FROM CURRENT LITERATURE

RADIOACTIVITY OF THE FREE NEUTRON

It has long been known that the mass of the neutron is not only greater than the mass of the proton, but also greater than the sum of the masses of the proton and the electron. The mass difference¹

\[ n^1 - H^1 = 1.25\ \mathrm{MeV} \]

shows that the free neutron must be \(\beta\)-radioactive. Since the mass of the electron is roughly \(0.5\ \mathrm{MeV}\), the upper limit of the energy of the \(\beta\)-particles arising in neutron decay must be equal to \(\sim 0.75\ \mathrm{MeV}\). For the decay period of the neutron, the theory of \(\beta\)-decay gives approximately 30 min. Strangely enough, indisputable direct evidence of the \(\beta\)-radioactivity of the free neutron had until now been lacking, and only very recently, in May 1950, brief communications were published on experiments in two different laboratories,²˒³ which give convincing proof of the sought radioactivity.

In the work² carried out at Oak Ridge, a beam of neutrons from a uranium–graphite reactor passed along the axis of an evacuated cylinder. The beam entered the cylinder and left it through thin aluminum windows. The decay protons arising in the region of the neutron beam were accelerated by an electric field of \(8\ \mathrm{kV}\) and concentrated on the first enlarged electrode of an electron multiplier, while the \(\beta\)-particles, after passing through an aluminum filter \(0.003\) inch thick, were recorded by two counters connected in coincidence. Under these conditions the multiplier gave a considerable number of pulses—about 1500 per minute. It was necessary, however, to make sure that these pulses were due specifically to neutron-decay processes and were not caused by various ionization processes on the walls of the cylinder or in residual gas. To this end, both electron counters and the multiplier were connected in coincidence, so that only triple coincidences were counted. A series of experiments was carried out in which the circuit operated on coincidences without any time delay between the pulses in the counters and in the multiplier. These coincidences, however, did not depend on the presence or absence of the electric field and, consequently, were due to secondary effects; such coincidences were not taken into account. Finally the circuit was arranged so that it responded when the pulse in the electron counters lagged behind the pulse in the multiplier by \(0.25\) microseconds. This time, \(0.25\ \mu\mathrm{sec}\), according to [[unclear: word beginning “pod-”]]

to the count, approximately equal to the time required for a decay proton to reach the collector. Such delayed coincidences were counted. Their number, in the presence of an electric field concentrating the decay protons on the first electrode of the multiplier, was on average \(0.74 \pm 0.05\) per minute; when the field was switched off, the number of coincidences fell to \(0.08\). These \(0.08\) coincidences are evidently of accidental origin; the remainder, \(0.74 - 0.08 = 0.66\), gives the number of true coincidences caused by the \(\beta\)-decay of the neutron. The coincidences disappeared when: 1) the neutron beam was interrupted by a screen of boron or cadmium, or 2) the reactor was switched off and the detectors were actuated by \(\gamma\)-radiation from \(\mathrm{Co}^{60}\). On the other hand, filling the cylinder with hydrogen did not change the number of coincidences.

On the basis of the totality of their results, the authors conclude that triple coincidences with a delay of \(0.25\) μsec are caused by the spontaneous transformation of free neutrons into protons with liberation of \(\beta\)-electrons whose energy is less than \(0.9\) MeV. Precise determinations of the decay period could not yet be made; however, a rough estimate gave \(10\)–\(30\) min for the period.

A shortcoming of the work described is that it is not proved that the positive ions causing the multiplier to operate are precisely protons. This shortcoming is compensated for by another piece of work,[^3] in which the mass of the positive ions was determined with the aid of a magnetic spectrometer. The positive ions arising in the region of the neutron beam were directed by an electric field into a magnetic spectrometer with a fine magnetic lens. As the ion counter, as in the preceding work, a multiplier was used. The curve of the dependence of the number of ions on the current strength in the lens winding showed a sharp maximum at the current strength corresponding to the proton mass. With an accelerating potential of 10 kilovolts, the spectrometer should have revealed a maximum corresponding to positive ions of molecular hydrogen, had such ions been present. However, no maxima, apart from the maximum corresponding to the proton mass, were observed. Further, in order to establish whether protons arise as a result of any side effects in the residual gas in the apparatus, the gas pressure was deliberately increased by a factor of 10. However, no substantial effects were thereby detected. The author concludes, therefore, that the protons detected by means of the magnetic spectrometer arise as a result of the transformation of neutrons.

Additional experiments were carried out to determine the decay period of the neutron. These experiments showed that the lifetime of the free neutron lies between 9 and 25 minutes.

E. Shpolsky

LITERATURE CITED

1. E. V. Shpolsky, Atomic Physics, Vol. II, p. 510, Gostekhizdat (1950).
2. A. H. Snell, F. Pleasonton and R. V. McCord, Phys. Rev. 78, 310 (1950).
3. J. M. Robson, Phys. Rev. 78, 311 (1950).