Takao Aoki

Feasibility study on epithermal neutron field for cyclotron-based boron neutron capture therapy.

To realize the accelerator-based boron neutron capture therapy (BNCT) at the Cyclotron and Radioisotope Center of Tohoku University, the feasibility of a cyclotron-based BNCT was evaluated. This study focuses on optimizing the epithermal neutron field with an energy spectrum and intensity suitable for BNCT for various combinations of neutron-producing reactions and moderator materials. Neutrons emitted at 90 degrees from a thick (stopping-length) Ta target, bombarded by 50 MeV protons of 300 microA beam current, were selected as a neutron source, based on the measurement of angular distributions and neutron energy spectra. As assembly composed of iron, AlF3/Al/6LiF, and lead was chosen as moderators, based on the simulation trials using the MCNPX code. The depth dose distributions in a cylindrical phantom, calculated with the MCNPX code, showed that, within 1 h of therapeutic time, the best moderator assembly, which is 30-cm-thick iron, 39-cm-thick AlF3/Al/6LiF, and 1-cm-thick lead, provides an epithermal neutron flux of 0.7 x 10(9) [n cm(-2) s(-1)]. This results in a tumor dose of 20.9 Gy-eq at a depth of 8 cm in the phantom, which is 6.4 Gy-eq higher than that of the Brookhaven Medical Research Reactor at the equivalent condition of maximum normal tissue tolerance. The beam power of the cyclotron is 15 kW, which is much lower than other accelerator-based BNCT proposals.

Experimental studies on the neutron emission spectrum and induced radioactivity of the 7Li(d,n) reaction in the 20–40 MeV region

To improve the data accuracy of the neutron emission spectrum of the 7 Li(d,n) reaction and the radioactivity ( 7 Be, 3 H, etc.) accumulated in the 7 Li target in IFMIF, we have measured the neutron emission spectrum and the radioactivity of Be induced in the lithium target for 25 MeV deuterons at the Tohoku University AVF cyclotron (K = 110) facility. Neutron spectra were measured with the time-of-flight (TOF) method at four laboratory angles by using a beam swinger system and a well collimated TOF channel. Induced radioactivity was measured by detecting the gamma-rays from Be with a pure Ge detector. Experimental results are compared with other experimental data. The present result of neutron emission spectra are in qualitative agreement with other experimental data but that of 7 Be production was much larger than expected by the recent codes. Measurement will be extended to several incident energies up to 40 MeV.

Measurements of Differential Thick Target Neutron Yields and 7Be Production in the Li, 9Be (d, n) Reactions for 25 MeV Deuterons

Neutron emission spectra and production rates of radioactive nuclide 7Be in the (d, n) reactions on thick lithium and 9Be targets have been measured for 25 MeV deuterons at the Tohoku University AVF cyclotron (K=110) facility to provide basic data for the design of the intense neutron source, International Fusion Reactor Material Irradiation Test Facility (IFMIF). Neutron spectra were measured with the time-of-flight method at ten laboratory angles between 0° and 90° by using a beam swinger system. Data were obtained down to ~1 MeV from the highest energy, using a two-gain NE213 detector system. Induced radioactivity of 7Be was measured by detecting the 478 keV γ-rays from 7Be with a pure Ge detector. The experimental results of neutron spectra revealed clearly the entire shape of the neutron emission spectrum and the angular dependence including so called “high energy tail” extending up to ~40 MeV. The present data at 0° are in reasonable agreement with corresponding value by Lone et al. at 23 MeV above ~5 MeV, but show large discrepancy in the lower energy region. The results of 7Be production for both nuclei were much larger than expected by the calculation with a recent code IRAC.

Measurements of Fast Neutron-induced Prompt Neutron Fission Spectra of 233U, 238U and 232Th

Prompt fission neutron spectra of 233U, 238U and 232Th for mono-energetic fast neutrons were measured by using the time-of-flight (TOF) method at Tohoku University 4.5 MV Dynamitron accelerator facility. Data were obtained for 233U at En=0.55, 1.9, 4.1 MeV, 238U at En=1.9, 4.1 MeV, and 232Th at En=4.1 MeV using solid samples. The data of 233U and 232Th were obtained in absolute scale (mb sr–1 MeV–1) by normalizing the data to the 238U spectrum as a standard.

Experiments on Neutron Scattering and Fission Neutron Spectra

Experiments on the 1) neutron elastic scattering for 55, 65 and 75 MeV neutrons, and 2) prompt fission neutron spectrum for MeV neutrons are described. The neutron elastic scattering cross sections have been measured for C, Si, Fe, Zr, and Pb at the 7Li(p, n) quasi-monoenergetic neutron source facility at TIARA, JAERI using the TOF method. Data were obtained at twenty-five laboratory angles between 2.6° and 53° that could clarify the angular distributions and angle-integrated values. Fission spectrum data were obtained for 233U, 238U and 232Th at 0.6, 1.9 and 4.1 MeV. The data at 1.9 and 4.1 MeV were given in absolute scale by taking the 238U data as the standard to clarify the problem of data files.

Double Differential Hydrogen and Helium Production Cross Section of Oxygen and Nitrogen for 75 MeV Neutrons

Double differential (n,xp), (n,xd), (n,xt) and (n,xα) cross sections of oxygen and nitrogen were measured for 75 MeV incident neutrons at 25°, 65° and 125° angles using a specially designed spectrometer. The spectrometer has three counter telescope consisting of a low pressure gas proportional counter, a thin SSD and a BaF2 scintillator. The energy dependence of the BaF2 was calibrated with direct proton, deuteron and helium beams. The results were compared with the LA-150 data library. The library represents the experimental results of proton, triton and α-particle generally well, but shows the marked difference in the high energy part of deuteron one.