Reduction of background radiation effects for positron lifetime measurements in the slow positron beamline at the Kyoto University Research Reactor

Positron annihilation lifetime spectroscopy (PALS), which is recognized as one of the major analytical methods of positron annihilation spectroscopy, can directly detect information related to the size of vacancy-type defects from lifetime values. PALS measurements performed under high background radiation have been previously reported. It is well known that coincidence techniques such as age-momentum correlation (AMOC) measurements are effective for the background reduction, but count rates decline significantly. In this study, a preliminary experiment was performed to reduce the influence of the background radiation without the coincidence technique in the pulsing system of the Kyoto University research Reactor (KUR) slow positron beamline. This experiment involved the introduction of a gate circuit for the background radiation discrimination using a dynode signal from a single scintillation detector (photomultiplier). After introducing the gate circuit, the time resolution and the lifetime value of Kapton were 308 ps and 388 ± 3 ps, respectively, with count rates of ∼400 counts/s at a KUR 5 MW operation. In the AMOC measurement, the time resolution and the lifetime value of Kapton were 297 ps and 380 ± 7 ps, respectively, with count rates of ∼40 counts/s at a KUR 5 MW operation. When the single detector with the gate circuit was used, the count rate was ∼1 order of magnitude higher than those of the AMOC measurements, while the time resolutions of the two methods were comparable.

Digitized detection of gamma-ray signals concentrated in narrow time windows for transient positron annihilation lifetime spectroscopy

A pulsed slow-positron beam generated by an electron linear accelerator was directly used for positron annihilation lifetime spectroscopy without any positron storage devices. A waveform digitizer was introduced to simultaneously capture multiple gamma-ray signals originating from positron annihilation events during a single accelerator pulse. The positron pulse was chopped and bunched with the chopper signals also sent to the waveform digitizer. Time differences between the annihilation gamma-ray and chopper peaks were calculated and accumulated as lifetime spectra in a computer. The developed technique indicated that positron annihilation lifetime spectroscopy can be performed in a 20 μs time window at a pulse repetition rate synchronous with the linear accelerator. Lifetime spectra of a Kapton sheet and a thermally grown SiO2 layer on Si were successfully measured. Synchronization of positron lifetime measurements with pulsed ion irradiation was demonstrated by this technique.

Positron annihilation in cardo-based polymer membranes

Positron annihilation lifetime spectroscopy (PALS) is applied to a series of bis(aniline)fluorene and bis(xylidine)fluorene-based cardo polyimide and bis(phenol)fluorene-based polysulfone membranes. It was found that favorable amounts of positronium (Ps, the positron-electron bound state) form in cardo polyimides with the 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) moiety and bis(phenol)fluorene-based cardo polysulfone, but no Ps forms in most of the polyimides with pyromellitic dianhydride (PMDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BTDA) moieties. A bis(xylidine)fluorene-based polyimide membrane containing PMDA and BTDA moieties exhibits a little Ps formation but the ortho-positronium (o-Ps, the triplet state of Ps) lifetime of this membrane anomalously shortens with increasing temperature, which we attribute to chemical reaction of o-Ps. Correlation between the hole size (V(h)) deduced from the o-Ps lifetime and diffusion coefficients of O2 and N2 for polyimides with the 6FDA moiety and cardo polysulfone showing favorable Ps formation is discussed based on free volume theory of gas diffusion. It is suggested that o-Ps has a strong tendency to probe larger holes in rigid chain polymers with wide hole size distributions such as those containing cardo moieties, resulting in deviations from the previously reported correlations for common polymers such as polystyrene, polycarbonate, polysulfone, and so forth.

Simulations of slow positron production using a low-energy electron accelerator

Monte Carlo simulations of slow positron production via energetic electron interaction with a solid target have been performed. The aim of the simulations was to determine the expected slow positron beam intensity from a low-energy, high-current electron accelerator. By simulating (a) the fast positron production from a tantalum electron-positron converter and (b) the positron depth deposition profile in a tungsten moderator, the slow positron production probability per incident electron was estimated. Normalizing the calculated result to the measured slow positron yield at the present AIST linear accelerator, the expected slow positron yield as a function of energy was determined. For an electron beam energy of 5 MeV (10 MeV) and current 240 μA (30 μA), production of a slow positron beam of intensity 5 × 10(6) s(-1) is predicted. The simulation also calculates the average energy deposited in the converter per electron, allowing an estimate of the beam heating at a given electron energy and current. For low-energy, high-current operation the maximum obtainable positron beam intensity will be limited by this beam heating.

