The 3\(\alpha \) Process Studied Through Pair Conversion Transitions from the Hoyle State in ¹²C

Abstract

The E0 and E2 pair transitions from the Hoyle state have been measured, with the aim of deducing the radiative width and 3α reaction rate by a new approach. The 3α process is the only way carbon is synthesised in stars and is a bottleneck in stellar nucleosynthesis. The new method, which requires the ratio of the pair transitions, is expected to reduce the uncertainty from 10% to 5%. We recently observed the E2 pair transition for the first time, confirming the feasibility of the method. However, more statistics are needed to obtain a precise value for the radiative width.KEYWORDS: 3α process, Hoyle state, pair conversion Stellar synthesis of carbonThe exceedingly short half-life of 8 Be (T 1/2 = 6.7 · 10 −17 s) inhibits the stellar p−p chain, and fusion of three α particles becomes the only gateway to carbon synthesis in stars. This opens in helium burning red giants where the α concentration is sufficient, and occurs via two steps [1]: First, two α particles fuse and form 8 Be. Then, before it decays, the 8 Be is fused with a third α particle, forming 12 C in the excited 7.654-MeV 0 + Hoyle state. This is because the 8 Be ground state energy is close to that of two α particles, and the Hoyle state is close to the 8 Be + α energy. The proximity of the Hoyle state to the 3α threshold enhances the 3α process by a factor of 10-100 millions [2]. However, the process is still very unlikely as the excited carbon nucleus α-decays ∼99.96% of the time. Stable carbon is only formed if the excited carbon nucleus decays electromagnetically, and the process thus depends directly on the electromagnetic decay properties of the Hoyle state. The radiative decay occurs either by a 7.654-MeV E0 transition to the 0 + ground state, or by a 3.215-MeV E2 transition to the first excited 2 + state. Both transitions synthesise carbon. In order of intensity, transitions occur by E2 γ decay, and then by E0 and E2 pair conversion, respectively. Conversion electrons are negligible. The 3α process is a bottleneck in stellar nucleosynthesis, and precise knowledge of the reaction rate is imperative for proper stellar modelling [3].