Masaharu Numajiri

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 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.

Free-volume characteristics and water absorption of novolac epoxy resins investigated by positron annihilation

A positron-annihilation lifetime technique was applied to four kinds of epoxy resin (cresol novolac, trishydroxy-phenyl-methane (THPM), tetra-methyl-biphenol and bis-phenol A) in order to study the relation between the intermolecular-space holes and water absorption. The sizes of the holes were measured, based on the life-time (τ3) of the ortho-positronium. Since THPM has a larger τ3 value (hence, a larger intermolecular-space hole) and absorption rate than that of the other samples, a correlation between τ3 and the absorption rate has been shown to be evident. Assuming a very simplified model, the shapes of the holes for the smallest unit cell were calculated using a computer code of molecular dynamics; the shape for THPM was found to be different from those of the other three samples.

Proton irradiation on p-bulk silicon strip detectors using 12 GeV PS at KEK

Abstract P-bulk n-strip silicon strip detectors were irradiated with a 12 GeV proton beam at the KEK Proton Synchrotron in order to investigate a radiation damage due to high fluence of high energy protons. Primary 12 GeV protons extracted at the EP1-A beam line was used for the irradiation. The detectors were irradiated with the fluences of 1.1 × 10 14 and 4.3 × 10 13 protons/cm 2 for the high and low fluence exposures, respectively. Bias voltage for achieving the full depletion of the irradiated p-bulk detectors was observed to be significantly higher than that for the n-bulk detectors. The full depletion voltage did not increase monotonically as the fluence increased; it showed little variation up to about 5 × 10 13 p/cm 2 and then started to increase. The behaviour could be explained by assuming a contribution from three processes: effective acceptor creation, persistent acceptor component, and acceptor removal.

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.

Structure dependence of gamma-ray irradiation effects on polyethylenes studied by positron annihilation

Abstract The irradiation effects on five kinds of polyethylenes (PEs) were studied from the viewpoints of mechanical properties, free radicals and positron annihilation. The degradation of the mechanical strength of samples irradiated by 60Co γ-rays was evaluated based on the elongation at break and the tensile strength. Fully stretched PE with a crystallinity larger than 99% was greatly affected by γ-irradiation. In low-density PEs free radicals were detectable only above 70 kGy, and sharply increased above 100 kGy. A detectable number of free radicals was obtained even at 0.1 kGy in PEs haaving high crystallinity. At around 100 kGy, the increase in the intensity (I3) of ortho-positronium (o-Ps), seems to be associated with an increase in the mechanical strength. Contrary to the expectation that the o-Ps seeks intermolecular space holes only in amorphous regions, the I3 obtained in PEs with nearly 100% crystallinity was almost half that low-density PEs, and τ3 had similar values. This suggests that even in crystalline regions the o-Ps seeks intermolecular space holes comparable to those in the amorphous regions.

Subthreshold antiproton production in pA, dA and αA reactions

Abstract An enormous enhancement of antiproton production in deuteron- and α-induced reactions has been observed in the subthreshold energy region between 2 and 5 GeV/nucleon. Antiprotons produced at 5.1° with a momentum range of between 1.0 and 2.5 GeV/ c were measured by a beam-line spectrometer and identified by the time-of-flight method. The production cross sections in the deuteron- and α-induced reactions at an incident energy of 3.5 GeV/nucleon were 2 and 3 orders of magnitude larger than those in proton-induced reaction at the same energy. The enhancement in light-ion reactions could not be explained by the internal motion in the projectile and target nuclei. The target-mass dependence (C, Al, Cu and Pb) of the cross sections has also been studied. Further, the cross sections of π and K productions were measured.

Radiation hardness of undoped CsI crystals against high energy protons

Abstract We compared radiation damages of undoped CsI for 12 GeV protons and for 60Co γ-rays between 104 and 108 rad. Degradation in optical transmission was similar in magnitude both for protons and γ-rays; it is as small as 1–3% per 1 cm thickness at 104 rad, while it is as large as or larger than 5% above 105 rad. The above feature is different for BGO or GSO:Ce, whose radiation hardness is roughly two orders of magnitude smaller regarding protons than regarding γ-rays. Recovery of damage is qualitatively similar both for proton and γ-ray irradiations; spontaneous recovery is only partial and the annealing effect of UV exposure is negligibly small.