Β-delayed Two-proton Emission Research Articles

Observation of a Strongly Isospin-Mixed Doublet in ^{26}Si via β-Delayed Two-Proton Decay of ^{26}P.

β decay of proton-rich nuclei plays an important role in exploring isospin mixing. The β decay of ^{26}P at the proton drip line is studied using double-sided silicon strip detectors operating in conjunction with high-purity germanium detectors. The T=2 isobaric analog state (IAS) at 13 055keV and two new high-lying states at 13 380 and 11 912keV in ^{26}Si are unambiguously identified through β-delayed two-proton emission (β2p). Angular correlations of two protons emitted from ^{26}Si excited states populated by ^{26}P β decay are measured, which suggests that the two protons are emitted mainly sequentially. We report the first observation of a strongly isospin-mixed doublet that deexcites mainly via two-proton decay. The isospin mixing matrix element between the ^{26}Si IAS and the nearby 13 380-keV state is determined to be 130(21)keV, and this result represents the strongest mixing, highest excitation energy, and largest level spacing of a doublet ever observed in β-decay experiments.

Observation of β-delayed 2He emission from the proton-rich nucleus 22Al

The β-delayed two-proton emission from 22Al was investigated experimentally through the implantation-decay method. A β-delayed two-proton decay branch from 22Al were identified based on the coincidence of the charged particles and γ-ray signals. The relative momentum (qpp) and the opening angle (θpp) distributions of the two β-delayed protons are measured and a strong peak at qpp∼20MeV/c, as well as a peak at θpp∼30∘ are observed clearly. The β-delayed 2He emission from 22Al with a probability of 29 (13)% by fitting the experimental data with the results of Monte Carlo simulations.

β -delayed proton emission from P26 and S27

Delayed emission of protons following β decay of neutron deficient nuclei P26 and S27 was investigated at the ACCULINNA separator in the Flerov Laboratory of Nuclear Reactions at Dubna. Ions of interest, identified in flight, were implanted into the active volume of the gaseous optical time projection chamber, which allowed us to record tracks of charged particles emitted in the decay. Total branching ratios for β-delayed proton emission and for β-delayed two-proton emission were determined. In addition, energy spectra for delayed protons below 2 MeV were established. Our findings for P26 agree with results of previous experiments. In the case of S27, however, the observed probability of delayed proton emission is an order of magnitude larger than reported in literature. Two new strong proton transitions were identified representing decays of the first two excited states of P27 to the ground state of Si26. The probability ratio of γ-to-proton emission from these states is discussed.

Observation of β-delayed two-proton emission in the decay of 22Si

The decay of the lightest nucleus with Tz=−3 , 22 Si, was studied by a silicon array. A charged-particle group at 5600 (70) keV in the decay-energy spectrum was identified experimentally as β -delayed two-proton emission from the isobaric analog state (IAS) of 22 Al. Experimental results of the IAS fed by a superallowed Fermi transition were compared with our large-scale shell-model calculations. The ground-state mass of 22 Si was obtained indirectly in the experiment for the first time. Two-proton separation energy for 22 Si is deduced to be −108 (125) keV, which indicates that it is a very marginal candidate for two-proton ground-state emission.

Direct and β-delayed multi-proton emission from atomic nuclei with a time projection chamber: the cases of 43Cr, 45Fe, and 51Ni

The two-proton radioactivity of 45Fe was studied experimentally with a time projection chamber. The aim of the experiment was the reconstruction of the proton tracks in three dimensions. Energy and angular correlations have been determined and the data are compared with theoretical models, in particular with a three-body model. Moreover, the decay of 43Cr was studied and β-delayed one-, two- and three-proton emission could be established. The correlations observed for β-delayed two-proton emission favour a sequential emission. Finally, β-delayed two-proton emission was observed for the first time for 51Ni.

The decay of proton-rich nuclei in the mass [formula omitted] region

In a series of experiments at the SISSI/LISE3 facility of GANIL conducted between 1999 and 2004, we have collected decay information for proton-rich nuclei between 36Ca and 56Zn. The data allowed us to study the decay properties of 26 nuclei. The main experimental information obtained for all nuclei is their β-decay half-life and their total β-delayed proton emission branching ratio. For many nuclei, individual proton groups and γ rays were identified and allowed us to establish first partial decay schemes for some of the nuclei studied. In addition, mass-excess values have been determined for some of the nuclei by means of the isobaric multiplet mass equation. For 50Ni, the decay via β-delayed two-proton emission could be tentatively identified. The decay of 49Ni allowed for the first time to identify the first 2 + state in 48Fe. The experimental data are confronted to model predictions for the half-life and the mass-excess values.

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.

Beta-delayed two-proton emission at ISOLDE

The process of β-delayed two-proton emission has attracted much interest over the past two decades, since it was hoped that the study of this process would provide information on correlations inside nuclei. At ISOLDE 31Ar has been extensively studied in three experiments during the past decade. We review these studies by comparing the new physics results that have emerged from the different experiments and by highlighting the technical development leading to these results. The β-delayed two-proton emission is, in most cases, simply an extension of β-delayed single proton emission to high lying states that are unstable to proton emission, thus no sign of correlations has been seen.

β-decay of [formula omitted]Ca

The β-decay of the Tz=−52 nucleus 35Ca was studied at the LISE3 spectrometer at GANIL. The 35Ca decay scheme was deduced from its β-delayed proton emission into the ground and excited states of 34Ar and from its β-delayed two-proton emission. The 35Ca half-life was determined to be 25.7±0.2 ms. The measured transition strength function B(GT) is compared to results obtained from large-scale sd-shell model calculations.

Spectroscopic studies of the βp and β2p decay of 23Si

23Si isotopes have been produced as projectile fragments of a 36Ar primary beam at 95 MeV/nucleon at the LISE3 spectrometer of GANIL. After implantation in a detector telescope, β-delayed one-proton and β-delayed two-proton emission has been observed. The main one-proton peaks are at (1.32±0.04)MeV, (2.40±0.04)MeV, and (2.83±0.06)MeV. The total decay energy for the β2p decay is (6.18±0.10)MeV for the decay to the ground state and (5.86±0.10)MeV for the decay to the first excited state in the daughter nucleus. However, energetically possible decays via βpα and β3p emission have not been identified. The spectra allowed us to determine the excitation energy of the isobaric analogue state in 23Al. This enabled us to calculate the coefficients of the T = 5/2 isobaric multiplet mass equation for A = 23. The mass excess of the 23Si ground state was deduced. This value is compared to different theoretical predictions. Additionally, we determined the branching ratios for the different decay branches. A half-life measurement yielded T1/2 = (40.7±0.4)ms.

The spectroscopy of 22Al: a βp, β2p and βα emitter

In an experiment at the LISE3 facility of GANIL, we produced the proton-rich isotope 22Al by the fragmentation of a 36Ar primary beam at 95 MeV/nucleon. After implantation in a detector telescope, the decay of 22Al via β-delayed proton emission, β-delayed two-proton emission as well as, for the first time, via β-delayed α emission has been studied. An α peak has been observed at (3.27 ± 0.04) MeV with a branching ratio of (0.31 ± 0.09) %. The comparison of the different decay channels to those of the mirror nucleus 22F and to shell-model calculations favor a 3 + state as being the ground state of 22Al. A half life of T 1/2 = (59 ± 3) ms has been measured.