Neutron Source For BNCT Research Articles

Study of the out-of-field dose from an accelerator-based neutron source for boron neutron capture therapy

One important issue in Boron Neutron Capture Therapy is the delivered dose to the tissues outside the tumor. An international standard for light ion beam systems sets two recommended limits for out-of-field dose based on distance from the field edge: maximum absorbed dose from all radiation types shall not exceed 0.5 % of the maximum dose at distances 15 cm to 50 cm from the field edge. At distances >50 cm from the field edge, the maximum absorbed dose shall not exceed 0.1 %. This paper is a continuation of our previous works focused on the design of an accelerator-based neutron source for BNCT. We already designed a novel Beam Shape Assembly which meets the IAEA criteria for BNCT treatments. Using this BSA, in the present work, we characterize by Monte Carlo simulations the dose outside the neutron field. The out-of-field dose has been assessed via estimates using the ambient and equivalent dose. Also the boron uptake in healthy tissues has been analyzed for the equivalent dose computation. It is concluded that our design for a future accelerator-based source for BNCT meets reasonably well the criteria defined from other forms of radiotherapy on both equivalent and effective dose outside the field.

Verification for dose estimation performance of a Monte-Carlo based treatment planning system in University of Tsukuba

The University of Tsukuba is developing not only a linac-based neutron source for BNCT (iBNCT) but also a multi-modal treatment planning system (Tsukuba-Plan) for BNCT. We are currently performing several verifications. Phantom experiments performed in iBNCT were simulated by the Tsukuba-Plan, and the calculation results were compared with the measurements from the experiments. The calculations were in good agreement with the measurements. The results demonstrated that the Tsukuba-Plan can perform to estimate doses properly for BNCT treatment at iBNCT.

Techniques to Measure Absolute Neutron Spectrum and Intensity for Accelerator Based Neutron Source for BNCT

Techniques to Measure Absolute Neutron Spectrum and Intensity for Accelerator Based Neutron Source for BNCT

Investigating a cyclotron HM-30 based neutron source for BNCT of deep-seated tumors by using shifting method

We have successfully proposed a simulation of a neutron beam-shaping assembly using MCNPX Code. This simulation study deals with designing a compact, optimized, and geometrically simple beam shaping assembly for a neutron source based on a proton cyclotron for BNCT purpose. Shifting method was applied in order to lower the fast neutron energy to the epithermal energy range by choosing appropriate materials. Based on a set of MCNPX simulations, it has been found that the best materials for beam shaping assembly are 3 cm Ni layered with 7 cm Pb as the reflector and 13 cm AlF3 the moderator. Our proposed beam shaping assembly configuration satisfies 2 of 5 of the IAEA criteria, namely the epithermal neutron flux 1.25 × 109 n.cm-2 s-1 and the gamma dose over the epithermal neutron flux is 0.18×10 -13 Gy.cm 2 n -1. However, the ratio of the fast neutron dose rate over neutron epithermal flux is still too high. We recommended that the shifting method must be accompanied by the filter method to reduce the fast neutron flux.

Neutron intensity monitor with activation foil for p-Li neutron source for BNCT – Feasibility test of the concept

Proton-lithium (p-Li) reaction is being examined worldwide as a candidate nuclear production reaction for accelerator based neutron source (ABNS) for BNCT. In this reaction, the emitted neutron energy is not so high, below 1MeV, and especially in backward angles the energy is as low as about 100keV. The intensity measurement was thus known to be difficult so far. In the present study, a simple method was investigated to monitor the absolute neutron intensity of the p-Li neutron source by employing the foil activation method based on isomer production reactions in order to cover around several hundreds keV. As a result of numerical examination, it was found that 107Ag, 115In and 189Os would be feasible. Their features found out are summarized as follows: 107Ag: The most convenient foil, since the half life is short. 115In: The accuracy is the best at 0°, though it cannot be used for backward angles. And 189Os: Suitable nuclide which can be used in backward angles, though the gamma-ray energy is a little too low. These would be used for p-Li source monitoring depending on measuring purposes in real BNCT scenes.

