Observation of the $^7$H excited state

TL;DR

This study reports the observation of an excited state in the unbound nucleus $^7$H, providing measurements of its energy levels and decay properties through a specific nuclear reaction experiment.

Contribution

First experimental identification and energy measurement of the $^7$H excited state using a $^8$He beam and missing mass spectroscopy.

Findings

$^7$H excited state observed at 6.5(5) MeV

Indications of the $^7$H ground state at 2.0(5) MeV

Low population cross section of about 10 μb/sr

Abstract

The $^7$H system was populated in the $^2$H($^8$He,$^3$He)$^7$H reaction with a 26 AMeV $^8$He beam. The $^{7}$H missing mass energy spectrum, the $^{3}$H energy and angular distributions in the $^7$H decay frame were reconstructed. The $^7$H missing mass spectrum shows a peak which can be interpreted either as unresolved $5/2^+$ and $3/2^+$ doublet or one of these states at 6.5(5) MeV. The data also provide indications on the $1/2^+$ ground state of $^7$H located at 2.0(5) MeV with quite a low population cross section of $\sim 10$ $\mu$b/sr within angular range $\theta_{\text{cm}} \simeq 6^{\circ} - 30^{\circ}$.

Full text

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Introduction. — The 7H nucleus is a special system in the “world of nuclides”. It is the heaviest conceivable hydrogen isotope with the largest $A/Z=7$ ratio, which is closer to “neutron matter” than any other known nuclide. The closed $p_{3/2}$ neutron subshell of its ground state (g.s.) implies special stability relative to its isobaric neighbors. The 7H g.s. decays via unique five-body 3H+$4n$ decay channel. This form of nuclear dynamics has not yet been studied at all, and it was discussed that in 7H this decay mechnism may lead to such an exclusive phenomenon as $4n$ radioactivity Golovkov et al. (2004); Grigorenko et al. (2011). Unfortunately, there is no definite reliable information about such an interesting system.

The search for 7H has a long, but not fortunate history. It was searched but not found among the nuclear-stable products of ternary fission of 252Cf Aleksandrov et al. (1982) and in pion double charge-exchange 7Li($\pi^{-}$,$\pi^{+}$) reaction Gornov et al. (2003). Since the emergence of the radioactive ion beams (RIB), the evident way to search for 7H is proton removal from 8He. The 1H(8He,2He) reaction was used in Ref. Korsheninnikov et al. (2003) and evidence for intense population of 7H spectrum right above the 3H+$4n$ threshold was demonstrated. Low energy resolution ($1.9$ MeV) and high background did not allow to draw a quantitative conclusion in this work. The 2H(8He,3He) reaction at 21 AMeV on a thick cryogenic deuterium target was used in Ref. Golovkov et al. (2004) for the specific task of searching for extreme low-lying (and therefore long-living) 7H g.s. Together with theoretical estimates for the lifetimes in the five-body decays, this allowed to establish the lower decay-energy limit $E_{T}>50-100$ keV for 7H. The decay energy $E_{T}$ and missing mass (MM) mean the same value having zero value at the 3H+$4n$ decay threshold. The observation of a quite low-lying 7H resonance state with $E_{T}\sim 0.57$ MeV produced in the 12C(8He,7H)13N reaction was declared in Ref. Caamaño et al. (2007). An important deficiency of this work was the difficulty of the reaction-channel identification. The observed events could belong also to 6H or to 5H continuum. The next attempt to obtain 7H was made using the 2H(8He,3He) reaction carried out at the 8He projectile energy 42 AMeV Nikolskii et al. (2010). Quite a smooth excitation spectrum was obtained in this work and authors pointed out a peculiarity at $E_{T}\sim$ 2 MeV at a cross-section level of $\sim 30$ $\mu$b/sr.

Though the 7H production from 8He seems to be a straightforward idea, it had not provided a decisive result within the last 15 years of research. In the present work we for the first time obtain a reliable quantitative results for the 7H energy spectrum coming closer to the solution of the 7H g.s. problem.

Experiment. — It was performed at the Flerov Laboratory of Nuclear Reactions (JINR) at the ACCULINNA-2 fragment separator Fomichev et al. (2018). This facility was commissioned in 2017, and this run was the first one performed with the full intensity primary beam. The 33.4 AMeV 11B beam was delivered by the U-400M cyclotron with the intensity of about 1 p$\mu$A. It was focused in the 5-mm spot on the 1 mm thick beryllium production target. The secondary 8He beam with energy of $26$ AMeV and $\sim 90\%$ purity, having intensity of $\sim 10^{5}$ pps, was focused into a 17-mm spot on the deuterium gas target. The $D_{2}$ target was cooled to 27 K, and its thickness made $\sim 3.8\times 10^{20}$ cm*-2*. Beam tracking was provided by two multi-wire proportional chambers located by 27 and 82 cm upstream the $D_{2}$ target and giving the individual 8He hit positions on the target with 1-mm accuracy. The time-of-flight detector system, which identified each particle in the secondary beam and measured its energy, consisted of two thin plastic scintillators with 12.3 m flight path having 0.2 ns time resolution.

