Target and Miniball
Figure 1. The Miniball detector array at CERN.
Figure 2. The detector array
With the construction of the first generation of radioactive-beam facilities in Europe it became apparent that a new generation of highly efficient γ-detector arrays is needed to make optimum use of these low intensity and expensive beams. In contrast to the Euroball or Gammasphere arrays the total full-energy peak efficiency and not the resolving power has to be optimized. With the Miniball array a factor of two larger full-energy peak efficiency compared to these high-spin devices is achieved at about 1/5 of the costs. The compact Miniball array is, however, mainly suited for the detection of events with low γ-ray multiplicities. Hence, it complements the high-spin physics addressed with, e.g., Euroball.
German, Belgian and French physicists have developed a γ-detector array called Miniball. The Miniball array has eight cluster detectors and each cluster consists of three individually encapsulated six-fold segmented high purity germanium detectors. The development of these compact arrays of segmented germanium detectors was pursued in a joint effort, solving problems like detector segmentation, densely packed electronics etc. together.
A common development for germanium arrays is the high granularity obtained by segmentation of the detectors. While the Exogam design contains 20 x 4 x 4 = 320 segments, the Miniball has 8 x 3 x 6 = 144 segments. This granularity is absolutely necessary to reduce the Doppler-broadening of γ-rays emitted by nuclei with velocities of up to 10% of the speed of light to a reasonably small value. The determination of the velocity vector of the γ-emitting nuclei by ancillary particle detectors will be required in many cases. Algorithms to improve the energy resolution and the localization of the primary γ-ray interaction via pulse-shape analysis have been investigated [1-4]. A localization of 1.3 MeV γ-rays in the radial position with a resolution of about 1 cm can be achieved. This results in an increase of granularity by about a factor of three. It also leads to a resolution of about 7 keV at Eγ = 1.3 MeV and v/c = 0.045 for the most problematic observation angle of 90° with respect to the flight direction of the γ-emitting nucleus (GEANT simulations).
Figure 3. A closer look at the target chamber surrounded by Miniball cluster detectors
In the Miniball array the new technology of encapsulated germanium detectors is used, where each detector module is kept under UHV vacuum in a thin-walled aluminium can. The advantage is that the single detectors of a cluster can be replaced easily in the common dewar. Thus the malfunction of an individual detector does not influence the remaining cluster. The importance of this technology increases with the number of energy signals from a sub-unit. Furthermore, the annealing of detectors is simplified. This new technology of encapsulated germanium detectors and their combination in cluster detectors has been proposed and realized in the framework of the Euroball project .
To prove that the encapsulation of segmented detectors is a reasonable technique it also has to be demonstrated that the isolation between different segments is maintained over several annealing cycles. Based on the requirements of the low multiplicity (Mγ < 15) experiments the following design values were formulated for the Miniball array:
Granularity = 144 segments in 4π, Ω
= 65 %
Figure 4. Signals of the core
The Edinburgh Nuclear Physics Group has developed a new particle detector, named the Compact Disc detector, based on the Double-Sided Silicon Strip Detector technology that has successfully been used by the group in recent years. The main motivations for the quest for a complete new design of such a device were the experimental conditions that are imposed by its usage at REX. To meet the needs the REX-ISOLDE detector set-up incorporates a compact and highly segmented silicon strip detector, which provides sufficient angular and energy resolution to account for the Doppler-shift broadening that will occur in the γ-spectra of the ions under investigation, due to their decay in flight. In addition, a timing signal will have to be provided for each detected ion as well. With this in mind, the Compact Disc detector design was developed to supply sufficient particle detection capabilities. In addition, an angular range of 10° - 40° at a distance of approximately 50 mm from the target will have to be covered.
The CD detector is a segmented DSSSD device, which is composed of four quadrants. The front of the CD consists of 16 annular p+ strips per quadrant at 2 mm pitch, while the back consists of 24 sector n+ strips at 3.5° pitch. This results in a total of 160 discrete detector elements. Consequently, information on the angular distribution of particles, with Δ = 3.5° and Δθ = 2.0°, can be extracted. The inter-strip distance will be between 35 μm and 100 μm. The total area of the CD detector is 5000 mm2, of which approximately 93% is active. The thickness of the silicon wafer will be between 50 μm and 1000 μm having a dead layer of 0.3 - 0.8 μm aluminium. The thickness will depend on whether energy loss or total kinetic energy measurement of the impinging particles is required. An energy loss detector in combination with a stop detector will provide particle identification. Another possibility to achieve this, is to derive a timing signal from the CD detector to measure time-of-flight with respect to the high frequency of the accelerator.
Each signal from the CD detector strips is fed into Edinburgh/RAL Preamplifiers type RAL108. The signals from these preamplifiers are then passed to Edinburgh/RAL Shaping Amplifiers to provide analog signals which reflect the deposited energy. These signals are subsequently fed into the analog-to-digital converters of the REX-ISOLDE data acquisition. In addition, Edinburgh/RAL Discriminators type RAL109, are used to produce a timing signal.
Total area = 50 cm2 (93% active)
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