EPA-R2-73-207
January 1973 Environmental Protection Technology Series
Development of Improved Systems
for Obtaining Time Integrated
Measurements of SC>2, NC>2, NOX,
and Other Pollutants
Office of Research and Monitoring
U.S. Environmental Protection Agency
Washington. D.C. 20460
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EPA-R2-73-207
Development of Improved Systems
for Obtaining Time Integrated
Measurements of SC>2> NC>2, N(3X>
and Other Pollutants
by
Victor R. Huebner
Instra-Tech, Inc.
1223 South State College Boulevard
Fullerton, California 92631
Contract No. 68-02-0318
Program Element No. 1A1010
EPA Project Officer: Andrew E. O'Keeffe
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND MDNITORING
U. S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, B.C. 20460
January 1973
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
ii
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TABLE OP CONTENTS
Section Title
1 Introduction and Summary ].
2 Overall Objectives 2
3 Basic Experimental Approach 3
3.1 General Background 3
3.2 Specific Characteristics of the Meloy Analyzer 3
3.3 Secondary Standards 5
4 Direct Photographic Film Detection 6
4.1 Basic Problems 6
4.2 Basic Film Sensitivity Considerations 7
4.3 Comparison of Different Films 10
4.4 Laboratory Tests of Film Sensitivity 11
4.5 Reaction Chamber Tests 18
.4.6 Gas Permeation Studies 31
4.7 Direct Air Injection 33
5 Alternative Detectors 34
6 Photon Counting 37
6.1 General Background 37
6.2 Electronic Considerations 39
6.2.1 General 39
6.2.2 Power Supply 4i
6.2.3 Photomultiplier Tube 4l
6.2.4 Amplifier Circuit 44
6.2.5 Digital Division 46
6.3 Results tip
7 Suggestions for Future Work 53
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LIST OF TABLES
Title
Relative Light Emission from the Meloy
Reaction Chamber 4
2 Comparison of Film Sensitivity 12
3 Polaroid 3000 Sensitivity 13
1» Tri-X Sensitivity 17
5 Diffusion Rate Parameters for a 1.5mm
I.D. by 10mm Long Tube 20
6 Relative Intensity Obtained from
Different Reaction Chambers 24
7 Relative Reflectivity of Various Tubes 27
8 Effect of Diffusing Ethylene into the
Reaction Chamber 31
9 Response of Various Photodetectors 35
11
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LIST OP FIGURES
Figure Title Pagi
1 Typical Response Curve for Photographic
Film 9
2 Chemllumlnescent Camera 16
3 Chemllumlnescence Reaction Chanber 2.1
4 Effect of Gas Velocity on Signal 23
5 Effect of Reaction Chamber Length 29
6 Gas Permeation Tube Test 32
7 Basic Components of the Chemllumlnescence
Photon Counter 40
8 PMT, Preamp, and Comparator Circuit 43
9 Photon Counter/Readout Circuit 47
10 Count Rate vs Current at Various
Threshold Levels 50
11 Threshold vs Counts at Various Light
Levels 52
12 Correlation of Count Frequency with
:Display Reading 54
13 PMT Current vs Meter Reading 55
14 .Ozone Concentration vs LED Matrix Readout 56
15 Appendix A - Digital Division Circuitry 59
16 Appendix B - Photon Counter Description 60
111
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1. Introduction and Summary
This program was primarily directed toward Investigating new
and improved methods of detecting and recording the weak light
emission resulting from the chemiluminescent reaction of air pollutants
with specific gaseous reactants. Major emphasis was placed on
attempting to use photographic film as both a detector and integrator
for the light emission. The intent was to expose film to the
chemiluminescent reaction, develop the film, and relate film density
to accumulated air pollutant. The major problem was that it requires
almost a million times as much light to produce a detectable response
on a one Inch square film plate as Is required for a photomultiplier
tube. This disadvantage can be minimized by concentrating the reaction
over a very small surface area. Consequently reaction chambers
having extremely small frontal areas were developed. Chambers that
were ,5mm diameter provided 1400 times as much light Intensity.
Unfortunately, this still wasn't enough to overcome photographic
film's poor sensitivity.
Although the photographic film approach was unsuccessful, several
avenues were explored that may be of value in the future. One of
the more important findings was that the chemiluminescence reaction
is primarily limited by physical mixing efficiency of the reactants.
Long, narrow reaction chambers provided much better mixing efficiency.
With excessively narrow tubes, however, optical reflectance losses
counteracted any Improved mixing efficiency. Nevertheless, moderately
small diameter reaction chambers should yield better signals and make
it possible to use small diameter detectors. The other major finding
of interest was that permeation tubes could be effectively used to
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meter in the reactant gas. This may make it possible to eliminate
the need for auxiliary reactant gas cylinders.
Photon counting was then explored as a means of increasing the
reliability and reducing the cost of chemiluminescent detectors.
This approach was quite successful. A system was developed that
amplified each photoelectron, discriminated against noise pulses,
accumulated all the photon counts, and displayed the sum of the
photon counts on a 5 x 7 light emitting diode display. The photon
counting system was packaged in a housing and was shipped to EPA
laboratories for further tests.
2. Overall objectives
Chemiluminescence techniques for monitoring air pollutants such
as S02, N02, NOx, ozone, etc. are extremely sensitive and reliable.
Their major advantage is that they require minimal mechanical components,
and no wet chemicals. Although Chemiluminescence methods are basically
simple and reliable, they impose severe sensitivity requirements upon
the optical detector system required to monitor light emission.
Photomultipller tubes coupled to high impedance analog amplifiers
are utilized almost exclusively in commercial Chemiluminescence
instruments. These Instruments generally perform quite well under
controlled laboratory conditions where the temperature is held constant
and technicians are available to zero and calibrate the instruments.
One of the purposes of this program was to develop methods of detecting
light emitted by Chemiluminescence which are more stable and reliable
than those presently used. The other prime objective of this program
was to develop a simplified method of time-integrating and recording
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the outputs of chemilumlnescence instruments. Data reduction
through time Integration la a practical necessity for any large
scale monitoring program. For time Integration to be effective,
instrument stability must be extremely good.
Present instruments must either have periodic manual calibration
or have very expensive self-correcting circuitry if long term
stability is required. Photomultlpller tube gain changes caused
by temperature variations or power supply changes are excessively
high. Even the best amplifiers are somewhat marginal in respect
to zero drift for this application. Consequently primary effort
was placed on elimination of the PMT system by utilizing photographic
film as the detector as well as recorder. When this approach proved
to be unsuccessful, digital photon counting methods were explored.
3. Basic Experimental Approach
3*1 General background
This program was oriented primarily towards the "readout"
portion of air pollution monitors rather than towards the chemical
transducer components. Consequently, a commercial ozone analyzer,
the Meloy Laboratories, Inc. OA320, was purchased for this program.
This analyzer is typical of the current state-of-the-art in pollution
analyzers based on the chemlluminescence principle.
3»2 Specific Characteristics of the Meloy Analyzer
The Meloy ozone analyzer has a 19mm diameter by approximately
20mm deep chamber with concentric 6.3 and 3.2mmO.D. tubes terminating
at the front glass wall to carry ethylene and air, respectively.
This IR essentially identical to the Nederbragt design. Recommended
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flow rates are 1 liter per minute for air and 28ml/min. for ethylene.
A UV lamp Is used to provide ozone for calibration purposes, and
gave a reading of 0.22ppm ozone under normal operating conditions.
When a Kodak Wratten 2B filter (400aim long pass) was Inserted
between the reaction chamber and the PMT, the ozone reading decreased
from 0.22ppm to O.l4ppm. When a Wratten 12 filter (510nm long pass)
was used, the ozone reading decreased to 0.02ppm. These results
Indicated that 36% of the light emission Is below 400nm, 555? Is
between 4oOnm and 510nm, and only 9% of the light emission Is greater
than 510nm. These results agree with those of Hodgeson, et al1.
In another series of tests, masks with various size center holes
were placed over the reaction chamber. These results are shown In
table 1. The data Indicate that light emission probably is evenly
TABLE 1
Relative Light Emission from the Meloy Reaction Chamber
Diameter hole
In mask Hole Area PMT Reading Intensity/mm2
6.0mm 28mm .Olppm 3.6
10.5 86 .08 9.3.
