NEUTRINO*

F. Reines and C. L. Cowan, Jr.

Every new discovery in the natural sciences broadens and deepens our knowledge of the universe. But at times these advances in the study of the universe raise new and more profound questions than those to which answers have already been given. Such was the case with the discovery and investigation of the radioactive process of beta decay. In this process the atomic nucleus spontaneously emits a negative or positive electron and is thereby transformed into another element, with the same mass number but with a nuclear charge differing from that of the original element by one electronic charge. As might have been expected, intensive investigation of this interesting alchemy of nature shed much light on problems relating to the atomic nucleus. However, a new problem arose at the very beginning, when it turned out that beta decay is accompanied by a mysterious loss of energy by the decaying nucleus¹ and that this energy cannot be intercepted by the apparatus in which the decay takes place². One possible explanation was that the conservation laws (on which the entire edifice of modern science rests) are not applicable in the domain of subatomic dimensions. Another explanation, in which the conservation laws were to remain valid, was proposed in 1933 by Wolfgang Pauli, who postulated the existence of a new elementary particle³ in order to explain the loss of energy by the nucleus. This particle, according to Pauli’s hypothesis, should be emitted by the nucleus simultaneously with the electron; it should carry no electric charge, but should carry off the missing energy and momentum, and moreover it should escape from the laboratory apparatus without being detected.

The concept of this ghostly particle was used by Enrico Fermi (who named it the “neutrino”) in constructing his quantitative theory of beta decay⁴. As is well known, this theory, with only minor modifications, has had ever greater success in its application to nuclear problems, which in itself is a very convincing argument in favor of the validity of Pauli’s hypothesis. However, numerous additional experimental tests were proposed which were intended to support the neutrino hypothesis and to provide additional information about its properties. The most characteristic property of this particle, which is precisely what makes its assumption plausible—namely, its ability to carry off energy and momentum without being available for detection—limits these tests to measurements of observable details of the decay process itself: the energy spectra, momentum vectors, and energy states associated with the emitted electron and with the daughter nucleus of the decay⁵. In this way, for example, the upper limit of the rest mass of the neutrino, equal to one five-hundredth of the mass

* F. Reines, C. Cowan, jun. The neutrino. Nature 178, 446 (1956).

rest mass of the electron, was established by a careful measurement of the beta-energy spectrum of tritium decay near its endpoint[^6]; it is usually assumed that the rest mass of the neutrino is identically zero.

At a time when there are no theoretical grounds for expecting a finite value of the neutrino rest mass, one may expect that there exists, although small, a finite magnetic moment of the neutrino—perhaps of the order of \(10^{-10}\) Bohr magnetons. This estimate is based on taking into account virtual states in which the neutrino can exist during the transformations of other particles[^7]. An upper limit of \(2\cdot 10^{-10}\) electron Bohr magnetons was established for the magnetic moment by calculations relating to the maximum transfer of heat to the Earth from the Sun by means of neutrinos[^8]. Recently we found a refined upper limit of \(10^{-9}\) electron Bohr magnetons, using a large scintillation detector located near the reactor of the Savannah River plant of the United States Atomic Energy Commission. The number of individual pulses was observed in the energy interval \(0.1\)—\(0.3\) MeV in 370 gallons of liquid scintillator. In this, all changes due to changes in the reactor power were attributed to recoil electrons in the liquid, arising as a result of interactions with the magnetic moments of neutrinos. It may be hoped that this limit will be further improved by measuring the background of gamma rays and neutrons.

Pauli—Fermi theory requires that neutrinos carry not only energy and momentum acquired from beta-decaying nuclei, but also angular momentum, or spin. The simplest of the beta processes is the decay of a free neutron[^9], proceeding according to the equation

\[ \mathrm{n}^{0} \to \mathrm{p}^{+} + \beta^{-} + \nu_{-}. \tag{1} \]

Since the particles taking part in process (1)—the neutron, proton, and beta particle—all have half-integral spin, it is necessary also to assign to the neutrino the spin quantum number \(1/2\) in order to balance the angular momenta in equation (1), where any two of the three particles on the right-hand side must have their spin vectors oriented antiparallel. Therefore all four particles appearing in equation (1) are fermions and obey the relativistic Dirac equation for particles of spin \(1/2\). It follows from this that each of these particles must have a corresponding antiparticle, of which only two have so far been identified: the antielectron (or positron) and the antiproton. The antiparticle corresponding to the neutrino in equation (1) can be obtained by rearranging the terms as follows:

\[ \mathrm{p}^{+} \to \mathrm{n}^{0} + \beta^{+} + \nu_{+}. \tag{2} \]

