Radioactive Electrostatic High-Voltage Generator

Beginning in 1913, descriptions of voltage generators based on the principle of charging a capacitor by charged particles from radioactive substances appeared from time to time in the literature. However, because of their low power, these generators did not become widespread. To construct a generator with a power of several milliwatts, it would be necessary to use a radioactive source on the order of a curie. In the past, such co-

FROM CURRENT LITERATURE

quantities of radioactive material were practically inaccessible. However, in recent years, thanks to well-known advances in the production of radioactive substances, the situation is changing substantially, and one may hope that in the near future large quantities of radioactive substances will be available. For this reason alone, it is of definite interest to consider the most recent of the radioactive electrostatic generators described in the literature.

A diagram of the generator is shown in Fig. 1.

The radioactive source—a layer of $\mathrm{Sr}^{90}\mathrm{NO}_3$—is deposited on the inner surface of cylinder $K$, made of nickel foil 20 microns thick. The diameter of the cylinder is 2 cm, its length 3.8 cm. The spheres at the ends of the cylinder serve to reduce the field gradient at these points. The source is connected to the spherical electrode $A$ by means of the metal rod $B$. The electron collector $C$ is insulated from electrode $A$ and the source by a quartz tube $D$. The collector is made of copper, coated on the inside with aluminum in order to reduce the effect of secondary emission of electrons and gamma rays of bremsstrahlung; it also serves as a vacuum chamber and at the bottom has a branch for connection to a vacuum pump. Fig. 2 shows the general appearance of the generator with the vacuum pump.

Fig. 1.

Fig. 2.

As the radioactive source, strontium 90 was used, having a half-life of 30 years. This isotope emits electrons with an energy of 0.65 MeV; the resulting isotope—yttrium 90—is also radioactive and emits electrons with an energy of 2.16 MeV. The half-life of $\mathrm{Y}^{90}$ is 62 hours. The advantages of $\mathrm{Sr}^{90}$ as a radioactive source are the following: availability, low cost, and the possibility of obtaining samples of high

specific activity, a long half-life, and the emission of high-energy electrons. Finally, the source used is readily amenable to chemical treatment and does not release gaseous products at low air pressures.

In the generator described, about 250 millicuries of \( \mathrm{Sr}^{90} \) were used. The short-circuit current was of the order of \(1.05 \cdot 10^{-9}\) ampere.

If the generator is regarded as an equivalent constant-current source charging a capacitor \(C\), in parallel with which is connected a resistance \(R\) (the load resistance and the internal resistance parallel to it), then the dependence of the potential difference on the capacitor on the charging time would be expressed by the formula \(V = Ri_0(1 - e^{-t/RC})\), where \(i_0\) is the magnitude of the charging current. In fact, this formula is approximately correct only for small potential differences, since as the potential difference increases the charging current begins to decrease owing to the repulsion of low-energy electrons.

Thus, to each value of the generator voltage there corresponds a definite current, which can be found from the energy spectrum of the \(\beta\)-rays of \(\mathrm{Sr}^{90}\)--\(\mathrm{Y}^{90}\), by determining the number of \(\beta\)-particles with energies greater than \(V\). With the aid of the resulting volt-ampere characteristic one can determine, for any load, the maximum expected voltage.

If, for example, we take \(R = 2 \cdot 10^{15}\) ohms, which corresponds to the resistance of the quartz insulator used at a relative air humidity of 25%, we obtain \(V_{\max} = 800\) kilovolts. Experimentally, the maximum potential difference that could be obtained was equal to 365 kv. The authors explain this circumstance by breakdown in the vacuum between the collector and the source, arising as a result of the emission of secondary electrons from the anode due to ion bombardment and the emission of positive ions from the source due to electron bombardment. This explanation is consistent with the oscillographic observations of the discontinuity of the discharge pulses and also with the fact that the limiting voltage depends on the nature of the electrode material.

It is also interesting to note that there exists an optimum gas pressure that ensures the maximum voltage. In the present device this pressure was of the order of \(10^{-3}\) mm Hg. At higher pressures the influence of gas ionization is manifested. The decrease in the maximum attainable voltage at lower pressures is apparently connected with the influence of the residual gas on the state of the electrode surfaces, which, at the optimum pressure, hampers the emission of ions and secondary electrons.

The principal shortcoming of the generator described is its low power, which is due to the small magnitude of the charging current, i.e., in the final analysis, to the small quantity of active material. As a result, the current leakage between the electrodes is of the same order of magnitude as the charging current and, consequently, the voltage fluctuations are very considerable.

The use of radioactive sources with activities of the order of 200 curies would make it possible to construct a more stable generator with a power of the order of one watt. However, even at present, less powerful radioactive voltage or current generators may prove useful in the design of various special vacuum devices, for which portability is required.

L. B.

References

1. E. Zinder, S. Christian, J. Appl. Phys., 23, 1213 (1952).
2. P. Lobanov and A. Belyakov, Doklady AN, 47, 337 (1945).