13.5 145 .14 9.7
19 (no mask) 284 .22 7.7
distributed throughout the reaction cell. The lower value for the
smallest hole probably Is due to a lower volume of gas since this
space Is occupied by the inlet tubes. The decline for the largest
area probably is due to a restricted angle of view because of the
chamber walls. Thus, it appears that light Is emanating uniformly
1. Hodgeson, J.A., Martin, B.E., and Baumgardner, R. E. Laboratory
Evaluation of Alternate Chemlluminescent Approaches for the Detection
of Atmospheric Ozone. ACS Meeting, Chicago, Sept. 1970.
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from all portions of the chamber. This would be the case If Incomplete
or Just barely complete mixing of the ethylene and oxygen were
occurring.
The response of the entire system was found to be 9x10-9 amperes
per ppm ozone. If we assume reasonable values for typical PMT's,
this corresponds to approximately IxlO-10 lumens, or llxlO-11 lumens
per centimeter. As wll] be shown In table 2 this is about 100 fold
less than the sensitivity threshold level for photographic film.
The response was inversely proportional to air flow rate (when the
ozone generator was used), and was moderately sensitive to ethylene
flow rate. As will be seen later, this is symptomatic of incomplete
reaction between ozone and ethylene.
3.3 Secondary standards
Several types of secondary light sources were used instead of
the Meloy Chemiluminescence monitor in order to provide greater
versatility and convenience. A Monsanto MV-1 light emitting diode
(LED) was used in most cases. By appropriately adjusting the voltage
and current-limiting resistors, highly reproducible light levels
could be easily achieved. The LED Ijght output was calibrated by
measuring its output with the Meloy analyzer PMT, and recording the
output in "equivalent parts-per-mlllion ozone". In this manner, the
relative sensitivity of experimental detection modes could be easily
determined. The primary disadvantage of the LED standard was that
light emission occurred at 6lOnm rather than at the ^30nm peak
wavelength of the ozone chemiluminescent reaction.
The other secondary standard consisted of a 24v. tungsten
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filament lamp driven by a 2lJvdc power supply through a series of
current limiting resistors. This lamp was optically coupled to the
detector under test (or to the Meloy PMT for calibration) through a
60 mm long by 6.3 mm D. quartz fiber optic tube, a Schott BG 18
infra-red filter, and a Kodak Wratten 36 filter. This combination
produced light with maximum intensity at 420nm. and a half-band width
of ^nm. Although the tungsten lamp's output Is less controllable than
a LED1s, it's better spectral properties was needed for some tests.
In all cases, light output was cross-correlated with the response of
the Meloy PMT system (indicated in ppm ozone), so that meaningful
comparisons could be made.
4. Direct Photographic Film Detection
4.1 Basic Problems
Photographic film was utilized in an attempt to eliminate
the present photomultiplj.er tube detection system because it has
a long history of usage in low light level application. Photographic
film also appeared to be highly advantageous for its simplicity in
integrating and recording the relative light intensity. Unfortunately,
photographic film also has two major disadvantages - reciprocity failure
and a dependance upon light intensity rather than total quantity of
light. Photographic film has a sensitivity threshold below which
an image will not be formed regardless of exposure time. Consequently,
low light levels will either have a very non-linear relationship to
film density or will go undetected. The mode of sensitizing -
through light intensity rather than light quantity further compounds
this problem. Whereas PMT detectors linearly respond to total light
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with no regard for light distribution, film response Is based upon a
light Intensity per unit area relationship. The only feasible method
of Improving this situation Is to concentrate the available light of
reaction into a smaller area. However, since the chemlluminescence
reaction is a gaseous reaction, the mixing problem becomes more
severe as reaction tube area is decreased.
4.2 Basic Film Sensitivity Considerations1
Essentially all film manufactured today is based on the silver
bromide reaction to light. The primary factor affecting film
sensitivity is the concentration of sulflde specks at the surface
of the silver crystals. These specks originate from high molecular
weight compounds in the gelatin used as the film base (synthetic
sulfur compounds give poor sensitization) and congregate at imperfections
in the silver bromide lattice. These sulflde specks require a
significant number of photon-generated electrons before they are
capable of promoting silver bromide sensitization through an electron
transfer process. This requirement for high concentrations of photon-
generated electrons arises because of a competing electron capture
process within the sulfide specks. Once this threshold level has been
reached, the silver bromide conversion can proceed at a rate governed
by light intensity. Larger silver bromide crystals hold more sulfide
specks and absorb more light than smaller crystals, thereby having
better sensitivity.
Sensitivity characterists of photographic film can be described
by a density vs. light exposure curve similar to the one shown in
1. Extracted from "Photographic Film" by P. Glatkides, Fountain Press,
London, 1958.
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figure 1. All film exhibits a nominal density called "fog" with no
light exposure. As the light Is Increased, the density begins increasing
at the threshold point. The curve then assumes a curved toe, a linear
section, and a curved shoulder. Very high light intensities create
a density decrease called "solarizatlon". Various systems are used
to designate film speed. Perhaps the poorest system Is the ASA system,
which has no scientific basis, Is only valid for high light levels,
and cannot be determined with any reasonable degree of precision. For
our purposes, the threshold level Is of paramount importance and re-
ferences to "sensitivity improvement" will refer to a decrease In the
quantity of light required to reach the threshold level. Consequently,
many of the observations made may appear to be contradictory to
popularly accepted notions.
It is generally believed that film sensitivity can be improved
by different developing procedures. Unfortunately, high speed
developing techniques only Improve contrast - they have no effect on
the film threshold level. Since the ASA ratings are based on contrast
ratios, darkroom manipulations will Improve the ASA value. They can't
make an invisible latent image appear if the film threshold hasn't
been reached.
The addition of sensitization chemicals to unexposed film can
greatly enhance sensitivity to long wavelength (red) light. They
cannot improve the basic film sensitivity at its maximum sensitivity
wavelength (350nm.) Thus, they serve only to extend the wavelencth
response characteristics and can do nothing to improve film response
to the ozone chemllumlnescence reaction, which already occurs near the
maximum wavelength sensitivity.
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Figure 1
Typical response curve for photographic film
solarization
Optical
Density
1
Quantity of light
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The most important sensitization techniques have consisted of
pre- or post-exposure of the film to light, mercury vapor, ozone, sulfur
dioxide, and various organic chemicals. These methods generally are
optimized by exposing the film to a level Just below its minimum
threshold level, so that the image of interest can easily trigger the
exposure. These methods require very careful control, and sensitization
beyond a factor of 3 improvement generally is considered to be too
tricky and unreliable.
4.3 Comparison of Different Films
Most of the faster types of film are compared in table 2. Column
1 lists the type of film, while its ASA value is listed in column 2.
The third column lists the basic threshold of the film. These values,
expressed In meter-candle-seconds, represent the lowest light level
that gives a barely perceptible increase in density over normal film
"fog". The threshold values were estimated in most cases from manu-
facturer's published curves. The next column lists the approximate
ratios of sensitivity at 400 nm. versus that at 500 nm. This is
important because film threshold values are determined at $00 nm,
whereas the ozone chemilumlnescense emission is near 400 nm. The
columns listing the sensitivity ratios for longer exposure times
(slOOsec/ slOOsec/ reflect the increase in density with
/slsec, /slOOsec.)
increased exposure time. Values of 100 would represent perfect linearity,
while lower values are the result of reciprocity failure. The conversion
to lumens per era2 puts film sensitivity on a radiometric basis. These
values represent the lowest concentration of 400 nm. light that is
capable-of creating a film Image..
Type laO is an astronomy emulsion that is coated on glass slides.
10
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It Is characterized by a relatively slow speed jut very little
"reciprocity failure". Thus, for very long exposure tines, it is more
sensitive than some of the "fast" film. Tri-X is a high speed'film
that is widely used by photographers. It is more sensitive than laO
for exposure times less than 100 seconds, but its poor reciprocity
characteristics make it less sensitive than laO at longer exposure
times. Royal Pan ^1*11 has approximately the same sensitivity as Tri-X.
Polaroid 3000 film is definitely faster for exposure times less than
100 seconds, but has very poor time-integration properties with longer
exposure times. Polascope 410 is not significantly better than
Polaroid 3000 film.
In summary it can be concluded that Polaroid 3000 film is
significantly more sensitive than laO, but has approximately the
.some sensitivity as do the other films listed in table 1 J f the exposure
time it> approximately 100 seconds. For very long exposure times, the
excellent reciprocity characteristics of laO make it superior to other
films. However, since photomultiplier tubes typically have sensitivities
~\ li ">
of l-5x lO"-1 lumens per cm"", even the best flirts have approximately
10,000 times poorer sensitivity than do photomultipller tubes.
