good as other sizes. Therefore un- 
separated Du Pont 2B1 phosphor was 
used in all other tests. 
Scintillator Response 
The response of the scintillator to 
gamma rays, slow neutrons and fast 
neutrons is shown in Fig. 5. It is clear 
that gamma rays of the intensity pres- 
ent in the Po-Be neutron source (0.1 
mr/hr when neutron flux was 24 n/cm?2/ 
sec) can be easily discriminated against. 
Although the hydrogen content of the 
scintillator makes it slightly sensitive 
to fast neutrons, the counting efficiency 
for them is very small. In critical 
measurements of thermal-neutron 
fluxes, correction for fast-neutron 
counts can be made by Cd difference. 
Absolute efficiency. A 2.40-cm- 
diameter scintillator was used in meas- 
uring the absolute efficiency of boron- 
plastic slow-neutron scintillators of 
optimized thickness (1.2 mm) and 
ZnS(Ag):BP ratio (2:1). The experi- 
ment was performed with the scintilla- 
tion counter set in front of the cali- 
brated slow-neutron source. Using 
the thermal neutron flux calibrated at 
the surface of the spherical source, to- 
gether with the 1/r? flux dependence 
and the measured area of the scintilla- 
tor, it was possible to evaluate the 
absolute efficiency straightforwardly. 
With discriminator set so that the 
response to gamma rays from the Po- 
Be source (0.1 mr/hr when neutron flux 
was 24 n/cm?/sec.) was slightly less 
than 1% of thermal-neutron response 
the efficiency of the scintillator for ther- 
mal neutrons was 21%: Note that if 
gamma-rays are of much greater rela- 
tive intensity than from the Po-Be 
source, the interference at this dis- 
crimination setting can be appreciable. 
Conclusions 
The scintillation counter has advan- 
tages of ruggedness, better time resolu- 
tion, and superior geometrical defini- 
tion when compared with BF; counters. 
The boron-containing neutron scintil- 
lator developed in the present investi- 
gation is believed to be more efficient 
for thermal-neutron scintillation count- 
ing than other similar scintillators. A 
measured efficiency of 33% for thermal 
neutrons has been obtained with the 
boron-plastic-type scintillator having 
one corrugated surface; from this an 
efficiency of 37%, can be estimated for 
a similar scintillator of the HBO, type. 
This scintillator can be further and 
markedly improved by using pure or 
enriched B! instead of natural boron, 
which contains only 19 wt% B". 
Because of the high neutron counting 
efficiency of this scintillator and the 
large-area, thin-sheet geometry of the 
sensitive volume, it is possible to ob- 
tain a large solid angle for interception 
of neutrons from uncollimated sources. 
The neutron scintillator developed 
here is also being used in photographic 
detection of neutron patterns.* The 
photographic sensitivity has been found 
to be considerably greater than any 
previously known. These results will 
be reported later. (See p. 81—Ed.) 
* * * 
We are grateful to Michael Giuliano for his 
assistance in investigating the use of HBO: in 
slow-neutron scintillators. 
BIBLIO GRAPHY 
1. K. H. Sun, Method of detecting elementary 
particle, U. S. Patent 2,534,932 (Applied 
June 19,1947—Granted December 19, 1950) 
. F. Sachs, Neutron detectors, Y-B4-18 (1951) 
. D. E. Alburger, A slow-neutron detector, 
BNL-1233 (1952) 
4. E. Gatti, E. Germagnoli, A. Persano, E. Zim- 
mer, Boron-layer scintillation neutron detec- 
tors, Nuovo Cimento 9, No. 9, 1012 (1952) 
6. P. G. Koontz, G. R. Keepin, J. E. Ashley, 
ZnS(Ag) phosphor mixtures for neutron detec- 
tion, Rev. Sci. Instr. 32, 154 (1955) 
6. C. O. Muehlhause, Neutron scintillation coun- 
ters, NucLeonics 14, No. 4, 38 (1956) 
7. J. A. De Juren, H. Rosenwasser, Absolute 
calibration of the NBS standard thermal- 
neutron density, J. Research Natl. Bur. 
Standards 62, No. 2, 93 (1954) 
& wt 
*In cooperation with E. O. Wollan 
of the Oak Ridge National Laboratory. 
at which the neutron-scintillator mixture becomes workable. A 
Metal mold for neutron phosphor 
this way, the BP. contains about 19.6 wt % boron and has a 
density of 1.6 gm/cm’. 
HBO». Not too well known is the fact that H3BO3 can be con- 
verted at moderate temperatures to a clear liquid of the approxi- 
mate composition HBO: About 6 hr at 210° C is required to 
effect this for 200 gm of H;BO; in a covered 600-ml beaker. The 
HBO, glass prepared in this manner can be poured onto a cold 
aluminum plate to harden for dessicator storage. The measured 
boron content of this material is 25.5-26.5 wt % as compared 
with 24.7% computed for the formula HBO». 
Assembly of Slow-Neutron Scintillator 
The final step in fashioning a neutron scintillator from the 
boron compounds just described consists of remelting, stirring in 
appropriate amounts of ZnS(Ag) powder, pouring or pressing 
tnto suitably shaped molds, and cooling. Much experimentation 
transpired before the present procedures were evolved. 
Using boron plastic (BP). A specific amount of BP was 
weighed out and remelted in an oil bath at 160° C, the temperature 
proportionate weight of ZnS(Ag), also preheated to 160° C, was 
added to the melt with constant stirring and mixing. To prevent 
charring the boron plastic from prolonged heating, mixing was 
completed in one minute or less. The mixture was then trans- 
ferred to a metal mold such as that shown here, covered with a 
\4-in.-thick glass plate, and the excess mixture squeezed out in a 
small press with heavy metal platens. To prevent premature 
cooling and hardening of the mixture during this operation, the 
mold, glass cover plate, and press platens were preheated to 160° C, 
and the transfer and pressing were effected as quickly as possible. 
Self-release of the scintillator from the mold after cooling and 
contracting was aided by treatment of both mold and cover glass 
with a mold-release agent, Dow Corning 20. 
The resulting scintillator disk was backed with aluminum foil 
on one side to increase the light output from the opposite side, and 
the whole wrapped in Saran film for protection against atmos- 
pheric moisture. A transparent multipurpose type adhesive 
such as Cello-stick was used to improve optical contact between 
the Saran film and the scintillator surface. Scintillators pre- 
pared in this way were kept in a dessicator while not in use be- 
cause of the hygroscopic nature of the boron plastic component. 
Large flat sheets of this scintillator have been made this way. 
Using HBO». The preceding method was also followed with 
HBOs2, except that the operating temperature was kept at about 
210° C instead of 160° C. It was difficult thoroughly to mix the 
ZnS(Aqg) with the molten HBO», because of the higher viscosity 
of this boron compound. One way to improve mixing of these 
two materials is to reduce the HBO, to a 68-mesh (or smaller) 
powder and mix with the ZnS(Aqg) powder prior to heating. A 
second and more reproducible method was to put the previously 
mized ZnS(Ag) and HBO: powders into a mold to overflowing, 
cover the mold with a thick glass plate, and place in a heated 
press. Press temperature could be as low as 170° C. 
25 
