Yoshihiro Nakane

Transmission Through Shields of Quasi-Monoenergetic Neutrons Generated by 43- and 68-MeV Protons —I: Concrete Shielding Experiment and Calculation for Practical Application

The energy spectra of neutrons that penetrate 25- to 200-cm-thick concrete shields are measured using 40- and 65-MeV quasi-monoenergetic neutron sources at the 90-MeV AVF cyclotron of the Takasaki Ion Accelerator Facility for Advanced Radiation Application at the Japan Atomic Energy Research Institute.

Transmission through Shields of Quasi-Monoenergetic Neutrons Generated by 43- and 68-MeV Protons —II: Iron Shielding Experiment and Analysis for Investigating Calculational Method and Cross-Section Data

Neutron spectra in the energy range above 10{sup {minus}4} eV transmitted through iron shields succeedingly are measured with a BC501A liquid scintillation detector and the Bonner Ball detector using quasi-monoenergetic neutron sources generated via the {sup 7}Li(p,n) reaction by 43- and 68-MeV protons. Using the collimated source neutrons, the spectra are obtained on the neutron beam axis and at off-center positions. The calculations using the MORSE-CG and DOT3.5 codes with the DLC119 group cross-section data sets as well as the HETC-KFA2 code are carried out and compared with the measurements.

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.

Neutron Dosimetry in Quasi-Monoenergetic Fields of 244 and 387 MeV

This paper describes the results of neutron spectrometry and dose measurements using a Bonner Sphere Spectrometer (BSS) at the ring cyclotron facility of the Research Center for Nuclear Physics (RCNP), Osaka University, Japan. Quasi-monoenergetic neutron fields were generated using the 7Li (p,n)7Be reaction and 246 and 389 MeV protons. Neutrons produced at 0° and 30° emission angles were extracted into a time-of-flight (TOF) tunnel, and the energy spectra were measured at a distance of 35 m from the target. To deduce the corresponding neutron spectra from thermal to the nominal maximum energy, the BSS data were unfolded using the MSANDB code and response functions were calculated by Monte Carlo (MC) methods. These spectra are compared to spectral measurements using NE213 organic liquid scintillators applying the TOF method. The results are discussed in terms of ambient dose equivalent H* (10) and compared with the readings of other instruments operated during the experiment.

Radiator effect on plastic nuclear track detectors for high-energy neutrons

Abstract The effect of radiators supplying charged particles to plastic nuclear track detectors has been investigated both experimentally and theoretically in order to apply them to personal dosimeters for high-energy neutrons. Performance of four types of radiator materials, CH 2 , CD 2 (deuterized hydrocarbon), LiF (lithium fluoride) and C (graphite), was checked in a quasi-monoenergetic neutron field generated by p-Li reactions. The efficiency has been numerically calculated based on a model with a special attention to the angular dependence of cross sections and data of characteristic response to light ions. The effect of respective radiator candidates has been evaluated as a function of the neutron energy. A two-layer radiator has also been proposed to adjust the energy dependence of the relative sensitivity to conversion factor for dose-equivalent.

BENCHMARK EXPERIMENT OF NEUTRON PENETRATION THROUGH IRON AND CONCRETE SHIELDS FOR HUNDREDS-OF-MeV QUASI-MONOENERGETIC NEUTRONS-II: MEASUREMENTS OF NEUTRON SPECTRUM BY AN ORGANIC LIQUID SCINTILLATOR

Abstract A shielding benchmark experiment has been performed to obtain the spectra of neutrons penetrating 10- to 100-cm-thick iron shields and 25- to 200-cm-thick concrete shields and to investigate the accuracy of various calculation codes using a 137-MeV quasi-monoenergetic neutron source. The source neutrons are produced from a 1.0-cm-thick lithium target bombarded with 140-MeV protons, and the energy spectra are measured with the time-of-flight (TOF) method using a NE213 organic liquid scintillator. The neutrons emitted in the forward direction were collimated with a 150-cm-thick iron collimator with 10- × 12-cm aperture. TOF and unfolding methods are applied to obtain the energy spectra behind the shield for the peak energy region and continuous-energy region, respectively. Monte Carlo calculations with PHITS and MCNPX are compared with the measured data. The comparison shows that the calculated spectra are in good agreement with the measured spectra.

