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 ------- 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. ------- 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 ------- 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. ------- 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. ------- 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 ------- 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 ------- 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» ------- '-,«:: so x 10 TO THC ... INCH 358-1 1 i\"i K> '.if ML.'-' ':!..« \..'.>. ". ', *. 11 bv-a tior Max. 8erf<^. Energy "" ------- IV-'..'^' :O X 1O TO THK '.. INCH 35S-11 f V (':-."_ M ut I I I .1 '. ;.! H *.<.'. v .. -. ft WFfcrf I r .1 nr.ij" "i±i£ili ------- rr.~ 10 X 1O I'U THF. ' .: INCH 358-1 1 s4ein 3 SEj.hr. ._,, i P 1 ~" i I JTTTT ~ I . i ' i.. _L_|_1.1__-..-. L--.-J i I : :!::!:.:: 1 I | ..... I ... i j -\ J I .;-.;.[__.lu--._j L : i r I ; '' | - a.,o ------- loo \OOO6 ------- l^ 10 X 1O TO THU CENTIMCTCH 46 151O ------- |