T. Itoga

Measurement of Neutron Emission Spectra in Li(d,xn) Reaction with Thick and Thin Targets for 40-MeV Deuterons

Abstract To improve the data accuracy of the neutron emission spectra of the natLi(d,xn) reaction that will be used as the neutron source in the International Fusion Materials Irradiation Facility, the authors have measured the neutron emission spectra from thick and thin lithium targets bombarded by 40-MeV deuterons at the AVF cyclotron (K = 110) facility of Tohoku University. The neutron spectra were measured at nine laboratory angles between 0 and 110 deg with the time-of-flight method using a beam-swinger system and a well-collimated neutron flight channel, obtained over almost the entire energy range of secondary neutrons using a two-gain method. The data obtained revealed the shape and angular dependence of emission spectra up to the high-energy region called a “high-energy tail.” The experimental results are compared with other experimental data and calculations.

Fast collimated neutron flux measurement using stilbene scintillator and flashy analog-to-digital converter in JT-60U

A line-integrated neutron emission profile is routinely measured using the radial neutron collimator system in JT-60U tokamak. Stilbene neuron detectors (SNDs), which combine a stilbene organic crystal scintillation detector (SD) with an analog neutron-gamma pulse shape discrimination (PSD) circuit, have been used to measure collimated neutron flux. Although the SND has many advantages as a neutron detector, the maximum count rate is limited up to ∼1×105counts∕s due to the analog PSD circuit. To overcome this issue, a digital signal processing system (DSPS) using a flash analog-to-digital converter (Acqiris DC252, 8GHz, 10bits) has been developed at Cyclotron and Radioisotope Center in Tohoku University. In this system anode signals from photomultiplier of the SD are directory stored and digitized. Then, the PSD between neutrons and gamma rays is performed using software. The DSPS has been installed in the vertical neutron collimator system in JT-60U and applied to deuterium experiments. It is confirmed t...

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.

High repetition rate neutron flux array using digital-signal processing and scintillators for study of high performance plasma at JT-60U

Aiming at increasing the count rate capability of the radial neutron collimator system in JT-60U, a digital-signal processing system has been extended to seven channels for data acquisition of stilbene crystal detector using analog-to-digital converters equipped with a fast flash memory. In this method, the detected pulses were postprocessed concerning pulse shape and pulse height. Using this system, the radial neutron collimator system can provide the information on the temporal evolution of emission rate of d-T neutrons and gamma rays as well as d-D neutrons. On the course of the development, the fast and stable handling of the large data around 7 GB is undertaken for a discharge.

Development of a new multi-moderator spectrometer for epithermal neutrons

Measurement of the neutron energy spectrum in the epithermal energy region is very important for accelerator-based Boron Neutron Capture Therapy (BNCT), but the measuring technique is not well established. We developed a new multi-moderator spectrometer using boron-loaded siHcon rubber and polyethylene for epithermal neutrons. In this paper, the response functions of the new multi-moderator neutron spectrometer were calculated with the MCNPX code, and were compared with the measured results in a neutron calibration field using a 252Cf neutron source. This spectrometer can be applied to measure the neutron spectrum to be used in accelerator-based Boron Neutron Capture Therapy.

Influences of Neutron Source Spectrum and Thermal Neutron Scattering Law Data on the MCNPX Simulation of a Cyclotron-Based Neutron Field for Boron Neutron Capture Therapy

In our previous study, the simulation of a cyclotron-based neutron field for boron neutron capture therapy (BNCT) using a (p,n) spallation source with the MCNPX code was validated through measurements of the neutron energy spectrum behind the moderator assembly and the thermal neutron distribution in an acrylic phantom using reaction rates of 198Au. These validations showed that the simulation generally well reproduced the measurements. However, some discrepancies between the measurements and the calculation remained for clinical trials. In this paper, we investigated the influences of neutron source spectrum and thermal neutron scattering law data in the simulation to resolve those discrepancies. We also compared measured and calculated neutron doses behind the moderator assembly with results obtained using a tissue equivalent proportional counter. We clarified that the neutron source spectrum calculated using the LA150 data led to the overestimation of high-energy neutrons in a phantom, but this overestimation did not significantly affect the neutron dose distribution in a phantom, because a dominant part of the absorbed dose is due to neutrons of energies below 1MeV. The study of the influence of neutron scattering law data in a phantom also indicated that the use of selected S(α,β) data led to an improvement in the simulation of thermal neutron behavior.

Response measurement of a Bonner sphere spectrometer for high-energy neutrons

Response measurements of a Bonner sphere spectrometer (BSS) were performed for high-energy neutrons up to 400 MeV in quasi-monoenergetic neutron fields at Research Center for Nuclear Physics (RCNP), Osaka University. The quasimonoenergetic neutron field was generated by injecting protons on to the Li target with the spectra consisting of a monoenergetic high-energy peak followed by an unwanted continuum tail down to the low energy region. Two-angle differential measurements were performed to obtain the monoenergetic response by subtracting the tail contribution. Discussions were made on data analysis for tail subtraction. While each Bonner sphere had different energy responses, good agreements were seen in the monoenergetic responses between the MCNPX calculation and the measurement with a suitable subtraction. Response calculations were also conducted for the whole spectra obtained by unfolding the measured BSS data and gave the results reproducing well the measured responses.

Development of fast measurement system of neutron emission profile using a digital signal processing technique in JT-60U

Neutron emission profiles are routinely measured in JT‐60U Tokamak. Stinbene neuron detectors (SNDs), which combine a Stilbene organic crystal scintillation detector (Stilbene detector) with an analog neutron‐gamma pulse shape discrimination (PSD) circuit, have been used to measure neutron flux efficiently. Although the SND has many advantages as a neutron detector, the maximum count rate is limited up to ∼1× 105 counts/s due to the dead time of the analog PSD circuit. To overcome this issue, a digital signal processing (DSP) system using a Flash‐ADC has been developed. In this system, anode signals from the photomultiplier of the Stilbene detector are fed to the Flash ADC and digitized. Then, the PSD between neutrons and gamma–rays are performed using software. The photomultiplier tube is also modified to suppress and correct gain fluctuation of the photomultiplier. The DSP system has been installed in the center channel of the vertical neutron collimator system in JT‐60U and applied to measurements of n...

Measurement of Differential Thick Target Neutron Yields (TTY) from Fe, Cu(p,n) Reactions at 35, 50, and 70 MeV

Differential thick target neutron yields (TTY) for natFe, natCu(p,xn) reactions were measured for 35, 50 and 70 MeV incident energy at several laboratory angles between 0‐ and 110‐deg. using the time‐of‐flight method with the Tohoku Univ. AVF cyclotron and a beam‐swinger system. We determined neutron energy spectra from ∼ 0.6 MeV to the highest secondary neutron energy. The experimental results were compared with the LA150 data library.

Development of Wide-Area Radiation Monitor Using an Optical Fibe

We have developed a method for radiation distribution measurement by combining an optical fiber of wave-length shift type with plastic scintillators,and studied its properties to apply as a wide-area radiation moniter. The detector employs two photomu1tipliers in both ends of the fiber and locates the radiation position by using the difference of light arrival time from scintillators. We tested the detector with gamma-rays and neutrons concerning with the position-response and pulse-height respons of the detector. From the experiment,we confirmed the proper operation of the detector and position response with spatial resolution of 30-60 cm.