The Argonne-Notre Dame BGO Gamma-ray Facility consists of two main components: an outer ring of 12 high-resolution Compton-suppressed Ge spectrometers (CSS) surrounding an inner array of 50 BGO elements with high gamma-ray stopping power and detection efficiency. These two figures show horizontal and vertical sections of the detector assembly.
This instrument is designed for a complete characterization of the electromagnetic decay of excited compound nuclei by providing the following quantities:
These quantities are measured on an event-by-event basis. Thus, it is possible to set gates on parameters, either individually or in a multidimensional space, to select of filter a specified class of events. For example, selection of high multiplicity and low sum-energy will enhance events with high spin but low excitation energy above the yrast line prior to gamma-ray decay.
The inner array is designed for bulk-property measurements of the [gamma]-ray spectrum (sum energy and multiplicity), i.e. this detector has to have a relatively good multiplicity resolution and sensitivity which is achieved with large solid angle coverage and good granularity. The array covers 78% of the 4[pi] geometrical solid angle around the target with 50 hexagonal elements. Each BGO element has a typical energy resolution of 17% (for 661-keV gamma-rays emitted from a 137Cs source) and a time resolution of 3 ns (measured with a 60Co source). The configuration of 50 separated BGO elements provides a sensitive measurement on [gamma]-ray multiplicity. For example, the measured multiplicity distribution for 30 [gamma]-rays, each of 1 MeV, peaks at 22.3 and has a resolution (FWHM) of ~5.5 (23%).
For the high-resolution photon energy measurements, in order to achieve high resolving power, the sensitivity of each individual Ge detector needs to be improved over that of a bare Ge crystal. In the BGO setup, the Ge detectors with BGO anti-Compton shields typically yield a peak/total (P/T) ration of ~0.62, compared to a P/T=0.18 for unsuppressed mode (again for a 60Co source). This improvement is further amplified in high-fold coincidence measurements: the fractions of events in full energy peaks for two-fold and three-fold coincidence measurements follow a (P/T)n law and will be 0.38 and 0.24, respectively; the corresponding values for bare Ge detectors being 3.2x10-2 and 5.8x10-3, respectively. Twelve CSSs offer sufficient pair combinations and solid angle coverage for optimal two-fold coincidence measurements.
In one of our experiments, using a 172-MeV 36S beam on a 160Gd target, typical count rates of the BGO inner ball were 180 K/sec and the corresponding count rate in each CSS was 2.2 K/sec.
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The general philosophy of a [gamma]-[gamma] coincidence experiment is that the [gamma]-rays detected by the detectors at almost identical time are considered to be in coincidence and, hence, are assumed to come from the de-excitation of a single compound nucleus. The circuitry must then determine the coincidence overlap from timing measurements, assure that all the required information (energies, patterns, times, ...) is recorded properly and also provide the capability of subtracting uncorrelated random events. A wide variety of computer programs are available which provide assistance with setting up the experiments (gain matching, adjustment of constant fraction discriminators, offset settings on ADCs, etc.), with data reduction and analysis (sorting of events into gamma-gamma coincidence matrices under a variety of gating conditions, peak fitting routines, etc.).
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A scattering chamber for coincidence measurements between [gamma]-rays and particles identified by either [Delta]E-E or time-of-flight techniques is also available. This equipment has been constructed by the University of Kansas group. A plunger apparatus for recoil-distance measurements of nuclear lifetimes has been developed by the Notre Dame group. The device fits entirely inside the BGO array. Dedicated chambers have been constructed for other experiments as well (g-factor measurements, fission-fragment coincidence measurements, etc.).
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