4.4 Laboratory tests of film sensitivity
In addition to a theoretical assessment of the film sensitivity
problem, a large number of film tests were made using an MV-1 LED to
simulate chemilumlnescence. The standard procedure followed in all
cases was to prepare the test set-up, attach the Meloy PMT and
associated electronics, and then operate the LED at various current
levels in order to obtain a plot of input current vs. "equivalent ppm
ozone". In all cases, the LED current was less than one-tenth as great
as the current, required to be barely perceptible to a trained observer
11
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TABLE 2
COMPARISON OF FILM SENSPr
Film
laO
Tri-X Pan
Royal Pan
Polaroid 3000
ASA value Threshold (s)
slOOsec/ slOOsec/
25
320
400
3,000
Polascope lJ10 10,000
.01 meter candle
secor.ds
.001
.001
.0003
.0001
x500nm
1.0
1.0
0.6
1.0
1.0
s Isec
100
15
30
10
5
slOOsec
70
5
10
2
2
lOOsea
1x10-8
7x10-9
6x10-9
3x10-9
2x10-9
' "ij
1x10-10
1x10-9
6x10-10
1x10-9
1x10-9
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after 5 minutes of acclimation in a totally dark room. In other words,
the light intensities were exceedingly low.
After the calibration curve for a particular test set-up had been
prepared, the PMT was removed and photographic film was placed in the
same relative orientation as the PMT cathode had been. Various current
levels were than applied to the LED (one level for each film negative)
until it was determined that no image had been recorded. The lowest
current level applied to the LED that had resulted in an image
(usually Just barely perceptible) was then recorded as the minimum
film threshold value. By referring to the current vs. equivalent
ozone chart, this minimum threshold value could be reported in terms
of equivalent ppm ozone".
The test procedure previously described was applied to Polaroid
3000 film. Results of these tests are shown in table 3« The exposure
areas were estimated by measuring the developed negative's images. The
LED current used is only of academic Interest since the actual light
reaching the film was greatly influenced by the test set-up. The
TABLE 3
Polaroid 3000 Sensitivity
LED test set-up
One inch from film
One inch from film
Exposure
time
1 min
60
Through 1.6mm x 45cm
filter optic 5
Through 1.6mm x 30cm
tube 3
Through 0.8mm x 30cm
tube 3
Exposure
Area
?
3,0cm.
3.0
.070
.018
.005
LED
Current
7xlO~5A
3xlO~5
Equiv.
Ozone
70ppm
30
Ozone/
cm'
1.6x10-5 2
5x10'
.1
3x10
-5
23ppm
10
28
55
80
13
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equivalent ozone values were determined by comparing the LED current
required to produce a barely perceptible Image with the PMT output
response for that current. The ozone per cm2 values are simple
arithmetic derivations based on exposure area and equivalent ozone
threshold values.
The first test was made with the uncolllmated LED one inch from
the film. This resulted in a dense exposure in the center which diminished
in intensity towards the edges. This non-uniformity would have a tendency
towards lower equivalent ozone values. As may be seen, the 60 minute
exposure only resulted in a factor of two sensitivity improvement over
the one minute exposure time. This is the result of Polaroid film's
poor reciprocity characteristics. When the LED light was transmitted
through a fiber optic bundle, the total sensitivity was much enhanced
(due to the smaller area over which the light was spread) but the
sensitivity on a per unit area basis was approximately the same. Where
a 1.6 mm or 0.8 mm teflon tube was used to transmit the light, their
smaller exposure area created a better gross sensitivity value, but
the sensitivity per unit area wasn't appreciably affected (within
experimental error). It thereby appeared that Polaroid 3000 film has
an apparent sensitivity threshold of between 23 and 80 ppm ozone when
<•>
a one cm area is utilized. This sensitivity is only improved by a
factor of two when very long exposure times are used. This estimate
is biased strongly in favor of the film, since the film sensitivity
is essentially the same at ^30 nm, (ozone chemilumlnescence) or 610 nm.
(LED), whereas the Meloy PMT has only 5% as much sensitivity to 610 nm.
radiation as it does to 430 nm radiation. Thus, the true equivalent
sensitivity of Polaroid film is In the neighborhood of 1000 ppm. ozone/cm2.
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p
Since.PMT systems can easily detect lOppb ozone, a one cm Polaroid
system Is approximately 100,000 times less sensitive than a PMT. This
value agrees quite well with the factor of 10,000 calculated for the
best film in a theoretical system. Thus, 'the film system must overcome
a 10,000 - 100,000 fold disadvantage before it can compete with a PMT
system. One method, which Is feasible only for astronomy emulsions
such as Kodak Ta-0, is to Integrate the available light over exceedingly
long time intervals. This may provide up to a factor of 100 improvement
under Ideal conditions. The other technique is to reduce the frontal
area of the reaction chamber so that the chemiluminescence is concentrated
in a smaller area, thereby Increasing the intensity per unit area.
Simple gas dynamic considerations dictate that the optimal chamber
configuration would be a long narrow tube with infinitely good internal
reflectivity. These types of chambers will be discussed In section 4.5.
In order to accomodate the astronomy type film plates and to hold
the long tubular types of reaction chambers that appeared to be needed,
a special camera chamber was designed and built. This is shown in
figure 2. The rubber gasket was sandwiched between the upper and
lower halves to provide a light-tight, gas-tight seal. The lower
chamber contains an aluminum film holder that can be positioned by
means of a rod sealed by 0-ring bushings. The upper section contains
a light-emitting dJ.ode holder, a holder for chemiluminescence reaction
chambers, and a vacuum line.
All initial tests of this camera chamber were made with 16mm.
Trl-X film. The film was held on the film plate by double sided
Scotch tape. In a typical test, the film would be positioned in place,
it would be exposed under prescribed conditions, and then moved about
15
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1.25cm D. hole to accept
reaction chamber
To vacuum
Rubber gasket
Actuating rod
Felt
Film holder
Position Indicator
Figure 2 Chemiluminescent Camera
16
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1/JJ inch to a new position where different exposure conditions were
used. The film was moved between each exposure until all tests were
completed or untJ.l the entire film strip was used. Development by
Mlcrodol-X (according to the manufacturer's recommended procedures)
always immediately followed the last exposure.
The MV-1 LED produced an image l.lmm. in diameter for very low
light levels. Higher light levels resulted in a central dense spot
about l.lmm. in diameter, surrounded by a 5mm. diameter spot having
about one-third the density of the central spot. The values shown in
table lJ indicates some of the relative densities obtained under varying
conditions. The equivalent ozone values were obtained as before by
TABLE 4
Trl-X Sensitivity
LED CURRENT Equlv. Ozone Exposure time Relative Density
0
1
3
6
0
3
5
determining the Meloy PMT response to varying currents through the
LED. The relative density values were subjective appraisals on a 0-10
scale where 0 Indicated no perceptible image. It may be seen that a
light exposure equivalent to 0.18 ppm ozone produced no image, even
for 60 minute exposure times. Increasing the exposure time from 20
minutes to 60 minutes resulted in a marked increase in density for the
same exposure levels, and a decrease in the exposure level required to
produce a given density. This indicates that Tri-X has reasonably
good reciprocity characteristics. This is in marked contrast to prior
1 uA
2
H
8
1
2
3
0.18 ppm
0.7
2.6
9.5
0.18
0.7
1.6
20 min.
20
20
20
60
60
60
17
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results with Polaroid 3000 film where a 60 fold increase in exposure
time only resulted in a 2-fold improvement in sensitivity.
The results shown in table 4 Indicate that the minimum threshold
level for Tri-X film is about 0.3 ppro ozone for a 1.1 mm. spot, or
approximately 30 ppm/cm2. This is essentially the same as was found
for Polaroid 3000 film. Since this film also has a "flat" spectral
response characteristic, it's true sensitivity also is 20 times poorer
(due to the poor response of the PMT to the light wavelengths emitted
by the LED). This "true" sensitivity then becomes 600 ppm. ozone/cm2.
Clearly then, even with reasonable good photographic, films, the entire
-M -R
chemiluminescence reaction must ocnur within an area of 10 to 10 2
cm before photographic film can be comparable to PMT's.
