FROM CURRENT LITERATURE

MULTIPLE NUCLEAR DISINTEGRATIONS (“STARS”) PRODUCED BY 100 MeV X-RAYS

As is well known, the study of nuclear disintegrations produced by quanta of high energy was until recently greatly hampered by the lack of suitable sources giving sufficient energy. Nevertheless, in the tables of the isotopes, according to Seaborg,^2 about 35 \((\gamma, n)\) reactions have been registered. Determination of the minimum energy required to liberate a neutron in these reactions gave values from 1.6 MeV for beryllium to 19 MeV for carbon. The energy required to liberate a proton is somewhat greater, since in order for a proton to escape from the nucleus it must overcome the Coulomb potential barrier, which is absent in the case of the neutron.

The recently constructed 100 MeV betatron has greatly expanded experimental possibilities. X-ray photons obtained at such excitation energies can liberate both neutrons and protons with practically equal probability. The residual nucleus, moreover, will be “heated” to such a high temperature that the possibility opens up of the “evaporation” of several more particles. It may be expected that at excitation energies of 100 MeV reactions will be observed with the liberation of 6 to 8 particles. Such multiple disintegrations have hitherto been observed only in cosmic rays.

For the study of multiple disintegrations one may use photography of the tracks of the liberated particles by means of a Wilson chamber, or the thick-layer emulsion method. In both cases it is possible to observe the direction of emission of the particles and to determine their relative energies. However, this method has the drawback that with its aid it is impossible to detect the emission of neutrons, so that the picture of the disintegration is incomplete. Moreover, since a mixture of elements is always present in the Wilson chamber, the nature of the nucleus undergoing disintegration can rarely be established with certainty.

Another method, in some respects more convenient, consists in observing the artificial radioactivity arising upon irradiation of the sample with X-rays.

G. Baldwin and G. Stanley Klaiber^1 used a 100-million-volt betatron constructed by Charlton and Westendorp to observe photodisintegration of nuclei by both of the above methods. A horizontal Wilson chamber 30 cm in diameter was built, which gave an expansion once per minute, synchronously with the operation of the betatron. Owing to this, the observed tracks had great sharpness. The chamber was filled with air and a mixture of water and alcohol vapors. It was placed at a distance of 25 feet from the source of X-rays; the beam intensity, after being intentionally reduced, was \(10^5\) times smaller than the maximum possible. In all, 1500 Wilson-chamber photographs were taken. In 105 of them, tracks of single protons with energies up to 9 MeV were obtained; on 7 photographs there were

Multiple Nuclear Disintegrations

tracks were obtained resembling the picture of disintegrations under the action of fast neutrons: one of these particles was a proton or an α-particle, and the other a heavy recoil nucleus. Finally, in three photographs there were found “stars” of four particles; moreover, in two of them these particles were identified in the following way: one α-particle, two protons, and the residual recoil nucleus. In addition, in each of the star-like disintegrations at least one more neutron was to have been liberated, since otherwise the law of conservation of momentum would have been violated. One of the “stars” obtained by the authors is shown in the figure.

The magnetic field in which the chamber was located in this case was 1350 oersteds. The tracks were identified as follows: 1—an α-particle with a range of 3 cm and an energy of 4.5 MeV; 2 and 3—protons; 4—probably a recoil nucleus. In addition, as has already been said, at least one neutron must have been liberated. If the nucleus undergoing disintegration was a nitrogen nucleus (which is most probable), then the residual recoil nucleus must belong to Li7 or Li6. A “star” of three particles was also obtained; one of these particles the authors identify with a meson (the photograph taken by the authors is, unfortunately, irreproducible). This identification, apparently, is not confirmed.

Observations by the method of artificial radioactivity, carried out with 13 elements (Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, Pb), made it possible in most cases to establish the types of reactions and the isotopes formed. Thus, for example, in the case after irradiation (ordinary irradiation was performed with x-rays of 100 MeV for one hour) carbon showed strong activity with a period of 20.5 min. This activity belongs to C11 (see Seaborg’s tables), and the reaction is therefore as follows: C12 (γ, n) C11. In the case of oxygen, 2 periods were established: 2.1 min. (O15) and 20 ± 2 min. (the most probable carrier—C11). Thus, in this case the reaction O16 (γ, n) O15 is possible for the two-minute activity, and the reactions O16 (γ, αn) C11 or O16 (γ, 2p 3n) for the 20-minute activity. In the cases of aluminum and phosphorus, along with activities that could easily be interpreted (for example, in the case of Al, periods of 63 sec. and 14.8 hours could be interpreted as consequences of the reactions Al27 (γ, 2p) Na25 and Al27 (γ, 2pn) Na24), there were also activities whose interpretation proved difficult. Namely, in the case of Al the 10-minute activity could belong to Mg27, but in that case the reaction 13Al27 (γ, ?) 12Mg27 would mean the possibility of a process of charge loss without loss of mass. Similarly, in the case of phosphorus the activity of 2.6 hours could be identified with Si31, but this also meant the conversion of 15P31 → 14Si31. The authors consider the possibility of side reactions such as Al27 (n, p) Mg27 in the case of aluminum to be excluded on the basis of their control experiments. In the summary of their work the authors believe that these results should be regarded

as an indication of the possibility of reactions of the indicated type, i.e., transformations with loss of positive charge without loss of mass.

Experiments with lead were carried out in order to establish the possibility of a photofission reaction in this case. However, the result obtained was negative. All the results obtained by the authors are summarized in the following table:

References Cited

1. George C. Baldwin and G. Stanley Klaiber, Phys. Rev. 70, 259 (September 1 and 15, 1946).
2. Glenn T. Seaborg, Rev. Modern. Physics. Russian translation, UFN 28, issue 2—3, 285 (1946).

E. Shpolskii