Simulation of the Charge Produced by Protons inside a Tissue Equivalent Ionization Chamber in a Mixed Neutron/Gamma Field in BNCT

The neutron and gamma dose in Boron Neutron Capture Therapy (BNCT) can be determined by using ionization chambers of different materials. However, inexplicable results, such as negative doses, are sometimes calculated. Computer simulations using MCNPX can help to understand the behavior of ionization chambers. This article deals with a part of this investigation: the contribution of protons to the total measured charge in a Tissue Equivalent (TE) ionization chamber that is flushed with methane-based TE gas. The inherent problem is that the Monte Carlo code MCNPX cannot track protons below 1 MeV. Methods and Materials: A custom-made program, called PPICC, calculates the deposited energy and thus the charge in the TE gas per proton. For this, it uses the stopping powers for protons in TE plastic and gas. MCNPX provides the total number of protons produced by all neutron interactions near the gas. To check this new procedure, measurements and simulations have been performed using a validated mixed beam of neutrons and gammas. The neutron fluence consists of 12% fast and 87% epithermal neutrons. In one set-up the chamber is free-in-air (epithermal/fast neutron field) and in the other in a cubic PMMA phantom at 25 mm depth (thermal/epithermal neutron field). Results: The total charge is the sum of the charges due to electrons, originating from primary and neutron induced gammas, and protons from 1H(n,n)1H and 14N(n,p)14C reactions. The total measured and calculated charges in the two set-ups have acceptable uncertainties and are in good agreement. Conclusions: The charge collected in a TE ionization chamber can be simulated in a mixed field of neutrons and gammas. The charge resulting from proton recoil in the gas is unexpectedly large.

ROCA Antoaneta;  LIU Yuan-Hao;  MOSS Raymond;  STECHER-RASMUSSEN Finn;  NIEVAART Victor Alexander; 

2009-12-17

AMER NUCLEAR SOCIETY

JRC48344

0029-5450,   

http://www.new.ans.org/pubs/journals/nt/v_168:1,    https://publications.jrc.ec.europa.eu/repository/handle/JRC48344,