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Performances of EXOGAM

p1001700.jpg        The performance of EXOGAM.

 


 

The sensitivity of the array for detecting weak structures depends on the total peak efficiency and the quality of the spectra. The optimum performance for the best sensitivity is obtained by maximising the efficiency whilst maintaining spectrum quality. The calculations presented below take into account loss factors that reduce the performance of an array in an experiment (neutrons, double hits due to high multiplicity etc.).

The spectrum quality is determined by the signal to noise or peak to total ratio and the energy resolution. The peak to total ratio is governed by the size of the Ge crystals and design of the suppression shield. In practice with a good design of the suppression system the best peak to total that can be obtained is 0.6-0.7 at 1.3 MeV.

The energy resolution DE of the system is a combination of several factors, most arising from Doppler effects. The major factors are:

  • Intrinsic resolution of the detectors.
  • Doppler broadening arising from the opening angle of the detectors.
  • Doppler broadening arising from the angular spread of the recoils.
  • Doppler broadening arising from the velocity (energy) variation of the recoils.

Under experimental conditions several factors affect the performance of the array. These include the Doppler broadening of the gamma-ray lineshape and the response of the array to a cascade of high multiplicity gamma rays and neutrons. These reduce the efficiency and peak to total. These loss factors include:

  • The isolated hit probability which accounts for a second gamma-ray or neutron hitting the Ge detectors causing the event to be removed from the photopeak.
  • The probability of the first hit being a neutron.
  • False vetoes generated in the suppression shields caused by scattering of gamma-rays or neutrons or from background radiation

In addition, very high singles rates can reduce the efficiency due to pile up in the electronics. The efficiency and the peak to total of the array can be calculated including these loss factors.

The configurations considered in the calculations are the cube geometry with 4 segmented CLOVER detectors with full suppression shields and the 16 segmented CLOVER geometry in configurations A and B. The total photopeak efficiency and peak to total values for the CLOVERS used at 662 keV and 1333 keV are taken from GEANT calculations and given in table 2.

  Photopeak efficiency
(%)
Peak-to-total
(%)
  662 keV 1.3 MeV 662 keV 1.3 MeV 
EXOGAM configuration Aa 28 20 57 47
EXOGAM configuration Bb 17 12 72 60
Gamma-Cubec 15 10 72 60

a partial suppression shield close packed configuration A, 16 CLOVERs at 114.1 mm
b full suppression shield pulled back configuration B, 16 CLOVERs at 147.4 mm
c full suppression shield, 4 CLOVERs at 68.3 mm.
Table 2: Total photopeak efficiency and peak to total for EXOGAM.

2.7  Performance Examples.

It is not possible to present in detail the performance of EXOGAM for all reactions and experimental set-ups that are envisaged. Therefore, a few examples have been chosen to indicate the performance of an array of 4 and 16 segmented CLOVERs for different experimental situations, different g-ray multiplicity and recoil velocity. The examples are taken from the physics case in section 2.

It should be noted that the performance of EXOGAM is significantly improved by the use of segmented rather than non-segmented CLOVERs. For example, for the reaction to study nuclei near 100Sn, see table 5, if the segmented CLOVERs are directly replaced with non-segmented CLOVERS of the same size then the resolution is calculated to be 22 keV for configuration A and 19 keV for configuration, at 1333 keV. The corresponding values for the EXOGAM CLOVERs are 13 and 11 keV.

2.7.1  Coulomb excitation of exotic nuclei.

The performance of EXOGAM is calculated for a Coulomb excitation experiment using beams of neutron rich Ar isotopes. The aim of this experiment is to study the sudden onset of collectively for neutron rich nuclei with magic numbers of nucleons. Of particular interest is the study of neutron rich nuclei with N = 28. The experiment is centred on 46Ar. Neutron rich Ar beams (e.g. 46Ar) at a level of 107 particles per second are expected in the early stages of SPIRAL since the noble gases have favourable diffusion and effusion properties leading to fast and efficient extraction from the ion source.

The performance in table 4 is calculated for Eg = 662 keV and 1333 keV, brecoil = 7.5%, g-ray multiplicity Mg = 2 and neutron multiplicity Mn = 0. The calculation assumes that the recoil angle has been measured using a suitable detector to within 3°.

  Photopeak efficiency
(%)
Peak-to-total
(%)
Resolution
(keV)
  662 keV 1.3 MeV 662 keV  1.3 MeV 662 keV  1.3 MeV
EXOGAM configuration A  26  18  55  45  7.6  14.9 
Gamma-Cube  13 67  56  11.4  22.8 
Table 3: Performance for a Coulomb excitation example.


  Photopeak efficiency
(%)
Peak-to-total
(%)
Resolution
(keV)
  662 keV  1.3 MeV  662 keV 1.3 MeV 662 keV 1.3 MeV
EXOGAM configuration A  20  14  40  32  2.0  2.3 
Gamma-Cube  43  35  2.0  2.3 
Table 4: Performance for a K-isomer experiment.


  Photopeak efficiency  Peak-to-total Resolution 
  (%) (%) (keV)
  662 keV  1.3 MeV 662 keV  1.3 MeV 662 keV  1.3 MeV
EXOGAM configuration A  17  13  38  31  6.5  12.6
EXOGAM configuration B  13  10  60  49  5.6  10.7 
Table 5: Performance for an experiment to study mass 100 nuclei.

