Cd Ditterence (eps) 
Qe st 2 3.354 §. 6 -7 8 
(1/e®%) x 105 (em-2) 
b w 
° ° 
ow 
°o 
Relative Counting Rate 
FIG. 2. 
position. 
FIG. 1. 
1.0 
Ratio of ZnS to Boron Plastic by Weight 
2.0 
Relative efficiency of neutron phosphor as a function of com- 
Phosphor thickness = 1.25 mm 
Counting rate for a spherical neutron 
source vs inverse square of the distance. 
Al- 
9 though not well shown by the figure, effect of 
room scattering becomes appreciable at large r 
High-Efficiency Slow-Neutron 
A mixture of boron-containing plastic and ZnS(Ag) phosphor has been optimized 
to give slow-neutron counters with 33% efficiency yet only 1% gamma-ray contribution 
when counting radiations from a Po-Be source. 
A SLOW-NEUTRON SCINTILLATOR such 
as the ZnS(Ag)—-boron-compound type 
(1-6) described here operates in three 
principal, closely related stages: 
1. A slow neutron passing through 
the scintillator is captured by a B?° 
nucleus. 
2. The resultant energetic alpha par- 
ticle reaches a ZnS(Ag) granule with 
sufficient residual energy to cause a 
scintillation. 
3. Light from the scintillation travels 
to the photomultiplier photocathode 
and reaches it with sufficient intensity 
to cause a recognizable pulse at the 
anode. 
The net efficiency of the scintillator 
is related to the probability that all 
three of these events proceed in succes- 
sion, that is, to the product of the 
separate probabilities for each of the 
three events. 
The probability for event 1 is 
P, = 1 — e-¥%*, where o is the micro- 
scopic slow-neutron capture cross sec- 
tion of B', and N is the areal density of 
B® nuclei. This equation holds so 
long as there is little competition with 
slow-neutron capture in the scintillator; 
this has been the case with all the scin- 
tillators examined in this study. The 
> 
22 
larger N is, the greater is P., but that 
as P, approaches unity, further in- 
creases in N accomplish very little. 
The probabilities for 2 and 3 are 
not susceptible to such direct analysis. 
The probability of 2, of the resultant 
alpha particle reaching the ZnS(Ag), 
depends on the density of material, the 
path length involved, and the minimum 
residual energy required. These, in 
Construction and optimization are given 
turn, depend on secondary factors such 
as relative amount of ZnS(Ag) in the 
material, size and shape distribution of 
the ZnS(Ag) particles, and the mini- 
mum usable light per scintillation. 
The probability of 3, of sufficient 
scintillation light reaching the photo- 
multiplier, depends on the strength of 
the scintillation, the transmissivity of 
the light path to the phototube, the 
Grooved Scintillators Are Best 
A thin scintillator made with a grooved 
or corrugated surface provides more scin- 
tillating material, and hence more B'° per 
unit projected area of scintillator, while 
maintaining a usable light transmittance. 
Such a scintillator was made by using a 
transparent Bioplastic mold, cast from a 
negative steel mold (see figure). The 
scintillator was cast at about 180° C and 
moderate pressure in the plastic mold. 
It was used with the grooved scintillator 
surface facing the photomultiplier, and 
with an aluminum foil covering the oppo- 
site surface. The improvement in scin- 
tillator efficiency was ~60%, resulting 
in an over-all efficiency of about 33 % for 
| included angle 
Colorless x Neutron phosphor 
polyester d 
resin mold Aluminun foil 
Improved slow-neutron scintillator 
thermal neutrons at a discriminator 
setting such that Po-Be gamma rays gave 
only 1% as many counts as thermal 
neutrons. 
