New information on $\beta$-delayed neutron emission from $^{12, 14}$Be

The $\beta$-delayed neutron branches from neutron-rich Be-isotopes have been measured with a setup sensitive down to thermal energies. For $^{14}$Be a new value of P$_{n}$= 101 $\pm$ 4% has been found for the total neutron-emission probability and an upper limit of 2.4% (95% confidence level) is imposed on the combination P$_{2n}$ + 3P$_{3n}$ of multi-neutron emission probabilities. Moreover, an improved value of P$_{n}$ = 0.50 $\pm$ 0.03% has been obtained for $^{12}$Be. A time-correlation method, previously developed for a continuous radioactive beam to determine neutron branching ratios, has been revised and adapted to the pulsed secondary beam at ISOLDE, CERN.