Chronology of Milestone Events in Particle Physics - SNELL 1948
Chronology of Milestone Events in Particle Physics

SNELL 1948

Snell, A.H.; Miller, L.C.;
On the Decay of the Neutron
Phys. Rev. 74 (1948) 1217;

Full Text
A collimated beam of neutrons, three inches in diameter, emerges from the nuclear reactor and passes axially through a thin-walled, aluminum, evacuated cylindrical tank. A transverse magnetic field behind the thin entrance window cleans the beam of secondary electrons. Inside the vacuum, axially arranged, an open-sided cylindrical electrode is held at +4000 volts with respect to ground. Opposite the open side a smoothed graphite plate is held at -4400 volts. The field between these electrodes accelerates and focuses protons which may result from decay of neutrons, so that they pass through a 27 / 8 x 15 / 8 inch aperture in the center of the graphite plate, and strike the first dynode of a secondary electron multiplier. The first dynode is specially enlarged so as to cover the aperture. Readings are taken
(1) with and without a thin B20 shutter in the neutron beam;
(2) with and without a thin foil over the multiplier aperture;
(3) with and without the accelerating voltage.
In a total counting rate of about 300 per min., about 100 are sensitive to operations (1), (2), and (3). In the absence of the accelerating field or with the foil (2) in, operation (1) does not change the counting rate. Assuming all of the 100 c.p.m. to be due to decay protons, preliminary estimates of the collecting and counting efficiency (10 percent) and of the number of neutrons in the sample (4 x 104) give for the neutron a half-life of about 30 minutes. It is at present much safer however to say that the neutron half-life must exceed 15 minutes. Coincidences are presently being sought between the disintegration betas and the collected protons.

Accelerator REACTOR Detectors CNTR

  n p e neutral

Particles studied
  n w

Record comments
First evidence for neutron beta decay.
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