The shape and normalisation of the β-delayed α spectrum from 11 Be was measured by implanting 11 Be ions in a segmented Si detector. The spectrum is found to be dominated by a well-known transition to the 3/2 + state at Ex = 9.87 MeV in 11 B. A significant increase in the observed decay strength towards the higher end of the Q β window means, however, that the 9.87 MeV state cannot alone be responsible for the transition. Using the R-matrix framework we find that the inclusion of an extra 3/2 + state at Ex = 11.49(10) MeV is required in order to obtain a satisfactory description of the spectrum. Both states show large widths towards α decay, exhausting significant fractions of the Wigner limit, a typical signature of α clusterisation. The observed Gamow-Teller strength indicate large overlaps between the two states and the ground state of 11 Be.

A multi-particle decay experiment was successfully performed at the ISOLDE Decay Station. In this new permanent station, devoted to β-decay studies, the novel MAGISOL Si-Plugin Chamber was installed to study the exotic decay modes of the proton drip-line nucleus 31 Ar. The motivation was to search for β3p and β3pγ channels, as well as to provide information on resonances in 30 S and 29 P relevant for the astrophysical rp-process. Description of the experimental set-up and preliminary results are presented.

The ACtive TARget and Time Projection Chamber (ACTAR TPC) is a novel gas-filled detector that has recently been constructed at GANIL. This versatile detector is a gaseous thick target that allows the tracking of charged particles in three dimensions and provides a precise reaction energy reconstruction from the vertex position. A commissioning experiment using resonant scattering of a 3.2 MeV/nucleon 18 O beam on an isobutane gas (proton) target was performed. The beam and the heavy scattered ions were stopped in the gas volume, while the light recoil left the active volume and were stopped in auxiliary silicon detectors. A dedicated tracking algorithm was applied to determine the angle of emission and the length of the trajectory of the ions, to reconstruct the reaction kinematics used to built the excitation functions of the 1 H( 18 O, 18 O) 1 H and 1 H( 18 O, 15 N) 4 He reactions. In this article, we describe the design of the detector and the data analysis, that resulted in center of mass reaction energy resolutions of 38(4) keV FWHM and 54(9) keV FWHM for the proton and alpha channels, respectively.

We use a sequential R-matrix model to describe the breakup of the Hoyle state into three α particles via the ground state of 8 Be. It is shown that even in a sequential picture, features resembling a direct breakup branch appear in the phase-space distribution of the α particles. We construct a toy model to describe the Coulomb interaction in the three-body final state and its effects on the decay spectrum are investigated. The framework is also used to predict the phase-space distribution of the α particles emitted in a direct breakup of the Hoyle state and the possibility of interference between a direct and sequential branch is discussed. Our numerical results are compared to the current upper limit on the direct decay branch determined in recent experiments.

While the 12 C(α, γ) 16 O reaction plays a central role in nuclear astrophysics, the cross section at energies relevant to hydrostatic helium burning is too small to be directly measured in the laboratory. The β-delayed α spectrum of 16 N can be used to constrain the extrapolation of the E1 component of the S-factor; however, with this approach the resulting S-factor becomes strongly correlated with the assumed βα branching ratio. We have remeasured the βα branching ratio by implanting 16 N ions in a segmented Si detector and counting the number of βα decays relative to the number of implantations. Our result, 1.49(5) × 10 −5 , represents a 24 % increase compared to the accepted value and implies an increase of ≈ 13 % in the extrapolated S-factor.

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Abstract.12 C is synthesised in stars by fusion of three α particles. This process occurs through a resonance in the 12 C nucleus, famously known as the Hoyle state. In this state, the 12 C nucleus exists as a cluster of α particles. The state is the band-head for a rotational band with the 2 + rotational excitation predicted in the energy region 9 -11 MeV. This rotational excitation can affect the triple-α process reaction rate by more than an order of magnitude at high temperatures (10 9 K). Depending on the energy of the resonance, the knowledge of the state can also help determine the structure of the Hoyle state. In the work presented here, the state of interest is populated by beta decay of radioactive 12 N ion beam delivered by the IGISOL facility at JYFL, Jyväskylä. IntroductionThe 12 C nucleus has a cluster structure in the Hoyle state, which is a 0 + resonance just above the triple-α threshold. With the non-spherical structure of the state, 12 C has a rotational band built upon this state. A recent measurement by Zimmerman et al.[1], along with the other observations [2][3][4][5] suggest that the first excitation of this band, the 2 + state, is to be found in 9 -11 MeV energy region. The second and higher lying member of this rotational band, 4 + has been found experimentally [6], but the 2 + excitation has proven difficult to study because of the the presence of a broad 0 + resonance at 10.3 MeV [4,7].At higher temperature in stars (10 9 K), this rotational excitation state becomes relevant to the triple-α process, affecting the reaction rate. In addition to its importance in determining the 12 C production rate, the state is also important when considering the structure. With the energy of this first rotational excitation known, the determined moment of inertia can settle the debate on the structure of the 12 C nucleus in the Hoyle state as different theoretical models predict different arrangements of the cluster in the Hoyle state [8].The experiment was performed at JYFL, Jyväskylä, Finland. The IGISOL (Ion-Guide Isotope Separator OnLine) facility delivered a beam of the radioisotope 12 N which β decays to 12 C, populating the states of interest. Due to the selection rules of β decay, this method gives

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