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
Mechanical accuracy: 0.3mm , see below
Homogeneity of the light output along the crystal axis: < 3.5% , see
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
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
- Crystal Preparation:
Unwrapped, air gap coupling to the photomultiplier (PMT).
Type EMI 9822QB S/N 20041, HV=1850V.
- Read Out:
Digital scope TDS740 with fixed settings, over IEEE488 bus with
the HIX data acquisition system running on a PC.
- Software Gate Definitions:
Gates starting 15 ns before peak, fast gate lenght: 100ns, total gate length:
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