These programs fit spectra containing peaks due to up to
50 elements. Nonlinear least squares Gaussian fits proceed from
a library of parameters which describe the spectrum of each ele-
ment in terms of the peak energies, full widths at half maxima
and relative ratios of peak values. For example, the carbon
spectrum consists of three peaks while the more complex Al
spectrum requires 15 to define it. Following fitting, the areal
2
density (mass/cm ) is calculated from cross sections determined
from standard (usually gravimetric) samples. Since this method
is absolute, it need not be completely calibrated for all ele-
ments on every unknown determination but only checked with a
few standards at the beginning and end of an extended run.
15
-------
An unusual advantage of this type of program is its
use to synthesize spectra and thus determine sensitivity
limits for assumed loadings on a particular type of backing.
This specific sensitivity determination is the only really
meaningful one since it accounts for both the underlying
continuum and elemental peak interferences.
Typical standard spectra are shown in Figures 8 and
9. Figure 8 shows peaks due to carbon, nitrogen and oxygen
obtained by bombardment of the plastic, Kapton. Figure 9
shows the considerably more complex spectrum due to bombard-
ment of a thin aluminum foil.
Five non-linear parameters allow necessary small
shifts in the energy calibration and full width at half
maximum necessary when fitting from a predetermined spectral
library. An additional ten linear parameters are employed to
achieve the fit to the continuum lying beneath the peaks.
These parameters were developed by trial and error during
the extended period over which the code was being developed.
Three or four iterations produce convergence in times of
2
one minute with an overall x in the range 0.5 to 1.1.
Output of the calculations is available from a line
printer or an x - y point plotter. Aerosol concentrations
are printed for all elements in the library with 3a detection
limits indicated. In the plot mode the raw data, fitted curve
and continuum background curve are all displayed.
User input to operate the program is kept as simple
.16
-------
RUN 1643 KRPTON
N
'0 100 200 300 400 500 600 700 800 900 1000 1400
CHANNEL NUMBER
Figure 8. Calibration spectrum using the plastic Kapton.
RUN 3124 flL FOIL
100 200 300 400 500 600 700 800 900 1000 1100
CHANNEL NUMBER
Figure 9. Calibration spectrum from an aluminum foil.
17
-------
as possible. It consists of only two cards to define starting
values of the non-linear parameters, one card for the run
numbers to be analyzed and cards to select which elements are
to be used from the library.
With insights gained during the development of SCAT,
initial thoughts have developed on an even more powerful
approach. In order to allow variation in peak shapes (due to
the detector or samples which are thick or non-uniform) it
appears feasible to integrate the Gaussian distribution over
the line shape of the data instead of fitting the Gaussians
to the spectrum. With this new approach a 6-function line
shape results in a Gaussian peak but a rectangular or trape-
zodial line results in distorted peaks resembling those due
to non-uniform or very thick samples. This method has been
incorporated into an experimental program and initial results
are most promising.
18
-------
SECTION 7
TOTAL MASS DETERMINATION
Elements measurable by PESA represent most of the mass
of a typical aerosol sample. PESA was used to determine the
content of a filter-type sample and the total mass was compared
to that determined gravimetrically.
Nuclepore filters 25 mm in diameter of 0.4 ym pore size
were used. From 4 to 7 liters/minute was drawn through a cir-
cular spot on the filter 8 mm in diameter for 24 hours. Flow
rates were measured every hour and the total volume of air
sampled was calculated.
The filters were weighed on a Mettler balance accurate
to ± 5 yg in a controlled (relative humidity 50%) atmosphere be-
fore and after the samples were taken. Before each weighing
the samples were allowed to equilibrate for at least 12 hours in
the controlled atmosphere. The total mass of particulate matter
collected was then obtained by subtraction.
The samples were placed in a vacuum chamber and irradi-
ated with 16 MeV protons. The scattered protons were detected
at 120° laboratory angle. The resultant spectra were fitted
using an interactive program which used a straight line for the
background and assumed the peaks to be Gaussian. Yields were
obtained in this manner for spectra taken on and off the spot
19
-------
Total Mass Sample (3)
3/iC 16 MeV protons
Indoor,0.4/x Nuclepore
10 100 200 300 400 500 600 700 800 900 1000
CHANNEL NUMBER
Figure 10. Scattering spectrum for an indoor aerosol s_aj:tipl.e_*
10
LU
lio3
o
CO
I"2
O
to1
10
Total Mass Blank (3)
3/xC 16 MeV protons
C Indoor 0.4/z Nuclepore
100 200 300 400 500 600 700 800 900 1000
CHflNNEL NUMBER
Figure 11. Background scattering spectrum taken a few milli-
meters distant from the spectrum of Figure 10.
20
-------
containing the collected particulate matter. The difference
thus represents yields from the aerosol particles only. A
library of absolute cross section given in terms of counts per
2
yg/cm was used to calculate the cross sectional density.
Scattering spectra for a 24 hours indoor air sample from
the Physics Department machine shop are shown in Figure 10 (beam
on sample) and Figure 11 (beam on filter immediately adjacent to
sample). Large peaks are seen corresponding to elastic and in-
elastic scattering C and 0. In addition peaks due to N,
9*7 OQ "31: 07 o Q Af\
*'A1, °S, '° Cl, K and uCa may be quantitatively evaluated.
The total mass (see Table 2) observed by proton scattering was
647 yg while gravimetrically 998 yg was measured. From past
I
experience we would estimate that the unobserved elements
(hydrogen and elements above calcium) would contribute about
86 yg to make a total of 733 yg by summing weights of individual
elements. The differences of 998-733 yg may be attributable to
water loss when the sample is placed in vacuum, but further work
is needed to clarify this situation.
Comparison of the weight of a sample before and after
bombardment was hampered by the necessity of gluing the filter
to a holder before irradiating it. Further experiments might be
able to circumvent this difficulty.
21
-------
TABLE 2. MASSES OF LIGHT ELEMENTS FOR AN
INDOOR INDUSTRIAL FILTER SAMPLE
ELEMENTS
C
N
0
Al-Si
S
Cl
K-Ca
MASS
(yg)
330±2
36±0.5
109±3
143±4
14±0.5
5±0.2
10±0.3
*
45
5
15
19
2
<1
1
NET
YIELD
125,833
6,266
8,834
1,137
837
556
1,233
Estimated mass of
unobserved elements
86
12
22
-------
SECTION 8
CARBON DETERMINATION
Determination of the amount of carbon in a sample pre-
sents special problems. Most filters and backing materials used
in air sampling work contain carbon as a major constituent. In
order to minimize the error due to background subtraction,
either the filter must be very heavily loaded or the filter it-
self must be made thinner. Heavy loading is an unattractive
alternative since in x-ray analysis it makes self absorption cor-
rections difficult to impossible for softer x-rays. On the
other hand, thinner filters are within the means of present
technology and may even be purchased on special order. Still
another approach would be the use of special low carbon content
filters for separate carbon determinations; however, the addition
of a separate system for a single element appears unwarranted
if the thinner filter method is developed.
Proton scattering spectra for two types of thin com-
mercially available filters are shown in Figure 12. Both spectra
were measured for the same number of incident protons (Q = 2yC)
and show that the Millipore contains less carbon than the Nucle-
pore. On the other hand, its use in aerosol analyses is made
far less attractive by its considerable nitrogen content. A
similar situation is also found for possible impaction surface
23
-------
Nylon. As shown in Figure 13 the Nylon contains sufficient ni-
trogen as to make it an unattractive substrate when nitrogen
determinations are desired.
