Chizuo Mori

Detection of extremely low level radioactivity with imaging plate

Imaging plate (IP) is useful for in-situ detection and distribution measurements of extremely low-level radioactivity. Exposure of IP for several hours makes it possible to detect down to about 10−4 Bq in a spot with an area of around 1 mm2 or less. Methods to determine radionuclide species by only a single exposure are proposed; the several sheets lamination method and the several times successive read-out method. Although the latent image degraded with time after exposure, simultaneous exposure of α-, low-energy β- and high-energy β-calibration sources together with the specimen led to a quantitative analysis.

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.

A neutron detector using silicon PIN photodiodes for personal neutron dosimetry

Abstract A highly sensitive and small sized neutron detector was devised for use as a personal neutron dosimeter using a silicon PIN photodiode and a gadolinium-foil converter. A detection efficiency of 5.6% was obtained for thermal neutrons by detecting internal-conversion electrons emitted from neutron-captured gadolinium. The γ-ray component contained in the output pulses was cancelled almost completely by using a twin photodiode with a tin foil instead of the gadolinium-foil converter. A low detection limit for neutron flux density of 3.1 × 10 2 cm −2 s −1 was obtained for thermal to 10 keV neutrons with a measurement time of 3 s under γ-ray background with a dose rate of 25 μSv/h.

Radioactivity and geometrical distribution measurements of α-emitter specimens with the imaging plate

Abstract Imaging plate detectors, having a sensitivity of about a hundred times greater than that of conventional X-ray films, are finding extension into many fields as a two-dimensional position detector for radiations. The imaging plate has a large area, of 20 cm × 25 cm, or even larger 35 cm × 43 cm, and a spatial resolution of about 100 μm × 100 μm (i.e. a pixel). The sensitivity of the plate to α-rays was investigated here; even a single incident α-ray can be detected and the signal output was found to be linear up to at least thi thirty incident α-rays per pixel. Plutonium fine particles in the environmental air of a nuclear fuel processing cell were collected on a filter paper. The plate was then exposed to α-rays emitted from the filter and the diameters of the fine particles could be determined by the output signal intensity from the plate.

Measurement of the radioactivity distribution of material surfaces with an imaging plate

Abstract An imaging plate has a very high sensitivity, and it is useful for in situ measurements of the distribution of extremely low levels of radioactivity, down to about 10 −3 Bq/mm 2 , of material surfaces such as stones, vegetables etc. A method to determine the radioactivity is proposed in such applications.

Gamma-ray emission probability measurement by a two-dimensional 4πβ-γ coincidence system

Abstract A two-dimensional data-acquisition system has been applied to a coincidence system composed of a new 4πβ proportional counter operated with CF 4 gas and an HPGe detector. This system was capable of measuring the disintegration rates of short half-life nuclides. Making the most merit of this system, the emission probabilities of the principal γ-rays for 128 I were measured; they are smaller than the evaluated values by about 25% and the uncertainties have been reduced by more than one order.

Precise measurement of the gamma-ray emission probability for 175Yb

The emission probabilities for the γ-rays of 175Yb were accurately measured by a 4πβ (ppc)-γ (HPGe) coincidence apparatus using a live-timed bi-dimensional data acquisition system. The emission probability of the main 396 keV γ-rays was 0.1315 ± 0.0016, a value that was about twice the evaluated values.

Self-induced space charge effect on gas gain in proportional counters

Abstract In the measurement of radiation using a proportional counter, energy linearity is lost at high gas gain, which is called the self-induced space charge effect. The dependence of the effect on the diameter of anode wire and the pressure of counting gas was investigated using methane gas flow proportional counters for X-rays with energies of 0.64, 1.5 and 5.9 keV. A theoretical explanation of the effect is also given. The effect increases with both increasing X-ray energy and pressure of the counting gas and with descreasing anode wire diameter.

Photoelectric Emission of Metals by X-Rays in the KeV Region

Electron emission yield from the metal surface under the irradiation of nearly monochromatic X-rays covering the energy range from 1.5 KeV to 7.9 KeV was measured. The yield was about 10-1 for gold and about 10-2 for aluminium, depending on the incident X-ray photon energy. The ratio of the electrons having the energy more than 30 eV to the total electrons depends also on the incident X-ray photon energy and on the material. According to the emission yield obtained theoretically, Auger electron emission is dominant when incident X-ray photon energy is just above the absorption edge of the material.

Position Sensitive Proportional Counter for Measurement of Tritium Labelled Gas Movement

A position sensitive proportional counter of a charge division type with a single resistive anode wire was constructed for the measurement of the movement of 3H labelled gas which is flowing or diffusing in a pipe. The introduction of resistors between the anode wire and pre-amplifiers brought a uniform detection efficiency for 3H β-rays throughout the counter. The position resolution was 3.1mm FWHM. Detection efficiency was almost 100% uniformly over about 700 mm in the total anode length of 740 mm. The movement of 3H labelled gas. could be measured effectively.