NUCLEONICS DATA SHEET No. 21 Radiation Detection 
Gamma-Detection Efficiency 
of Organic Phosphors 
By K. I. ROULSTON 
and S. |. H. NAQVI 
Department of Physics 
University of Manitoba 
Winnipeg, Canada 
0.5 
Discriminator 
O4F bias=5 kev-- 
7. '0 kev- 
THIS NOMOGRAM and the accompany- 
ing curves make possible a rapid deter- 
mination of the detection efficiency of 
an organic phosphor. 
For detection a certain minimum en- 
ergy must be transferred to a Compton 
electron. The cross section can be cal- 
culated by integrating the differential 80 100 200 00 \ 
: Incident photon energy (kev) 
Compton cross section from the corre- 
sponding minimum photon scattering 
angle to 180 deg. 
One starts with the minimum accept- 
able energy transfer (discriminator- o mm ry 24 px 
bias setting). From Fig. 1 one then 0.5 0.5 1.00.1 0.05 
finds the cross section for detectable 
energy transfer, ¢. To determine the 
corresponding absorption coefficient yu 0.4 0.8 
(= po) we use the three nomogram 
scales at left. The nomogram then 
determines the efficiency (1 — e~#7) 0.2 
from u and the thickness z. 0.3 0.6 
The method assumes equality of 
total linear absorption coefficient and ~ 0.5 0.2 
detection absorption coefficient. This i 2 20 
is generally justified by crystal geome- ae 
tries in which a photon that is scattered 0.2 s 0.4 
by an event not leading to detection 
cannot escape from the crystal with- 
out traversing the remainder of the 
path lengths. 
With 25-kev bias we observe effi- 
ciencies that correspond to our curve 
above 200 kev. Below 200 kev the 
cross section is larger than calculated, 
apparently due to multiple collisions. 0.1 
Example: Discriminator bias is at 
25 kev, and we are detecting 200-kev 
photons in a 10-cm-thick plastic phos- 
phor. From the curves we find a de- 
tection cross section of 0.27 barns. A 
representative phosphor has p = 0.34 
x 1074 clectrons/em*. With this as- aoe 
sumption we find a detection efficiency 
of 60%. 0.05 0.005 0.1 “1,000 a 
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Detection cross section (barns /electron) 
Linear absorption coefficient (cm-') 
Electron density (102 electrons /em?)’ / 
Detection efficiency (%) 
Thickness (cm) 
90 
98 
99 
229 