Examples of carbon measurements on aerosol samples are
shown in Figures 14 and 15. Figure 14 is the scattering spec-
trum for 16 MeV protons incident upon a single-orifice impactor
sample (particles > 4jam aerodynamic diameter) for l.lm of air
from the coastal region north of Los Angeles, California. The
numbers above each of the peaks are the masses of the elements
in micrograms per cubic meter of air. In this case, with a
2
paraffin coated polystyrene backing (z 100 yg/cm ), the carbon
in the sample represents 26% of the carbon present. The amount
of carbon in the backing was measured by bombardment of the
backing at a location about 2mm away from the sample.
A spectrum resulting in determination from bombardment
of a deposit on a carbon Nuclepore filter is shown in Figure 15.
In this case analysis of a six-hour summertime sample from
downtown St. Louis, Missouri, yields a carbon value of 16% of
the total carbon present and a ±14% statistical error. Another
feature of interest in this figure is the overall broadening of
all peaks which is a measure of total mass loading.
24
-------
Figure 12.
PESA spectra
for Nuclepore
and Millipore
filters.
1000
500
§ 100
o
o
50
10
500
100
O
O
50
10
PROTON SCATTERING
8L-I20° Ep-l6MeV
Q 2fj.C NUCLEPORE
r<>'
L. 1 *i
500
CHANNEL NUMBER
600
700
25
-------
1 1685 Lfl ST-1 (PESO)
_j
UJ
2
Z
a
i
o
X
1
2
O
Q
4000F
3500
3000
2500
2000
-
1500
-
m
m
-
X2 I2C
\ (total 2.3)
I2c'
* ,1
', 1
1 1
1000
500
o-
i j
1 /
/
W
i r
ri. i
L J *
A. ^
sample 0.6
I6Q
15.
t
I3C
28^.1
I
TN
/ t^i
/ V\
27Ai+28Si
14.
t
r
23Na
No 55*
* A/
s m**** i>* *' 9^%h***M*i*^M\^ N*/^
-------
IO4
io
LU
LU
Q_
in io4
'O
CJ
io
io
500
«
I6f)
/
c
.
Proton Scattering
Ep= 15.980 MeV eL=l20°
0=30-0
Nuclepore - Blank
600 700 800
CHANNEL NUMBER
900
1000
Figure 15.
Spectrum for Nuclepore filter sample of sufficient
thickness to permit carbon content determination.
27
-------
REFERENCES
1. Cohen, B. L., K. C. Chan, L. Shabson, G. Wedberg and
H. Rudolph. Trace Element Studies at University of
Pittsburgh. In: Proceedings of the Third Conference on
Application of Small Accelerators, Vol. 1, ERDA CONF-
741040-P1, NTIS Springfield, Virginia, 157-162 pp.
28
-------
BIBLIOGRAPHY
Anttila, Al. et al. 1972. Use of Backscattering in Elemental
Analysis. Int. Jour. Appl. Rad and Isotopes 23, 315.
Cohen, L. and R. A. Moyer. 1971. Analysis for Impurities by
Nuclear Scattering. Anal. Chem 4_3, 123.
Jolly, R. K. and H. B. White, Jr. 1971. Elemental Analysis
by Elastic Scattering. Nucl. Inst. and Meth. 97, 299.
Krivan, V. 1972. Analytische Charakterisierung von Stoffen
durch Strevung von geladenen Terchen. Z. Anal, Chem.
262, 1.
Nelson, J. W. 1977. Proton Induced Aerosol Analyses: Methods
and Samples in X-ray Fluorescence Analyses of Environ-
mental Samples edited by Thomas G. Dzubay, Ann Arbor
Science Publishers, Ann Arbor, Michigan.
Nelson, J. W. and D. L. Meinert. 1975. Proton Elastic Scatter-
ing Analyses - A Complement to Proton Induced X-ray
Emission Analyses of Aerosols in Advances in X-ray
Analysis Volume 18 / Plenum Press, New York, N. Y.
Williams, I. E. 1973. Quantitative Analysis of Elements by
Proton Scattering. Florida State University Thesis
(unpublished).
29
-------
PROTON SCATTERING FOR ANALYSIS OF ATMOSPHERIC PARTICULAR MATTER*
K. R. Akselsson, J. w. nelson, and J. W. Winchester
The Florida State University
Tallahassee, Florida 32306
Proton scattering has been applied to the problem of elemental quantitative analy-
sis of air particulate matter. Elements up through chlorine may bo resolved using 16
MeV protons incident upon targets up to about 1 mg/cm2 in thickness. Using the FSU
Super FH Tandem Accelerator and a large area solid state proton detector, an analysis
can be performed in several minutes. Combination of this technique with proton induced
X-ray emission analysis provides a means of quantitative analysis for all elements.
These accelerator based methods are being applied to studies of the composition of air
particulata matter in diverse locations such as St. Louis, Mo.; Los Angeles, Ca.;
several cities in Florida; and Bermuda.
"This research was supported in part by the U.S. Environmental Protection Agency,
Grants R-803913 and R-802132, and the National Science Foundation for accelerator
operations support.
(proton scattering; quantitative analysis; air particulate matter)
Introduction
Direct measurement of the most abundant elemental
constituents of atmospheric aerosol particle size frac-
tions is important in relating aerosol particle physics
theory to the variation of chemical composition with
particle size. The former depends on physical interac-
tions leading to particle coagulation and removal from
the atmosphere, which are functions of particle mass,1
and the latter is important in describing the transport
of specific chemical substances in the atmosphere, many
of which are of practical public health importance.2
The direct measurement of the most abundant constituents,
carbon, nitrogen, and oxygen, however, is difficult by
most analytical techniques and in practice is almost
never attempted in atmospheric research requiring large
numbers of separate elemental analyses. Therefore,
full advantage has not been taken of our present under-
standing of the physics of aerosols in accounting for
the occurrence of chemical components of aerosol par-
ticles.
The present research is directed toward developing
sensitive and rapid technique for determining the
light elements up to sulfur and chlorine in non-vola-
tile, components of aerosol particle size fractions by
proton elastic scattering analysis, PESA.3"5 Elements
sulfur and heavier can be determined by proton induced
X-ray emission, PIXE,6"8 and the two methods in combi-
nation should provide a determination of all elemental
constituents which contribute significantly to Che
total aerosol particle mass. The present paper pre-
sents evidence that this approach is feasible, and the
two techniques can be applied to the same atmospheric
samples by proton bombardment in a Van de Graaff accel-
erator.
Experimental
The experimental arrangement is similar to that
used in our previous reports.3"5 A collimated proton
beam of 16 MeV is used as an optimum of high enough
energy to provide resolution of protons elastically
scattered by isotopes of adjacent elements up to chlorine
and of low enough energy to be stopped completely in the
available Si detector of 1500 micron depletion d«pth.
The detector used in the present study is 6 :rro x 50 cm
active area collimated by a 1 rm elliptical curve run-
ning the length of the detector, so as to accept a
narrow range of baekscattered proton energies from each
target nuclide at the 120° detector angle and still
have a large enough effective detector surface are« to
3SC3
3809
J3Z3
ZCCC
1503
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«.
.
1 66
'Y
it
r:
it
|i
i t
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! .' 1
W '>,
tee
i LH 5T-I (PESaj
X2 :
("a]
**^»!WW V»~S*S -xA*^A
230 3aa »oe s:o =o
ZC '
I (roioi 23)
sample 0.6
ij
"0
! 15.
! i
I 3.
bi!
jl
| ^1^'
r >
! ^3ci
H j [if^ 40
' j'n? k "Na : ' \ 2.7 C0
!.Ay ? -r^o 3.5: -;S>,«
^^V Uv7^ ^'v
e 7ce ace see uoo u
CHANNEL NunBER
fig. 1 Proton scattering spectrum for 16 MeV protons incident upon an air particulate sample from the coast
north of Los Angeles, California. This sample was obtained from 1.1 m3 of air using a single orifice cascade
impactor. The numbers above each peak are the mass of that element in micrograjns per cubic meter of air.
