8He Nuclei Stopped in Nuclear Track Emulsion

A nuclear track emulsion (NTE) is exposed to 60 MeV 8He nuclei. Measurements of decays of 8He nuclei stopped in NTE allow one to evaluate possibilities of α-spectrometry. Thermal drift of 8He atoms is observed. Knowledge of the energy and emission angles of α-particles allows one to derive the energy distribution of α-decays Q2α. The presence of a “tail” of large values Q2α is established.

Scheme of a major channel of the cascade decay of 8 He isotope; light circles correspond to protons, dark ones neutrons 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). 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 forward-backward 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.
Arrival directions and stopping points of ions 8 He, their decay vertices and stops of decay α-particles were determined for 136 "whole" and 142 "broken" events. In "broken" events the decay points were determined by extrapolating the electron tracks. The distribution of the opening angles of α-particle pairs 2α has a mean value (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 and their energy values are determined by spline interpolation of calculations in the SRIM model [12]. The mean value of the α-particle ranges is (7.4 ± 0.2) µm at rms (3.8 ± 0.2) µm corresponding to their mean energy (1.70 ± 0.03) MeV at rms 0.8 MeV. Correlation of ranges of α-particles in pairs is clearly manifested. The distribution of the range differences 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 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 of fragments, that is, M * 2 = ( P i ) 2 . In a case of an α-particle pair i is equal just 1 and  2. In general, the distribution of Q 2α (Fig. 3) corresponds to the 8 Be decay from the state 2 + . However, the mean value of 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 both α-particle ranges <12.5 µm and opening angles > 145 • , provides a mean value of Q 2α equal to (2.9 ± 0.1) MeV at rms (0.85 ± 0.07) MeV, which corresponds to the 2 + state.
The measurements are continued to saturate statistics in the high energy "tail" of Q 2α . The insertion in Fig. 3 shows Q 2α with additional 98 α-pairs having both ranges 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 will allow one to verify calculations of spatial structure of 8-nucleon ensembles emerging as α-pairs of the 2 + state decays [13].
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 ( 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). Uniformity of distributions of events over these parameters and absence of a clear correlation indicate on a thermal drift of the atoms 8 He. The mean value of L( 8 He-8 Be) amounting to (5.8 ± 0.3) µm at rms (3.1 ± 0.2) µm, can be associated with a mean range of atoms 8 He. The low value of a mean speed of the atoms 8 He defined as ratio of the mean value of L( 8 He-8 Be) to the half-life of the nucleus 8 He supports a pattern of diffusion.
Observation of the diffusion points to the possibility of generating of radioactive atoms 8 He and pumping them out of sufficiently thin targets. Increasing of the mean speed and drift length is achievable due to heating of the target. There is a prospect of accumulating of a significant amount of 8 He atoms. In particular, a 8 He radioactive gas can be used to measure the half-life of the 8 He nucleus at a new level of precision and for laser spectroscopy of this isotope. Applied interest consists in studies of thin films by pumping atoms 8 He and their deposition on α-detectors. Such opportunities are developing intensively with respect of the 6 He isotope [14,15].
The practical result of this work is the demonstration of the opportunities of recently reproduced NTE in a way of exposure in a beam of 8 He nuclei. Nuclear track emulsion made it possible to identify decays of stopped 8 He nuclei, estimate possibilities of α-range spectrometry and observe the drift of 8 He atoms. The high quality of the beam of radioactive 8 He nuclei at the ACCULINNA fragment separator was confirmed. The presented analysis of the decay of nuclei 8 He can serve as a prototype for studying the decays of stopped nuclei 8,9 Li, 8,12 B, 9 C and 12 N. Statistics of hammer-like decays found in this study is a small part of the flux of 8 He nuclei, and the measured decays-about 10% of this share. This limitation is defined by "reasonable expenses" of human time and labor. However, NTE in which radioactive nuclei are implanted provides a basis for the application of automated microscopy and image recognition software, allowing one to rely on unprecedented statistics. Thus, a synergy of classical technique and modern technology can be achieved.