S. N. Murray

A new concept kinetic-ejection negative-ion source for rib generation

Abstract Chemically active radioactive species are often released from target-materials in a variety of molecular forms. For example, 17F, is principally released from Al2O3 target-material as Al17F. Because of the low probability of simultaneously dissociating such molecular carriers and efficiently ionizing their atomic constituents with conventional hot-cathode, electron-impact ion sources, species of interest are often distributed in several mass channels in the form of molecular side-band beams. Consequently, beam intensities of the desired radioactive species are diluted. The sputter negative-ion beam generation technique is particularly effective for simultaneously dissociating molecular carriers and efficiently ionizing highly electronegative atomic constituents. Therefore, a new concept kinetic-ejection negative-ion source (KENIS), based on this principle, was conceived to address this problem. The source has proven to be highly efficient for simultaneously dissociating and negatively ionizing sputter-ejected atomic fluorine from cesiated surfaces. The source has been successfully employed on-line to generate high intensity 17F− beams for use in the astrophysics research program at the Holifield Radioactive Ion Beam Facility (HRIBF) using the 16O(d,n)17F reaction. The mechanical design features, principles of operation, operational parameters, beam quality information (emittance data) and efficiencies for forming intense beams of 17F− during off-line and on-line operation of the source are presented in this report.

The status of the spallation neutron source ion source

The ion source for the spallation neutron source (SNS) is a radio-frequency, multicusp source designed to deliver 45 mA of H− to the SNS accelerator with a pulse length of 1 ms and repetition rate of 60 Hz. A total of three ion sources have been fabricated and commissioned at Lawrence Berkeley National Laboratory and subsequently delivered to the SNS at the Oak Ridge National Laboratory. The ion sources are currently being rotated between operation on the SNS accelerator, where they are involved in ongoing efforts to commission the SNS LINAC, and the hot spare stand (HSS), where high-current tests are in progress. Commissioning work involves operating the source in a low duty-factor mode (pulse width ∼200 μs and repetition rate ∼5 Hz) for extended periods of time while the high-current tests involve source operation at full duty-factor of 6% (1 ms/60 Hz). This report discusses routine performance of the source employed in the commissioning role as well as the initial results of high-current tests performe...

An efficient negative surface ionization source for RIB generation

Abstract A high-efficiency, negative surface ionization source, equipped with spherical-sector LaB6 ionizer, has been developed for generating radioactive beams of highly electronegative species for accelerator-based nuclear physics and nuclear astrophysics research. The source utilizes direct-surface ionization to form negative-ion beams resulting from interactions between highly electronegative atoms or molecules and a hot (∼1722 °C) LaB6 surface. Even though this type of ionizer has a widely publicized propensity for being easily poisoned, no evidence of this effect was experienced during testing of the source. The source has been extensively evaluated off-line in terms of ionization efficiency for generating beams of Cl− and Br− by feeding AlCl3 and AlBr3 vapors at low-feed rates into the source. The geometry of the ionization volume is designed to minimize direct losses of neutral particles through the extraction aperture and to enhance the probability for striking the LaB6 ionizer. The source is reliable, stable and easy to operate, and ionizes Cl− and Br− with efficiencies for formation and extraction of 30% and 20%, respectively. The design features, principles of operation, off-line performance, operational-parameter and beam-quality (emittance) data for the source are presented in this article.

Efficient negative-ion sources for radioactive ion beam applications (abstract)

Direct extraction, negative-surface ionization, and sputter-type sources have been designed to efficiently ionize specific members of the group VII A elements (F, Cl, Br, I, and At) for use in the nuclear structure physics and nuclear astrophysics research programs at the Holifield Radioactive Ion Beam Facility (HRIBF). A negative surface ionization source that utilizes a solid, spherical geometry, LaB6 ionizer (φ≅2.7 eV) for negatively ionizing Cl, Br, l, and At. During off-line evaluation, the ionization efficiency for Cl− generation ranged between 15% and 20% and for Br− generation, between 15% and 25%. Chemically active elements, such as fluorine, are often released from refractory host materials in a variety of molecular forms. Consequently, the LaB6 surface ionization source, described above, is ineffective for simultaneously dissociating and negatively ionizing this element. To overcome this problem, a new concept source, referred to as the kinetic ejection negative ion source (KENIS), was develope...

Prototype negative-ion sources for radioactive ion-beam generation

Radioactive ion beams (RIBs) of 17F and 18F are of interest for investigation of astrophysical phenomena such as the “hot” CNO cycle and the rp stellar nuclear synthesis processes. In order to generate useful beam intensities of atomic F−, the species must be efficiently and expediently released from the target material, thermally dissociated from fluoride release products during transport to the ionization chamber of the ion source, and efficiently ionized in the source upon arrival. We have conceived and evaluated two prototype negative-ion sources for potential use for RIB generation: (1) a direct extraction source and (2) a kinetic-ejection source. Both sources utilize Cs vapor to enhance F− formation. The mechanical design features, operational parameters, ionization efficiencies for forming atomic F−, and delay times for transport of F and fluoride compounds for the respective sources are presented. The efficiency η for formation and extraction of F− for the direct extraction negative-ion source is ...

