The methods that use scintillation counting with ZnS(Ag) scintillation detector are widely used for gross alpha activity determination. The common criteria for the selection of optimum counting condition for a ZnS(Ag) scintillation detector do not consider simultaneously operating voltage and discrimination level variation. In presented method a relationship between voltage and discrimination level is derived for counting efficiency.Fifth International Yugoslav Nuclear Society Conference : September 27-30, Belgrade, 2004

Kandić, Aleksandar

Novković, D.

Vukanac, Ivana

Đurašević, Mirjana

Repository of the Vinča Nuclear Institute (VinaR)

Fifth Yugoslav Nuclear Society Conference  YUNSC - 2004████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████305████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████THE DETERMINATION OF THE OPTIMUM COUNTINGCONDITIONS FOR A ZnS(Ag) SCINTILLATION DETECTORM.M. ĐURAŠEVIĆ, A.B. KANDIĆ, D.N. NOVKOVIĆ and I.S. VUKANACInstitute for Nuclear Sciences Vinca, Beograd, Serbia and Montenegro, mirad@vin.bg.ac.yuABSTRACTThe methods that use scintillation counting with ZnS(Ag) scintillation detector are widelyused for gross alpha activity determination. The common criteria for the selection ofoptimum counting condition for a ZnS(Ag) scintillation detector do not considersimultaneously operating voltage and discrimination level variation. In presented method arelationship between voltage and discrimination level is derived for counting efficiency.Key words: ZnS(Ag) scintillation detector, gross alpha activity, optimum countingconditions1. IntroductionThe important aspect of environmental protection and remediation is the assessment of theradiological quality of samples. Alpha-particle-emitting sources could be detected by the proportionalcounters, silicon surface-barrier, liquid scintillation and ZnS(Ag) scintillation detectors. Thescintillation ZnS(Ag) detectors are the common choice for measurements of alpha-emitters. Theadvantages of these detectors are: long term stability, ruggedness, and relative easy samplepreparation. The results of measurements by ZnS(Ag) scintillation detector could be used as an initialindicator for decision, in order to identify and quantify the presence of alpha-emitters in samples, arethe further radiological analyses necessary.For growing needs in environmental radioactivity monitoring the preliminary measurements withZnS(Ag) scintillation detector are developed. The procedure for determination of the optimumconditions for operating of the ZnS(Ag) scintillation detector has been presented.2. Criteria for determination of the optimum counting conditionsIn setting up a nuclear counting system, it is often desirable to establish an operating point, which willprovide maximum stability over long periods of time [1]. The reaching of the optimum countingconditions is possible by choosing the appropriate values of the high voltage supply and discriminationlevel.The optimisation of the specific device could be performed by figure of merit, FM, defined asBEFM /2= , where E is the counting efficiency and B is the background counting rate [2, 3].Fifth Yugoslav Nuclear Society Conference  YUNSC - 2004████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████306████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████The efficiency of ZnS(Ag) scintillation detector depends on high voltage supply and discriminationlevel (LLD) for the given device gain (preamplifier and amplifier). It is recommended not to use toohigh voltage supply, because the lifetime of the phototube is longer at the lower voltage supply.However, the operating voltage should be taken above the threshold voltage of the plateau. It shouldbe chosen where the counting rate has a minimum variation due to voltage supply and discriminationlevel drifts. This criterion involves the selection of the operating voltage at one third of the plateaulength [2]. Maximum figure of merit is achieved by adjusting the operating voltage for a givendiscriminator setting or by adjusting the discriminator setting for given operating voltage [2].The new approach for determination of the optimum counting conditions of ZnS(Ag) scintillationdetector takes into account the both mentioned criteria, simultaneously [4].In this paper the results obtained by means of three different methods of determination of optimumcounting conditions are presented.3. ExperimentThe measurements were carried out by the ZnS(Ag) scintillation detector (Bicron 3M.125PP/3M-X)with photomultiplier tube (Canberra 2007). The maximum cathode to anode voltage for the PMT is2000 V. The discrimination level varies from 0 V to 10 V.The alpha spectroscopic source used in this experiment is 239Pu with the intensity of 1560 α s-1 in 2π(LMRI, No. 2796). The source-detector distance is 4.1 mm.The characteristic curves were determined by measuring the alpha source by means of ZnS(Ag)scintillation alpha detector for high voltage supply between 800 V and 1400 V and the discriminationlevel between 0.1 V and 2.0 V. The statistics of one measurement was from 10000 to 90000 counts foreach combination of voltage and discrimination level. This experiment is based on the previouslyperformed measurements [5]. The results obtained in the previous experiment point out that thementioned discrimination level range is sufficient and that a smaller steps in this range should be done.4. Results and discussionDetermination of the optimum conditions for ZnS(Ag) scintillation detector could be done by: plateaustudy, determination of figure of merit, determination of combination of efficiency and discriminationlevel for the certain operating voltage.Figure 1. illustrates a typical characteristic curves for the ZnS(Ag) scintillation detector. It wasobserved that plateau became shorter with increasing of the discrimination level. Thus, in the range ofdiscrimination level from 2 V to 3 V, the plateau disappeared [5]. This was also a limitation on thediscrimination level value. The values for operating voltage and discrimination level obtained fromFigure 1. are 1050 V and 0.5 V, respectively.Figure 2. illustrates FM as a function of voltage and discrimination level in a ZnS(Ag) scintillationdetector. From a graph we could observe flat zone, which take place in the discrimination level rangefrom 0.3 V to 0.5 V for FM 2.0, with a small variation of the background and efficiency. The value foroperating voltage obtained from Figure 2. is 1000 V, since it should be selected in flat zone.Fifth Yugoslav Nuclear Society Conference  YUNSC - 