4.5 Reaction Chember Tests
It was previously shown that the chemiluminesoenoe must be
concentrated in a very small area if adequate sensitivity is to be
achieved with photographic film. Theory dictates that it is impossible
to concentrate light by simple optical lenses or mirrors. Some preliminary
tests with lenses, mirrors, light guides, verified that normal optical
concentration techniques were useless. One possible optical concentration
technique would be to collect light emitted within a glass sphere with
fiber optics arranged in "porcupine" fashion. The fiber optics could
then be bent and arranged in a single bundle, thereby creating a rough
collination of the heretofore diffuse light. This collimated light
could then be concentrated by conventional optical methods. The
mechanical problems associated with this concept would be exceptionally
severe and it did not appear warranted to follow this path.
18
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It was shown that the chemilumlnescense reaction must be concentrated
-i| -52
in an area of approximately 10 to 10 J cm . This is equivalent to a
circle 0.04 to 0.11 mm. in diameter. Since this small size precludes
a spherical reaction chember, a ]ong tubular-shaped chamber was
dictated. Although the fact that the Meloy chamber exhibited marginal
mixing behavior even though it had a very large size was discouraging,
we proceeded with the deve]opment of smaller reaction chambers. The
first chamber had a 0.76mm O.D. teflon tube with a 3.2mm O.D. x 1.52mm
I.D. teflon tube that opened 15 mm back of the viewing port. This
pair of tubes was inserted into a tube that could be evacuated and
had a //O cover slip as its window. All Joints were sealed with
silicone rubber. This chamber was positioned against the PMT. Various
air flows and ethylene flow rates were tested. The ethylene flow
rate had only slight influence on a signal. The signal response was
essentially linear with the reciprocal of the air flow rate. Using
the inner tube for ethylene and the outer tube for air gave about
50$ greater response than the reverse case.
When this reaction chamber was operated under identical conditions
to those used for the Heloy chamber, a reading of . OOUppro ozone was
2 P
obtained. With the smaller area of the new chamber (l.Smnr vs. 28^1111^),
this represented an intensity per unit area improvement of 3.0.
In viev; of the relatively poor improvement shown by the new
chamber, some basic diffusion parameters were calculated using Pick's
principles. These are shown In table 5, The first column lists various
flow rates, while the second column indicates the degree of mixing
occurring between ehtylene and air. For example, a flow rate of 105ml/
minute would mix 105? of the gases. It Is obvious that the normal flow
19
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rate of one liter per minute is responsible for less than \% mixing
efficiency.
TABLE 5
Diffusion Rate Parameters for a 1.5mm I.D. by 10mm Long Tube
Plow Rate Mixing
Requirement Homogeneity
7.4ml/min 99?
15 90
42 50
105 10
280 1
Since it appeared that a variety of tests v/ould be needed before
a small chamber that provided adequate mixing could be obtained, a
more versatile chamber was built. This is shown in figure 3.
In operation, the smaller assembly was inserted through the rubber
grommet until it touched the cover slip. The large housing could be
mated with either the PMT or the camera. The 1.45jrmO.D. tube was
used for ethylene. This tub* was considerably longer than Its outer
teDescoping tube so that its end position could be varied without
disassembling the chamber. Although ethylene flow rates were varied
with all chambers tested, no large differences were ever noted.
Consequently, a standard ethylene flow rate of 28ml./min. was used
for all tests.
In the first test, the ethylene tube was positioned 4.0 cm. back
of the outlet. When a one liter per minute air flow rate was used
with the ozone generator on, a reading of 0.078ppm ozone was obtained
from the PMT. After a mask with a 4mm D. hole was used to block out
all but the tube Itself, a reading of only 0.0l8ppm was Indicated.
This Indicated that the bulk of the reaction was occuring after the
?0
-------
Figure 3
Chemiluminescence reaction chamber
#0 cover 3Jip~~-^|J
^1
1.3cm D. opening.
1.5cm O.D.
.10cm
1.5mm O.D.
teflon
.2mm D. tube to vacuum
I.D. rubber gromrcet
3mm O.D. x 1.5mm
I.D. Tfeflon
6.3mm O.D. x 3.2mm I.D.
Teflon tube
-------
gases had left the reaction tube.
The relationship between air flov/ and ozone reading for this
3.3mm I.D. Teflon tube configuration is shown in figure 4.
It may be seen that a very linear relationship was obtained
between the reciprocal of the air flow and output signal (as with
the Meloy chamber). A 3.Omm I.D. aluminum tube inserted into the
chamber did not change the values, thereby indicating that it was
not sufficiently reflective to exhibit light-piping effects, or
sufficiently reactive to Increase ozone consumption. Results of the
previous 1.5mm I.D. teflon tube chamber also are shown in figure [l.
Since this chamber had more light attenuation as well as a greater
gas velocity, its light output would be expected to be less.
The other two "orifice" curves in figure ^ are of greatest
interest. In these cases, a 3.0min O.D. by 3.0cm. long by either
1.5mm I.D. or 0.76nunI.D. was inserted in the throat of the chamber.
The ethylene tube v/as moved Just back of the insert. Both of these
curves exhibited increasing signals with decreasing flows at high
flow rates, but then reached a plateau at the lower flow rates. This
probably indicated complete consumption (due to complete mixing)
of the available ozone. The smaller orifice had a. lower signal probably
because of optical attenuation. It also required a lower flow rate
before the plateau was reached, presumably because of the higher
velocity characteristics of small tubes.
When data are plotted against the reciprocal of the air flow
rate as in this case, it is really analogous to plotting the data
against ozone concentration, since the ozone concentration decreases
as the air flow increases. Although it may appear that a straight line
22
-------
Figure 4
Effect of gas velocity on signal
.20
.15-
EQU!valent
Ozone
(ppm)
.1C.
^"^. 5mm
5mm prlflce
(X*76mn orifice
3.3mm Teflon tube
3mm alum, tube
1.5mm Teflon tube
1.0 1.5 2.0
1/Alr Flo* (liters)
-------
function should occur with no plateau, this is not the case if
efficient mixing occurs. The correct model for this type of test is
to assume that there are three gas flows - ethylene, pure ozone, and air.
Since the ozone generator produces a constant flow of ozone regardless
of air flow, the air merely serves as a diluent. Consequently, under
complete mixing conditions, one would expect a constant response
regardless of air flow rate.
The preceding data are tabulated as a function of chamber area
in table 6. In all cases, the values obtained at an air flow rate
TABLE 6
Relative Intensity Obtained
from different reaction chambers
Type of Ozone Reading/ Relative
Chamber Diameter Reading Diameter2 Intensity
Original 1.95mm .?2ppm o,06 1.0
Teflon tube 1.5 .004 0.17 2.8
Teflon tube 3.3 .017 0.16 2.6
Aluminum tube 3.0 .017 0.19 3.1
3cm long orifice 1.5 .20 8.6 113
3cm long orifice 0.76 .05 8.6 113
of one liter per minute were used. It may be seen that the straight
tubes provided an intensity increase of about 3 regardless of
diameter or material. The orifice tubes which provided a turbulence-
producing edge, had 11? times as great a light intensity as the
original tube. The 1.5mm inch orifice chamber had essentially the
same total quantity of light as did the original chamber. In view
of the large signal as well as the plateau formation (fig. 4), we can
safely assume that essentially complete reaction is occurring. The
0.76mm inch orifice tube did not show the expected 4 fold Increase in
intensity. Since the air flow curve shape indicated complete mixing,
this lack of additional improvement simply was due to loss of available
24
-------
light due to the small hole. In spite of the 100 fold .intensity
improvement;, it was still not possible to obtain a photographic
image.
If one assumes that the average light-producing collision
occurs in the middle of the tube (1.5cm back of the viewing port),
the viewing angle even with perfectly straight chamber walls is only
6° and 3° for the 1.5mm and 0.76mm chambers, respectively. This
corresponds to a total light gathering efficiency of only .025$ for
the l.5mm chanber, and .006? for.the 0.76mm chamber.
Other chamber lengths and ethylene tube placements resulted in
either the same intensity or decreased intensity. An interesting
observation was that a significant ozone response v/as found for as
long as three hours after the ethylene was turned off. With the
smaller orifice chamber, this response was essentla]ly the same as
when the ethylene v/as on. Extensive flushing of all ethylene tubes
with air Immediately after ethylene shut-off did not alter this
situation. It thus appears that ethylene probably is being absorbed
in the tubing, and exceedingly small quantities of ethylene are
required if a well designed chamber is used.
The next step was to explore the possibility of utilizing internal
reflectance in order to increase the light gathering efficiency. It
was hoped that reaction chambers with good internal reflectivity properties
would permit the use of longer tubes (for better mixing) and smaller
dlataeter tubes (for better light concentration).
