ISAC Facility Research Articles

Application of Low Energy Spin Polarized Radioactive Ion Beams in Condensed Matter Research

The new ISAC facility at TRIUMF (Canada), produces some of the world's most intense radioactive ion beams (RIBs). Although the primary scientific motivation for these facilities lies in nuclear physics and nuclear astrophysics, RIBs also have applications in condensed matter (CM). Many pioneering CM experiments have already been performed at the ISOLDE facility (CERN). Continuing on this path at ISAC we have recently polarized a beam of 8Li+ to be used as a magnetic probe of ultra-thin films and interfaces. We report here on progress in developing this new application of low energy RIBs, which is based on the technique of β-detected nuclear magnetic resonance (β- NMR)

Radioactive beam facilities of North America

There are several nuclear physics laboratories in North America that have on-going research using energetic and stopped radioactive beams. These include the large ISOL-based programs ISAC at TRIUMF in Canada and HRIBF at Oak Ridge and the in-flight fragmentation program at the NSCL of Michigan State University. There are also smaller, more specialized, programs using a variety of techniques at ATLAS at Argonne, the Cyclotron Institute of Texas A&M University, the Nuclear Structure Laboratory at Notre Dame University, the 88-inch cyclotron of Berkeley, and the Nuclear Structure Laboratory at SUNY/Stony Brook. A major upgrade of the NSCL, the Coupled Cyclotron Project, was recently completed, resulting in driver beams at higher energies and intensities than previously available. A major improvement of the ISAC facility is also in progress. ISAC is being upgraded to provide higher energies and a broader range of masses in the ISAC II project with an ECR charge state booster and a superconducting linac to provide energies of 6.5 MeV/u for masses up to A=150. In the United States the construction of a next-generation facility, the Rare Isotope Accelerator (RIA), is being planned. RIA will have a very high-intensity, up to 400 kW, superconducting heavy-ion driver linac which will provide beams from protons to uranium. This facility utilize both ISOL and in-flight production mechanisms. Many laboratories and universities nation-wide are participating in the R&D program for this future facility.

On-line collection and transfer of radioactive noble gas isotopes

Techniques have been developed for on-line collection and transfer to a measurement cell of rare noble gas isotopes. A low-energy isotope separator beam is stopped in a foil. After collection, the foil is heated, and the noble gas atoms are cryopumped to a cold finger and then transferred with a buffer gas to a measurement cell. This has been tested with 120Xe produced from a 120Cs beam at the TRIUMF ISAC facility, and greater than 40% efficiency for transfer from the foil to the cell has been demonstrated. The technique will be used for electric dipole moment measurements with radon isotopes and may also be useful for other measurements with gaseous targets including laser spectroscopy.

First laser ions at an off-line mass separator of the ISAC facility at TRIUMF

For efficient and in particular for selective production of radioactive ion beams at on-line mass separator facilities the technique of resonance ionization laser ion sources (RILIS) has become the most powerful tool. In facilities like ISOLDE at CERN they nowadays represent the most commonly used type of ion source for rare short-lived isotopes, delivering highest suppression of isobaric contaminations. For a first off-line demonstration preparing the development and installation of such a laser ion source at the new ISAC facility at TRIUMF in Vancouver (Canada), an all solid state laser system developed at the University of Mainz (Germany), was transferred and tested there at an off-line test separator. The laser system consists of three tunable titanium:sapphire lasers with a repetition rate of 12 kHz, pulse length of ∼30 ns, up to 2.5 W output power in the infrared to red spectral region and features additional frequency doubling units. With this system first RILIS studies were performed in a number of elements. Here we report on stable isotope measurements in Ca and Ga.

Nuclear astrophysics with radioactive beams: a TRIUMF perspective

Explosive nuclear burning in stellar environments involves reactions with a wide range of isotopes. For isotopes that are unstable, information on relevant reaction rates can only generally be obtained at radioactive beam facilities. The ISAC facility at TRIUMF is purpose built to provide a wide range of radioactive beams for nuclear astrophysics purposes as well as a range of other science.

High power target developments at ISAC

TRIUMF, Canada’s national research facility for particle and nuclear physics is currently operating the ISAC facility. A high-energy proton beam from the H - TRIUMF cyclotron is used to generate short-lived radioactive species in a thick target. An ion source at the target creates a radioactive beam, which is then injected into the ISAC beam lines and accelerator system. The ISAC facility is designed to accept proton beam intensity up to 100 μA at 500 MeV. At present our target design can only sustains 40 μA at maximum. Beyond this point the target has to be cooled. A new target equipped with fins has been developed that may sustain proton beam up to 100 μA. The fined target has been tested off-line and a thermal simulation using ANSYS ® has been conducted and the results are reported here.

