Search for novel decay modes using $^{11}$Be

In 2014, the transmutation of $^{11}$Be to $^{10}$Be was observed using accelerator mass spectrometry and attributed to a previously unobserved mode of radioactive decay: $\beta^{-}$-delayed proton ($\beta^{-}p$) emission. The branching ratio of this decay channel was deduced to be nearly two orders of magnitude larger than expectations based on theoretical models. Confirmation of this discrepancy, through detection of emitted protons, would yield valuable input for these models and potentially relate the $\beta^{-}p$ rate to the neutron halo in $^{11}$Be.

A more exotic decay-process was proposed in early 2018. The neutron lifetime anomaly might be explained by a small decay branch to a ``dark neutron’’ that cannot be observed directly. If this were true, then a neutron could spontaneously disappear from a weakly neutron-bound nuclide such as $^{11}$Be, converting it to $^{10}$Be, without proton emission.

We have performed a measurement of the charged particle radiations from $^{11}$Be at the National Superconducting Cyclotron Laboratory. The measurement employed the recently-commissioned GADGET system, a gas-filled detector based on a MICROMEGAS structure for charge amplification surrounded by the SeGA array of high purity Germanium detectors for gamma-ray detection.