This process is observed in the positron decay of radioactive nuclei with excess protons, where the proton and the daughter neutron are both nucleons of the nucleus. A further rearrangement leads to the reaction

\[ \beta^{-} + \mathrm{p}^{+} \to \mathrm{n}^{0} + \nu_{+}. \tag{3} \]

This equation describes the capture of an electron belonging to one of the inner shells of an atom by a nuclear proton; it is equivalent to equation (2). Thus there arises the question of the identity of the neutrino \(\nu_{+}\), appearing in equations (2) and (3), with the neutrino \(\nu_{-}\), appearing in equation (1). Since neither the terminal mass nor the magnetic moment has been measured for either of the two kinds of neutrino, we have no grounds for assuming that they are in fact identical. The rule of algebraic conservation of fermions, which states that fermions arise or disappear in particle–antiparticle pairs, is the rule

requires that the particle \(\nu_-\) in equation (1) be called an “antineutrino,” since it is emitted simultaneously with a negative electron. The identity or nonidentity of the neutrino \(\nu_+\) and the antineutrino \(\nu_-\), although not accessible to observation in ordinary beta decay, can be established by measuring the decay constant of the double beta decay of certain isotopes. This process was studied theoretically by Goeppert-Mayer \({}^{10}\) for the case in which the neutrino is not identical with the antineutrino, and by Fermi \({}^{11}\) for the case in which the two neutrinos are identical, as was assumed by Majorana \({}^{12}\). As a typical case, the double \(\beta\)-decay of \(Nd^{150}\) was considered:

\[ Nd^{150} \to Sm^{150} + 2\beta^- + 2\nu_- \quad (\text{Dirac--Mayer}), \tag{4a} \]

\[ Nd^{150} \to Sm^{150} + 2\beta^- \quad (\text{Majorana--Fermi}). \tag{4b} \]

If the neutrino and antineutrino are identical, then the virtual emission of one neutrino and its immediate reabsorption by the nucleus are equivalent to the real emission of two neutrinos, i.e., equation (4b) must take place. This mutual “cancellation” (in the algebraic sense) is impossible if the neutrino and antineutrino are different. The half-life periods for the processes (4), as was shown by Primakoff \({}^{13}\) and Konopinski \({}^{14}\), are very different in the two cases: namely, for equation (4a) the period is of the order of \(10^{19}\) years, and for (4b) of the order of \(10^{15}\) years, the decay energy being \(5.4\) MeV. Further, in the Majorana--Fermi case one should expect the total-energy spectrum to be linear.

That no period exists agreeing with equation (4b) was shown for a number of protected isotopes \({}^{15}\) by Kallstein and Libby, then by Fireman and Schwarzer in the case of \(Sn^{124}\), by Ashwalom—for \(Ca^{48}\), and, finally, by our collaborators and us—for \(Nd^{150}\). In the experiment with \(Nd^{150}\) a lower limit of the lifetime \(4 \cdot 10^{18}\) years was established for decay of the Majorana--Fermi type. This limit should be compared with the reasonable value \(1.3 \cdot 10^{15}\) years, computed, as already stated, for decay according to the Majorana--Fermi scheme; and for identical neutrinos one obtains (under the most stringent assumptions) \(6 \cdot 10^{17}\) years. Thus, it must be concluded that the neutrino and antineutrino are different particles with a still undetermined “distinction.” This conclusion is further confirmed by the negative result of an experiment, recently described by R. Davis \({}^{16}\), who used the reaction

\[ Cl^{37} + \nu^+ \to Ar^{37} + \beta^- . \tag{5} \]

The chlorine targets consisted of 1000 gallons of carbon tetrachloride located near a large reactor; the liquid was tested for the presence of \(A^{37}\). The fission fragments, rich in neutrons, should emit only antineutrinos \(\nu_-\).

