Chapter 1 Introduction
2.3 Experiments and set-ups
2.3.2 AFRODITE array
AFRODlTE is an acronym for AFRican Omnipurpose Detector for Innovative Techniques and Experiments. The AFRODITE array is mounted on a rhom- bicuboctahedron frame with 16 detector positions [New98]. The target chamber has the same geometry (see Figure 2.7). The array normally consists of two types of high purity germanium (HPGe) detectors, namely, Compton suppression clover detectors and low energy photon spectrometers (LEPS). The clover detectors are similar to those of EUROGAM II and EUROBALL III [Duc99]. Each clover de- tector is surrounded by a bismuth germanate (BGO) Compton suppression shield.
Each clover consists of four 50 × 70 mm HPGe crystals and the eight clovers subtend a solid angle of 11% of 4π. These detectors are very suitable for measur- ing high-energy γ-rays due to their large volume. The total active Ge volume is approximately 470 cm2.
Moreover, the crystals are closely packed without any material between them, thus enabling good resolution for signals added from more than one crystal. The distance between the front face of the detector and the center of the AFRODITE array is 17.6 cm [New98]. The target chamber is positioned at the center of the AFRODITE array as shown in Figure 2.8. There is a target ladder that moves
vertically and it controls the target position inside the target chamber. The target ladder has four slots. In this experiment two slots were used for targets, one for empty frame, while in the last slot a ruby viewer was mounted. The ruby viewer was used for focusing the beam. The ruby viewer can be moved perpendicular to the beam direction using the target ladder. The viewer is a metal plate with a small hole at the center and is used to align the beam from the SSC. The verification of visual alignment is done using a camera connected to the small transparent window on the target chamber. The changing of the target can be done automatically using the control panel. The measured full energy peak efficiency of the AFRODITE array for nine clovers is 1.8% at 1.332 M eV.
Figure 2.7: The AFRODITE array. The target chamber with Compton sup- pression clover detectors and LEPS detectors are visible.
Chapter 2. Experimental Techniques and Equipment 21
Figure 2.8: The AFRODITE array with its two hemi-spheres open. The black insulated pipes to the detectors dewars are used to fill them with liquid nitrogen.
2.3.2.1 Clover detectors
Clover detectors are ideal for detecting high energy γ-rays because of their high efficiency in add-back mode. A clover detector is shown in Figure 2.9. These detectors are composite detectors. A composite detector is made up of several Ge crystals packed closely together in the same cryostat. In this way a large detector can be created with high photo-peak efficiency and high resolving power, since the individual crystals minimize the effect of Doppler broadening. Doppler shift depends on the direction of the emitted γ-ray with respect to the direction of the recoiling nucleus. One can apply Doppler correction to improve the energy resolution when the recoiling nucleus is not stopped in the target. However the accuracy of the Doppler correction is better when the detector has small opening angle. An array of composite detectors has high granularity and small opening angle for each crystal, while it has high efficiency. The clover detectors consist of four n-type coaxial HPGe crystals arranged in compact geometry. The crystals with the dimensions of 70 mm in length, 50 mm in diameter, 36 mm tapering length and 41mmside of the crystal surface are packed in a common cryostat, as shown Figure 2.10 and Figure 2.11.
Figure 2.9: A clover detector
Each HPGe crystal element has its own preamplifier, which allows energies de- posited in more than one element of the clover detector due to Compton scatter- ing to be added. The clover detector can be operated in two different modes, the direct mode and the add-back mode. In direct mode, each of the four crystals is considered independently as a separate detector. If the number of hits is more than one, the full energy peak is considered to be a sum of the individual energy peaks of the four segmented crystals. The efficiency is;
εdirect =
4
X
i=1
εi (2.19)
Chapter 2. Experimental Techniques and Equipment 23
Figure 2.10: Schematic of the 4 HPGe crystal of a clover detector.
Figure 2.11: Left: Individual HPGe crystal. Right four: Closely packed HPGe crystals in a clover detector.
In add-back mode coincidence hits in the different crystals are considered and summed together in order to reconstruct the full energy peak. Conventionally, only double hit events in adjacent crystals should be included in the add-back process, while diagonal or three hit events in a clover are assumed to correspond
to more than one incident γ-ray and are most likely accidental events. Therefore, the full energy peak in the add-back mode is the sum of one fold events efficiency and two fold (excluding diagonal scattering) events efficiency. For this work the mode used for data analysis was direct mode.
The advantages of using clover detectors can be summarized as follows:
• Reduces the effect of Doppler broadening by increased granularity because of the reduced opening angle of the clover crystals, in comparison with a large volume detector.
• Good energy resolution.
• Good timing response.
Chapter 2. Experimental Techniques and Equipment 25 Specifications of the clover detectors in the AFRODITE array [New98];
• Distance from the crystal surface to the target centre: Dtc = 196 mm
• Distance from the detector end-cap to the crystal surface: Dec = 20 mm
• Total opening angle per detector: θ= 23.2◦
• Solid angle per detector: ΩGe = 1.34% of 4π(for a 0.2 mmdistance between crystals)
• Photopeak efficiency (for 1.33M eV): εph = 20×10−4
• Add-back factor (for 1.33M eV): 1.56
• Peak-to-total ratio (for 1.33 M eV): (P/T)Ge= 0.30
2.3.2.2 Compton suppression
In order to reject theγ rays that Compton scatter out of the clover detector, the Ge crystals are housed inside a bismuth germanate (BGO) suppression shield. Thus, BGO is a highly efficient scintillator for the detection of γ rays. For this reason it is well suited to veto the Compton scatteredγ rays that leave the HPGe detec- tor volume. Spectrum obtained using the HPGe detectors show some unwanted background that comes when a portion of the energy of the incident photon is deposited inside the detector due to Compton scattering. Hence, weak peaks are completely masked by the Compton continuum. Compton suppression is one way of reducing this unwanted background. The HPGe detectors are surrounded by bismuth germanate (BGO) shield. The BGO shield is made of scintillation de- tectors that have comparatively low light output as opposed to other scintillator detectors. They are best use used as veto for Compton scattered events. In order to prevent the detection of γ rays which have not been scattered from the germa- nium crystal, hevimet collimators are placed in front of the BGO detector. The peak to total ratio improves to about 45%. Figure 2.12 shows a schematics of a Ge detector with BGO shield.
Figure 2.12: Schematic cross section of a clover-BGO system.
2.3.2.3 Low Energy Photon Spectrometer (LEPS)
The LEPS detectors are made of a single HPGe crystal of p-type with 10mmthick- ness and 60 mm of diameter and are electrically segmented into four quadrants.
The signals from each quadrant are processed separately. The LEPS detectors are more suitable for detection of low energy γ-rays due to the small volume of their Ge crystals. They have high relative efficiency at low energy between 30 keV to 300 keV.
Some technical specifications of the AFRODITE array for LEPS detectors [New98]:
• The distance from the crystal surface to the target centre: Dtc = 119 mm for a 4 mm gap between the target chamber and the end-cap.
• Total opening angle per detector: θ= 28.3◦
• Solid angle per detector: ΩGe = 1.57% of 4π
• The distance from the detector end-cap to the crystal surface: Dec = 15 mm