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Insights into Nuclear Clusters in 28^{28}Si via Resonant Radiative Capture Measurements

International audienceThe heavyion radiative capture reaction 12C(16O,$\gamma$)28Si has been studied at three energies on( ELab = 20.0 and 21.2 MeV) and off( ELab = 20.7 MeV) resonance at Triumf (Vancouver) using the stateoftheart Dragon 0° spectrometer and its very efficient associated BGO $\gamma$ array. Intermediate states around Ex = 11.5 MeV, carrying a large part of the resonant flux have been observed for the first time in this system. The nature of those doorway states is discussed in terms of recently calculated cluster bands in 28Si. The results are compared to a recent similar investigation of the 12C(12C,$\gamma$)24Mg reaction

Analog E1 transitions and isospin mixing

We investigate whether isospin mixing can be determined in a model-independent way from the relative strength of E1 transitions in mirror nuclei. The specific examples considered are the A=31 and A=35 mirror pairs, where a serious discrepancy between the strengths of 7/2--->5/2+ transitions in the respective mirror nuclei has been observed. A theoretical analysis of the problem suggests that it ought to be possible to disentangle the isospin mixing in the initial and final states given sufficient information on experimental matrix elements. With this in mind, we obtain a lifetime for the relevant 7/2- state in 31S using the Doppler-shift attenuation method. We then collate the available information on matrix elements to examine the level of isospin mixing for both A=31 and A=35 mirror pairs

Baseline variability in onshore near surface gases and implications for monitoring at CO2 storage sites

The measurement of gas concentrations and fluxes in the soil and atmosphere is a powerful tool for monitoring geological carbon capture and storage (CCS) sites because the analyses are made directly in the biosphere in which we live. These methods can be used to both find and accurately quantifying leaks, and are visible and tangible data for public and ecosystem safety. To be most reliable and accurate, however, the measurements must be interpreted in the context of natural variations in gas concentration and flux. Such baseline data vary both spatially and temporally due to natural processes, and a clear understanding of their values and distributions is critical for interpreting near-surface gas monitoring techniques. The best example is CO2 itself, as the production of this gas via soil respiration can create a wide range of concentrations and fluxes that must be separated from, and not confused with, CO2 that may leak towards the surface from a storage reservoir. The present article summarizes baseline studies performed by the authors at various sites having different climates and geological settings from both Europe and North America, with focus given to the range of values that can result from near surface processes and how different techniques or data processing approaches can be used to help distinguish a leakage signal from an anomalous, shallow biogenic signal

Decay Modes of Narrow Molecular Resonances

présenté par Sandrine Courtin (DRS-IPHC)The heavy-ion radiative capture reactions $^{12}C(^{12}C,\gamma)^{24}Mg$ and $^{12}C(^{16}O,\gamma)^{28}Si$ have been performed on and off resonance at TRIUMF using the Dragon separator and its associated BGO array. The decay of the studied narrow resonances has been shown to proceed predominantly through quasi-bound doorway states which cluster and deformed configurations would have a large overlap with the entry resonance states

Level structure of Si26 and its implications for the astrophysical reaction rate of Al25(p,γ)Si26

A study of the level structure of Si26 using in-beam γ-ray spectroscopy is presented. A full level scheme is derived incorporating all states lying below the proton threshold energy. The results are in good agreement with shell model predictions and one-to-one correspondence is found with states in the mirror nucleus Mg26. Additionally, a γ-decay branch is observed from a state at 5677.0(17) keV, which is assigned to a 1+ resonance important in the astrophysical reaction Al25(p,γ)Si26. The newly derived resonance energy, Er=159.2(35) keV, has the effect of decreasing the reaction rate at the novae ignition temperature of 0.1 GK by a factor of 2 when compared with the previous most precise measurement of this state

Level structure of 22Mg: Implications for the22Na(p, γ)22Mg astrophysical reaction rate and for the22Mg mass

The level structure of 22Mg has been studied with high-sensitivity γ-ray spectroscopy techniques. A complete level scheme is derived incorporating all subthreshold states and all levels in the energy region relevant for novae burning. The excitation energy of the most important astrophysical resonance is measured with improved accuracy and found to differ from previous values. Combining the present result with a recent resonance energy measurement of this state leads to a derived 22Mg mass excess of -400.5(13) keV

Terminating states in the positive-parity structures of As 67

The energy levels and γ-ray decay scheme of the positive-parity states in the Tz=12 nucleus As67 have been studied by using the Ca40(Ar36,2αp)As67 reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson-Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three g92 particles and that both possess noncollective terminating states

γ-ray spectroscopy of the A=23, T=1/2 nuclei 23Na and 23Mg: High-spin states, mirror symmetry, and applications to nuclear astrophysical reaction rates

Background: Obtaining reaction rates for nuclear astrophysics applications is often limited by the availability of radioactive beams. Indirect techniques to establish reaction rates often rely heavily on the properties of excited states inferred from mirror symmetry arguments. Mirror energy differences can depend sensitively on nuclear structure effects. Purpose: The present work sets out to establish a detailed comparison of mirror symmetry in the A=23, T=1/2 mirror nuclei 23Na and 23Mg both to high spin, and high excitation energy, including beyond the proton threshold. These data can be used to benchmark state-of-the-art shell-model calculations of these nuclei. Methods: Excited states in 23Na and 23Mg were populated using the 12C(12C,p) and 12C(12C,n) reactions at beam energies of 16 and 22 MeV, and their resulting γ decay was measured with Gammasphere. Results: Level schemes for 23Na and 23Mg have been considerably extended; highly excited structures have been found in 23Na, as well as their counterparts in 23Mg for previously known rotational structures in 23Na. Mirror symmetry has been investigated up to an excitation energy of 8 MeV and spin-parity of 13/2+. Excited states in the region above the proton threshold have been studied in both nuclei. Conclusions: A detailed exploration of mirror symmetry has been performed which heavily constrains expectations as to how mirror energy differences should evolve for different structures. Agreement with shell-model calculations provides confidence in using such estimations where real data are absent

Mirror energy differences in the A=31 mirror nuclei, S31 and P31, and their significance in electromagnetic spin-orbit splitting

Excited states in S31 and P31 were populated in the C12(Ne20,n) and C12(Ne20,p) reactions, respectively, at a beam energy of 32 MeV. High spin states of positive and negative parity have been observed in S31 for the first time, and the yrast scheme of P31 has been extended. Large mirror energy differences between the first 9/2- and 13/2- states were observed, but only small differences for the first 7/2- and 11/2- levels. The significance of these observations is discussed in relation to the electromagnetic spin-orbit effect and the relative binding energy of the levels