Whereas a careful consideration of the experimental evidence for all terms of the \(\beta\)-process equation, except the neutrino, can confirm the existence of the neutrino, its reality can be convincingly proved only by direct observation of the neutrino itself. If the neutrino is a real particle carrying away the missing energy and momentum from the place where \(\beta\)-decay occurs, then the detection of these missing energy and momentum at another place will prove the reality of the neutrino. Thus, if the negative \(\beta\)-decay represented by equation (1) can be connected elsewhere with the reaction

\[ \nu_- + p^+ \to \beta^+ + n^0, \tag{6} \]

which will be observed with the predicted rate, then the cycle will be closed. The expression for the effective cross section was obtained by apply-

application of the principle of detailed balance to equation (1) with the aid of known data: the decay constant and the electron energy spectrum for the β-decay of the free neutron

\[ \sigma=\left(\frac{G^2}{2\pi}\right)\left(\frac{\hbar}{mc}\right)^2 \left(\frac{p}{mc}\right)^2 \frac{1}{\frac{v}{c}} \ \mathrm{cm}^2, \tag{7} \]

where \(\sigma\) is the cross section in \(\mathrm{cm}^2\), \(G^2\) is the dimensionless β-coupling constant based on neutron decay\({}^9\); \(p\), \(m\), \(v\) are, respectively, the momentum, mass, and velocity of the emitted positron, \(c\) is the speed of light, and \(2\pi\hbar\) is Planck’s constant—all in CGS units. For neutrinos with energy \(3\ \mathrm{MeV}\), incident on free protons, this cross section is \(10^{-43}\ \mathrm{cm}^2\). The explicit solution of equation (6) for the cross section as a function of neutrino energy gives

\[ \sigma=1.0\cdot 10^{-44}(E-a)\sqrt{(E-a)^2-1}\ \mathrm{cm}^2, \tag{8} \]

where \(a+1(=3.53)\) is the threshold energy for the reaction, and \(E\) is the neutrino energy, both quantities being in units of \(m_e c^2\). The threshold for a proton bound in a nucleus is higher by the amount of the energy difference between the target nucleus and the daughter nucleus. It is interesting to note that the penetrability of matter for neutrinos of low energy \((E<(a+1))\) is equal to infinity, and it is very large for neutrinos with energies of only a few MeV; in the latter case the mean free path for absorption is comparable with the radius of the universe.

Equation (6) can be used in an experiment in which a large number of hydrogen atoms serve as targets for an intense neutrino flux, while a detector is used that is capable of registering the simultaneous appearance of a positron and a neutron. Such a direct experiment became possible thanks to the large number of β-emitters in the form of fission products, concentrated in multi-megawatt reactors, and to advances in detection techniques using liquid scintillators. An estimate of the neutrino flux from large reactors shows that in 50 liters of water placed near a reactor, several protons per hour should undergo reaction (6). In such a case the problem consists in observing these events against the background of neutrons and γ-rays from the reactor, natural radioactivity, and cosmic rays. An attempt in this direction was made by us in 1953 in an experiment carried out at the Hanford plant of the Atomic Energy Commission\({}^{17}\). The proton targets consisted of 300 liters of liquid scintillator (toluene plus traces of terphenyl and alpha-naphthyl phenyl oxazole, in which cadmium propionate was dissolved). The number of delayed coincidences of pairs of pulses was observed, the first pair being attributed to positron annihilation and the second to neutron capture by cadmium; the observed counting rate was \(0.4\pm0.2\) per minute, in agreement with the predicted value, provided that the above-mentioned background was substantially reduced. The ratio of the signal to the total background was, however, still very low (1/20), in view of which further study of the signal was impractical and the results could be regarded only as preliminary. Nevertheless, on the basis of the Hanford experiment it was evident that the problem of detection was in a certain sense soluble, and a second experiment\({}^{18}\) was designed with the aim of further reducing the background and of making it possible to check each term of equation (6) independently of the others.

In Fig. 1 the detection scheme used in this experiment is shown. The sequence of events represented in this scheme is as follows: a neutrino produced in the decay of a fission fragment in the reactor causes the conversion of a target proton into a neutron with the simultaneous emission of

NEUTRINO

positron. The positron is captured by one of the electrons of the water, producing two annihilation $\gamma$-photons with an energy of $0.51$ MeV each. These $\gamma$-photons must be detected simultaneously by counters I and II. The neutron is slowed down and diffuses for several microseconds and ultimately is captured by cadmium, giving several $\gamma$-photons (a total of $9$ MeV), which in turn are recorded by counters I and II. Thus, we obtain a prompt coincidence, followed after several microseconds by a second prompt coincidence, thereby recording a quite definite sequence of events.