A NEW CONCEPT POSITIVE (NEGATIVE) SURFACE IONIZATION SOURCE EQUIPPED WITH A HIGH POROSITY IONIZER

Abstract A versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed, fabricated, and initial tests completed which can operate in either positive- or negative-ion beam generation modes without mechanical changes to the source. The highly permeable, composite Ir/C has an intrinsic work function of φ=5.29 eV and can be used directly for the generation of positive-ion beams of highly electropositive elements. For negative-surface ionization, the work function is lowered by dynamic flow of a highly electropositive adsorbate such as Cs through the ionizer matrix. The results of initial testing indicate that the source is reliable, stable and easy to operate, with efficiencies for Cs+ conservatively estimated to exceed 60% and as high as ∼50% for F− generation. However, the overall ionization efficiency for F− formation and extraction is ∼1% due to the low efficiency for thermal dissociation of the carrier molecules (assumed to be CsF and AlF). The design features, operational principles, and initial performance of the source for generating Cs+ and F−, when operated with Cs, are discussed in this article.

Initial tests of the Spallation Neutron Source H- ion source with an external antenna

The ion source for the Spallation Neutron Source (SNS) is a radio-frequency (rf) multicusp source designed to deliver H− beam pulses of 40mA to the SNS accelerator with a normalized root-mean-square emittance of less than 0.2πmmmrad, with a pulse length of 1ms and a repetition rate of 60Hz. In order to achieve this performance the source must operate with both high-pulse rf power, ∼50kW, and high average rf power, ∼3.5kW, over a continuous operational period of 3weeks. During operation at these power levels the plasma-immersed, porcelain-coated rf antenna is susceptible to damage, limiting source lifetime. We are therefore developing an ion source where the plasma is separated from the Cu antenna by an Al2O3 discharge chamber. This article describes the ion source, presents initial beam extraction measurements, and details our ongoing effort to develop this concept into a suitable ion source for the SNS.

Effusive-flow characterization of arbitrary size and geometry target/vapor transport systems: RIB applications

The principal factors that limit intensities of short-lived radioactive ion beams produced by the isotope separator on-line technique are time delays due to diffusion of radioactive species from solid or liquid target materials and their effusive-flow transport to the ion source. Although diffusion times can be reduced by proper design of short diffusion length, highly refractory targets, effusive-flow times are more difficult to assess. After diffusion from the target material, the species must travel through the target material and vapor transport system to the ion source. The time required for effusive-flow transport to the ion source depends on the conduction path, chemical reactions between the species and target material and materials of construction, as well as the physical size and geometry of the transport system. We have developed a fast valve (0.1 ms closing time) for introducing gaseous or vapor-state species into the target/vapor transport/ion source/system that permits measurement of effusive-flow times for any gaseous or vaporous species (chemically active or chemically inactive) through any vapor transport system, independent of size and geometry. Characteristic times are determined from the exponential decay of the momentum analyzed ion beam intensity for the species during effusive flow through the vapor transport system under evaluation. This article describes the effusive-flow apparatus and presents characteristic time spectra and characteristic effusive-flow time data for noble gases flowing through a serial-flow target reservoir system.

Effusive flow delay times for gaseous species in a compact rf ion sourcea)

A rf ion source is presently being developed and evaluated as a potential candidate for use in generating radioactive ion beams (RIBs) for the experimental research program at the Holifield Radioactive Ion Beam Facility (HRIBF) now under construction at the Oak Ridge National Laboratory. For this application, any time delays that are excessively long with respect to the half‐life of the radioactive species of interest can result in significant losses of the RIB intensity; therefore the times for effusive flow through the ion source are of fundamental importance since they set limits on the minimum half‐life of radioactive species that can be processed in the source. Complementary experimental and computational techniques have been developed which can be used to determine the characteristic delay times for gaseous species in low‐pressure ion source assemblies. These techniques are used to characterize the effusive delay times for the stable counterparts of various atomic and molecular radioactive species i...

Advances in the performance and understanding of the Spallation Neutron Source ion source

The ion source developed for the Spallation Neutron Source (SNS) is a radio-frequency, multicusp source designed to produce ∼40mA of H− with a normalized rms emittance of less than 0.2πmmmrad. To date, the source has been utilized in the commissioning of the SNS accelerator and has already demonstrated stable, satisfactory operation at beam currents of 10–40mA with duty factors of ∼0.1% for operational periods of several weeks. Ultimately the SNS facility will require beam duty factors of 6% (1ms pulse length, 60Hz repetition rate). To ascertain the capability of the source to deliver beams at this duty factor over sustained periods, ongoing experiments are being performed in which the ion source is continuously operated on a dedicated test stand. The results of these tests are reported as well as a theory of the Cs release and transport processes that was derived from these data. The theory was then employed to develop a more effective source-conditioning procedure and a direct-transfer Cs collar, which ...