-------
assure good counting efficiency for small aerosol par-
ticle samples in the target position. The detector is
located outside the vacuum system of the scattering
chamber, and scattered protons pass through a 1/4-mil
({.25 |ffl) Mylar window and about 2 mm of air before
entering the detector, causing negligible loss of reso-
lution in comparison with other factors in the arrange-
ment.
Results
Figure 1 presents the elastically scattered pro-
ton spectrum from a sample of aerosol particles
greater than 4 urn aerodynamic diameter collected during
« 16 hour period along a coastal location near Los
Angeles, September 1974. The sample is stage 1 of a
cascade impactor9 operating at a 1 liter/min air flow
rate, representing 1.1 m-3 total air volume, and the
ample is supported by polystyrene film of thickness
100 ug/on2, figure 2 is an X-ray spectrum produced
by bombardment of the same sample with 3.75 KeV pro-
ton* and detection in a Si(Li) detector. At the pre-
sent time the PIXE procedure is precisely calibrated8
permitting routine quantitative analysis, and the PESA
procedure is undergoing calibration verification. The
sample from stage 1 is large and has a diameter of
about 2 am and the beam was colligated to about 1.5mm.
Thus, not all of the sample was analyzed. The numbers
I
S
10
Bermuda Aerosol
let. 1973
PESA
54321
Impactor Stage
Fig. 3 Size fraction analysis of a Bermuda aerosol sara-
ple using proton elastic scattering analysis (PESA) for
C,N,O, and S, and proton induced x-ray emission analysis
(PIXE) for S,C1, and Ca. Impactor stages S through 1
represent particles of equivalent aerodynamic diameters
0.25-0.5,0.5-1,1-2,2-4, and >4 urn, respectively.
30908
JS999
iseea
1473 LO ST-1
CI3.2 :»
Pe
1.87
Zn Br
.038 .OS4
Hi Ti 5 Cul PbLa/
! ;OI6 !| CM.] .202 PtiLp
380
see see
CMONNEL
709 889 908 1998 11(10
rig, 2 Proton induced X-ray emission analysis of the same air paniculate sample as shown in fig. 1. Numbers
above each peak are the mass of that element in mierograms per cubic meter of air.
of mierograms of each element given in Fig. 1 have
been calculated after normalizing the PESA Ca + K
value to that of PIXE, supposing the elements have
the same distribution in the sample. It ia seen that
the sample size for PESA analysis is sufficient for
detection of most elements from carbon to calcium and
till small enough not to cause proton energy disper-
sion and loss of resolution. The sample size is also
adequate for detection of about ten elements by PIXE.
Figure 3 presents the distribution of seven ele-
ments with particle size for a sample collected during
72 hour period in October 1973 by cascade iapactor
from a tower in Bermuda.-0 This analysis, performed
before the absolute PESA calibration vas quantified,
represents concentrations of S, Cl, and Ca by PIXE and
concentrations of C, tt, 0, and S by PESA with norma-
lisation of S to the PIXE value all stages. Carbon
values are approximate owing to uncertainties in poly-
styrene backing thickness. The particle size distri-
butions show high large particle abundances for Na.Cl,
and Ca, which may originate from sea water dispersion,
and high small particle abundances for much of the N,
S, and 0, where composition of (NH^SOj is expected,
and for some of the carbon. The combination of PESA
and PIXE appears to be valuable for presenting
enough elemental data to suggest chemical composition
relationships such as these. PIXE alone would be
insufficient.
Because of the special interest in measurements
of nitrogen and sulfur related to the atmospheric
chemistry of these elements, test bombardments have
been carried out on targets which could serve as
standards. Figure 4 shows the scattering spectrum for
Kapton film, a material with a precisely known atomic
ratio 0/N * 2.5. The spectrum is simple and the ele-
ments are clearly resolved and can be readily analyzed
by suitable computer programs. This material provides
a convenient means to determine the relative N/0
-------
IT
Fig. 4 Proton scattering spectrum for 16 HeV proton*
incident upon a 7.5 urn thick Kapton film.
fig- 5 Proton scattering spectrum for 16 MeV protons
incident upon a sulfur target. The target was prepared
by vacuum evaporation of elemental sulfur onto a Mylar
becking and then over sprayed with Krylon to stabilize
the sulfur to bombardment in vacuum.
scattering cross sections. Figure S shows the scat-
tering spectrum for elemental sulfur on Mylar, with
Krylon binder. The elastic scattering cross section
for sulfur is dominant over lower energy inelastic pro-
ton groups in the spectrum. Our identification of
peaks between channels 100 and 600 is not yet complete,
but a few inelastic sulfur scattering peaks are appar-
ently present and may be useful in quantitative reso-
lution of spectra from complex mixtures as may be en-
countered in analysis of environmental samples. Current
emphasis on high quality analyses of particulace sulfur
in atmospheric samples, because of its pollution signi-
ficance, makes it desirable to have alternate methods
for sulfur determination. FESA and FIXE can both be
applied to sulfur determination in the same samples.
High quality elemental analyses may be produced
providing the proton scattering cross section is not
highly sensitive to variables in a routine procedure
end providing that variations in saiipla composition and
thickness do not degrade the spectrum to the point of
not being.able to resolve the elemental constituents.
As exemplified by Fig. 1, impactor sampling time may
reedily be chosen for obtaining a sample of size large
enough for detection of elements by PESA and small
enough to permit their resolution. It should be pointed
out that thick samples tend to cause proton energy dis-
persion, not proton particle loss, and some dispersion
can be tolerated if adjacent energy peaks can be resol-
ved, tn contrast, PIXE and any other X-ray methods for
elemental analysis are vulnerable to x-ray attenuation
et low x-ray energies, and this makes determination of
elements lighter than sulfur uncertain if the required
sample self-absorption corrections are large. Thus, in
many practical cases, determinations of elements in the
region of sulfur by both PIXE and PESA offer a decided
advantage over determinations by one method alone.
In order to define the sensitivity of the PESA
method to instrumental variables, measurements of effec-
tive cross section with angle and proton energy have
been made. Figure 6 shows how the relative cross sec-
tion varies with scattering angle for 12C,160, and 32S.
(These are relative values only. The variation of effec-
tive cross section at 120° with mass number of nuclides
?Li to *0ca has been given in Fig. 9 of reference 5.)
Figure 6 indicates that the sulfur cross section varies
imperceptibly over a broad angle interval from 100O to
135O, oxygen has a gentle minimum centered about 113°,
and carbon decreases almot monotonically with angle
from 100° to 135°, being relatively flatter around 12QO
than angles greater or less thin this. For convenience
in our experimental arrangement a.id for the least prac-
tical sensitivity to variations in scattering angle, we
have chosen 120° for most of our further calibration
experiments. This choice of angle also affords a rela-
tively high carbon cross section and permits precise
carbon measurements where subtraction of backing mate-
rial contribution is required. We also note that more
forward angles suffer from apparently less satisfactory
peak to background ratios, and the peaks are kir.'emati-
cally more closely spaced. These and other practical
considerations for the resolution of elemental consti-
tuents in environmental samples indicate that the opti-
mum angle chosen for analysis is not a trivial problem
end should be explored with great care. At present,
our choice of 120° appears to be a good one.
2 15
I
10
r
Proton Scattering
Ep« 15.930 MeV
100 HO 120 130
Scattering Angle (dig)
140
Fig. 6 Differential cross section in arbitrary units
versus scattering angle for three isotopes. The lines
through the points are guides for the eye.
Figure 7 shows relations calculated which influence
the resolution which can be achieved for 12c and J5ci.
