AN EXPERIMENT PROPOSED TO DETECT THE FREE NEUTRINO

The authors of the notes under review \(^{1,2}\) propose the following experiment for detecting the free neutrino. Neutrinos formed in the \(\beta\)-decay of fission fragments in a powerful chain reactor pass through a scintillation counter of large volume, of the order of \(0.3\ \mathrm{m}^3\). The cross section of the counter protons for the process \(\nu + p \to n + e^+\) is \(10^{-44}\ \mathrm{cm}^2\). The authors believe that several tens of such events per minute can be obtained in the detector. To reduce the background, boron or cadmium compounds are placed in the counter and delayed coincidences of \(\gamma\)-quanta arising from neutron capture with positron and annihilation radiation are recorded. It is also necessary, in order to reduce the background, to use boron-paraffin shielding and screens of lead and steel. To discriminate against double pulses from \(\mu\)-meson decay, stars, etc., Geiger–Müller counters must be connected in anticoincidence.

The most important part of the apparatus is the large scintillation counter \(^{2}\), which must satisfy the following requirements: 1) the presence of a large number of protons in the sensitive volume, 2) good efficiency with respect to the registration of \(\beta\)-particles, \(\gamma\)-rays, and neutrons, 3) sharp energy resolution, 4) a controllable short mean capture time of the neutrons formed when the neutrino is captured. Three such cylindrical counters were made: an experimental brass model with a volume of \(0.07\ \mathrm{m}^3\) and two counters with a volume of \(0.3\ \mathrm{m}^3\), made of stainless steel and cold-rolled steel, respectively. After fabrication the counter tanks were annealed to remove adsorbed oils, and the inner surface was covered with a layer of white paint. The three solutions filling the counters consisted of: 1) toluene containing terphenyl, 2-(1-naphthyl)-5-phenyloxazole (\(\alpha\) NPO), and methyl borate (or cadmium propionate in methanol); 2) tritylbenzene containing 2.5 diphenyloxazole, \(\alpha\) NPO, and methyl borate; 3) purified mineral oil containing 2.5 diphenyloxazole, \(\alpha\) NPO, and methyl borate.

These solutions contained from \(4.6 \cdot 10^{22}\) to \(7.2 \cdot 10^{22}\) protons/\(\mathrm{cm}^3\). By regulating the cadmium or boron content one can bring the mean neutron-capture time down to \(5\ \mu\mathrm{sec}\). Absorption of the scintillation light by the solutions was reduced by purification and by using \(\alpha\) NPO to modify the scintillation spectrum. To detect the scintillations, 32 photomultipliers were placed around the experimental model and 90 photomultipliers around each of the other counters. A corresponding electronic system was made, consisting of linear amplifiers, pulse-height analyzers, and delayed-coincidence analyzers. Automatic monitoring of the liquid levels in the counters and of leaks due to possible failures was carried out. A nitrogen atmosphere above the solution prevented hydrolysis and excluded boron radioactive contamination from the air.

The manufactured counter was used to study the counting rate and pulse spectrum from \(\alpha\)-, \(\beta\)-, and \(\gamma\)-sources placed at various points in the counter volume, and from external neutron sources. The authors found that the counters are sufficiently insensitive to the position of the source and that the uniformity of the readings is considerably better for large counters than for the experimental model of small volume. The efficiency of the counter for \(\gamma\)-rays with an energy of \(0.5\ \mathrm{MeV}\) is 75%. In the case of neutrons (the counter was surrounded by a lead screen 15 cm thick), a peak of the reaction \(p(n,\gamma)d\) was obtained, clearly distinguished from the background of recoil protons and \(\gamma\)-rays arising in the lead screen upon neutron capture.

The delayed-coincidence scheme was checked by measuring the decay time of \(\mu\)-mesons of cosmic rays stopping in the scintill-

...in the scintillator. The mean lifetime was found to be \(2.2 \pm 0.5\ \mu\text{sec}\). The large detector area made it possible to accumulate sufficient statistics in 30 minutes.

The authors note that large scintillation detectors can find a number of interesting applications, and they give examples of such applications carried out by them in the course of the investigation.

V. S.

References

1. F. Reines, C. L. Cowan, Phys. Rev. 90, 492 (1953).
2. C. L. Cowan, F. Reines, F. B. Harrison, E. G. Anderson, F. H. Hayes, Phys. Rev. 90, 493 (1953).