Measurement of the radiative branching ratio for the Hoyle state using cascade gamma decays

This thesis focuses on a new determination of the radiative width, Γrad of the Hoyle state from the observation of proton-gamma-gamma triple coincidences using the ¹²C(p, p′)¹²C reaction at 10.7 MeV bombarding energy. The angular correlation of the cascading 3.215 MeV and 4.439 MeV γ-rays corresponding to the 0₂⁺2 - 2₁⁺ - 0₁⁺ transitions in ¹²C was also measured for the first time, which uniquely confirms a spin and parity of 0+ for the Hoyle state. The two γ-rays from the Hoyle state were studied in coincidence with inelastically scattered protons using the CACTUS and SiRi arrays at the Oslo Cyclotron laboratory. The CACTUS γ-ray array consists of twenty-six 5′′ by 5′′ NaI detectors covering nearly 15% of the 4π solid angle. The SiRi particle array has 64 ΔE-E telescopes, allowing the observation of proton-γ coincidences and the determination their time differences. In a 12 day long experiment, 5.97×10⁹ events were recorded. Complementary experiments were carried out using the ²⁸Si(p, p′) reaction to calibrate the detector system. Comprehensive sorting and analysis procedures have been developed in the present thesis. The knowledge of the radiative width of the Hoyle state is fundamentally important to evaluate the triple–alpha reaction rate, responsible for carbon production in the universe. Our experimental approach was very similar to the work of Obst and Braithwaite [1], the only previous credible experiment based on cascading gamma–ray measurements, carried out in 1976. However the detector system and analysis procedure used here permitted the examination of the extremely rare triple coincidence events in greater detail. The main result of the thesis is the identification of 529(29) triple-coincidence events, which were used to determine the ratio of radiative width to the total width of the Hoyle state as Γrad/Γ=4.07(22)×10⁻⁴. Our results are in good agreement with the previously adopted Γrad/Γ value based on several experiments carried out between 1961 and 1976 and using different methods. Using the present value of Γrad/Γ and the adopted values of Γ/Γπ(E0) and Γπ(E0), the radiative width of the Hoyle state was determined as Γrad =3.8(4) ×10⁻³ eV. Additionally the γ − γ(θ) angular correlation of the 3.215 MeV and 4.439 MeV cascade has been also measured at nineteen separation angles, identifying a 0+ spin and parity for the Hoyle sate in 12C in the first time