&. 
_ Relative Counting Rote 
Pas 
aha 1Q «2,0 Bi 
Besant, . Phosphor Thickness (mm) 
TEE i Se, 
~v 
{2} 
Oo 9 
cojeuse) 
: Relative Gounting Rote 
an & a o 
5 
Oo | 
os 
FIG. 4. Relative efficiency of neutron phos- 
Zn$:BP = 2.0 
phor vs. phosphor thickness. 
FIG. 3. Transparency of neutron phosphor 
to the Po-a scintillation of ZnS 
Scintillation Counters 
By K. H. SUN, P. R. MALMBERG, and F. A. PECJAK 
Westinghouse Electric Corp. 
East Pittsburgh, Pennsylvania 
sensitivity of the photomultiplier, and 
the amount of background noise 
present. This probability is further 
complicated by secondary factors such 
as length of light path, transparencies 
of the media comprising the path, num- 
ber of interfaces between different 
media in the light path, and trans- 
missivities of such interfaces. 
Consideration of these various fac- 
tors shows that a change in certain of 
them may be favorable to one of the 
three main stages involved in counting 
a slow neutron, and unfavorable to one 
or both of the other two. Thus, in- 
creasing the proportion of ZnS(Ag) in 
the scintillator increases the probability 
of an energetic a-particle reaching the 
ZnS(Ag), but reduces the probability 
for neutron capture and renders the 
scintillator more opaque. Because of 
the complexity of these many factors 
and their interrelationships, develop- 
ment and optimization of the slow- 
neutron scintillator has been on an 
empirical basis. 
What was optimized? We aimed 
primarily at high efficiency, ease of 
manufacture, and low cost. Although 
the primary consideration of merit for 
a scintillator is usually the counting 
efficiency for a specific type of radia- 
tion, other factors can be important, 
depending on the intended use. Exam- 
ples are: sensitivity to other types of 
radiation, inherent noise level, usable 
counting life, and ruggedness. 
Test Equipment 
To test the scintillators we needed 
only slow-neutron sources and some 
standard electronics. 
Slow-neutron sources. A slow-neu- 
tron source with minimal spurious 
radiation was required to obtain the 
relative efficiencies of the neutron scin- 
tillators as a function of a given design 
parameter, and to measure absolute 
efficiencies of the better scintillators. 
Two sources were used, each consisted 
of a Po-Be fast-neutron source sur- 
rounded by paraffin moderator. The 
Po-Be sources were 0.7-in.-diameter 
0.7-in.-long cylinders and emitted 
~2 X 10° fast neutrons/sec. 
In one of the slow-neutron sources, 
the Po-Be source was fixed at the cen- 
ter of a 6-in. cube of paraffin in a thin 
steel case. This source was used for 
obtaining most of the comparative 
data. 
The second source consisted of an 
Cio 4m. Oo 7 Be Sc 10. 
Phosphor Thickness (mm) 
_--Ra. gamma source 
(~20x y-strength 
of Po- Be source } 
e a 
| 3S _-Po-Be slow- 
; = “ neutron source 
} o 
i = 
| € 
PJ 
: -a 
eS) 
| & _-Po-Be fost~ 
| 3 neutron source 
| & : 
| 
| .- Background 
50 
; Discriminator Setting (volts) 
been eb v Popa eure Le I 
FIG. 5. Response to neutrons and 
gamma rays vs discrimination level 
8-in.-diameter sphere of paraffin in a 
14g-in.-thick aluminum shell, the Po- 
Be source being fixed at the center. 
J. A. DeJuren of the National Bureau 
of Standards calibrated the thermal- 
neutron flux at the surface of this sphere 
against the Bureau’s absolute-neutron- 
flux standard, using manganese-foil- 
activation and cadmium-difference 
techniques (7). The flux was deter- 
mined with an accuracy of +10%, and 
was about 24 thermal neutrons/em?/ 
sec at the outer surface. 
This spherical neutron source pro- 
vides a simple 1/r? relationship of neu- 
tron flux to distance from source. 
Thus the absolute counting efficiency 
can be determined for a given scintil- 
lator at an arbitrary distance r from 
the source center. Careful measure- 
ments of relative thermal neutron flux 
versus distance from the source center 
were made with the cadmium-differ- 
ence method, using one of the better 
scintillators shielded from stray or ex- 
ternally reflected radiation by suitably 
disposed sheet cadmium. Figure 1 
demonstrates that the 1/r? relationship 
does hold quite well for this source. 
Circuitry. Both the 2-in. DuMont 
6292 and 5-in. DuMont K1198 photo- 
23 