2004████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████307████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████                                                                                                                                                                                                                                                                                        800 900 1000 1100 1200 1300 14000100002000030000400005000060000700008000090000100000CountsVoltage [V] LLD=0.1 LLD=0.3 LLD=0.5 LLD=0.7 LLD=1.0 LLD=1.3 LLD=1.5 LLD=1.7 LLD=2.0Fig. 1. Typical characteristic curves in a                           ZnS(Ag) scintillation at different LLD                           settings                                                                                                                                                                                                                                                                                        0.1 0.2 0.3 0.4 0.5 0.6 0.780085090095010001050110011501200125013001350140014500.51.01.52.02.52.01.51.00.5Voltage [V]LLD [V]Fig. 2. FM as a function of voltage and LLD inZnS(Ag) scintillation detectorFigure 3. presents counting efficiency as the function of the operating voltage for differentdiscrimination level. The fitted exponential function for the ZnS(Ag) scintillation detector is alsoshown.We found a good agreement between the trends in the data and the fitting function, with correlationcoefficients in the interval [0.97, 1.00]. Although the maximum theoretical absolute detectionefficiency for 2π geometry alpha particle detector is 50 %, the experimental absolute countingefficiency will be below this value. This discrepancy can be explained in terms of backscattering,absorption in window and air layer between the detector and source, and the geometry less than 2π [6].In our experiment the maximum value for efficiency is about 30 %. In further analysis, studiedefficiency is ranged from 29.3 % to 29.7 %.                                                                                                                                                                                                                                                                                        800 900 1000 1100 1200 1300 14002526272829303132Ef=29.7 %Ef=29.3 %Ef = y0+A1e-U/t1Efficiency [%]Voltage [V] LLD=0.1 V LLD=0.3 V LLD=0.5 V LLD=0.7 V LLD=1.0 VFig. 3. Counting efficiency as a function of                          operating voltage for different LLD                                                                                                                                                                                                                                                                                        0.0 0.2 0.4 0.6 0.8 1.01000110012001300U = y0+A1e-LLD/t1Voltage [V]LLD [V]Ef=29.3%Ef=29.4%Ef=29.5%Ef=29.6%Ef=29.7% Fig. 4.  Voltage as a function of LLD for                            the different counting efficienciesFigure 4. presents voltage as a function of discrimination level for different counting efficiencies, basedon Figure 3. and fitted function. For the efficiency range considered above, the expression given inFifth Yugoslav Nuclear Society Conference  YUNSC - 2004████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████308████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████Figure 4. is applied. In this case, we found a good agreement between the trends in the data and theempirical relationship, with correlation coefficients in the interval [0.99, 1.00].The derived relationship provides a quick method for determining of the discrimination level for thevoltage selected and for the efficiency ranged from 29.3 % to 29.7 %. For the operating voltage of1050 V and 1100 V, combination of efficiency and discrimination level are presented in the Table 1.Therefore, the optimum parameter values for our ZnS(Ag) scintillation detector are 1100 V fordiscrimination level 0.5 V or 1050 V for discrimination level 0.3 V.Table 1. Combination of efficiency and discrimination level for the operating voltage of 1050 V and 1100 VDiscrimination level [V]Operatingvoltage [V] Ef=29.3% Ef=29.4% Ef=29.5% Ef=29.6% Ef=29.7%1050 0.28 0.22 0.15 - -1100 0.50 0.44 0.35 0.26 0.175. ConclusionThe method described in this paper offers several advantages:a) it is based on the selection of the of the operating voltage in the plateau region;b) relationship between voltage and discrimination level is derived;c) the method considers simultaneously operating voltage and discrimination level variationto rich the best counting conditions.Using this method we determined the optimum counting conditions for our ZnS(Ag) scintillationdetector: operating voltage 1100 V and discrimination level 0.5 V.The aim of this experimental work was to develop a method for absolute measurements of a thickalpha sources with ZnS(Ag) detector system. This method does not require the use of standards,special calibrations, or complicated radiochemical procedures. Applications of this method include thequantitative determination of radon and progeny in air, water, and charcoal, and measurement of thealpha activity in soil and on air filter samples, as well as for purposes related with reactordecommission.6. References[1] G.F. Knoll, Radiation Measurements (1989)[2] Overman, R. T., Clark, H. M., 1960. Radioisotope Techniques, McGraw-Hill, New York[3] Draganić, I., ed., 1985. Radioactive isotopes and radiations, Institute for nuclear sciences “BorisKidrič”, Belgrade, (in Serbian)[4] Pujol, Ll., Saurez-Navarro, J. A., Montero, M., 2000. A method for the selection of the optimumcounting conditions in ZnS(Ag) scintillation detector, Applied Radiation and Isotopes 52 (4), pp.891-897[5] Đurašević, M.M., Kandić, A.B., Novković, D.N., Vukanac, I.S., 2004. A method for theoptimisation of counting conditions for a ZnS(Ag) scintillation detector, Proceedings of the XICongress of Physicists in Serbia and Montenegro, Petrovac n/m, 3 - 5 June 2004, pp. 8-61 - 8-64,(in Serbian)[6] Blanco Rodriguez, P., Martin Sánchez, A., Vera Tomé, F., 1997. Experimental studies of self-absorption and backscattering in alpha-particle sources, Appl. Radiat. Isot. 48 (9), pp. 1215-1220

The determination of the optimum counting conditions for a ZnS (Ag) scintillation detector

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The determination of the optimum counting conditions for a ZnS (Ag) scintillation detector

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