TENIS — ThErmal Neutron Imaging System for use in BNCT

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Highlights

  • Thermal neutron flux imaging based on the detection of 2.22 MeV prompt gamma-rays.

  • Image reconstructed from 17 scintillator responses using appropriate algorithm.

  • Optimizing collimator holes and using B4C as neutron absorber led to desired image.

  • Cross-talk between detectors is not significant due to the energy range definition.

  • Response uniformity achieved in a polished scintillator with extra isolated length.

Abstract

A thermal neutron flux measurement tool with two perpendicular sets of plastic scintillator arrays was designed and simulated (Ghal-Eh and Green, 2016) with the MCNPX code (Version 2.6.0, with ENDF/B-VII cross section library (ENDF, 2011)). The proposed system aimed to provide a thermal neutron map based on the detection of 2.22 MeV gamma-rays resulting from 1H(nth, γ)2D reactions. In the present work, using Monte Carlo code FLUKA and its scintillation light transport capability, several important upgrades were carried out to include the light transport modeling in the response of plastic scintillators, analyze the cross-talk phenomenon, optimize the system geometry, and also provide a new approach in thermal neutron image reconstruction. The results showed that the last two cases played a significant role in improving the longitudinal profile of thermal neutron flux.

Introduction

Radiation measurement instruments are essential equipment in any medical radiotherapy scenario that is used to ensure the effectiveness of treatment and also the safety of both patients and staff. In this research, a reliable, real-time, and at the same time, cost-effective tool for measuring thermal neutron flux has been studied, which can be especially used in boron neutron capture therapy (BNCT). BNCT is a type of radiotherapy that has a unique property of tumor-cell-selective heavy-particle irradiation which justifies its clinical use in the treatment of malignant tumors. It can form large dose gradients between cancer- and normal cells, even if the two types of cells are mingled at the tumor margin. This treatment is undertaken through the selective accumulation of 10B-including agents in tumor cells and then irradiation with thermal or epithermal neutrons. The large cross-section of this isotope for thermal neutrons and the short range of by-product heavy charged particles allow high doses to be delivered in the tumor region by their high linear energy transfer (Suzuki, 2020, Gambarini et al., 2014).

Information on how thermal neutron flux distributes in the body exposed to neutron radiation and where the peak occurs is one of the key factors in ensuring the quality of BNCT treatment. This measurement can be performed by a thermal neutron flux imaging tool, named TENIS, which was first introduced in the work of Ghal-Eh and Green in 2016 (Ghal-Eh and Green, 2016). The present study is a step towards completion and optimizing the previous design to approach a practical prototype construction. In this section, the features of the proposed system are briefly summarized. Then, in the next section, the required changes undertaken to optimize and improve the image quality are described, and finally, the effect of scintillation light transport modeling on system performance is investigated.

Although the preliminary simulation study of the proposed detection system with the MCNPX (Version 2.6.0) Monte Carlo code (Hendricks et al., 2008) represented promising results, it was decided to perform a set of simulations with the FLUKA code (Ferrari et al., 2005) to take advantage of the scintillation light transport feature which is not embedded in the MCNPX code (Yazdandoust et al., 2019). It should be noted that since the number of required events registered in the detectors were small, to obtain desired accuracies and also avoid extremely long computer runs, the mesh weight-window and importance biasing (with USIMB.f user-routine) variance-reduction techniques were used in MCNPX and FLUKA simulations, respectively.

TENIS is a real-time measurement equipment based on commercial plastic scintillators that can be used to map the thermal neutron flux within a hydrogen-containing volume as described in the following.

The basic performance of TENIS is very similar to orthogonal position-sensitive detectors used in cosmic-ray studies. It consists of seventeen parallelepiped NE102 plastic scintillators (seven 20-cm long horizontal and ten 14-cm long vertical scintillators), each with a cross-sectional area of 2 × 2 cm2, which are located surrounding a small rectangular water phantom used in the BNCT studies at the University of Birmingham, U. K. This set of orthogonal scintillators divides the phantom into seventy equal water voxels (i.e., 2 × 2 × 15 cm3), such that their xz projection forms a seventy-pixel mesh, each with 2 × 2 cm2 area (Fig. 1).

The thermal neutron mapping in TENIS is based on the detection of prompt gamma-rays resulting from 1H(nth, γ)2D reactions. Neutrons undergo (nth, γ) reactions with hydrogen nuclei as soon as they are thermalized inside the water phantom, producing 2.22 MeV gamma-rays. As a result, the number of gamma-rays produced in a specific voxel inside the water phantom can be considered as a measure of the thermal neutron flux in that region. The gamma-rays that reach the scintillators along a straight path will retain the information on the location of thermal neutron interaction. To this purpose, an appropriate parallel-hole lead collimator assembly was designed on both sides, between the water phantom and the plastic scintillators, to allow straight passages of gamma-rays and eliminate the rest. Since the 2.22 MeV gamma-rays are generated isotopically in the water phantom, they have no preferred direction, and also both the geometric and detection efficiencies are identical for all photons passing through the collimators. Therefore, the number of gamma-rays that reach the specific scintillator can be considered as a measure of thermal neutron flux in the voxels just in front of that scintillator.

The collimator assembly consists of two large 10-cm thick lead blocks, in which there are four 6-mm diameter air pipes in each 2 × 2 cm2 of their cross-sectional area. Moreover, a thin cadmium layer is placed between the phantom and the collimator on both sides to prevent scattered neutrons from entering the scintillators (Fig. 2). Finally, the thermal neutron flux inside the water voxels is estimated by multiplying the responses of horizontal detectors by vertical ones in the energy range of 2.0–2.3 MeV. The result of this estimation can be plotted as a two-dimensional xz map.

Section snippets

Materials and methods

The evaluation of thermal neutron flux by TENIS showed that it can appropriately reconstruct a two-dimensional image of thermal neutron flux inside the water phantom and also it predicts that the thermal neutron flux peaks at the second row of the phantom voxels, 2 cm far from the 1 keV neutron beam entrance. However, as far as the comparison with real (i.e., direct MCNPX-simulated) thermal neutron flux shows, the reconstructed image is more diffuse toward the +z direction (Ghal-Eh and Green,

Results and discussion

As discussed earlier, TENIS aims to visualize the thermal neutron flux inside a water phantom using plastic scintillators for use in the pre-treatment stage of the BNCT. This is implemented by processing the responses of orthogonal, rectangular scintillators embedded on both sides of the phantom to detect 2.22 MeV gamma-rays.

Fig. 10 shows a two-dimensional map of thermal neutron flux inside seventy voxels of a rectangular water phantom which is exposed to a broad beam of 1 keV neutrons along

Conclusions

Providing adequate thermal neutron flux at the tumor site is one of the factors that guarantee efficient treatment in the BNCT. Therefore, it is necessary to know how the thermal neutron flux is distributed in the region being irradiated by an epithermal neutron beam. Besides, the introduction of a simple and cost-effective tool for a precise estimation of thermal neutron flux is a step forward in the pre-treatment quality assurance procedures in the BNCT. In this study, the idea of a real-time

CRediT authorship contribution statement

H. Yazdandoust: Software, Writing, Investigation. N. Ghal-Eh: Co-supervision, Writing, Validation. M.M. Firoozabadi: Co-supervision, Writing, Validation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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