TIGRESS

The TRIUMF-ISAC Gamma-Ray Escape Suppressed Spectrometer (TIGRESS) is a state-of-the art gamma-ray spectrometer designed for a broad program of nuclear physics research with the accelerated radioactive ion beams provided by the ISAC-II superconducting linear accelerator at TRIUMF. The radioactive ion beams delivered by ISAC-II are accelerated to energies (continuously variable between 1.5 and 6.5 MeV/nucleon for heavy nuclei and up to 15 MeV/nucleon for light nuclei) sufficient for them to undergo Coulomb excitation, nucleon transfer, and/or nuclear fusion reactions in thin foils supported in a reaction chamber at the centre of the TIGRESS spectrometer. The high frequency photons, or gamma rays, emitted by the excited atomic nuclei produced in these reactions are measured by TIGRESS to study the structure of the nucleus and the forces that hold it together. TIGRESS design, development, and installation was supported by an $8.06M Research Tools and Instruments grant awarded by NSERC in 2003 to a collaboration of researchers from across Canada (the University of Guelph, Université Laval, McMaster University, Université de Montréal, Simon Fraser University, University of Toronto, and TRIUMF), with leadership by the University of Guelph. The TIGRESS spectrometer (Figure 1) is now fully operational and being used in a wide range of experiments at ISAC-II.

The "heart" of the TIGRESS spectrometer is an array of 32-fold segmented high-purity germanium (HPGe) gamma-ray detectors. As shown in Figure 2, each of the four large single crystals of germanium in a TIGRESS detector has an outer electrical contact with 8 separate segments, for a total of 32 such outer contacts per detector. In addition to the electronic signals from the inner core contacts, which give high-resolution measurements of the total energy deposited by gamma-rays in each crystal, signals from the 32 outer segment contacts provide information on the locations of the gamma-ray interactions within the detectors.

 

All of the TIGRESS detector signals are continuously digitized 100 million times per second (100 MHz) in custom-designed 14-bit 10-channel (TIG-10) waveform digitizer modules (Figure 4). The shapes of these digitized waveforms, from segments in which gamma rays interact as well as the induced signals in neighbouring segments, depend of the exact locations of the gamma-ray interactions within the HPGe crystals. The detailed analysis of these digitized waveforms thus allows the gamma-ray interaction locations to be determined with much finer resolution than the physical size of the detector segments. An average position sensitivity for single gamma-ray interactions of 0.44 mm has been achieved by these techniques (see Ref. 2 below).

 

The ability to determine gamma-ray interaction locations within the TIGRESS detectors enables accurate correction of the measured gamma-ray energies for the Doppler shifts inherent in experiments with ion beams accelerated to several percent of the speed of light, while allowing each HPGe crystal to subtend a large solid angle about the reaction point at the centre of the spectrometer. TIGRESS thereby attains the excellent gamma-ray energy resolution that is the defining feature of HPGe detectors, while simultaneously achieving the very high gamma-ray detection efficiency required for experiments with accelerated radioactive ion beams.

The TIGRESS HPGe clover detectors are surrounded by Compton suppression shields constructed from the high-efficiency scintillator crystals bismuth germanate (BGO) and cesium iodide (CsI). These shields detect gamma rays that scatter out of the HPGe crystals without depositing their full energy, and the subsequent vetoing of these unwanted events significantly improves the signal to background in the gamma-ray spectra recorded by TIGRESS. As illustrated in Figures 5 and 6, the TIGRESS Compton suppression shields, each of which contains 20 optically isolated scintillator segments, have a modular design that enables rapid reconfiguration of the entire spectrometer between a "maximum efficiency" configuration in which the HPGe detectors are close-packed at 11.0 cm radius from the reaction centre, and an "optimal suppression" configuration in which the HPGe detectors are withdrawn to 14.5 cm and the BGO front shields are inserted radially to form a full Compton suppression shield around each HPGe detector.

 

In both configurations of the TIGRESS array, an inner sphere of 11.0 cm radius is available to accommodate the auxiliary detection systems necessary to detect reaction products in coincidence with the gamma rays measured by the surrounding TIGRESS detectors. The initial Coulomb excitation, inelastic scattering, and transfer reaction experiments with TIGRESS at ISAC-II were performed with the segmented Si CD detectors of the Bambino (Figure 7) array developed by collaborators at Lawrence Livermore National Laboratory and the University of Rochester in the United States. The Silicon Highly-segmented Array for Reactions and Coulex (SHARC) (Figure 8) developed by collaborators from the University of York in the United Kingdom and Louisiana State University and Colorado School of Mines in the United States, has been used for (d,pγ) transfer reaction studies with TIGRESS at ISAC-II. The TIGRESS Integrated Plunger (TIP) developed by collaborators from Simon Fraser University provides a powerful system for nuclear excited state lifetime measurements with TIGRESS (Fig. 9). Most recently, the Spectrometer for Internal Conversion Electrons (SPICE) (Fig. 10), has been employed in first in-beam internal conversion electron measurements with TIGRESS. Auxiliary detectors that can be located downstream of TIGRESS at ISAC-II include the DESCANT neutron detector array developed at the University of Guelph and the ElectroMagnetic Mass Analyser (EMMA) currently being developed at TRIUMF. These combined systems provide a powerful facility to pursue nuclear structure, nuclear astrophysics, and nuclear reactions research with the high-quality accelerated radioactive ion beams from ISAC-II through the detection of conversion electrons, light charged particles, neutrons, and heavy ion recoils in coincidence with the gamma rays measured by TIGRESS.

 

Additional technical details related to TIGRESS are given in the publications listed below, while the most recent physics results from our research programs with TIGRESS can be found in our publications and theses lists.