A series of tests were made to determine the reflectivity properties
of various tubes. A controlled diffused light source was oriented
at one end of the tubes, while the Meloy Labs PMT was used at the
?5
-------
other end to determine relative light Intensity values. Twelve Inch
tubes were tested in all cases. Results of these tests are shown In
table 7. The first column lists the type of tube tested, the second
column Indicates the Inside diameter, the third column shows the
amount of light passed by the tubes (In equivalent ppm ozone), and the
last column relates the average light Intensity per unit area. It
may be seen that the glass tubes gave the best results, with the small
bore capillary providing an intensity gain of 3^ fold. If we assume
a critical angle of 30° the Increase in light Intensity should have
been a factor of ^0,000 gain. The relatively low gain could be
accounted for if the glass only reflected an average of 96% of the
light with each bounde. Although a factor of 3*» gain can be achieved,
much higher gains are possible if the average reflectivity could be
Increased above 96%. However, this may not be practical for a field
Instrument.
The calculated efficiency for the small bore tube Is about 0.1^.
If we take a rather simplistic view and assume that the average
chemiluminescent collision occurs in the midpoint of the tube, the
overall efficiency should be 3.2%. A 3QCm long by 0.5 mm reaction
chamber should then exhibit an Improvement of .0327.052 or 12.8 fold
over a one cm^ chamber.
In order to evaluate actual chamber performance, various tests
were made with small bore capillary tubing. The 6.3mm O.D. x 0.5 nun I.D.
capillary tube was mounted in a 13mm vacuum tube equipped with a. #00
cover slip and a suitable adaptor for mounting on the photomultiplier
tube. Air and ethylene were mixed right at the entrance to the tube.
Ethylene flow rates of 0.25, 0.5, 1.0, 1.5, and 2.0 liters per minute
26
-------
TABLE 7
Relative reflectivity of various tubes
Type of
Tube
None-holes spaced
30cm apart
Inside
Di ameter
1.8mm,
Light
Intensity
1.35
Relative
Intensl ty
mm..
1.0
Teflon
6.3mm O.D. glass
1.5mm O.D. pi
Stainless
.75
.50
1.0
1.1
.74
3.80
4.30
.87
3.0
3^
10
1.6
27
-------
were tested. Three different tube lengths - 23, 30, and 53cm
tested. Results of these tests are shown In figure 5.
In all cases, the lower ethylene flow rate showed a moderate
decrease In sensitivity. The 23cm chamber had very good response but
the plateau was not as flat as previously experienced with larger
diameter chambers. This Indicates that complete mixing Is not occurring
In the tube, and longer tubes are Indicated. The 30cm chamber still
didn't have a flat plateau, and had considerably less response than
did the 23cm chamber.The lower response is due to greater reflectance
losses. These losses are further indicated in the 53cm chamber. Here,
only the very high flow rates gave a signal. At normal air flows,
the reaction apparently occurred so far down the tube that its light
was completely absorbed before reaching the PMT.
Both the 23 and 30cm reaction chambers showed better response than
would have been Indicated by the internal reflectance tests. Whereas
the reflectance efficiency for the 30cm had been calculated at 3.2?,
the actual response was roughly 50% as great as the original Meloy
chamber. This discrepancy probably is due to a better mixing efficiency
in the capillary tube than the Meloy chamber, since the Meloy chamber
never has shown any plateau characteristics on a response vs. velocity
plot. Other possibilities are an underestimation of the reflectivity
(a 1% error in the estimated reflectance per bounce would have a
large effect on total efficiency) or to the bulk of the reaction
occurring forward of the tube's midpoint. In any case, the test
results looked very encouraging. If we compare the response per unit
area of the 23cm tube (0.5mm I.D.) with the original Meloy chamber
(19mm I.n.), we find a net decrease in area of 1,400 fold coupled with
-------
.35
.30
.25
I
n
c
o
a
>/)
0)
o
M
o
.20
.15
.05
Figure 5
Effect of reaction chamber length
X53CIH long
long
long
28 ml./min. ethylene
7 ml./min. ethylene
3 « 5
I/Air flow in litera/min.
29.
-------
only a slight decrease in total response. Thus, the brillance was
Increased 1,000 foil1. In comparison to a one cm4- area cell, brilliance
was increased approximately 250 fold. Since previous calculations had
indicated that the brilliance of a hypothetical 1cm area chamber must
be increased by between 10 and lO-5 before photographic film v:onld be
comparable to a PMT, we were still approximately two orders of magnitude
away from our goal. Since the use of even smaller diameter reaction
tubes would begin to pose serious pressure drop, mixing, and plumbing
problems, we decided to terminate any further work on smaller chambers
and concentrate on the photographic film aspects.
Various capillary tube reaction chambers including the 23cm
0.5mm I.D. tube, a 7.6 by O'.lcm I.D.tube, and others were used in
conjunction with various photographic films. These films included
Polaroid 3000, Tri-X, and Ia-0. Exposure times up to 4 hours were
used, and all developing was by manufacturer's recommended procedures.
Air flow rates and ethylene flow rates were varied over wide ranges.
The Meloy ozone generator (0.3ppm ozone) was used In all testa, and
proper operation was confirmed by checking light emission with the
Meloy PMT prior to each series of photographic tests. In no instance
did we obtain any perceptible images on the films. Even optical coupling
of the capillary reaction tube to the surface of the film (by using
appropriate quartz lenses) did not yield any images. These results
merely confirmed the theoretical calculations which stated that several
orders of magnitude more sensitivity was still needed. In view of the
increasing difficulty of obtaining additional significant sensitivity
improvements, this approach was terminated. Perhaps the future will
30
-------
yield filmj with better sensitivities (some of the newer solid state
imace storage plates already look promising). In that event, the
present v/ork on improved mixing efficiency and smaller viewing areas
should prove valuable.
*l.6 Gas Permeation Studies
Even though the photographic film recording approach had not
proved fruitful, the use of small capillary tube reaction chambers
had opened up the possibility of using gas permation rather than
direct injection of ethylene into air.
One of the major logistic disadvantages of chemiluminescent o^one
a.nalyzers is the necessity to supply ethylene. If this could be
replaced by a reactive liquid whose vapors diffused into 'the chamber,
a much simpler system could be developed. Consequently, the feasibility
of using a diffusion membrane was tested.
Figure 6 describes the test setup used. The air flowed past the
silicone rubber tube, through the capillary tube, and to the vacuum
port. The ethylene was allowed to flow at 15ml. per minute through
the reservoir surrounding the diffusion tube. Response dropped
severely when ethylene flow was stopped, presumably due to rapid
depletion. The diffusion device was tested against direct injection
of ethylene into the end of the capillary tube. The results are
shown in table 8.
Table 8
Effect of Diffusing Ethylene into the Reaction Chamber
(Responses given in equivalent pprn, ozone)
Air flow Ethylene @ Ethylene @ Ethylene
rate 28ml. /min. 7ml. /min. diffused in
2.01pm .110 .095
1.5 .1^5 .120 .090
1.0 .190 .165 .120
0.5 .275 .2^5 .175
0.25 .280 .265 .170
31
-------
Figure 6
Gas permeation tube test
to ethylene
vent
1mm. I.D. x 30cm glass tube
#00 cover slip
PMT
5mm O.D. silicon rubber wlth it3cin exposed
-------
In all cases, the usual pattern of increasing response with
decreasing air flow, finally reaching a plateau, was found. The response
decreased about 10/5 as the ethyl ene flow was reduced, from 28ml./min.
to Trcl./min. With the diffusion device, response dropped another
30%. Nonetheless, the response obtained by diffusing ethylene through
silicone rubber was quite respectable. It opens the possibility of
diffusing liquid reaotants, thereby eliminating the gas bottle
requirement.
4.7 Direct Air Injection
Film manufacturers have a problem of film fogging due to the
chepiiluminescence reaction of ozone in the atmosphere with gelatin.
In order to overcome this problem, they use sealed shipping containers
and add antioxidants to the film. It was attempted to use this
reaction ar, the basis of an ozone detector.
One end of a 3«2mm Teflon tube was connected to the ozone
generator and the other end was positioned about 3.2mm from the film
plate. Vacuum was applied to the camera case to create a flow rate
of one liter per minute. No reaction v/ith Kodak Tri-X 16mm. film
could be found after one hour exposures.
