Gross alpha/beta measurements in water samples using liquid scintillation counter

GROSS ALPHA/BETA MEASUREMENTS IN W ATER SAMPLES USING LIQUID SCINTILLATION COUNTER

Nilgun CELEBI. Sedat YASAR, Gursel KARAHAN, Mehmet SONMEZ, Bulent AKAR, Mehmet KOCAK Qekmece Nuclear Research and Training Center P.O.Box 1,

Ataturk Airport 34831 Istanbul TURKEY

ABSTRACT

Liquid scintillation techniques provide the detection and quantification of alpha and beta emitters in aqueous sample. Liquid Scintillation Counter (LSC) techniques using Pulse Decay Analysis (PDA) allow counting of alpha and beta radiation in the same sample simultaneously. PDA uses special pulse decay time discrimination electronics to differentiate alpha pulses from beta pulses in a liquid scintillator. In this experiment, Packard Tricarb 2770 TR-SL LSC has been used. Ultima Gold LLT produced by Packard Instrument Company was used as the liquid scintillator. The optimum counting parameters and Pulse Decay Discriminator (PDD) settings were provided for the best alpha and beta separation. PDD was verified by counting a pure alpha 241 Am and a pure beta 36C1. Spill of beta in alpha and alpha in beta was found around 0.1% at the optimum discriminator setting of 139. The counting efficiencies were 100% and 97% for alpha and beta counting respectively. Water samples were pre-concentrated to achieve the Turkish maximum permissible levels of 0.037 BqL 1 for gross alpha radioactivity and 0.37 BqL 1 for gross beta radioactivity in drinking water. After counting, the alpha and beta spill corrections were made and the gross alpha and beta radioactivities were calculated. LSC method results were compared with the gas flow proportional counters.

INTRODUCTION

Natural waters contain a number of both alpha and beta radionuclides in widely varying concentrations [1]. Liquid Scintillation Spectrometry has become of widespread interest during the last twenty years [2,3], Some of the difficulties that arise when measuring low-energy beta and alpha particles using conventional methods are completely avoided. Sample is counted directly in a homogeneous solution of an appropriate organic scintillator. Under these conditions, problems relating to sample self-absorption, attenuation of particles by detector windows, and beta backscattering from detector are solved.

Alpha and Beta emitting radionuclides may be counted simultaneously in the same liquid scintillation sample. A scintillation pulse consists of a prompt (initial) component and delayed (slow) component. These components occur in different proportions in alpha and beta pulses, with the result that alpha pulses are longer than beta pulses. LSC uses a special pulse decay discriminator to categorize the pulses as either alpha events or beta events and stores the events

appropriately in separate multichannel analyzers (MCA) [2,3]. A time-based Pulse Decay Discriminator may be adjusted by the operator, or the system, to determine the optimum setting for specific sample conditions. PDD is affected by the sample chemistry, vial type, geometry and degree of quenching [4].

MATERIALS AND METHODS

In this work Packard Tricarb 2770 TR-SL LSC has been used. Standard 20 mL plastic vials were used. Packard Ultima Gold LLT was used as the cocktail.

Sample: Scintillant ratio- The optimum sample:scintillant ratio was determined using the total available volume of 20 mL. For the best alpha /beta separation minimal sample volume was required. The optimum sample:scintillant ratio was found as 3-mL water in 12-mL scintillant. Background was minimum and Figure of Merit (FOM) was maximum at this ratio for this ratio.

PDD Optimization- PDD is optimized to accurately separate the pulse types and store the events in separate MCA’s. Only a pure beta and a pure alpha standard are required to establish this optimum setting. A plot of percent spillover of alpha events into the beta MCA and beta events into the alpha MCA is automatically generated by the instrument. The optimum discriminator setting is automatically calculated from the intersection of the two curves [5]. The optimum pulse decay discriminator value was determined using pure alpha 241Am and a pure beta 36Cl standards about 50000 dpm (Figure 1). For the most accurate results, standards were prepared as identical as possible to the samples and counted individually at defined alpha/beta standards counting protocol to establish the PDD value. PDD was found as 139 setting the alpha channel to 150-600 keV and the beta channel to 10-800 keV. Misclassification of alphas into beta MCA (as a fraction) was 0.0159; misclassification of betas into alphas MCA (as a fraction) was 0.0144 (Figure 2).

Counting Efficiency and Quench - The prepared standard samples were measured and the corresponding efficiency was calculated from the count rate of the known activity. The alpha counting efficiency was found as 100%, the beta efficiency was 97%. Adding small quantities of CCl4 to the prepared standard samples searched the effect of quenching on the PDD [6]. The quenching was monitored by the transformed Spectral Index of the External standard (tSIE) which is calculated from the Compton spectrum induced in the scintillation cocktail by an external 133Ba source. The calibration curves were prepared and used finding the efficiency of the unknown quenched sample.

Sample Preparation for LSC- To achieve the Turkish maximum permissible levels of 0.037 BqL-1 for gross alpha radioactivity and 0.37 BqL-1 for gross beta radioactivity in drinking water [7], water samples were pre-concentrated. 400 mL sample was evaporated down to dryness on a hot plate and then reconstituted in 1 M HCl to 3 mL. The sample solution was transferred into a 20 mL plastic vial and added ULTIMA GOLD LLT (12 mL). The vial was shaken until aqueous and organic phases were mixed completely. The vials were placed in the counter and

protocol for 300 minute each. Distilled water was used to prepare background sample. The measured count rates were corrected for background and the spillover.