Low energy spin polarized radioactive beams as a probe of thin films and interfaces

A spectrometer for β-detected nuclear magnetic resonance (β-NMR) has been commissioned at the ISAC facility at TRIUMF. A beam of low energy highly spin polarized 8Li + can be decelerated and implanted into ultra-thin structures 6–400 nm thick. β-NMR provides local information on the electronic and magnetic properties of materials which is similar to conventional NMR but can be used as a sensitive probe of ultra-thin films, interfaces and other nanostructures. We report here on the status of the spectrometer and preliminary results on a simple metal film.

Advances at ISOL facilities

A brief outline of current progress of a selected number of ISOL facilities is given, with special emphasis on the ISAC facility at TRIUMF.

Developments in high-intensity two-step target design

The two-step fission-product target concept is analyzed to design a prototype target for testing at the ISAC facility. A two-step target consists of an inner cylinder of heavy metal irradiated by an energetic light ion beam; neutrons produced emerge from the target into a coaxial secondary target (blanket) of fissionable material. With this approach, the production of fission product activities is enhanced compared to other products that can interfere in experiments, and significantly less energy is deposited in the secondary target than for direct irradiation. The design is mechanically simple and provides independent control of operating temperatures. The analysis determined appropriate target and blanket dimensions, and energy deposition profiles, temperature distributions and fission production rates. The production rates of some neutron-rich isotopes for selected elements are summarized. A conceptual design is presented, along with issues on cooling the primary target and heating of the secondary target.

RNB post-accelerator for ISAC at TRIUMF— present and future

The post-accelerator for ISAC includes a 35 MHz RF Quadrupole (RFQ) to accelerate beams of A/ q⩽30 from 2 keV/u to 150 keV/u and a post-stripper, 105 MHz variable energy drift tube linac (DTL) to accelerate ions of 3⩽ A/ q⩽6 to a final energy from 0.15 to 1.5 MeV/u. Recently funds have been allocated for an extension to the ISAC facility, ISAC-II, to permit acceleration of radioactive ion beams up to energies of at least 6.5 MeV/u for masses up to 150. The design concept and present status of both the ISAC-I project and the future ISAC-II project will be presented.

Online isotope separation at ISAC with a 10 μA proton driver beam

After initial operation using a 1 μA proton driver beam for radioactive ion beam production, the ISAC facility at TRIUMF has progressed to operating at higher proton beam intensities, in the range of 10 μA p +. ISOL targets designed for 1 μA p + levels have been successfully operated for extended periods at the higher beam intensities, with only minor modification. Thin niobium foil targets with reduced volume were employed to maximize the release of short lived Rb product nuclides. Production yields were measured by depositing the beam on a movable tape in a collection station with both beta- and gamma-counting capabilities for product identification. The yield per μA p + of 75Rb ( t 1/2∼19 s) was observed to increase in a nonlinear manner with increasing proton current on target.

TRIUMF-ISAC target station and mass separator commissioning

The ISAC facility includes: a new building with 5000 m 2 of floor space, a beam line with adequate shielding for up to 100 μA protons at 500 MeV from the TRIUMF H − cyclotron, two target stations, remote handling facilities for target services, a high resolution mass separator and a linear accelerator. The first radioactive ion beams were produced in the fall of 1998 and now the experimental program has started. A novel approach for the target station permits the operation of the ISOL target up to 100 μA. Furthermore, several diagnostics tools were developed for the mass separator helping considerably the RIB tuning.

Contributions from X-ray and gamma-ray observations

An intense beam of 26gAl has been developed for accelerated beam experiments at TRIUMF’s ISAC facility. Studies of the on-line production of Al radionuclides from thick silicon carbide targets have been performed as part of a program of beam development for astrophysical reaction studies at ISAC. While the release of short-lived Al nuclides from SiC was found to be slow, development of new target material forms and high-power target containers has allowed operation of SiC targets with proton currents of up to 70 μA on target. In addition, operation with the TRIUMF resonant ionization laser ion source (TRILIS) has produced 26gAl beam intensities of 5.1 × 1010 s−1.

Complementarity of low-energy spin polarized radioactive nuclei and muons

The recently completed ISAC facility at TRIUMF is the most intense source of low-energy radioactive ions in the world. Beams of short-lived radioactive ions have applications in many different areas of physics including condensed matter research. At ISAC we are developing the technique of beta-detected nuclear magnetic resonance (β-NMR) as a tool for investigating thin structures and dilute impurities. Large nuclear spin polarization can be generated using a fast collinear laser method so that a beam of highly spin polarized ions can be implanted at low energies (1–90 keV). In this paper we describe the facility and the initial experimental program with emphasis on how this technique can complement μSR.

