With the recent change of the equivalent dose limit for the lens of eyes from 150 to 20 mSv/yr., averaged over 5 years, it is of great importance to investigate the beta-ray spectrum in the maintenance areas of the nuclear power plants, where workers are exposed to mixed beta-gamma fields. A beta-ray spectrometer consisting of a thin silicon detector and a plastic scintillator has been developed, which can accept only beta detection events while rejecting gamma events via coincidence. Based on the spectroscopy system that has been built, a comprehensive upgrade of software and hardware has been conducted. The data acquisition software MC2 was replaced with CoMPASS, developed by CAEN to achieve higher stability and functionality. Optimal shaping parameters and coincidence time window were determined by instruction and sample tests. The fast event signal from the plastic scintillator does not provide enough energy information. Therefore, three approaches were executed, including adding a capacitor, adding a preamplifier, and changing the digital pulse processor to solve the problem. The method of changing digitizer was accepted as the final solution to obtain the best spectrum through benchmark tests. The experimental measurements characterized the response of the beta-ray coincidence spectrometer under the mixed beta/gamma radiation field with a various count rate ratio from 0.00625 to 0.8. Experimental result shows excellent and stable performance of the detector system under a higher beta to gamma ratio. Gamma contribution of the coincidence spectrum, especially in low energy region, significantly increased when beta count rate was controlled to a minor level. Monte Carlo simulations are also carried out using the MCNP6 code to validate the measurements. / Thesis / Master of Science (MSc) / With the recent change of the equivalent dose limit for the lens of eyes from 150 to 20 mSv/yr., averaged over 5 years, it is of great importance to investigate the beta-ray spectrum in the maintenance areas of the nuclear power plants, where workers are exposed to mixed beta-gamma fields. A beta-ray spectrometer consisting of a thin silicon detector and a plastic scintillator has been developed, which can accept only beta detection events while rejecting gamma events via coincidence. Based on the spectroscopy system that has been built, a comprehensive upgrade of software and hardware has been conducted to slow down the signal and achieve higher stability. The experimental measurements characterized the response of the beta-ray coincidence spectrometer under the mixed beta/gamma radiation field with a various count rate ratio from 0.00625 to 0.8. Experimental result shows excellent and stable performance of the detector system under a higher beta to gamma ratio. Gamma contribution of the coincidence spectrum, especially in low energy region, significantly increased when beta count rate was controlled to a minor level. Monte Carlo simulations are also carried out using the MCNP6 code to validate the measurements.

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<p>U disertaciji je izvr&scaron;ena optimizacija detektorskog IZ&nbsp;&nbsp; sistema Quantulus 1220^TM radi razvoja metoda za određivanje pojedinačnih radionuklida ³H,²²²Rn i⁹⁰Sr, kao i ukupne alfa/beta aktivnosti u vodama. Predstavljeni su eksperimenti u svrhu kalibracije sistema, istraživanja uticaja relevantnih parametara na rezultate određivanja aktivnosti alfa i beta emitera, i pode&scaron;avanja optimalnih vrednosti ovih parametara.&nbsp;Posebna pažnja je posvećena variranju&nbsp; PSA diskriminatora, odnosno pode&scaron;avanju analizatora oblika impulsa na optimalnu vrednost u svrhu optimalnog razdvajanja alfa od beta događaja, kao i ispitivanju efekata prigu&scaron;enja u uzorcima i metodama&nbsp;njihove korekcije.&nbsp; Predstavljene razvijene metode su brze, efikasne i jednostavne, &scaron;to je naročito važno u slučaju radijacionog akcidenta, kada je potrebno brzo odrediti sadržaj radionuklida u&nbsp; uzorcima vode, a njihova validnost je pokazana u okviru međunarodnih<br />i međulaboratorijskih interkomparacija.</p> / <p>Dissertation involves optimization of detector AB&nbsp;&nbsp; system&nbsp; Quantulus 1220^TM for development of methods for determination of individual radionuclides ³H, ²²²Rn and ⁹⁰Sr, as well as gross alpha/beta activity in waters. Presented experiments had purpose of calibration of the system,&nbsp; investigation&nbsp; of&nbsp; relevant parameters&nbsp; influence&nbsp; on&nbsp; activity&nbsp; measurements&nbsp; of alpha and beta emitters, as well as&nbsp; setting the optimum values of&nbsp; these parameters. Special attention&nbsp; was devoted to&nbsp; PSA&nbsp; discriminator value&nbsp; setting, i.e. adjusting the pulse shape analyzer level at optimum value for an optimal separation of alpha from beta events, as well as testing the effects of quenching in the samples and methods for quench&nbsp; correction. Presented developed methods are fast, efficient and simple, which is especially important in the case of a radiation accident, when there is a need to quickly determine the content of radionuclides in water samples, and their validity has been demonstrated within the framework of international and inter-laboratory&nbsp; ntercomparison measurements.</p>