Estimation of Thermal neutron Fluence by Monte Carlo Simulation and Experimental Measurement of TLD

Abstract

In dosimetry, where it is necessary to measure (or estimate) the presence of thermal neutron fluxemanating from nuclear reactors, boron neutron capture therapy (BNCT) beam facilitiesand other neutron sources, the activation foil methodiscommonly used; however, it has the disadvantage of requiring a well-timed readout prior to the decay of the activation products as well asexpensiveHPGe detectors to count the emitted photons from the irradiated sample.In addition, it is prone to noise from unwanted activation products. The objective of this study is to present, evaluate and verify an alternative method that uses LiF based thermoluminescent dosimeters (TLD) to obtain thermal neutron fluence. This method is an alternative to the commonly used activation foil method.Thermoluminescence dosimeters are widely used in clinical settings to estimate photon and neutron doses andtheir response and characteristics have been well established. This study evaluatedthe neutron flux spectrumemitted from a Cf-252 neutron source through a moderating shadow cone using Monte Carlo calculations, and then experimentally verified its dose deposition, and H*(10) ambient dose equivalent. To account for thermal neutron fluence, a cadmium sheet was folded to shield groups of TLDs from direct and scattered thermal neutrons in the experiment. It was possible to account for thermal neutron fluence by simply subtracting the shielded TLD dose reading from the unshielded onesbecause cadmium is an excellent shield against thermal neutrons. The shadow cone was used as a neutron moderator between the californium source and the TLDs; it thermalizes fast neutrons emanating from the californium source. Monte Carlo simulations showed that a majority of energy deposit in TLD was caused by thermal neutrons and that scattered neutrons from the walls of this irradiation room were non-negligible and must be accounted for. The thermal neutron fluence calculated by Monte Carlo was converted into H*(10) ambient dose rate equivalent according to the ICRP recommendations, this was then verified against calibrated ³He detector readings. The difference between Monte Caro and measured results wasless than 3.5%. The ratio of simulated thermal neutron fluence among position A (unshielded), position B (shielded in the front by a Cd sheet) and position C (shielded insidea folded Cdpocket) were in good agreement with experimental values obtained from TLD neutron dose readings at the same positions. In conclusion, the developed method is sufficient as an alternative to the activation foil method to determine the thermal neutron flux for the pre-calibrated TLD system. Sincethe results from Monte Carlo simulations were in fair agreement with calibrated measurements, the method aided by Monte Carlo can be further used tocalibratethe TLD system.

BNCT시설,원자로,그리고다른중성자선원에서발생하는열중성자속의측정 (또는평가)이필요한경우주로방사화박을이용하게된다.그러나방사화박 (activation foils)은방사화붕괴시간에의한추가시간의소요,원하지않은물질의방사화에의한노이즈그리고방출되는광자계측을위한고가의HPGe 계측기의필요로단점을나타낸다. 본연구의목적은LiF 기반열형광선량계를이용하여다른방법으로열중성자속을측정및평가하는것에있다.이방법은의료계에서광자와중성자를측정하는것에널리사용되고반응도와특성이잘파악되어있는열형광선량계를이용함으로흔히사용되지만복잡한방법론과고가의장비를필요로하는방사화박을대체할수있다. 본연구는몬테카를로계산을이용하여감속물질(그림자원뿔, shadow cone) 을통과한Cf-252중성자선원에서나온열중성자를평가하고,실험적으로중성자속, 선량축적량그리고 H*(10) 주위선량당량을측정한다.열중성자를설명하기위해카드뮴시트를접어열형광선량계에직접적으로입사하는중성자와튕겨져입사하는중성자를나누었다.열중성자속은카드뮴시트를이용하여열중성자를막은그룹과막지않은그룹의측정값에서단순히차이를구하는것으로알아낼수있었는데, 이는카드뮴이열중성자를높은확률로막을수있기때문이다.감속물질은Cf-252선원과열형관선량계사이에위치하였고,Cf-252에서발생한고속중성자를열중성자화하는역할을하였다. 몬테카를로계산에의하면열형광선량계에축적된에너지의대부분은열중성자에의한것이었고,열중성자중에서조사실벽에서튕겨져나온중성자의영향이반드시고려를해야할만큼컸다.열중성자속또한몬테카를로시뮬레이션을이용하여추정하고, H*(10) 주위선량당량으로변환되었으며,교정된3He 계측기의값으로검증하여3.5% 이내의차이를확인했다.위치A (Cd시트없음), B (한쪽면을Cd시트로방호), C (접은Cd시트를사용하여양쪽면방호)에서열중성자속과열중성자속비율의몬테카를로계산결과는열형광선량계로측정한값과잘맞았고해석적인계산값또한잘맞았다. 결론으로,본연구에서개발된방법이열중성자속을측정하는목적으로기존의방사화박방법을대체하기에충분하였다.몬테카를로시뮬레이션결과가측정결과와잘맞았기때문에추후열형광선량계시스템을교정하는데이러한방법이사용될수있을것이다.