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This paper reports four nuclear emulsion observations in which a stopped multiply charged fragment decays into two alpha particles and a proton after irradiation by high energy protons. Track ranges, particle identifications, momentum balance, and estimates of kinetic and excitation energies are used to exclude fragmentation, neutron emission, and alternative scattering explanations, leading the authors to interpret the events as decay of boron 9 through an intermediate beryllium 8 state. The inferred excitation energies are compared with known boron 9 and beryllium 8 levels, and the apparent preference for decay to the first excited beryllium 8 level is discussed as possibly reflecting spin selection effects. The observed flight times and, in one case, a fast electron track are taken to support production of carbon 9 followed by beta decay into boron 9.
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PHYSICS
V. F. DAROVSKIKH, M. M. MAKAROV, and V. I. OSTROUMOV
OBSERVATION OF THE DECAY OF THE NUCLEUS $\mathrm{B}^{9}_{5}$ IN NUCLEAR EMULSION
(Presented by Academician B. P. Konstantinov on 17 VI 1961)
In examining nuclear emulsions irradiated with high-energy protons, we recorded 4 cases in which a multiply charged particle, at the end of its range, decays into 3 charged particles. The decay is observed at the end of the cone-shaped track of the primary particle, which excludes the possibility of explaining the cases under consideration as nuclear fragmentation. Comparison of the tracks of these particles with tracks of particles of known nature showed that the charge of the primary particle is equal to 5 or 6. Two secondary tracks are similar to the tracks of $\alpha$ particles from the decay of $\mathrm{Be}^{8}_{4}$ observed in emulsion; the third track was identified as the track of a singly charged particle. Assuming that the two short secondary tracks belong to $\alpha$ particles and one to a proton, the energies of these particles were determined. The characteristics of the cases are given in Table 1.
Table 1
| Case No. | Length of primary track, $\mu$ | Ranges, $\mu$ — $\alpha$ particles | Ranges, $\mu$ — proton | Energy, MeV — $\alpha$ particles | Energy, MeV — proton | Excitation energy of $\mathrm{B}^{9}_{5}$, MeV | Excitation energy of $\mathrm{Be}^{8}_{4}$, MeV | Type of splitting |
|---|---|---|---|---|---|---|---|---|
| 1 | 35.5 | 3.1 1.6 |
13.0 | 0.9 0.5 |
0.9 | 2.0 | 1.3 | Single track $f$ |
| 2 | 15 | 3.1 3.5 |
6.1 | 0.9 1.1 |
0.6 | 2.3 | 1.9 | Star $2p + f$ |
| 3 | 68 | 8.4 7.1 |
15.1 | 2.3 2.0 |
1.05 | 5.0 | 4.1 | Star $1p + f$ |
| 4 | 102 | 3.3 3.7 |
14.0 | 1.0 1.2 |
1.0 | 2.9 | 2.1 | Star, 16 prongs |
Cases 1–3 were found in P-9 emulsion, insensitive to electrons, which was irradiated with protons of energy 660 MeV; case 4 was found in electron-sensitive PR emulsion exposed to a proton beam of energy 9 BeV. In the latter case, in addition to the tracks of heavy particles, there is observed the track of a fast electron emerging from the center of the secondary star and, possibly, one more track of a relativistic electron separated from the center of the secondary star by a considerable distance.
Table 1 indicates the type of splitting in which an unstable fragment is emitted. A check showed that the total momentum of all three secondary particles (assuming that 2 $\alpha$ particles and one proton are emitted) is in all cases close to zero to an accuracy of 5–10%. Consequently, neutron emission does not occur (at least with an energy greater than 0.2 MeV). It could be assumed that the observed cases are examples of the decay of the nucleus $\mathrm{B}^{8}_{5}$ according to the scheme
$$ \mathrm{B}^{8}_{5} \xrightarrow{\beta} \mathrm{Be}^{8}_{4} \to 2\alpha', $$
with the appearance of a proton at the point of separation of the $\alpha$ particles being the result of an elastic collision of one of the $\alpha$ particles with a hydrogen atom. It could also be admitted that elastic scattering of the fragment $\mathrm{B}^{8}_{5}$ on a hydrogen nucleus of the gelatin had occurred.
