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.
Results:
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.
| Crystal | npe (temp.corr.) |
Time [ns] |
| without lacquer | with
lacquer | without lacquer | with lacquer |
| S004 | 83.7 | 112.9 | 0.50 | 0.59 |
| S029 | 59.3 | 78.7 | | 0.61 |
| S064 | 67.2 | 78.4 | 0.57 | 0.56 |
| S067 | 52.2 | 84.5 | 0.44 | 0.69 |
| S114 | 63.7 | 75.3 | 0.53 | 0.53 |
| S161 | 56.2 | 78.3 | 0.57 | 0.55 |
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
krause@psi.ch
B. Krause, 10. April 1997
