Observation of a two-neutron cascade from a resonance inO24

Two-neutron sequential decay ofO24

“…2 (c). The best fit for the three-body decay gives an energy of E = 715 ± 110 (stat) ±45 (sys) keV, and Γ < 2 MeV, which agrees with previous measurement [23]. Only an upper limit can be put on the width, since the width is dominated by the experimental resolution.…”

“…Two unbound resonances have been observed in 24 O above the one-neutron separation energy [19,23,24], and there is also evidence for resonances above the two-neutron separation energy [19,23] around 7.5 MeV. The first tentative evidence that one of these resonances decays by sequential emission of two neutrons was deduced from a measurement of two discrete neutron energies in coincidence similar to a γ-ray cascade [23]. In this Rapid Communication we present the first observation of a twoneutron sequential decay, exposed by energy and angular correlations, in 24 O.…”

“…The structure of 24 O, the heaviest bound isotope for which the neutron drip-line is established [17], has been well studied and there is substantial evidence for the appearance of a new magic number N = 16 [18][19][20][21][22]. Two unbound resonances have been observed in 24 O above the one-neutron separation energy [19,23,24], and there is also evidence for resonances above the two-neutron separation energy [19,23] around 7.5 MeV. The first tentative evidence that one of these resonances decays by sequential emission of two neutrons was deduced from a measurement of two discrete neutron energies in coincidence similar to a γ-ray cascade [23].…”

“…In this analysis, we require D 12 > 50 cm, and V 12 > 12 cm/ns which is the beam velocity. This technique has been used in several measurements of two-neutron unbound nuclei to discriminate against 1n scatter [11,13,23,[32][33][34][35][36][37]. To further enhance the 2n signal, an additional threshold of 5 MeVee is applied to every hit recorded in MoNA-LISA.…”

“…The two-neutron decay from 24 O was modeled following the formalism of Volya [45] and was treated as a single state in 24 O which could decay sequentially through either of the two states in 23 O. In this formalism, a distribution for the relative energy of the two neutrons E r = ǫ 1 − ǫ 2 is calculated as a function of the total decay energy E. The first neutron, with kinetic energy ǫ 1 , decays from an initial state with energy E 1 and width Γ 1 , to an intermediate unbound state E 2 , Γ 2 , which proceeds to decay by emitting another neutron with kinetic energy ǫ 2 .…”