The angle subtended by the detector is determined by the
width of the elliptical curve which fonr.s she collimatcr
of our detector face, currently about 1°. since the
intrinsic detector resolution is in the region of 40 keV.
it is desirable to keep additional energy dispersion due
to detector angle well below this value. This is espe-
cially true if resolution of :7A1 and J8Si is to be
achieved, as we consider necessary in atmospheric
aerosol studies.
32
-------
ISO
160
140
JI2C
Joo
Jeo
!<°
s
Proton Scottering
AEviAS
E?'I6 MaV
'"01 2 3 4 5
Angli Subtendsd by Ottectcr
A9 (dsg)
Fig. 7 Variations in scattered proton energy versus
angle subtended by the proton detector. In the analy-
sis system at FSU, angles between 1 and 2 degrees are
employed.
10
- 4
s z
5
doYdfl vs Ep
23,
'No
15.90 15.92
I5S4 15.96 15.98
Incident Prolon Energy (MeVI
16.00
Fig. 8 Excitation functions (or the ground and first
excited states of '^Na over tne range Of interest for
the FSO analysis system. The lack of structure is
desirable in that quantitative analyses do not depend
upon sample thickness. The line indicates 30 keV,
the maximum sample thickness analyzable with good
resolution.
Finally, Fig. 8 shows the differential scattering
cross section as a function of proton energy for i3Na
as measured from a 60 ug/cm* Ma Cl sample. The uncer-
tainty in the repeatability of the proton energy scale
was ±10 keV. Data for the two major peaks observed in
the sodium spectrum, elastic scattering from the ground
state and inelastic scattering to form the first exci-
ted state, are given for a range of energy. This range
is several times greater than we anticipate will be
caused by extreme variations in sample thickness which
can be accepted and still give the energy resolution
required for a successful analysis for individual
elements. Normally, no thickness greater than that
equivalent to a 30 kaV energy loss would be accepted,
and neither sodium cross section is found to vary over
this range. Also, it is seen from fig. a that tedious
energy calibration procedures of the incident proton
energy are not necessary for sodium measurements.
Clearly, such tests must be carried out for every
element for which analysis is attempted. For those
we have completed to date, similar results have been
found, indicating no difficulties are foreseen due to
cross section sensitivity to this effect. It is also
fortunate that the ratio of cross sections for the
elastic and inelastic groups of scattered protons from
sodium is constant with energy, because then both
groups can be used in the quantitative analysis for
sodium in complex mixtures by straightforward computer
fitting programs.
Conclusions
Experiments performed to date indicate that proton
elastic scattering analysis can be a practical, rapid,
and sensitive means of elemental analysis when applied
to atmospheric aerosol samples. It has inherent sim-
plicity, relative freedom from interference between
elements present in the sample, and potential for
automated nondestructive analysis. In combination
with PIXE, PESA has the capability for determining all
elemental constituents of aerosol samples that contri-
bute significantly to the total aerosol mass, a capa-
bility which is unique in contrast to alternative ana-
lytical techniques. Future research and development
of PESA should be directed to detailed cross section
measurements of the nuclides of interest over the
range of variables of energy, angle, and other para-
meters encountered in practical elemental analysis. An
effort should be made to develop computer hardware and
software which will automate the procedure so that ana-
lyses can be carried out in minutes or less of time
and data can be handled readily for interpretation.
Finally, the PESA technique should be field tested
extensively so that relationships in elemental compo-
sition of environmental samples revealed by the tech-
nique can be evaluated. Such evaluations may indicate
directions for further improvement of the technique.
References
1C. E. Junge, Air Chemistry and Radioactivity, Academic
Press, Hew York, 1963.
'T. B. Johansson and J. w. Winchester, Proc. 2nd Int.
Conf. Duel. Meth. Environ. Res., Columbia, Mo. July
1974 (In press. Tech. Info. Center, USAEC, Oak Ridge,
Tenn.).
3j. w. Nelson et al., IEEE Trans. Hucl. Sci. NS21.618
(1974).
4j. w. Nelson and 0. L. Meinert, Advances in x-Ray
Analysis 18, (197S) (in press,Plenum).
5j. w. Nelson et al., Proc. 3rd Conf. on Applications
of Small Accelerators, Denton, Texas, Oct. 1974 (in
press, USAEC Tech. Info. Center).
«T. 8. Johansson et al., Nucl. Instr. £ Meth. 84
(1970) 141.
?T. B. Johansson et al., Advances in X-Ray Analysis JL£
p. 373 (Plenum Press, 1972).
8T. B. Johansson et al,, (accepted Anal. Chen.).
9R. I. Mitchell and J. M. Pilcher, Indus, and Engr.
Chem. 5.1, 1039 (1959).
100. L. Meinert, U.S. thesis, The Florida State univer-
sity, June 1974 (unpublished).
33
-------
NUCLEAR INSTRUMENTS AND METHODS 142 (1977) 251-257 ; © NORTH-HOLLAND PUBLISHING CO.
REX - A COMPUTER PROGRAMME KOR P1XE ANALYSIS*
HENRY C. KAUFMANN
Department of Physics. Florida Slate University. Tallahassee. Fla 32306. U.S.A.
K. ROLAND AKSELSSON
Department of Environmental Health, University of Lund. 22362 Lund, Sweden
and
WILLIAM J. COURTNEY
Department of Physics, Florida A&M University. Tallahassee, Fla 32307, U.S.A.
The implementation of a physics-based model for non-linear least-squares analysis of proton-induced X-ray emission
spectra via a Fortran programme REX is discussed. The modelling of distinct spectral components and of the physical
effects involved is briefly discussed. Results are presented in graphical and tabular form. The limitations of the present
model are discussed and future refinements indicated.
1. Introduction
Proton-induced X-ray emission analysis (PIXE)
has been shown to be a fast, inexpensive, reliable
and convenient method for routine multi-elemen-
tal trace analysis'). The main divisions in such an
analysis are: sample preparation and bombardment
and decomposition of the measured pulse-height
spectra. Intense work is going on in all these fields
to find suitable, time-saving sample preparation
techniques, to build bombardment and data-taking
systems which are fast, reliable and automated
and to develop inexpensive, reliable and automat-
ed computer programmes for resolving PIXE
spectra. Kaufmann and Akselsson2) and Kauf-
mann et al.3) have earlier described versions of a
computer code, REX, based on a model of the
physics in PIXE-analysis. This report describes the
current version of REX, some results obtained
with it and a discussion of further improvements
which can be made.
The parameters of the model are found using a
least-squares minimization based on a technique
developed by Kaufmann4).
2. The model
The radiation environment due to the photons
present during proton bombardment of a sample
may be deduced from the pulse-height spectra ob-
tained from a Si(Li) detector. The photon radiation
spectrum consists of discrete quanta due to char-
Research supported in pan by US Environmental Protection
Agency, Grants R-803913 and R-803887.
acteristic X-rays and a continuum. The continuum
proper has its origin in two phenomena: (1) brems-
strahlung of secondary electrons and of protons2'5)
and (2) Compton-scattered electrons from high-en-
ergy X-rays and y-rays which originate in or near
the sensitive volume of the detector. For typical
samples, the bremsstrahlung component is domi-
nant: however, the sample thickness and compo-
sition and the amount of absorptive material be-
tween the sample and the detector can alter the
relative contribution of the Compton electron
component. To the first order, both phenomena
scale with sample thickness and, while the brems-
strahlung component will be attenuated by any
absorptive material present, the Compton electron
component will not.
The putative Compton electron component is
modelled in a fashion which implicitly allows for
phenomena such as bremsstrahlung escape and
backscattered photoelectrons. Both of these con-
tributions to the continuum are more properly mo-
delled in the detection process. Preliminary at-
tempts at implementing a more precise model for
these components will be deferred to a later sec-
tion. The model discussed here does not include
the silicon escape peaks for the characteristic ra-
diation: inclusion of these in the model presents
some difficulty as will be discussed later.
