Spin Spectrometer

The Spin Spectrometer is a unique device which is capable of measuring simultaneously the gamma-ray multiplicity and the total gamma-ray energy on an event-by-event basis. This 4-pi array, consisting of 72 close-packed NaI(Tl) detectors, will be available to users of the Holifield Radioactive Ion Beam Facility from the beginning of operations.

Each crystal of the Spin Spectrometer is 178 mm from the target and covers about 1.34% of the total solid angle. Stable performance is best obtained at count rates below 20k per detector. Each detector is enclosed in an aluminum can and is 178 mm thick. The average energy resolution ranges from 8.6% at 662 keV to 6.3% at 1332 keV. Typical time resolution is 2.1 ns at FWHM. More detail concerning the response and performance of the Spin Spectrometer can be found in the reference below.

In addition to the standard operational mode, up to 20 of the NaI(Tl) detectors may be replaced with Compton-suppressed Ge(Li) detectors. Other auxiliary detectors which have been used in conjunction with this array include X-ray detectors, neutron counters, barium fluoride detectors, and several different charged particle detectors. The reaction chamber at the center of the array has an inner diameter of 32 cm and is equipped with 4 access ports and internal hardware for supporting auxiliary detectors. Particle detectors which have been used in the past include silicon telescopes, a 4-pi plastic phoswich array, a 4-pi CsI(Tl) array, and parallel plate avalanche counters.

The energy calibrations of the array are automated and accomplished using some subset of the following gamma-ray sources; Se-75, Cs-137, Bi-207, Co-60, Y-88, and Na-24. A typical calibration is 5 keV/channel which gives 10 MeV full range (2048 channel ADC). Response calibrations are also automated and take into account the detector efficiencies, detector-to-detector scattering, coincidence summing in each element, and pulses from neutrons and/or high energy protons. Neutrons are identified using a time-of-flight technique as a function of detected energy. The Spin Spectrometer operates best when some auxiliary device determines the time of the event, but algorithms exist which can be used to determine the time of an event from the distribution of the triggering times in the NaI(Tl) detectors when the number of detected gamma-rays is at least three. Due to changes in computer platforms, some automated features discussed above, may not be currently available.

Reference