Properties of the CsI Crystals

by Ch. Brönnimann

The requirements on the mechanical and optical properties of the pure CsI Crystals for the calorimeter of the PIBETA detector are the following:

Ratio of the fast (100ns) to the total (1000ns) component of the CsI-pulse: F/T>70% , see below Fast/Total Measurements

Mechanical accuracy: 0.3mm , see below Distance Measurements

Homogeneity of the light output along the crystal axis: < 3.5% , see Tomography

Fast/Total Measurements

The Fast/Total value is a very important parameter of a pure CsI crystal; it is defined as the ratio of the fast integral (100ns) to the total integral of the raw CsI pulse and must be >70%.. The figure below shows a typical CsI pulse and the definition of the integration gates:

Since these measurements are very sensitif to various parameters like electronic noise, integration starting points etc..., we choosed, after various tests, the following setup: The tested crystal is coupled to a dedicated PMT, which is connected to a digital scope TDS740 read out by a PC. With cosmic muons penetrating the CsI crystal one gets after ca. 10 min a sample of 1000 raw CsI pulses as shown above, which are analysed in software.

The Fast/Total measurements are performed on all the CsI crystals as a first entry control when they are delivered at PSI. In order to compare the results we performe these measurements always under the same conditions:

The figure below shows the result of Fast/Total measurements on two different crystals:

Distance Measurements

The distance measurements to determine the mechanical accuracy of the crystals are performed at PSI using the computer controlled 3 Coordinate Measuring Machine from the company WENZEL Precision. The machine is programmed to automatically scan the surface of a crystal with a specific shape; each plane is scanned at 6 points with a dedicated touch head and planes are fitted through these points. Finally, the vertices of the crystal are calculated, which then can be compared with the theoretical values. The precision of the system is about 0.007mm. The tolerances for the mechanical deviations are 0.3mm. The following pictures are showing a Csi crystal ready to be scanned by the 3 Coordinate Measuring Machine.

Ch. Broennimann, 5. Jan 1995