For the case in which the size of a thin sample
is smaller than the area of the proton beam, the
yield of a characteristic K. X-ray transition is given
by
I = nNacokTi T2sfl/(4n), (1)
VI. DETECTORS AND ELECTRONICS
34
-------
H. C. KAUFMANN el al.
where n is the proton flux in particles per cm2, N
is the number of atoms of the elements, a is the
ionization cross section for the K.-shell, w is the
fluorescence yield for the K-shell, k is the branching
ratio for the emission of the characteristic X-ray, r, rs
the transmission between the sample and the de-
tector, TI is the mean transmission for X-rays
through the sample itself, e is the detector effi-
ciency and Q is the solid angle subtended at the
detector by the collimator.
The proton flux, n, is deduced from the dead-
time-corrected integrated charge and the values for
er, w, and It are taken from refs. 6, 7 and 7 re-
spectively.
The factors JT,, T2 and £ in eq. (1) depend on the
X-ray energy as do the continuum components to
be discussed later. Thus, it is most convenient to
express the model in terms of "equivalent" X-ray
energy rather than as pulse-heights. The "equiv-
alent" energy corresponding to a particular pulse-
height is taken here to be the energy of fictive
monoenergetic X-rays which would give their full-
energy peak at that pulse-height. The parameters
of the transformation of the spectra from
pulses/channel to pulses/energy interval are in-
cluded in the least-squares fit.
The form of the transformation depends on the
characteristics of the pulse-height analysis system.
For optimum speed, a linear calibration is crucial
and in this case the transformation is given by
£ = p, + p2(x-;0), (2)
where p, and p2 are parameters, x is the channel
number and j0 is a fixed channel number chosen
to optimize the calculational procedures.
When applied to a thin sample on a thin back-
ing, the model gives the following expression for
the yield of pulses per energy unit:
f(P, L, E) = CON 1 (P, E) + T, (P, E) T2 (P, E)
CON2(P, £) + £ L(I) T, (P, £(/, J))
T2(P, £(/,J))-PEAK(P, £(/,./)), (3)
where / is the number of pulses per energy unit,
P is an array of non-linear parameters, L is an ar-
ray of linear parameters, T\(P, E) is the trans-
mission through absorbers external to sample,
Ti(P. E) is the mean transmission through the
sample material, CONl(/>, £) is the continuum
unaffected to the first order by absorption effects
such as, e.g., from Compton scattered electrons,
CON2(/>, E) is the continuum affected by absorp-
tion such as, e.g., bremsstrahlung from secondary
electrons and PEAK(/>, £(/, J)) is the characteristic
X-ray line shape convoluted with a detector re-
sponse-function.
The indices / and J refer to the atomic number
of the element, e.g. Fe, and to specific character-
istic X-ray transitions of the element, e.g. K.,,, re-
spectively. The parameter arrays, P and L, are
found by minimizing the function
(4)
where y, is the number of pulses in channel
/, w, = 1/y, if y,>0, otherwise y, = 1 and d£/d* is
the derivative of the transformation from channel-
space to energy-space. In the case of a linear
transformation [eq. (2)], the component d£/dx is
folded into the linear parameters and is thus in-
cluded in /.
It is the simultaneous fitting to all the model
parameters describing the entire pulse-height spec-
trum which distinguishes the present approach
from others. As is illustrated by the fitted data
shown later, this approach provides a versatile so-
lution to such common problems as element inter-
ferences and the use of unorthodox absorption
schemes.
3. Discussion of the components in the model
3.1. CHARACTERISTIC X-RAY TRANSITIONS
Although the detector response to a X-ray
transition is better described as the sum of a
Gaussian and a low-energy electron backscatter
tail8), to minimize computing-time, we have chos-
en to approximate the system response by a Gaus-
sian without tails. The full width at half maxi-
mum (fwhm) of this distribution is dependent on
the energy of the detected radiation. In REX, this
dependence is taken to be of the form
fwhm-p, [£/£«]", (5)
where p} and p4 are suitable parameters and £0 a
reference energy which serves the same purpose
as y» in eq. (2).
The intensity of the X-rays from each element
are parameterized with a single amplitude and the
different transitions for each element are scaled to
this parameter using known relative intensities.
This procedure lessens the ambiguity in dealing
with interfering transitions from different ele-
ments. Transitions from a particular element are
included individually if they are separated by more
than 50 eV.
35
-------
REX
Obtaining reliable measurements of the elemen-
tal masses based on the yield of K X-rays depends
mainly on the accuracy of the elemental stan-
dards. For L X-rays, the relative intensities are sig-
nificantly dependent on the proton energy. Thus,
elemental standards for L X-rays have to be ana-
lysed at the same energy as the samples. For thick
samples, these variations in the relative transition
probabilities give rise to complications which can-
not yet be resolved using REX.
3.2. CONTINUUM
The continuum component not affected by first-
order absorption, CON1, is mainly caused by
Compton scattered electrons passing through the
sensitive volume of the detector. Other processes
are, however, also involved and it is not straight-
forward to model this component from basic phy-
sics of the process. The following empirical expres-
sion has been found to give good fits:
CON1 = p11/E + pl2-r-p13z + p14z2 + p15z3, (6)
wherez = x-y'i,x is the channel number and J, a
reference channel chosen to give a suitable range
for the numbers in the calculations. In spectra
having no peaks above 15 keV, the parameters pti,
PH and p]S are held fixed at zero.
The bremsstrahlung component has been thor-
oughly investigated5). However, to save comput-
ing-time, we have used the approximation
CON2 = exp(-pt,z)-(p7 + p8z + p9r2 + p10z3), (7)
where z=x-jt,x is the channel number and y'2 a
reference channel. Empirically this expression is
found to give good fits.
3.3. TRANSMISSION
In earlier versions of REX2-3) four nonlinear pa-
rameters were used to describe the transmission
through the sample itself and through external ab-
sorbers. Since each nonlinear parameter adds sig-
nificantly to the computing-time and since three
of these parameters did not vary between runs, we
have remodelled the transmission factors.
Blank Nuclepore
I mg/cm*.0.4^.
Absorber: None
Hole:-
' 0 100 200 300 100 SOO
CXI1KNEL NUMBER
Blonk Nuclepore
Img/cm1.0.4^4
Ft Absorber :350/im Mylar
Hole: None
\, .
V-V:^STI
10 0 10o 200 300 UOO 500
CHANNEL NUMBER
Blank Nuclepore
I mo/cm* ,0.4 fj.
Absorber :350|im Mylar
Hole: 9%
200 300 KOO
CHPNKEL NUMBER
SOO
Fig. 1. Fits to proton induced X-ray emission spectra obtained from bombarding nominally blank Nuclepore niters using a variety
of detection schemes. Note in particular the fit to the spectrum obtained by using an absorber with hole (bottom).
VI. DETECTORS AND ELECTRONICS
36
-------
H. C. KAUFMANN et al.
The external absorber is a multi-component ab-
sorber consisting of foils to maintain vacuum in
the sample chamber and the detector housing, of
the gold layer and the dead-layer of the detector,
of air and of an optional absorber. The "trans-
mission" through this system is modelled by
T, = £TOP, = exP(fl£1') [1 - exp(C£')] Top,, (8)
where E is the detector efficiency and 7"opl the
transmission through an optional absorber. The
contants a, b, c, and d in the expression for the de-
' lector efficiency9) are initially obtained by a least-
squares fit and are then held fixed.
