GRIFFIN

Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei (GRIFFIN) is a state-of-the-art new high-efficiency gamma-ray spectrometer (Fig. 1) for decay spectroscopy research with the low-energy radioactive ion beams provided by the ISAC and ARIEL facilities at TRIUMF. GRIFFIN supports a broad program of basic and applied research in the fields of nuclear structure, nuclear astrophysics, and fundamental particle interactions.

 

The "heart" of the GRIFFIN spectrometer is an array of 16 large-volume "clover-type" high-purity germanium (HPGe) gamma-ray detectors (Fig. 2), each of which contains 4 large germanium single crystals 90 mm in length and 60 mm in diameter with the outer edges tapered at 22.5 degrees over the first 30 mm of their length for close-packing in the 16-detector array.

 

The GRIFFIN HPGe gamma-ray detectors provide excellent energy resolution (~ 1.9 keV FWHM for 1.33 MeV gamma rays) and, in the close-packed configuration in which the front faces of the detectors are located 110 mm from the centre of the array, a very high gamma-ray detection efficiency. As shown in Fig.3, by adding the energies deposited in the 4 crystals of each clover detector an absolute detection efficiency of 19% for 1 MeV gamma rays is achieved, while an even higher absolute efficiency of 25% at 1 MeV can be obtained in low gamma-ray multiplicity experiments by also adding together the energies deposited in neighbouring clover detectors. Figure 3d) compares this performance of GRIFFIN to that of the former 8pi Spectrometer used in the decay spectroscopy research program at ISAC-I until December 2013. In the "clover mode" GRIFFIN is 17 times more efficient that the 8pi Spectrometer was for 1 MeV gamma rays and 37 times more efficient by 10 MeV, while the "summed neighbour" mode of GRIFFIN is 22 times more efficient at 1 MeV and more than 70 times more efficient than the 8pi Spectrometer was for 10 MeV gamma rays. As most GRIFFIN experiments are performed in a coincidence mode in which at least 2 gamma rays from the same nuclear decay are measured in coincidence, the relevant figure of merit for the performance of the spectrometer is the square of the above efficiency ratio, i.e. a factor of ~ 300 - 500 increase in gamma-gamma coincidence efficiency compared to the previous 8pi Spectrometer for typical experiments with ~ 1 MeV gamma rays. The enormous increase in gamma-ray detection efficiency provided by GRIFFIN is revolutionizing the decay spectroscopy research program at ISAC, enabling both high-precision measurements of complex beta decay schemes near stability and detailed spectroscopic studies of the most exotic rare isotopes that will be produced by the new ARIEL facility at TRIUMF.

In addition to the 16 HPGe gamma-ray detectors, GRIFFIN has also been designed to accommodate a powerful suite of auxiliary detection systems that has been developed by our collaboration over the past decade. These auxiliary detectors include:

 

The Scintillating Electron-Positron Tagging Array (SCEPTAR) comprised of 20 thin plastic scintillator beta detectors that surround the implantation point of the radioactive ion beam inside the central GRIFFIN vacuum chamber. SCEPTAR detects the beta particles emitted in radioactive decays with high efficiency (~ 80%), as well as providing information on their directions of emission in order to veto background in the surrounding GRIFFIN HPGe detectors from the bremsstrahlung radiation produced by the stopping of the energetic beta particles.

 

The Zero Degree Scintillator (ZDS), a fast plastic scintillator located immediately behind the radioactive beam implantation location on the thin tape that can replace one hemisphere of the SCEPTAR array for beta detection in fast timing experiments.

 

An in-vacuum moving tape collector (MTC) system developed by collaborator Prof. E.F. Zganjar from Louisiana State University. This continuous loop tape system intersects the radioactive ion beam from ISAC-I at the centre of GRIFFIN, threads around the plastic scintillators and light guides of SCEPTAR (Fig. 4) or the ZDS (Fig. 5) inside the central vacuum chamber, and is collected in a box behind a lead shielding wall outside of the spectrometer (see Fig. 1). Long-lived daughter activities and/or contaminants in the radioactive ion beam are thus removed from the view of the GRIFFIN detectors. The timing of the beam implantation/counting/tape movement cycles are user programmable and optimized for each experiment.

