The Clover Detector

Figure 1:Schematic views of the Clover detector

The Clover detector has been proposed and developed by the France / U.K. collaboration as part of Euroball. It consists of four coaxial n-type Ge detectors, arranged like a four-leaf clover. Each detector has a square front face with round edges obtained by tapering it on two adjacent faces with an angle of 7.1 degree starting at around the half of the length and by cutting the two remaining faces parallel to the crystal axis and along its whole length. This enables a close packing of the detectors in the front - the Ge-Ge distance is about 0.2mm - and retains most (89%) of the original crystal volume. For Ge detectors of 50mm diameter and 70mm length, the remaining active volume in the Clover detector is about 470 cubic centimeters. The detectors are held only by the rear-side which reduces the amount of material around the detector resulting in a good P/T ratio. The four detectors are mounted in a common cryostat of tapered rectangular shape. To save space the gap between the tapered edges of the detectors and the inside of the cryostat is as small as 3.5mm.

To enable the close packing of the Ge detectors, the outer surfaces of the detectors are at ground whereas the high voltage (common for the 4 detectors) is applied on the inner contact. Energy and timing signals are obtained through AC-coupling for each detector.

The advantages of the Clover detector are:

• a reduction of the Doppler broadening
• a high sensitivity to the linear polarisation of gamma-rays
• a reduced vulnerability to neutron damage
• a good timing particularly for low energy gamma-rays

The average photopeak efficiency of the individual detectors at 1.33MeV is 2.58 * 10 ^-4 at 25cm from the 60Co source, whereas the total photopeak efficiency of the Clover detector in the add-back mode is 16.1 * 10 ^-4. This value corresponds to a large add-back factor (total photopeak efficiency in add-back mode divided by the sum of the individual photopeak efficiencies) of 1.56. The P/T response of the bare Clover detector is 0.30.

The time response of the individual detectors measured against a small BaF-scintillator with gamma-rays from a 60Co source is about 5.5ns FWHM and the FWTM/FWHM is smaller than 3.0 which is good regarding the low CFD threshold (50keV). For selected gamma-ray energies larger than 300keV the FWHM and the FWTM are 4.5 and 11.3ns respectively.

Cross-talk between the four channels has been investigated. The tests do not show parasitic peaks down to 10-20keV in the Ge detectors neighbouring the one detecting the full energy of a gamma-ray.

The correction for Doppler broadening has been studied for the Clover detector by means of Monte-Carlo simulation calculations taking into account both the detection of the full energy of a gamma-ray and the scattering of this gamma-ray between two detectors. The simulation calculations demonstrate that all the scattering interactions happen close to the common surface of the firing Ge detectors. The half width of the spatial distributions is smaller than 1.5cm and is almost independent of the gamma-ray energy ranging from 200keV to 5MeV. Since most of the scattered events (more than 75%) occur in a region of +-2cm measured from the surface separating the responding detectors, it is reasonable to use the centre angle of the Clover detector for Doppler shift correction. Only a few scattered events result from the regions beyond 2 cm. Therefore a simple Doppler broadening correction method can be used. After a proper orientation of the Clover detector relative to the beam axis, it can be divided in two parts perpendicular to this axis. If a gamma-ray is only detected in the right (left) hand side of the Clover detector, then the angular position of the detector placed in the right (left) hand side is used to correct for the Doppler shift. The center angle of the Clover detector is used when the gamma-ray is scattered from one side to the other. The photo peak of the gamma-line will after the correction have a good FWHM, only the FWTM will be slightly worsened.

A common escape-suppression shield consisting of BGO scintillation detectors surrounds the Clover cryostat containing the Ge detectors. It protrudes about 5 cm inward beyond the detector surface to be sensitive to backscattered gamma-rays and is 24 cm long. The maximum thickness of the BGO shield is 2 cm at about the middle of the Ge detectors. The P/T ratio of the escape-suppressed Clover detector is calculated to improve to 0.60.

The Clover detector is used in the 4pi array Eurogram II at Strasbourg which consists of 30 coaxial tapered Ge detectors at forward and backward angles and 24 Clover detectors in the sector around 90 degrees.