Nuclear Instruments and Methods最新文献

The Argonne FN tandem accelerator and standard components of its experimental heavy-ion research facility have been used as a highly-sensitive mass spectrometer to detect several long-lived radioisotopes and measure their concentrations by counting accelerated ions. Background beams from isobaric nuclei have been eliminated by combining the dispersion from the energy loss in a uniform Al foil stack with the momentum resolution of an Enge split-pole magnetic spectrograph. Radioisotope concentrations in the following ranges have been measured: ¹⁴C/¹²C = 10⁻¹² to 10⁻¹³, ²⁶Al/²⁷Al = 10⁻¹⁰ to 10⁻¹², ³²Si/Si = 10⁻⁸ to 10⁻¹⁴, ³⁶Cl/Cl = 10⁻¹⁰ to 10⁻¹¹. Particular emphasis was placed on exploring to what extent the technique of identifying and counting individual ions in an accelerator beam can be conveniently used to determine nuclear quantities of interest when the measurement involves very low radioisotope concentrations. We are able to demonstrate the usefulness of this method by measuring the ²⁶Mg(p, n)²⁶ Al(7.2 × 10⁵ yr) cross section at proton energies in the astrophysically interesting range just above threshold, and by accurately determining the previously poorly known half-life of ³²Si.

A group is working at the Centre de Recherches Nucléaires in Strasbourg on the design of a compact superconducting cyclotron (K = 500). The main parameters of the accelerator required to cover the needs expressed by the physicists are presented and discussed. Three different modes of operation are considered at present: internal source, external high charge state source and upgraded MP tandem injection. Some recent results, obtained in Strasbourg for ion stripping and of interest in the injection problem are also presented.

Various beam-scanner systems are reviewed and some common problems, such as the need for gain-switched preamplifiers and the difficulty in obtaining a comprehensible and correctly phased display, are discussed. A system that overcomes these problems by means of logarithmic AGC preamplifiers and electronic trace splitting and phase locking is described.

A new heavy-ion accelerator facility is under construction at the Oak Ridge National Laboratory. A brief description of the scope and status of this project is presented with emphasis on the first operational experience with the 25 MV tandem accelerator.

The column structure of the XTU tandem Van de Graaff generator at Legnaro has been successfully tested up to 20.3 MV in +7.3 bar SF₆ gas. The laddertron charging system adopted has been able to deliver up to 400 μA charging current per chain.

We decided to use a laddertron at the Orsay MP tandem in place of the belt, the lifetime of which was too short. The new charging system was installed at the beginning of 1979 and, after various failures during the first months of operation, is now reliable. A short review of the accelerator's improvements is given, as well as the operating characteristics since the installation of the laddertron.

The main features of the six North American MP tandem Van de Graaff accelerators are summarized as well as some of their main performance characteristics. The latest accelerator-related studies, developments and improvements are described for each facility. Ongoing and proposed future developments at some of these laboratories are briefly discussed.

The fabrication and properties of ion implanted shallow layers <250 Å thick are outlined in the context of future devices where scaling down of device dimensions will require the formation of thin layers, if there is not to be a deterioration in device performance. It is shown, furthermore, that the high solubility of the common dopants that can be obtained using ion implantation enables high fields to be produced in silicon leading to abrupt changes in potential over interatomic distances. The application of these thin layers in hot electron devices leads to a number of novel device concepts including Schottky barrier height control, the formation of majority carrier diodes in the bulk of silicon, and the fabrication of monolithic hot electron transistors.

Surface acoustic waves (SAW) have been used in the present work to study changes of mechanical properties of surface layers during ion implantation. The SAW propagation velocity is measured during implantation as a function of implanted ion type and dose. The possibility of travelling SAW visualization to study the formation of macroscopic clusters in the implanted surface layers is demonstrated.

Channelling techniques are used to study the annealing of implanted diamonds and to determine the final lattice sites the implants occupy. Potential donor ions (Sb, P, Li) and Ge have been implanted into heated (1000–1100°C) natural diamond, and the remaining damage was studied by the RBS channelling technique. It was found that the diamond lattice remains nearly undamaged even after rather high dose implantations (≈ 10¹⁶ cm⁻²), the residual damage probably being due to extended defects. The lattice location of the implants was determined from channelling experiments where for each ion the most suitable signal characterizing a close encounter was chosen (RBS, PIXE and a nuclear reaction). Phosphorus and Ge are found to occupy partly substitutional sites, Li is partly interstitial while Sb is found to be mostly non-substitutional.