Abstract
A natural silicon target was investigated in a ^{text {nat}}Si(mathbf {gamma },gamma ^{prime }) photon-scattering experiment with fully linearly-polarised, quasi-monochromatic mathbf {gamma } rays in the entrance channel. The mean photon energies used were langle {E}_{gamma }rangle = 9.33, 9.77, 10.17, 10.55, 10.93, and 11.37 MeV, and the relative energy spread (full width at half maximum) of the incident beam was varDelta E_{gamma }/langle {E}_{gamma }rangle approx 3.5–4 %. The observed angular distributions for the ground-state decay allow firm spin and parity assignments for several levels of the stable even-even silicon isotopes.
Highlights
For a massive star to end its lifecycle in a type-II core collapse supernova, modern three-dimensional simulations have demonstrated the necessity for including neutrino-nucleus interactions as an additional heating mechanism
Considering that a massive star in the last weeks of its lifecycle fuses lighter nuclei to form 28Si, which subsequently acts as a seed to synthesise in the very last stage elements up to 56Ni/56Fe, it can be safely assumed that there is a considerable amount of silicon, in particular 28Si, in the collapsing iron-nickel core or the shell surrounding it
As in Ref. [15], the first objective of the photon-scattering experiments presented in this work was to exploit the wellknown excitation energy of the 11446-keV level as an energy calibration point for other nuclear resonance fluorescence (NRF) measurements
Summary
For a massive star to end its lifecycle in a type-II core collapse supernova, modern three-dimensional simulations have demonstrated the necessity for including neutrino-nucleus interactions as an additional heating mechanism (see, e.g., Refs. [1,2]). The latter work used partially polarised bremsstrahlung, which allows firm spin and parity information to be extracted for strongly excited levels [14]. [15], the first objective of the photon-scattering experiments presented in this work was to exploit the wellknown excitation energy of the 11446-keV level as an energy calibration point for other nuclear resonance fluorescence (NRF) measurements. The opportunity to measure more energy settings in combination with the high sensitivity of the experimental setup allowed for an unambiguous assignment or confirmation of spin and parity combinations for excited levels in 28Si and 30Si. polarised, quasi-monochromatic photon beam. Using the direction of the incident photon beam as the quantisation axis (polar angle θ ), the angular distribution of the emitted γ rays contains information about the angular momentum, J , of the excited level. In the phase convention of Krane, Steffen, and Wheeler [19], the angular distributions for an even-even nucleus are given as:
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