PROPERTIES OF OPTICAL GREASES FOR BaF 2

Nuclear Instruments and Methods in Physics Research A254 (1987) 85-87
North-Holland, Amsterdam
85
P R O P E R T I E S OF OPTICAL G R E A S E S F O R BaF 2 S C I N T I L L A T O R S
W. K L A M R A , Th. L I N D B L A D , M. M O S Z Y i Q S K I * a n d L.O N O R L I N
Research Institute of Physics, S-104 05 Stockholm, Sweden
Received 18 September 1986
Properties of different optical oils and greases in the UV light region were studied using transmission and photoelectron
measurements. The results show that not all of the greases commonly used for scintillators are applicable to the BaF2 detector, which
have a fast component in the UV region.
Results obtained for a spectrometer consisting of a cylindrical well-polished 48 mm diam. × 50 mm BaF/ crystal indicate
improved properties for the fast component, compared to the nonpolished crystals.
1. Introduction
2. Experimental procedure
Recent studies have shown unique properties of BaF2
crystals, both for ),-ray and charged particle detection
[1-4]. BaF2 scintillators have two light components, a
fast one at 220 nm with a decay time of 0.6 ns and a
slow one at 310 nm with a decay time of 620 ns. The
fast component may be used to obtain very good timing
signals. Consequently, a good utilization of the light
from the BaF2 detector in the UV re#on is of main
importance. In previous studies [2] a reduction of the
photoelectron collection efficiency for the UV light (and
thus a relative reduction of the intensity of the fast
component) was observed. Hence, it is essential to minimize the loss of fast photons in the collection process.
One of the problems is connected with absorbtion of
the UV light in the optical grease used in the coupling
between the crystal and the photocathode of the photomultiplier. Therefore within the frame o f the tests of
different configurations of the BaF2 counters for the
Nord Ball multidetector system [5], it was decided to
check the properties of some optical greases and oils
used in scintillation techniques.
Two methods were employed to test the oils and
greases. In the first one, the transmission was measured
using a grating monochromator. In the second method,
the number of photoelectrons released from the photocathode of the XP 2020Q photomultiplier was determined both for the total light and the fast component
alone. The results of those tests will be presented below
together with some properties of a 48 mm diam. × 50
mm length cylindrical BaF2 detector.
The set up for the UV light transmission measurements consisted of a Heath EVE-700 grating monochromator with an EMI 9789 QB photomultiplier. The UV
light from a Hg lamp was directed to the entrance slit of
the grating spectrometer after passing through two
quartz windows with a thin layer of the studied oil in
between.
The photoelectron studies were performed with a 48
mm diam. ×50 mm length cylindrical BaF2 crystal
manufactured by BDH Chemicals. The well-polished
crystal was coated with teflon tape in order to increase
the light output. The scintillator was then coupled to a
XP 2020Q photomultiplier working with the B' dynode
chain [6]. The number of photoelectrons per energy unit
produced by the scintillator in the photomultiplier was
determined for both the total light pulse and the fast
component alone. The method employed has been described by Bertollacini et al. [7]. Briefly, this method use
a comparison between the mean value of the single
photoelectron pulse height distribution (which determines the gain of the photomultiplier) and a characteristic point in the detected energy spectrum of the
59 keV ),-line from 241Am. The measurements for the
total light pulse were performed using a simple spectrometric system of a charge sensitive preamplifier and a
main amplifier with an 30 #s integration and 2 #s
shaping time constants, respectively. The extraction of
the fast component was achieved in a somewhat different manner. An ORTEC 454 timing filter amplifier was
used for fast differentiation of the pulse (time constant
of 10 ns). Secondly, we used a fast integrating preamplifier with an integration time constant of 100 ns
and, finally, a spectroscopic amplifier with a 0.5 /is
shaping time constant.
* Institute of Nuclear Studies, 05-400 Swierk-Otwock, Poland.
0168-9002/87/$03.50 © Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
86
W. Klamra et al. / Properties of optical greases
Table 1
Oils and greases studied in the UV light transmission measure
ment
3. Experimental results
3.1. Transmission measurements
The studied oils a n d greases are listed in table 1 a n d
the results are s h o w n in fig. 1. As can be seen in fig. 1,
the transmission curves show two kinds of behaviour:
(i) a c o n s t a n t dependence of X or (ii) a threshold-like
effect for X < 290 nm. The last one is very valid for the
G 688, V 788 a n d V 789 oils. Since the transmission for
> 270 n m is very low, these oils are hardly applicable
for B a F 2 spectrometry.
