Mixed-field dosimetry measurement of a target assembly for an accelerator-based neutron source for boron neutron capture therapy

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Abstract

The objective of this work was to measure the neutron and gamma-ray absorbed dose components for a target assembly for an Accelerator-Based Neutron Source (ABNS) for Boron Neutron Capture Therapy (BNCT), and to compare these measurements with MCNP calculations in order to verify the calculations of the in-air neutron and gamma-ray absorbed dose components. The measurements were made using the paired ionization chamber technique. The measured neutron and gamma-ray specific absorbed dose components agreed with calculations within experimental errors, which were approximately 10%. The measured gamma-ray specific absorbed dose rate of 140 cGy s−1 A−1 ±12% is consistent with reported yields of 478 keV gamma rays due to the 7Li(p, p′)7Li* reaction. This specific gamma absorbed dose rate is significant and should not be neglected in moderator assembly design.

Introduction

As a part of on-going efforts at The Ohio State University (OSU) to test an Accelerator-Based Neutron Source (ABNS) for Boron Neutron Capture Therapy (BNCT), the importance of verifying the gamma-ray production of a thick lithium-7 target bombarded by 2.5 MeV protons has become apparent. Until now, the focus of ABNS research at OSU and elsewhere has been in determining in-air and in-phantom neutron doses from the epithermal neutron beams from ABNS sources 1, 2. Gamma-ray absorbed dose analyses have been performed, but they typically have included only the contributions to the gamma ray absorbed dose from (n, γ) reactions occurring in the moderator assembly and in phantoms.

Gamma-rays are produced in the Li target by the interaction of the proton beam with the target. The 7Li(p, p′)7Li* and 7Li(p, n)7Be* reactions produce 7Li* and 7Be* which de-excite by emitting 478 and 430 keV photons, respectively. Also, 7Be is radioactive and emits a 478 keV gamma with a 53 d half-life. In addition to these primary production mechanisms, a low probability reaction, proton capture, emits 16 and 19 MeV photons [3].

The OSU design of an ABNS for BNCT includes a moderator assembly between the target assembly and the patient. The gamma-rays produced in the target are generally of relatively low energy compared to those produced by the n, γ reactions in the moderator assembly and in the patient. These low energy gamma-rays can be significantly attenuated by the moderator assembly depending on the design on the moderator assembly. Nevertheless, the production rate of these low energy gammas in the target must be well known in order to perform accurate calculations of the gamma-ray absorbed dose in the phantom.

This paper describes measurements of the photon and neutron absorbed doses produced by bombarding a thick lithium target with 2.5 MeV protons. The measurements were made in-air using the paired ion chamber (IC) technique, with a tissue equivalent (TE) walled IC flushed with tissue equivalent gas and a magnesium walled IC flushed with argon gas. This paper describes these measurements and then compares the measured gamma-ray and neutron absorbed doses per Coulomb of charge on target to Monte Carlo N-Particle (MCNP) Transport Code calculations [4]of the same quantity. This comparison serves to verify the accuracy of the source term and geometry used in the MCNP target model at OSU.

Section snippets

Experimental equipment and setup

The experimental equipment used to perform the paired ion chamber measurements consisted of one set of paired ionization chambers, gas flow system, high voltage supply, and an electrometer. The center of each detector was positioned on the beamline axis at a distance of 2.41 cm from the downstream face of the neutron beam monitoring device (see Fig. 1).

The paired ICs which were used were supplied by Far West Technology (FWT). The pair consisted of a TE-walled and a Mg-walled ion chamber. The ICs

Measurement results

The absorbed dose components were determined with , using the measured responses of the ionization chamber, and the appropriate chamber sensitivities. The responses of the ion chambers were corrected for temperature, pressure, and leakage, and were normalized to the amount of charge deposited on the target during the time interval for which the response was measured, qtarg. The neutron and gamma absorbed dose per unit charge on target, hereafter called the specific absorbed dose (and

Discussion and conclusions

The purpose of the work which is reported in this paper was to confirm by measurement neutron and gamma-ray absorbed dose rates calculated with MCNP in the mixed field immediately downstream of the target assembly for an ABNS for BNCT. The measurements were made using the paired IC technique. The neutron specific absorbed dose was measured to be 3800 cGy/C. The measured neutron specific absorbed dose is within one standard deviation of the neutron specific absorbed dose of 4120 cGy/C predicted by

Acknowledgements

The efforts of the authors were supported, in part, by the Department of Energy under Award No. DE-FG02-93ER61612. The help of the Ohio University Edwards Accelerator Lab faculty and staff is gratefully acknowledged.

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