Daniel M. MontgomeryAnalytics, Inc.

the low-activity levels, the filter and charcoal car-tridge are normally counted in close proximity to thegamma-ray detector (1 cm or less). The cartridge is

counted with the inlet side facing the detector-"face" counting. This practice minimizes the count-

ing time necessary to achieve a given sensitivity(LLD); however, it may introduce significant errors

in the measurement as will later be discussed.Efficiency calibrations for filters are rather

straight forward. Filter standards with an eight ra-dionuclide mixture (109Cd, 57 Co, 139Ce, 203Hg, 113Sn,

137 Cs, 88y, and 60Co) covering the energy range from

88 keY to 1836 keVare commercially available or canbe prepared from a standard solution of this mix-

ture. It is important that the activity be uniformlydistributed across the same active area as the sam-

ples to be counted. Significant coincidence summingmay occur for 88y and 60Co, and corrections may be

necessary in the energy range from 898 keY to1836 keY, depending on the detector efficiency andthe proximity of the sample to the detector. The ra-dionuclides 1311, 1331 and 1351, with the longest half-lives (8.0 d, 20.9 h and 6.7 h, respectively), emitstrong gamma rays (at 364, 529 and 1260 keY, re-

spectively) that are essentially free of coincidencesumming even when the sample is counted near the

detector housing. If the 1260-keV gamma ray from

1351 is used for quantification, coincident summingcorrections for the standard may be necessary. How-

ever, an alternative would be to use the 420-keVgamma ray from 1351 which is outside the energy

onitoring radioiodine in ambient airor gaseous effluent streams requiresconcentration of radioiodine fromlarge volumes of air to achieve the sen-

sitivity required by various regulatory agencies. Thisis accomplished by passing an air sample through aparticulate filter followed by a charcoal cartridge.Since radioiodine may be present in various chemi-cal forms including elemental iodine, organic iodidespecies, and other inorganic species, the charcoal car-tridges are impregnated with KI or TEDA( tetraethylenediamine) to convert organic iodidespecies to forms that are collected on charcoal. Inthe presence of high noble gas activity (i.e., accidentmonitoring), a silver zeolite cartridge is recom-mended over a charcoal type cartridge since noblegases are not retained on silver zeolite but are highlyretained on charcoal and would significantly lowerthe sensitivity for radioiodine.

The preferred method to analyze the filter andcartridge is to count them separately with a germa-nium gamma-ray spectrometer (counting the filterand cartridge together is discouraged). Then, the ac-tivities on the filter and cartridge are combined togive the total radioiodine activity. The contributionof particulate radioiodine is generally small; how-ever, studies1 have shown that particulate radioio-dine may constitute as much as 40% of the total. Thefraction of particulate radioiodine is variable and de-pendent on the source of radioiodine and other con-ditions such as pre-release treatment of the effluentstream (e.g., HEPA filter, charcoal train). Because of

Reprint of "From the Counting Room", Vol. 1, No.2, 1990 47

Radioactivity 6- Radiochemistry

The Counting Room: Special Edition

tected, NRC requires quantification and reporting ofthese radionuclides. Therefore, it is wise to calibratethe detectors over a wide enough energy range topermit analysis of all the iodine radionuclides (i.e.,up to 1300 keY). Additionally, the software utilizedby most commercially available gamma-ray spectros-copy systems require 8-10 efficiency! energy pairs atappropriate energies to accurately define an effi-ciency equation over the energy range from 88 to1836 keY.

One method of preparing charcoal and silverzeolite cartridge standards involves disassemblingthe cartridge and gravimetrically adding a knownquantity of the eight radionuclide mixture (as de-scribed above) to a portion of the charcoal. Theamount of spiked charcoal is based on the depth ofspiked charcoal desired when reloaded into the car-tridge. The spiked charcoal is homogenized andplaced back in the cartridge with a barrier betweenthe active and inactive layers to prevent mixing. Theactive layer can be varied from approximately 5 mmto the total active depth of the cartridge (homogene-ous).

