Pair conversion spectroscopy of the Hoyle state

The triple-alpha reaction leading to the formation of stable carbon in the Universe is one of the most important nuclear astrophysical processes. The radiative width of the so-called Hoyle state, involving the 7.654 MeV E0 and the 3.2148 MeV E2 transitions, is known with a 10-12% accuracy. A novel, more direct approach to determine the radiative width was proposed recently, based on the measurement of the E0 and the E2 internal pair conversion intensities. We report on the development of a new magnetic pair spectrometer with high sensitivity for electron-positron pairs and with excellent energy resolution.