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Target and Miniball |
Target and Miniball
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Figure 1. The Miniball detector array at CERN. |
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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).
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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 [5].
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:
- Full-energy
peak efficiency: εph
≥ 20 % at Eγ=
1.3 MeV, εph
≥ 5 % at Eγ=
11.7 MeV.
- High
counting-rate capability: ≈ 10 kcs-1 per individual Ge-detector.
- Granularity:
> 200 in 4π and > 600 in 4π including the radial
information from pulse-shape processing.
- Minimum inner
free sphere for ancillary detectors: radius ≥ 10 cm.
- High
modularity and flexibility for arrangements in different geometries.
- Geometrical
compatibility of the Miniball modules with the composite detectors in use
in Euroball.
Data - Miniball
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Granularity = 144 segments in 4π, Ω
= 65 % |
Compact disc detector
Motivation
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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.
Technical
Description
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.
Electronics
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.
Data - Particle
detector
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Total area = 50 cm2 (93% active) |
References
0. http://www.miniball.york.ac.uk
1. S. Schebel et al.,
annual report, MPI für Kernphysik, Heidelberg, 1992 23
2. F. S. Goulding and D.
A. Landis, IEEE Trans. Nucl. Sci. 41, No. 4 (1994)
3. T. Kröll et al.,
Nucl. Instr. Meth. A371 (1996) 489
4. Ch. Gund, Diploma
thesis, Heidelberg, 1996
5. J. Eberth et al.,
Nucl. Instr. Meth. A369 (1996) 135
6. P. Reiter et al.,
Nucl. Phys. A701 (2002) 209c
7. J. Eberth et al.,
Progress in Particle and Nuclear Physics 46 (2001) 389