A compact Ge-detector array
GEORGINA is a compact Ge-detector array consisting of 5 detectors each with a relative γ-efficiency of 100%. The design criteria of this array were:
- Large efficiency for energies up to 12 MeV.
- Compact and versatile design to fit into tight spaces, to allow effective shielding, and to adopt the array to a wide variety of experimental needs (e.g., in combination with Si-detector arrays or other γ-detectors).
- Modest granularity; in most experiments of interest the γ-ray multiplicity will be ≤3.
This has been realized by 5 individual 100% n-type Ge-detectors with their crystals mounted at 90° to the cold finger as shown below
Placing the centers of the crystals on the corners of a regular pentagon (as shown in the schematic 3-d drawing above) leaves an opening large enough to accommodate a standard 2” beam line. This allows placing the array just upstream of a target in which the nuclear reaction takes place (as indicated in the schematic by the golden disk representing the target and the arrow the beam direction). In this way it is possible to cover most of the upstream solid angle with Ge-crystals resulting in a large γ-efficiency, often a prerequisite for the measurements of low reaction cross sections. A typical set-up is shown in the picture to the left where one of the detectors has been removed for clarity.
In many low-energy reactions with beam energies below 2-3 MeV the beam is stopped in the target itself and an additional large volume Ge-detector can be placed just downstream of the target which approximately doubles the γ-efficiency. Because of its modularity GEORGINA can also be used to measure γ-γ coincidences between members of a cascade. In a recent experiment one such a cascade was observed which consisted of a 4.4 MeV γ-ray in coincidence with a 4.9 MeV γ-ray. Setting a coincidence gate on the 4.9 MeV line observed in GEORGINA resulted in a γ-spectrum in the downstream detector shown below (note: x-scale in channel number with a dispersion of 2 keV/ch):
This spectrum is background free and represents the response function of a Ge-detector to 4.4 MeV γ-rays. Using this method and selecting appropriate cascades it is possible to experimentally determine absolute full-energy as well as total detector efficiencies up to energies in excess of 10 MeV without the need of calibrated sources and summing corrections. Because of the good performance of the GEORGINA detectors, they can also be used individually; for example detectors have been recently used in combination with RHINO and ICEBALL.