Speaker
Christian Aa. Diget
(Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark)
Description
The triple alpha process, the fusion of three alpha particles, is responsible for the main stellar production of
12C. It is known that the rate of this process is dominated by the 7.65 MeV resonance predicted by Hoyle and
identified in 1953 [1].
At present two reaction rates are widely employed [2,3]. Work is ongoing to improve these rates both at the
most important temperature range [4], and for temperatures where other natural parity resonances in 12C
play a role [5] which is where the two existing rates actually differ. To clarify the latter, we present a detailed
experimental analysis of the 0+ and 2+ strength in the triple alpha continuum above the Hoyle state and
investigate their influence on the triple alpha process.
Since the 0+ and 2+ states in question show a strong coupling to the triple alpha continuum, they are broad
and not easily distinguished among the other states in this 12C energy region. Our approach is to selectively
fed the states through the beta decay of 12N and 12B, and we detect the subsequent triple alpha breakup to
identify the states.
A new experiment (IG301) with improved detector setup has been performed at the IGISOL separator, Finland,
to allow a detailed analysis of the properties of the states. We use a setup of segmented silicon detectors to
measure triple alpha coincidences with a sufficient detection efficiency in most regions of three particle phase
space. This has for example permitted us to observe previously unknown breakup channels involving the 8Be
2+ exited state.
Since the 12C states overlap in energy and some have the same spin and parity they interfere. This is evident
from the data and our analysis takes this into account. In this way we determine: properties of the states; their
interference; and their coupling to the possible breakup channels 8Be(0+)+alpha and 8Be(2+)+alpha.
With this knowledge we investigate the influence from these states and breakup channels on the reaction rate
of the triple alpha process, and thus explore the influences on the triple alpha process beyond the Hoyle state.
References:
1. F. Hoyle, Astrophysical Journal Supplement Series 1, 121 (1954)
2. G.R. Caughlan and W.A. Fowler, Atomic Data and Nuclear Data Tables 40, 283 (1988)
3. C. Angulo et al., Nuclear Physics A 656, 3 (1999)
4. S.M. Austin, Nuclear Physics A 758, 375c (2005)
5. H.O.U. Fynbo et al., Nature 433, 136 (2005)
Author
Christian Aa. Diget
(Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark)
