ITR-21
CALIBRATION OF 4- INCH LOW BACKGROUND BETA COUNTING
SYSTEMS FOR COUNTING AIR FILTERS
By
R. E. Jaquish
Southwestern Radiological Health Laboratory
U.S. Department of Health, Education and Welfare
Public Health Service
Consumer Protection andEnviromnental Health Service
Environmental Control Administration
Bureau of Radiological Health
May, 1969
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ABSTRACT
The 4-inch low background beta counters were recalibrated
and alternate methods of preparing standards were studied.
It was concluded that the placement of a standard solution
directly on a 4-inch stainless steel counting planchet is
an acceptable approximation of activity on the surface of
an air filter.
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LIST OF TABLES
TABLE Page
I. Beta Calibration - System 01 4
II. Beta Calibration - System #2 5
III. Beta Calibration - System #3 6
LIST OP FIGURES
FIGURE Page
1. Efficiency vs. Beta Energy - System #1 7
2. Efficiency vs. Beta Energy - System #2 8
3. Efficiency va. Beta Energy - System #3 9
4. Fission Product Average Maximum Beta Energy vs. Time
After Fission 10
5. Coated Filters - Efficiency vs. Energy 11
ii
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In January 1969 a recalibration of the beta systems used for counting
4-inch glass fiber filters from the Air Surveillance Network was
initiated. The objective was to determine the effect of the resultant
difference, if any, between the current method of calibration by
placing a known anount of activity directly on a 4-iisch stainless steel
planchet and an alternative nothod of depositing the activity on a
filter. This ^calibration was performed with three types of samples
as follows:
a. The standard solution was uniformly distributed directly
over the surface of a 4-inch stainless steel planchet.
b. The standard solution was uniformly distributed over the
surface of a 4-inch glass fiber filter attached to a
4-inch stainless steel planchet.
c. The standard solution was uniformly distributed on the surface
of a 4-inch glass fiber filter coated with acrylic lacquer
attached to a stainless steel planchet.
1. Sample Preparation
All samples were prepared by pipetting a standard solution directly
onto tho planchet and filter, as specified with an Eppendorff
micro-pipette. The volume of the solution used was either 500
or 1000 depending on tho activity of tho standard. The range
of activity on the samples at tho time of count was between
10,000 and 100,000 transforcations/min. Tho solution was placed
on the sample in small droplets iron the cento? out to the edge of
the sample in a spoke-like pattern.
The samples with a filter on the planchet were prepared by spraying
the planchet with spray adhesive and placing the glass fiber
filter directly on the planchot. Tho filters used were Gelman
Type E glass fiber filter with an area of 81 en2.
The coated filters were prepared by spraying the filter with an
acrylic lacquer frca a pressurized can. The filters were allowed
to dry under a heat lamp beforo the solution was pipetted onto
the coated filter. Four of tho filters were weighed before and
after the acrylic coating was applied to determine the weight of
the acrylic applied. The average weight of material on the filter
was 2.7 mg/cm. This thin film was considered to be essentially
zero and the added backscattcr material would have practically
no effect.
When the standard solution was placed on the coated filter, the
solution beaded-up on the filter and did not soak into the filter
during the entire drying time.
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The standards used for the recalibration were:
Max. Beta
Nuclide Energy (MeV)
C 0.156
99Tc 0.229
185W 0.429
J37Cs 0.559
36C1 0.714
904T190 °*766
Sr 1.463
32P 1.710
137
All of these are pure single beta emitters except the Cs and
90Sr-Y. Pure beta enitters were used whenever possible to eliainate
the problem of conversion electrons. The beta counter is very
insensitive to gaoma radiation with less than 1% of the photons
being detected.
137
For the Cs the value used for conversion electrons was 9.5%;
therefore, 1.095 betas/transfomation was used. The energy
distribution of the betas froa 137Cs is:
1.180 HeV 4.8%
0.518 K9V 95.2%
0.662 HeV 9.5%
137
The average maximum energy for Cs is 0.559 MeV. This average
is not exact since the beta values for the 1.18 and 0.518 MoV betas
are maxinun energies with an average energy of about 1/3 the
maxiouQ. The 0.662 MeV conversion electrons are monoenergetic.
