The CLOVER ARRAY

This section contains the description of the array to be coupled to the PRISMA spectrometer, based on the composite EUROBALL CLOVER detectors [7]. This detectors are composed of four Ge-HP crystals, each with a diameter of 50 mm (see figures  6  7), mounted in a single cryostat.
The energy signals from the four crystals are acquired independently and, since one $\gamma -$ray can interact with more than one crystal, add-back algorithms are used off-line to determine the $\gamma -$ray energy. To make use of the escape-suppression technique, the detectors are surrounded by a BGO anti-Compton shield, allowing a consistent improvement of the peak-to-total ratio. EUROBALL in its last phases (III and IV) incorporates 26 such detectors.

Figure 6: Ge-HP crystals in the CLOVER detector

Figure 7: The CLOVER detector with the anti-Compton shield.

The definitive design for the $\gamma -$array to be coupled to PRISMA is a shell of CLOVER detectors distributed in a hemisphere. The array first characterized by Monte-Carlo simulations and latter redesign to fit the mechanical requirements is shown in Fig. 8. In this configuration the Clover detectors are placed at backward angles (see Tab. 8) between $\theta=103^o$ (12 detectors) and $\theta=180^o$ (1 detector), with respect to the entrance direction of the spectrometer, which allows the maximum number of detectors, and therefore to obtain maximum efficiency, without using the $\theta=90^o$ angle, where the Doppler broadening is largest for products at high velocity.

Figure 8: Array built with 25 Clover detectors. The drawings correspond to the definitive configuration of the array designed by our collaborators from the University of Manchester.

To estimate the performance of the Clover array, Monte-Carlo simulations have been performed with the GEANT3 [8] detector design and simulation tool library. The simulation of the interaction of $\gamma -$ray with the detectors has been carried out including all relevant physics processes, with the geometry shown in figure 9 and under the kinematic conditions expected when measuring with PRISMA. The tungsten collimator has been modelled as an hemisphere with an inner radius of 19cm and a thickness of 3.5cm. The collimator holes have been designed to accept any $\gamma -$ray emitted within 5mm from the target position. The iron volumes of the PRISMA magnets, in front of the $\gamma -$array, have been also included in the simulation.

Table II: The angular position of the 25 Clover detectors identified in the figure  8.

Number	$\theta$	$\phi$	Number	$\theta$	$\phi$	Number	$\theta$	$\phi$
1	102.86	0.00	13	128.57	0.00	22	134.64	38.04
2	102.86	30.00	14	154.29	0.00	23	134.64	141.97
3	102.86	60.00	15	180.00	0.00	24	134.64	218.04
4	102.86	90.00	16	154.29	180.00	25	134.64	321.97
5	102.86	120.00	17	128.57	180.00
6	102.86	150.00	18	128.57	90.00
7	102.86	180.00	19	154.29	90.00
8	102.86	210.00	20	154.29	270.00
9	102.86	240.00	21	128.57	270.00
10	102.86	270.00
11	102.86	300.00
12	102.86	330.00

Figure 9: CLOVER array built with 25 detectors View from behind (left) and section (right).In the left panel, the white volume in front of each detector corresponds to the collimator holes.

Figure 10: Two views from the CLOVER array displaying the position of the Ge crystals with respect to the PRISMA magnets drawn in blue.

From the simulations we have obtained the following performance figures for the complete 25 detectors array:

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Total peak efficiency $\approx$3.3% for $E_\gamma$=1.3MeV.

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Peak/Total ratio $\approx$50%.

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Energy resolution $\approx$10 keV for v/c=10% and $E_\gamma$=1.3MeV.

The efficiency as a function of the $\gamma -$ray cascade multiplicity is shown in figure 11.

Figure 11: Simulated peak and total efficiency of the array as a function of the $\gamma -$ multiplicity of the cascade.

In figure 12 we see the PRISMA spectrometer and a schematic drawing of the CLOVER array at the PRISMA target position. The $\gamma -$detector system, installed on a mobile platform, will rotate together with the spectrometer, in such a way that reaction products detected in the spectrometer focal plane, in coincidence with the $\gamma -$rays, will have a forward trajectory with respect to the array. The PRISMA start detector (micro channel plate) allows one to determine the trajectory of the products with an angular resolution $\Delta\theta <1^o$. Because the high accuracy in the direction of the nuclei emitting the $\gamma -$rays the final Doppler broadening will be due to the angular aperture of the Ge crystals alone.

Figure 12: The PRISMA spectrometer (left) and a schematic drawing of the CLOVER array at the target position (right). The trajectories of the outgoing products can be traced up to the focal plane detector.

In addition to the performance figures given above, a feature that increases our interest in the use of the CLOVER detectors, is the possibility of measuring the degree of linear polarization of the $\gamma -$rays emitted by the products. The CLOVER composite detectors have an excellent sensitivity for Compton polarization measurements (see figure 13). In nuclear reactions where the outgoing products have a preferential direction for the alignment of the angular momentum, the polarization measurements together with the angular distributions or correlation (DCO) information, allows one to determine the character and multipolarity of the electromagnetic transitions and thus to obtain fundamental information on the spins and parities of the excited states.

Figure: Polarization sensitivity of a EUROBALL II CLOVER detector, both measured and calculated by Monte-Carlo simulation [9]