5. Properties and Performance of the Pure CsI Crystals

In the preceding chapters, the proposed pibeta experiment at PSI was described. The experimental technique was presented as a relative measurement between the decay rates for p ⁺-> p ⁰e⁺ n _e and p ⁺->e⁺ n _e. In order to achieve the attempted goals with this method the detector needs to meet specific requirements. The calorimeter, being the essential part of the detector, is requested to

* consist of a very fast scintillating material, capable of handling high event rates, and

* detect positrons and photons of 70 MeV with good energy resolution.

As mentioned in 4.2, the scintillator material of the pibeta calorimeter was chosen to be pure Cesium Iodide (CsI). The requests mentioned above condition a very careful examination of the delivered crystals.

The aim of this chapter is threefold:

1. The presentation of the properties of pure CsI.

2. Technical specifications of the crystals which have to be fulfilled by the manufacturers. This requires precise measurements of the specified quantities with dedicated methods. The quality control of the delivered crystals is of major importance since the success of the experiment strongly depends on the quality of the scintillator material.

3. It is shown how the properties of the individual crystals are defined in the MC simulation and what the resulting energy resolutions for the processes of interest are. A realistic simulation of the calorimeter response is essential for the calculation of the low energy tail and for the correction for background in the final experimental analysis.