Naoto Takeda

Generation and application of Laser-Compton gamma-ray at ETL

Abstract Laser-Compton gamma-ray facility has been developed at Electrotechnical Laboratory. Generation of the Laser-Compton gamma-ray and its applications for the establishment of the standard field of high-energy gamma-rays, for the nuclear physics research and for the beam diagnostic of the electron beam circulating in the storage ring is presented.

Energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523

Abstract The energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523 was examined for the neutron energy range between 1.2 and 14 MeV. Pulse height spectra measured with monoenergetic neutrons evidenced that they were characterized by either a single peak with a shoulder or by double peaks. A Monte Carlo simulation of the neutron behavior in the detector reproduced the spectra by considering the nonlinear light yield against recoil proton energy. Detection efficiencies ranging from 10% for 1.2 MeV to 0.6% for 14 MeV neutrons were obtained with a 12.7 cm diameter × 7.6 cm long scintillator. A high background rejection with a ratio of 2200 to 1 obtained without using any γ-shielding makes the present neutron spectrometer attractive for the application to low-level environmental neutron measurements.

NEUTRON SPECTROMETER WITH POSITION-SENSITIVE PROPORTIONAL COUNTERS

A spherical neutron counter for spectrometry and dosimetry has been developed. The counter consists of a spherical polyethylene moderator and three slender 3He position-sensitive proportional counters inserted into the moderator. The thermal neutron distribution in the moderator, which is a function of the energy of the incident neutrons is obtained with the proportional counters. Characteristics of the counter have been studied with experiments and Monte Carlo simulations. Also shown are energy spectra of a 252Cf neutron source that are obtained by an unfolding method using the thermal neutron distributions as response functions.

A multipurpose spherical neutron counter

We have developed a unique neutron counter applicable to measurements of fluence, energy distribution and the dose equivalent of neutrons with very wide energy range, and to estimations of incident direction of neutron beams. This 4/spl pi/ sensitive neutron counter consists of a spherical polyethylene moderator and three charge-division-type position-sensitive proportional counters (PSPCs) inserted into the moderator orthogonally with each other. The three PSPCs give thermal neutron distributions in the spherical moderator that bring us the above information. The characteristics of this neutron counter have been studied with Monte Carlo simulations and with experiments using monoenergetic neutrons. >

Determination of focal mechanisms of nonvolcanic tremor using S wave polarization data corrected for the effects of anisotropy

We propose a method to determine the focal mechanisms of nonvolcanic tremors using anisotropy-corrected S polarizations measured directly from noisy tremor signals. The successful retrieval of polarization angles in noisy signals owes much to the observation that, in general, tremors propagate slowly and therefore do not change their location immediately. This feature of tremors enables us to use longer time windows to compute polarization angles (e.g., 1 min or longer), resulting in a stack of particle motions. We applied the method to a tremor episode that occurred beneath the Kii Peninsula in April 2013. Although the majority of inferred focal mechanisms suggested that tremors manifest shear slip on the plate interface, some events with strike-slip components were also found to have occurred. The proposed method provides us with a tool for investigating smaller-scale spatial distributions and temporal variations of focal mechanisms throughout tremor zones.

Scintillating fiber camera for neutron dosimetry in spacecraft

Abstract A scintillating fiber camera for three-dimensional imaging was newly developed for radiation dosimetry in spacecraft. The camera consists of a scintillating fiber stack, an image intensifier unit and photomultipliers for triggering events. The scintillating fiber stack has 100 scintillating fiber layers. The layers are alternatively stacked up to be perpendicular to each other. The stack is coupled to a two-stage image intensifier and then coupled to a CCD camera for the track readout. Each fiber layer consists of 100 scintillating fibers and the fiber stack composed of 100 layers leads us to a sensitive volume of 50×50×50 mm 3 . Each fiber has a cross-section of 0.5×0.5 mm 2 . It is found that the camera has the capability to clearly identify charged particles, neutrons and γ-rays by observing individual three-dimensional images of those tracks. The threshold energy for identification of neutrons and γ-rays is 5– 10 MeV for recoil proton energy when the coincidence signals from 2 photomultipliers are used as triggers and is expected to be 2–3 MeV when the triggers from either photomultiplier are used. The whole energy region for neutron dosimetry will be covered by the combination with the Bonner spheres for the energy region lower than ∼10 MeV .

Performance of a spherical neutron counter for spectroscopy and dosimetry

A spherical neutron counter for spectroscopy and dosimetry has been developed. The counter consists of a spherical polyethylene moderator and three slender /sup 3/He position-sensitive proportional counters. Rough measurements of neutron fluence spectra and ambient dose equivalents, H*(10), have been done in the moderated-neutron field of JAERI. Characteristics of the counter are examined and compared with those of a Bonner sphere neutron spectrometer.

A small scintillating fiber camera consisting of 0.25 mm square fibers for space dosimetry

A small scintillating fiber camera for space dosimetry was developed. The camera consists of a scintillating fiber stack bundled with 0.25-mm square fibers, two image intensifier units whose input windows are 40 mm in diameter, and two photomultipliers. Neutron events are distinguished from gamma-ray events by observing three-dimensional track images of recoil protons and electrons. Experimental results show that events caused by recoil protons with energies above 5 MeV can be distinguished from gamma-ray events. The purpose of this camera is to evaluate the LET, of charged particles produced in the interaction of neutrons with the nucleus in scintillating fibers. The camera is expected to measure the dose contribution from higher energy neutron, especially, more than /spl sim/10 MeV, which is the upper limit energy of the present measurement in space.

Nonlinearity of pulse height of recoil helium in /sup 3/He proportional counters

The pulse height distribution in /sup 3/He proportional counters has been investigated to accurately quantify the response function to be used in fast neutron spectroscopy. The measurements were performed as a function of the operating voltage, the counting gas mixture, and the incident neutron energy. The special emphasis of this study has been put on the nonlinearity of the pulse height of a recoil helium induced by the /sup 3/He(n,n)/sup 3/He reaction. A distinct deviation of 14% at maximum to lower energies was found in the position of the recoil edge with the energy axis calibrated by the full energy peaks of the /sup 3/He(n,p)T reaction produced with neutrons of thermal energy and 2.413 MeV. It was confirmed that the nonlinear effects, such as an initial recombination and a space-charge effect, take place more significantly in the energy loss process of a recoil helium because of its higher linear energy transfer (LET) in the filling gas compared to that of a proton and triton pair emitted from the /sup 3/He(n,p)T reaction. The deviation becomes larger with an increase of the average LET in the gas mixture. The experimental results suggested that the nonlinear effect caused by a higher LET recoil helium is inevitable for the filling gas, even if the operating voltage is changed to an acceptable range.

Relation between the coupling and tremor rates in the transition zone around the Shikoku region

We examine the spatiotemporal distributions of the interplate couplings and the tremor count rate in the Shikoku region, which is located in the western part of the Nankai subduction zone in Japan, from July 2007 to June 2015 by comparing the results of static geodetic inversion using onshore global navigation satellite system data and the published tremor catalog. We observe that the temporal fluctuations in these two rates correlated well at a few spot-like points in the tremor-occurrence zone at a deeper location next to the strongly coupled area. The well-correlated points are densely distributed around the long-term slow slip area in the Bungo Channel and are randomly dispersed in the central and eastern portions. The correlation coefficients of the well-correlated points differ from point to point; however, at some points, zero tremor counts can be expected when the coupling rates reach the plate convergence rate. This indicates that tremors occur on the plate interface and that the tremor count rates can be used as proxies for monitoring the small fluctuations in interplate coupling.