The Cluster Detector

The Cluster detector [1], developed in a collaboration of Univserity of Köln, KFA-Jülich and the company Eurisys, consists of seven large hexagonal tapered Ge detectors closely packed in a common cryostat. They are surrounded by a common BGO escape-suppression shield. By adding all signals of the individual detectors typically a total-absorbtion efficiency of 29 percent and a peak to total ratio of 61 percent (39 percent without shield) is achieved. At 5 MeV gamma-energy the efficiency is about twice that of a large individual detector. The energy resolution at 1.33 MeV gamma-energy is typically 2.4 keV.

Figure 1:Schematic view of a Cluster detector

Figure 2:Design of a Cluster detector

Figures 3 and 4: Encapsulated Ge-crystal

To protect the sensitive intrinsic surface of the detectors and to improve the reliability each crystal is encapsulated. The capsules [2] are made from aluminum with a wall thickness of 0.7 mm. It is hermetically sealed by electron welding of all feedthroughs and of the capsule lid. A pocket in the lid contains a getter material which keeps the vacuum inside the capsule below 10 ^ -6 mbar even at annealing temperatures of 110 degree Celsius. The cold part of the preamplifier is mounted on the capsule lid. Therefore it can be easily exchanged in case of damage.

The Ge crystals have a length of 78 mm and a diameter of 70 mm at the cylindrical end. The distance of the Ge surface to the inner wall of the capsule is only 0.5 mm which gives a distance of 2.5 mm between the surfaces of two neighbouring detectors in a Cluster.

15 Cluster detectors (plus one spare) have been ordered by Germany (8), Italy (4), Sweden (2), Danmark (1) and U.K. (1) for the Euroball III array. They will form a one pi shell at backward angles with a total-absorption efficiency of about 4 percent. The high granularity of the Cluster detector allows the Doppler broadening in accelerator based experiments to be minimized and the isolated-hit probability for high multiplicity events to be improved.

Prior to their implementation in Euroball III, the already available Cluster detectors are being employed in smaller set-ups. Early in 1995 experiments at the S-Dalinac in Darmstadt and the FN-Tandem in Köln have demonstrated the large sensitivity of these detectors for gamma-rays in the energy range 3 to 8 MeV. The Cluster campaign at the Max-Plack-Institut für Kernphysik, Heidelberg started in May 1995 with six Cluster detectors. Cluster experiments at GSI will start in spring 1996.

[1] J. Eberth, Conf. on Physics from large gamma-ray Detector Arrays, Berkeley, LBL-35687 (1994).

[2] J. Eberth et al., Nuclear Physics News, Vol. 3, No. 3 (1993) 8.

last update march 1st, 1999 / Euroball at GSI / HJW