β-Delayed Deuteron Emission fromLi11: Decay of the Halo

Abstract

The deuteron-emission channel in the β-decay of the halo-nucleus 11 Li was measured at the ISAC facility at TRIUMF by implanting post-accelerated 11 Li ions into a segmented silicon detector. The events of interest were identified by correlating the decays of 11 Li with those of the daughter nuclei. This method allowed the energy spectrum of the emitted deuterons to be extracted, free from contributions from other channels, and a precise value for the branching ratio B d = 1.30(13) × 10 −4 to be deduced for Ec.m. > 200 keV. The results provide the first unambiguous experimental evidence that the decay takes place essentially in the halo of 11 Li, and that it proceeds mainly to the 9 Li + d continuum, opening up a new means to study of the halo wave function of 11 Li. The nuclear halo [1] is among the most peculiar features discovered in unstable nuclei. The 11 Li nucleus is the showcase of a two-neutron halo system, with its very extended matter distribution related to the small energy necessary to remove the neutrons, S 2n = 378(5) keV [2]. Considerable effort has been expended to determine the characteristics of this unstable, short-lived nucleus (T 1/2 = 8.5(2) ms [3]). Among the available probes, the β-decay has the advantage of being described by a wellestablished theory, and thus provides a valuable tool for the investigation of the properties of the ground state of the parent nucleus.In the case of 11 Li, one decay channel is of special interest: the β-delayed deuteron emission 11 Li −→ β 9 Li + d. This mode is related to the possibility that in halo nuclei the core and the halo particles could decay, more or less independently, into different channels [4]. Evidence of the β-decay of the 9 Li core in 11 Li with survival of the two-neutron halo was reported in Ref. [5]. On the other hand, according to calculations [6,7,8] the deuteronemission channel should be dominated by the decay of a neutron in the halo: a "halo decay". This may be measured via the 11 Li −→ β 9 Li + d decay probability, or the branching ratio B d . In addition, the β-delayed deuteron decay of 11 Li could proceed directly to the continuum, without forming an intermediate state (resonance) in the daughter nucleus 11 Be. The information on the initial 11 Li wave function would then be more easily accessible. Both B d and the energy distribution of the emitted deuterons are related to this.We report here the first measurement of the relevant quantities, B d and the energy of the emitted 9 Li and

Unique decay process:β-delayed emission of a proton and a neutron by theLi11halo nucleus

“…There is indeed the possibility that the decay occurs in the halo, releasing the halo nucleons. This process has been observed in the β-delayed deuteron decay of 6 He and 11 Li [1][2][3][4][5][6]. It is, however, severely limited by the energy conservation condition S 2n < B( 2 H) + (m n − m p − m e ) c 2 ≈ 3.007 MeV, (1) where S 2n is the two-neutron separation energy of the halo nucleus, B( 2 H) is the binding energy of the deuteron, and m n , m p , and m e are the neutron, proton, and electron masses, respectively.…”

“…The most probable total energies of the proton and neutron should lie between 0.15 and 0.3 MeV. In any case, the branching ratio is much smaller than for the deuteron and triton channels, i.e., (1.3 ± 0.13) × 10 −4 [6] and (0.93 ± 0.08) × 10 −4 [17], respectively. It is even much smaller than for the hindered deuteron decay of 6 He, (2.6 ± 1.3) × 10 −6 [3].…”

“…It is, however, severely limited by the energy conservation condition S 2n < B( 2 H) + (m n − m p − m e ) c 2 ≈ 3.007 MeV, (1) where S 2n is the two-neutron separation energy of the halo nucleus, B( 2 H) is the binding energy of the deuteron, and m n , m p , and m e are the neutron, proton, and electron masses, respectively. Only a few nuclei have low enough separation energies to allow this decay: 6 He, 8 He, 11 Li, 14 Be, 17 B, 27 F, ....…”

“…The 9 Li + n + p final state is in the threebody continuum of 11 Be. The calculation of wave functions in this continuum is much more complicated than in the threebody continuum of 6 He [11,12]. The construction of threebody scattering states for 9 Li + n + n would already be more difficult than for α + n + n because of the poor knowledge of the 9 Li + n interaction.…”

Clusters in the Beta-Decay of Light Exotic Nuclei

“…19 The measurement of the β-decay of 11 Li (T 1/2 = 8.5 ms) with the implantation technique took place in TRIUMF, Vancouver. 20 The identification of the deuteronemission channel was achieved BY correlating a 11 Li decay with a subsequent decay of the radioactive daughters in the same pixel; in this case, the decay (T 1/2 = 178 ms) of 9 Li, the partner emitted together with the deuteron in the decay channel of interest of 11 Li. The amount of 9 Li nuclei produced in this way was extracted by fitting the total daughter-decay spectrum with the shapes of the energy spectra of the decay of 9 Li, 8 Li and 11 Be, plus a background of 11 Li decays.…”