Fig. 1. Schematic diagram of the neutrino detector. 1 — $\gamma$-quantum from neutron capture in cadmium; 2 — path of the diffusing neutron; 3 — $\gamma$-quantum from positron annihilation; 4 — liquid scintillation detector II; 5 — proton from the target; 6 — target ($\mathrm{H_2O} + \mathrm{CdCl_2}$); 7 — liquid scintillation detector I; 8 — neutrino from the reactor; 9 — $\gamma$-quantum from positron annihilation.

The total volume of the apparatus is determined by the number of events expected per hour per liter of water, and also by the detection efficiency that can be hoped for. The most important factor in the geometry of the apparatus and in the detection efficiency is the absorption of the annihilation radiation by the water itself. Preliminary experiments and estimates showed that the optimum thickness of the water should be $7.5$ cm. Since, in order to obtain several counts per hour, the overall efficiency predetermined a water volume of 200 liters, two tanks of size $1.9\,m \times 1.3 \times 0.07\,m$ each were used. The depth of the liquid scintillation detector (61 cm) was such as to absorb the $\gamma$-rays from neutron capture by cadmium with good efficiency and to transmit the resulting light to the ends of the detector with minimal losses. The scintillation liquid (triethylbenzene, terphenyl, and POPOP—a wavelength shifter) was viewed from the ends of each detector tank by 150-inch DuMont photomultipliers, the number of which was determined primarily by the amount of light emitted by a single scintillation. The apparatus was assembled in the form of a “sandwich,” with two target tanks between three detector tanks, forming two substantially independent triads that jointly used the central detector tank. The entire detector was enclosed in a lead-paraffin protective box and placed deep underground, near one of the reactors of the Savannah River plant of the U.S. Atomic Energy Commission. Signals from the detectors were transmitted through coaxial cables to an electronic trailer located outside the reactor building. The signals were analyzed with respect to pulse height and coincidence time and, when this was possible, were recorded photographically by means of three-beam oscilloscopes. In Fig. 2 a recording of an event in the lower triad is presented. The entire system was calibrated with a plutonium-beryllium neutron source and a dissolved $\mathrm{Cu}^{64}$ positron source in the target tanks. Standardized pulse sources were also used to monitor the stability of the electronics. In addition, the response of the detector to cosmic-ray $\mu$-mesons was used to monitor the operation of the detector. After the instrument had been in operation for 1371 hours, including both the time when the reactor was operating and the time when it was not operating, it was found that$^{19}$:

1) The signal dependent on reactor operation is $2.88 \pm 0.22$ counts/hour, in agreement with the predicted$^{20}$ cross section $(6 \cdot 10^{-44}\ \mathrm{cm}^2)$ at

conditions, that the ratio of signal to background depending on the reactor is equal to 20/1. The ratio of signal to background not depending on the reactor is equal to 3/1.

2) Dilution of the solution in light water in the heavy-water target tank, in order to reduce the proton density by half, causes a twofold decrease in the signal dependent on the operation of the reactor. At the same time the neutron-detection efficiency, measured with a plutonium-beryllium source, remained unchanged.

Fig. 2. Typical record. Each of the three oscilloscope traces shown corresponds to a definite detector tank. The recorded event occurred in the lower triad. First of all, coincident pulses of $\gamma$-rays are seen, arising in the annihilation of a positron in each tank; following them, after 5.5 microseconds, come the more significant “neutron” pulses. The amplification in this case was chosen so that the “neutron” pulses could be measured. A second oscilloscope also operated in parallel with the first, but with greater amplification, so that the positron pulses could be measured.

3) It was shown that the first pulse of a pair of detectors is due to the annihilation radiation of the positron. The following tests served as controls: the radiation spectrum coincided with the spectrum of the positron annihilation radiation of $Cu^{64}$ dissolved in water; the radiation was absorbed in the expected manner by a thin layer of lead placed between the target tank and one of the detectors.

4) The second pulse of a pair of detectors was identified as being caused by cadmium capture of a neutron that had been born simultaneously with the positron. The basis was the distribution of capture times, in comparison with both the calculated distribution and that observed with a neutron source. The spectrum of the second pulse agreed with the spectrum of $\gamma$-rays from cadmium capture, and removal of the cadmium entailed the disappearance of the reactor signal.

