EXCITED NUCLEONS*)

S. F. Powell

In the course of the last year, data have accumulated indicating the existence of a new type of nuclear excitation, apparently due to the presence inside the nucleus of a neutral hyperon**). This particle, denoted by the symbol $\Lambda^0$, can probably combine with other nucleons of the nucleus, creating a relatively stable formation. The decay of such a nucleus is ultimately caused by the decay of the $\Lambda^0$ particle.

The existence of the $\Lambda^0$ particle (until recently called the heavy neutral $V$-particle) was established with the aid of a Wilson chamber in investigations of cosmic rays. It was shown that, among the various neutral particles capable of producing “neutral $V$ events,” discovered by Rochester and Butler$^1$ in 1947, hyperons are encountered most often. The majority of the processes mentioned are caused by particles of two types: 1) $\Lambda^0$, decaying into a proton and a negative $\pi$-meson ($\Lambda^0 \to P + \Pi^{-}$) with the release of energy of 35–40 MeV$^2$, and 2) $\theta^0$, heavy neutral mesons, transforming into two light mesons, probably into $\pi$-particles ($\theta^0 \to \pi^+ + \pi^-$) with the release of energy of 210 MeV$^{3,4}$. The corresponding mean lifetimes are $t_{\Lambda^0}\sim 3\cdot 10^{-10}\ \text{sec}$ and $t_{\theta}\sim 2\cdot 10^{-10}\ \text{sec}$.

At present it has been established that $\Lambda^0$ particles can be formed in such nuclear reactions in which the kinetic energy of the participating particles is less than the energy corresponding to the rest mass of the hyperon$^{5,6}$. It is therefore supposed that the latter arises upon collision as a result of a transformation of a nucleon. The noted fact, and also the observed transformation of a $\Lambda^0$ particle into a proton in its decay, indicate that this particle is expediently

*) S. F. Powell, Nature 173, No. 4402, 469 (1954).

**) The terminology used was proposed by Amaldi et al. (Nature 173, 123 (1954)); see UFN 53, 289 (1954). A hyperon is a particle having a mass intermediate between the mass of the neutron and that of the deuteron.

...similarly be regarded as an “excited” nucleon. As is known, an excited atom returns to its normal energy state by emitting a quantum of radiation—a photon; by analogy one may suppose that an excited nucleon is de-excited by emitting a “heavy quantum” of the nuclear field, i.e. a π-meson.

Recently it has become possible to measure quite accurately the mass of the \(\Lambda^0\)-particle by the photographic method. In a stack of photographic emulsions exposed at high altitude, Friedlander, Keefe, Menon, and Merlin\(^7\) found 8 cases of decay of \(\Lambda^0\)-particles; moreover, in all cases the secondary proton and the negative π-meson stopped inside the stack. Thus it was possible to measure the range and, consequently, the energy of the particles. These data, as well as the measured value of the angle between the tracks of both emitted particles, made it possible to determine with good accuracy the amount of energy released. The value obtained, \(37.2 \pm 0.5\) MeV, agrees well with the value obtained in measurements in a Wilson chamber. The mass of the \(\Lambda^0\)-particle turns out to be equal to \(2182 \pm 2m_e\). The quoted error in the value of the energy was obtained from the scatter of the results of individual measurements. It is possible that, with refinement of the relation between the range and the energy of protons, the final value will have to be corrected somewhat.

The first observation indicating that hyperons can exist not only as free particles but also in a bound state inside a nucleus was made by Danysz and Pniewski\(^8\). In a photographic plate exposed to cosmic rays, these authors found the track of a heavy nuclear fragment having charge \(5e\). At the end of its range the fragment decayed with the emission of three or four charged particles.

The remarkable feature of this event was that the decay occurred at the end of the fragment’s range and, consequently, this parent particle was at rest or was moving with a very small velocity. The secondary transformation, therefore, could not have been due to a collision of the fragment with another nucleus. In this connection it was suggested that the secondary transformation was due to the spontaneous decay of the fragment. This supposition, however, encounters the following difficulty. It is known that, when a nuclear particle—for example, a proton—collides with a nucleus, a large amount of energy may be transferred to the latter, as a result of which the nucleus becomes strongly excited. The situation is regarded as follows: the nucleons enter a state of rapid motion—“thermal excitation” occurs—and, as a result, some of the nucleons “evaporate,” i.e. the nucleus disintegrates. It is commonly assumed that such a process occurs in a time of the order of \(10^{-20}\) sec. In the exper...

in the case under consideration, the flight time, reckoned from the moment of emission of the fragment to the moment of its stopping, was more than \(3\cdot 10^{-12}\) sec. Thus, the lifetime of the fragment turned out to be a million times greater than the lifetime which could have been expected on the basis of our usual ideas about nuclear excitation.

