Teste | Abstract: 115-1 | ||||
Abstract:The Beta 2 Secondary Standard (BSS2) was developed by the German National Institute of Metrology, Physikalisch-Technische Bundesanstalt (PTB), for calibration of personal dosimeters and survey instruments for beta radiation. Since 2004, the Dosimeter Calibration Laboratory of the Center for Development of Nuclear Technology (LCD/CDTN) has conducted and published studies on beta radiation metrology using the BSS2 irradiation facility. The use of 90Sr/90Y, 85Kr and 147Pm beta radiations sources that were calibrated by PTB/Germany and by following the requirements of ISO6980 standards make the BBS2 a reliable and traceable system. Absorbed dose rates and personal or directional equivalent doses in standard conditions can be calculated based on calibration certificates. In non-standard conditions, those quantities may be determined by extrapolation chambers. To correlate the absorbed dose rates at different tissue depths, it is necessary to determine the transmission factor, which, experimentally, is given by T = T’. ka, where T’ = I/ I0, and I is the ionization current for different absorber thicknesses, and I0 is the current for the 0.07 mm absorber thickness. And ka = [(a – a1) / a]2 where a is the source-detector distance and a1 is the thickness of the absorber. Since the transmission factor is hard to be experimentally determined, Monte Carlo simulations are needed. In this case, T = [(a – a1) / a]2. [Em(d) / Em(0)], where Em(d) is the average energy deposited in a sensitive volume surrounded by an absorber with thickness d, and Em(d) is the average energy deposited in the same volume and absorber with 0.07 mm thickness. The objective of this work was to reproduce and validate the BSS2 geometry for the 85Kr beta radiation source of the BSS2 in the LCD/CDTN with the Particle and Heavy Ion Transport code System (PHITS) Monte Carlo particle transport simulation code. The geometry of the 85Kr source was reproduced according to published papers and other BSS2 features were taken from the manual and specifications provided by the manufacturer. The results of the simulations showed a maximum error of 9 % when the transmission factor was compared to the experimental published values and a maximum error of 5 % when compared to those values provided in the PTB calibration certificate. Although the differences between the simulated values and the PTB reference values were lesser than or equal to 5%, an unexpected behavior was observed in the simulations. The transmission factor apparently reduced in simulations with thicknesses less than 0.04 mm. Future work will focus on studying the causes of this behavior, including an increase in the number of stories simulated and the inclusion of photons emitted at the 85Kr source. Keywords: phits, simulations, beta radiation, transmission factors |