First observation of ${}^{15}$Be

The neutron-unbound nucleus ${}^{15}$Be was observed for the first time. It was populated using neutron transfer from a deuterated polyethylene target with a 59 MeV/u ${}^{14}$Be beam. Neutrons were measured in coincidence with outgoing ${}^{14}$Be particles and the reconstructed decay energy spectrum exhibits a resonance at 1.8(1) MeV. This corresponds to ${}^{15}$Be being unbound by 0.45 MeV more then ${}^{16}$Be thus significantly hindering the sequential two-neutron decay of ${}^{16}$Be to ${}^{14}$Be through this state.

Figure 1

Element, isotope, and ${}^{14}$Be reaction product identification: (a) Energy loss (DE) versus time of flight of the fragments. The different elements are cleanly separated. (b) Isotope identification parameter xtx versus time of flight. The identified beryllium isotopes, along with ${}^{14}$Be reaction products are indicated in the figure.

Figure 2

Neutron time of flight. The black data points corresponds to events in coincidence with neutrons and the ${}^{14}$Be reaction products. The red data points also require an energy of at least 3 MeV deposited in the MoNA bars.

Figure 3

${}^{15}$Be decay energy spectrum. The data are shown by the circles with statistical error bars. The best fit to the data (solid black line) is a sum of an $l=2$ resonance (green short-dashed line) and background contributions approximated by $l=0$ (red long-dashed line) and $l>0$ (blue dotted line) components. In the inset the decay energy for the deuterated polyethylene target (black solid points) is compared to the carbon target (red open squares). The carbon data is normalized to the same counts as the deuterated polyethylene data.

Figure 4

Partial experimental level scheme for neutron rich beryllium isotopes. The height of the grey boxes represents the uncertainties of the states. The data for the excited state in ${}^{14}$Be is from [29], the lower limit for the 3/2${}^{+}$ is from Ref. [9], the ${}^{15}$Be 5/2${}^{+}$ state is from the present work and the two-neutron separation energy for ${}^{16}$Be is from Ref. [4].