Beam characteristics using stable isotopes from a multicusp source for the TRIUMF ISAC facility

A multicusp source for positive ion beams has been designed and constructed in collaboration with the Ion Beam Technology Department of LBNL for the TRIUMF ISAC project. This type of source has demonstrated a high yield of singly charged ions, a low energy spread, a good emittance, and is compact and simple. Several stages of tests and measurements using nonradioactive beams to characterize the source performance are being carried out both at LBNL and at TRIUMF prior to the final phase of radioactive target-source system tests. Results of these nonradioactive tests and certain problems encountered are reported and discussed in this article.

OPPIS development at TRIUMF

Optical pumping has proven to be a very efficient technique for the production of polarized beams and polarized targets. Polarization transfer collisions between protons, atomic hydrogen or deuterium, and optically pumped alkalimetal vapor atoms are successfully implemented in the high current optically pumped polarized ion sources (OPPIS) and the laser-driven sources (LDS) for target applications. The feasibility of a 20 mA polarized H − ion current has been demonstrated. Pulsed OPPIS applications for the future RHIC and HERA polarized beam facilities are discussed. Nuclear polarized 8Li beams produced by optical pumping, for β-NMR surface studies with a new ISAC facility at TRIUMF, are also considered.

A facility for studying radiative capture reactions induced with radioactive beams at ISAC

The measurement of low energy fusion reactions of importance to nuclear astrophysics scenarios are a prime objective of the physics program of the new ISAC facility, located at TRIUMF in Vancouver, Canada. Intense radioactive beams of 19Ne, 14,15O, 20,21Na, 17,18F and other low Z species with energies in the range of 0.15 to 1.5 MeV/mass unit will be available to measure cross sections and resonance strengths of alpha and proton induced reactions. An important component of the experimental configuration will be a new Reaction Products Detection Facility (RPDF) consisting of a windowless gas target, surrounded by a gamma array, while the recoils are separated from the intense radioactive beam using a Recoil Mass Spectrometer (RMS). The RMS will be based on a Wien filter. The recoiling reaction products will then be detected using either a Si μ-strip array or a gas filled detector. Using these devices along with coincidence requirements and time of flight conditions a background reduction factor of the order of 10 +15 is the present goal.

An ion source test stand for the ISAC facility at TRIUMF

The properties of ion sources and the beams extracted from them are critical to the successful design of the ISAC facility at TRIUMF. To better understand these properties, an off line 60 kV test stand has been constructed to test and evaluate targets and ion sources intended for use at the future ISAC facility. The test stand also presents the opportunity to evaluate diagnostics, beam monitoring and system control techniques required for ISAC. The test stand vacuum chamber mimics the ISAC target module. Ion beams are produced using a fixed geometry multielectrode extraction column and transported through 5m of beamline using electrostatic elements. Mass analysis is achieved using a 45° magnetic dipole midway along the flight path. Diagnostic elements positioned throughout the beamline are used to determine beam properties.

A facility for studying radiative capture reactions induced with radioactive beams at ISAC

The measurement of low energy fusion reactions of importance to nuclear astrophysics scenarios are a prime objective of the physics program of the new ISAC facility, located at TRIUMF in Vancouver, Canada. Intense radioactive beams of 19Ne, 14,15O, 20,21Na, 17,18F and other low Z species with energies in the range of 0.15 to 1.5 MeV/mass unit will be available to measure cross sections and resonance strengths of alpha and proton induced reactions. An important component of the experimental configuration will be a new Recoil Product Detection Facility (RPDF) consisting of a windowless gas target, surrounded by a gamma array, while the recoils are separated from the intense radioactive beam using a Electro-Magnetic Separator (EMS) employing Wien filters. The recoiling reaction products will then be detected using either a Si μ-strip array or a gas filled detector. Using these devices along with coincidence requirements and time-of-flight conditions a background reduction factor of the order of 10 +15 is the present goal.

The proposed ISAC facility at TRIUMF: A status report

An accelerated radioactive beams facility for nuclear astrophysics studies was proposed for installation at the TRIUMF facility (Vancouver, Canada) several years ago. It would consist of a front-end on-line separator (ISOL) coupled with a post-accelerator booster stage. The resulting radioactive ion beam would have sufficient energy to perform rate measurements of sub-barrier fusion reactions, involving short-lived reactants. A status report is presented herein along with detailed information on the installation and use of TISOL, a prototype of the frond-end isotope separator device.