Most laboratories use a "face" loaded cartridge-which means that the activity is loaded preferentiallyon the inlet or front "face" of the cartridge. Experi-ence has shown that a 5-mm "face" loaded standard,counted with the inlet or "face" side toward the de-tector cryostat for the entire counting period, willgenerally provide an adequate efficiency calibrationfor charcoal cartridges. However, a better alternativeis to count the standards (and samples) with the in-let side down ("face" side) for half of the countingtime and then with the inlet side up ("flip" side) forthe remainder of the counting time. The spectrum iscollected over the entire counting interval. This willgive results, for both standards and samples, that areless sensitive to the distribution of iodine.

The latter part of this article provides experi-mental data on the effect of radioiodine distributionon detector efficiencies for counting with inlet sidedown ("face" counting) and counting by the "flip"technique. Detector efficiencies may vary consider-ably for different models and types of cartridges.Calibrations are generally valid only for the specifictype or brand of cartridge used for the calibration.

range where coincidence summing corrections ofthe standard are necessary.

Efficiency calibrations for charcoal cartridges aremore complex since the distribution of iodine in thecartridge is dependent on many factors including:the iodine species, humidity, temperature, sampleflow rate, and length of exposure. These factors canall affect the distribution and the penetration depthof the radioiodine in the cartridge. Charcoal car-tridges can be counted directly on the detector, orthe charcoal can be removed from the container, ho-mogenized and counted.

The practice of removing the charcoal beforecounting has, in principle, the advantage of yieldinga homogeneous sample. A calibration standard canbe prepared by adding a known quantity of mixedgamma-ray standard to the appropriate mass of char-coal and thoroughly mixing to ensure homogeneity.Disadvantages of this technique include: a lowercounting efficiency than when "face" counted; alonger analysis time due to disassembly of the car-tridge and mixing of the charcoal in another con-tainer, and possible sample loss during transfer. Dueto these problems, most nuclear power plant count-ing rooms do not use this technique.

Based on my experience with the Nuclear Regu-latory Commission (NRC) and as a consultant to thenuclear power industry, the most common practicefor counting charcoal cartridges is to "face" countthe sample within a few centimeters of the detector.The preparation of an appropriate standard requiressome knowledge of the radioiodine distribution inthe charcoal cartridge samples. Based on the discus-sion above, the distribution may be quite dependenton the particular monitoring situation (i.e., environ-mental monitoring, long term stack monitoring,grab samples for respiratory protection purposes),and environmental conditions.

This paper focuses on 1311; however, other short-lived radioiodine species may be present in effluentsamples. These include: 1321, tl;2 = 2.3 hours;1331, tl/2 = 20.9 hours; 1341, tl;2 = 50.2 minutes, and1351, tl;2 = 6.7 hours. In most nuclear power plantsthe concentrations of short-lived radioiodines ingaseous effluents are low due to the holdup time as-sociated with the effluent treatment systems. How-ever, if short-lived radioiodine radionuclides are de-

48 Reprint of "From the Counting Room", Vol. 1, No.2, 1990

"Calibrating Germanium Detectors for Assaying Radioiodine in Charcoal Cartridges"

Daniel M. Montgomery

3 mm from detector

Efficiency, %

FB Ratio Face Flip

3 cm from detector

Efficiency, %

FB Ratio Face FI

2.08 1.34 1.001.92 1.28 0.971.73 1.23 0.971.44 1.10 0.93

1.22 1.01 0.921.02 0.95 0.92

6 cm from detector

Efficiency, Ofo

Fe Ratio Face Rip1311 Load

O-mm2-mm5-mm10-mm

15-mm25-mm

2.822.552.251.66

1.33

1.04

4.08

3.903.59

3.00

2.73

2.45

2.762.722.592.42

2.382.41

1.75

1.68

1.56

1.32

1.20

1.00

0.610.580.560.510.49

0.46

0.48

0.46

0.46

0.45

0.45

0.46

Flip efficiency refers to counting efficiency when the source is counted With the inlet side facing the detector for half of the countingperiod and the inlet facing away for half of the counting period.

Face efficiency refers to counting efficiency when the cartridge is counted with the inlet side facing the detector.FB is the ratio of activity observed counting with the inlet side facing the detector to the activity observed when counting the inlet

side facing away from the detector.

Table 1 Measured front-to-back ratios and counting efficiencies.

