FIG. 6. Model B-50 generator. 
4 X 10!2 ohms, which can be calculated 
by inverse of the slope or 
V 
ae 
(4) 
On continued irradiation, the current- 
voltage characteristic (upper curve of 
Fig. 4) showed a decrease in slope or 
increase in resistance to R; = 8 X 10! 
ohms, both values of R agreeing with 
the results in Fig. 3. 
Next, to measure this apparent 
change in resistance after irradiation, a 
fresh sheet was inserted between the 
electrodes of Fig. 2 and a constant 
voltage applied while the source was 
covered with an absorber. The ab- 
sorber was removed after the normal 
polarization current decreased to a low 
value (the corresponding volume re- 
sistivity p = 10'8 ohm-cm) and the 
induced current recorded. The calcu- 
lated resistivity was found first to drop 
to about 10!* ohm-cm, decrease further 
for a few minutes to a minimum of 
about 101° ohm-cm, and then actually 
increase in resistance after one day 
with approximately the square root of 
the time. Details of this effect were 
given in a previous paper (8). 
Thus, the equivalent circuit in Fig. 5 
of a generator using a solid insulator 
can be deduced from Kq. 1 and from 
the observed induced conductivity 
effect in solid insulation. This circuit 
is seen to be similar to the vacuum 
generator except that the internal 
resistance R, is a variable, being a func- 
tion of the total time of irradiation and 
of the incident beta-current density. 
All good insulation (i.e., unirradiated 
p > 10'° ohms-cm) showed this effect; 
however, an accelerated life test using 
Seer Gold foil anode 
—-- Potting 
compound 
-- Steel tube 
— Polystyrene 
bog 
cates Isotope 
--Lead shield 
== -~- Aluminum 
collector 
WFLT LD LF LT LF LET AT LT LFA LE EG LPF LAL 
TT LT LS I 
~ ~--Cathode 
Assembled, unpotted battery at left 
a 250-me source eliminated all plastic 
insulators but polystyrene due to physi- 
cal degradation. For example, the 
fluorocarbons Teflon and Kel-F in- 
creased in resistance for a few weeks, 
then became brittle, and shorted the 
electric circuit; polyethylene increased 
in resistance for a week, then began to 
decrease due to slow degradation. 
Polystyrene, on the other hand, has in- 
creased in resistance for over 1 yr. 
On the basis of these results, the test 
source of Fig. 2 was potted in plastic 
and demonstrated in January, 1952, as 
a radioactive battery. Since that 
time, models B-50 and D-50 have been 
built, reducing the size and weight and 
increasing the safety for the same cur- 
rent output of 50 uya. 
Battery Design, Characteristics 
Model B-50 is shown in Figs. 6 and 7. 
A beta source of 10 me of Sr?-Y°% is 
welded between two strips of 0.0005-in.- 
thick gold foil, 2 in. long and 14 in. 
wide. The active area extends 14 in. 
from the bottom end and is surrounded 
FIG. 7. Parts of B-50: shield, insulator, and 
gold-encased source 
by a ‘‘U’’-shaped aluminum collector, 
¥g in. thick, which is supported by a 
lead shield. 
A slot for insertion of the source and 
insulator was formed by pouring the 
lead around a stainless-steel jig, which 
held the aluminum collector in position 
in a steel tube. The jig was removed 
after the lead cooled and the source 
was placed in two polystyrene bags and 
inserted in the slot. One bag, 3¢ in. 
wide and 114 in. long, was inside a 
larger bag, 19 in. wide and 13¢ in. long. 
Both bags were made by folding 
0.005-in. Plax polystyrene along the 
long dimension and heat sealing along 
a 1¢-in., ‘“‘L’’-shaped border, leaving 
the top open. ‘“Biwax”’ potting com- 
pound was poured around the outside 
of the unit, and the output character- 
istics were measured. 
First, the average output current for 
twenty-five batteries at zero voltage 
was found to be J, = 40 X 107 
ampere out of a total available current 
of 120 X 107!* ampere, a 33% current- 
collection efficiency. Most of the 67% 
loss was due to absorption in the gold 
foil of most of the Sr%® beta particles 
and some of the weaker Y% beta 
particles. 
The initial rate of charge of the out- 
put voltage was then measured across 
an electrometer voltmeter to be 1.2 
volts/sec and the capacity was calcu- 
lated for Eq. 3 to be C = 30 upf. This 
capacity is slightly higher than the 
capacity of 25 wuf measured on a 
bridge, since a high-resistance contact 
to the surface of the insulator by 
ionization of the air between the source 
and collector reduces the effective 
spacing between the source and col- 
lector electrode. Also, polarization 
current and trapped beta particles 
were found to be a significant part of 
the current when the voltage was 
changed. For example, it was neces- 
sary to wait several minutes on short- 
ing the terminals before the current 
reduced to its original value at zero 
voltage. 
The maximum charging voltage 
across an electrostatic voltmeter after 
two months was increased from a few 
hundred volts to 6,000 volts, which 
from Eq. 2 corresponds to an internal 
insulator resistance of 1.5 X 10!4 ohms. 
Model B-50 had a higher current- 
collection efficiency than the original 
unit of Fig. 2, since the collector elec- 
trode surrounds the source. The dense 
gold source electrode, however, in- 
139 
