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Figure 1 : The TIGRESS Gamma-Ray Spectrometer located on its dedicated experimental beamline at the ISAC-II facility at TRIUMF. |
The TRIUMF-ISAC Gamma-Ray Escape Suppressed Spectrometer (TIGRESS) is a state-of-the art new gamma-ray spectrometer designed for a
broad program of nuclear physics research with the accelerated radioactive ion beams now being 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 higher 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
of light, 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 has been 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 operational and being used in a wide range of experiments at ISAC-II.
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Figure 2 : Schematic of the 4 high-purity germanium (HPGe) crystals of a TIGRESS "clover" detector, showing the 8-fold segmentation of the outer electrical contact of each crystal. |
Figure 3 : A single TIGRESS 32-fold segmented HPGe
clover-type detector in the testing laboratory. |
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Figure 4 : TIGRESS digital data acquisition system designed and constructed by Universite de Montreal TIGRESS collaborators. |
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 still 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.
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Figure 5 : Schematic of the TIGRESS HPGe crystals surrounded by their modular front, side, and back Compton suppression shields. |
Figure 6 : The maximum efficiency (left) and optimal suppression (right) configurations of the TIGRESS spectrometer. |
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Figure 7 : One Si CD detector of the Bambino detector inside the target chamber of TIGRESS. The accelerated radioactive beam from ISAC-II enters from the left. |
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Figure 8 : The SHARC Double-Sided Silicon Strip (DSSD) detector array being assembled (left), and installed in a specialized
target chamber at the centre of TIGRESS (right). |
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 have all been 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. Most recently, 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 employed in its first (d,pg) transfer reaction study with TIGRESS
at ISAC-II. Additional detectors that will be located downstream of TIGRESS at ISAC-II include the DESCANT neutron detector
array under development at the University of Guelph and the ElectroMagnetic
Mass Analyser (EMMA) being developed
at TRIUMF. These combined systems will provide a powerful new 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 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.
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