To resolve this difficulty, Danysz and Pniewski proposed that the observed phenomenon is due to the presence of an excited nucleon (i.e., a hyperon) among the nucleons of the fragment. In this case the observed decay is attributed to the decay of the hyperon.

Another possibility was also admitted, namely that in the initial decay, together with the fragment, a meson is emitted, rotating around the fragment in a bound orbit. In such a case the decay of the fragment could be explained by the capture of the meson and the transfer to the nucleons of an energy corresponding to the rest mass of the meson.

Such splittings, caused by the capture of free \(\pi^-\)-mesons by nuclei, are often observed in photographic plates exposed to cosmic rays.

Soon Tidman et al.\(^{9}\) discovered a second similar case, with the charge of the emitted nuclear fragment equal to 2 or 3. The importance of this observation lies in the fact that it practically eliminated the possibility (already improbable) that in the first case there had been an independent splitting occurring by chance at the end of the range of the nuclear fragment.

Several months later Crussard and Morel\(^{10}\) and Freier\(^{11}\) observed two more similar events, arguing against the supposition that the phenomenon under discussion is due to a \(\pi\)-meson situated in a closed orbit. In each of these two cases it was established that at the end of its range a nuclear fragment (having a small charge) decayed, emitting three charged particles, one of which was a \(\pi\)-meson. This phenomenon cannot be explained by the capture of a \(\pi\)-meson by the nucleus, since there is not enough energy for the decay. The difficulty is removed if one assumes that the captured meson had a larger mass, for example was a \(K\)-meson.

Meanwhile, in these two cases as well the amount of kinetic energy released was of the order of \(40\) Mev, which is precisely in agreement with the original supposition concerning the presence of a \(\Lambda^0\)-particle among the nucleons of the emitted fragment. Two still later observations also speak convincingly in favor of this latter explanation. The simplicity of the nuclear systems involved makes it possible to analyze these two cases with very good accuracy.

In the event observed by Hill et al.^12, the emitted particle was probably a nucleus with mass number 4 and charge 2. At the end of its range this fragment decayed into a \(\pi\)-meson and two other charged particles. If the transformation is described by the equation

\[ {}^{4}\mathrm{He}^{*}_{2}\to {}^{1}\mathrm{H}_{1}+{}^{3}\mathrm{He}_{2}+\pi_{-1}, \]

then the sum of the momenta of the secondary charged particles turns out to be equal to zero, and the amount of energy released is \(\sim 33.8\) MeV. A detailed analysis using the exact value of \(m_{\Lambda^0}\) shows that if there was a \(\Lambda^0\)-particle inside the initial fragment, then the binding energy of the latter is \(\sim 4\) MeV. It is of interest to compare this value with the energy required to remove a neutron from an \(\alpha\)-particle, which, as is known, is about 20 MeV.

The event recently described by Bonetti et al.^13 can be subjected to an even more detailed analysis. It was possible to determine the mass of the emitted fragment, whose charge is \(e\); this mass turned out to be \(\sim 5500m_e\), i.e. approximately the mass of a triton. At the end of its range the particle decayed into two charged particles moving in opposite directions, i.e. both tracks were collinear. One of the secondary particles proved to be a \(\pi^{-}\)-meson with a long range; on stopping in the stack of photographic plates the meson was captured by a nucleus and caused the decay of the latter. The other particle produced a short recoil track. If this latter particle is a helium-3 nucleus, which must be assumed in order to conserve mass number and charge, then from the range of the particle we find that its momentum was equal to the momentum of the \(\pi\)-meson.

Thus the transformation can be described by the equation

\[ {}^{3}\mathrm{H}^{*}\to {}^{3}\mathrm{He}_{2}+\pi_{-1}. \]

The total amount of energy released is 41.5 MeV. Corresponding to this, the binding energy of the \(\Lambda^0\)-particle with the two other nucleons of the excited triton must be of the order of 1.5 MeV.