In other tests an air pump was connected to the ozone generator
inlet, and positive pressure was used to direct a one liter per
minute air flow onto the film. Again, no exposure could be seen
after development
It thus appears that either the 0.3 ppm ozone concentration
cannot overcome the antioxidant, or the one hour exposure time is much
too short. In any case, this does not appear to be a fruitful approach.
33
-------
5.0 Alternative Detectors
Even though the direct film exposure concept proved to be
unfeasible, a drastic need still existed for a reliable, low cost,
long term recording monitor for air pollutants. Although the
chemiluminescence reaction Itself Is simple and reliable, the extremely
low light emission level has made it necessary to use photomultipller
tubes as sensors. The gain as well as zero level of PMT's is markedly
aJtered by changes in temperature and voltage. Consequently, a portable
instrument would require a thermostatted oven (very high power consumption),
and a very well regulated power supply (expensive). In an effort to
overcome these disadvantages, various other photodetectors were tested.
In most cases, the test setup consisted of an 2^v. power supply
with 1.5 Kohm, 1.2 Kohm, 1.0 Kohm and 820 ohms current llmiters
(positions 1,2,3,11), driving a 2l
-------
TABLE 9
Response of Various Photodetectors
Detector
r:elov PUT
reloy PI.T
RCA 1P28 PMT
?CA 1F28 PI-T
CL902L
PD 1900-Sl
PD 1900-S10
DT 1737
CLR 2060
CLR 2180
Test Condition
O^one amplifier, 1200v.
Electrometer, 1200 v.
Ozone amplifier, 800v.
Electrometer, 800v.
Electrometer, 15v.
Electrometer, 15v.
Electrometer, 15v
1 r-eg x 1000 (lmv=lpa.)
Electrometer, Iv.
Electrometer, Iv.
Dark Lamp 1 Lamp 2 Lamp 3 Lamp 1
Onpm. 0.3 5.5 Overran^e
OuA .01 .13 1.38 3.18
Oppm
1.37nc
2.00na
25.Oca. 25.2
I2.0pa. 12.0
41.7pa
.01
1.13
2.01
5.2
2.0
1.85
6.53
.086
1.85
2.04
25.2
12.0
4fi 7
TO . (
1.87
6.67
.25
2.75
2.05
28.0
43.0
h Q 7
T y . /
2.09
6.77
1.10
8.72
2.25
29.0
11.3
P Q n
O j ,U
2.31
21.1
OJ
-------
The first test with the Meloy PMT showed the correlation between
our simulated light conditions and the chemiluinlnescent light emission.
The actual current output of the Meloy PMT was directly related to the
ozone reading. The 1P28 PMT had considerably less response than the
Meloy PMT. This would be expected in view of the lower operating
potential. The Clairex CLQ02L cadmium sulfide photoresistor tracked
the simulated light conditions a.lmost as well as the 1P28. It had a
higher relative dark current and its response was exceedingly slow.
The next three detectors are vacuum photodiodes manufactured by
the Tung-Sol division of V/arner Electric Corporation. The PD 1900-S4
appeared to follow the light source somewhat, but the signal to dark
current ratio was very poor. Also, the current increase equivalent
to 5•5ppm ozone was only 0.2 picoamperes, which makes it difficult to
design suitable amplifier circuits. The PD 1900 with S10 response
characteristics gave even poorer response, presumable due to its
greater sensitivity to red light, which J.s a noise and dark current
producer in this application. The DT 1737A is a PD1900-S10 photodiode
coupled to a PET amplifier. It is advertised as being equivalent to
photomultiplier tubes. Our tests indicated that it did indeed follow
our simulated signals, but that its noise and drift characteristics
were unacceptable.
The CLR 2060 and CLR 2180 detectors are silicon photodarlingtons
sold by Clairex. They differ only in that the CLR 2180 has a. lens
to collect light. Even though these detectors have maximum sensitivity
to 900nm light and only about 5% as much sensitivity to l»00nm light,
both detectors were able to measure light intensities equivalent to
ozone or greater. In going from darkness to lamp ^, CLR 2060
36
-------
Increased by 0.46na. while CLR 2180 .increased by l4.6na., thereby
showing the importance of good light collecting ability.
The preceding results cannot be strictly related to detectability
of cheip.ilurnlnescence because of their disproportionate sizes. Since
these tests were made with a small diameter (3.2mm)light source, all
detectors collected a high proportion of the available light. With
a large diameter (over 2.5cm) reaction chamber as used in the Meloy
ozone analyzer, only the Meloy PMT (which has an end-on configuration)
will collect the light efficiently. The 1P28 PMT, which has a side
window configuration collects less than one percent of the light. No
signals due to the ozone reaction were obtained with the 3.P28. The
vacuum photodiodes would collect about 20? of the light but their
responsivity characteristics are poor. The solid state detectors collect
very small proportions (less than 0.15?) of the light and have poor
responsivity characteristics. It was concluded that only end-on
photomultjp]ier tubes are capable of monitoring the ozone-ethylene
chemllumlnescent reactions at this time.
6. Photon Counting
6.1 General Background
All light originates as discrete energy particles termed "photons".
The eye, most detectors, and most amplifiers are too slow to respond
to individual photons. Consequently, the output of most photodetectors
appears to be an analog signal, the magnitude of which is proportional
to light intensity. With a sufficiently fast response system (risetimes
less than one microsecond), the detector output appears as a number of
sharp spikes, each of which represents a single photon. The height of
37
-------
the spikes remains constant regardless of light intensity. Increased
light only results in Increased numbers of spikes.
A recent article by Zatzick (1) reviews the advantages and
general state of the art of photon counting. Photon counting is
universally agreed to be the ultimate method for obtaining the best
reliability and signal-to-noise ratios in low light level applications.
In photon counting only the number of photons are counted. Electronic
noise is eliminated by threshold circuits and dc drift is eliminated
by using ao coupled amplifiers. With conventional analog systems,
not only the number of photons affect the signal, but also their peak
height, the frequency of cosmic rays, electronic, noise, and electronic
drift. The apparent photon peak height is governed by the gain of the
photomultlpller tube and amplifier. Although amplifier gain Is stable,
photomultiplier gain Is greatly influenced by power supply voltage and
is moderately Influenced by temperature. This means that a very well
regulated power supply must be used, and a. controlled temperature oven
must be provided for the PMT If it is used in the field. Cosmic rays
strike the photomultiplier tube at a rate of approximately one/cm2/sec.
Since they are orders of magnitude larger than photon signals, they
can have an appreciable effect on the baseline at low light levels.
Electronic noise is another source of baseline drift. Although it is
simple to remove this source of error by threshold limiting with a
photon counting circuit, it becomes an appreciable signal in analog
circuits. Electronic drift of both the PMT and amplifiers is an
exceedingly difficult problem to solve with standard analog circuits.
1. ZatzJck, M.R. Electro-Optical Systems Design, June 1972, pp. 20-27
38
-------
Photon counting circuits always are AC coupled so as to eliminate this
problem. Thus, the photon counting technique eliminates many error
sources which limit the sensitivity and reliability of conventional
systems.
6.2 Electronic Considerations
6.2.1 General
Although photon counting has nany theoretical advantages over
standard analog integration procedures, the electronic circuitry must
be relatively sophisticated to avoid introducing nev/ problems. Fig.
7 illustrates the basic components required to monitor chemiluminescence
reactions by photon counting techniques. A photomultiplier and its
power supply are needed Just as in an analog system. The pre-amplifier
serves to lower the impedance of the signals. The comparator discriminates
between the photoelectron signals and noise, and amplifies the photoelectron
signals to a saturation level voltage. The divider network reduces
the total counts to manageable levels, while the readout system permits
a number representative of the pollutant level to be recorded. Perhaps
the most difficult area is in obtaining adequate risetime speeds from
the PMT amplifier. If the response speed is too slow, the photoelectron
signals will appear as broad peaks instead of sharp spikes. This
results in a poorer signal-to-noise ratio and in peak overlap at higher
light levels. The peak overlap causes doublet and triplet photoelectron
peaks to appear, which results in an exponential power of 2 or 3
relationship between counts and light intensity, instead of the normal
linear relationship. The other major problem area if? in handling the
massive number of photoelectron counts. Since approximately 1000 photons
riust be counted each second, a 24 hour accumulation would result in
39
-------
Co
K
o
o
•o
o
ct
High
Voltage
Power
Supply
PMT
Pre-amn
Corcparato
Divider
Readout
O
3
H-
D
OQ
C
0>
—J
o
0)
3
O
ill
•a
D'
O
ct
O
3
o
o
ct
ft
-------
approximately 100 million counts. This is handled most easily Ly using
a "brute force" approach of digital division.
