Target Wheel Research Articles

A multisample, high-intensity Cs sputter negative ion source for accelerator mass spectrometry applications

The development of a high-intensity Cs sputter negative ion source has been completed. The source is equipped with a spherical tungsten ionizer and a target wheel with 18 sample positions. The wheel is separated from the hot sputter region by a gate valve in order to change the samples rapidly and reduce cross contamination. A drive system can push the selected sample into the sputter position and pull it back to the wheel. A metal-ceramic bonded ring is used to support all the parts at sputter potential. The ring is protected from vapor depositions by a specially designed shielding structure, resulting in reliable insulation between the ionizer and the sputter samples. No insulation trouble has occurred during the work for over one year. Beams of 10-μA BeO−, 5-μA Al−, 4.5-μA Fe−, and 350-μA C−, etc. have been delivered. The normalized beam emittance ranges from (2–4) πmm mrad MeV1/2. The memory effect has been evaluated. More than 1000 and 500 times of attenuation for C− and BeO− are observed, respectively, in 10 min after a sample change. The ionization efficiency for 12C− is higher than 5.4% for a graphite sample.

Materials impurity analysis by means of nuclear resonance reactions

An automated beam line for measurement of trace impurities in materials is installed on the FN tandem in the Center for Accelerator Mass Spectrometry, LLNL. The major purpose of the beam-line system is to determine the diffusion of water into glasses by use of the 19F(p, αγ) 16O reaction. This study is stimulated by the radiation-waste-isolation program; however, the system is applicable to studies of other impurities in many material matrices. The target wheels, each of which holds twenty-four samples, can be changed in less than an hour. The beam-line control system both indexes the target wheel and controls the tandem energy as it steps through the requested measurement protocol. The tandem control system, in response to a request, adjusts the terminal potential and other transport elements to maintain constant spot size and location.

Uranium targets sandwiched between carbon layers for use on target wheels and on a wobbler in heavy-ion bombardments

Uranium layers of ∼0.4 mg/cm 2 are evaporated by means of a 6 kW electron-beam gun onto 0.04 mg/cm 2 thick carbon films in a high-vacuum process; a protecting layer of ∼0.01 mg/cm 2 of carbon is added in the same vacuum cycle. The evaporation- and deposition yields are discussed and measurements of target characteristics are described. C/U/C sandwich targets in the shape of a sector of an annulus are prepared for use on rotating target wheels of 155 mm radius to be bombarded with a pulsed beam of heavy ions. One type of circular targets of 20 mm in diameter is mounted to a target wobbler. Both, wheel and wobbler, distribute the intensity of the heavy-ion beam to a larger area to reduce radiation damages. Examples of target applications will be mentioned.

Influence of heavy-ion bombardment on the structure of targets for atomic and nuclear interaction studies

An accurate knowledge of the absolute magnitude of the energy loss of heavy ions inside targets is necessary in many experiments. The energy loss and target properties are directly correlated. Even if the initial status of a target is known, the heavy-ion bombardment may lead to a drastic change of its properties or even to its total deterioration. The degree and timescale of a possible variation depends on the type of the projectile beam and on the target properties itself, the latter being influenced by the chemical composition, the physical characteristics, and the mechanical arrangement of the target system. Structural changes of a target system limit its useful lifetime; it can be prolonged by special techniques like the application of target wobblers or target wheels. 1.6 MeV proton backscattering has been employed to investigate series of targets prior to and after their irradiation in heavy-ion beams in order to gain information on structural changes for the interpretation of experimental results obtained in heavy-ion positron spectroscopy.

The part of targets in heavy-ion positron spectroscopy

Quasi-atomic K-hole and positron-production experiments together with the positron-line phenomena are presented which emphasize the importance of target proporties in heavy-ion collisions near the Coulomb barrier with beams of 208Pb, 232Th, and 238U. Various self-supported, supported, or sandwiched targets — either metallic or as chemical compounds — have been used; preparation techniques (high-vacuum sublimation- and evaporation condensations, pack rolling, sputter deposition, and molecular plating) are mentioned casually for targets ranging from C, Ti, Ni, Ta, and Pb up to radioactive samples of 232Th, 238U, and 248Cm. The influence of target deterioration is discussed in view of the energy loss and the charge states of the ions. In-beam heavy-ion scattering as well as proton backscattering are exemplified as target control procedures. A target wobbler and a target wheel are given as examples for the reduction of deteriorations due to heavy-ion bombardments.

