CLover Array for Radioactive ION beams
The Ge array consists of eleven segmented Clover detectors and ten elements
of the old ORNL Compton Suppressed Spectrometer (CSS) array.
is deployed on the RMS beam line and surrounds a 15-cm radius target chamber
with a forward fantail extending some 30 cm from the target. The
majority of the detectors are placed in the back hemisphere,
approximately 25 cm from the target. This large distance reduces
Doppler broadening and allows the use of auxiliary detectors such as
The Clover detectors are slightly modified versions of those used in Eurogam II. Each Clover detector consists of four individual crystals sharing a common cryostat. Each crystal is electronically segmented into two halves, and has an individual efficiency of 25% relative to a 3-inch by 3-inch NaI detector. By minimizing the dead layer between the adjacent crystals, the signals from all four crystals may be added together to obtain the total pulse height for an incident gamma ray that scatters from one crystal to one or more of the other three crystals. This so called add-back mode increases the photo efficiency of a Clover detector from about 100% to more than 150% for Co-60 lines, and provides excellent efficiency for detection of high-energy gamma rays. When operated in the add-back mode, a single Clover detector has a photopeak efficiency of 0.2% for detection of 1.33 MeV gamma rays. Furthermore, since the segmented Ge crystals have an effective width of 2 cm, the Doppler broadening resulting from is minimized. The peak-to-total ratio of the Compton-suppressed clover detectors, when operated in add-back mode, is approx. 0.55 for Co-60. The four individual crystals of a Clover detector may also be used as a polarimeter.
The CSS elements consist of 25% Ge detectors equipped with BGO Compton Suppression Shields. The Ge crystals have a length of approximately 7 cm, a diameter of 5 cm and provide a peak-to-total of nearly 0.5 for Co-60 lines when operated in the Compton-suppressed mode. At a distance of 25 cm, the 10 detectors provide a total photopeak efficiency of 0.3% for a 1.33 MeV gamma-ray.
The combination of 10 CSS elements and 11 Clover detectors placed 22 cm from the target will have a total photopeak efficiency of about 2.6% for a 1.33 MeV gamma ray. The Clovers may be placed at three target-to-Ge distances of 20, 21.7, and 23.5 cm. Five of the Clover detectors are placed at 90 deg. with respect to the beam axis; these detectors can provide linear polarization information. The angles of the remaining Ge detectors are: 5 CSS at 57 deg., 1 CSS at 90 deg., 4 CSS at 120 deg., 4 Clovers at 132 deg. and 2 Clovers at 155 deg. No Ge detector is placed at very forward angles, where Rutherford scattering of the radioactive ion beams would cause excessive background of positrons and 511 keV annihilation radiation. The design of the array shell is such that the positions of the detectors are interchangeable. However, due to the large face of the Clovers, some crystals may lie face-to-face resulting in enhanced detection of annihilation radiation.
Details of angular positions for the Ge crystals, and source codes for gain-matching, add-back and Doppler correction, may be obtained from firstname.lastname@example.org.
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This file last modified Monday January 08, 2007