DELAYED DECAY OF HEAVY FRAGMENTS FROM NUCLEAR FISSION

The first case of delayed decay of a heavy fragment was observed by Daniels and Perkins¹ in G-5 photographic plates irradiated in the stratosphere (Fig. 1). From a “star” $A$ of type $21 + 18p$, a fragment $f$ with charge $\sim 5e$, energy $\sim 150$ MeV, and range $90\mu$ is emitted. On stopping, or almost stopping, the fragment produces a “star” $B$ with total energy $\geq 140$ MeV. Similar phenomena were observed by other authors²–⁹. Data concerning these cases are given in Table I.

*) Different energies correspond to different decay schemes.
**) In all cases not specially stipulated, the energy is indicated.

Table 1

under the assumption that the particle is a proton.

On the basis of an analysis of the data presented, the following conclusions are drawn:

1. The appearance of the “star” \(B\) at the end of track \(f\) cannot be explained by an accidental coincidence (in \(1000\ \mathrm{cm}^3\) of emulsion one such event may be observed with probability \(10^{-4}\), two events with probability \(10^{-8}\), etc.).

2. The formation of the “star” \(B\) from the collision of fragment \(f\) with a nucleus of the emulsion is impossible, since in the majority of cases the fragment stops

Fig. 1.

or almost stops, and its kinetic energy is much less than the energy of the “star” \(B\).

1. In all cases the flight time of the fragment was greater than \(10^{-12}\) sec. and, consequently, too large in comparison with the lifetime of the excited nucleus having an energy on the order of the energy of the “star” \(B\) (for an excitation energy of \(100\ \mathrm{MeV}\) the lifetime is on the order of \(10^{-20}\) sec. \({}^{10}\)).

2. On leaving nucleus \(A\), a heavy fragment may capture onto one of the quantum orbits a \(\pi^-\)-meson, which is then absorbed by the same fragment

at point \(B\). In this case the energy of the “star” \(B\) cannot differ greatly from \(140\ \mathrm{MeV}\), which contradicts the experimental results.

1. The most probable is the hypothesis put forward in [1], according to which fragment \(f\) contains one excited nucleon (a \(V_1^0\)-particle), decaying at point \(B\) with the release of \(175\ \mathrm{MeV}\) of energy (including the rest mass of the \(\pi\)-meson). The \(V_1^0\)-particle in the nucleus can decay according to two schemes: meson-

Fig. 2.

ic,

\[ V_1^0 \to p + \pi^- \]

and

\[ V_1^0 \to n + \pi^0 \; (?) \]

and non-mesonic, according to which the particle, interacting with a nucleon of the nucleus, is transformed into a nucleon:

\[ V_1^0 + p \;(\text{or } n) \longrightarrow n + p \;(\text{or } n) + 175\ \mathrm{MeV}. \]

In this case, besides the slow particles, there can be in the “star” \(B\) no more than one fast \((E_{\mathrm{kin}}\sim 80\ \mathrm{MeV}\), with a large scatter) proton.

A characteristic example of the decay of bound \(V_1^0\)-particles according to the mesonic scheme is event 9 (Figs. 2 and 3). From a “star” of type \(22+3n\) a particle was emitted which, after traversing \(\sim 12\ \mathrm{mm}\) in the emulsion, stops and decays

Fig. 3.

into two particles flying off in opposite directions: a \(\mathrm{He}_2^3\) nucleus and a \(\pi^-\)-meson, which stopped after \(23.8\ \mathrm{mm}\) and formed a 5-prong star. The authors interpret this event as a decay according to the scheme:

\[ \mathrm{H}^3{}^* \longrightarrow \mathrm{He}_2^3 + Q, \]

where

\[ Q = 41.7 \pm 1 \ \mathrm{MeV} \]

(\(\mathrm{H}_1^{3*}\) is a nucleus containing a \(V_1^0\)-particle).

An example of nonmesonic decay is case 11 (Fig. 1).

On the basis of these schemes, in a number of cases an estimate has been made of the binding energy of the \(V_1^0\)-particle in the corresponding nuclei.

In case 9 a value of \(1\ \mathrm{MeV}\) was obtained, as compared with a binding energy of \(6.24\ \mathrm{MeV}\) for a neutron; in case 8, \(4\ \mathrm{MeV}\) instead of \(20\ \mathrm{MeV}\) for a neutron\(^8\) \((\mathrm{He}_2^5 \to p + p + p + \pi^- + Q)\); in case 11, \(1 \pm 5\ \mathrm{MeV}\), as compared with \(1.7\ \mathrm{MeV}\) for a neutron, and \(4 \pm 5\ \mathrm{MeV}\) instead of \(19\ \mathrm{MeV}\) for the different decay schemes, respectively\(^9\):

\[ \mathrm{Be}^{9*} \longrightarrow \mathrm{He}^4 + \mathrm{He}^4 + n \]

and

\[ \mathrm{Be}^{8*} \longrightarrow \mathrm{He}^3 + \mathrm{He}^4 + n . \]

G. T.

Cited Literature

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