Resonance state in 7H

Impact of the decay width in Breit-Wigner formula on Maxwellian-averaged cross section for neutron capture on 16O

Neutron capture on 16O may serve as a crucial neutron poison reaction in weak s-process occurred in massive stars. In our previous study [Zhang, S. S., Xu, S. Z., He, M. et al. Eur. Phys. J. A 57, 114 (2021)], we found that the contribution from low-lying resonances to Maxwellian-averaged cross section (MACS) progressively increases as the energy goes beyond 70 keV. In Breit-Wigner formula for resonant cross section, the decay width is a decisive quantity. In this paper, we discuss the impact of three kinds of decay widths, i.e. a constant width and two energy-dependent widths, on reaction cross sections and MACSs. The penetration factor adopts semi-classical WKB approximation and the asymptotic solution, respectively. We clarify that energy-dependent width are necessary for a reasonable behavior of resonance cross section around resonance peak and low-energy region far from the peak. The difference of two energy-dependent widths decreases from 3.7 to 1.5 with the energy increasing from 0.01 keV to 1000 keV. It results in similar behavior of resonance cross sections, but a slight difference by 1% for MACSs when E > 50 keV.

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

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 ∼25  μb/sr within angular range θ_{c.m.}≃(17°-27°).

Structure of superheavy hydrogen 7H

The properties of nuclei with extreme neutron–to–proton ratios reveal the limitations of state-ofthe-art nuclear models and are key to understand nuclear forces. 7H, with six neutrons and a single proton, is the nuclear system with the most unbalanced neutron–to–proton ratio ever known, but its sheer existence and properties are still a challenge for experimental efforts and theoretical models. We report here the first measurement of the basic characteristics and structure of the ground state of 7H; they depict a system with a triton core surrounded by an extended four-neutron halo, built by neutron pairing, that decays through a unique four–neutron emission with a relatively long half-life. These properties are a prime example of new phenomena occurring in almost pure-neutron nuclear matter, beyond the binding limits of the nuclear landscape, that are yet to be described within our current models.

Nuclear structure experiments along the neutron drip line

Invariant mass measurements and breakup reactions of neutron-rich nuclei have been instrumental in the study of nuclear structure effects at the limit of nuclear existence. The measurements of neutron-unbound states rely on the detection of neutrons in coincidence with fragments at energies between 100 and 1000 MeV/u. Charged particle and γ-ray coincidence measurements yield additional information. The production and detection methods for these experiments and examples of results in light nuclei are presented. Future opportunities with new facilities and the development of new detectors are described.