The experimental setup is shown in Fig. 1. Choosing the same ($d$,3He) reaction as in Nikolskii et al. (2010), we, however, had to optimize the setup in a different way. Energy resolution for the 7H missing mass measurement, estimated by Monte-Carlo method, at a level of $\sim 1.1$ MeV which is two times better than in Nikolskii et al. (2010). A set of the two identical $\Delta E$-$E$-$E$ telescopes was the key installation of the experiment destined to detect the low-energy 3He recoil nuclei emitted in the 2H(8He,3He)7H reaction in the range of $9-20$ MeV. Each telescope consisted of three Si strip detectors — one 20-micron SSD ($50\times 50$ mm, 16 strips) and two 1000-micron SSDs ($61\times 61$ mm2, 16 strips), where the second 1000-micron detector operated as veto. The telescopes were located $166$ mm downstream from the $D_{2}$ target covering an angular range of $\sim 8^{\circ}-26^{\circ}$ in laboratory system. Finally, tritons originating from the 7H decay and moving in a narrow cone of forward angles, $\theta_{t}\leq 6^{\circ}$, were detected by the $61\times 61$ mm2 telescope which was installed at zero laboratory angle $280$ mm downstream from the target. It consisted of one 1500-micron thick Si DSD ($32\times 32$ strips) and a set of 16 square CsI(Tl)/PMT modules (the CsI(Tl) crystals were 50 mm thick). The 3H telescope provided angular resolution of $\sim 0.5^{\circ}$ and energy resolution of $\sim 2\%$.

Missing mass spectrum. — All together 107 events were detected in the experiment. Fig. 2 (a) shows correlation plot between the 7H MM and 3H energy in the 7H center-of-mass (c.m.) frame. It can be seen that the majority of data is in agreement with the hypothesis of 7H population and its subsequent decay. The events outside the kinematicaly allowed region are very few and evenly distributed. The MM spectrum of 7H is shown in Figs. 2 (b), (c) in different representations. In this spectrum the peak with energy $E_{T}=6.5(5)$ MeV, width $\Gamma=2.0(5)$ MeV, and population cross section of $\sim 30$ $\mu$b/sr can be well identified. This peak is interpreted as the first excited state of 7H, though the $5/2^{+}$ and $3/2^{+}$ doublet of the lowest excited states cannot be excluded. There is also a compact group of events at $E_{T}\sim 2$ MeV emerging at 7H c.m. angles $17^{\circ}-27^{\circ}$. This group has population c.m. cross section of $10$ $\mu$b/sr, and we associate it with the 7H ground state. Such an interpretation is at the limit of statistical significance and deserves special discussion. Figs. 2 (b), (c) show that the MM spectrum at $E_{T}>12$ MeV can be explained by the combination of rapidly growing 5-body phase volume and rapidly falling detection efficiency.

Discussion of the 7H ground state evidence. — We consider the group of events with $0.5<E_{T}<2.5$ MeV as candidate for the 7H ground state. Because of small statistics (5 events) this group can be regarded only as an indication of the possible ground state. To increase confidence in this interpretation, let us consider all the candidate events in details.

Fig. 3 demonstrates the good quality of the 3He recoil identification. It is clear that the 3H fragment identification in the zero-angle telescope is much better. Thus the decay channel identification is unambiguous for all the events in Fig. 2 (a). The channel identification was checked especially carefully for the individual g.s. candidate events. It can be also seen in Fig. 2 (a) that all the 7H g.s. candidate events are located within the kinematical locus associated with the 7H decay hypothesis.

The angles of the respective 7H g.s. candidate events are shown by arrows in Fig. 4 (a). Our setup was not suited for the 7H g.s. detection in the forward-angle cross-section maximum of the 2H(8He,3He)7H reaction. It can be seen that all the g.s. candidate events are concentrated in the region predicted to be the second diffraction maximum for the calculated cross section of the $1/2^{+}$ state.

