This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Bloch, C.

Bowyer, S.M.

Bowyer, T.W.

Pate, S.F.

Pickar, M.A.

English

IUScholarWorks

Pb-GLASS DETECTOR EFFICIENCY TESTING AT THE 
UNIVERSITY OF ILLINOIS TAGGED PHOTON FACILITY 
C. Bloch, S.M. Bowyer, T.W. Bowyer, S.F. Pate 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 74 05 
M.A. Pickar 
U n i u e r ~ i t y  of Kentucky,  Lexington, Kentucky 40506 
In the upcoming @(n',d)y experiment (E328), photons will be detected in coincidence 
with deuterons. The small total cross section for this reaction, x 10 pb, combined with a 
luminosity of x lo2' ~ m - ~ s - ' ,  results in a total production ra.te of about 1 s-'. This low 
count rate dictates the use of very efficient photon detectors. Recently, IUCF acquired 160 
Pb-glass Cerenkov detectors from the University of Basel. These detectors are blocks of 
Schott F2 glass measuring 4.2 by 6.4 cm on the front face, and 6.4 by 6.4 cm on the back 
faxe, and 50 cm in length. With an absorption length of 5.2 cm in the energy range of 
interest, 80-120 MeV, the 50 cm length of these detectors ensures almost 100% detection 
efficiency provided only a small portion of the electromagnetic shower escapes the detector. 
The p'(n',d)y experiment will use these detectors in stacks 4 blocks wide by 5 blocks 
high. The question of interest is then: by adding together the pulse heights of all the 
detectors in a stack, wha8t is the photon detection efficiency as a function of incident 
photon position on the face of the stack? To answer this question, a 3 by 3 stack of de- 
tectors was tested in July of 1989 using a tagged photon beam facility at the University 
of Illinois. This facility uses a primary electron beam that illuminates a converter foil, 
producing photons via electron bremsstrahlung. The residual electrons, primary electrons, 
a.nd photons proceed through a magnet that bends residual electrons of a selected energy 
range onto a, counter axray (See Figure 1). The firing of a particular array element deter- 
mines the photon energy and thus, provides tagging. The tagged photon beam energy was 
70 f 3 MeV with a width (a.t l / e  of maximum intensity) of about 2 cm. A plastic scintil- 
la,tor paddle, placed on top of the stack, served as a cosmic ray trigger. Coincidence events 
between this paddle and the Pb-glass stack provided a means of tracking the gains of the 
detectors since these events have a fixed energy. A laser bea.m aligned along the path of 
the photon beam allowed accurate positioning of the stack in the beam, and the response 
at 57 positions over the face of the sta.ck was mapped. Figure 2 shows the efficiency of 
this stack of detectors versus distance of an incident photon from the edge of the staxk, 
where the efficiency was normalized to the data taken with the photon beam centered on 
the stack. The finite photon beam size was taken into account. 
Our measurements indica,te that there is no loss of efficiency when crossing the bound- 
ary between two detectors. There is, however, a significant loss in efficiency as one asp- 
proaches the edge of a stack. Using this measured function, the relative efficiency of a 4 
by 5 stack of detectors (the configuration tha,t will be used in the p'(n',d)y experiment) was 
computed to be 0.88 k 0.04. Since the depth of these detectors span about 10 absorption 
lengths, the interaction probability is approximately 100% for photons that travel down 
the center of the detector. Thus, the total efficiency of a 4 by 5 stack will be nearly 88%. 
Figure 1. Schematic of the University of 
Illinios tagged photon facility. 
Figure 2. Relative photon detection ef- 
ficiency vs. distance of incident photon 
from the edge of a Pb-glass detector stack 
(in cm). 
Rirnory 
0 
/ 
/ 
/ 
0 
/ 
Electron 
Counter 
Arroy 
I Converter 
Foil 
e' Beom 
Distance from Edge of Stack (cm) 
We would like to acknowledge the invaluable assistance of A.M. Nathan and M. Lucas 
in performing these tneasurement s. 


Pb-Glass Detector Efficiency Testing at the University of Illinios Tagged Photon Facility

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Pb-Glass Detector Efficiency Testing at the University of Illinios Tagged Photon Facility

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