5) The radiation originating from the reactor in the form of neutrons and $\gamma$-rays was excluded as a source of the signal by two experiments. In the first, an intense americium-beryllium neutron source was placed outside the protective shielding of the detector. It turned out that this source was not only ineffective in producing suitable delayed coincidences, but also that the spectrum of its first pulse did not correspond to the required signal in that it decreased monotonically with increasing energy. In the second experiment, additional shielding, which should have reduced the reactor neutrons and $\gamma$-rays by at least a factor of 10, caused no changes in the reactor signal, apart from the statistical fluctuations mentioned in item 1.

Thus, the experimental verification of equation (6) showed that the free neutrino is accessible to observation near a powerful reactor.

The production of sufficiently intense neutrino fluxes from reactors opens up a number of interesting possibilities. One of them is connected with the use of heavy water for diluting proton targets, as was described above. This test is expedient because the threshold ...

the value of the energy for the interaction of a neutrino with a deuteron by 2.2 MeV, i.e., by the binding energy of the deuteron, above the threshold energy for equation (6), as a result of which the cross section will be smaller by an order of magnitude. Other considerations reduce it to an even lower value.

However, the neutrino–deuteron interaction is interesting in itself, since there are two possibilities:

\[ \nu_- + D \to \beta^+ + n + n, \qquad (8a) \qquad \nu_- + D \to \beta^+ + n_2, \qquad (8б) \]

where \(n_2\) is a bound state of the bineutron \({}^{21}\), which has not yet been observed. If observation were to confirm reaction (8a), then a careful measurement of its intensity in comparison with reaction (6), together with knowledge of the fission-neutrino spectrum, would make it possible to carry out a direct determination of the ratio of the coupling constants of Fermi and Gamow–Teller beta decay. This follows from the fact that the coupling constant in equation (6) contains a mixture of both types, whereas in equation (8a) it represents only the Gamow–Teller constant. If, on the other hand, the reaction according to equation (8б) were observed, then not only would the considerations just set forth apply, but the existence of a bound state of the bineutron, which by virtue of the Pauli principle would have to be a singlet state (antiparallel spins), would give a direct answer to the question of the dependence of nuclear forces on charge. This follows from the fact that the singlet state of the \((n,p)\) system is known as an unbound state. Since the two neutrons in equation (8a), being produced by fission-fragment neutrinos, may have an energy of only a few kilovolts, and since they leave the event in states with antiparallel spins, the conditions appear favorable for the formation of a bineutron even if its binding energy were only a few tens of kilovolts.

Since Pauli’s proposal of the neutrino hypothesis, and the success of this hypothesis in its application to Fermi’s theory of \(\beta\)-decay, the participation of similar particles has been assumed in the observed decay of a certain number of mesons \({}^{22}\). The question arises of the identity of these neutrino-like particles with the neutrino of nucleon decay. It should be noted that in nuclear \(\beta\)-decay the initial and final nuclei both, obviously, interact strongly with nuclei. This is not the case in \((\pi,\mu)\)-decay, where the emission of the “neutrino” transforms the interaction of heavy particles from strong to weak. Furthermore, despite the apparent equality of the matrix elements of nuclear \(\beta\)-decay and the matrix elements associated with \((\mu,\beta)\)-decay, both the initial and the final products of the latter interact only weakly with nuclei.

The neutrino is the smallest object of physical reality thus far known to man; the largest object is the universe. An attempt to understand something about one of these objects by means of the other means an attempt to span the scale within which all manifestations of the laws of nature lie. And so, despite our vague knowledge of these extreme limits, problems arise that seize our imagination. If nuclear reactions play a role in the catastrophic “birth” of the universe—as is assumed—what fraction of the primordial energy was rapidly transformed into the energy of an irreversible neutrino field? Do these neutrinos remain captured by the ordinary gravitational field, and if so, what are their present density, their energy spectrum, and angular distribution? If the neutrino has zero rest mass, then in discussing their gravitational potential should they be regarded as “material” particles, or in connection with the electromagnetic radiation field? The problem of discovering these final products of all processes of the emergence of nuclear energy and measuring their characteristics constitutes a bold challenge to physics today.

Properties of Neutrinos

Spin \(1/2\,\hbar\).

Mass \(< 1/500\), if at all different from zero.

Charge 0.

Magnetic moment \(< 10^{-9}\) Bohr magneton.

Cross section for the reaction \(\nu_- + p^+ \to \beta^+ + n^0\) at \(3\) MeV \(= 10^{-43}\,\mathrm{cm}^2\).

The neutrino \(\nu_+\) is not identical with the antineutrino \(\nu_-\).

