The spin spectrometer: Design, instrumentation and response characteristics of 4 πγ-ray multidetector system

Abstract

Construction of a new type of spectrometer for the investigation of nuclear structure at high angular momentum and of the mechanisms of heavy-ion induced reactions is reported. The spectrometer consists of 72 NaI(Tl) detectors closely packed in a 4 π arrangement. It is capable of recording on an event-by-event basis for each detector that fires (1) the identification number, (2) the pulse height, (3) the time of flight, and (4) the pulse width. From these data we can obtain for each event: the γ-ray multiplicity, the total pulse height and thus the total γ-ray energy, the neutron multiplicity, the angular correlations of the emitted γ-rays, and the time relationships between the γ-rays in a cascade. Pulse height spectra of the γ-rays and of the neutrons associated with the above parameters can also be obtained. The geometric arrangement, the performance of the individual detectors, the associated electronics, the data acquisition system, the response of the entire system to incident γ-cascades of given multiplicity and total energy (including the effect of coincidence summing and detector-to-detector scattering), and the response to incident neutrons are discussed for various approximate and realistic inputs. The unfolding of populations of total pulse height and coincidence fold to yield populations in excitation energy and γ-ray multiplicity is described. The unfolding of pulse-height spectra observed in individual detectors to yield γ-ray energy spectra is also discussed.