The mean transmission through the sample it-
self is modelled by
T2 = [1 - «p{-ji(E)ps}]/&i(E)p,], (9)
where //(£) is the absorption coefficient of the
sample matrix. For certain standard matrices, /j(£)
is tabulated in REX.
The optional absorber may be a solid Mylar
sheet. However, in the usual case with samples
predominantly composed of low Z elements but
with traces of high Z elements, an absorber with
a small hole may be an optimum. This type of se-
lective absorption is easily introduced without ad-
ding more parameters by writing
ropl = (l-W)exp{-/((£)rf} + H, (10)
where H is the fraction of the solid angle Q sub-
tended by the hole, n(E) the absorption coefficient
as a function of the X-ray energy E and d the
thickness of the absorber.
4. Results
The coding of the model in the Fortran pro-
gramme REX has been undertaken with speed of
calculation and memory requirements being of pri-
mary concern. At present the programme is com-
piled in 42k of memory in a Datacraft 6024/3
computer. Execution time per spectrum evaluation
varies between 20 and 180s depending on the
quality of the starting parameters and the number
of fixed parameters.
10S
u
10
a
103
102
..1
Nelson Streoker
c. Absorber: None
« Hole:-
* j A
T A ( .
few ,
J' --U' '
7 -V p
' ^..
'^ LA A <»>,,»,,,
' "^"f-^-s^^'i^
10S
u
d
i
U 3
COUNTS/
5M °c
,
ca
I
:|V^
V '
J
Nelson Streaker
' Absorber : 350 urn Mylar
J! Hole: None
,|
W '
"^^y '. w
\^ /; [; tm
^5" i '; A '':
'V1 'lI'lV*1"
XiB i to (* .*,
x^/uV<
bH
Zr
^ **
5 3
5 to3
10 o
100 ZOO 300 HOO SOO 600
CHANNEL NUMBER
CHANNEL NUKBER
Nelson Streaker
Absorber: 350/j.m Mylar
Hole: 9%
300 100
CHANNEL NUMBER
Fig. 2. Fits to proton induced X-ray emission spectra obtained from bombarding an aerosol sample obtained with the Nelson
Streaker. For quantitative comparison see table 1.
37
-------
REX
TABLE I
Comparison of results from runs using different absorbers. The sample analyzed is from 2 mj air and the count rate in all three
analyses was lUOOcps analyzed for 500s each.
Element
Al
Si
P
S
Cl
K
Ca
Ti
V
Mn
Fe
Ni
Cu
Zn
Pb
Br
Sr
Zr
Absorber: None
Hole:
Amount Error MDL"
(ng/cm2) (%) (ng/cm2)
1914
6743
426
1633
233
557
3300
133
24
1404
23
36
74
556
120
41
45
53
39
40
32
23
20
19
17
20
14
19
15
15
16
14
19
21
205
53
36
21
18
13
12
11
11
11
7
7
6
6
27
6
10
15
Absorber: 350 //m
Hole: None
Amount Error
(ng/cm2) ('*)
543
3486
165
12
33
1625
10
28
69
693
119
44
26
20
19
17
25
16
14
17
13
14
16
14
14
15
Mylar
MDL"
(ng/cm2)
1030
32
13
6
5
3
2
2
2
2
5
1
2
3
Absorber: 350 tim
Hole: 8.9'Ai
Amount Error
(ng/cm2) (%)
2013
5740
203
1314
90
504
2893
135
26
1338
9
27
61
573
110
42
30
37
32
36
30
24
19
19
16
16
14
19
14
14
16
14
14
15
Mylar
MDL"
(ng/cm2)
231
59
40
24
20
14
11
7
7
5
3
3
2
2
8
2
2
4
Minimum detection limit.
The quality of the present model may best be
assessed by examining fitted spectra from a varie-
ty of detection and sample conditions. In this sec-
tion, we present such an assessment.
The continuum model was tested by obtaining
photon spectra from a nominally blank hydrocar-
bon filter material such as Nuclepore. In fig. 1 fit-
ted spectra for data obtained using the three com-
mon absorption schemes in the aerosol research
programme at FSU are shown. The continuum
model provides a good description of the data in
all three cases.
A test of the complete model on real data is
presented in fig. 2. The data are from the proton
bombardment of an aerosol sample obtained with
a continuous filter sampler10). Once again the
spectra were obtained using the three common ab-
sorption schemes and are from bombardments of
the same spot on the filter. As a further illustra-
tion of the overall versatility and reliability of the
analysis, the masses obtained for each element in
the three analyses are tabulated in table 1.
Careful study of the figures and table 1 indi-
cates some of the inadequacies of the present
model. The silicon K-shell edge absorption affects
the measured values of P, S, and Cl significantly.
The fitted spectra deviate from the functional
form of the data at both the lowest and the high-
est energies in the spectra. However, the analytical
errors associated with these effects are comparable
to the instrumental systematic errors.
The ease with which elemental interferences are
handled is illustrated in fig. 3. In the upper part
of the figure is displayed the result of a fit to a
PIXE analysis of an aerosol sample from Chacal-
taya, Brazil") in which As has been omitted from
the element request list. The fit for the Pb X-rays
is seen to be very poor. The lower figure includes
both As and Pb in the request list and the result-
ing fit is excellent. The element library and the si-
multaneous fitting of all model parameters greatly
facilitates the correct accounting of element inter-
ferences.
5. Discussion
We have presented a status report describing
the continuing development of a programme to
obtain reliable quantitative measurements of trace
elements using proton induced X-ray emission.
The model used is based on the physics of X-ray
VI. DETECTORS AND ELECTRONICS
38
-------
H. C. KAUFMANN et al.
100 200 300 <400 500 600 700
Chocoltoyo .Bolivia
Atsi»be.:J50,mM,lar
Hol<:9%
100 200 300 100 SOO 600 700
CHBNNEl. NUHBER
Fig. 3. Fits to a spectrum from an aerosol sample without (up-
per) and with (lower) arsenic in the request list.
production and detection. The data presented at-
tests to the overall suitability of the model and of
its implementation.
Pointers to future refinements of the model are
shown by the inadequate accounting of the con-
tinuum as can be seen in the fitted data in figs.
1 and 2. At present, the errors associated with
these errors are comparable to the errors associated
with the instrumentation and do not therefore
greatly lessen the accuracy of the analysis.
Some of the difficult problems which must be
solved are displayed in fig. 4. The upper figure is
a spectrum from a long accumulation of the X-
rays from a 55FeCl2 source. The spectral properties
which we do not explicitly include are (a) the Si
K X-ray escape peaks and (b) possible radiative
Auger emission12). Although we can, in principle,
include the escape peaks in the present model, in
practice the inclusion is made difficult by the ex-
cessive width of the escape peak compared to that
of a full energy X-ray of the same energy. The
radiative Auger transition is very weak [a 0.5% of
the intensity of the ^-transition12)) and thus
should only present an analytical problem when
small amounts of a lower Z element are to be
55FeCl2 Source
RATiRodKJtive Auge' Transition
MnK.
1 0 100 200 300 UOO SOO 600 700 800
CWWNEL KIM8ER
Lanthanum Standard
UaorOa-.U ea fm
LoK.
10
300 WOO 500
CHANNEL NUMBER
Fig. 4. Two spectra showing features which the current ver-
sion of REX is unable to fit. These are silicon escape peaks
(ESC) and radiative Auger emission (RAT) in the upper spec-
trum and the low-energy shoulders on the peaks and the broad
bump between channels 150 and 500 in the other spectrum.
measured in the presence of an abundant interfer-
ing element of higher Z.
The lower part of fig. 4 shows a spectrum taken
during proton bombardment of a La standard. The
La L X-rays have been absorbed so that the detec-
tor response to the K X-rays can clearly be seen.
In this particular case, the causes of the shoulders
on the low energy sides of the peaks and of the
rise of the continuum at energies below about
channel 450 are not unambiguously known.
