MEASUREMENT OF ENERGY-LOSS RATES BY SLOW NUCLEI OF H$^1$, H$^2$, He$^4$, AND Li$^6$*)

Comparatively few works have so far been devoted to the experimental study of the energy loss of slow protons, deuterons, and other light nuclei.

To a considerable extent this is explained by the difficulties connected with registering such particles by counters, ionization chambers, and photographic plates. In the present work the author used an electron multiplier as the detector.

The source of light nuclei is a Cockcroft–Walton accelerator giving a regulated voltage from 40 to 430 kV. The beam under investigation is deflected by magnetic analyzer 1 (see Fig. 1) and, through a diaphragm, is directed onto target 2, which is used simultaneously for registering the passage of a definite charge.

) H. A. Wilcox, Phys. Rev., 74*, 1743 (1948).

For producing homogeneous beams of nuclei two methods were used. The first of them is based on scattering of the beam particles in a very thin layer of gold deposited on the polished surface of a beryllium target. In this case, for a given scattering direction a sharp maximum is obtained in the distribution of particles by energy, with

\[ E_m=\frac{197-A}{197+A}E_B, \]

where \(A\) is the mass number of the incident particles, \(197\) is the mass number of the scatterer (gold), \(E_B\) is the energy of the incident particles, and \(E_m\) is the energy of the particles at the maximum of the distribution curve.

In the second method the scattering target is replaced by a nickel disk coated with a layer of Be. As a result of the reactions

\[ \mathrm{Be}^{9}(p,d)\mathrm{Be}^{8} \quad\text{and}\quad \mathrm{Be}^{9}(p,\alpha)\mathrm{Li}^{6} \]

the nuclei \(\mathrm{H}^{2}\), \(\mathrm{He}^{4}\), and \(\mathrm{Li}^{6}\) are formed.

Fig. 1. Diagram of the apparatus:
\(1\)—magnetic analyzer, \(2\)—target, \(3\)—foil, \(4\)—particle-energy analyzer, \(5\)—electron multiplier.

On leaving the target the beam enters the energy analyzer \(4\)—a system of concentric cylinders \(1/4\) of a circumference long. The potential of the inner cylinder varies from 0 to 50 kV; the outer cylinder is grounded.

Labels in Fig. 2:
Deuteron energy in keV. Proton energy in keV. Energy loss rate of protons and deuterons in gold. Protons (scattering method). Deuterons (scattering method). Deuterons (reaction method). Vertical axis: energy loss rate in keV per micron. Horizontal axis: square of particle velocity in \(10^{16}\ \mathrm{cm}^{2}/\mathrm{sec}^{2}\).

Fig. 2. Energy-loss rates in gold for protons and deuterons.

The analyzer selects nuclei with energy \(E=19.5ZV\) (\(Z\) is the nuclear charge, \(V\) is the voltage on the analyzer), which are then registered by an electron multiplier (5) and a counting circuit.

In the course of the measurements, the relative number of particles of various energies per unit charge was determined. The same curve was taken for the case when gold or aluminum foil was placed between the target and the analyzer. Figure 2 presents the result of a study of the rate of energy loss by protons and deuterons in gold. Whereas, according to existing theory, these losses should be identical, experimentally the rate of energy loss for the deuteron is substantially higher. This discrepancy is explained by the authors as due to elastic collisions, the influence of which is usually neglected. An elementary calculation gives, for deuterons, losses due to elastic collisions more than four times greater than the losses of protons of the same velocity.

B. R.