I. Mukha

The β2p decay mechanism of 31Ar

Abstract We have measured the beta-decay of 31 Ar with a high granularity setup sensitive to multiparticle decay branches. Two-proton emission is observed from the isobaric analog state in 31 Cl to the four lowest states in 29 P and furthermore from a large number of states fed in Gamow–Teller transitions. The mechanism of two-proton emission is studied via energy and angular correlations between the two protons. In all cases the mechanism is found to be sequential yielding information about states in 30 S up to 8 MeV excitation energy. Improved data on the β -delayed one-proton branches together with the two-proton data provide precise information about the beta-strength distribution up to 15 MeV excitation energy.

Invariant mass spectrum and alpha-n correlation function studied in the fragmentation of He-6 on a carbon target

Momentum distributions and invariant mass spectra from the breakup of He-6 ions with an energy of 240 MeV/u interacting with a carbon target have been studied. The data were used to extract information about the reaction mechanism which is influenced by the structure of He-6. It is found that the dominant reaction mechanism is a two-step process: knock out of one neutron followed by the decay of the He-5 resonance. The shape of the (alpha+n) two-body invariant mass spectrum is interpreted as mainly reflecting the 5He ground state which is a J(pi) = 3/2(-) resonance. However, no evidence for correlations between cu particles and neutrons is observed in the momentum widths of the distributions. It is demonstrated that a combined analysis of the two-body invariant mass spectrum and an appropriate correlation function may be used to determine the properties of the intermediate resonance

Beta decay of 31Ar

Abstract A complete study of 31 Ar beta decay has been made by high-resolution charged-particle and gamma-ray spectroscopy. Beta-delayed radiation was detected by an array of three charged-particle detectors and a large-volume germanium detector. Fifteen new energy levels were discovered in 31 Cl. The beta-strength distribution, measured to 14.5 MeV, is compared with a shell-model calculation in the full sd space. The quenching of the Gamow-Teller strength and the isospin impurity of the IAS in 31 Cl are discussed.

Observation of the Li-11(beta d) decay

Experimental data from the decay of Li-11 show for the first time the presence of beta-delayed deuterons with a branching ratio larger than 10(-4). To distinguish between beta d and beta t events the decays of the daughter nuclei Li-9 and Li-8 were identified in the energy and decay time spectra. Furthermore, a time correlation analysis between the beta d events and the subsequent daughter decays was performed.

Probing the 11Li halo structure through β-decay into the 11Be∗(18 MeV) state

An experimental study of beta-delayed charged particles (H, He, Be) from Li-11 shows a beta-feeding to an excited state at 18.15(15) MeV in Be-11 with a large Gamow-Teller strength, B-GT greater than or equal to 1.6. Branching ratios and reduced widths of 2-, 3- and 5- particle decay channels of this state are extracted, A strong suppression of the neutron branch from the 18 MeV state to the Be-10(g.s.) is observed, We suggest that the measured partial width may be sensitive to the (p(1/2))(2) component in the Li-11 halo, A rough estimate indicates that the (p(1/2))(2) component is not the dominant one in the halo wave-function.

TWO-PROTON EMISSION IN THE DECAY OF 31AR

Several beta-delayed two-proton branches were observed in the decay of Ar-31, the most intense ones proceeding through the isobaric analogue state (IAS) in Cl-31. The mechanism of the two-proton emission is studied via the energy and angular distributions of the two protons, Simultaneous emission of the two protons fits the present data well, sequential decays might also describe them. Independent of the decay mechanism, a spin of 5/2 for the IAS is suggested, An improved limit on the direct two-proton emission fi om the ground state of Ar-31 is presented, (C) 1998 Elsevier Science B.V.

Determination of the spin of 31Ar

Abstract The beta-delayed proton emission from the lightest Ar-isotopes has been recorded with a high-granularity, large solid-angle Si-detector set-up. Proton energy shifts due to beta-recoil have been measured. We demonstrate how this allows the spin of 31 Ar to be determined as 5/2. The method can be applied at decay rates as low as 1 s −1 .

Fine structure in the beta-delayed proton decay of Ar-33

Low-energy beta-delayed protons from Ar-33 have been measured for the first time, The data reveal states, which, despite unfavourable barrier penetrability values, strongly decay to the first excited 2(+) state in S-32. The observation is discussed in terms of the standard shell model, A natural explanation is provided by the large spectroscopic amplitudes, involving s(1/2) and d(3/2) orbitals, as well as the l = 0 barrier penetrability, favouring the decay to the 2(+) state.

TWO-PROTON DECAY OF THE ISOBARIC ANALOGUE STATE OF 31AR

Abstract We have studied several two proton branches in the β-decay of 31Ar, the most intensive ones proceeding through the isobaric analogue state (IAS) in 31Cl. The energy and angular correlations of the two protons originating from the IAS have been measured. An indication of non-isotropic emission has been observed in the p+p+29P branch. The energy spectra of protons from the IAS 2p-branches seem to be continuous, which is difficult to explain by a sequential emission through a few intermediate levels in 30S. The model of non-sequential or democratic three-particle decay gives the best fit to the present data favoring the IAS spin-parity 5 2 + .

Proton dripline studies at ISOLDE: 31Ar and 9C

In this contribution examples of the application of new technologies to disentangle the mechanism of beta-delayed multiparticle emission are given. In particular the mechanism of β-delayed two-proton emission from 31Ar has been resolved and proved to be sequential, a preview of 9C-decay data is discussed.