Because of its very high photopeak efficiency the EXOGAM array is particularly well suited for spectroscopy using the Coulomb excitation mechanism or for the search for events which will have a weak signature because of the expected low beam intensity. Although the v/c for this reaction is large, the segmentation of the Ge detectors maintains the energy resolution within reasonable values. The calculations show that an early implementation with 4 segmented CLOVERs would enable a spectroscopy study of the first few collective states of these exotic nuclei.

2.7.2  High spin spectroscopy of a stable system :K isomer studies.

The performance of EXOGAM is calculated for the high spin spectroscopy of the stable nucleus 181Ta. This nucleus can be populated by the reaction 176Yb(9Li,4n)181Ta at a beam energy of 40 MeV. The aim of this experiment is the study of multi-quasiparticle states to high spin and the population of the predicted high K isomers in this nucleus.

The performance in table 4 is calculated for Eg = 662 and 1332 keV, brecoil = 0%, Mg = 10 and Mn = 4. A thick target is used to stop the recoils. The resolution obtained corresponds to the intrinsic detector resolution.

The choice of this example illustrates very nicely the effect of the g-ray multiplicity as a function of the solid angle subtended by the Ge detectors and of the granularity of the array. When the multiplicity is 10, which corresponds to a common situation for nuclear reactions induced with radioactive beams, EXOGAM fulfills completely the design specifications. The efficiency of a system for the close packed cube geometry is dramatically reduced. This effect is obviously due to the pile-up of more than one g-ray within the single crystals and forbids the use of such a configuration at intermediate multiplicities.

2.7.3  Nuclear Structure around 100Sn.

It is possible to make compound nuclei above 100Sn on the N = Z line. Reactions such as 72Kr on 32S or 72Kr on 40Ca can be used to populate 104Te and 112Ba, respectively. These compound systems decay via (xp,ya,zn) channels to nuclei around 100Sn. A charged particle ball and neutron detectors would benefit this type of spectroscopy. The performance in table 5 is calculated for Eg = 662 and 1332 keV, brecoil = 6.3%, Mg = 15 and Mn = 1.

This reaction, which generates not only a large v/c but also a relatively high g-ray multiplicity, corresponds to the most unfavoured conditions and demonstrates the EXOGAM capabilities. The high efficiency is maintained even though the multiplicity is relatively high and despite the very large recoil velocity, the energy resolution is reasonable. Note that without segmentation of the Ge crystals, these values would be worse by more than 50 %. In table 5 the performance of EXOGAM when the Ge detectors are pulled back from the target (configuration B) is also calculated. The distance between the target and the Ge is ~ 15 cm which leads to an improvement of the energy resolution and moreover, to a substantial increase in the peak to total due to the adjunction of the additional side-suppression elements and thus to higher spectra quality. This possibility to use EXOGAM in different configurations allows the physicist to optimise a given parameter (efficiency or spectra quality) depending on the experimental conditions.

2.7.4  Study of a stable system at high spin.

The performance is calculated for the spectroscopy of a stable nucleus at high spin. The nucleus 172Yb can be populated by the reaction 130Te(46Ar,4n). If such a reaction could be investigated with a stable beam then the experimental set-up would be rather simple. However, 46Ar is a b- emitter and the build up of scattered beam in the target region gives rise to a high background. This background contributes to the singles rate in the detectors and hence careful design of the target chamber and shielding has to be carried out. This background can however be removed from the recorded data by collecting additional information which is correlated to the compound nucleus reaction. This can be achieved using ancillary detectors such as neutron detectors or a recoil detector or by correlating the events by the beam pulse. The performance in table 7 is calculated for Eg = 662 and 1332 keV, brecoil = 2.2%, Mg = 15 and Mn = 4.

  Photopeak efficiency
(%)
Peak-to-total
(%)
Resolution
(keV)
  662 keV  1.3 MeV 662 keV  1.3 MeV 662 keV  1.3 MeV
EXOGAM configuration A  17  13  36  29  2.9  4.9
EXOGAM configuration B  13  10  57  47  2.7  4.3 
Table 6: Performance for an experiment to study high spin in a stable nucleus.


  Photopeak efficiency
(%)
Peak-to-total
(%)
Resolution
(keV)
  662 keV  1.3 MeV 662 keV  1.3 MeV 662 keV  1.3 MeV
EXOGAM configuration A  21  15  44  35  4.8  9.1
EXOGAM configuration B  15  10  64  52  4.2  7.8 
Table 7: Performance for an experiment to study mass 60 nuclei.

 

2.7.5  Superdeformed spectroscopy in the neutron deficient mass 60 region.

The predicted superdeformed minimum in the self conjugate nuclei in the mass 60 nuclei can be optimally populated using, for example, the reaction 34Ar + 40Ca. This reaction populates a range of N = Z nuclei depending on the beam energy. At 150 MeV, 60Zn is populated strongly at high spin via the 3a,2p channel and with a large cross-section 150 mb. This nucleus can be populated using a stable beam but with a much lower cross-section, few mb. This reaction would benefit enormously from a charged particle inner ball or a spectrometer for kinematic reconstruction to improve the resolution of the peaks. The performance in table 7 is calculated for Eg = 662 and 1332 keV, brecoil = 4.5%, Mg = 10 and Mn = 1. The recoil angle is measured to within 3°.

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