Masayoshi Aoki

Looping variation observed in environmental gamma-ray measurement due to atmospheric radon daughters

Abstract The contribution of gamma-rays originating from the atmospheric radon daughters to the environmental radiation is observed to be from ∼ 10 to 25%. In the measurement of diurnal variation of the environmental gamma-ray intensity and the atmospheric radon daughters concentration, we have found a looping correlation between them. By using a model calculation, the cause of this phenomenon is confirmed to be the diurnal variation of the altitudinal radon daughters concentration.

Automatic Measuring Instrument for Radon Daughters Concentration of Precipitation

An automatic measuring instrument for the radon daughters concentration of precipitation has been constructed and used successfully. The precipitation collected by a poly-ethylene funnel during 15 min is led to a glass cell set in a well-type NaI(Tl) scintillator, and after 5 min counting the water sample is pumped out. A sequence of operations of sampling, measurement, recording, draining out and correction for the volume of the water sample is performed automatically, and is repeated at every 15 min. The instrument has sensitivity and durability enough for practical use at outdoors. This instrument enables the measurement of radon daughters concentration as low as 0.5 Bq/ml at precipitation rate as small as 0.5 mm/h. For the improvement of accuracy of the monitoring system of environmental 7-radiation, the effect of radon daughters accumulated on the ground surface by precipitation can be eliminated on real time by means of providing it with the output of this instrument. This instrument can be used for...

TSEE from Surfaces of LiF and CaF2 Crystals

The TSEE glow peaks from a LiF crystal after being exposed to ionizing radiation were observed at 125, 180 and 230°C with a Q-gas flow counter. For irradiation with X-rays (20 R), the TSEE glow peak at 125°C was more dominant than the other peaks. We found that the 125°C peaks for the (110) and (111) surfaces were more intense than for the (100) cleaved surface. When methane was used as a counting gas, the glow peak of the (100) surface at 125°C was weaker than for the case of Q-gas, and the ratio of these intensities was less than one tenth. The TSEE glow peaks from a CaF2 crystal were observed at 85, 180 and 270°C after X-ray excitation. The TSEE intensity at 85°C for CaF2 cleaved surfaces showed almost the same tendency as for LiF with regard to the effects of crystal orientation.

Consideration on the measurement of α and β rays with the exoelectron dosimeter

Abstract With the thin thermalox BeO disk, we have measured TSEE (thermally stimulated exoelectron emission) of α and β rays, and have found that it depends only on the stopping power of the thin surface layer of the material. It has been made clear that absorbed dose of high energy β-rays in a rather deep layer from the surface does not contribute to the exoelectron emission. This is a serious problem to measure the surface absorbed dose by the solid state detector.

Efficiencies of TSEE Excited by Various Types of Ionizing Radiation

The efficiency of thermally stimulated exoelectron emission (TSEE) was studied for the surfaces of exoelectron materials excited by α, β and γ-rays. The measured efficiencies correspond reasonably well with the mean dE/dx of a surface thin layer. There remain problems regarding the counting efficiency of the 2π flow counter system and the number of the missing exoelectrons that escaped before a measurement. Regarding the β-ray excited group, it can be concluded that the origin of the observed exoelectrons was in a thin surface layer with a thickness of several hundred angstroms and that the contribution of the bulk origin was negligible.

Thermoluminescence and Thermally Stimulated Exoelectron Emission from Evaporated LiF and CaF_2 Films

It was found that thin films evaporated onto a substrate in vacuum exhibit both thermoluminescence (TL) and thermally stimulated exoelectron emission (TSEE) when they are heated without any stimulative pre-treatment. These phenomena were observed only once after evaporation, and succeeding repeated heating showed neither TL nor TSEE. The heating rates used were 0.5°C/s for TL and 1°C/s for TSEE. In this study, the temperature at the main glow peak of TL or TSEE from the unirradiated films was found to be higher than that from these irradiated with gamma-rays.