50 Years of Neutrino Physics Early Experiments
Part of the Ettore Majorana International Science Series book series (EMISS, volume 12)
It is now 50 years since Pauli (1) formulated the neutrino hypothesis and 47 years since Fermi (2) cast it in its essentially modern form.
KeywordsNeutron Capture Beta Decay Fission Fragment Fission Reactor Target Proton
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- 1.W. Pauli, Jr., address to the Group on Radioactivity, Tübingen, Germany, 4 December 1930 (unpublished); Rapports du Septieme conseil physique Solvay, Bruxelles, 1933 (Gautier-Villars, Paris, 1934), chap. 1, p. 324.Google Scholar
- 3.A popular account of these early days was written by Cowan in 1964 [“Anatomy of an Experiment: An Account of the Discovery of the Neutrino”, Smithson. Inst. Annu. Rep. 4626 (1964), p. 409 ]. The status of the neutrino in 1936 was reviewed by H.A. Bethe and R.F. Bacher [ Rev. Mod. Phys. 8, 82 (1936)]. Attempts to detect the neutrino up to 1948 were summarized by H.R. Crane [ibid., 20, 278 (1948)]. B. Pontecorvo [ Inverse Beta Decay (Division of Atomic Energy, National Research Council of Canada, Chalk River; declassified and issued by the Atomic Energy Commission in 1949)] and L.W. Alvarez [Univ. Calif. Radiat. Lab. Rep. UCRL-328 (1949)] suggested a radiochemical method using a fission reactor based on the reaction ν+37Cl → 37Ar + e−. They did not pursue the method. Alvarez was dissuaded by his estimates of the background to be anticipated from cosmic rays; these estimates later proved to be correct for the reactors then available. As we know now, the neutrino produced in fission is ν¯e and the neutrino required for the 37Cl reaction is νe so the reactor result would have been negative even though the neutrino exiCrossRefGoogle Scholar
- 4.N. Bohr, J. Chem. Soc, p. 349 (1932).Google Scholar
- 6.The technique of scintillation counting followed the discovery by W. Crookes and by J. Elster and H. Geitel [ Phys. Z. 4, 439 (1903)] of the scintillation properties of zinc sulfide exposed to alpha particles [described by E. Rutherford, J. Chadwick, and C.D. Ellis, Radiations from Radioactive Substances (Cambridge Univ. Press, Cambridge, England, 1930)]. It received great impetus from the development of the photomultiplier tube and the crucial observation [H. Kallmann, Phys. Rev. 78, 62 (1950)Google Scholar
- M. Agena, M. Chiozotto, R. Querzoli, Atti Acad. Naz. Lincei Cl. Sci. Fis. Mat. Nat. Rend. 6, 626 (1949)Google Scholar
- G. T. Reynolds, F.B. Harrison, G. Salvini, Phys. Rev. 78, 488 (1950)] that liquids could be made to scintillate with high efficiency when the scintillating compound was at low concentration. We recognized that with a sufficiently transparent scintillator and enough photocathode area, one should, in principle, be able to make a detector of almost arbitrarily great size — just what was needed for neutrino detection. Our first large detector, nicknamed El Monstro, was a 1-m3 bipyramidal brass tank containing toluene and viewed on the top and bottom by four 2-inch photomultiplier tubes. Our subsequent detectors employed many more photomultipliers to increase light collection and so obtain the desired energy resolution.CrossRefGoogle Scholar
- 10.At that time R. Davis, Jr., reassessed the 37Cl approach and decided to make an effort to observe the reactor neutrino by using that reaction. We called to his attention the existence of other well-shielded, powerful SRP reactors, and he placed 4000 liters of CCl4 near one of them. He obtained a negative result (R. Davis, Jr., paper presented at the American Physical Society Meeting, Washington, D.C., 1956) which, taken together with our observation, proved that although the ν¯e existed it was incapable of inverting 37Ar decay. This suggested that the neutrino emitted by neutronrich fission fragments (e− decay), ν¯e, was different from the νe emitted in e+ decay, which at that time was one of the two possibilities to be checked.Google Scholar
- 11.This prediction incorporated the then held belief that parity is conserved in the weak interaction. In view of the large experimental errors and the poorly known ν¯e spectrum, we considered this crude agreement consistent with the ν¯e origin of the signal and continued our program to make this comparison more precise. (Our initial analysis grossly overestimated the detection efficiency with the result that the measured cross section was at first thought to be in good agreement with prediction.) As commented on later in this account, the effect of parity nonconservation is to increase the predicted cross section by a factor of 2. In the two-component theory the electron neutrino has only two states — one, ν¯e with its spin angular momentum parallel, and one, νe, with its spin angular momentum antiparallel to its linear momentum. The old four-component theory allowed each neutrino to have two spin states.Google Scholar
- R.L. Garwin, L.M. Lederman, M.W. Weinrich, ibid., p. 1415Google Scholar
- 13.T.D. Lee and C.N. Yang, Phys. Rev. 104, 1671 (1956), (parity non-conservation in β decay).Google Scholar
- 16.In the fall of 1956, following the observation experiments, we measured the cross section with equipment built for that purpose in 1954–5, but we did not publish the result until an improved measurement had been made of the ν¯e spectrum from fission (1957), which made possible a more precise comparison with theory [F. Reines and C.L. Cowan, Jr., in Second United Nations International Conference on the Peaceful Uses of Atomic Energy (A/Conf. 15/P/1026, United Nations, New York, 1958); R.E. Carter, F. Reines, J.J. Wagner, M.E. Wyman, in ibid. Google Scholar
- 17.T.D. Lee and C.N. Yang, ibid., p. 254.Google Scholar
- 18.B. Pontecorvo, Electron and Muon Neutrinos (Reprint P-376, Joint Institute for Nuclear Research, Dubna, Soviet Union, 1959).Google Scholar
© Plenum Press, New York 1982