Misclassification Calculations- The misclassification calculations are made to determine the actual cpm. The equations for true count rate due to alpha (AT) and beta disintegrations (BT) are given below respectively:

At =A0 A0 X p B 0 X p 1 - X p - X a

B B 0 - B 0 X a ~ A0 X a

d t = ---T 1 - X a - X p

A0= observed net count rate in alpha MCA

B0= observed net count rate in beta MCA

Xa= alpha misclassification as beta Xp= beta misclassification as alpha

In this experiment, Xa= 0.0159 and Xp=0.0144 for PDD=139.

Corrected cpm values both alpha and beta were used to calculate alpha and beta radioactivities [5].

Method Validation- In order to test the validity of the method, three check sources were prepared containing about 1:10, 1:1, 10:1 activity ratios of 241Am and 36Cl in the same chemistry as the unknown samples. The results were given in Table 1.

RESULTS AND DISCUSSION

The gross alpha and beta activities of water samples were determined by Liquid Scintillation Counter with alpha/beta discrimination and compared with the results of Gas Flow Proportional Counters (GFPC). Results were given in Table 2. It has been demonstrated that liquid scintillation counter is applicable for gross alpha and beta analysis. The results for water samples counted by LSC and GFPC agree within ± 2a for different mixtures of alpha and beta. The use of LSC with PDD for routine is advantageous because of the counting of alpha and beta radiation in the same sample simultaneously. In terms of sample preparation and counting time, LSC provides the best means of insuring that the time and steps required are minimal. In addition, Tri-Carb alpha/beta LSC’s can count 408 large vials (20 mL) automatically. GFPC has some disadvantages when compared to LSC. Counting efficiency for alphas are 40%, or less, compared to 100% for LSC and for betas are 45%, compared to 97 % for LSC. The LSC PDD method will provide good sensitivity.

The result of Table 1 has shown that all measurements were statistically indistinguishable from known activities and, therefore, it was concluded that the method is suitable for the determination of gross alpha and beta activities in water samples in routine work for a high range of activity ratios.

REFERENCES

1. United Nations Scientific Committee on the Effects of Atomic Radiation. A Handbook o f Radioactivity Measurement Procedures. NCRP Report 58 (Washington, DC: NCRP Publications) (1978).

2. Knoll, G.F. Radiation Detection and Measurement. New York, John Wiley&Sons. ISBN 0- 471-49545-X. (1979).

3. Sanchez-Cabeza, J.A. and Pujol, LI. A Rapid Method fo r Simultaneous Determination o f Gross Alpha and Beta Activities in Water Samples Using a Low Background Liquid Scintillation Counter. Health Physics. 68, 674-681 (1995).

4. Dazhu, Y. Yongjun, Z. and Mobius, S. Rapid Method for Alpha Counting with Extractive Scintillator and Pulse Decay Analysis. Journal of Radioanalytical and Nuclear Chemistry.

147:1,177-189 (1991).

5. Passo, C.J and Cook, G.T. Handbook o f Environmental Liquid Scintillation Spectrometry. Packard Instrument Company, Meriden, CT 06450 USA (1996).

6. Mobius, T.L. Alpha-Beta Discrimination by LSC. Determination of Radionuclides in Environmental Samples. IAEA Regional Advanced Training Course. 23 June-11 July 1997.

7. T.C. Resmi Gazete. Dogal Kaynak, Maden ve I?me Sulari ile Tibbi Sularin Istihsali. Ankara, Sayi:23144, 18 Ekim 1997.

Pulse Decay Discriminator Setting Chart

% Spillover vs. Time Discriminator Setting

/

/

Pulse Decay Discriminator Setting

■ Alpha in Beta " Beta in Alpha

Figure 2. Alpha/Beta Crossover Plot-241Am and 36Cl

Table 1 . Measurement of prepared checked source by LSC

Alpha activity (241Am) / Beta Activity (36Cl)

Known (BqL-1 ) Measured (BqL-1 )

0.25±0.05 0.22±0.052.39±0 0.21±0.07 0.19±0.05

0.24±0.05 .050.23±0.05 0.28±0.04 2.33±0.06

Table 2. Some of the determination of sample activities by two methods

Sample

Gas Flow Proportional Counters

Liquid Scintillation Counter

GrossAlpha (BqL-1) GrossBeta (BqL-1) GrossAlpha (BqL-1) GrossBeta (BqL-1) Bursa Kaynak 0.044±0.010 0.40±0.02 0.039±0.02 0.260±0.060 Bursa Imalathane 0.022±0.007 0.06±0.02 0.019±0.01 0.06±0.020 AnkaraAltinMemba 0.036±0.011 0.095±0.02 0.033±0.02 0.097±0.002 Baspinar 0.013±0.003 0.04±0.02 0.019±0.003 0.06±0.020 Halkapinar Ivriz 0.009±0.002 0.05±0.02 0.012±0.004 0.03±0.004 £ene 2 0.037±0.011 0.07±0.02 0.03±0.03 0.05±0.02 Bimpas 1 0.180±0.021 0.11±0.03 0.16±0.02 0.11±0.04 Bimpas 2 0.150±0.010 0.14±0.05 0.08±0.03 0.11±0.02 Bimpas 3 0.130±0.010 0.14±0.05 0.09±0.03 0.12±0.02 Karamandere 0.020±0.010 0.05±0.02 0.01±0.002 0.05±0.02 Gokberk 0.030±0.013 0.06±0.03 0.02±0.03 0.04±0.03 ABE 0.035±0.011 0.20±0.01 0.02±0.02 0.24±0.02