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SYNOPSISPositron lifetimes were measured for four kinds of polyethylene samples and were resolved into four components. The temperature dependence of the two longlived components was examined in detail. In agreement with other results, the longest lived component could be reasonably assigned to ortho-positronium located in amorphous regions. This component was shown to be sensitive to the defects in high-density polyethylene introduced in the course of its production. Both the intensity and the lifetime of the second longest lived component were structure insensitive, i.e., they did not change even on passing through the melting point. This component has been tentatively assigned to a positronium compound state. The effect of gamma-ray irradiation was also examined. Although the intensity (Z4) of the longest lived component was reduced by the irradiation, correlation between Z4 and the free radical concentration was poor, and the reduction in I4 caused by the irradiation is considered to be due to structure change and not to chemical reasons. Keywords: positron annihilation in polyethylene, before and after irradiation polyethylene, positron annihilation in, effect of radiation on 517

The experimental method of measuring neutron production double-differential cross sections was tested a t incident proton energies of 0.8 and 1.5GeV with C and Pb targets. Neutrons were measured with liquid scintillators by the time-of-flight method. The use of very weak secondary beam forced us to take a typical flight path length a s short a s l m. Preliminary experimental consideration led to the results that the amount of back ground neutrons were made low by the short flight path measurement, and a two-gate integration method was suited for the pulse shape discrimination between high energy neutrons and gamma-rays. The cross sections were obtained in the neutron energy range up to 100MeV with acceptable energy resolution. The experimental results obtained a t the incident proton energy of 0.8 GeV were consistent with the data taken with much longer flight paths. It was confirmed that the timeof-flight method with bare detectors a t the short flight path was reliably applicable to the spallation neutron measurement.

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SYNOPSISFour kinds of epoxy resins: cresol novolac, tris-hydroxyphenylmethane, tetramethylbiphenol, and bisphenol A, were cured with phenol novolac epoxy resins. Characteristics of these epoxy compounds were studied by the positron annihilation lifetime (PAL) technique. Glass transition temperatures, thermal expansion coefficients, and volume of intermolecularspace holes among polymer chains were obtained from the lifetime, T~, of the long-lived component of ortho-positronium. It was revealed that, at the glass transition temperature, Tg, the volume of the hole created among polymer chains expanded 1.4 times the volume at room temperature. The smaller flexural modulus of tris-hydroxyphenylmethane than that of the other samples was explained by the volume of intermolecular-space holes obtained from T~. Aging effects were seen in the data of the intensities, Z3, of ortho-positronium, which became smaller after heating the samples above T g . I3 and 7 3 were strongly affected by the density of cross-linkings and their chemical structures. The larger the density of cross-linkings, the smaller I3 and higher Tg were obtained. Epoxy compounds with the higher water-absorption rates had larger intermolecular-space holes. 0 1993 John Wiley & Sons. Inc.