Toshiya Sanami

Development of monoenergetic neutron calibration fields between 8 keV and 15 MeV

For characterization and calibration of neutron dosimeters and spectrometers, monoenergetic neutron calibration fields have been developed at eight energy points between 8 keV and 15 MeV (8 and 27 keV, 0.25, 0.55, 1.0, 2.0, 5.0 and 15 MeV). Monoenergetic neutrons are obtained by the Sc(p, n) reactions for 8 and 27 keV, and by the 7Li(p, n), T(p, n), D(d, n) and T(d, n) reactions between 0.25 and 15 MeV. Care was taken to reduce background neutrons by parasitic reactions and the scattering of primary neutrons at the target. The neutron spectrum was characterized by the time-of-flight technique, and the neutron fluence was measured with two independent methods within ±5%. The field has been applied successfully for calibration and characterization of various neutron dosimeters and spectometers.

Absolute number of scintillation photons emitted by alpha-particles in rare gases

In order to determine absolute scintillation yields due to alpha particles in high pressure rare gases, the number of scintillation photons N/sub p/ was measured by using a VUV sensitive photodiode (PD) with a MgF/sub 2/ window and a Cs-Te photocathode with spectral quantum efficiency q/sub e/(/spl lambda/) measured as a function of wavelength /spl lambda/. The number of photoelectrons from the photocathode N/sub pe/ was measured absolutely by using charge-sensitive preamplifier calibrated in numbers of electrons. A collection efficiency F/sub ce/ at the photocathode for scintillation photons can be determined from solid angles subtended by the photocathode at a scintillation point under the condition that there is no photon reflected at surrounding wall. Then, N/sub p/ was determined from N/sub p/=N/sub pe//(Q/sub e/F/sub ce/), where Q/sub e/ is effective quantum efficiency calculated from q/sub e/(/spl lambda/) and a relative intensity I(/spl lambda/) of scintillation in rare gases at wavelength /spl lambda/. Although luminescence spectrums from rare gases emitted by radiation have been measured by many researchers, these spectrums were scarcely corrected by an efficiency of apparatus (e.g. efficiency of monochromator and scintillation detector) for /spl lambda/. In order to exactly determine the luminescence spectrums, these were also measured on our own terms. And, since it was reported that scintillation intensity from rare gases change with a pressure of rare gases, this experiments was carried out in a pressure range from 1.0/spl times/10/sup 5/ Pa to 1.0/spl times/10/sup 6/ Pa. The measurements were carried out in gaseous argon, krypton and xenon. In xenon of 1.0/spl times/10/sup 5/ Pa, N/sub p/ was measured to be 1.6/spl times/10/sup 5/.

Simultaneous measurements of absolute numbers of electrons and scintillation photons produced by 5.49 MeV alpha particles in rare gases

The absolute numbers of scintillation photons and electrons produced by 5.49 MeV alpha particles were measured simultaneously in argon, krypton and xenon in the gas pressure range from 1.01/spl times/10/sup 5/ Pa to 1.01/spl times/10/sup 6/ Pa. The ratio of the number of excited atoms to the number of electron-ion pairs is an important quantity for understanding the energy pathway of the absorbed radiation energy and was found to be 0.52, 0.55, and 0.60 in argon, krypton and xenon, respectively. The ratios were determined by measuring the number of scintillation photons originating from the excited atoms and the number of electrons. The value of W/sub s/, which is defined as the average energy to produce one photon, in the case that all of the electron-ion pairs recombine was estimated to be 17.5, 15.4, and 13.0 eV in argon, krypton and xenon, respectively. From the relation between the numbers of electrons escaping from the recombination with ions and the numbers of scintillation photons, it is confirmed experimentally that one scintillation photon is emitted from one recombination process. This means that an excited molecule caused by three-body collisions is not de-excited without emitting a scintillation photon in the vacuum ultraviolet region.

Fast-neutron profiling with an imaging plate

Abstract A method for taking the fast-neutron profile has been developed using an imaging plate combined with a polypropylene converter. By this method, we can obtain the spatial distribution of fast neutrons without any γ-ray contamination by taking the difference between the data with and without the converter. We have studied properties of this method concerning the neutron energy, converter thickness, fading, dynamic range and spatial resolution. Under appropriate conditions, the method has more than a 102 dynamic range with linearity and less than 1.0 mm spatial resolution for 5 and 15 MeV neutrons. By using this method, we successfully obtained a fast-neutron profile after a collimator and radiography of a 5 cm thick iron block.

