Automatic F positron source supply system for a monoenergetic positron beam

A system which supplies an intense ¹⁸F (half life 110 min) positron source produced by an AVF cyclotron through ¹⁸O(p,n)¹⁸F reaction has been constructed. Produced ¹⁸F is transferred to a low background experiment hall through a capillary. It is electro-deposited on a graphite rod and used for a source of a slow positron beam. In the meantime the next batch of target ¹⁸O water is loaded and proton irradiation proceeds. This system makes it possible to perform continuous positron beam experiments using the ¹⁸F positron source.     

High energy proton beam bombardment of polyaniline

Irradiation effects of 35 MeV proton beam bombardment on polyaniline (PAn) have been studied. PAn was made by standard MacDiarmid method, and the irradiation was performed at ambient temperature lower than 20°C. A 511 keV γ photon was detected by γ spectrum analysis, and a nuclear reaction ¹⁴N(p,α)¹¹C in collaboration of ¹²C(p,pn)¹¹C took place. By means of chemical ionization, mass spectra of samples before and after ion bombardment were recorded. Experimental results showed that products such as

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and cyclic compounds like

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and

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were produced. IR spectra and SEM micrographs gave results of vibrational frequency shifts and morphology consistent with mass spectrometry. Finally we conclude that high energy proton beam bombardment resulted in main-chain scission and carbonization of PAn. Although the electrical conductivity reduced from 0.72 to 0.62 S cm^-1, it retained at the same level. This implies that PAn might be used under the influence of high energy radiation.     

Characterization of polymer sub-surface using slow positron beam

A new pulsed mono-energetic slow positron beam as well as the conventional positron annihilation lifetime spectroscopy (PALS) have been applied to study the sub-surface and the bulk of epoxy polymer. Significant changes of o-Ps parameters were found at a short distance from the surface. The lifetime of o-Ps was observed to decrease with increasing the positron implantation depth, while its intensity increased. The temperature effect on o-Ps parameters at sub-surface was also investigated. The glass transition temperature for the sub-surface was lower than that for the bulk. Furthermore, the thermal expansion coefficient of the sub-surface was found smaller than that of the bulk.     

The design of a spin-polarized slow positron beam

Spin-polarized low-energy positrons were considered as useful probes for studying electron spin states of both surface and bulk materials. Due to the spin-dependent interactions between electrons and positrons, the formation of positronium (Ps), an electron-positron bound system, can be distinguished from different electron spin states. Recently, a positron source of ¹⁸F has been developed for a spin polarized slow positron beam at the institute of physical and chemical research (RIKEN). The design of an electrostatic positron beam will be discussed in conjunction with a spin rotator.     

A bright, slow cryogenic molecular beam source for free radicals

We demonstrate and characterize a cryogenic buffer gas-cooled molecular beam source capable of producing bright beams of free radicals and refractory species. Details of the beam properties (brightness, forward velocity distribution, transverse velocity spread, rotational and vibrational temperatures) are measured under varying conditions for the molecular species SrF. Under typical conditions we produce a beam of brightness 1.2 × 10(11) molecules/sr/pulse in the X(2)Σ(+)(v = 0, N(rot) = 0) state, with 140(m/s) forward velocity and a rotational temperature of ≈ 1 K. This source compares favorably to other methods for producing beams of free radicals and refractory species for many types of experiments. We provide details of construction that may be helpful for others attempting to use this method.     

Characterization of the surface layer of LB-films using a slow positron beam

Langmuir–Blodgett films were studied using a variable energy slow-positron beam. We measured the energy spectra of positron annihilation radiation for Cd and Mg eicosanoid films and obtained the V- and S-parameters as a function of the incident positron energy, E. In the V-E curves of Cd eicosanoid films, there were dips at the positron energy whose mean implantation depth corresponding to the first and second Cd²⁺ layers from the surface. These dips are interpreted as the result of inhibition of Ps formation by the Cd²⁺ ions. The S-parameter was found to be sensitive to chemical composition of the film and also to possible structural change due to heat treatment. Our results suggest that positron beams provide valuable information about the microstructure of the Langmuir-Blodgett films.     

Production and application of pulsed slow positron beam using an electron linac

Slow positron beam is quite useful for non-destructive material research. At the Electrotechnical Laboratory (ETL), an intense slow positron beam line has been constructed by exploiting an electron linac in order to carry out various experiments on material analysis. The beam line can generate pulsed positron beams of variable energy and variable pulse period. Various capabilities of the intense pulsed positron beam is presented, based on the experience at the ETL, and the prospect for the future is discussed.     

Tunable pores in mesoporous silica films studied using a pulsed slow positron beam

Positron annihilation lifetime spectroscopy (PALS) based on a pulsed slow positron beam was applied to study mesoporous silica films, synthesized using amphiphilic PEO–PPO–PEO triblock copolymers as structure-directing agents. The pore size depends on the loading of different templates. Larger pores were formed in silica films templated by copolymers with higher molecular-weights. Using 2-dimensional PALS, open porosity of silica films was also found to be influenced by the molecular-weight as well as the ratio of hydrophobic PPO moiety of the templates.     

