creased the bremsstrahlung and, con- 
sequently, the shielding. 
Model D-50. In the third model, 
D-50 (Figs. 8 and 9), the radioisotope 
is deposited directly in the poly- 
styrene insulator, which is machined 
in the form of a cup 34¢ in. in diameter 
and 3¢ in. long with a }42-in. wall. 
Contact is made to the isotope source 
by a 0.005-in.-diameter monel-wire 
loop, which is soldered to a }¢-in.- 
diameter copper rod held in a }4-in.- 
diameter polystyrene plug. A solvent 
such as toluene is used to seal the open 
end of the capsule to a groove in the 
plug after the isotope is inserted. The 
plug and capsule fit in a cylindrical 
aluminum collector 3g in. in diameter 
and 34 in. long that is held in the lead 
shielding and copper tube. The wax 
potting compound used in the B-50 
was replaced with polystyrene to 
reduce surface leakage. When the 
complete battery was assembled, swab 
tests were made to detect any radio- 
isotope leakage. The measured radi- 
ation of the D-50 was approximately 
the same as model B-50; however, the 
volume, including shielding, was re- 
duced from 4 in.? to 1 in.’ by the 
elimination of the gold source elec- 
trode. In the D-50, the only dense 
material within range of the beta 
particles is the monel wire, which has 
a small cross-section. 
Next, the electrical characteristics 
were taken as in the case of the B-50. 
The current at zero voltage was again 
40 upa; the battery capacity was 
approximately 5 wf for a charging rate 
of 5 volts/sec across an electrometer, 
which added 3 wuf; and the maximum 
voltage was 7,000 volts after two weeks 
shelf life. This beta current represents 
a 33% collection efficiency of the 
120 X 10-1? ampere emitted from the 
isotope. Most of the lost current in 
this case is absorbed in the 142-in.- 
thick insulator wall. This thickness, 
however, increases the breakdown 
voltage to approximately 30,000 volts. 
For special applications where the 
maximum voltage required is only a 
few hundred volts, the insulator thick- 
ness can be correspondingly reduced 
to obtain an increase in current (at 
a sacrifice of mechanical strength). 
Also, for lower current applications, 
the shielding can be decreased for the 
same external radiation. 
The output voltage decreased when 
a battery was exposed to room atmos- 
phere in a fully charged condition for 
140 
| 
UAE 
1] ue 
2 
FOU 
FIG. 8. Model D-50 generator. 
more than a few hours due to electro- 
static precipitation of dust and, conse- 
quently, surface conduction across the 
external insulation. Contamination of 
the polystyrene surface was less serious 
than in the case of the wax potting 
compound. In either case, however, 
the dust could be removed with soap 
and water followed by a thorough 
drying. 
Development work is continuing on 
improving the mechanical seals to de- 
crease the possibility of any isotope 
leakage. It appears, however, that 
the present geometry gives about the 
minimum volume with a 10-mc source 
for an external radiation that is below 
tolerance at the surface. For lower 
currents, the shielding can be scaled 
down accordingly. 
Applications 
Batteries using radioisotopes as an 
energy source offer, in principle, the 
advantages of long life under extreme 
FIG. 9. 
Parts of D-50: shield, aluminum 
collector, polystyrene cup insulator, and 
polystyrene plug with wire anode lead 
wer Polystyrene plug 
_---- Monel wire 
aluminum 
jos === Isotope 
--------Lead shield 
POLL LL ee eee 
“"~--=-Copper tube 
~>Potting compound 
Cathode 
Assembled, unpotted battery at left 
operating conditions, as the nuclear 
process itself is essentially unaffected 
by temperature or pressure. The 
half-life of the Sr%-Y isotope is 
approximately 25 yr; however, a wide 
choice of half-lives is available. The 
extrapolated life of the polystyrene 
insulation from the 250-mc tests has 
exceeded 25 yr for the 10-mc batteries, 
or 250 yr for a 1-me battery. As no 
chemical reactions or solid-state effects 
are used in the power generation, the 
insulation appears to be the only 
critical feature. 
Thus, for applications requiring 
small currents, such as_ ionization 
chambers, these batteries should offer 
the advantages of smaller size, less 
weight, and lower cost,* since most of 
the power available from conventional 
chemical batteries is not used. Also, 
in applications requiring operation 
over a period of many years at low 
temperatures, these batteries appear 
to be more reliable than batteries that 
depend completely or in part on a 
chemical reaction. 
BIBLIOGRAPHY 
1. H.G.J. Moseley, Proc. Roy. Soc. (London) A88 
471 (1913) 
2. E. G. Linder, S. M. Christian, J. Appl. Phys. 
28, 1213 (1952) 
3. E. G. Linder, Phys. Rev. 71, 129 (1947) 
4. I. A. Lobanev, A. P. Beliahov, Compt. Rend. 
Acad. Sci. URSS 47, 332 (1945) 
5. P. H. Miller, Phys. Rev. 69, 666 (1944) 
6. R. Hofstadter, Nucizontcs 4, No. 4, 2 (1949) 
7. O. Sisman, C. D. Bopp, ORNL-928 (1951) 
8. J. H. Coleman, D. Bohm, J. Appl. Phys. 24, 
497 (1953) 
*It is estimated that an ionization 
chamber power supply producing up to 
10,000 volts with a charging current of 10 
ua could be manufactured in quantity for 
under five dollars at the current price of 
Sr°° of fifty cents per mc. 
