G. D. Alton’s research while affiliated with Oak Ridge National Laboratory and other places

The advent of the fast-valve device, described previously, permits measurement of molecular-flow times of chemically active or inactive gaseous species through radioactive ion beam (RIB) target–ion–source systems, independent of size, geometry and materials of construction. Thus, decay losses of short-half-life RIBs can be determined for a given target/vapor-transport system in advance of on-line operation, thereby ascertaining the feasibility of the system design for successful processing of a given isotope. In this article, molecular-flow-time theory and experimentally measured molecular-flow time data are given for serial- and parallel-coupled Ta metal RIB vapor-transport systems similar to those used at ISOL based RIB facilities. In addition, the effect of source type on the molecular-flow time properties of a given system is addressed, and a chemical passivation method for negating surface adsorption enthalpies for chemically active gaseous species on Ta surfaces is demonstrated.

Isochronal anneal studies have been made on n- and p-type germanium bombarded with germanium and several substitutional impurities, using the four-point and thermoelectric probes as means of indicating changes in sheet resistivity and majority carrier type. Electrical properties of diodes made from type converted samples were also determined. All bombardments were performed at 40 keV in a 24-in.-radius electromagnetic isotope separator at room temperature. These studies revealed the following. (1) Heavy-particle bombardment of germanium introduces acceptor states regardless of the identity of the bombarding species. (2) Annealing curves for n-type germanium indicate a two-stage process; details of the annealing process are dependent on the ion species used and the crystal orientation bombarded. (3) There is a crystallographic and mass dependence on the amount of change in sheet resistivity produced in n-type germanium. (4) Type conversion temperatures in p-type germanium were observed to occur at 450–475 °C for phosphorus, ~500 °C for arsenic, and slightly greater than 500 °C for antimony, suggesting that the ease of moving a particular species into an active lattice position is related to its atomic size. (5) Voltage–current and capacitance measurements indicate that large-area diodes of relatively good properties and reproducibility can be made by ion implantation in germanium.

Releases of short-lived species from ISOL targets are simulated with computer codes. Analytic solutions to the diffusion equation are compared with those obtained from a finite-difference code for radioactive isotope diffusion release from simple geometry targets. The Monte Carlo technique as a practical means for vapor transport system design is demonstrated by simulating the effusive-flow of neutral particles through complex target–vapor transport systems. Particle release curves involving decay losses in both diffusion and effusive-flow are computed; and a numerical procedure is proposed to measure the diffusion coefficients and the characteristic effusion times of rare isotopes in target–ion source systems.

Decay losses, associated with the times required for particles to diffuse from ISOL production targets and to effusively-flow to an ion source, must be reduced to as low as practically achievable levels in order to deliver useful beam intensities of short-lived isotopes for research at ISOL based Radioactive Ion Beam (RIB) facilities. We have developed a fast-valve system and complementary 3-D Monte-Carlo code which can be used separately or in combination to assess the effusive-flow properties of vapor-transport systems, independent of size, geometry and chemical properties of the transport species. In this report, we describe the fast valve and present time spectra and characteristic time data for noble gases flowing through serial- and parallel-coupled vapor-transport systems similar in geometry but longer than those used for RIB generation at the HRIBF with and without target coating matrices.

Isobaric contaminants are often problematical in accelerated negative ion beams for research at certain radioactive ion beam (RIB) and accelerator mass spectrometry (AMS) facilities since their presence in low-intensity rare isotopic beams seriously compromise experimental results. This article describes a non-resonant, laser-based photo-detachment apparatus for use at these facilities, which, according to calculations efficiently removes isobaric contaminants from these beams. The advantage of the system for isobaric contaminant removal over other systems proposed to date lies in its ability to efficiently capture easily transportable energetic negative ion beams with low, intermediate or high energy spreads by a superconducting solenoid magnetic field. The ability to change the diameter of captured beams by adjusting the magnetic field strength permits optimum control of the radial overlap of the laser/negative ion beam profiles over an extended interaction region under high vacuum conditions without retarding optical affect, collision-cooling or capture losses.