Near threshold 7Li(p,n) 7Be reaction as neutron source for BNCT

7Li(p,n)7Be is an endothermic reaction and working near its threshold (1.88MeV) has the advantage of neutron spectra with maximum energies of about 100keV, considerably lower than at higher beam energies, or than using other neutron-producing reactions or as for the uranium fission spectrum, relevant for BNCT based on nuclear reactors. With this primary energy it is much easier to obtain the energies needed for treating deep seated tumors by BNCT (about 10keV). This work studies bombarding energies up to 2.05MeV, different beam incidence angles and the effect of the undesirable gamma production via the 7Li(p,γp’) 7Li reaction.

Liquid Li based neutron source for BNCT and science application

Liquid lithium (Li) is a candidate material for a target of intense neutron source, heat transfer medium in space engines and charges stripper. For a medical application of BNCT, epithermal neutrons with least energetic neutrons and γ-ray are required so as to avoid unnecessary doses to a patient. This is enabled by lithium target irradiated by protons at 2.5MeV range, with utilizing the threshold reaction of 7Li(p,n)7Be at 1.88MeV. In the system, protons at 2.5MeV penetrate into Li layer by 0.25mm with dissipating heat load near the surface. To handle it, thin film flow of high velocity is important for stable operation. For the proton accelerator, electrostatic type of the Schnkel or the tandem is planned to be employed. Neutrons generated at 0.6MeV are gently moderated to epithermal energy while suppressing accompanying γ-ray minimum by the dedicated moderator assembly.

Near-threshold 7Li(p,n)7Be neutrons on the practical conditions using thick Li-target and Gaussian proton energies for BNCT

The near threshold 7Li(p,n)7Be neutrons generated by incident proton energy having Gaussian distribution with mean energies from 1.85 to 1.95MeV, were studied as a practical neutron source for BNCT wherein an RFQ accelerator and a thick Li-target are used. Gaussian energy distributions with the standard deviation of 0, 10, 20 and 40keV for mean proton energies from 1.85 to 1.95MeV were surveyed in 0.01MeV increments. A thick liquid Li-target whose dimensions were established in our previous experiments (i.e., 1mm-thick with 50mm width and 50mm length) was considered in this study. The suitable incident proton energy and physical dimensions of Pb layer which serves as a gamma absorber and a Polyethylene layer which is used as a BDE were surveyed by means of the concepts of TPD. Dose distribution were calculated by using MCNP5. A proton beam with mean energy of 1.92MeV and a Gaussian energy distribution with a standard deviation of 20keV at a current of 10mA was selected from the viewpoint of irradiation time and practically achievable proton current. The suitable thicknesses of Pb gamma absorber was estimated to be about 3cm. The estimated thickness of the polyethylene BDE was about 24mm for an ideal proton current of 13mA, and was 18mm for a practical proton current of 10mA.

Effects of Proton Energy on Optimal Moderator System and Neutron-induced Radioactivity of Compact Accelerator-driven 9Be(p,n) Neutron Sources for BNCT

Accelerator-based BNCT systems using 9Be(p, n) neutron sources have been constructed and under construction. This reaction become preferable over about 10 MeV since the neutron production becomes more efficient, but the radioactive products increase with the proton energy. In case of construction near a hospital we should consider not only the neutron production rate but also the radioactivity. Therefore, it is necessary to obtain information on the required beam power of an accelerator and the activity as a function of the proton energy to decide the optimal proton energy. We performed simulation calculations and first decided optimal thickness of MgF2 moderator and Fe filter to fulfill the neutronic conditions of BNCT. Then, we deduced accelerator power and found that the required accelerator power is approximately 30kW around 10 MeV proton energy and decreases gradually with the proton energy. On the other hand, at energies less than 11 MeV, the photon dose rate around the target became lower for level for working around the target after seven days cooling time. Furthermore, the dose rate at the opening of a collimator reduced drastically after one hour cooling time at energy less than 10 MeV and got to a dose level at which 3hours work/day can be done within the limits of regulation. We have therefore concluded that the optimal proton energy for BNCT is around 10 MeV from the results obtained here.