90
For Sr-Y the energy distribution is:
>-
90y _
0.526
2.27
90_
MeV
MeV
100%
100%
» .4 «
The average maximum energy for
2. Counting
All samples were counted for five minutes on each of the three beta
systeas.
3. Data
The data from the counting of standards are listed in Tables I, II,
-------
III. The activity added is as of the time the sample was counted.
4. Calibration and Conclusions
Calibration curves for each systea indicating the three geometries
are shown in Figures 1, 2, and 3. For all three systems the
calibration curves are sinilar. At the lower energies there is
greater self-absorption in the filter; therefore, the efficiency
values show a wider difference. The calibration with the solution
placed directly on the stainless steel planchet and the activity
of the coated filter are very similar.
Figure 4 shows the average maximum beta energy for mixed fission
products as a function of tice after fission. Accordingly an
average value of 1 KeV could bo assumed at anytime between two and
100 days after fission. Also shown is the average maximum beta energy
for plowshare devices with a predominance of tungsten components.
Note that the average naximun beta energy is somewhat lowor, being
constant at 0.5 MeV after 10 days.
The efficiency used for beta emitter on the three counters is 50%.
This efficiency is an acceptable value as it lies between efficiency
for beta emitters from Plowshare devices and pure fission devices.
The efficiency for 1 HGV beta particle is 54% and for 0.5 MeV botas
is about 47%.
36
For all three counts the calculated efficiency for the Cl was
high. The activity of the solution was checked by having an aliquot
4i7~ beta counted and the resulting count was less than 3% different
frost the calibration from the supplier. On previous calibrations
the Cl was also high. The T1! is low. This could be due to
the fact that there is a disagreement on the half-life of rri in
the references and this standard was several years old.
Figure 5 shows the calibration curves for each system for the
coated filter geometry. It is felt that this geometry is most
representative of a filter sample with activity deposited on its
surface. It is noted that all three systems respond nearly
identically.
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Table I
Nuclide
14C
11
11
"TC
it .
tt
185
"
137
Cs
Ck "
it
36ci
11
It
204
Tl
11
tf
9°Sr-Y
IT
8!sr
B Energy
(Max.)
.156
II
It
.292
II
tl
.429
tt
tT
.514
tl
tt
.714
It
It
.766
Tl
Tf
1.41
It
II
1.463
ft
ti
Half-life
5730y
Tl
it
2.12xl05y
It
II
75 d
; t
"
30y
rt
3xl06y
tl
tl
3. Sly
tl
1 T
27. 7y
Tt
11
50. 4d
rt
M
dpm
Activity
Added
24,400
It
I r
1.12xl05
it
Tl
5
2.40x10
ri
1 1
23,700
i r
it
16,400
Tl
11
12,500
Tl
tl
25,610
10,250
25,610
31,350
tt
II
1.710 14.28d
32,000
BETA CALIBRATION
System #1
Date of CPM CPM
Count Planchet Filter
1-9-69
1-9-69
1-9-69
3-10-69
2-12-69
3,671
2-14-69 40,992
3-27-69 115,800
1-30-69 11,377
10,356
6,226
14,460
18,340
1-28-69 19,202
514
21,499
71,000
9,326
8,971
5,087
5,185
17,228
CPM
Coated
Filter
2293
35,262
95,900
11,003
9,592
5,684
14,580
17,912
19,240
% Eff.
Planchet
15.0
36.6
48.2
48.0
63.1
49.8
56.5
58.5
60.0
% Eff.
% Eff. Coated
Filter Filter
2.1
19.1
29.6
39.3
54.7
40.7
50.6
55.0
9.4
31.5
40.0
46.4
58.5
45.5
56.9
57.1
60.1
18,873
59.0
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Table II
32
BETA CALIBRATION
System #2
Nu elide
14
C
it
11
"TC
it
ti
185W
tf
tf
137
Cs
tt
1 1
36ci
II
II
204T1
II
It
90
Sr
ft
tt
89
Sr
B Energy
. (Max.)
