Functional tests on plain crystals

The CsI slabs constituting the GRBM detectors have been manufactured and preliminary tested by Bicron Corporation, New Bury, OH, USA. The most relevant tests performed in Bicron, with detectors illuminated from the inside are:

• measurements of gain and energy resolution of each slab by using radioactive monochromatic sources of gamma-rays with energies between 22 and 662 keV; the crystals showed a very good linearity and energy resolution values and dependency on energy very close to that reported in table 1.4 and estimated from the calibrations reported in next section;
• estimate of the gamma-equivalent energy of the light pulsers inserted in each slab as gain calibrator, which resulted to range from 350 to 450 keV;
• testing of gain and energy resolution variations with HV changes; no significant energy resolution changes occur within HV variations of $\pm$200 Volts;
• uniformity tests: signal amplitude as a function of the site of photon interaction in the crystal was tested using a collimated ⁵⁷Co (122 keV) radioactive source. The irradiation spots on the crystal had a 10 mm diameter. As expected and as shown in Fig. 2.1 for LS1, the signal amplitude is much higher for photons incident directly underneath PMTs and minimum at the edges. This results in different PHA channel assignments for signals generated by the detection of photons of the same energy. This disuniformity, expressed as $\frac{maximum~channel-minimum~channel}{average~channel}$ ranges from 68% to 77% depending on the slab.

  

Figure 2.1: Mapping of the gain of LS1 at 122 keV