Yasuyuki Nagashima

A new positron lifetime spectrometer using a fast digital oscilloscope and BaF2 scintillators

Abstract A new positron lifetime spectrometer has been constructed and tested. It consists of a state-of-the-art digital oscilloscope (LeCroy Wavepro 960) and scintillation detectors based on Hamamatsu H3378 (R2083Q) photomultiplier tubes and BaF 2 scintillators. A very good time resolution is obtained with scintillators of commonly used sizes (28xa0mm in diameter, 10–20xa0mm thick) and wide energy window settings (40–50%). The time resolution of 144xa0ps full-width at half-maximum (FWHM) is achieved for the positron lifetime measurement, in which time differences are measured between the arrivals of a 1.27xa0MeV γ-ray from 22 Na and one of the two positron annihilation γ-rays of 0.511xa0MeV. The time resolution is further improved with a ‘double-stop’ setup, where timing information carried by the positron annihilation γ-ray pair is combined to determine the time of the annihilation. The time resolution with this setup is 119xa0ps FWHM. This improvement is interpreted in terms of the energy deposit in the scintillation detectors by γ-rays. Applications of the new spectrometer to other scintillation timing measurements are also described.

MOMENTUM-TRANSFER CROSS SECTION FOR SLOW POSITRONIUM-HE SCATTERING

The momentum-transfer cross section for positronium-He scattering has been determined by analysing the thermalization process of orthopositronium in the gas. The momentum distribution of the positronium is observed by using the one-dimensional angular correlation of the annihilation radiation method. Silica aerogel is used to form a sufficient amount of positronium in a restricted region necessary for the high-resolution measurements. The average energies of orthopositronium for mean lifetimes ranging from 3.2 to 86 ns are determined by applying static magnetic fields. A thermalization model which includes the momentum-transfer cross section as an adjustable parameter is fitted to the average energy. The cross section thus obtained is for positronium in the energy range below 0.3 eV.

A high-efficiency positron moderator using electro-polished tungsten meshes

Abstract Positron moderators made with stacks of electro-polished tungsten meshes have been tested. The wires of the meshes were thinned to about 10xa0μm in diameter by an electro-polishing treatment. The moderation efficiency was measured to be 7.5×10−4 for 12 overlapping meshes.

Intense positron beam at KEK

Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e + /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

A new two-dimensional angular correlation of annihilation radiation apparatus using position-sensitive photomultiplier tubes

Abstract A new two-dimensional angular correlation of annihilation radiation apparatus is described. Position-sensitive photomultiplier tubes coupled with two-dimensional arrays of small BGO scintillator blocks make simple and compact position-sensitive γ-ray detectors. With a sample-detector distance of 5 m, an angular resolution of 1.1 mrad FWHM and a coincidence count rate of ∼2.4 cps per mCi are obtained. Its performance is demonstrated by the result of a test measurement for KI crystal in which non-localized positronium exists at low temperatures.

Momentum-transfer cross sections for slow positronium–gas collisions

The momentum-transfer cross sections for positronium (Ps)–gas molecule scattering in the low energy region are determined by analysing the thermalization process of ortho-positronium (o-Ps) in various gases. The momentum distributions of Ps in Ne, H2, CH4, N2, C2H4, n-C4H10, i-C5H12 and neopentane are measured as functions of mean lifetime, by using the one-dimensional angular correlation of annihilation radiation method. The average energies obtained for o-Ps with mean lifetimes of 3–90 ns are analysed, to yield the momentum-transfer cross sections for the Ps–gas molecule scattering. The cross sections thus determined are σm = (20 ± 8) × 10−16 cm2 for Ne, (16 ± 3) × 10−16 cm2 for H2, (17 ± 6) × 10−16 cm2 for CH4, (37 ± 10) × 10−16 cm2 for N2, (35 ± 8) × 10−16 cm2 for C2H4, (100 ± 30) × 10−16 cm2 for n-C4H10, (110 ± 50) × 10−16 cm2 for i-C5H12 and (180 ± 60) × 10−16 cm2 for neopentane, in the Ps energy range below 0.3 eV. The Ps is thermalized mainly through elastic scattering with diatomic and polyatomic molecules, as in the case of scattering with inert gases.

A position-sensitive scintillation detector for two-dimensional angular correlation of annihilation radiation using metal-package position-sensitive photomultiplier tubes

Abstract We have constructed and tested a prototype of a new position sensitive γ-ray detector which consists of an array of 2.6×2.6×18xa0mm 3 BGO scintillator blocks, a light guide, and four metal-package position-sensitive photomultiplier tubes (R5900-00-C8) recently developed by Hamamatsu Photonics Co. Ltd. Scalability of the detector of this type makes it possible to construct a larger detector using many PS-PMTs, which will be useful for the two-dimensional angular correlation of annihilation radiation apparatus.

Measurement of positron spin polarization by using the Doppler broadening method

Abstract A new positron spin polarimeter based on the measurement of the Doppler broadening of annihilation radiation is described. The polarization is determined from the magnetic field dependence of the S-parameter of the Doppler broadening spectra for amorphous SiO2. In order to examine the sensitivity of this polarimeter, the spin polarization of the positrons from a 22Na source is measured and compared with the value theoretically expected. The result is also compared with the polarization measured by using the angular correlation of annihilation radiation method.

Spot 18F positron source electro-deposited on a graphite rod

Abstract 2.2xa0GBq of 18F (half-life 110xa0min) was effectively deposited on a graphite electrode of diameter 3 or 5xa0mm. The fraction of the electro-deposited 18F was as high as 97% of the total 18F produced in the solution. The fraction of the positrons emitted out of the electrode was 44% of the total β+. Thus, as much as 36% of the β+ from the produced 18F is available for the source of slow positron beams.

Irradiation-induced voids in alumina single crystal studied by positron annihilation

Abstract Angular correlation of annihilation radiation (ACAR) has been measured on α-alumina single crystals irradiated with fast neutrons to a dose of 3 × 10 24 n/m 2 at about 470 K. After post-irradiation annealing above 900 K, remarkable narrowing in ACAR curves has been observed. These ACAR curves can be decomposed into three Gaussian components: a narrow (N), an intermediate (M) and a broad (B) component, with FWHM of 2–5, about 6 and 11 mrad, respectively. The N and M components are attributed to positrons trapped in the voids. A magnetic quenching effect is observed for the N component but not for the M component. This shows that the N component is due to two photon self-annihilation of positronium (Ps) formed in voids. The FWHM of the N component indicates, through the momentum of zero-point motion of Ps, that the average void diameter is 0.6 nm after annealing at 1000 K and 1.7 nm after annealing at 1525 K Striking similarity of the M component to a void-surface trapping component in metallic Al, together with nearly the same lifetime as that of voids in Al, suggests the existence of metallic surface-conduction-electron states in the voids in α-alumina.