Affects of microwave bandwidth on the high-charge-states of ion beams extracted from a conventional minimum-B-geometry ECR ion source are first demonstrated. The high-charge-state intensities, produced with broadband microwave radiation are observed to be factors ⩾2 than those produced with narrow bandwidth microwave radiation at the same power level.

The charge-state enhancing effects of broadband microwave radiation are demonstrated by comparing the charge-state distributions and intensities of Arq+ ion beams, extracted from a conventional B-minimum geometry, 6.4GHz ECR ion source, when operated with traveling-wave-tube (TWT) amplified microwave signals generated, respectively, from a “white” noise generator (WNG, bandwidth: 200MHz) and a conventional narrow-bandwidth local oscillator (LO, bandwidth: ∼1.5MHz). Clear enhancement (factors>2) is obtained for high-charge-state Ar ion beam intensities, produced with broadband radiation, over those obtained with narrow-bandwidth radiation at the same power level. The high-charge-states (q>9) exhibit an anomalous, abrupt increase in intensity at an optimum pressure while the total X-ray count and plasma potential suddenly dip at approximately the same pressure, seemingly in correlation. These effects are not understood in terms of rf adsorption or principal charge-state limiting atomic physics phenomena. Future experiments are proposed for evaluating the full potential of the broadband heating method enhancing the high-charge-state ion beam capabilities of conventional minimum-B ECR ion sources.

The lifefimes of the (2p)2P3/2 level in Si XII and the (2p2)3P2,1,0, (2s2p)1P1 and the (2p2)1D2 levels in Si XI have been determined using the beam-foil method. It is proposed that existing discrepancies between experimental and theoretical f values for the (2s2)1S0-(2s2p)1P1 transition in some ions of the Be sequence may be the result of the inability of the experimentalist to account for strong cascading from the (2p2)1S0 level.

As clearly demonstrated at several laboratories, the performances of electron-cyclotron resonance (ECR) ion sources can be enhanced by increasing the physical sizes (volumes) of embedded ECR zones. En-larged ECR zones have been achieved by engineering the central magnetic field region of these sources so they are uniformly-distributed "volumes" in resonance with single-frequency rf power. Alternatively, the number of ECR surfaces in conventional minimum-B geometry sources can be increased by heating their plasmas with multiple, discrete frequency microwave radiation. Broadband rf power offers a simple, low cost and arguably more effective means for increasing the physical sizes of the ECR zones within the latter source type. In this article, theoretical arguments are made in support of the volume effect and the charge-state enhancing ef-fects of broadband microwave radiation (bandwidth: 200MHz) plasma heating are demonstrated by comparing the high-charge-states of Ar ion beams, produced by powering a conventional minimum-B geometry, 6.4GHz ECR ion source, equipped with a biased disk, with those produced by conventional bandwidth (bandwidth: ∼1.5MHz) radiation.

In order to generate useful intensities of short-lived radioactive species at Isotope Separator On-Line (ISOL) based research facilities, species of interest must be diffused from the interior of the production target-material, effusively transported to an ion source, ionized, extracted, mass-analyzed, and accelerated to research energies in times commensurate with their lifetimes. The intensities at such facilities are principally limited by decay losses associated with times required for diffusion-release from target materials and transport from the target to the ion source and by the maximum permissible primary beam power depositional density that can be tolerated without deleteriously affecting target integrity or ion source efficiency. Consequently, it is imperative to minimize delay times associated with the independent diffusion and effusive-flow processes by choosing small dimensioned (short-diffusion length) highly refractory target materials and formatting them in highly permeable mechanically and thermally robust structures that can withstand high-power beam irradiation for extended periods of time. This article provides basic information on methods, procedures and principles for selecting target materials; for designing fast diffusion-release targets; for designing fast effusive-flow vapor transport systems; and introduces methods for controlling target temperatures as required for optimizing intensities of short-lived radioactive ion beams at ISOL based research facilities.