with a large transfer of energy to the latter. However, a simple calculation shows that neither of these possibilities is realized in the observed cases. First, this follows from the angular and energy relations of the $\alpha$ particles and the proton; second, from probabilistic considerations (the probability of elastic scattering of an $\alpha$ particle on hydrogen over a length of no more than $1\,\mu$ from the point of decay of $\mathrm{Be}^{8}_{4}$ is of the order of $10^{-6}$, which, given the available statistics of $\mathrm{Be}^{8}_{4}$ decay cases, makes this supposition unrealistic). The second possibility is also excluded by the consideration that the fragment after scattering on a proton would have a sufficient range, and the point of decay of $\mathrm{Be}^{8}_{4}$ and the beginning of the proton track would be separated by a distinctly observable interval of $2$–$3\,\mu$.
It is therefore natural to consider all the indicated cases as examples of the disintegration of the nucleus $\mathrm{B}^{9}_{5}$ according to the scheme
\[ \mathrm{B}^{9}_{5} \to \mathrm{Be}^{8}_{4} + \mathrm{p}. \tag{1} \]
The fact that the energies of the $\alpha$ particles in each pair are rather close to one another suggests that the decay proceeds through the intermediate nucleus $\mathrm{Be}^{8}_{4}$. The values of the excitation energy of $\mathrm{Be}^{8}_{4}$ are given in Table 1. From the known masses of the particles participating in process (1), and from the sum of their kinetic energies, one can estimate the excitation energy of the nucleus $\mathrm{B}^{9}_{5}$ (see Table 1). The values obtained turn out to be close to the known levels of $\mathrm{B}^{9}_{5}$: $2.3$, $2.9$, and $4.9$ MeV (1). The values of the excitation energy of $\mathrm{Be}^{8}_{4}$ apparently correspond to the broad level at $2.9$ MeV (width about $1$ MeV). It is surprising that the decay of $\mathrm{B}^{9}_{5}$ proceeds to the first excited level of the nucleus $\mathrm{Be}^{8}_{4}$ even when the initial energy is sufficient only for transition to the “tail” of the resonance curve of the level, and that among the 3 observed cases with low excitation energy of $\mathrm{B}^{9}_{5}$ ($2.3$ and $2.9$ MeV) there is not a single one in which the ground state of $\mathrm{Be}^{8}_{4}$ would be realized, although in this case the energy release would be considerably larger. If this is not a matter of statistical fluctuation, then it remains to assume that there is a prohibition on the transition of $\mathrm{B}^{9}_{5}$ to the ground state of $\mathrm{Be}^{8}_{4}$ because of the large difference in spins.
Since the spin of the nucleus $\mathrm{Be}^{8}_{4}$ is zero, and that of the first excited level is two, it follows from the above supposition that the first excitation levels of the nucleus $\mathrm{B}^{9}_{5}$ have large spins, so that the transition to the ground state of $\mathrm{Be}^{8}_{4}$ is associated with the emission of a proton with a large orbital angular momentum. From the observed range of the fragments one can determine their time of flight before stopping, which gives a lower limit on the lifetime of the unstable nucleus (of the order of $10^{-12}$ sec). This value is too large for it to be possible to take it as the lifetime of the nucleus $\mathrm{B}^{9}_{5}$. Most likely we are dealing with the appearance, in nuclear disintegrations, of nuclei $\mathrm{C}^{9}_{6}$, which by $\beta$ decay transform into the nucleus $\mathrm{B}^{9}_{5}$. A similar case was found in Ref. (2). We note that the energy release in the decay of $\mathrm{B}^{9}_{5}$ discovered in (2) was considerably larger (more than $12$ MeV). Case 4, containing the track of a fast electron, supports the hypothesis of $\beta$ decay of the nucleus $\mathrm{C}^{9}_{6}$.
Radium Institute named after V. G. Khlopin
Academy of Sciences of the USSR
Received
17 V 1961
CITED LITERATURE
- J. Saji, J. Phys. Soc. Japan, 15, 367 (1960).
- M. S. Swami, J. Schneps, W. F. Fry, Phys. Rev., 103, 1134 (1956).