Noriaki Nakao

Measurements of Secondary Neutrons Produced from Thick Targets Bombarded by High-Energy Helium and Carbon Ions

The angular and energy distributions of neutrons produced by 100 and 180 MeV/nucleon He and 100, 180, and 400 MeV/nucleon C ions stopping in thick C, Al, Cu, and Pb targets were measured using the Heavy-Ion Medical Accelerator in Chiba of the National Institute of Radiological Science (NIRS), Japan. The neutron spectra in the forward direction have broad peaks of {approximately}60 to 70% of the incident particle energy per nucleon due to the break-up process, and they spread up to almost twice the projectile energy per nucleon. The neutron spectra are similar for the same incident energy of 100 MeV/nucleon for both He and C ions. The phenomenological hybrid analysis, based on the moving source model and the Gaussian fitting of the break-up process, could well represent the measured thick target neutron spectra. The experimental results are also compared with the calculations using the heavy-ion code, and the calculated results agree with the measured results within a factor of 2 margin of accuracy. This systematic study on neutron production from thick targets by high-energy heavy ions is the first experimental work performed by NIRS and will be useful for designing the shielding for the high-energy heavy-ion accelerator facility.

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.

Measurements of Secondary Neutrons Produced from Thick Targets Bombarded by High Energy Neon Ions

Following our preceding study on thick target neutron yields by He and C, we measured angular and energy distributions of neutrons produced by 100, 180 and 400 MeV/nucleon Ne ions stopping in thick carbon, aluminum, copper and lead targets using the heavy ion medical accelerator of the National Institute of Radiological Sciences. The neutron spectra in the forward direction have broad peaks which are located at about 60 to 70% of the incident particle energy per nucleon due to break-up process and spread up to almost twice as much as the projectile energy per nucleon. The neutron spectra at all angles consist of two components of cascade neutrons and evaporation neutrons. The phenomenological hybrid analysis of the moving source model for these two components and the Gaussian fitting of break-up process could well represent the measured thick target neutron spectra. The experimental results are also compared with the calculations using the HIC code, and the calculated results generally agree with the meas...

Measurements of High-Energy Neutrons Penetrated Through Concrete Shields Using Self-TOF, NE213, and Activation Detectors

Abstract Neutron energy spectra penetrated through concrete shields were measured using three types of high-energy neutron detectors: the Self-TOF detector, an NE213 organic liquid scintillator, and Bi and C activation detectors, which have been newly developed by a group at the Heavy-Ion Medical Accelerator in Chiba (HIMAC) facility of the National Institute of Radiological Sciences, Japan. Neutrons were generated by bombarding 400 MeV/nucleon C ions on a thick (stopping-length) copper target. The neutron spectra were obtained through an unfolding code with their response functions and compared with LAHET and MCNPX calculations combined with the LA150 cross-section library. The calculations tend to overestimate with increasing the shielding thickness compared to the experimental results. The neutron fluence measured by the NE213 detector was simulated by the track length estimator in the MCNPX code, and the contribution of the room-scattered neutrons was evaluated. The neutron fluence attenuation length was obtained from the experiment for each detector and the calculation in the energy range of 20 to 800 MeV.

Measurement of Neutron Production Cross Sections by High-Energy Heavy Ions

The double-differential cross sections (DDX) of neutron production by high energy heavy ions were measured using the RIKEN Ring Cyclotron of the Institute of Physical and Chemical Research, Japan, and the Heavy Ion Medical Accelerator in Chiba (HIMAC), the National Institute of Radiological Sciences. The neutron energy spectra from 0 to 110 degrees were obtained by using the time-of-flight method. The neutron spectra in the forward direction have a sharp peak which is located at the incident particle energy per nucleon due to the knock-on process, and spreads out to about 3 times the incident particle energy per nucleon. The total neutron production cross sections were obtained by integrating the energy spectra above 20 MeV over a hemisphere from 0 to 90 degrees. The experimental results are compared with calculations using the intranuclear cascade-evaporation Monte Carlo code, HIC, and the quantum molecular dynamics code, QMD. The QMD codes give better agreement with the experimental results than the HIC code.