The last two of the cases considered apparently definitively exclude the possibility of explaining the phenomenon under consideration by the capture of a meson moving in an orbit around the nucleus. In the case of light \((\pi)\)-mesons, such an explanation is refuted by the observed emission of \(\pi^{-}\)-mesons from the decaying fragment. There remains the possibility of the participation of a heavy meson. However, in the last two observations the momentum of all the charged particles turns out to be equal to zero, and the total amount of kinetic energy released is about 40 MeV. If one assumes that the observed splittings were caused by capture

EXCITED NUCLEONS

of a heavy negative meson by a nucleus, then the fate of an energy of the order of 300 MeV remains uncertain. In this case one would have to assume the emission of one or several neutral particles having high energy and large momentum. But then it is necessary to regard as accidental the momentum balance described by Bonetti et al., as well as the value of the binding energy of the \(\Lambda^0\)-particle, which is so easily explained under the alternative assumption. Such an accidental coincidence is quite implausible and so unlikely that it may be rejected. Thus, we arrive at the conclusion that the only acceptable explanation is the assumption of the participation of a \(\Lambda^0\)-particle.

Taking into account that at present many laboratories are working on the problem under consideration, one may be confident that new cases permitting detailed analysis will soon be discovered. In time it may become possible to determine also the binding energy of \(\Lambda^0\)-particles in various nuclei.

Chestong and Primakoff \(^{14}\) pointed out that a case of an excited deuteron (not yet observed experimentally) would be of particular interest:

\[ {}^{2}\mathrm{H}_{1}^{*}\to{}^{1}\mathrm{H}_{1}+{}^{1}\mathrm{H}_{1}+\pi_{-1}. \]

However, it is not known whether the bond between the \(\Lambda^0\)-particle and the proton is strong enough for such a system to possess the stability necessary for observation.

It was said above that it is expedient to regard the \(\Lambda^0\)-particle as an excited nucleon. However, the limitations of such a representation are already clear. Let us recall that an excited hydrogen atom consists of a proton and an electron in a state of higher energy than in the normal atom. By analogy one might suppose that an excited nucleon consists of a proton and a bound \(\pi^{-}\)-meson, i.e. that \(\Lambda^0\) is a composite particle. As long as the available data concerned the decay of free \(\Lambda^0\)-particles, such a picture was still acceptable. However, the relative stability of \(\Lambda^0\)-particles that are constituent parts of nuclei argues against this possibility. The point is that, when a \(\pi^{-}\)-meson is captured by a nucleus, the time of interaction of the meson with the nucleons is small in comparison with \(10^{-12}\) sec, and the decay energy is obtained at the expense of rest mass; meanwhile, in the event discovered by Bonetti et al., the nuclear fragment existed without decay for more than \(2\cdot10^{-10}\) sec. If the \(\Lambda^0\)-particle consists of a \(\pi^{-}\)-meson bound to a proton, then it is difficult to understand the reason for the relatively long lifetime of the nuclear fragment. One would expect that the bound \(\pi^{-}\)-meson would cause the same splitting of the nucleus as upon its capture from outside.

The considerations presented indicate that the $\Lambda^0$ particle may be regarded as an excited nucleon, but in a sense different from that which follows from our usual notions. We are entering a new domain, for which fundamentally new concepts must be established; nevertheless, it seems reasonable to regard the transition of a nucleon into an excited state as the result of a change in its internal structure. If this is so, then we are beginning to penetrate into what Maxwell called the “strange layers of the material world,” i.e., we are beginning to penetrate into the world of the nucleon. It appears that this world is inexhaustible.

References

1. Rochester and Butler, Nature 160, 855 (1947).
2. See the discussion in Bagnères Conf. Report (1953).
3. Barker, Proc. Roy. Soc. A, 221, 328 (1954).
4. Thompson, Buskirk, Cohn, Katzmark and Rediker, Bagnères Conf. Report, p. 30 (1953).
5. Fretter, Gregory, Johnston, Lagarrigue, Meyer, Muller, and Peyrou, Bagnères Conf. Report, p. 26 (1953).
6. Fowler, Shutt, Thorndike and Whitmore, Phys. Rev. (in press).
7. Friedlander, Keefe, Menon and Merlin, Phil. Mag. (in press).
8. Danysz and Pniewski, Phil. Mag. 44, 348 (1953).
9. Tidman, Davis, Herz and Tennant, Phil. Mag. 44, 350 (1953).
10. Crussard and Morellet, C. R. Acad. Sci. Paris, 236, 64 (1953).
11. Freier, report by Ney, Bagnères Conf., mimeographed report, p. 253 (1953).
12. Hill, Salant, Widgoff, Osborne, Pevener, Ritson, Crussard and Walker, Bull. Amer. Phys. Soc. 29, 60 (1954).
13. Bonetti, Levi Setti, Panetti, Scarsi and Tomasini, Nuovo Cimento 11, 210 (1954) and private communication.
14. Cheston and Primakoff, Phys. Rev. 92, 1537 (1953).