Correlation patterns anticipated for the decay of the core+$4n$ systems were studied in the recent paper Sharov et al. (2019). Among the correlations considered in Sharov et al. (2019) only the energy distribution of 3H in the 7H frame can be reconstructed from experimental data of the present work. These distributions are expected to have profile with quite a narrow low-energy peak. Their shape can be affected by the decay dynamics of 7H as well, see Fig. 5 (a). For comparison with the measured data we calculated also the angular distribution of 3H relative to the reconstructed 7H flight direction in laboratory frame, see Fig. 5 (b). In contrast with the 3H energy in 7H frame, the mentioned angle is defined with much higher precision. Namely, the 3H direction is defined with precision of $0.5^{\circ}$ by the forward telescope, and the 7H direction is deduced with precision of $0.5^{\circ}$ based on the momentum vectors of the incoming 8He beam and the 3He recoil. It can be seen in Fig. 5 (b) that all the candidate g.s. events nicely fit in the theoretically predicted angular distribution peak.

The 7H g.s. position at $E_{T}=2.0(5)$ MeV, suggested here, is consistent with the observation of near-threshold anomaly in Ref. Korsheninnikov et al. (2003). Our spectrum of 7H for $E_{T}<8$ MeV is consistent with the spectrum of Ref. Nikolskii et al. (2010), see Fig. 2 (d). The latter was obtained in the same reaction at different energy and with worse energy resolution. The g.s. energy inferred in our work strongly differs from the value $E_{T}=0.57^{+0.42}_{-0.21}$ MeV reported in Caamaño et al. (2007), far beyond the declared experimental errors. Another subject of concern is the the large cross section reported in Caamaño et al. (2007) for the 12C(8He,7H)13N reaction populating the 7H g.s., while this reaction is less preferable than the ($d$,3He) reaction, e.g. due to the $Q$ value (see also discussion of this issue in Nikolskii et al. (2010)).

Discussion of the 7H excited state. — What can be the nature of the 6.5 MeV state in 7H? It should be noted that 7H has closed $p_{3/2}$ subshell. Systems with shell closure typically have quite poor low-lying excitation spectrum, and the easiest expectation is that the lowest is the $2^{+}$ excitation formed by pushing neutrons to the $[p^{2}_{3/2}p^{2}_{1/2}]_{2}$ configuration. The $2^{+}$ excitation of valence neutrons should be coupled with core spin to the $5/2^{+}-3/2^{+}$ doublet. The separation of the doublet members is questionable, and here we can refer only to the experience of the 5H excited states’ studies in Ref. Golovkov et al. (2005) where this separation was found to be insignificant.

The systematics of the lowest excited states for light systems with closed $p_{3/2}$ is given in Fig. 6. It can be seen that excited states which can be related to the excitations of the neutron configurations have typical energies $E^{*}\sim 3.5-4.5$ MeV. In this plot the 7H excitation energy is determined assuming that the group of events at $E_{T}=2.0$ MeV represents the g.s. position, which gives excited state position $E^{*}\sim 4.5$ MeV, fitting well the systematics. If we admit lower $E_{T}$ values for the g.s., for example $E_{T}<1$ MeV, we get unexpectedly high energies for the 7H excited state, $E^{*}>5.5$ MeV. This can be considered as additional argument supporting our prescription of the 7H g.s.

The 7H c.m. angular distribution for the 6.5 MeV excitation region is shown in Fig. 4 (b). The experimental angular distribution corresponds well to the $5/2^{+}$ and $3/2^{+}$ distributions calculated by FRESCO code FRE with the setup efficiency taken into account.

Conclusion. — The following major results are obtained in this work:

(i) For the first time, the 7H excited state is observed at $E_{T}=6.5(5)$ MeV with $\Gamma=2.0(5)$ MeV. This state can be interpreted as unresolved $5/2^{+}$ and $3/2^{+}$ doublet, built upon the $2^{+}$ excitation of valence neutrons, or one of the doublet states.

(ii) Indications for the 7H g.s. at $E_{T}=2.0(5)$ MeV are found in the measured energy and angular distributions.

(iii) The measured c.m. population cross section of the presumed 7H g.s. is about 10 $\mu$b/sr, which clarifies why the previous searches for the 7H g.s. required so much time and effort without bringing reliable assignments of such a remote isotope.

The obtained results represent an important step towards resolving the problem of the 7H observation and also demonstrate the high potential of the “newcomer” ACCULINNA-2 facility.

Acknowledgments. — We acknowledge important contribution of Prof. M.S. Golovkov to the development of the experimental setup. This work was supported in part by the Russian Science Foundation grant No. 17-12-01367 and MEYS Project (Czech Republic) LTT17003.