Microstructural characterization of rapidly solidified AlMnCr alloys

In order to further increase the intensity of ion beams used in high-current experiments, e.g. the synthesis of heavy and superheavy elements, it is necessary to develop methods for cooling the targets sufficiently below the melting point which is particularly low in the case of metallic lead and bismuth targets. In this paper we describe measurements for various fixed targets and rotating target wheels in vacuum as well as in a stationary He-gas and a He-gas flow as cooling medium. As a result improvements of the target system are suggested, which will allow for a considerable increase of the beam intensity.

The proposed spallation neutron source and modernized reactor as possible sites for a low temperature irradiation facility in Germany

A feasibility study for a Spallation Neutron Source (SNQ) in Germany was completed in June 1981. In this project an intensity-modulated LINAC (100 pps) would provide a proton beam of energy 1100 MeV and time-average current I ̄ p = 5 mA . Spallation neutrons are produced in the lead material of a rotating target wheel and moderated in a hybrid arrangement consisting of both a small H 2O volume and a large D 2O tank. Here the maximum values of the peak and time-average thermal fluxes are \\ ̂ gf th ≈ 1.3 × 10 16 cm −2 s −1 and \\ ̄ gf th ≈ 6.5 × 10 14 cm −2 s −1 , respectively. A low temperature irradiation facility (LTIF) has been proposed to allow irradiations in the temperature range of 4.5 to 450 K with either thermal neutrons ( \\ ̄ gf th ≈ 1 × 10 14 cm −2 s −1 ) or fast neutrons ( \\ ̄ gf f ≈ 2 × 10 13 cm −2 s −1 ). The advantages and disadvantages of having this LTIF at the SNQ are discussed with respect to the alternative of installing it at a fission reactor. Finally, the example of a possible modernization and upgrading of the Munich research reactor FRM is used to discuss the performance of such a reactor and the concept of a LTIF in this case, and to point out the complementarity of an optimized SNQ (high- \\ ̂ gf applications) and such a modernized reactor (high- \\ ̄ gf applications).

A rotating wheel system for the detection of spontaneously fissioning nuclides from heavy ion reactions

A rotating wheel system for the detection of spontaneously fissioning reaction products from heavy ion reactions was developed. In this system products recoiling from a rotating target wheel are stopped in a catcher foil stack which can be rotated at various velocities up to 80 rotations per second. All products emitted within a wide emission cone are stopped in the catcher foils and are rotated into shielded positions with stationary fission-track detectors positioned on both sides of each rotating foil. This technique allows a sensitive detection of spontaneously fissioning nuclides over a broad range of half-lives from 1 ms to about 1 d. By post-bombardment exposure of the catchers to new fission-track detectors, the half-lives covered can be extended to about 1 y. From their range and angular distributions within the catcher foils, heavy-ion fusion products can be distinguished from nuclides produced in damped collisions, and excitation functions can be extracted.

The velocity filter SHIP, performance and survey of current experiments

The velocity filter SHIP is used to investigate unslowed evaporation residues from heavy ion fusion reactions. The main subjects of investigation are α emitting and spontaneously fissioning nuclei very far from stability with halflives as short as microseconds and formation cross-section down to the nanobarn region. The efficiency of the filter is calculated by a Monte Carlo method taking into account momentum transfer from nucleons and α particles evaporated from the excited compound system as well as energy loss, scattering in the target, and the ionic charge distribution. The calculations are compared with experimental results. For the irradiation of low melting point targets with high beam currents a rotating target wheel has been developed. Light evaporation residues are identified by an ionisation chamber. α emitting and spontaneously fissioning nuclei are investigated by implantation into surface barrier detectors. The high background rejection and sensitivity of detection are demonstrated for the case of the identification of Fm and element 104.

A rotating target wheel with thin targets for heavy ion beams of high current densities

A target wheel rotating synchronously with the pulsed heavy ion beam of the GSI UNILAC is described. Isotopically enriched Pb targets of 4.5 × 1.5 cm 2 and of thicknesses of 0.5 to 1.2 mg/cm 2, covered with thin C layers on both sides, were mounted on the wheel and irradiated by up to 2 × 10 12 s −1 Ar ions of 5 MeV/u on a beam spot of 0.25 cm 2 without being destroyed. The total particle dose on the targets was 4 × 10 16 ions.

An intensity-distribution monitor for the SIN external proton beam

A profile monitor of the SIN extracted proton beam at the external pion-production targets has been developed using a special target wheel and a beam-spill detector.