Speaker
Description
The second 0+ state of $^{12}$C at 7.654 Mev (the so-called Hoyle state) is one of the most studied states among all nuclei because of its well defined $\alpha$-cluster structure and for its strong astrophysical implications. Nevertheless, its structure still presents aspects to be
clarified. In particular, the hypotheses of the presence of excitations of the Hoyle state date back to 1956 when Morinaga first suggested it. In recent years, a 2+ [1-5] and a 4+ [6] excited state were found and it was claimed they belong to a rotational band built on the Hoyle state. Such assignment was based on the spin and parity of the states and on their energy.
A stronger proof that these levels are excitations of the Hoyle state may come from the study of their electromagnetic properties, in particular by measuring the B(E2). These are strongly enhanced in transition among states that belong to the same rotational band.
In particular the B(E2,$2^+_2\rightarrow0^+_2$) would provide extremely important information on the nature of the Hoyle state. And even an upper limit on such B(E2) would be enough to put significant experimental constraints to the models that try to describe $^{12}$C structure and in particular the Hoyle state.
We propose to measure the the B(E2) for the transition from the second 2+ (10.03 MeV) to the second 0+ state (7.654 MeV).
The measurement is very difficult to realize and has not been performed up to now because of the very small gamma decay probability of the state (around 10$^{-8}$) due to the fact that the state has a well pronounced $\alpha$-cluster structure and it is well above the $\alpha$-decay threshold. Only using a large solid angle and high efficiency $\gamma$-detector array, like AGATA, it will be feasible to perform such a kind of measurement.\
The initial state can be populated via an inelastic scattering reaction, $^{12}C(p,p')^{12}C^*$, with a proton energy of 28 MeV. The excited state of interest in the $^{12}$C can decay in two ways: either into three $\alpha$-particles
$^{12}C^* \rightarrow^{4}He+^{8}Be$, or gamma-rays $^{12}C^*\rightarrow^{12}C^* + \gamma$.
By measuring the branching ratio $\gamma / \alpha$ and from the total width of the state known from literature it is possible to extract directly the B(E2).
The $\alpha$-decay will be measured using an array of high-granularity silicon detectors (DSSSD) to be placed inside the AGATA scattering chamber. The $\gamma$-decay will be measured using AGATA in coincidence with the proton ejectile and the three alpha-particles from the breakup of the daughter state following gamma decay: the Hoyle state.
The cross section of the $2^+_2$ state populated via ^{12}C(p,p')^{12}C^*$ has been estimated to be 1 mbar from experimental data taken from two experiments [3,7]. Assuming such a cross-section value, from 4 to 10 gamma per day are expected to be detected depending on the value of the B(E2) used for the calculation.
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[2] M. Itoh, et al., Phys. Rev. C, vol. 84, p. 054308, Nov 2011.
[3] M. Freer, et al., Phys. Rev. C, vol. 80, p. 041303, Oct 2009.
[4] M. Freer, et al., Phys. Rev. C, vol. 86, p. 034320, Sep 2012.
[5] W. R. Zimmerman, et al., Phys. Rev. Lett., vol. 110, p. 152502, Apr 2013.
[6] M. Freer, et al., Phys. Rev. C, vol. 83, p. 034314, Mar 2011.
[7] M. HARADA, et al., Journal of Nuclear Science and Technology, vol. 36,
no. 4, pp. 313–325, 1999.