Experimental studies of shielding and irradiation effects at high-energy accelerator facilities

Abstract Experimental studies of shielding and radiation effects are carried out at Fermi National Accelerator Laboratory (FNAL) under collaboration between FNAL and Japan, aiming at benchmarking simulation codes and studying irradiation effects for the upgrade and design of new high-energy accelerator facilities. The purposes of this collaboration are (a) acquisition of shielding data in a proton beam energy region above 100 GeV, (b) further evaluation of predictive accuracy of the PHITS and MARS codes, (c) modification of physics models and data in these codes if needed, (d) characterization of radiation fields for studies of radiation effects, and (e) development of a code module for an improved description of radiation effects. The first campaign of the experiment was carried out at the Pbar target station and NuMI experimental station at FNAL, which use irradiation of targets with 120-GeV protons for antiproton and neutrino production, respectively. The generated secondary particles passing through steel, concrete, and rock were measured by activation methods as well as by other detectors such as a scintillator with a veto counter, phoswich detector, and a Bonner ball counter on trial. Preliminary experimental and calculated results are presented.

Detection of explosives and illicit drugs using neutrons

A procedure developed for the determination of the flux perturbation factor required for the thermal neutron activation analysis of bulky samples of unknown composition has been extended for epithermal neutrons using hydrogenous and graphite moderators. Measurements on the diffusion and backscattering of thermal neutrons in soil components were carried out for the development of novel nuclear methods in order to speed up the humanitarian demining process. Results obtained for the diffusion length were checked by MCNP-4C calculations. In addition, the effect of the weight and density of the explosives on the observation of the anomaly in the reflected thermal neutrons was examined by using different dummy landmines.

Measurements of (n, xp), (n, xd) double differential cross sections of carbon and aluminum for 65 and 75 meV neutrons

Double differential (n, xp), (n, xd) cross sections of carbon and aluminum were measured for 65 and 75 MeV neutrons at angles between 12° and 70° using ΔE-E telescopes at a 7 Li(p, n) neutron source facility at TIARA. The data at 12° were obtained by employing an annular geometry. The telescopes consisting of SSD and Nal(Tl) scintillator, experimental methods and the data reduction procedures are presented as well as the results. The carbon data are compared favorably with existing experimental data. The (n, xp) spectra of both carbon and aluminum agreed fairly well with theoretical calculations based on the intra-nuclear cascade model and the multi-step Hauser-Feshbach model including preequilibrium effects, but the (n, xd) spectra differ significantly from those calculations.

Advantages and limitations of thermal and epithermal neutron activation analysis of bulk samples

Systematic investigations have been carried out to extend the thermal neutron activation method for elemental analysis of bulk samples. A new method developed for the determination of the flux perturbation factor renders the thermal and epithermal neutron activation analyses of bulky samples of unknown compositions possible both in hydrogenous and in graphite moderators. The flux perturbation, F, depression, H, and self-absorption, G, factors are given for different samples. The limitation of the epithermal neutron activation analysis for hydrogenous bulky samples is also discussed.

Studies on thermal neutron perturbation factor needed for bulk sample activation analysis

Abstract The spatial distribution of thermal neutrons produced by an Am–Be source in a graphite pile was measured via the activation foil method. The results obtained agree well with calculated data using the MCNP-4B code. A previous method used for the determination of the average neutron flux within thin absorbing samples has been improved and extended for a graphite moderator. A procedure developed for the determination of the flux perturbation factor renders the thermal neutron activation analysis of bulky samples of unknown composition possible both in hydrogenous and graphite moderators.

Measurements of double-differential neutron emission cross-sections of 238U and 232Th for 2.6, 3.6 and 11.8 MeV neutrons

Abstract Double-differential neutron emission cross-sections (DDXs) of 238 U and 232 Th were measured for 2.6, 3.6 and 11.8 MeV incident neutrons using a time-of-flight (TOF) method at the Tohoku University 4.5 MV Dynamitron accelerator facility. In DDXs for En=2.6 MeV, discrete structures by vibrational level groups are visible clearly around excitation energy (Ex) around 0.75 and 1.15 MeV, while inelastic scattering to continuum-levels become dominant at En=3.6 MeV for both nuclei. For En=11.8 MeV, continuum structures with strong forward peaking are dominant with some discrete structures at around 0.7–4.0 MeV excitation energy for both of 238 U and 232 Th. Partial cross-sections for the elastic and inelastic scattering processes were also derived from the DDX data.