multipliers were used with conventional 
positive-grounded dynode voltages and 
cathode-follower output feeding a linear 
amplifier, discriminator, and scaler. 
Optimizing the Scintillator 
An iterative procedure is generally 
required to optimize this type of 
scintillator. Parameter limits for a 
reasonable scintillator were established 
by a series of rough preliminary tests. 
These were followed by a more exacting 
series. The criteria examined most 
closely were sample thickness and ratio 
of ZnS(Ag) to boron plastic.* Other 
factors investigated included particle 
size, type, or brand of ZnS(Ag) and its 
particle size, light attenuation charac- 
teristic of a fixed-composition scintilla- 
tor, ratio of boric acid to glycerol in the 
boron plastic, and response of the scin- 
tillator to fast neutrons and gammas. 
ZnS(Ag):boron plastic ratio. In- 
itial work showed that a high ZnS(Ag): 
boron plastic (BP) ratio would be ad- 
vantageous and that the optimum 
thickness would be ~1-2 mm. On 
this basis, a series of scintillators 5 em 
in diameter X 1.2 mm thick of varying 
ZnS(Ag): BP ratio were compared in a 
slow-neutron flux. These tests, the 
results of which are shown in Fig. 2, 
* Preparation is discussed below. 
established that the neutron counting 
efficiency increases with ZnS(Ag):BP 
ratios up to 2.2:1; maximum efficiency 
seemed to be not far beyond that ratio. 
Experimentally, this ratio is limited by 
mixing difficulties at the higher values, 
where the mixtures become quite dry. 
For this reason, the quite-workable 
ratio of 2:1 was used subsequently. 
Thickness. The variation with scin- 
tillator thickness of its transmittance 
for light from an alpha-particle scintil- 
lation was studied briefly. A Po?!° 
alpha-source was covered with a ~10-p 
layer of ZnS(Ag) and the combination 
used as a scintillation light source. 
Scintillators of 2:1 ZnS(Ag):BP 2.4 
cm in diameter and of various thick- 
nesses were interposed between this 
light source and the photomultiplier 
tube. Figure 3 shows that scintillators 
of this composition become rather 
opaque to the scintillation light when 
thicker than 1 mm orso. These meas- 
urements are at a discriminator setting 
such that maximum sensitivity without 
excessive background is obtained. 
More detailed experiments on thick- 
ness versus scintillator efficiency were 
conducted using a series of scintillators 
ranging from 0.5 to 10 mm in thickness. 
The results are shown in Fig. 4. It 
will be noted that a pronounced maxi- 
mum occurs when ~1.2 mm thick. 
Boric acid:glycerol ratio. To find 
the best ratio of boric acid to glycerol 
in the boron plastic 5-cm-diameter X 
1.2-mm-thick disks having a ZnS(Ag): 
BP ratio of 2:1 were used. Past a 
boric acid: glycerol weight ratio of 4:1, 
further increases in boric acid did not 
materially increase the fractional boron 
content nor, consequently, the counting 
rate. However, the higher ratios make 
the material less hygroscopic. Hence, 
the earlier decision to use a 6:1 ratio 
for most studies emerged as a good one. 
Types and sizes of ZnS(Ag). Using 
2:1 ratio of ZnS(Ag):BP and 5-cm- 
diameter X 1.2-mm-thick disks as be- 
fore, relative counting efficiencies were 
determined for five commercial ZnS 
(Ag) phosphors—Du Pont 2B1 (former- 
ly 1410), Sylvania CR20 and 132, and 
RCA 33Z20A and F2030. The results 
indicate that the Du Pont 2B1 phos- 
phor is somewhat superior for this par- 
ticular application. The average grain 
size of this phosphor is about 10 yu. 
To get some idea of the effect of par- 
ticle size on efficiency, fractionated 
Du Pont 2B1 of grain sizes 6 and 10 p, 
and 25-u Du Pont 1101 were compared 
with the regular unfractionated 2Bl1 
phosphor. These tests showed that 
the unfractionated phosphor was as 
Preparation of Slow-Neutron Scintillators 
To make a good slow-neutron detector of the type described, 
the boron-containing material should be transparent and color- 
less, contain a large proportion of B', and be susceptible to 
intimate mixing with the ZnS(Ag). There are two methods that 
suggest themselves for the combining of ZnS(Ag) powder with 
the boron-containing material. One, using the boron substance 
also in the powdered form, is simply to mix the two powders 
together and then press the mixture into a compact disk. The 
second is to employ a boron material which can be prepared 
initially in liquid form, mixing the ZnS(Ag) with it while liquid, 
and subsequently solidifying the combination. The second 
method, while usually more difficult, provides more intimate con- 
tact between the ZnS(Aqg) and the boron compound and is the 
method used for most of the scintillators considered here. 
A number of boron compounds fulfilling the requirements for 
transparency and freedom from coloration can be found, such 
as BN, B03, or H;BO3. The BN contains more boron (43.5 
wt %) but is refractory and does not melt at temperatures below 
the sublimation temperature for ZnS. Therefore, it is unusable 
in a liquid form. B203, containing 33.1 wt % boron, melts at 
about 580° C. It is very viscous even at 1,000° C and hence 
difficult to manipulate. Although it is possible to lower the 
viscosity of molten BO; by addition of small amounts of oxides 
or fluorides of alkali metals, there remains the high melting 
temperature which tends to harm the luminescent properties of 
ZnS(Ag). 
Preparation of Boron Plastic (BP) and HBO» 
Two boron compounds that have proved especially suitable for 
this application are 
1. A borate polyester plastic containing as high as 20% boron, 
24 
hereafter referred to as boron plastic or BP, 2. HBO, glass, con- 
taining about 26% boron. Both are quite readily incorporated 
with the ZnS(Ag) without harming it. 
BP. The combination of boric acid and glycerol (C3Hs0s) 
to form a colorless, transparent polyester plastic has been known for 
some time. The molar proportions commonly used are 1:1. To 
increase the relative proportion of boron in this plastic, various 
weight ratios of boric acid to glycerol, ranging up to 10:1, were 
examined. At this high ratio, the viscosity of the polyester melt 
becomes high, and there is danger of charring. The boric 
acid: glycerol weight ratio of 6:1 (about 9:1 molar ratio) proved 
to be the highest that would permit ease of handling of the plastic, 
and is the.ratio which was used for most of the work. 
Boron plastic of 6:1 weight ratio was prepared according to the 
following receipt: 20 gm of glycerol is first heated to near boiling 
in a round-bottom 200-ml flask. Then 120 gm of crystalline 
HBO; is gradually added with continuous shaking and heating. 
The initial reaction is rapid, the H;BO; disappearing quickly 
with considerable liberation of steam. As the reaction proceeds, 
solution of H3BO; takes place more gradually. At the moment 
the final H;BO3 has been added and dissolved, the clear liquid 
boron plastic is poured onto a cold thick aluminum plate. The 
slightly cloudy and brittle plastic which results wpon cooling is 
then stored in a dessicator in polyethylene bags until subsequent 
remelting and combining with ZnS(Ag). This intermediate 
cooling state facilitates the many weighings necessary in com- 
bining the BP and ZnS(Aq) in varying proportions for efficiency 
comparisons, but could be dispensed with in preparing a large 
number of scintillators of fixed composition. If the heating of 
the molten BP is prolonged at the last stage, the cooled material 
will be clear and glasslike, and further heating will initiate 
charring. The heat delivered in the remelting stage will usually 
remove sufficient additional water to render the BP, as incorpo- 
rated in the neutron scintillator, clear and colorless. Prepared 