Oral administration of tritiated water (HTO) in mouse. II. Tumour development

Previously we reported haematopoietic death as an effect of tritiated water (HTO) in drinking water in the concentration range from 5.92 x 10(11) to 1.85 x 10(10) Bq/dm3. In the present study the effects of HTO in a lower concentration range from 9.25 x 10(9) Bq/dm3 (0.240 Gy/day) to 3.70 x 10(8) Bq/dm3 (0.096 Gy/day) are reported. Female (C57BL/6N and C3H/He)F1 mice were maintained on drinking water containing various levels of HTO. Mice survived for > 150 days with a high incidence of tumour development (70 to 80%). In the dose-rate range from 9.25 x 10(9) Bq/dm3 (0.240 Gy/day) to 1.85 x 10(9) Bq/dm3 (0.048 Gy/day) the main cause of death was thymic lymphoma. However, at a dose-rate of 9.25 x 10(8) Bq/dm3 (0.024 Gy/day) the incidence of thymic lymphoma sharply decreased, while the incidence of other tumours increased. The tumour type became more diverse at lower concentrations of HTO. The latent period of tumour development was shorter and the life-shortening effect was more marked by 3H beta-irradiation in this study than b X- or gamma-irradiation reported in other investigations.

Micro-PIXE (particle induced X-ray emission) analysis of aluminum in rat-liver using MeV heavy ion microprobes

Heavy ion microprobes (HIM) such as 3 MeV Si2+ and 3 MeV p2+ have been applied to the elemental analysis by PIXE (proton-induced X-ray emission). It was found that silicon and phosphorus microprobes have several times higher sensitivity for aluminum K alpha X-rays than 2 MeV proton microprobes, and detection limits were more favorable in a phosphorus microprobe. Using a 3 MeV P2+ microprobe, the liver of a rat, which had been injected with aluminum-lactate, was investigated and it was found that aluminum segregates in areas with a dimension of about 10 microns. These areas could hardly be observed with 2 MeV proton microprobes.

HTO oral administration in mice. I: Threshold dose rate for haematopoietic death

Tritiated water in various concentrations was orally administered continuously to (C57BL/6N and C3H/He)F1 female mice in a closed animal chamber. Tritium radioactivity in various organ tissues was measured periodically after initiating tritiated water intake using an automatic sample combustion system and a liquid scintillation counter. After 7 days the specific radioactivity reached a plateau. These data allowed absorbed organ doses to be estimated. Within a range of 1.48 x 10(11) to 5.92 x 10(11) Bq/dm3 as the concentration of tritiated water in drinking water, the time of death after initiating the administration was about 2 weeks, a typical time for haematopoietic death. A linear relationship of times of death with tritiated water concentrations in drinking water was observed, on a log-log scale, between 1.85 x 10(10) Bq/dm3 and 1.48 x 10(11) Bq/dm3. At concentrations lower than 9.25 x 10(9) Bq/dm3, mice no longer died from haematopoietic failure. We conclude, therefore, that there should be a threshold dose rate for haematopoietic death.

Chronic toxicity of 2-(2-furyl)-3-(5-nitro-2-furyl) acrylamide (AF-2) in mice, with special reference to carcinogenicity in the forestomach

Chronic toxicity of 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2), which had been widely used in Japan as a food additive, was studied in both sexes of ICR/JCL mice by feeding a diet containing AF-2 in a concentration of 0.4% or 0.08%. The experiments confirmed its definite carcinogenicity in the forestomach, with dose dependency; more than 70% of mice fed the higher dose developed tumors, the majority of which were squamous cell carcinomas with metastatic growths, whereas, with a lower dose, fewer tumors occurred, following a longer latent period. In addition, possible leukemogenicity was suggested by a higher incidence in AF-2-fed females (10.8%) over that in control females (3.2%). The development of non-neoplastic lesions in the liver and kidneys, and of amyloidosis in various organs was also observed in some of AF-2-fed mice.