9Be(d,n)10B-based neutron sources for BNCT

In the frame of accelerator-based BNCT, the 9Be(d,n)10B reaction was investigated as a possible source of epithermal neutrons. In order to determine the configuration in terms of bombarding energy, target thickness and Beam Shaping Assembly (BSA) design that results in the best possible beam quality, a systematic optimization study was carried out. From this study, the optimal configuration resulted in tumor doses ≥40Gy-Eq, with a maximum value of 51Gy-Eq at a depth of about 2.7cm, in a 60min treatment.The optimal configuration was considered for the treatment planning assessment of a real Glioblastoma Multiforme case. From this, the resulted dose performances were comparable to those obtained with an optimized 7Li(p,n)-based neutron source, under identical conditions and subjected to the same clinical protocol.

Beam shaping assembly of a D–T neutron source for BNCT and its dosimetry simulation in deeply-seated tumor

This article involves two aims for BNCT. First case includes a beam shaping assembly estimation for a D–T neutron source to find epi-thermal neutrons which are the goal in the BNCT. Second issue is the percent depth dose calculation in the adult Snyder head phantom. Monte-Carlo simulations and verification of a suggested beam shaping assembly (including internal neutron multiplier, moderator, filter, external neutron multiplier, collimator, and reflector dimensions) for thermalizing a D–T neutron source as well as increasing neutron flux are carried out and our results are given herein. Finally, we have simulated its corresponding doses for treatment planning of a deeply-seated tumor.

小型加速器に基づくBNCT用のBe-9(p, n)中性子源の性能に及ぼす陽子エネルギーの影響

小型加速器に基づくBNCT用のBe-9(p, n)中性子源の性能に及ぼす陽子エネルギーの影響

Design of a model for BSA to meet free beam parameters for BNCT based on multiplier system for D–T neutron source

Extensive research has recently been carried out for the development of high-energy D–T neutron generators as neutron sources for BNCT. The energy of these high-energy neutrons must be reduced by designing a Beam Shaping Assembly (BSA) to make them usable for BNCT. However, the neutron flux decreases drastically as neutrons pass through different materials of BSA. Therefore, it is very important to find ways to treat the neutrons economically. In this paper the possibility of using natural uranium as a neutron multiplier is investigated in order to increase the number of neutrons emitted from D–T neutron generator. According to the simulations and performed calculations, a sphere containing natural uranium as neutron multiplier was used to increase the number of neutrons generated by the D–T neutron generator. The energy of fast neutrons that are generated by D–T fusion reaction and amplified by neutron multiplier system is decreased using proper materials as moderators and fast neutron filters in BSA. The gamma rays which are generated as a result of neutron interaction with moderators are removed from neutron spectrum using bismuth as the gamma filter. Also, a thermal neutron absorber omits undesired low-energy neutrons which lead to a high radiation dose for the skin and soft tissues. The results show that passing neutrons through such a BSA causes the establishment of free beam parameters yet the reduction of the output beam intensity is unavoidable. The neutron spectrum related to our BSA has a proper epithermal flux and the fast and thermal neutron fluxes are compatible with the IAEA recommended values.

Thermal neutron field with D-T neutron source for BNCT

Thermal neutron field with D-T neutron source for BNCT

Fission reactor based epithermal neutron irradiation facilities for routine clinical application in BNCT—Hatanaka memorial lecture

Based on experience gained in the recent clinical studies at MIT/Harvard, the desirable characteristics of epithermal neutron irradiation facilities for eventual routine clinical BNCT are suggested. A discussion of two approaches to using fission reactors for epithermal neutron BNCT is provided. This is followed by specific suggestions for the performance and features needed for high throughput clinical BNCT. An example of a current state-of-the-art, reactor based facility, suited for routine clinical use is discussed. Some comments are provided on the current status of reactor versus accelerator based epithermal neutron sources for BNCT. This paper concludes with a summary and a few personal observations on BNCT by the author.

Be target development for the accelerator-based SPES-BNCT facility at INFN Legnaro

An accelerator-driven thermal neutron source for BNCT, planned to be installed at the INFN Laboratori Nazionali di Legnaro (LNL), is in progress in the framework of the SPES (selective production of exotic species) research program. The most critical element of such a facility is the construction of a reliable neutron converter based on the 9Be(p,xn) nuclear reaction, working at a high power level (150 kW) and 5 MeV beam energy, due to the SPES driver constraints. Two original, beryllium-based, target concepts have been designed for such a purpose. The present status of the neutron converter, as well as the test results performed so far on prototypes constructed, is reported here.