We hope to resolve some of these questions by
measuring the detector response to monoenergetic
photons obtained by single-crystal diffraction.
References
') T. B. Johansson. R. E. Van Grieken, 1. W. Nelson and }. W.
Winchester. Anal. Chem. 47 (1975) 855.
2) H. C. Kaufmatm and K. R. Akselsson, Advan. X-ray Anal. 18
(1975)353.
3) H. C. Kaufmann. K. R. Akselsson and W. ). Courtney, Ad-
van. X-ray Anal. 19(1976)355.
4) H. C. Kaufmann, to be published.
39
-------
REX
5) F. Folkmann.J. Borggrenand A. Kjeldgaard, Nucl. Instr. and ') H. M. Schupferling, Nucl. Instr. and Meth. 123(1975)67.
Meth. 119(1974) 117. ') W. J. Gallagher and S. J. Cipolla. Nucl. Instr. and Meth. 122
6) K. R. Akselsson and T. B. Johansson, Z. Physik 266 (1974) (1974) 405.
245. I0) i. W. Nelson, B. Jensen, G. G. Desacdeleer. K. R. Akselsson
7) W. Bambynek, B. Crasemann, R. W. Fink, H.-U. Freund, H. and J. W. Winchester, Advan. X-ray Anal. 19 (1976) 403.
Mark, C. D. Swifi. R. E. Price and P. Venugopala Rao, Rev. ") F. Adams, private communication.
Mod. Phys. 44 (1972) 716. 12) G. Presser, Phys. Lett. S6A (1976) 273.
VI. DETECTORS AND ELECTRONICS
40
-------
NUCLEAR INSTRUMENTS AND METHODS 142 (1977) 127-132 ; © NORTH-HOLLAND PUBLISHING CO.
LIGHT ELEMENT ANALYSIS BY PROTON SCATTERING
J. W. NELSON*
Department of Physics. Florida Slate University, Tallahassee, Florida 32306, U.S.A.
and
W. J. COURTNEY'
Department of Physics. Florida A & M University, Tallahassee, Florida 32306. U.S.A.
A system for quantitative elemental analysis by proton scattering at 16 MeV has been developed. Samples of thicknesses
up to 1 mg/cm2 may be analyzed for the light elements up to Cl. Examples of spectra for atmospheric paniculate matter
and biological specimens are shown.
1. Introduction
Proton induced X-ray emission analysis (PIXE)
may be used to perform quantitative analyses of
any element on many types of samples. No single
system design is optimum for measurement of all
elements in samples ranging from very thin to in-
finitely thick to the proton beam. Using proton
beams of the Florida State University Tandem
Van de Graaff accelerator, we have developed an
energy dispersive PIXE analysis system1) for thin
solid samples (thicknesses up to approximately
1 mg/cm2). Attenuation of the softer X-rays in
both the sample and windows, limits the useful-
ness of our system to analyses of elements having
Z>13. Proton elastic scattering analysis (PESA)
has been developed for light elements in order to
complement our PIXE technique.
While quantitative PIXE analysis may be per-
formed for even the lightest elements, absorption
by the sample itself imposes severe limits on the
useable thicknesses of solid samples. In planning
our system primarily for determinations on air par-
ticulate matter, individual particle linear dimen-
sions up to 10-50 nm were of interest. For selected
elements the thicknesses needed to reduce the in-
tensity of a beam of X-rays by one-half as a func-
tion of photon energy are shown in fig. 1. For par-
ticles in the 10/im region attenuation is seen to be
a major correction in quantitative measurements.
For this reason proton scattering was investigated
as a complementary method for analysis of light
elements.
Research supported in part by EPA Grant R802913030.
2. Proton scattering analysis
A light element method to supplement PIXE
analysis should possess similar characteristics and
sample requirements as PIXE. Proton elastic scat-
tering analysis has properties which make it quite
complementary. It is absolute, multi-element, rap-
id, subject to automation and non-destructive of
the prepared sample. Sample thicknesses up to
about 1 mg/cm2 may be used. Unlike the X-ray
analysis in which too thick a sample attenuates
but does not degrade the quality of the spectrum,
PESA spectra exhibit loss of resolution which im-
mediately signal that sample thickness is too large.
Although nuclear scattering cross sections are
several orders of magnitude smaller than those for
X-ray production, comparable analysis times (min-
100
in
c/)
LU
(J
I
<
I
10
0.1
I 2.3 4 5
PHOTON ENERGY (KeV)
Fig. 1. Thicknesses needed to reduce the intensity of a beam
of X-rays to half value as a function of photon energy for
selected elements.
IV. USE OF HEAVY IONS
41
-------
J. W. NELSON AND W. J. COURTNEY
10'
4
10'
K X-RAYS
Ep 5MeY
L X-RAYS
Ep SMeV
M X-RAYS
Ep SMeV '
Elastic Protons
Ep 16 M«V
6L IZO-
IO°LI I I I I I I I I I I I I I I I 1 I I I I I I I I I I I I I I \-\
2 6 10 14 18 22 26 30 34 38 42
B« 0 Ma S
ATOMIC NUMBER
50M58626670747882
Br n
Fig. 2. Overall efficiency of the Florida State University combined PESA-PIXE quantitative analysis system.
16
I
u ,4
§
513
PROTON SCATTERING
Ep'ICMeV
I3S-
CSkiV
16
14
13
0 20 4O 6O 80 KDO 120 140 160 ISO ZOO
SCATTERING ANGLE (DEC)
Fig. 3. Kinematics of 16 MeV elastic proton scattering for some of the light elements.
42
-------
LIGHT ELEMENT ANALYSIS
utes) are achieved on air paniculate samples. This
is in part due to the presence of larger amounts of
many light elements. In fig. 2 the overall efficien-
cy of the FSU system2) is shown. Only those ele-
ments for which the production cross section has
been determined for our system are shown. The
ordinate is not the cross section as usually defined
but the readily useable number of outgoing parti-
cles per //C of incoming protons per ^g/cm2 of
sample.
Values of elastic scattering cross sections are
generally an order of magnitude higher than the
inelastic cross sections. Thus elastically scattered
protons were chosen for analysis rather than
inelastic groups or the gamma rays accompanying
inelastic scattering. Further choices of the incident
proton energy and angle of detection were dictated
by other considerations. An incident proton ener-
gy 16 MeV was selected on the basis of depend-
able accelerator operation (1 MV below maximum
rated voltage), greater energy difference between
adjacent mass nuclei, and smallest energy loss in
the sample. In fig. 3 the kinematics of 16 MeV
elastic proton scattering are presented. Selection of
a rearward scattering angle- is necessary for a suf-
ficient energy difference to permit resolution of
target mass 27 from mass 28. Assuming an overall
system resolution of 30-40 keV is maintained, a
scattering angle of 135° will result in a 65 keV sep-
aration to allow determination of both aluminum
and silicon. From fig. 3 protons scattered at 135°
from elements carbon through chlorine will have
energies between 12 and 15 MeV and may be
100
>
«
i 80
o:
UJ
z
UJ
z 60
? 40
<
5 20
Ep=l&OOMeV
8
01234
ANGLE SUBTENDED BY DETECTOR(DEC)
Fig. 4. Energy variation of scattered protons versus the angle
subtended by the detector.
stopped in large area (300 mm2) 1500 yum depletion
depth detectors.
In the desire to attain the highest counting rate
consistent with 30-40 keV overall resolution, the
energy broadening contribution due to the angle
subtended by the detector was calculated and is
shown in fig. 4. At a scattering angle of 135° we
utilize detector collimators which subtend l°-2° in
order to constrain this source of loss of resolution
RUN 1643 KRPTON
0 100 200 300 400 SOO 600 700 800 800 1000 1100
CHRNNEL NUMBER
Fig. S. PESA spectrum Tor Kapton plastic.
