Use of an Electron Multiplier Tube as a New Technique in Disintegration Experiments

Abstract

An electron multiplier tube with 12 electrodes has been used as a detector in the bombardment of beryllium by protons of energy 239, 268, and 397 kv. The disintegration particles from the two possible reactions, ${}_4{}^{}{}_{}{}^{}\mathrm{Be}_{}^9{}_{}{}^{}(p,\alpha ){}_3{}^{}{}_{}{}^{}\mathrm{Li}_{}^6{}_{}{}^{}$ and ${}_4{}^{}{}_{}{}^{}\mathrm{Be}_{}^9{}_{}{}^{}\mathrm{}(p,d)\mathrm{}{}_4{}^{}{}_{}{}^{}\mathrm{Be}_{}^8{}_{}{}^{}$, were selected by an electrostatic analyzer which measured directly their energies. The results for each bombarding voltage are given as an energy distribution curve which shows a complete resolution of the various peaks. All the particles, except the unstable ${}_4{}^8{}_{}{}^{}\mathrm{Be}$, were detected, an interesting feature being the detection of the recoil nucleus ${}_3{}^6{}_{}{}^{}\mathrm{Li}$ in its three states of ionization. The perfect resolution of the peaks is due to the fact that the target used was extremely thin and that no windows were used in the complete path of the particles from the target, through the analyzer and unto the detector, since the multiplier tube works in vacuum. Thus, no straggling was observed, and the width of the peaks coincide with the values predicted by the theory of the electrostatic analyzer for the slits used in these experiments. The ratios in which the three lithium ions appear are given and the problem of the probability for capture or loss of electrons by the ions is briefly discussed, on the basis of the ratio $\gamma$ of the electron orbital velocity to the velocity of the ion itself, as suggested by Bohr and used by Knipp, Teller, and Brunings in their treatment of heavy ions. The calculated values for $\gamma$ under our conditions are: for the process ${\mathrm{Li}}^{+++}$⇄${\mathrm{Li}}^{++}$, $\gamma =1.22\mathrm{}$, and for ${\mathrm{Li}}^{++}$⇄${\mathrm{Li}}^{+}$, $\gamma =1.09\mathrm{}$, which have a net effect of favoring the doubly ionized state of lithium. The actual counting rates observed indicate, in fact, that the ${\mathrm{Li}}^{++}$ is the more abundant of the three ions.