Study on surface structure of plasma‐treated polydimethylsiloxane (PDMS) elastomer by slow positron beam

In this paper, the variations in surface structure of polydimethylsiloxane elastomers before and after argon plasma treatments have been investigated by X‐ray photoelectron spectroscopy, slow positron beam, and scanning electron microscope. An inorganic silica‐like layer was probed by X‐ray photoelectron spectroscopy after 3 minutes or longer time of treatments, and the sample surface turned into totally hydrophilic. Short time (1 and 2 min) plasma exposure mainly removed preexisting low molecular weighted (LMW) siloxanes on sample surface. By using slow positron beam, the thicknesses of silica‐like layer for 3‐, 5‐, and 10‐minute–treated samples were estimated to be around 30, 66, and 91 nm, respectively. Beneath the silica‐like layer, a loose polymeric structure was also detected, which was ascribed to the accumulation of LMW siloxanes. Scanning electron microscope images showed that the silica‐like layer cracked after 10 minutes of plasma treatment, which provided direct diffusion pathways for LMW siloxanes. Hence, 10‐minute–treated sample showed rather low organic composition near surface. Slow positron beam provides valuable depth profile information for evaluating the surface aging condition of polydimethylsiloxane composite.     

Search for molecular effects in range corrections: Lithium determination by proton bombardment

Three different proton-induced reactions were used to analyze 19 pure lithium compounds of known composition. Prompt alpha-particles were measured from the reaction⁷Li(p, )⁴He at E_p=4.5 MeV and prompt gamma-rays of 429 and 479 keV from the ractions⁷Li(p,n)⁷Be and⁷Li(p,p)⁷Li, respectively, at E_p=4.5 MeV. Elemental stopping powers were calculated from tables and used to compute the stopping power of the target matrices by Bragg's Law. Apparent discrepancies in the measured yield could point to deviations from Bragg's Law and hence to molecular effects. The maximum value for any molecular effect was found to be <5.5%.     

Effect of radiation damage on luminescence of erbium-implanted SiO2/Si studied by slow positron beam

The effect of damage on 1.54 μm luminescence for 30 keV-Er-implanted SiO₂ films has been studied by positron annihilation and cathodoluminescence. It was found that S-parameter in the films decreased after implantation, indicating the suppression of positronium formation. The luminescence appeared with the recovery of the S-parameter after 600°C annealing. The intensity reached a maximum at 900°C annealing whereas the S-parameter did not change significantly. It seems that most damages recover at 600°C and thereafter Er ions transform to an optically active state at 900°C.     

18F-labeling techniques for positron emission tomography

Much research effort has been made to understand various biological processes at levels of molecules using molecular imaging techniques.Because of great sensitivity,high resolution,and rapid detection,positron emission tomography(PET)imaging is becoming one of the most used imaging techniques for medical diagnose and pre-clinical studies.Here we provide a review on molecular imaging and PET imaging.An introduction is also provided on18F-fluorine labeling techniques for the preparation of PET imaging probes.A summary and comparison of currently available18F-fluorine labeling methods is provided.The perspectives for18F-fluorine labeling techniques are also given.     

Contamination-free separation of stable nuclides produced in an iron target by proton bombardment

An application of the temperature gradient tube method is described, which has allowed to chemically separate the elements, Be, Ca, Sc, Ti, V, Cr, Mn produced in an iron target irradiated by high energy protons, without noticeable contamination by natural impurities. It is suggested that this method could be useful in other works on small quantities of stable isotopes.     

Analysis of peptides by means of a single focusing mass spectrometer equipped with a molecular beam solid analysis/fast atom bombardment source

The use of a molecular beam solid analysis/fast atom bombardment source in conjunction with a single focusing mass spectrometer of limited specification is described for the identification of peptides. This ionization method is shown to have significant advantages for such analyses and the performance of the mass spectrometer is shown to be adequate for this work.     

Production of 7Be from proton bombardment of light elements

The formation cross sections of ⁷Be from proton bombardment of natural Be, B, N and O have been measured as a function of E_p up to E_p = 52 MeV.     

Synthesis of 11C, 18F, 15O, and 13N radiolabels for positron emission tomography

Positron emission tomography (PET) is a powerful and rapidly developing area of molecular imaging that is used to study and visualize human physiology by the detection of positron-emitting radiopharmaceuticals. Information about metabolism, receptor/enzyme function, and biochemical mechanisms in living tissue can be obtained directly from PET experiments. Unlike magnetic resonance imaging (MRI) or computerized tomography (CT), which mainly provide detailed anatomical images, PET can measure chemical changes that occur before macroscopic anatomical signs of a disease are observed. PET is emerging as a revolutionary method for measuring body function and tailoring disease treatment in living subjects. The development of synthetic strategies for the synthesis of new positron-emitting molecules is, however, not trivial. This Review highlights key aspects of the synthesis of PET radiotracers with the short-lived positron-emitting radionuclides (11)C, (18)F, (15)O, and (13)N, with emphasis on the most recent strategies.