IV. USE OF HEAVY IONS
43
-------
J. W. NKLSON AND W. J. COURTNEY
to about 20 keV. A further practical consideration
is the mechanical problem of milling elliptically
shaped collimators for large area detectors which
subtend a significant fraction of the cone of equal
energy outgoing particles. From the standpoint of
the kinematics alone, the most extreme backward
scattering angle approaching 180° is desirable. Me-
chanical problems of target chamber construction
with detector outside the vacuum chamber influ-
enced us not to use this approach. On the other
hand the 90° scattering angle offers the possibility
of the use of linear shaped collimators but at a
cost of less kinematic energy separation of adja-
cent masses (see fig. 3).
3. A particular PKSA system
One system has been developed at FSU using
16MeV incident protons and detecting scattered
protons at 120° with respect to the incident proton
direction. Typical PESA spectra are shown in figs.
5 and 6. In fig. 5 the spectrum of Kapton plastic
is shown. Referring to fig. 2 it may be observed
that carbon and nitrogen are accentuated relative
to oxygen at this scattering angle and incident
energy. Thus a suitable evaluation computer code
must be developed to account for the relative
cross sections. Such a program is in the final
stages of development and is closely patterned af-
ter our PIXE analysis program REX3). For exam-
100 200 300 UOO SOO 600 700 800 900 1000 1100
CHPNNEL NUMBER
Fig. 6. PESA spectrum for the element Al. This is the most complex spectrum among those of the elements lighter than calcium.
62500
40000
; 22500
j 10000
2500
GM- STREAKER 15-D
xs
0 100 200 300 MOO SOO £00 700 800 900 1000 1100
CHANNEL NUMBER
Fig. 7. PESA spectrum obtained by bombarding a Nuclepore niter paniculate sample of relatively clean winter air from rural
Michigan.
44
-------
LIGHT ELEMENT ANALYSIS
pie I2C is characterized by both the elastic peak la-
belled in fig. 5 and the first inelastic group just be-
low channel number 100. Similarly UN is charac-
terized by the labelled elastic peak and that due to
the proton group to the second excited state which
occurs just above channel number 200. Although
the majority of light elements have spectra which
are simply described, the most complex case is de-
picted in fig. 6. The 27A1 spectrum requires about
IS Gaussian peaks for its description. This is simi-
lar to the situation with the L and M X-ray lines
of Pb which need 17 Gaussian peaks for their de-
scription in our PIXE analysis program. It should
be noted that in cases such as aerosol analysis
only the dominant- elastic "Al peak will be sig-
nificant.
4. Applications
The PESA system was designed primarily for
analysis of air paniculate samples. An example of
this application is shown as fig. 7 which is the
scattering spectrum obtained by bombarding a Nu-
clepore filter paniculate sample of relatively clean
winter air from a rural area of southern Michigan.
1685 LA ST-1 (PESO)
total 23)
sample 0.6
see see 7ee see see leee nee
CHONNEL NUMBER
Fig. 8. PESA spectrum of a size fraction of paniculate matter from a site near Los Angeles, Ca. The numbers above the peaks
are the mass of that element in micrograms per cubic meter of air.
0 100 200 300 <400 SOO 600 700 600 900 1000 MOO
CHHNNEL NUMBER
Fig. 9. PESA spectrum resulting from the bombardment of 2 mm of the root end of a human hair.
IV. USE OF HEAVY IONS
45
-------
J. W. NELSON AND W. J. COURTNKY
10 0 100 200 300 tOO 500 600 700 800 900 ~1000 1100
CHRNNEL NUMBER
Fig. 10. Clam heart spectrum. Sample prepared by flattening between glass plates and freeze drying.
In this spectrum the I2C and "0 peaks are dom-
inated by those isotopes contained in the filter it-
self. By contrast the spectrum4) shown in fig. 8
was taken by bombarding a thin polystyrene
backed (100 #g/cm2) sample in which case subtrac-
tion for the I2C in the backing is feasible. The
numbers above the peaks are the mass of that ele-
ment in micrograms per cubic meter of air.
Although suitably thin biological samples are
usually difficult to prepare, results have been
achieved in several cases. Fig. 9 is the spectrum
resulting from the bombardment of 2 mm of the
root end of a single human hair. The somewhat
broad peak labelled 32S contains lesser amounts of
P and Cl which were detected by P1XE analysis.
For hair specimens of thicker cross section the res-
olution of adjacent peaks is not as complete as for
those in fig. 9 but indications are that they can be
evaluated with the aid of a computer code. Ano-
ther example of a biological sample analysis is the
clam heart spectrum shown as fig. 10. Clam hearts
are normally too thick to permit resolution of ad-
jacent peaks. This difficulty was overcome by flat-
tening the heart between two glass plates.
5. 'Conclusions
A quantitative elemental analysis system utiliz-
ing proton elastic scattering analysis for light ele-
ment determinations has been developed. For thin
samples (1 mg/cm2 or less) the method is absolute,
multi-element, rapid (minutes) and non-destruc-
tive of the prepared sample. In conjunction with
proton induced X-ray emission analysis, it forms a
system capable of measurement of all elements
within its detection limits. Extensive application to
the analysis of atmosphere particulate matter have
begun and feasibility studies for biological samples
are being explored.
References
') T. B. Johansson, R. E. Van Grieken, J. W. Nelson and J.
W. Winchester, Anal. Chem. 47 (1975) 855.
2) J. William Nelson, Proc. EPA Symp. and Workshop on X-
ray fluorescence oj environmental samples. Chapel Hill, N.C.
(1976) (in press. Ann Arbor Science Publ.).
3) H. C. Kaufmann, K. R. Akselsson and W. J. Courtney. Ad-
van. X-ray Anal. 19 (1975) 355.
4) K. R. Akselsson. J. W. Nelson and J. W. Winchester. Null.
Bu. of Stds. Special Publ. 425, U.S. Govt. Printing Office
(1975).
5) E. C. Henley. J. W. Nelson and M. E. Kassouny, Proc. 10th
Annual Conf. on Trace substances in environmental health (Ed.
D. D. Hamphill; in press USAEC).
46
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-6QO/2-78-213
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
PROTON SCATTERING ANALYSIS FOR LIGHT ELEMENTS IN
AIR PARTICULATE MATTER
5. REPORT DATE
1Q7R
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
J. Nelson. G. Hudson, H. Kaufmann, W. Courtney,
I. Williams, K. Akselsson, D. Meinert, J. Wincnester
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Physics Department
Florida State University
Tallahassee, Florida 32306
10. PROGRAM ELEMENT NO.
1AD712 BB-38 (FY 781
11. CONTRACT/GRANT NO.
R - 802913
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Protection Agency - RTP, NC
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park. NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final 5/74 - 7/78
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Proton scattering analysis has been developed to provide analyses of elements
with atomic numbers ranging from 6 to 20 as a complementary method to X-ray
analysis. Samples of air particulate matter from both filter and impaction
devices have been analyzed and time sequence and particle size information
obtained. In combination with X-ray analysis, total mass may be determined by
summation of elemental masses; however, further investigation is needed to
compare with the gravimetric method. Like X-ray analysis, it has a broad range,
and is rapid, subject to automation, and non-destructive of the prepared sample.
Analyses of typical atmospheric samples may be performed in five minutes or
less. With further development, the method can readily be advanced from experimental
to routine usage.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
* Air pollution
* Particles
* Chemical Analysis
*Proton Scattering
* Chemical Elements
Light weight elements
13B
07D
2 OH
07A
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCT.ASSTFTF.n
21. NO. OF PAGES
57
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
47
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