Daiki Satoh

Measurement of Response Functions of a Liquid Organic Scintillator for Neutrons up to 800 MeV

Response functions of a BC501A liquid organic scintillator for neutrons up to 800 MeV have been measured at the heavy-ion accelerator of the National Institute of Radiological Sciences, Japan. A thick graphite target was bombarded with 400-MeV/u C ions and 800-MeV/u Si ions to produce high-energy neutrons whose kinetic energy was determined by the time-of-flight method. The measured response functions were compared with the results obtained using SCINFUL-QMD code, and the accuracy of the code was experimentally verified up to 800 MeV. This work will contribute to extending the energies measurable with our new radiation dose-monitoring system (DARWIN), which is based on the BC501A scintillator.

Measurement of microdosimetric spectra with a wall-less tissue-equivalent proportional counter for a 290 MeV/u 12C beam

The frequency distribution of the lineal energy, y, of a 290 MeV/u carbon beam was measured to obtain the dose-weighted mean of y and compare it with the linear energy transfer (LET). In the experiment, a wall-less tissue-equivalent proportional counter (TEPC) in a cylindrical volume with a simulated diameter of 0.72 microm was used. The measured frequency distribution of y as well as its dose-mean value agrees within 10% uncertainty with the corresponding data from microdosimetric calculations using the PHITS code. The ratio of the measured dose-mean lineal energy to the LET of the 290 MeV/u carbon beam is 0.73, which is much smaller than the corresponding data obtained by a wall TEPC. This result demonstrates that a wall-less TEPC is necessary to precisely measure the dose-mean of y for energetic heavy ion beams.

Development of Dose Monitoring System Applicable to Various Radiations with Wide Energy Ranges

A new inventive radiation dose monitor, designated as DARWIN (Dose monitoring system Applicable to various Radiations with WIde energy raNges), has been developed for monitoring doses in workspaces and surrounding environments of high energy accelerator facilities. DARWIN is composed of a phoswitch-type scintillation detector, which consists of liquid organic scintillator BC501A coupled with ZnS(Ag) scintillation sheets doped with 6Li, and a data acquisition system based on a Digital-Storage-Oscilloscope. Scintillations from the detector induced by thermal and fast neutrons, photons and muons were discriminated by analyzing their waveforms, and their light outputs were directly converted into the corresponding doses by applying the G-function method. Characteristics of DARWIN were studied by both calculation and experiment. The calculated results indicate that DARWIN gives reasonable estimations of doses in most radiation fields. It was found from the experiment that DARWIN has an excellent property of measuring doses from all particles that significantly contribute to the doses in surrounding environments of accelerator facilities—neutron, photon and muon with wide energy ranges. The experimental results also suggested that DARWIN enables us to monitor small fluctuation of neutron dose rates near the background-level owing to its high sensitivity.

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.

Age-dependent dose conversion coefficients for external exposure to radioactive cesium in soil

To estimate effective doses for members of the public exposed to external radiation from radioactive cesium (134Cs and 137Cs) deposited on the ground by the Fukushima nuclear accident, we calculate the conversion coefficients for converting activity concentration to effective dose rate by using the Particle and Heavy Ion Transport Code System. The data were produced from different age groups within the public (newborns; 1-, 5-, 10-, and 15-year-old children; and adults) for the situations in which radioactive cesium is distributed uniformly in the soil over a planar area and at specific depths of 0.0, 0.5, 2.5, 5.0, 10.0, and 50.0 g/cm2. On the basis of the results, we also derive the conversion coefficients for exponentially distributed volumetric sources. In addition, we obtain the conversion coefficients that give the effective dose accumulated over the first and second months, the first year, and over a lifetime (50 years) because of the contamination remaining on the ground. These calculations indicate that the conversion coefficients to obtain the effective dose rate are higher for the younger ages compared with adults, but do not exceed the ambient dose equivalent rate. Furthermore, we find that the difference between the calculated effective dose rates according to the International Commission on Radiological Protection 1990 and 2007 Recommendations is small (7% maximum) for a ground contamination of radioactive cesium.

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.

Analysis of the effect of structural materials in a wall-less tissue-equivalent proportional counter irradiated by 290 MeV u(-1) carbon beam.

Effects of structural materials in a wall-less tissue-equivalent proportional counter were evaluated based on the calculation of energy deposits by EGS5 and the measurement of lineal energy distributions using 290 MeV u(-1) carbon beams. It is found that the correction of measured data based on simulation is necessary for understanding the energy deposition spectra in the homogeneous condition in tissues.

Calibration of a Bonner sphere spectrometer in quasi-monoenergetic neutron fields of 244 and 387 MeV

This paper describes the results of calibration measurements for a Bonner sphere spectrometer (BSS) with 3He proportional counter performed in quasi-monoenergetic neutron fields at the Research Center for Nuclear Physics (RCNP) at the University of Osaka, Japan. Using 246 MeV and 389 MeV proton beams, neutron fields with nominal peak energies of 244 MeV and 387 MeV were generated via 7Li(p,n)7Be reactions. At high energies, the neutron spectra were measured by means of the time-of-flight (TOF) method. The low-energy part of the neutron spectra were determined by BSS measurements down to thermal energies using the MSANDB unfolding code and three different sets of response functions. These were obtained by means of Monte Carlo (MC) calculations including various codes and intra-nuclear cascade (INC) models. Unfolded BSS fluence rates were additionally confirmed by GEANT4 calculations. For calibration of the BSS, measured count rates were corrected for low-energy contributions and compared with count rates calculated using TOF data and various response functions. In addition, measured response values were compared with mono-energetic response calculations, and best agreement was found with GEANT4 results using the Bertini INC model.

Study on Response Function of Organic Liquid Scintillator for High‐Energy Neutrons

Response functions of liquid organic scintillator for neutrons up to 800 MeV have been measured at the Heavy‐Ion Medical Accelerator in Chiba (HIMAC) of National Institute of Radiological Sciences (NIRS). 800‐MeV/u Si ions and 400‐MeV/u C ions bombarded a thick carbon target to produce neutrons. The kinetic energies of emitted neutrons were determined by the time‐of‐flight (TOF) method. Light output for neutrons was evaluated by eliminating events due to gamma‐rays and charged particles. The measured response functions were compared with calculations using SCINFUL‐QMD and CECIL codes. It was found that SCINFUL‐QMD reproduced our experimental data adequately.