Descriptions of nuclear structure deduced from the long-lived isotopes of day-to-day existence are not appropriate for nuclei far from stability whose structure is relevant to, for example, stellar nucleosynthesis environments. Breakdown of shell structure, emergence of new “magic” numbers, and formation of neutron-proton pair condensates are all predicted modes of nuclear behaviour that can be probed with ISAC-II exotic heavy-ion beams that should be available in 2005. The energy of these beams should be above the Coulomb barrier of many projectile-target combinations of interest. Population of excited states with these projectiles will produce gamma rays which could be Doppler energy shifted by up to 10% of their energy. Much of the information of the predicted modes of nuclear behaviour is contained within the intrinsic properties of these gamma rays. It is therefore essential to correct for those Doppler shifts while preserving high efficiency, as is done with Exogam and Miniball.

The TRIUMF-ISAC Gamma-Ray Escape Suppressed Spectrometer (TIGRESS) combines the proven technologies of multi-crystal high-purity germanium (HPGe) “clover” detectors and scintillator-based escape suppression, with high outer-contact electrical segmentation and digital signal processing, to measure gamma ray production in ISAC-II experiments. The clover technology provides high absolute efficiency, while the escape suppressors reduce continuum background from gamma rays that are not fully absorbed in the HPGe. Analysis of the pulse shapes of the highly segmented outer contacts is expected to determine the angle of gamma ray emission in the lab frame to ~3^o, compared with the ~20^o subtended by each crystal. This will bring the emission-angle Doppler-broadening component of an 800 keV gamma ray from a source moving at 5% the speed of light (typical of ISAC-II experiments) to 2.1 keV, comparing favourably to 1.8 keV intrinsic HPGe resolution and 13 keV broadening one would obtain with unsegmented crystals in the same array.

A prototype HPGe module was funded by the Canadian Foundation for Innovation, Ontario Innovation Trust, the University of Guelph, and TRIUMF, and is shown in the photo to the left. This test determined that the detector was capable of <2 mm lateral position resolution for single interactions, well within the limits needed to obtain the afore-mentioned angular resolution. Following a technical review of the prototype detector in July 2003, the Natural Science and Engineering Research Council (NSERC) released funds to begin construction of a 12-detector array to be completed in 2009. A subarray is expected to be ready for the first ISAC-II beams. The TIGRESS collaboration comprises researchers from University of Guelph, McMaster University, Université de Montréal, University of Toronto, Université Laval, Simon Fraser University, and TRIUMF.

Greg Hackman