FROM CURRENT LITERATURE

A MASS SPECTROGRAPH OF A NEW TYPE¹

The author considers the motion of charged particles with the same value of $\frac{e}{m}$ in the field of a spherical capacitor. Under the condition that these

Fig. 1

Fig. 2

particles have as their source a common point between the plates of the capacitor (point $P$, Fig. 1), equal values of the initial velocities, and that their trajectories lie in the plane of a great circle of the sphere, such particles will be focused at a point $Q$, lying in a plane separated from that in which $P$ lies by the angle $\Phi$. The magnitude $\Phi$ is determined by the field gradient between the plates and by the velocity of the particles. The possibility of employing a spherical capacitor for these purposes was first indicated by Aston² in 1919.

In an analogous manner, part of the cavity of a spherical capacitor will act with respect to the points $P$ and $Q$ (Fig. 2), lying on a straight line passing through the center of the sphere.

The author carries out a theoretical study of such a device as a two-dimensional problem, neglecting space charge, as well as the edge effect at the entrance and exit of the capacitor. The author then introduces relativistic corrections for the case of very fast particles.

On the basis of the calculations performed, the author constructed an experimental model of a mass spectrograph with a spherical capacitor, the schematic drawing of which is shown in Fig. 3. Here 1 and 2 are the plates of the spherical capacitor (copper), 3 is the source

Fig. 3

particles, 4—the Faraday cylinder, 5—the cutting diaphragms, which compensate for the edge effect of the capacitor³, and 6—the glass bulb. Hatching indicates one of the focused particle beams. The spherical capacitor is mounted on the same leg as the particle source; the method of mounting is not shown, in order to avoid cluttering the drawing.

The author tested the apparatus he had constructed with electrons. In these experiments the cathode potential was 2000 V relative to the Earth (the Earth potential was applied to the diaphragms). The plate potentials were ±300 V. The apparatus showed good focusing.

The author considers the principal advantage of the apparatus to be that, being, so to speak, a “three-dimensional” spectrograph, at a given resolving power it has a much larger aperture. In particular, for the apparatus investigated the theoretical value of the reduced dispersion was 1010, while the aperture was 0.210, i.e. \(\frac{1}{60}\) of the full sphere. It turns out that considerably larger apertures can also be used without serious distortions due to the edge effect.

A drawback of the apparatus is the difficulty of its practical realization. Thus, for example, in order to place the capacitor in the bulb, the latter was first cut along the equator and then sealed again.

N. Khlebnikov, Moscow

Literature

1. E. M. Purcell, Phys. Rev., 54, 818, 1938.
2. F. W. Aston, Phil. Mag., 38, 710, 1919.
3. R. Herzog, Z. Physik, 97, 596, 1935.