Benchmark Experiment of Neutron Penetration through Iron and Concrete Shields for Hundreds-of-MeV Quasi-Monoenergetic Neutrons—I: Measurements of Neutron Spectrum by a Multimoderator Spectrometer

Abstract A shielding benchmark experiment has been performed to obtain the experimental data of neutrons penetrated through iron and concrete shields by using 140-, 250-, and 350-MeV p-Li quasi-monoenergetic neutrons. The quasi-monoenergetic neutrons were emitted from a 1-cm-thick Li target bombarded with 140-, 250-, and 350-MeV protons. The neutrons emitted in the forward direction were extracted into the time-of-flight room through a collimator of 12- × 10-cm aperture embedded in a 150-cm-thick concrete wall. The concrete and iron shield blocks were set at the exit of the collimator. Neutron energy spectra behind the shields were measured by a multimoderator spectrometer (3He proportional counter covered with polyethylene moderator of various thicknesses). Neutron energy spectra behind concrete and iron shields with different thicknesses were obtained down to thermal energy. The experimental results were compared with calculation results by the Monte Carlo simulation code PHITS. These experimental results will be useful as benchmark data to investigate the accuracy of various transport calculation codes.

Improvement of neutron detection efficiency with high sensitive CR-39 track detector

Abstract Two subjects have been studied for establishing an efficient detection technique applicable to neutrons with an energy higher than 20 MeV , for which personnel dosimetry becomes more important especially in space activities and radiation protection around high-energy accelerators. A performance of a new sensitive detector, recently developed copolymer of CR-39 and N-isopropylacrylamide, was checked in a D–T neutron field. It was confirmed experimentally that its sensitivity was several times as high as a pure CR-39 in the radiator effect and more than twice in the total efficiency. In the other experiment, CR-39 samples were exposed to 65 MeV quasi-monoenergetic neutrons, and the registration characteristics have been investigated from a fundamental point of view. The particles recorded were divided into three groups of proton relatives, α -particles and heavy ions by applying the growth curve method and the multi-step etching technique. The measured fractions were in good agreement with the values calculated by SSNERS code previously developed.

Monte Carlo calculation and measurement of energy response of a solid state nuclear track detector to neutrons from 100 keV to 20 MeV

Abstract A code system SSNRES using the Monte Carlo method has been developed for calculating the energy response of a solid state nuclear track detector (SSNTD) which has a polyethylene radiator to neutrons in the energy range from 100 keV to 20 MeV. To evaluate the accuracy of the code system, energy response of an SSNTD was measured for mono-energetic neutrons of 250 keV, 1 MeV, 5 MeV and 15 MeV, and for 252 Cf fission neutrons. Calculated responses were in good agreement with the experimental ones. Calculated results show that the contribution of recoil-proton was dominant in the detector response at the neutron energy below 10 MeV, while that of charged particles produced by the neutron interactions with carbon and oxygen was dominant at 20 MeV.

Absorbed dose measurements and calculations in phantoms for 1.5 to 50 keV photons.

A Monte Carlo code EGS4 expanded for low energy photon transport was validated by measuring absorbed doses in a phantom for 30 and 10 keV monoenergetic photons from synchrotron radiation. Using the EGS4 code, depth doses at 0.07 mm, 0.02 to 0.1 mm, and 10 mm in the ICRU slab phantoms were calculated for 1.5 to 50 keV photons using the updated photon cross section data PHOTX. The results show that the doses at 0.02 to 0.1 mm below 10 keV are practical indices of effective dose as calculated by others, based on the 1990 ICRP recommendations (1991).