Doppler correction

The great advantage of the segmentation of the Exogam germanium detectors is that the opening angle subtended by individual segments is reduced by a factor of two compared with a single crystal. The opening angle is a major contribution to the Doppler broadening of the photopeak for high v/c which means that the subsequent reduction in the photopeak width can be up to a factor of two. This improvement is fundamental when using radioactive beams where Ge detectors have to be configured in a close geometry. A comparison of the calculated resolution for a single crystal detector, and a segmented and non- segmented clover is shown in the left hand panel of fig. 4. The improvement in the effective resolution has been measured experimentally using a small segmented clover detector. This detector has crystal dimensions of 50 mm in diameter and 70 mm in length before shaping, compared to the Exogam detectors of 60 mm diameter and 90 mm length. It was tested using a 4.22 MeV/A (v/c = 8.5%) gold beam which was Coulomb excited by a carbon target. The resolution of the 547 keV transition improved from 23.6 keV for the full detector to 6.7 keV when the energies were Doppler corrected for each segment. This result is shown in the right hand panel of fig. 4.

  

Figure 4: Performance of the segmented clovers in Exogam in connection with Doppler broadening of energy peaks. Left: the calculated broadening as a function of energy (90 degrees, 60 mm and 0.075c) showing the progressive advantages of each segmentation level. Right: data for Coulomb excitation of ¹⁹⁷Au striking a carbon target (547 keV, 90 degrees, 0.085c) taken using a smaller prototype version of the detectors; (a) no correction, (b) correction according to the hit crystal, (c) correction according to the hit segment within the crystal.