First Observation of Ground State Dineutron Decay:Be16

Abstract

We report on the first observation of dineutron emission in the decay of 16Be. A single-proton knockout reaction from a 53 MeV/u 17B beam was used to populate the ground state of 16Be. 16Be is bound with respect to the emission of one neutron and unbound to two-neutron emission. The dineutron character of the decay is evidenced by a small emission angle between the two neutrons. The two-neutron separation energy of 16Be was measured to be 1.35(10) MeV, in good agreement with shell model calculations, using standard interactions for this mass region.

A Method for Unambiguous Detection of a Hypothetical Bound Two-Neutron System

“…Therein the emission of two correlated neutrons with low relative momentum from the excited 10 Be nucleus was observed and satisfactorily described in an effective three-body model: 8 Be gs + n + n. The only differences with respect to Ref. [1] are the way of production (n+ 9 Be → n +n+ 8 Be gs ) and somewhat higher excitation energy (E ex = 16 MeV).…”

“…A recent announcement of a discovery of bound di-neutrons existing in neutron halo nuclei [1] revived interest to these hypothetical, electrically neutral light nuclei. Certain level of excitement continues despite it became clear that the observed effect is, probably, due to emission of a correlated pair of neutrons originating from the virtual singlet spin state: the negative value of the scattering length 1 a nn = −18.7 fm was used to interpret the measured data for the three-body decay.…”

“…Bodek (B) M. Smoluchowski Institute of Physics, Jagiellonian University, Cracow, Poland E-mail: kazimierz.bodek@uj.edu.pl the reported observation [1] of a singlet neutron pair emitted from light nuclei at low excitation is not a new phenomenon. A similar effect has been observed already more than two decades ago [6].…”

The role of 14 MeV neutrons in light element nucleosynthesis

“…In experiment [4] 16 Be ground state was populated via single proton knockout reaction from 17 B. 16 Be is bound with respect to the emission of one neutron and unbound to two-neutron emission.…”

“…Reactions of interest: A step which might be of interest is 1 4 He+n has peak in the total cross section (σ T = 5 b) at approximately deuteron energy of 100 KeV (T=1.16x10 9 o K), while reaction 2 H+ 2 H→ 3 He+n total cross section is peaked at around deuteron energy of 2.0 MeV. During BBN, in the period of transition from quark-gluon plasma to baryonic matter, probability of 2n formation [n-n fsi or n-n bound state (?)]…”