6.2.2. Power Supply
The high voltage power supply can he relatively poorly regulated
since amplitude changes of the photoelectron signals do not have a
direct influence on the number of counts. The current capability of
this power supply need be only one-tenth as great as required for
analog systems, because the dividing chain can be composed of much
higher resistance values. This is possible because each photoelectron
places a constant current load upon the PMT, as opposed to the variable
load imposed when the PMT is operated into an integrating current amplifier.
Almost all of the lower cost high voltage power supplies operate
as dc-dc converters. The Input d.c. power is chopped at approximately
10KII-7. transformed to a higher voltage, and then rectified to obtain a
fj *
high voltage dc output. The chopper frequency appears at the output
as a ripple, which ultimately becomes noise. ?ince a photon counting
system can discriminate against this ripple, it can tolerate the vise of
these low cost power supplies. All studies on photon counting were
made with a Mi]. Electronics type VL15 power supply. This unit transforms
input voltage to a voltage 100 times greater. It was normally operated
with a 12vdc input and a 1200 vdc output.
6.2.3. Photomultlplier tube
Most of the requirements for a photomultiplier tube used for
photon counting are similar to those for normal analog usage. In
order to obtain best sensitivity and reliability, it should have
minimal response to longer wavelengths, since longer wavelength response
Increases the thermionic noise problem. It should also have the minimum
-------
possible cathode area, since excess cathode area increases thermionic
and cosmic ray noise. DC leakage is unimportant for photon counting
purposes, so dark current values are not especially meaningful. A
special requirement is that the PMT must have fast response characteristics.
The EMI 9524S PMT was chosen for this application. This 30mm.
end-on tube has an S-ll spectral response. It was primarily chosen
for its overall reasonable characteristics, rugged construction, and
low cost. It has a somewhat slow rise-time response of 18 nanoseconds,
but this is adequate if only low light levels are measured. Even
though this is a moderately small diameter tube much smaller tubes are
available at higher costs. Alternatively, a magnetic focusing lens
could be used to reduce the effective cathode diameter. This tube was
used as is, however, since it fit the existing Meloy reaction chamber
and permitted us to proceed immediately to the photon counting problem.
The dynode chain configuration used with the PMT is shown in
figure 8. This is a relatively standard configuration except that
the chain resistance is ten-fold greater than normally used, and
capacitors are used on the last three stages to provide a more constant
gain regardless of the photon rate. Higher resistance is possible with
photon counting circuits because less current is required at the low
light levels generally encountered. It should be noted that the plate
is tied to ground through a 22K ohm resistor. This permits the output
to be monitored either by the photon counting amplifier or by a
conventional dc analog amplifier. This permitted us a high degree of
flexibility in our experimental tests. A major disadvantage of this
configuration is that the PMT shield must be tied to the high voltage
supply. A much safer and lower noise configuration v/ould be to tie
-------
J> •'*
£N\1 V24S
3WI8LD
—/loov-
I I
T
00 f
I -OO/
0
7
-------
the cathode and shield to ground, and tie the load resistor to the
positive high voltage output. This would necessitate an ac coupled
circuit, which would preclude any do current measurements. Nonetheless,
if only photon counting were to be used, it would be recommended that
the cathode and shield be grounded.
6.2.^. Amplifier Circuit
Also shown in figure 8 is the final amplifier circuit delivered
in the breadboard, instrument. The 22 Konm load resistor was found to
be the largest possible resistance that would not unduly broaden the
photoelectron signals when unshielded wire was used between the plate of
the cathode and the preamp]ifier. With the 22Kohm resistor, it v/as
not possible to use shielded cables without broadening the signals due
to the extra capacitance. This did not prove troublesome since shielded
cables are not needed for photon counting purposes. The Harris HA-2000
preamplifier is a newly introduced FET Input, unity gain amplifier which
is capable of following 100 MHZ signals. This preamplifier was very
satisfactory and was used in all experimental configurations. A slow-
response amplifier was used at the output o.C 1;he preamplifier r;o that
EPA personnel could conveniently monitor dc current vs. photon counts.
Very little work was done with this amplifier since v/e preferred to
disconnect the PMT output lead and measure current directly whenever
a comparison test was made. The auxiliary slow amplifier is completely
functional and does reflect the true dc state of the chemiluniinescent
reaction. The output of this amplifier is quite rippled and drifts
womewhat since this is typical of analog systems. The Meloy amplifier
output appears smoother because it has approximately a 10 second time
constant.
-------
The output of the preamplifier also IP fed through a coupling
capacitor to an PET amplifier stage. A diode was used between these
stages to prevent negative peak overshoot, which would unnecessarily
increase peak doubling effects. This amplifier stage presented the
greatest difficulties in achieving adequate speed and stability. Single
transistors, as used here, are not thermally stable unless additional
compensation circuitry IP used. Integrated circuit amplifiers would
be the ideal choice if available. Various amplifiers, including the
LM101 with feedforward compensation and the Harris HA-2520, were tested
in various configurations. In all cases, a linear re] ationRhip
between current and count rate could not be achieved. Rest re
i
were obtained when the IIA2520 was direct.! y coupled to the KA2000
preamplifier in a standard two amplifier feed-back arrangement. Under
these conditions, a power slope factor of 1.1 was achieved under low
light level conditions. Even though the HA2520 has a very impressive
slew rate of 120v//us, the high gain demanded of this amplifier slows
it to a point where response time is greater than one microsecond.
This causes peak broadening, which results in some photon doublets
that tend to degrade linearity (note: if only doublets were present,
the photon count would be proportional to the square of the current).
In view of the poor thermal characteristics of the amplifier
section, its output is capacltively coupled to the comparator. This
comparator (a Falrchild uA710) is used with open loop gain so that
any peaks exceeding a preset threshold value become amplified to
the comparator saturation level. The comparator used in this circuit
performed quite well. However, a far superior comparator has recently
become available - the Advanced Micro Devices Am685. This comparator
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has a 2ns resetime Instead of the 40ns risetime of the yuA?10. Even
more important, it has excellent stability (10uV/°C. drift) which permits
i.t to be used directly with very small signals. This makes it possible
to couple it directly to the preamplifier, or to use an amplifier with
low gain, thereby minimizing rise time. Thus, this comparator makes
possible an amplification system made up entirely of stable temperature-
compensated integrated circuits. Unfortunately, this comparator came
on the market too late to be incorporated in our existing circuitry.
This comparator uses ECL logic outputs instead of TTL as does the older
710 comparator. While this does not represent a difficult interface,
it would have entailed making an entire new circuit board. Since the
photon counting technique could be easily shown to be far superior to
analog systems, the new comparator was not incorporated into the system.
The output of the comparator is available on an output jack
labeled "counter". This makes it possible to use any standard digital
counter to monitor the rate of photon production. The signal then
proceeds to the "digital division" portion of the circuit, where the
photon pulses are counted.
6.2.5 Digital Division
Figure 9 is a functional diagram of the decade division circuitry,
while the complete circuit is shown in appendix A. The pulsed output
of the comparator is fed to the array of decade dividers. F.ac'n stage
performs a 10-fold division of the number of counts. Thus, a single
pulse appearing at the output of the third decade divider would
represent ]000 counts at the input. A rotary switch is used to select
which decade divider output (the extent of division) rshall be permitted
to accumulate. The Dast decade divider outputs are decoded to binary
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form by a 7 bit binary counter. This counter provides a 7 bit parallel
output that is related to pollutant concentration. This provides a
resolution of one part in 127. Greater resolution is possible simply
by using larger counters. The 7 bit accuracy was chosen since a 5x7
LED matrix was chosen as the readout device. Naturally, once the
pulses are in digital form, any desired resolution can be obtained
simply by using binary converters and displays with greater capacity.
The counting time is regulated by an RC network attached to a
digital timer (SE 55). A rotary switch changes the timing period in
decade- steps from 0.1 minute to 100 minutes. At the end of each
timing period, a number of events occur. These are:
1. Replace the previous counts in the memory with the accumulated
counts from the last time period. These counts will be stoker! in the
memory during the entire next timing period.
