Lithuanian Journal of Physics article / Lietuvos fizikos žurnalo straipsnis

EFFICIENCY CALIBRATION OF HIGH-PURITY GERMANIUM DETECTOR USING MONTE CARLO SIMULATIONS INCLUDING COINCIDENCE-SUMMING CORRECTIONS: VOLUME SOURCE CASE
Marina Konstantinova, Darius Germanas, Arūnas Gudelis, and Artūras Plukis
Center for Physical Sciences and Technology, Savanorių 231, 02300 Vilnius, Lithuania
Email: marina.konstantinova@ftmc.lt

Received 9 November 2020; revised 15 January 2021; accepted 18 January 2021

The gamma-ray spectrometry by the instrumentality of Ge detectors is used for the detection of low activity environmental samples of different geometry (soil samples, air filters with aerosols, milk powder, etc.). Such measurements require separate calibration of the detector. The high purity germanium (HPGe) gamma-ray spectrometer of GC2520 series was used for experiments. For the efficiency calibration, three cylindrical containers filled with different ⁶⁰Co water solution levels were used, and the gamma-ray coincidence summing was modelled using MCNP6. The dimensions of the pure germanium crystal, provided by Canberra, were used for the simulations. The true coincidence summing takes place when two or more gamma quanta, which are emitted in a cascade from an excited nucleus, are detected within the resolving time of the detector. However, there is often a mismatch between the simulated and experimental efficiencies. The experimentally obtained and modelled spectra were compared: a good consistency of experimental and modelled results allows investigating the volume sources. During the simulation it was found that the factors affecting the accuracy of modelling are the thickness of the dead layer, crystal dimensions and the thickness of the Al detector cap. The analysis allows measuring the radionuclides activity concentration of samples placed in the containers with different filling heights having only standard shape calibration sources. The obtained accuracy is sufficient to fulfil criteria of 5–10% for such type of simulation to be applied for measurements of real samples in standard BURK-60 containers of various sample filling heights.

Keywords: HPGe detector, efficiency calibration, MCNP6, coincidence-summing correction

GRYNO GERMANIO DETEKTORIAUS EFEKTYVUMO KALIBRAVIMAS MONTE KARLO METODU SU SUTAPČIŲ SUMAVIMO PATAISOMIS: TŪRINIO ŠALTINIO ATVEJIS
Marina Konstantinova, Darius Germanas, Arūnas Gudelis, Artūras Plukis

Valstybinis mokslinių tyrimų institutas Fizinių ir technologijos mokslų centras, Vilnius, Lietuva

Gama spektrometrija naudojant germanio detektorius taikoma mažo aktyvumo skirtingos geometrijos aplinkos mėginiams (dirvožemio mėginiams, oro filtrams su aerozoliais, pieno milteliams ir pan.) tirti. Tokie matavimai reikalauja atskiro detektorių kalibravimo. Eksperimentams buvo naudojamas GC2520 serijos gryno germanio (HPGe) gama spektrometras. Buvo atsižvelgta į gamintojo pateiktus gryno germanio kristalo matmenis. Efektyvumui kalibruoti buvo naudojami trijų skirtingų užpildymo laipsnių cilindriniai indai su ⁶⁰Co vandeniniu tirpalu. Gama spinduliuotės sutapčių sumavimas buvo modeliuotas naudojant MCNP6. Tikru sutapčių sumavimu laikomas įvykis, kai du ar daugiau gama kvantų, kurie kaskadu sklinda iš sužadinto branduolio, aptinkami per detektoriaus skyrimo trukmę. Dažnai pastebima neatitiktis tarp modeliuotų ir eksperimentiškai nustatytų efektyvumo verčių. Palyginti eksperimentiškai gauti ir sumodeliuoti spektrai. Geras eksperimentinių ir sumodeliuotų rezultatų nuoseklumas leidžia tirti tūrinius šaltinius. Modeliuojant nustatyta, kad veiksniai, darantys įtaką modeliavimo tikslumui, yra neveikos sluoksnio storis, kristalų matmenys ir detektoriaus kriostato apvalkalo iš aliuminio storis. Analizė leidžia išmatuoti radionuklidų savitąjį aktyvumą skirtingo aukščio mėginių, įdėtų į BURK-60 indus, turint tik standartinius kalibravimo šaltinius. Mūsų modeliavimo tikslumas atitinka 5–10 % kriterijus, taikomus tokio tipo modeliavimui, ir šie rezultatai gali būti naudojami eksperimentuose imituojant įvairių užpildų ir aukščių standartinių BURK-60 konteinerių šaltinius.

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