Nuclear Physics Group

The 8π Spectrometer

After a decade of operation at ISAC-I, the 8pi Spectrometer was decommissioned in January 2014 to make way for
the new high-efficiency GRIFFIN spectrometer. The 8pi Spectrometer has now been moved to the
Nuclear Science Laboratories at Simon Fraser University.

Figure 1 : One hemisphere of the 8pi Gamma-Ray Spectrometer at ISAC-I. The removable white Delrin layer in front of each HPGe detector and Compton suppressor prevents high-energy beta particles from directly striking the gamma-ray detectors.

The Canadian 8pi Gamma-Ray Spectrometer is an array of 20 high-purity germanium (HPGe) gamma-ray detectors that provides high-resolution measurements of the gamma rays emitted from excited states of the atomic nucleus. These HPGe detectors are surrounded by Compton suppression shields formed from crystals of the high-efficiency scintillation detector bismuth germanate (BGO), which detect and veto unwanted events in which a gamma ray Compton scatters out of the HPGe crystal without depositing its full energy. The $5.0M 8pi Spectrometer was originally funded jointly by NSERC and Atomic Energy of Canada Limited and was used in experiments with accelerated stable ion beams at the Chalk River Laboratories. The spectrometer was redeployed to TRIUMF and reconfigured by our collaboration for optimal use in decay spectroscopy experiments with the low-energy (30-60 keV) radioactive ion beams provided by the ISAC-I facility.

The 8pi Spectrometer surrounded the target location of a dedicated low-energy beam transport line at the ISAC-I facility, where it was the primary facility for decay spectroscopy research between 2003 and 2013. The radioactive ions from ISAC-I were implanted in the centre of the spectrometer and gamma rays emitted following their radioactive decay detected and measured in the surrounding HPGe detectors of the 8pi. To augment the capabilities of the 8pi Spectrometer our collaboration developed a suite of powerful auxiliary detection system. These include:

The Scintillating Electron-Positron Tagging Array (SCEPTAR) developed at the University of Guelph and TRIUMF. SCEPTAR consists of an array of 20 thin plastic scintillator beta detectors that surround the implantation point of the radioactive ion beam inside the central vacuum chamber of the 8pi Spectrometer. 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 surround HPGe detectors from the bremsstrahlung radiation produced by the stopping of the energetic beta particles.

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 the 8pi Spectrometer, threads around the plastic scintillators and light guides of SCEPTAR inside the central vacuum chamber of the 8pi, and is collected in a box behind a lead shielding wall outside of the spectrometer (see Fig. 3). Long-lived daughter activities and/or contaminants in the radioactive ion beam are thus removed from the view of the 8pi and SCEPTAR detectors. The timing of the beam implantation/counting/tape movement cycles are user programmable and optimized for each experiment.

Figure 2 : One hemisphere of the SCEPTAR beta detector array inside one hemisphere of the 8pi Spectrometer. The radioactive ion beam from ISAC-I is implanted on a thin tape (not shown) at the mutual centre of the 8pi and SCEPTAR arrays. Figure 3 : The collection box and lead shielding wall (yellow) for the moving tape collector system of the 8pi Spectrometer. The tape travels inside the beam pipe to intersect the low-energy radioactive ion beam from ISAC-I at the centre of the arrays.

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 8pi 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 Dipentagonal Array for Nuclear Timing Experiments (DANTE), an array of 10 BaF_2 gamma-ray detectors. The BaF_2 scintillators have an extremely fast time response and 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.

Figure 4 : The Si(Li) conversion electron detectors of the PACES array inside the 8pi vacuum chamber. Figure 5 : One of the 10 BaF_2 detectors of the DANTE array.

The electronic signals from all of the above detectors were processed in a high-throughput and flexible data acquisition system developed at the University of Guelph and Lawrence Livermore National Laboratory. This data acquisition system allowed any combination of coincidence triggers to be selected for a particular experiment, and provided both macro event assembly from the independent array data streams and accurate control of total system deadtimes for high-precision measurements through time stamping of all events relative to a temperature stabilized 0.1 ppm precision 10 MHz reference clock.

The 8pi Spectrometer, with its powerful suite of auxiliary detection systems, formed a unique facility for beta-gamma-electron-X-ray coincidences decay spectroscopy studies with radioactive ion beams and supported a broad program of nuclear structure, nuclear astrophysics, and fundamental symmetries research.

Highlights from our research programs with the 8pi Spectrometer at ISAC-I can be found in our publications and theses lists.