2. Advance the LED readout by one row and display the new counts
during the next courting period. This results in a 7 bit nunber in one
row of the LED display. This row is advanced during the start of each
new timing period. In addition, the circuit shown in Appendix A also
incorporates a "Blip" capability in which the LED matrix is dark during
most of the timing period. The binary coded value only appears as a
0.1 second blip at the end of the timing period. If this is coupled to
a photographic recording system, a very low power system can result.
3. Reset all dividers to zero. This readies them for the next
timing period.
*J. Reset the timer.
This logic system thereby results in a system in which both the
amount of count division and counting time can be independently varied.
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This d'.-gree of flexibility probably wouldn't be required in a field
instrument, but it is helpful for R & D purposes. The f. x 7 LED matrix
displays a binary coded number that is proportional to pollutant
concentration on one of its 7 bit rows. This display row is advanced
one row at the end of each timing period in order to display the new
reading. This system has functioned very well with no failures at
any time.
6.3 Results
All data were obtained either by using the Meloy ohemilumlnescence
chamber or by using a light bulb simuDator having a filtered output of
410 nm. Photon, counts were plotted against "ppm ozone" or against
"analog output current" whose values were obtained by applying exactly
the same experimental conditions while obtaining the analog or digital
data.
When analog output currents were compared to photon counts, an
extremely non-linear relationship was obtained in most cases (in early
experiments). Reasonably linear relationships could be obtained in
many cases if the count rate was raised to an exponential power
between 1.5 and 3. Close examination of the photon shape on a
Tektronix oscilloscope revealed that peak widths in excess of one
microsecond were causing doubling-up of the photon signals. With
the high speed, circuitry shown in figure 8, however, good linearity
was achieved in practically all cases.
A plot of photon counts vs. analog current is shown for various
threshold level settings in figure 10. The count rate is displayed
on the Y axis, while the equivalent analog current is shown on the
X axis. The different corves are labeled according to the threshold
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""tgure
Count rate vs. Current at various threshold levels
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6 8 10 20 30 10 60 80 100 200
Current in Nanoamperes 50
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voltage applied to the comparator. Zero volts corresponds to essentially
no threshold offset, whereby all counts are recorded. This curve has
a slope less than unity because many of Its counts are derived from
electronic noise, which remains constant regardless of light level,
thereby yielding a fairly horizontal slope. The 0.8v. threshold
provided a slope greater than unity. This is presumably due to counting
many "doublet" photon signals composed of two photon signals appearing
at the same time, thereby yielding a signal twice as intense. All the
curves obtained with threshold voltages between 0.06 and 0.6 volts
are quite linear, thereby confirming the good high speed capabilities
of the electronic circuit. From these data, It may be Inferred that
most of the noise pulses are smaller than 0.06 volts, and that most
of the photon signals have amplitudes between 0.06 and 0.6v.
The data of figure 10 is reformated in figure 11 to show the
relationship between threshold voltage and count rate for various light
levels. As the threshold level is reduced from 0.06v. towards zero, the
dark count rate increases very rapidly. This is due to counting
electronic no3se pulses near the zero threshold level. The region
between 0.5 and 0.6 volts also is characterized by a steep slope. This
is presumably due to a transition from counting only single photon
signals to counting doublets. The region of least slope lies between
0.3 and 0.5 volts. This is presumably the peak height of the average
photon signal. If the photomultlplier tube provided precisely the
same gain for all photons., this region would be perfectly horizontal.
If this were the case, the photon count would be completely oblivious
to small changes In PMT gain. As it is, changes in PMT gain will affect
the count rate, but much less severely than If It were operated in the
51
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Threshold (Volts)
Figure 11
Threshold vs. Counts at Various Light Levels
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analog mode. lietter stability would he possible with gallium phosphide
PMT cathodes which generate 30 electrons per photon. These PMT's are
more expensive than the simple PMT used for these tests.
A plot of count rate vs. LED matrix readout is shown in figure 12.
As would be expected with a digital system, a perfect correlation exists
A plot of PMT output current vs. ozone reading is sh^wn in figure 13.
This also shows a good linear relationship, as would be expected. The
relationship between count rate and ozone concentration is shown in
figure 14. A threshold setting of 0.2v. was used to obtain these data.
It may be seen that very good linearity was obtained.
In summary, the photon counter exhibited very good linearity over
a wide dynamic range. The threshold voltage setting for the comparator
could be set between 0.06 and 0.6 volts and still achieve linearity.
Lower thresholds settings tended to Include electronic noise, while
higher thresholds Included primarily those photons that happened to
appear at the same time as another photon. The primary known weaknesses
of the present system - a cheap Pi'T and less than optimum analog
amplification probably were responsible for a dependency of count rate
upon threshold voltage setting.
7. Suggestions for future work
The feasibility of applying photon counting principles to
chemllumlnescence detectors has now been shown. Sensitivity is now
limited only by thermionic noise - amplifier drift, cosmic rays, power
supply drift, etc. create little or no interference. It is suggested
that the already good performance of the present photon counter can be
Improved at least 10 fold by utilizing a small area detector to decrease
the thermionic noise contribution. This will be especially important
53
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3 4567891
100K
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Matrix Readout
Time Base = 6 seconds
Figure 32 Correlation of Count Frequency with Display Reading
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Figure lii 3?;one Concentration vs. LED
Matrix Readout
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when longer wavelength reactions such as NO - ozone are monitored.
X
As a further aid to thermionic noise reduction, some of the
newer liquid crystal films such as N-(p-ethoxybenzylldene) -p-n-
butylaniline should be tested as a chopper. This will function to
alternately sense the background alone vs. the background plus signal
without the complexity of mechanical choppers. This type of light
chopper could be easily incorporated into photon counters simply by
using up-down counters. The "on" cycles would count up, while the
"off" cycles would count down. This then, would represent the ultimate
in high reliability signal detection.
On the commercial aspects, the present transistor amplifier
should definitely be replaced by an integrated system employing the
new AM685 voltage comparator. This will provide faster response
times, much greater reliability and stability, and a greater linear
dynamic range. Also, the "brute-force" chain of decade dividers
should be replaced by an integrated counter such as the Mostek MK5009P.
This single chip device can divide by as much as 3.6x10^, it's division
is completely programmable, and it uses considerably less power than
does a comparable number of TTL decade dividers.
The readout device can be any number of digital output devices.
This includes numeral displays, printers, a computer, teletype lines,
etc. In the device delivered, a 5 x 7 LED matrix was used with the
idea that a permanent record could be made on photographic film. A
more convenient readout would be a magnetic card such as used with
the Hewlitt-Packard or Monroe desk-type computers. With this type of
recording card, over a month's operation could be stored on one card.
The card could then be entered into a computer to be collated with other
57
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data. In any case, once the data Is in digital form, many readout
options are available,
r
8
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APPENDIX A
Digital Division Circuitry
59
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PAGE NOT
AVAILABLE
DIGITALLY
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APPENDIX n
Photon Counter Description
60
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C.
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APPENDIX B
Description of the Photon.Counter
1. Electrical connector - llOv., 50-60 HZ.
2. Photomultlplier tube - nates directly with Meloy reaction chamber.
3. Preamp output - 0 - O.lv. photoelectron signals prior to amplification.
4. Counter output - 0 5v. digital signals that can be connected to any
digital counter.
5. Meter output - 0 - 10v.t damped analog output. This signal corresponds
to the conventional meter output used In most commercial chemiluminescence
monitors.
6. On - off switch - this unit has essentially no warm-up time.
7. Indicator light - Operates from the main power supply to confirm that
the unit Is on.
8. Time adjust - sets the cycle rate to 0.1, 1, 10, or 100 minutes.
9. Divider - Divides the number of photon counts by exponent Indicated
(100 to 10 million). This should be set so that the LED readout (12)
provides a reasonable reading,
10. Overrange Indicator - Lights and stays on whenever the capacity of the
LED readout (12) Is exceeded (over 127 units).
11. Reset - This button should be pushed to turn the overrange Indicator off.
12. LED readout - Contains 7 horizontal lamps and 5 vertical rows. The
7 horizontal lamps Indicate a binary number between 1 and 127 which Is
proportional to ozone concentration. Each row normally indicates the
accumulated photon counts for the previous cycle. After the current
cycle time Is completed, the lighted row will advance upward one row
and display the latest reading.
13. Blip control - Inhibits the LED readout from constant indication. In
the "Blip11 position, the LED readout will display the accumulated counts
for 0.1 second at the end of each cycle.
14. Threshold control - A screwdriver adjust potentiometer that permits the
noise discrimination threshold to be Increased or decreased.
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