 

The Pentagonal Array of Conversion Electron Spectrometers (PACES), an array of 5 liquid nitrogen cooled Si(Li) conversion electron detectors also developed by collaborator Prof. E.F. Zganjar of Louisiana State University. PACES can replace one hemisphere (10 detectors) of SCEPTAR inside the GRIFFIN central vacuum chamber and detects the internal conversion electrons that compete with gamma-ray emission during transitions between nuclear states. PACES enables transition multipolarities to be determined through the measurement of internal conversion coefficients, and provides access to highly-converted low-energy transitions and electric monopole (E0) transitions for which gamma-ray emission is forbidden. The PACES Si(Li) detectors also serve as alpha-particle detectors for experiments with beams of heavy actinide isotopes.

 

Exterior to the central vacuum chamber, an array of 8 fast-timing gamma-ray detectors can also be deployed through the 8 triangular faces of the GRIFFIN rhombicuboctahedral geometry. These detectors allow the half-lives of excited nuclear states that exist for only picoseconds (trillionths of a second) to be measured directly by fast electronic timing methods. Any combination of up to 8 of the BaF2 detectors (Fig. 8) of the Dipentagonal Array for Nuclear Timing Experiments (DANTE) or up to 8, 51 mm x 51 mm cylindrical LaBr3(Ce) detectors (Fig. 9) can be used for these measurements. The LaBr3 detectors provide better gamma-ray energy resolution and higher detection efficiency, while the BaF2 detectors provide the ultimate in fast timing response.

Finally, by removing the 4 downstream HPGe clover detectors of GRIFFIN, it is also possible to couple the 70-element Deuterated Scintillator Array for Neutron Tagging (DESCANT) neutron detector array shown in Fig. 10. The combination of GRIFFIN and DESCANT provides a powerful capability for beta-delayed neutron-gamma coincidence measurements with the most neutron-rich rare isotope beams from ISAC and ARIEL.

The signals from the GRIFFIN HPGe, Si(Li), BaF2, and LaBr3 detectors are continuously digitized 100 million times per second (100 MHz), while the signals from the SCEPTAR, ZDS, and DESCANT detectors are continuously digitized 1 billion times per second (1 GHz). These digitized signals are then process and filtered in a hierarchy of custom digital electronic modules (see Fig. 11) that have been designed for complete data readout from the spectrometer with each of the 64 HPGe crystals recording as many as 50,000 gamma-ray interactions per second. This digital data acquisition system allows any combination of coincidence triggers to be selected for a particular experiment, and provides accurate control of deadtimes for high-precision measurements.

 

The first phase of the GRIFFIN project, comprised of the 16 large-volume HPGe clover detectors, their mechanical support structure, digital electronics, and associated instrumentation, was funded at a total project cost of $8.96M by the Canada Foundation for Innovation (CFI), the University of Guelph, and TRIUMF, with major in-kind contributions from TRIUMF and commercial vendors. The project was initiated in August 2011, and an Early Implementation of the spectrometer comprised of 12 HPGe clover detectors was commissioned at ISAC-I in September 2014 and the full complement of 16 GRIFFIN HPGe detectors began scientific operation in the summer of 2015.

 

The second phase, and completion, of the full GRIFFIN spectrometer through the addition of bismuth germanate (BGO) Compton and Background Suppression Shields (see Fig. 12) to veto background events in the GRIFFIN HPGe and LaBr3 gamma-ray detectors was funded by the Canada Foundation for Innovation (CFI), the Ontario Ministry of Research and Innovation (MRI), and the British Columbia Ministry of Advanced Education in 2015 and is currently under construction.