Production of Induced Radioactivity by High Velocity Protons

J. D. Cockcroft et al.

Editor’s Note

By the mid 1930s physicists were routinely producing new radio-isotopes by bombarding elements with alpha particles and protons. Here Cockcroft and colleagues describe the bombardment of graphite with high-energy protons, producing radioactivity that decreases with a half-life of about ten minutes. This suggested that the proton beam had created a significant amount of some radionuclide, presumably 13N. Some of the emitted particles were positrons, although most were ordinary electrons with high energies that might be explained as due to “kicks” by gamma rays. These in turn might have been created by annihilation of positrons in the chamber walls. This was among the first observations of electron–positron annihilation, a process that was not well understood until the 1940s.　　中文

CURIE and Joliot1 have reported that a number of new radioactive isotopes can be produced by the bombardment of various elements with α-particles, these isotopes emitting positive electrons. In particular, they showed that boron when bombarded by α-particles was transformed to the isotope N13, radio-nitrogen, this isotope having a half life of 14 minutes. They suggested that the isotope might be produced by the bombardment of carbon with heavy hydrogen, the product, N14, disintegrating with the emission of a neutron to radio-nitrogen.　　中文

We have bombarded a target of Acheson graphite with protons of 600 k.v. energy and have used a Geiger counter to search for any radiations produced after the bombardment ceased. After bombardment for 15 minutes with a current of about 10 microamperes of protons, the target was removed from the apparatus and placed against the Geiger counter. We then observed about 200 counts per minute, being about forty times the natural effect. The number of counts decayed exponentially with time, having a half life of 10.5±0.5 minutes.　　中文

We then carried out an experiment similar to that performed by Becquerel, in which the source was placed on one side of a 9 mm. thick lead plate with the counter on the opposite side, the whole being placed in a magnetic field, so that any electron emitted could only reach the counter by applying a field of appropriate sign and magnitude. We found that when the field was such that positive electrons could reach the counter, the number of counts increased by a factor of 3; when the field was in the reverse direction no definite increase was observed. We conclude, therefore, that the radiations consist in part at least of positive particles.　　中文

We have also taken about 250 Wilson chamber photographs in a field of 2,000 gauss, placing the activated source against the outside of the chamber wall, which was about 3 mm. thick. Under these conditions, we observed only two electrons of positive curvature which could possibly have come from the source, these electrons having energies of the order of 500 k.v. We observed, on the other hand, 48 tracks of Compton electrons starting in the gas, having energies ranging from 100 k.v. to 500 k.v., suggesting the emission of γ-rays of energy between 500 k.v. and 1 million volts. These γ-rays may result from the annihilation of the positive electrons, presumably in the glass wall of the chamber. The deflection experiments, whilst not at present precise, tend to confirm that few of the positive electrons would have sufficient energy to penetrate the glass walls. Further experiments will, therefore, be carried out with the source inside the chamber.*　　中文

The observations suggest that the unstable isotope N13 is produced by the addition of a proton to C12. The difference between the half life observed and that reported by Curie and Joliot may be due to the formation of N13 in a different excited state.　　中文

No marked increase in the number of counts was observed when a mixed beam of heavy hydrogen ions and protons was substituted for the proton beam.　　中文

We are very much indebted to Dr. K. T. Bainbridge, who supplied the Geiger counter with which the observations were made.　　中文

(133, 328; 1934)

J. D. Cockcroft, C. W. Gilbert and E. T. S. Walton: Cavendish Laboratory, Cambridge, Feb. 24.

Reference:

1. Comptes rendus, 198, 254; 1934.

* February 27. Experiments carried out with a counter having a mica window of small stopping power gave a great increase in the number of counts owing to the positive electrons now entering the counter. The absorption curve of the positive electrons is similar to that of negative electrons of 800 k.v. energy.