.156
ri
it
.292
f f
rt
.429
II
:t
.514
i i
tt
.714
tt
tt
.766
II
1 1
1.41
If
f I
1.463
Half-life
5730y
II
If
2.12xl05y
It
;t
75d
It
11
30y
1 1
II
3xl06y
r t
it
3. Sly
tl
'
27. 7y
II
tl
II
Activity
Added
24 , 400
1 1
i r
1.12xl05
1 1
It
2.40xl05
! 1
f r
23,700
rt
If
16,400
i f
1 1
12,500
ft
II
25,610
10,750
25,610
31,350
Date of
Count
1-9-69
IT
"
2-14-69
tf
rt
3-27-69
1 1
It
1-30-69
It
i I
1-9-69
11
1 1
1-9-69
f ;'
II
3-10-69
f 1
If
2-12-69
CPM
Planchet
3,157
-
-
40,273
-
-
119,100
-
-
11,460
-
-
9,805
-
-
6,173
-
-
-
-
14,764
17,481
CPM
Filter
-
-
476
_
21,414
-
_
73,000
-
-
9,368
-
_
-
8,649
-
5,074
-
5,422
-
_
CPM
Coated
Filter
-
2409
-
_
-
34,710
_
-
98,800
-
-
10,786
_
9,855
-
-
5,818
-
14,888
-
-
_
% Eff.
% Eff. % Eff. Coated
Planchet .Filter Filter
12.9
9.9
2.0
40.0
19.1
31.0
49.6
30.4
41.2
48.4
- -
39.5 45.5
59.8
60.0
52.7
49.4
46.5
40.6
58.1
52.8
57.6
55.8
1.710
14.28d
32,000
1-28-69 19,096
17,358
18,949
18,097
19,390
59.7
55-. 3
59.2
57.7
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Table III
BETA CALIBRATION
System #3
Nuclide
14c
ft
It
"TC
it
it
185
iB w
It
II
137Cs
It
It
36
Cl
tt
ii
204
Tl
It
II
90
Sr
ft
It
89
Sr
tl
II
32P
II
If
B Energy
(Max.)
.156
II
tl
.292
II
II
.429
"
r:
.514
ti
ii
.714
11
tl
.766
"
tt
1.41
11
11
1.463
tl
If
1.710
It
tr
Half-life
5730y
ti
M
2ol2xl05y
1 I
1 1
75d
"
"
30y
ir
1 1
6
3x10 y
II
II
3. Sly
it
"
27. 7h
II
tl
5014d
It
f 1
14.28d
II
II
Activity
Added
24,000
It
1 1
1.12xl05y
"
"
5
2.40x10
i r
H
23,700
if
11
16 , 400
i r
it
12,500
M
"
25,610
10,250
25,610
31 , 350
it
11
32,000
ti
Date of
Count
1-09-69
It
fl
2-14-69
1 1
1 1
3-27-69
i t
II
1-30-69
f 1
11
1-9-69
!f
II
1-09-69
II
ft
3-10-69
i:
n
2-12-69
f 1
ft
1-28-69
f|
CPM
CPM CPM Coated % Eff.
Planchet Filter Filter Planchet
4,568 - - 18.7
3,445
636
47,984 - - 42.8
24,310
41,530
137,600 - - 57.3
80,100
112,300
12,296 - - 51.9
9,853
11,830
10,244 - - 62.5
10,421
8,867
6,655 - - 53.2
6,454
5,319
15,223
5,295
15,036 r
17,826 - - 56*9
17,670
18,504
18,936 - - 59.2
19,161
1O CUT
% Eff.
% Eff. Coated
Filter Filter
_ _
-
2.6
. _
21.7
37.1
-
33.4
46.8
_ «-
41.6 49.9
-
-
63.5
54.1
-
51.6
42.6
59.4
51.7
-
_ _
56.2
59.0
_ _
59.9
KO r»
-------
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