There are several unpublished studies2,3 investi-gating the distribution of radioiodine in charcoalcartridges under actual monitoring conditions. In-vestigations by Dale Nix2 at TVXs Browns Ferry Nu-clear Plant showed that radioiodine was foundwithin the front 5 mm and the average depth wasabout 2 rnrn. These studies included various moni-toring locations in the plant and included cartridgesthat were in place for up to one week. Investigationsby McFarland3 at a radioiodine processing facilityshowed that greater than 95% of the radioiodine incharcoal cartridge samples was within the front5 mm. These samples were collected in ambient airover a period of 2-3 days. Spiking the first 5 mm onthe inlet side ("face") of a charcoal cartridge with theradionuclide standard, therefore, provides a reason-able approximation of the expected distribution(and activity gradient continuously decreasing fromthe front "face" to the back).

Loading effects on the measured counting effi-ciency for radioiodine were investigated by spikingcharcoal cartridges with 1311 at various depths andcounting at three different distances from a germa-nium detector. Commercially available TEDA im-pregnated charcoal cartridges, with approximate di-mensions of 5.7-cm wide by 2.54-cm thick, wereused in this work. The thickness of the radioiodinedistribution corresponded to O-mm, 3-mm, 5-mm,10-mm, 15-mm, and 25-mm (homogeneous). All

samples were prepared gravimetrically from 1311 so-lutions calibrated to within :t 3%. The O-mm depthwas prepared by distributing the activity on a filterusing a pattern of drops over the active area to ap-proximate a uniform distribution. The filter wasplaced in the cartridge, in front of the charcoal bedon the inlet side ("face"). Tests of the O-mm car-tridge showed that 1311 did not volatilize or redistrib-ute during the test period of approximately 2 days.Other thicknesses of 1311 spiked charcoal were pre-pared by adding a known amount of 1311 activity to apredetermined amount of charcoal, homogeneouslymixing it, and replacing it in the inlet side ("face") ofthe cartridge to provide the desired depth. A barrierwas inserted to separate the 1311 active from the inac-tive charcoal. The remainder of the inactive charcoalwas replaced and the cartridge resealed.

The counting system consisted of a 30-cm3 ger-manium detector interfaced to a computerized mul-tichannel analyzer system. After each count, the netpeak area and efficiency for the 1311, 364.8-keVgamma ray were determined. Each cartridge stand-ard was counted with the "face" down for 1000 sec-onds and processed to determine the net counts andefficiency. The cartridges were then turned over withthe "face" side up ("flipped") and counted for an ad-ditional1000 seconds by resetting the ADC live timeto 2000 seconds, without erasing the spectrum. Thecumulative count was then processed to give the net

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Radioactivity 6- Radiochemistry

The Counting Room: Special Edition

Figure 1 Efficiencies vs front-to-back ratios for "flip" and "face"

counting 3 mm from detector.

Figure 3 Relative efficiencies vs front-to-back ratios for "flip" and

"face" counting 3 cm from detector.

Figure 2 Relative efficiencies vs front-to-back ratios for "flip" and

"face" counting 3 mm from detector.

Figure 4 Relative efficiencies vs front-to-back ratios for "flip" and

"face" counting 6 cm from detector.

counting rate and efficiency for the 364.8-keVgamma ray. Front-to-back ratios were determinedfrom the following formula:

FCFB= TC-FC

"face" side down, to the counting rate for a cartridgecounted with the "face" side up ("flipped"). This ra-tio correlates with the distribution of radioiodine inthe cartridge with the highest value of FB for "face"loaded cartridges. The value of FB decreases as thedepth of loading increases and approaches unity forhomogeneously loaded cartridges (the ratio may notbe exactly one, since many cartridges are not manu-factured so that the front and back are precisely thesame).

The measured front-to-back ratios and countingefficiencies are presented in Table 1. Counting effi-ciencies for cartridges loaded at various depths were

where FB = front-to-back ratio, FC = counts of the364.8-keV gamma ray from counting with "face"down, and TC is the total counts of the 364.8-keVgamma ray from the "face" and "flip" counts.

