Katsuhisa Kudo

Accelerator‐based Neutron Fluence Standard of the National Metrology Institute of Japan

We report the present status of the national standard on accelerator‐based fast neutron fluences in Japan. Monoenergetic neutron fluence standards have been established at 144 keV, 565 keV, 5.0 MeV and 8.0 MeV by using a Van de Graaff accelerator and at 2.5 MeV and 14.8 MeV by using a Cockcroft Walton accelerator. These standards are prepared to measure the detection efficiency and the energy response of neutron sensitive devices, such as personal dosimeters and survey meters. Neutron production and absolute fluence measurement for these energies are described. We are developing a new standard in the energy region of a few tens of keV, which is also introduced here as well as our future plans.

Measurement of /spl gamma/-ray dose in a thermal neutron field by using a /sup 3/He-filtered GM counter

A new type of /sup 3/He-filtered GM counter has been used to measure the dose of /spl gamma/-rays existing in a thermal neutron field. The detector assembly consists of a commercially available GM counter (8.8 mm diameter and 31.8 mm height) surrounded by a cylindrical aluminum vessel (220.5 mm diameter and 163 mm height) filled with /sup 3/He gas in the pressure range up to 6 atmospheres. The contribution of secondary /spl gamma/-rays produced by thermal neutron captures in the GM counter itself can be excluded by increasing the gas pressure of /sup 3/He, because the thermal neutron capture reaction of /sup 3/He(n,p)T results in the emission of charged particles of a proton and a triton. In order to determine the response of the GM counter, the deposited energy distribution of the counter to monoenergetic /spl gamma/-rays was calculated by the EGS4 Monte Carlo code in the /spl gamma/-ray energy range from 60 keV to 10 MeV and then the response was derived through the calibration of the counter at /spl gamma/-ray energies of 60 keV, 662 keV and 1.17 MeV +1.33 MeV. On the other hand, the /spl gamma/-ray energy distribution in the thermal neutron field was calculated by the MCNP Monte Carlo calculation, assuming the three dimensional experimental assembly of a graphite pile including the concrete floor in the experimental room. As a result, the /spl gamma/-ray fluence and the effective dose equivalent in the thermal neutron field have been determined by using the GM counter response, the /spl gamma/-ray energy distribution in the field and the conversion factor from fluence to effective dose equivalent.

Studying the Effect of Sunlight on Etching Characteristics and Response of the CR-39 Track Detector

The effect of sunlight on the etching characteristics and response of the CR-39 detector is studied. Our results indicate that sunlight significantly affects both the etching charcteristics and sensitivity of the deterctor. The detector response to about 6.1 MeV α particles and neutrons from an Am-Be source drops by about 51% and 87%, respectively, after six months of exposure to sunlight.

Improvement in the Sample Mass-Change-Method used for Determination of the Bulk Etch Rate of CR-39 Neutron Detector

The bulk etch rate is an important parameter of any etch-track detector. An accurate measurement of the bulk etch rate is, therefore, highly desirable in the calibration of a CR-39-based neutron dosimeter. Amongst several available bulk etch rate measuring techniques, the sample mass change technique is claimed to be the most accurate one. We carried out systematic experiments and found that even this method has some limitations. The problems associated with this method are discussed. To circumvent these problems, modifications in the sample mass change technique have been suggested.

Standardization of DT neutron field and its application to detector calibration for fusion diagnostics

The standard irradiation field for DT neutrons has been developed by using an associated α-particle technique. Special attention has been paid to establish the DT neutron standard for fusion diagnostics at neutron energies from 14.0 MeV to 14.10 MeV by using special emission angles between 90° and 100° to the direction of an accelerated deuteron beam. The sharp distributions with the energy spread less than 10 keV at 96°–98° provide the standard energy points of DT neutrons at energies from 13.98 MeV to 14.00 MeV. A new technique with a 3He proportional detector has been applied to determine the energy spread as well as the precise mean energy of a DT neutron distribution after calibrating the detector at the reference energy point of 14.00 MeV.

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 .

CHARACTERIZATION OF THE 2.5 MEV NEUTRON FIELD FOR MONOENERGETIC FAST NEUTRON FLUENCE STANDARD AT THE NATIONAL METROLOGY INSTITUTE OF JAPAN

This paper describes the 3 MeV neutron field where the neutrons are generated in the D(d,n) 3 He reaction by bombardment of a deuterium-titanium target with a 230 keV deuteron beam from a Cockcroft-Walton accelerator. The neutron field is being prepared for a new national standard on neutron fluence in Japan. The neutron fluence was measured using a proton recoil neutron detector consisting of a silicon surface barrier detector with a polyethylene radiator and the results were compared with data obtained by measuring 3 MeV protons from the D(d,p)T reaction in the deuterium-titanium target in a manner similar to the associated particle method. Neutron spectra were measured using a newly developed recoil proton spectrometer composed of three position sensitive proportional counters and two silicon surface barrier detectors. The gamma rays existing in the field were also characterized using a liquid organic scintillation detector. The ambient dose equivalents were estimated to be of the order of a few µSv at a neutron fluence of 10 7 neutrons cm -2 .

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.

GAMMA DOSIMETRY WITH CR-39 ETCH TRACK DETECTOR

To preserve and improve the safety of food for commercial purposes, it is exposed to high gamma-ray doses. The gamma-ray doses used for this purpose range from 0.15 kGy to 50 kGy. At such high doses, the etching characteristics of CR-39 are severely affected. This property, therefore, can be used to develop a CR-39-based gamma dosimeter. In this context, systematic studies were carried out and the bulk etching rate was determined as a function of gamma-ray dose using different methods.

Effect of gamma energy, temperature and shelf life on the response of gamma and BD-100R neutron bubble detectors

The use of commercially available bubble detectors for gamma and neutron detection has been reported by several groups. We have carried out systematic investigations to study the response of gamma bubble detector as a function of energy, the effect of waiting time on sensitivity of BD-100R neutron bubble detector and the effect of shelf life on both types of bubble detectors. Our results show that the response of the gamma bubble detector is energy dependent. Waiting time of up to 260 hours for BD-100R neutron detector has no significant effect. Both the detectors have a limited shelf life. These factors must be taken into account while using these detectors.