The Capture of Electrons by Alpha-Particles

Arthur H. Barnes
Phys. Rev. 35, 217 – Published 1 February 1930
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Abstract

A stream of electrons is superposed upon a beam of alpha-particles. The alpha-particles are then deflected by a magnetic field and counted by the scintillation method. Alpha-particles which have captured electrons will not be deflected to the point where observation is made, and consequently, the decrease in scintillations will indicate the number of captures which occur.

Probability of capture as function of relative velocity.—It is found that electrons are captured when their velocity equals that of the alpha-particle, and also, at a series of discrete velocities both greater and less than that of the alpha-particle. Counting scintillations due to singly charged particles showed that at certain of these discrete velocities one electron is captured. At the other velocities two electrons are captured. This was determined by counting scintillations due to neutral particles which had passed undeflected through the magnetic field. Both single and double capture occur at zero relative velocity, the latter predominating as the electron density is increased.

Calculation of the kinetic energy of the electron with respect to the alpha-particle corresponding to each of the velocities at which single capture occurs, gives a series of energy values which can be associated with the energy levels in the ionized helium atom. Similarily, for the case of double capture, the sum of the kinetic energies due to each of the two electrons, corresponds to the energy of the quantum states of both the parhelium and orthohelium types of atom. There is indication that the hitherto unobserved ground state of orthohelium is being formed. The conclusion is reached that in order for capture to occur, it is necessary that the electron, due to some external field, possess a kinetic energy, which, with respect to the nucleus, is either zero, or equal to the energy of one of the quantum states of the atom. In the latter case double the normal energy must be radiated, and the question arises as to whether it is radiated as two quanta of normal frequency, or as one quantum of twice that frequency.

There is evidence that the penetrating power of the singly charged and neutral particles is less than that of the nucleus alone.

Time required for capture of electron by alpha-particle.—The construction of one of the tubes used permitted the time during which the alpha-particle was in the electron stream to be made less than 3×1010 sec without appreciably decreasing the percent of capture. The electron density under these conditions was of the order of 107 electrons per cc.

  • Received 24 December 1929

DOI:https://doi.org/10.1103/PhysRev.35.217

©1930 American Physical Society

Authors & Affiliations

Arthur H. Barnes

  • Physics Laboratories, Columbia University

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Issue

Vol. 35, Iss. 3 — February 1930

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