Cerenkov detectors are used to measure the velocity of particles in configurations designed to study the isotopic composition of galactic cosmic rays. The geometrical properties of the detector are outlined. Monte-Carlo simulations of photon propagation in a diffusive detector were undertaken. The scattering properties of diffusively reflecting white paint and of surface treatments for the radiator material were measured. It is found that the absorption of light in the radiator is an important light loss mechanism. The simulations are used to find optimal mapping techniques and data reduction strategies. The application of these techniques are discussed with respect to the large area isotopic composition experiment (ALICE) Cerenkov detector

Acharya, B. S.

Balasubrahmanyan, V. K.

Esposito, J. A.

Lloyd-Evans, J.

Ormes, J. F.

Streitmatter, R. E.

English

NASA Technical Reports Server

OG 9.1-4
262
HIGH RESOLUTIONCHERENKOVDETECTORSFORCOSMICRAY ISOTOPEEXPERIMENT
B. S. Acharya*, V. K. Balasubrahmanyan, J. A. Esposito**,
J. Lloyd-Evans**, J. F. Ormes and R. E. Streitmatter
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.
ABSTRACT
Cherenkov detectors can be used to measure the velocity of
particles in a variety of configurations designed to study
the isotopic composition of galactic cosmic rays. In order
to achieve the highest possible resolution it is necessary
to understand the geometrical properties of the detector in
detail. We have carried out Monte-Carlo simulations of
propagation of photons in a diffusive detector to help to
achieve that understanding. We have also measured the
scattering properties of diffusively reflecting white paint
and of surface treatments for the radiator material. We
find that the absorption of light in the radiator is an
important light loss mechanism. We also use these
simulations to find optimal mapping techniques and data
reduction strategies. The application of these techniques
will be discussed with respect to the ALICE Cherenkov
detector I .
I. Introduction. The use of the Cherenkov detector in balloon-bo_ne
cosmi_ r--a-ye-xpe-Fiments was first introduced by Webber and McDonald_.
Ever since, these detectors have become an indispensable tool in experi-
ments where the velocity of an incident particle of known charge is
measured. However, the resolution of these detectors often causes a
severe limitation in the resolution of ultimate physical parameters
measured in an experiment. For example, in experiments designed for
isotopic mass measurement using the Cherenkov versus range technique,
the resolution in mass number depends critically on the resolution of
Cherenkov detectors.
The resolution of a Cherenkov detector depends upon the total light
collection efficiency as well as spatial uniformity. The highly
directional nature of Cherenkov emission leads to further complications
of dependency of light collection efficiency on the energy and incident
angle of the projectile. Besides, background signals from a-electrons
above Cherenkov threshold, residual scintillation in the radiator,
Cherenkov emission from white paint (usually used in the integration
chamber) etc., limit the ultimate resolution of the detector. Though
the resolution is limited primarily by photo electron statistics, other
effects may become significant either at large pulse heights or in
detectors with high detection effic_gQcy as the resolution scales with
the number of photo electrons as n_ "_. Requirements on a
large area of these detectors further increases the importance of
spatial uniformity.
* NAS/NRCResearch Associate on leave from the Tata Institute of
Fundamental Research, Bombay, INDIA
** Also, Univ. of Maryland, Dept. of Physics, College Park, MD 20742
https://ntrs.nasa.gov/search.jsp?R=19850026617 2020-03-20T16:47:45+00:00Z
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High resolution is usually achieved by processes such as selecting a
radiator of high photon yield per cm., by placing the radiator material
in a white diffusion box with a high photo cathode to total area ratio,
by frosting the surfaces of the radiator to eliminate specular reflec-
tions, by resolution quality control of the selected Photomultiplier
(PM) tubes, etc.
In this paper we shall discuss the Monte-Carlo simulations carried
out to study the nonuniformity in the Cherenkov response due to the
geometry of the set up. Also discussed are the results of laboratory
tests on the various surface treatments for the radiator. All the
results refer in general to a detector of square geometry and in
particular to a Cherenkov detector to be employed in the large area
isotopic composition experiment ALICEI.
2. Properties of the Cherenkov Detector. The response of a Cherenkov
detector is determlned b-yMonte-Carlo s_-mulations. The actual geometry
of a Cherenkov detector to be used in ALICE is adopted in the simula-
tions. Briefly, the Cherenkov radiator consists of 113.7 x 113.7 x
2.25 cm3 Pilot 425, a plastic with an index of refraction of 1.5. The