The front-to-back ratio represents the ratio ofthe counting rate for a cartridge counted with the

50 Reprint of "From the Counting Room", Vol. 1, No.2, 1990

"Calibrating Germanium Detectors for Assaying Radioiodine in Charcoal Cartridges"

Daniel M. Montgomery

mized by using the "flip" method of counting evenat a sample-to-detector distance of 3 mm.

Because the data presented in this paper are de-pendent on many factors such as the detector dimen-sions, the type of charcoal cartridge used, and sam-pling conditions, these results may not bequantitatively applicable to other situations. How-ever, these studies demonstrate the value of the "flip"counting method for a variety of conditions. Site spe-cific studies may be conducted to determine the dis-tribution of radioiodine in samples collected underactual operating conditions using the techniques pre-sented in this article. The most appropriate loadingfor a calibration standard can then be selected.

{f a high degree of accuracy is desired or is neces-sary, a series of efficiency curves or tables can be de-termined for different distributions (i.e., S-mm-, 10-mm-, IS-mm-Ioaded cartridges) using mixedgamma-ray standards. The ratio of front-to-back ac-tivity for radioiodine would be calculated for eachsample and then the appropriate efficiency curve ortable would be utilized for the analysis.

When "face" counting of cartridges is practiced,it may be a good idea to periodically measure the ra-tio of front-to-back activity in charcoal cartridgesamples to ensure that calibrations and countingconditions are appropriate to meet the desired meas-urement accuracy.

AcknowledgementsThe author thanks Estie W. Belvin for her assistancein preparation of the charcoal cartridge standardsand John McCorvey for counting the standards.

plotted against the front-to-back ratios. These resultsare presented in Figure 1, for a source to detector dis-tance of 3.0 mm. As expected, counting cartridges"face" down in close proximity to the detector (Fig-ure 1) is very sensitive to loading. The 0- mm loadedcartridge has an efficiency that is 1.70 times that of ahomogeneously loaded cartridge.

The effects of the loading at source-to-detectordistances of 3 mm, 3 cm and 6 cm, for both "flip"and "face" counting, are shown in Figures 2-4, whererelative efficiencies are plotted against the front-to-back ratios. The counting efficiencies were normal-ized to 1.00 for cartridge efficiencies with a 5-mmthick loading. This assumes that the cartridge effi-ciency, with the 5-mm thick loading, is most repre-sentative of the expected loading for actual charcoalcartridge samples. Upper and lower limits weredrawn at relative efficiencies 1.10 and 0.90 to repre-sent an allowable deviation limit of plus and minus10% from the 5-mm value. The value of 10% issomewhat arbitrary; however, a 10% systematic er-ror would still allow one to meet the acceptance cri-teria used by the U.s. NRC's Confirmatory Measure-ments Program. The most restrictive NRC criteriawould allow a measurement to deviate up to 15%from the NRC measured value.

The problems associated with counting samplesvery close to the detector are illustrated in Figure 2.Although all points are within the limits when "flip"counted, only the 2-mm cartridge would be withinthe limits when "face" counted. Careful comparisonof Figures 2, 3 and 4 show that the deviation fromthe 5-mm cartridge efficiency decreases as thesource-to-detector distance increases. At a source-to-detector distance of 3 cm, all points are within 5%for "flip" counting and 3 values are outside of the10% limits for "face" counting. At a source-to-detec-tor distance of 6 cm, all points are within 5% of the5-mm value for "flip" counting, and all points exceptfor the 15-mm and homogeneously loaded car-tridges are within the 10% limits for "face" counting.

As demonstrated by the above measurements,potential errors associated with a variable distribu-tion of radioiodine in cartridge samples, can be mini-

References1. P.G. Voillegue, et al Evaluation of the Air-Vegetation-Milk Pathway for

1311 at the Quad Cities Nuclear Power Station", u.s. Nuclear Regu-

latory Commission Report NUREGjCR-1600, November 1981.

2. Personal Communication (1990), D.N. Nix, Tennessee Valley

Authority, Sequoyah Nuclear Power Plant, P.O. Box 2000, Soddy-

Daisy, TN 37379.

3. Personal Communication (1988), RC. McFarland, Analytics, Inc.,

1380 Seaboard Industrial Blvd., Atlanta, GA 30318.

R&R

Reprint of "Prom the Counting Room", Vol. 1, No.2, 1990