3.2. Photoelectron measurements
Oil/grease
Viscosity
(cSt)
Manufacture
DC 200
Rhodorsil SI 300
Rhodorsil 47 V
Viscasil
RTV 615 A + B a)
G 688
V 788
V 789
100000
60000
100000
60000
Dow Coming
Rhbne-Poulenc
General Electric
Visilox Systems
a) Mixing of two components RTV 615 A and RTV 615 B.
The results of the m e a s u r e m e n t s for all the oils are
given in table 2. One finds that the D C 200, 47 V a n d
Viscasil are the best ones, which is in agreement with
the transmission measurements.
It is interesting to note a reduction of the intensity of
the fast c o m p o n e n t c o m p a r e d to that given in ref. [1] for
the 2.4 cm diam. scintillators. O n the other h a n d , a
similar value is o b t a i n e d if the 2.4 cm diam. crystal is
coupled to a 2.8 cm diam. photomultiplier [2] (R 1668).
Thus, it indicates a reduced collection efficiency of
photoelectrons from the peripheral region of the p h o t o cathodes.
Table 2
Results of photoelectron measurements
Oil/grease
Fast component Total light Fast comp.
(phe/MeV)
(phe/MeV) /total light
(%)
DC 200
Rhodorsil SI 300
Rhodorsil 47 V
Viscasil
RTV 615 A + B
V 788
V 789
260
235
260
260
210
123
123
1580
1470
1575
1580
1495
1210
1200
1.5
1.0
1.0 I
1.0]~
1.0~
1.0
.i
t-
.o
u
•
.
*
~
•
=
"
•
•
"
....
w,-
=
-
-
"
*
•
"°o1
"°0I
"°0I
240
.
=~
DC 200
~
•
•
N
SI
300
•
•
•
=w
t,7
V
,,"
"
.....
[
•
-
= •
.
Viscosi[
%
--"
•
-
i
•
"
•
i
i
260
280
Wovelength
RTV615A+B
G 688
e
%
V 788
•
=.
V 789
I
l
300
(nm)
Fig. 1. Light transmission for studied oils and greases.
3:20
16.5
16.0
16.5
16.5
14.0
10.2
10.3
W. Klamra et aL / Properties of optical greases
80~ a ) F a s t
50~]~ component /
\ FWHM=28~
2o
-
80
c
b) Total' l lght
40
FWHX=ll~
~_
I
20
200
I I I I I I I I-800
1000
400
800
Energ g [keV]
Fig. 2, Energy spectrum of 3,-rays from 137Cs measured with a
BaF2 crystal for (a) fast component, (b) total light.
4. Performance of the 2 in.x 2 in. BaF2 scintillation
counter
In figs. 2a and b are shown spectra for the 662 keV
y-ray from 137Cs radioactive source measured for the
total light and the fast component alone. The energy
resolutions are 11 and 28%, respectively, while the obtained time resolution for 6°Co was 340 ps (measured
with Pilot U as reference counter). These results can be
compared to data reported by Beck et al. [8] for nonpolished BaF 2 crystals. Thus, the polished crystal gives
an improved energy resolution for the fast component
87
b u t not for the total light where a worsening is observed. It is, however, somewhat difficult to compare
the time resolution results, since the sizes of the studied
crystals were not the same. This because of the fact that
this quantity is in a way depending on the dimension of
the crystal, i.e. for small crystals a better time resolution
is observed. The dependence for the energy resolution is
less obvious, in particular for the total light.
Acknowledgement
We are much indebted to Dr. S. Mannervik for
assistance in the transmission measurements.
References
[1] M. Laval, M. Moszyhski, R. Allemand, E. Cormoreche, P.
Guinet, R. Odru and J. Vacher, Nucl. Instr. and Meth. 206
(1983) 169.
[2] M. Moszyhski, R. Allemand, E. Cormoreche, M. Laval, R.
Odru and J. Vacher, Nucl. Instr. and Meth. 226 (1984) 534.
[3] K. Wisshak and F. K~ippeler, Nucl. Instr. and Meth. 227
(1984) 91.
[4] S. Kubota, M. Suzuki, J. Ruan, F. Shiraishi and Y. Takami,
Nucl. Instr. and Meth. A242 (1986) 291.
[5] B. Herskind, Nucl. Phys. A447 (1985) 395c.
[6] Philips, Data handbook, Electron tubes, Book T9, 1985.
[7] M. Bertolaccini, S. Cova and C. Bussolati, Prec. Nucl.
Electr. Symp., Versailles, France (1968).
[8] F.A. Beck, Conf. on Instr. for Heavy Ion Nuclear Research, Oak Ridge, USA (1984).

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