radiator is housed in a chamber having dimensions 114.3 x 114.3 x
20 cm_. Th_ chamber interiors are coated with a diffusely reflecting
BaS04 paint which has a reflectivityof 95 percent in the visible light
region. The detector configuration utilizes 24 RCA $83006 photo-
multiplier tubes of 5" diameter, forming an effective detection area of3354 cm_. The PM tube has a maximum quantum efficiency of 30 percent
at a wavelength of ~ 400 nm.
The required percentage spatial uniformity in the response of this
Cherenkov detector is shown against resolution in mass number for the
ALICE detector assembly in Figure (1). It is seen that less than a
percent nonuniformity is required to achieve a mass resolution of~ 0.3
A.M.U. Usually, Cherenkov detector responses are mapped_, using
accelerator beams.
However, it is unwieldly to map large area detectors using
accelerators. Alternatively,one can simulate the Cherenkov chamber
response map and map the detector using cosmic ray muons or flight data.
Two extreme cases of surfacing of the radiator, namely, 'perfectly
polished' and 'ideal frosted' are considered in our simulation. In the
case of 'polished'surface, light is required to follow Snell's law at
the boundary while an isotropic angular distribution is assumed for the
'frosted'surface whenever photons pass the boundary of the radiator.
Each photon is followed until it is either absorbed by the radiator or
the white paint coated wall or detected by PM tubes. A photon is
considered to be absorbed if it is surviving even after 50 reflections
either in the walls or in the radiator. A five percent probability for
absorption of photons is assumed for the white paint coated wall and a
uniform angular distribution for the scattered photon off the wall. The
individual PM tube responses are assumed to differ only in parameters
describing the relative positions of impact point and center of the
photo cathode.
264
OG 9.1-4,
The simulated response of the detector for a particle passing
through with different incident angles is shown in Figure (2) for
'polished' surface, 'polished' and'frosted'surfaces of a radiator doped
with waveshifter. Frosting of the surface reduces the nonuniformityas
expected.
Laboratory tests were carried out to study the angular distribution
of outgoing light for different surface treatments. Four samples of
lucite of thickness 5 mm are chosen. One side of each sample is
polished while the other side is either polished,machined with a fly-
cut technique, sandblasted or sandpaper treated using 80 grit sandpaper.
A He-Ne laser beam (_ = 6320 A) is made to impinge on the polished
surface of the samples at right angles and the angular distribution oY
the forward component of light is measured at the other side. The
angular distributions are shown in Figure (3) for all samples along with
an angular distribution obtained for a surface coated with a white paint
of BaSO4 base. It is seen that for all test samples, the transmitted
light is far from isotropic.
It was also observed that the contamination of the surface due to
the sandblasting process is higher than the rest. Samples are kept
inside a light diffusion box mounted with a green Light Emitting Diode
(LED), (_ = 5700 A) and a photomultiplier. A significant absorption of
light due to contamination of the surface is observed in the case of
sandblasted samples. However, this absorption may be reduced if one
uses AlO2 for blasting the surface instead of sand.
The nonuniformitydue to a geometric dependent variation in the
response of the Cherenkov detector is studied through Monte-Carlo
simulations. Particles are made to impact at different positions in the
detector, and the Cherenkov response is mapped. The percentage of
detected photons is plotted in Figure (4) against the distance along the
diagonal for two attenuationmean free paths of 1 m and 10 m.
3. Conclusions. A sandpaper treated surface looks good for our
purpose. Radiators doped with waveshiftingmaterials and having a long
attenuation length tend to make the detector response uniform over the
area besides yielding higher light output. Further simulations are
being carried out and the results will be compared with the mapped
response of the Cherenkov detector, obtained using cosmic ray muons.
Also simulations are currently being extended for a detector of circular
geometryU.
4. Acknowledgement. We thank Dr. Walter Viehmann for results on
scattering experiments.
References
1. Esposito J. A., et al., This Conference, 0G-9.1-9.
2. Webber, W. R. an_ McDonald, F. B., 1955, Phys. Rev., 100, 1460.
3. Schult, J. B., and Shai, C. M., 1971, NASA/GSFC X-762-71-266.
4. Ahlen, S. P. et al., 1976, Nucl. Inst. Methods., 136, 229.
5. Rasmussen, I. L., et al., 1983, Proc. 18th ICRC, 8angalore,_8,77.
6. Streitmatter,R. E., et al., 1981, Proc. 17th ICRC, Paris, 8_, 54.
0G9.1-4
265
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High resolution Cherenkov detectors for cosmic ray isotope experiment

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