Comparison of the crystal non-uniformity with and without lacquer

Bernward Krause

During the last month I restudied the influence of the Russian lacquer on the uniformity of our CsJ-crystals. For this examination I used the RASTA apparatus. This apparatus is now upgraded to measure the number of photoelectrons per MeV (npe) and the time resolution of a penetrating cosmic particle. For the time resolution the crystal is sitting between two small round plastic scinillators (thickness 9.5mm, diameter 50mm). The time difference between the mean value of the upper and lower scintillator and the crystal TDC value is histogramed. The sigma of the distribution gives the time resolution value. To get enough events to determine the time resolution one need a two hour cosmic data taking. After this is finished the crystal is illuminating by a LED. Data are taken for ten attenuations. Together with the cosmic data the number of photoelectrons per MeV are determined. After this the non-uniformity of the crystal is measured (see Uniformity measurement with a 137Cs source and RASTA).

Realisation of the measurements:

First I took cosmic data with a non-painted crystal for two hours to get enough statistic for the time resolution result. Then I made the LED measurement before I get the data for the determination of the crystal uniformity. After these measurements the crystal was painted two times and wrapped in two layers of teflon and one layer of mylar. The surface where the PMT is glued on was covered with lacquer too. The same measurements (cosmics, npe, uniformity) were done with the painted crystal. Except the temperature which was measured all other parameters kept the same (high voltage of the PMT, attenuation factors, threshold). I used the following crystals: S004, S029, S064, S067, S114 and S161 to exclude the dependence of the crystal shape.


In a table I listed the values for the number of photoelectrons and the sigma of the time resolution before and after painting. The crystal surface treatment increases the photoelectrons. The time resolution doesn't change.

Crystalnpe (temp.corr.) Time [ns]
without lacquerwith lacquerwithout lacquerwith lacquer

Besides these numbers all data are histogramed for the different PAINTED crystals ( S004, S029, S064, S067, S114 and S161). But more important is the behaviour of the non-uniformity. In the following histograms I plotted the normalized results of the non-uniformity before and after the treatment with the lacquer (colored straight line: before painting the crystal).

In the following scatterplot the gradient of the first 9cm before and after painting is shown for each crystal.

Any comments or questions are welcome. Email to

B. Krause, 10. April 1997