The disintegration of lithium by fast protons

In this article, we will discuss the study of lithium disintegration by fast protons. We will clarify the method used to produce a continuous beam of fast protons with energies up to 600 kilovolts by applying high voltages, and we will explain the experiments conducted to measure the extent of the spread of these protons outside the tube. The same method was used to study the effect of bombarding a layer of lithium with a stream of these ions, where lithium was placed inside the tube at a 45-degree angle to the beam. A 2 cm thick mica window was mounted on the side of the tube, and the presence of radiation from the lithium was studied using the method of light emission outside the tube. The thickness of the mica window was more than sufficient to prevent any scattered protons from escaping into the air even at the highest voltages used.

Preliminary Observations

When an acceleration voltage of about 125 kilovolts was applied, a number of bright light emissions were immediately observed, which rapidly increased with the higher voltages used, namely 400 kilovolts. At this point, hundreds of light emissions were observed per minute using a low proton current of a few milliamperes. No light emissions were observed when the proton beam was cut off or when the lithium was shielded from it using a metal screen. The extent of the particles was measured by introducing mica screens into the path of the beam, and it was found to be about eight centimeters in air and did not change significantly with voltage.

Nature of the Particles

Experiments were conducted using a Chemi-Zoo expansion chamber, where a number of tracks resembling those of alpha particles were observed, consistent with the range precisely determined by the light emissions. It is estimated that at 250 kilovolts, one particle is produced for every 10^9 protons that collide. The brightness of the light emissions and the density of the tracks observed in the expansion chamber indicate that the particles are ordinary alpha particles. If this view proves correct, it is not unlikely that the lithium isotope with mass 7 occasionally absorbs a proton, resulting in a nucleus of mass 8 decaying into two alpha particles, each with a mass of four and an energy of about eight million electron volts. The energy release in this view is about sixteen million electron volts per decay, which approximately agrees with what can be expected from the mass deficit involved in such disintegration.

Future Experiments

Experiments are currently underway to determine the effect of other elements when exposed to a beam of fast protons and other particles.

References: NATURE, 129, 242, February 13, 1932.

Book Information: The study was conducted at the Cavendish Laboratory, Cambridge, on April 16.