Application of an ultraminiature thermal neutron monitor for irradiation field study of accelerator-based neutron capture therapy

摘要

Figure 1 is a schematic illustration of the SOF detector, the components of which included a probe with a small amount of plastic scintillator, a plastic optical fiber, a photo-multiplier tube, a charge pre-amplifier, a discriminator and a counter. The plastic scintillator (Bicron BC490, partially polymerized plastic scintillator), which was mixed with a small amount of LiF powder (enriched 95% 6Li), was attached to the tip of the plastic optical fiber (Mitsubishi Rayon MH4002, 1 mm-diameter optical fiber with 2.2 mm-diameter polyethylene shielding). The signal recorded by the SOF detector came from the reaction between the 6Li nuclei and the thermal neutrons that emitted charged particles (alpha and triton), which in turn produced scintillation photons in the plastic scintillator. The photon signals were transmitted through an optical fiber to the Photon Counting Head (Hamamatsu H7155), which was made up of the photo-multiplier tube, charge pre-amplifier and discriminator. These signals were converted into 30 ns-width TTL pulses, which were sent to a personal computer via a USB (Universal Serial Bus) connection for data processing. The SOF detector system was composed of two identical SOF detectors, one with 6LiF and the other without 7LiF (for gamma-ray and fast-neutron compensation). In this study, neutrons were generated using a Shenkel type single-end accelerator at the Hiroshima University Research Accelerator (HIRRAC), where the maximum accelerator voltage is 3 MV and the maximum beam current of H+ is 1 mA. A lithium metal target with a diameter of 25 mm and a thickness of 300 μm was used for neutron production via the 7Li(p,n) reaction. While the maximum attainable beam current of HIRRAC is 1 mA, the stable beam current achieved on the day of the experiment was only 50 μA; the HIRRAC beam current is highly dependent on the accelerator condition. With a pure lithium metal target at an accelerator voltage of 2.5 MV, the neutron production rate was ～8.83 × 1011 n/s/mA, which corresponds to a neutron production rate at the lithium target of ～4.42 × 1011 n/s for 50 μA [10]. One of the indicators of whether a neutron irradiation field will be suitable for BNCT is the thermal neutron flux distribution it generates in a water phantom. Currently, BNCT clinical studies are mainly performed for brain tumors; therefore, the evaluation of the accelerator-produced neutron beam was carried out using a cylindrical water phantom having dimensions similar to a human head under BNCT irradiation. An 18 cmφ (diameter) × 20 cm (length) cylindrical water phantom constructed from a 3 mm-thick polymethyl methacrylate (PMMA) material was used in the experiment. Figure 2 illustrates the setup for measuring the thermal neutron flux distribution in the water phantom with the SOF detector system. The water phantom was placed behind a 20 cmφ (diameter) × 20 cm (length) D₂O moderator enclosed in 1-mm-thick stainless steel. The detector probe was set at desired positions in the water phantom using a positioning device controlled by a stepper motor. Thermal neutron flux measurements using the SOF detector were performed at 2.5-mm increments from the inside surface of the phantom up to 20 mm from the outside surface of the phantom, then at 5-mm steps from 20 mm to 50 mm, and at 10-mm increments from 50 mm to 150 mm. Each measurement was performed until the accumulated proton charge was 5 × 10?3 coulomb (i.e. taking ～100 s). The scanned direction was along the beam axis, which was defined as the depth direction in this paper. In order to investigate whether the SOF measured data agreed with calculated data, MCNP simulations were carried out using a calculation geometry based on the experimental set-up shown in Fig. 2. The walls around the irradiation room were included in the calculation geometry because the irradiation room was small enough such that scattered neutrons from the walls were expected to have an appreciable contribution to the SOF measured signals. Neutron production via the 7Li(p,n)7Be reaction was calculated using the Fortran program LIYIELD.FOR developed by Lee et al. [10]. Here, a proton beam having a Gaussian energy distribution (σ = 0.1 MeV) and an average energy of 2.5 MeV was assumed to be incident on a pure lithium metal target with a diameter of 2.5 mm and a thickness of 300 mm. The thickness of the lithium target was chosen such that it would be sufficient to completely stop an incident 2.5-MeV proton.