[en] The nuclear structure of states in "1"2C have been a subject of interest for both theory and experiment since the early days of nuclear physics. Many open questions remain, especially concerning the existence and properties of 0"+ and 2"+ states in the triple alpha continuum. A series of experiments have been performed using β-decay of "1"2N and "12"B to probe these states. The latest experiment was performed at KVI using an implantation method, measuring the sum energy of the three α-particles directly. Preliminary results from this experiment will be presented. (author)

No abstract available

[en] A method is presented for the detection of charged particles emitted in the β-decay of exotic nuclei. Direct implantation of radioactive nuclei in a very thin double-sided silicon strip detector is now feasible thanks to the advances in the production and post-acceleration of ISOL beams. The high degree of pixelazation, and consequently the small active volume of the detector, and the precise energy of the radioactive ion beam are the key features of the method. Among the advantages of the method are a good precision of the overall normalization and a reduction of the background due to β-radiation. Several developments are described in the present paper. The detection threshold at low energies is determined using pulsed signals. An alternative energy calibration for the decay events, free from any dead-layer effects can be performed by implanting radioactive ions decaying to known narrow resonances, e.g. ²⁰Na. Time and position correlations in a small detection volume can be used to identify possible different decay channels. An overview of precision measurements using this technique is given to prove the strength of the method.

[en] A complete kinematics study of the ¹⁰B(³He,pααα) and ¹¹B(³He,dααα) reactions has been performed to study the multi-particle break-up of ¹²C resonances above the triple-alpha threshold. Four-particle coincidence detection gives us complete information on the direction and energy of the individual alpha particles from the decay of ¹²C, allowing us to extract new information on the structure of ¹²C which we shall present in this contribution. We have observed gamma de-excitation of the T = 1 15.11 MeV resonance using charged particle detectors, and have constructed Dalitz plots of the individual resonances in ¹²C using the complete kinematics information of the alpha particles which come from their break-up.

[en] The β-decays of the mirror nuclei ¹²B and ¹²N both populate states in ¹²C and they are therefore a precious source of information about this nucleus. Due to the selection rules of β-decay only 0⁺, 1⁺ and 2⁺ states are populated. This allows a very clean study of unbound states just above the 3α-threshold with those spin and parities. This probe has been applied in two experiments using two complementary experimental techniques: in the first the three α-particles emitted after β-decay are measured in coincidence in separate detectors using the ISOL method, while in the second method ¹²B and ¹²N are implanted in a detector and the summed energy of the three α-particles is measured directly. Preliminary results from the two approaches are presented

[en] Two complementary experimental techniques have been used to extract precise branching ratios to unbound states in ¹²C from ¹²N and ¹²B β-decays. In the first the three α-particles emitted after β-decay are measured in coincidence in separate detectors, while in the second method ¹²N and ¹²B are implanted in a detector and the summed energy of the three α-particles is measured directly. For the narrow states at 7.654 MeV (0⁺) and 12.71 MeV (1⁺) the resulting branching ratios are both smaller than previous measurements by a factor of ≅ 2. The experimental results are compared to no-core shell model calculations with realistic interactions from chiral perturbation theory, and inclusion of three-nucleon forces is found to give improved agreement

[en] Accurate measurements of the ⁸B neutrino spectrum are important for the interpretation of solar neutrino data. Experimentally, the ⁸B neutrino spectrum can be obtained from the measurement of the β-delayed α spectrum. We report on an α-α coincidence measurement performed at the IGISOL facility in Jyvaeskylae, Finland. Our measurement allows extensive cross-checks to be performed and gives a more intense neutrino spectrum at high energies compared to the present standard. The deviation reaches 4% at the end point of the spectrum. Below 11 MeV, the deviation is less than 1%.

[en] The β decays of ¹²N and ¹²B have been studied at KVI and JYFL to resolve the composition of the broad and interfering 0⁺ and 2⁺ strengths in the triple-α continuum. For the first time a complete treatment of 3α decay is presented including all major breakup channels. A multilevel, many-channel R-matrix formalism has been developed for the complete description of the breakup in combination with the recently published separate analysis of angular correlations. We find that, in addition to the Hoyle state at 7.65 MeV, more than one 0⁺ and 2⁺ state is needed to reproduce the spectra. Broad 0₃⁺ and 2₂⁺ states are found between 10.5 and 12 MeV in this work. The presence of β strength up to the ¹²N Q-value window suggests the presence of additional 0⁺ and 2⁺ components in the ¹²C structure at energies above 12.7 MeV.

[en] A complete kinematics study of the ¹⁰B(³He,pααα) and the ¹¹B(³He,dααα) reactions has been performed to study the multi-particle break-up of ¹²C resonances above the triple-alpha threshold. The values of energy and widths of some states has been improved, and in states of natural parity partial branches of decay through the ground state of ⁸Be have been extracted. The influence of the 'ghost' of the ⁸Be ground state has been taken into account in order to clarify the partial branches.

[en] The reaction ³He+¹¹B→d+¹²C* has been used to populate resonances in ¹²C up to an excitation energy of 15 MeV. The subsequent breakup to three α particles has been measured in complete kinematics. Dalitz plots are used to visualize and analyze the data. The Dalitz plot intensity distribution exhibits zero points characteristic of the total spin and parity of the 3α system allowing us to determine the spin and parity of a state in ¹²C at 13.35 MeV whose quantum numbers were hitherto not well established. The Dalitz plot intensity distributions of the 2^- state at 11.83 MeV and the 1⁺ state at 12.71 MeV are compared with the predictions of a recent three-body calculation as well as with simpler models. All are able to reproduce the gross structures seen in the Dalitz plot, but none give an accurate description of the detailed profile of the distributions.