A neutron producing target for BINP accelerator-based neutron source

An innovative accelerator-based neutron source for BNCT has just started operation at the Budker Institute of Nuclear Physics, Novosibirsk. One of the main elements of the facility is a lithium target producing neutrons via the threshold 7Li( p, n) 7Be reaction at 25 kW proton beam with energies of 1.915 MeV or 2.5 MeV. The design of an optimal target and results of the investigation of radiation blistering of the lithium layer were presented at previous NCT Congresses. During the last two years the neutron target has been manufactured, assembled and placed in the facility. Optimization of the target is carried out with the Monte Carlo simulation code MCNP. In this article, the design of the target is discussed, results of all previous investigations are summarized, results of target testing and neutron generation are described, and results of simulation of neutron spectra are presented.

A comparison between a TESQ accelerator and a reactor as a neutron sources for BNCT

In this work, the performance of an accelerator-based neutron source design has been compared with that of a modern fluoride-filtered reactor-based epithermal beam having near-optimal quality for treatment of deep seated tumors in relation to its applicability for BNCT. The accelerator is a Tandem-Electrostatic-Quadrupole (TESQ) accelerator which is a design under development at the National Atomic Energy Commission (CNEA) in Buenos Aires, Argentina based on the 7Li(p,n)7Be reaction, relatively close to its energy threshold. The reactor is the Massachusetts Institute of Technology reactor upgraded with a Fission Converter Beam (MIT-FCB) and improved with an 8mm thick 6Li filter. The comparison has been done by means of data reported on the MIT-FCB+6Li filter performance and MCNP simulations on our TESQ design considering the neutron fluxes provided by the two sources and the doses delivered in a human phantom by both devices. The results show a deeper advantage depth (AD) for the TESQ which turns out to be a promising alternative to a reactor-based BNCT treatment.

Benchmark experiments for cyclotron-based neutron source for BNCT

In the previous study, we found the feasibility of a cyclotron-based BNCT using the Ta(p,n) neutrons at 90° bombarded by 50 MeV protons, and the iron, AlF 3, Al and 6LiF moderators by simulations using the MCNPX code. In order to validate the simulations to realize the cyclotron-based BNCT, we measured the epithermal neutron energy spectrum passing through the moderators with our new spectrometer consisting of a 3He gas counter covered with a silicon rubber loaded with natB and polyethylene moderator and the depth distribution of the reaction rates of 197Au(n, γ) 198Au in an acrylic phantom set behind the rear surface of the moderators. The measured results were compared with the calculations using the MCNPX code. We obtained the good agreement between the calculations and measurements within ∼10% for the neutron energy spectra and within ∼20% for the depth distribution of the reaction rates of 197Au(n, γ) 198Au in the phantom. The comparison clarified a good accuracy of the calculation of the neutron energy spectrum passing through the moderator and the thermalization in a phantom. These experimental results will be a good benchmark data to evaluate the accuracy of the calculation code.

Feasibility of an epithermal neutron source for BNCT based on RFQ accelerator

The use of boron neutron capture therapy for the treatment of deep-seated tumours, such as glioblastoma multiforme, requires neutron beams of suitable energy and intensity. The analysis of the therapeutic gain shows that a high tumour control probability with sublethal dose at healthy tissues can be achieved, in most cases, by using neutron beams of a few keV energy, with a flux of about 10 9 neutrons/cm 2 s. Therapeutic neutron beams with high-spectral purity in this energy range could be produced by accelerator-based neutron sources through a suitable neutron-producing reaction. We investigate the feasibility of a solution based on a small radio frequency quadrupole for a proton beam current of 30 mA and an energy of 2 MeV. An appropriate choice of the function parameters of the RFQ (modulation, efficiency of acceleration, phase shift, etc., …) allows one to design relatively compact accelerators, which could eventually lead to setup hospital-based BNCT facilities.