Evidence for the First Excited State of $^{7}\mathrm{H}$

Evidence for the First Excited State of ${}^{7}H$

A. A. Bezbakh et al.

Phys. Rev. Lett. 124, 022502 – Published 16 January 2020

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 of the $1 / 2^{+}$ ground state of ${}^{7}H$ located at 1.8(5) MeV with quite a low population cross section of $\sim 25 μ b / sr$ within angular range $θ_{c . m .} ≃ (17 ° - 27 °)$ .

Received 25 June 2019
Revised 13 September 2019

DOI:https://doi.org/10.1103/PhysRevLett.124.022502

Physics Subject Headings (PhySH)

Direct reactions Few-body systems Nuclear structure & decays

Nuclear Physics

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Vol. 124, Iss. 2 — 17 January 2020

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Figure 1
The experimental setup. The inset presents kinematical conditions of the ${}^{2}H ({}^{8}{He}, {}^{3}{He}) {}^{7}H$ reaction at 26 AMeV. The solid lines correspond to the kinematical area which is accessible by the used setup.
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Figure 2
(a) Correlation between the ${}^{3}H$ energy in the ${}^{7}H$ c.m. frame and the ${}^{7}H$ MM energy. Red dashed line shows the kinematical limit for tritons coming from the ${}^{7}H$ decay. Grey triangle represents kinematically allowed region used for the reconstruction of the MM spectrum. Panels (b) and (c) show the ${}^{7}H$ MM spectrum shown with two different binning factors. The red dotted curve in (b) shows the fit to the data by the 6.5 MeV resonant state with $Γ = 2 MeV$ plus the contribution of the $t + 4 n$ five-body phase space. The blue dashed curve shows the fit with the addition of the 1.8 MeV ( $Γ < 300 keV$ ) and 12 MeV ( $Γ = 4 MeV$ ) resonances. The both curves were convoluted with the setup efficiency [green dotted curve in panel (c)] and MM resolution. (d) The data from Ref. [9]. Vertical dotted lines indicate the presumed positions of the ${}^{7}H$ ground and first excited states.
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Figure 3
The ${}^{3}{He}$ recoil identification.
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Figure 4
The c.m. angular distributions calculated for the ${}^{2}H ({}^{8}{He}, {}^{3}{He}) {}^{7}H$ reaction channels. The angles of the five ${}^{7}H$ g.s. candidate events are indicated by arrows in (a). The gray curves in (a) show the $1 / 2^{+}$ cross section for $\pm 300 keV$ variation of the ${}^{7}H$ g.s. energy. The gray histogram in (b) shows (in arbitrary units) the experimental angular distribution in the $E_{T} = 5 - 8 MeV$ range (27 events). The green dash-dotted curves, associated with the arbitrary-scale right axes, show the setup efficiency for the registration of ${}^{7}H$ with $E_{T} = 1.8 MeV$ and $E_{T} = 6.5 MeV$ , respectively.
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Figure 5
(a) Energy distribution of tritons (in the ${}^{7}H$ frame) from the ${}^{7}H$ g.s. decay. (b) Distribution of angles between triton and ${}^{7}H$ in the laboratory frame. The colored curves in (a) and (b) are predicted for the different ${}^{7}H$ g.s. decay energies, while the gray curves of the corresponding style show the distributions corrected for the experimental resolution. The energies and angles of the five ${}^{7}H$ g.s. candidate events are indicated by arrows. Panel (c) shows the likelihood functions for the ${}^{7}H$ g.s. energy deduced from Fig. 2 (black solid curve), ${}^{3}H$ energy distribution from panel (a) (blue dashed curve), and ${}^{3}H$ angular distribution from panel (b) (red dotted curve). The intersections of these functions with the horizontal dashed lines at 0.68 and 0.95 define the confidence intervals for $1 σ$ and $2 σ$ levels, respectively.
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Figure 6
Systematics of the lowest excited states for the isotones with closed $p_{3 / 2}$ neutron subshell. There exist two different prescriptions for the low-lying ${}^{8}{He}$ spectrum: (i) $2^{+}$ , $1^{-}$ [20, 21] and (ii) $1^{-}$ , $2^{+}$ [22].
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Physical Review Letters

Evidence for the First Excited State of H7

A. A. Bezbakh et al.

Phys. Rev. Lett. 124, 022502 – Published 16 January 2020

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Evidence for the First Excited State of ${}^{7}H$