References and Notes

1. Chadwick discovered that the \(\beta\)-spectrum is continuous. L. Meitner in 1922 drew attention to the fact that a quantized nucleus should not emit a continuous spectrum, while Ellis established nonconservation of energy from experiments with emitted electrons. J. Chadwick, Verh. Deutsch. Phys. Ges. 16, 383 (1914); C. D. Ellis, Internat Conf. on Physics 15, 209 (1934).

2. Ellis and Wooster, Proc. Roy. Soc. A117, 109 (1927); J. Chadwick and D. E. Lea, Proc. Camb. Phil. Soc. 30, 59 (1934); M. E. Nahmias, Proc. Camb. Phil. Soc. 31, 99 (1935); C. S. Wu, Phys. Rev. 59, 481 (1941).

3. W. Pauli, Rapports du Septieme Conseil de Physique Solvay, Brussels, 1933 (Gauthier Villars, Paris, 1934).

4. E. Fermi, Z. Physik. 88, 161 (1934).

5. We cannot describe here the many excellent and difficult experiments in which the recoil of nuclei emitting neutrinos (\(\sim 8\)–\(200\) eV) was measured. A review of these experiments may be found in the book: Siegbahn, Beta and Gamma Ray Spectroscopy (Interscience Publishers, Inc., New York, 1955).

6. L. M. Langer and R. J. D. Moffat, Phys. Rev. 88, 689 (1952); Hamilton, Alford and Gross, Phys. Rev. 92, 1521 (1953).
   This question is considered in detail in Wu’s article included in Siegbahn’s book (see 5). We give the upper limit cautiously estimated by Dr. Wu.

7. F. G. Houtermans und W. Thirring, Helv. Phys. Acta 27, 81 (1954).
   G. Bethe gave a detailed relation between the recoil spectrum of electrons and the energy and magnetic moment of the neutrino: Proc. Camb. Phil. Soc. 31, 108 (1935).

8. H. R. Crane, Revs. Mod. Phys. 20, 278 (1948).
   This article also summarizes attempts to discover the neutrino made before 1948. The state of the neutrino problem in 1936 is given in the article H. A. Bethe and R. F. Bacher, Revs. Mod. Phys. 8, 82 (1936).

9. A. H. Snell and L. C. Miller, Phys. Rev. 74, 1714 A (1948); A. H. Snell, F. Pleasanton and R. V. McCord, Phys. Rev. 78, 310 (1950); J. M. Robson, Phys. Rev. 78, 311 (1950), 83, 349 (1951).

10. M. Goeppert-Mayer, Phys. Rev. 48, 512 (1935).

11. W. H. Furry, Phys. Rev. 56, 1184 (1939).

12. E. Majorana, Nuovo Cim. 14, 171 (1937).

13. H. Primakoff, Phys. Rev. 85, 888 (1952).

14. E. J. Konopinski, Los Alamos Report LAMS 1949 (1955).

15. M. I. Kalkstein and W. F. Libby, Phys. Rev. 85, 368 (1952); E. L. Fireman and D. Schwarzer, Phys. Rev. 86, 451 (1952); F. B. Harrison, L. M. Langer and F. Reines, Nuovo Cim. 3, 649 (1956).

16. R. Davis (Jr.), Amer. Phys. Soc. Washington, D. C. Meeting, 1956. This experiment was originally proposed by Pontecorvo and discussed by Alvarez in report UCRL—328 (1949).

17. F. Reines and C. L. Cowan jun., Phys. Rev. 90, 492 (1953); 92, 830 (1953).

18. C. L. Cowan jun. and F. Reines, Amer. Phys. Soc., New York, Meeting, January 1954.

19. C. L. Cowan jun. and F. Reines, Postheadline Paper, Amer. Phys. Soc., New Haven Meeting, June 1956; Cowan, Reines, Harrison, Kruse and McGuire, Science 124, 103 (1956).

20. The neutrino spectrum was derived from the \(\beta\)-decay spectrum of fission fragments, measured by Moulthrop at Brookhaven National Laboratory. Dr. Moulthrop kindly communicated his results to the authors before their publication.

21. Arguments for and against the existence of the “bineutron,” also called the “dineutron,” are considered by B. T. Feld in his article on the neutron, included in the volume edited by E. Segrè, Experimental Nuclear Physics, vol. II.

22. S. Oneda and A. Wakasa discuss classes of interactions between elementary particles in Nucl. Phys. 1, 445 (1956).