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Effects of gamma-ray irradiation on high density (HDPE) and low density (LDPE) polyethylenes were measured by positron annihilation at temperatures between 100 and 420 K. The effect of the irradiation on the intensity I3 and the lifetime τ3 of the longest lived component was significant below the glass transition temperature Tg, while they were little affected above Tg. For LDPE a marked minimum was observed in the I3 vs T curve. The lower edge of the minimum, corresponding to glass II and glass I transitions, was found to shift to lower temperatures by the irradiation. In HDPE both I3 and τ3 decreased on irradiation mainly due to radiation induced free radicals. The gel fraction of HDPE was small when irradiated below Tm, while it rose sharply on irradiation near Tm. Positron lifetime parameters of this highly crosslinked HDPE showed a distinct difference compared to HDPE irradiated below Tm. Usefulness of the positron annihilation method is discussed in conjunction with studying micro-structure of polymers.

The anti-inhibition effects of positronium (Ps) formation in CH2Cl2 and CCl4 solutions of c-C6H12 were investigated for all fluorinated benzenes by measuring positron lifetime spectra. The relative capabilities of anti-inhibition for the fluorinated benzenes at a low concentration of 0.01 volume fraction in both CH2Cl2(0.2 M)/c-hexane and CCl4(0.1 M)/c-hexane solutions were compared with their Stoke's losses, and a very similar trend for both anti-inhibition effects and Stoke's losses has been found. It strongly suggests that the anti-inhibition effects of the fluorinated benzenes are mainly correlated to their excited states. At relatively higher concentration regions, a negative influence on Ps formation caused by molecular polarities has been found, too. Discussion based on the spur reaction mechanism is presented.

Positronium (Ps) yields were measured for CS2/THF, CS2/TEA, C6F6 THF and C6F6/TEA mixtures. Addition of C6F6 or CS2 into TEA or THF showed a similar influence which induces a decrease of Ps yield. This is ascribed to the non-dissociative electron scavenging by both CS2 and C6F6. On the other hand, the effect of addition of TEA and THF has been found to be different for C6F6 and CS2, i.e. both TEA and THF did not change Ps yield in CS2 substantially, while they caused a sharp decrease of Ps yield in C6F6. The latter sharp decreases was ascribed to quenching of excited state of C6F6. This interpretation is supported by pulse radiolysis emission measurements for the same system, in which it was found that TEA quenches the excited state of C6F6 efficiently.

Anti-inhibition effect on Ps formation has been found for some chorinated benzenes (BCI x ) in CCl 4 /c-C 6 H 12 solutions. The relative capabilities of the anti-inhibition effect of BCI x (except for those without the anti-inhibition effect) in CCl 4 (0.1M)/c-C 6 H 12 were compared with the previously reported results for flourinated benzenes, and a quite similar trend for both systems has been observed. The effects of the chlorinated benzenes on Ps formation in nonpolar molecular substances is summarized based on a new Ps formation mechanism—“energy compensation and resonant formation”—in molecular materials.

The effects of CH2Cl2 on positronium yields in c-C6H12, C6H12-C6F6(10 vol.%), C6H12-C6H5F(10 vol.%) and C6H12-CS2(10 vol.%) solutions with the concentration of CH2Cl2 being changed from 0 to 0.4 M were measured by positron lifetime spectra. It was found that the electron scavenger (CH2Cl2) can exert a small influence on the Ps yield in the C6F6 solution. In the C6H5F solution, the Ps yield was effected a lot by CH2Cl2, but C6H5F still can show a certain anti-inhibition effect even when the concentration of Ch2Cl2 was quite high (0.36M). In the CS2 solution, an additional inhibition effect on Ps yield was found. A discussion is made based on the spur reaction mechanism. The additional inhibition effect in the CS2 solution is ascribed to both direct electron scavenging and CS-2 anion scavenging (indirect electron scavenging or electron re-scavenging) by CH2Cl2. On the other hand the anti-inhibition effect of C6F6 and C6H5F is ascribed mainly to the exciting energy transfer which results in the formation of intermediate excited states (C6F6∗·e+) and (C6H5F∗·e+), which are a kind of Ps precursor and cannot be efficeintly quenched by CH2Cl2.

Positronium formation process has been simulated by Monte-Carlo method within the framework of the spur reaction model for various number of ion pairs and for different initial spur geometries. When only three charged particles (e+, e− and M+) are assumed, the most probable size of the positron spur appears to be 60–80 A. Electron scavenger effects have also been simulated, and it has been shown that the inhibition effect becomes less efficient when there are many ion pairs in the spur in accord with our previous observation.

Positrinium yield ( I 3 ) in binary mixturesof benzene derivatives( p -difluorobenzene 1,2,4,5-tetrafluorobenzene, pentafluorobenzene, perfluorobenzene and p -xylene) in n-hexane has been measured. As additives all of the aromatic compounds have been found to enhance Ps yield in n-hexane, but the effect was unexpectedly smaller for 1,2,4,5-C 6 H 2 F 4 . It is proposed that there are at least three processes which lead to the enhancement: anti-recombination by scavenging excess electrons, anti-recombination by scavenging positive holes, and electron abstraction from excited molecular states. Addition of a small amount of n-hexane in the fluorinated benzenes resultedin a decrease in the Ps yield to a different degree which strongly depended on the number of flourine atoms substituted. Discussion is made on the Ps formation mechanism taking into account possible role of excited molecular states.

Three different techniques of muon spin science, μ + SR, longitudinal μ + spin decoupling and μ + spin resonance, have been applied in search for the origin of the occurence of the “lost polarization,” which is the muon spin polarization unobservable either as coherent diamagnetic muon, muonium and muonium-substituted radicals. The results indicate that the lost polarization involves fast reactions of muonium on the time scale faster that about 1 ns by, most probably, spin exchange reactions, but some contribution from chemical reaction of muonium also seems probable. The μ + spin decoupling experiment has also revealed that there is a small amount (∼10%) of thermal muonium atoms in CCl 4 .

Positronium yield and emission spectra in C 6 F 6 / c -C 6 H 12 mixtures have been measured. An emission band in the range of 300–450 nm with the maximum at 370 nm and the lifetime of about 3 ns has been observed. This is ascribed to the lowest singlet excited state of C 6 F 6 . The dependencies of Ps yield and emission intensity on C 6 F 6 concentration were very similar to each other. Possible effect of excited states of C 6 F 6 on Ps formation is emphasized.

Single-photon counting techniques were applied to measure the time profiles of fluorescence induced by high-LET (He 2+ - and α particles) and low-LET (γ-ray) irradiation in cyclohexane and benzene containing 10 mM 2,5-diphenyloxazole as a fluorescent molecule. In cyclohexane, the lifetime of the fast decay-component was shorter for α-irradiation than that for γ-irradiation, but no significant differences were observed in the shape of the slow components. In benzene, on the other hand, the shape of the fast-decay component was almost the same for α- and γ-induced fluorescences, but there was no slow component for the latter. Electron scavengers (ethylbromide, chloroform and carbontetrachloride) suppressed the fluorescence intensity, and the effect in cyclohexane was found to follow the Warman-Asmus-Schuler's equation and the efficiency, described in terms of the reactivity parameter, was approximately the same for α- and γ-induced fluorescences. All these results have been appropriately interpreted considering overwhelming charge-transfer (for cyclohexane) or excitation-transfer (for benzene) mechanism of the fluorescence with an assumption that the high-LET induced fluorescence takes place mainly in the spurs formed by δ-rays.