An experimental study on radiation streaming through a labyrinth in a proton accelerator facility of intermediate energy

A radiation streaming experiment has been carried out at the Takasaki Ion Accelerator Facility for Advanced Radiation Application at the Japan Atomic Energy Research Institute in a room housing a Cu target irradiated with 68 MeV protons and in a labyrinth of three-legs having a total length of 29 m. In the experiment, neutron and gamma ray energy spectra, neutron reaction rates, and neutron and gamma ray dose equivalent rates were measured using various counters and dosimeters. The experimental data show the applicability of some empirical formulas for estimating the thermal neutron flux in a room and neutrons streaming in a labyrinth designed for a proton accelerator operating in the intermediate energy region. The data suggest that it is mandatory to estimate the gamma ray dose equivalent rate in a labyrinth, which is dominated by the secondary gamma rays due to the neutron capture reaction.

Development of Self-TOF Neutron Detector and its Application to Shielding Experiment at HIMAC

A new type detector, called ‘Self-TOF detector’, has been developed for high energy neutron spectrometry behind a shield by our group. The detector consists of a veto counter, a set of radiators with 20 thin detectors, a start counter and a stop counter of nine segments. The measurement of the detector response function for high energy neutrons and the concrete shielding experiment were done at the HIMAC(Heavy-Ion Medical Accelerator in Chiba) of NIRS(National Institute of Radiological Sciences), Japan. Neutron spectra were also compared with the LCS(LAHET Code System). Both results are in rather good agreement within a factor of 2.

Target dependence of beryllium fragment production in neutron- and alpha-induced nuclear reactions at intermediate energies

Summary The yields of 7Be from natC, 27Al, natCu, natAg and 197Au targets and 10Be from natCu, natAg and 197Au targets in alpha- (400 MeV) and neutron- (maximum end-point energy, E0 = 500 MeV) induced reactions were measured in order to compare them with the photonuclear-reaction yields at E0 = 1000 MeV. The target-mass dependences of the yields of 7Be and 10Be showed very similar trends among alpha-, neutron-, and photon-induced reactions. The fragmentation yields of 7Be decreased more slowly, and those of 10Be increased gradually with an increase in the target mass; both yields intersected at an approximate target mass of 125. The exponential increase in the fragmentation-yield ratios of 10Be to 7Be with an increase in the neutron-to-proton ratio of the targets suggests that pre-formation of the fragments inside the excited nuclei is strongly influenced by the target properties, such as N/Z. Furthermore, the effect of the incident particle was clearly observed in the yield ratios of 10Be to 7Be from the Cu and Ag targets.

Measurements of Thick Target Neutron Yields from 100 to 800 MeV/Nucleon Heavy Ions

We have measured angular and energy distributions of neutrons produced by 100 and 180 MeV/u He, 100, 180, 400 MeV/u C, 100, 180, 400MeV/u Ne, 400MeV/u Ar, 400MeV/u Fe, 400MeV/u Xe and 800MeV/u Si ions stopping in thick carbon, aluminum, copper and lead targets using the HIMAC (Heavy-Ion Medical Accelerator in Chiba) of NIRS (National Institute of Radiological Sciences), Japan. The neutron spectra in the forward direction have broad peaks of about 60 to 70 % of the incident particle energy per nucleon due to the break-up process and spread up to almost the twice of the projectile energy per nucleon. The neutron spectra are similar for the same incident energy for different projectile particles. The experimental results are compared with the calculations using the HIC code, and the calculated results agree with the measured results within a factor of 2. This systematic study on neutron production from thick targets by high-energy heavy ions is the first experimental work performed by NIRS and will be useful for designing the shield of the high-energy heavy-ion accelerator facility.

Experimental Analyses on Radiation Streaming through a Labyrinth in a Proton Accelerator Facility of Several Tens MeV

In order to estimate the accuracy of calculation methods such as empirical formulas and Monte Carlo codes for neutron and gamma ray fluxes through a labyrinth connected with a room having a Cu target irradiated by 68-MeV-proton, experimental analyses are carried out. As results of the analyses, the followings are obtained. (l)The MCNP calculation is applicable to estimation of neutron and gamma ray dose equivalent rates in the labyrinth, although cross sections in the process of neutron thermalization and gamma ray production cross section should be modified for more precise estimation. (2)The empirical formulas are useful for estimation of neutron dose equivalent rate. However, an additional safety factor of 2 should be required to apply to the estimation of total dose equivalent rates, because the contribution of gamma ray dose equivalent rate increases with the distance in the labyrinth and becomes comparable to that of neutron dose equivalent rate at the distance longer than 20 m.