$^8$He nuclei stopped in nuclear track emulsion

The fragment separator ACCULINNA in the G. N. Flerov Laboratory of Nuclear Reactions of JINR was used to expose a nuclear track emulsion to a beam of radioactive $^{8}$He nuclei of energy of 60 MeV and enrichment of about 80%. Measurements of decays of $^{8}$He nuclei stopped in the emulsion allow one to evaluate possibilities of $\alpha$-spectrometry and to observe a thermal drift of $^{8}$He atoms in matter. Knowledge of the energy and emission angles of $\alpha$-particles allows one to derive the energy distribution of $\alpha$-decays Q$_{2\alpha}$. The presence of a"tail"of large values Q$_{2\alpha}$ is established. The physical reason for the appearance of this"tail"in the distribution Q$_{2\alpha}$ is not clear. Its shape could allow one to verify calculations of spatial structure of nucleon ensembles emerging as $\alpha$-pairs of decays via the state $^8$Be$_{2+}$.


I.
At the energy of a few MeV per nucleon, there is a possibility to study decays of radioactive nuclei by implanting them into a detector [1][2][3][4]. In particular, population of 2α-and 3α-particle states is possible in decays of light radioactive nuclei. In this respect, the unique, although somewhat forgotten, possibilities of nuclear track emulsion (NTE) for the detection of slow radioactive nuclei are worthy to be mentioned. The advantages of this method are the best spatial resolution (about 0.5 µm), the possibility of observing the tracks in a full solid angle and a record sensitivity starting with relativistic singly charged particles with a minimum ionization. In NTE, the directions and ranges of the beam nuclei, as well as slow products of their decays can be measured, which provides a basis for spectrometry. More than half a century ago, hammer-like tracks from the decay of 8 Be nuclei through the first excited state 2 + of about 2.0 MeV were observed in NTE. They occurred in the β-decays of stopped 8 Li and 8 Be fragments, which in turn were produced by high-energy particles [5].
Another example is the first observation of the 9 C nucleus from the decay 2α + p [6]. When used with sufficiently pure secondary beams, NTE appears to be an effective means for a systematic study of the decay of light nuclei with an excess of both neutrons and protons.
In March 2012 exposure of NTE to nuclei 8 He of energy of 60 MeV [7] is performed at the fragment separator ACCULINNA [8] in the G. N. Flerov Laboratory of Nuclear Reactions, JINR. Features of decays of the 8 He isotope are shown in Fig. 1, according to the compilation [9]. Fig. 2 shows a mosaic macrophotograph of a decay of a nucleus 8 He stopped in NTE.
It is typical one among thousands observed in this study. Video recordings of such decays taken with the microscope and camera are collected [10].
When scanning the NTE pellicle with a 20× objectives on the microscopes MBI-9 a primary search for β-decays of 8 He nuclei was focused on hammer-like events (Fig. 2). The absence of tracks of the decay electrons in the event was interpreted as a consequence of an incomplete efficiency of observation. Often, in the events named "broken"ones gaps were observed between stopping points of primary tracks and subsequent hammer-like decays. In total 1413 "whole" and 1123 "broken" events were found. Decay vertices of 580 "broken" events were found to be laying in a backward hemisphere with the respect to arrival directons of ions. Corresponding to a half of the statistics this number indicate that the forwardbackward asymmetry is absent. The "broken" events were attributed to a drift of thermalized 8 He atoms that arose as a result of neutralization of 8 He nuclei. This effect is determined by the nature of 8 He and such events are identified them particularly reliably.
The coordinates of stopping pointsof the ions 8 He (as well as arrival directions), the decay vertices and stops of decay particles were determined for "hammers"of 136 "whole"and 142 "broken"events. In "broken"events the decay points were determined by extrapolating the electron tracks. The emission angles and the ranges of α-particles were obtained on this basis. The distribution of the opening angles of α-particle pairs has a mean value < Θ 2α > = (164.9 ± 0.7) • at rms = (11.6± 0.5) • . Some kink of "hammers"is defined by the momenta carried away by eν-pairs. The dependence of the α-particle ranges L α and their energy values are determined by spline interpolation of calculations in the SRIM model [11]. The mean value of the α-particle ranges is (7.4 ± 0.2) µm at rms (3.8 ± 0.2) µm corresponding to a mean energy <E( 4 He)> = (1.70 ± 0.03) MeV at rms 0.8 MeV. Correlation of ranges L 1 and L 2 of α-particles in pairs is clearly manifested. The distribution of the range differences L 1 -L 2 has rms 2.0 µm.
Knowledge of the energy and emission angles of α-particles allows one to derive the energy distribution of α-decays Q 2α . The relativistic-invariant variable Q is defined as the difference between the invariant mass of a final system M ⋆ and the mass of a primary nucleus M, that is, Q = M ⋆ -M, M ⋆ is defined as the sum of all products of the 4-momenta P i,k of fragments, that is, M * 2 = (P i ·P k ). In general, the distribution of Q 2α (Fig. 3) corresponds to the 8 Be decay from the first excited state 2 + . However, the mean value <Q 2α > is slightly higher than expected. This fact is determined by the presence of a "tail" of large values Q 2α , obviously not matched the description by a Gaussian function. Application of the selection criteria for ranges L 1 and L 2 less than 12.5 µm and opening angles Θ > 145 • , provides a value <Q 2α > = (2.9 ± 0.1) MeV at RMS (0.85 ± 0.07) MeV, which corresponds to 2 + state. Ranges L 1 and L 2 stay to be well correlated above 12. 5 µm. Therefore, enhanced ranges L 1 and L 2 can not be attributed to fluctuations of ranges or recombination of ions He +2 .
The targeted measurements are continued to saturate statistics in the high energy "tail "Q 2α and to establish its shape. The insertion in Fig. 3 shows Q 2α with additional 98 α-pairs having L 1 and L 2 above 12.5 µm. It should be noted that the highly energetic α-pairs are among better measurable ones despite to relatively rare appearance. The physical reason for the appearance of the "tail" in the distribution Q 2α is not clear. Probably, its shape 4 will allow one to verify calculations of spatial structure of 8-nucleon ensembles emerging as α-pairs of decays via the state 8 Be 2+ [12].
In the 142 "broken"events the distances L( 8 He-8 Be) between the stopping points of the 8 He ions and the decay vertices as well as the angles Θ( 8 He-8 Be) between directions of arrivals of the ions and directions from the stopping points of the ions towards the decay vertices are defined (Fig. 4)