The ISAC on-line isotope source at the Canadian TRIUMF laboratory recently achieved its full design energy. Paul Schmor and Jean-Michel Poutissou describe the experiments scheduled for the source.
The Isotope Separator and Accelerator (ISAC) at TRIUMF uses the on-line isotope separation technique that was developed at CERN to produce relatively intense beams of short-lived exotic nuclei for experiments in nuclear astrophysics and nuclear and condensed-matter physics. A 500 MeV beam of protons from the TRIUMF cyclotron is used to create the rare isotopes in a thick heated target, from which they effuse and are then ionized, extracted as a beam, separated by mass and accelerated. The first stage, ISAC-I, passed a major milestone on schedule last December when a 4He+ beam was accelerated through the continuous-wave radiofrequency quadrupole (RFQ) and drift-tube linacs to the design energy of 1.5 MeV nucleon.
Users have been running experiments with radioactive ion beams with an atomic mass of less than 30 at up to 60 keV since November 1998, and they have now begun taking data at full energy. The 20 µA proton beams used until now make this the highest-power ISOL source. The current is being raised to 40 µA, and a target has been tested to the full 100 µA design capability.
The importance of studying the properties of exotic nuclear isotopes has been increasingly recognized by physicists and astronomers in recent years (see OECD Megascience Forum report or Echoes of a report), and almost a dozen radioactive ion beam projects are now under way around the world. Research at ISAC-I focuses on the nuclear processes occurring in stars – where the high densities and huge numbers involved can lend importance to even short-lived isotopes – and on precision tests of the Standard Model of particle physics.
The ISAC ion source and target system is suspended at the bottom of a 2 m long iron shield block that can be lifted out of its vacuum enclosure and transported, as a unit for servicing, to a hot cell by a remotely operated crane. This system was designed to increase the useful target lifetime and decrease the exposure of personnel to radiation during servicing. In fact, the shielding permits the operation of thick uranium targets with up to 50 kW of proton beam.
Although the initial range of isotopes is limited by the use of a surface ion source to provide isotopes that can easily be thermally ionized, the list of available nuclei will expand considerably in 2002, when a 2.45 GHz electron-cyclotron resonance ion source (ECRIS) is scheduled to begin operation. Any isotope with an atomic mass of less than 240 and an energy of up to 60 keV can be transported, with the aid of electrostatic optics, through a magnetic mass analyser (mass resolution one part in 10 000), either to one of several low-energy experimental stations, or to the low-frequency (35 MHz) continuous-wave RFQ linac.
The 8 m long RFQ, which has a four-rod split-ring design, accelerates ions with a mass:charge ratio of less than 30 from 2-150 keV per nucleon. An 11.7 MHz pseudo-sawtooth prebuncher in the injection line is used to fill every third radiofrequency bucket in the RFQ, forming beam bunches at 87 ns intervals. The accelerated beam is then bunched again, stripped by a carbon foil to a higher charge state, rebunched and accelerated in a five-tank drift-tube linac (DTL) operating continuous wave at 105 MHz.
The DTL has a completely separated-function design, with the interdigital H-mode accelerating cavities separated by magnetic quadrupole triplets for transverse and three-gap split-ring bunchers for longitudinal focusing (the latter having been developed for ISAC by INR Moscow). The final beam energy, for selected ions with a mass:charge ratio of less than 6, is continuously variable at 0.15-1.5 MeV/nucleon.
The higher-energy experimental facilities reflect the emphasis on nuclear astrophysics. A large-acceptance recoil spectrometer system (DRAGON) is being commissioned to study the radiative capture reactions involved in explosive events like novae, supernovae and X- and gamma-ray bursts. To complement it, a large-acceptance scattering facility (TUDA) has been developed to locate resonances of interest in the corresponding compound nuclei.
The programme will focus on proton and alpha radiative-capture reactions in the low-mass (A<30) region. The first scheduled experiment will try to establish the rate for the 21Na(p, g) 22Mg reaction, which determines the production of sodium-22 in nova explosions of O-Ne-Mg-rich white dwarfs. With a 2.6 year half-life and a 1.275 MeV decay gamma ray, sodium-22 is the prime candidate for nova sightings in the next round of satellite-based gamma-ray searches, such as the ESA INTEGRAL mission.
The low-energy beams (up to 60 keV) are used in a broad programme covering fundamental symmetry tests, nuclear structure studies in exotic nuclei and condensed matter studies using light-polarized ions. Precision measurements of pure-Fermi beta-decay lifetimes, branching ratios and Q values (currently under way on rubidium-74) will improve the testing of the weak interaction theory and the determination of up-down quark mixing, while correlation studies in beta decay with trapped atoms (the TRINAT programme with metastable potassium-38 and polarized potassium-37) are placing constraints on extensions of the Standard Model.
A low-temperature nuclear orientation refrigerator (LTNO) for on-line nuclear magnetic resonance and perturbed angular correlation studies, and a large germanium gamma-ray detector array (the former Chalk River 8- spectrometer) are to be used in studies of nuclear deformation in transitional regions (mass band 80-100 and near 180).
Novel facility
A novel facility for beta-NMR (nuclear magnetic resonance) studies of condensed (especially superconducting) materials is being commissioned. Light polarized ions (currently lithium-8) are produced via collinear polarized laser beam excitation, while the spectrometer sits at an adjustable high voltage. The range of the ions can be adjusted so that they stop on the surface of the sample or at a prescribed depth, allowing studies of magnetism on surfaces, in thin layered materials and at interfaces.
With ISAC-I coming into full operation, TRIUMF is now turning to the construction of ISAC-II, funding for which was approved by the Canadian government last year for completion in 2005. This will involve adding a 6.5 MeV/nucleon superconducting linac and a new experimental hall, and will not only increase the ion energies but also allow the mass range to be extended to atomic masses of around 150. This will enable the electrostatic barrier to be overcome for all target nuclei, opening up a range of nuclear structure physics with proton- or neutron-rich projectiles – particularly studies of nuclei near the limits of stability, although a strong focus on nuclear astrophysics will remain.
