Kenjiro Kondo

Radiation damage in silicon microstrip detectors

Abstract A radiation damage effect on silicon strip detectors of pn-junction type is investigated using a high energy proton beam. In order to clarify the cause of leakage current increase, several variations of strip detectors with different surface structures were made and tested. No appreciable differences in leakage current increase are observed among these samples. A strong temperature dependence on leakage current is observed. This can be explained by a formation of radiation induced trap energy levels in the bulk silicon. A moderate room temperature annealing is seen. A pulse height degradation of about 10–20% is observed at a few Mrad of radiation. Some implications of the present results are discussed for possible application of silicon semiconductor detectors in future high energy hadron colliders.

Positron irradiation effects on polypropylene and polyethylene studied by positron annihilation

Abstract Positron irradiation effects on polypropylene (PP) and polyethylene (PE) have been studied using the positron source for positron annihilation (PA) experiments. From the intensities ( I 3 ) of the long-lived component of ortho -positronium ( o -Ps), it has been shown that the irradiation effect is larger in PP than in PE and that the samples with high crystallinity receive larger effect. The positron irradiation effect was compared with the effect of 60 Co γ-rays. The decrease in I 3 in 1 MGy γ-irradiated PP was smaller than that of positron irradiated samples, although the latter dose was estimated to be about two orders of magnitude smaller than the former. By decomposing the o -Ps component further into two components, it has been found that most of the decrease is concerned with the longest-lived component, indicating that it is mainly the Ps formed in the amorphous region that is affected by radiation.

Radiation damage of BGO crystals due to low energy γ rays, high energy protons and fast neutrons

Abstract Results are presented on the effects of radiation on BGO crystals. It is shown that BGO has more than two orders of magnitude higher resistance against low energy γ rays than SF5 lead glass. Darkening of the crystal is hardly visible even after an irradiation of 3.6 × 10 6 R. The darkening faded by more than one half after several days and it has almost completely disappeared after much longer periods. The tolerable rate of synchrotron radiation on BGO calorimeters is 100 R/h in a conservative estimate if the degradation in transmittance is required to be less than 1 % over a depth of 20 radiation lengths. In BGO calorimeters, the degradation of scintillation efficiency is less important than that of the transmittance, 99.999% pure crystals have a two orders of magnitude better resistance than 99.99% pure ones. Sample darkening caused by 12 GeV protons or fast neutrons is an order of magnitude more severe than that by low energy γ rays. Moreover, the recovery is not complete even after a period of more than 100 days. Some data on sample activation are also presented.

Radiation effect on positronium formation in low-temperature polyethylene

Abstract The irradiation effect of γ-rays on polyethylene (PE) has been studied by positron annihilation lifetime spectroscopy (PALS). In the case of non-irradiated PE samples, at a low temperature below the glass-transition temperature, the intensity of the long-lived component of positronium, I3, increases due to an increase in the concentration of trapped electrons. However, the increase in I3 obtained in a few MGy γ-irradiated samples became very small due to the effect of induced radicals. It has been observed that the trapped electrons were affected by the intensity of the positron sources used for a PALS experiment.

Radiation hardness of cerium-doped gadolinium silicate Gd2SiO5:Ce against high energy protons, fast and thermal neutrons

Abstract Degradation of Gd 2 SiO 5 :Ce in optical transmittance due to proton irradiation was negligibly small below 10 6 rad, smaller than 2%/cm at 10 7 rad and large at 10 8 rad. The radiation hardness of 10 7 rad against protons is by two orders of magnitude smaller than against low energy γ-rays. Long term spontaneous recovery of the proton-induced damage is not large (10–20% of the initial degradation in 84 days). Recovery upon exposure to UV light occurs to some extent. Degradation due to thermal neutrons was negligibly small for a fluence of 10 14 n/cm 2 . No degradation was observed for exposure to fast neutrons of about 10 13 n/cm 2 during one year in the extracted beam tunnel of proton synchrotron.

Radiation effect on polypropylene studied by the relaxational behaviour at low temperature using positron annihilation

Abstract The irradiation effect of γ-rays on polypropylene (PP) has been studied through relaxational behaviour, using the positron annihilation lifetime (PAL) method. At low temperature, the intensity of a long-lived component of positronium, I3, for unirradiated PP samples increased due to a termination of the local thermal motion of polymer structures. At 100 K, the increase in I3 of 1 MGy γ-ray irradiated samples became very small. However, the increase was observed again in the PP samples after 48 h of irradiation by positrons during the PAL experiment. This suggests a restructuring of the short polymer chains cut by γ-ray irradiation may occur.

Temperature and radiation effects on positronium formation

Abstract During positron annihilation lifetime spectroscopy (PALS) experiments of polymers, positrons emitted from the positron source induce radiation effects in the sample. Since these radiation effects can affect positronium (Ps) formation probability, PALS has some potential to be used as a new tool to study radiation effects. As a function of the contact time of the sample with the positron source, the Ps formation probability increases or decreases at low temperature or at room temperature, respectively. The decrease of the Ps formation probability at room temperature may be attributed to the accumulation of free radicals that can scavenge the electrons which could otherwise have combined with positron to form Ps. The increase at low temperatures may be attributed to the accumulation of trapped electrons that can be picked up by positrons to form Ps.

Effects of aging and oxidation on positronium formation in polyethylenes

Abstract Positron annihilation lifetimes (PAL) have been measured for polythylenes (PEs) under differenr conditions of heat treatment. It is shown that the intensity, I 3 , of the long-lived component of positronium (Ps) shows hystereses between the heating and cooling processes due to aging effect. It is also shown that about half of Ps observed at low temperatures below the glass transition is inhibited after the sample is heat-treated above the melting point in the atmosphere containing oxygen. It is suggested that carbonyl groups (C0) formed by thermal oxidation can inhibit Ps formation through trapping the precursors of Ps, e + and/or e - .

Positron annihilation and polymerization of epoxy resins

Abstract The polymerization of bisphenol-A epoxy resin mixed with a curing agent (methyltetrahydrophthalic anhydride) was followed from the liquid to the solid state using the positron annihilation lifetime (PAL) technique at several curing temperatures. With the commencement of polymerization, the intensity, I 3 , of ortho -positronium started to increase, followed by a sharp decrease of its lifetime, τ 3 . When the curing temperature was raised further, I 3 increased to a large value, indicating further propagation of the polymer networks. When the polymerization was terminated, τ 3 reached the smallest value and I 3 reached a large saturation value. PAL is a powerful technique for studying microscopic structural changes and state transitions of polymers.

Positron annihilation in polypropylene studied by lifetime and coincidence Doppler-broadening spectroscopy

Abstract The momentum density distributions (MDDs) of electrons taking part in the annihilation processes in polypropylene (PP) have been measured by coincidence Doppler-broadening spectroscopy. MDDs at the beginning of measurements to those at the saturation level of Ps formation have been compared in order to follow the possible changes in concentration of carbonyl groups (CG). A high initial CG concentration in PP has been observed, while for antioxidant-containing PP no significant presence of CG has been detected, and no changes have been observed during positron irradiation.