A new measurement of the pair-decay branch of the 12C Hoyle state

c50bf7af-1bb6-4b5f-9a6b-2eadd615ac98

Closed

STFC
STFC
STFC
STFC
STFC

£6,800

July 9, 2011

Sept. 9, 2011

This experiment will determine the rate of helium burning in stars, through which the carbon we consist of have been created. This is done through a measurement of key properties of the so-called Hoyle state, an excited version of carbon-12 which greatly speeds up the burning of helium. The measurement is important for the understanding of how the Universe was enriched with carbon prior to the formation of the Earth, as well as for the foundations of supernova explosions and the creation of a whole range of chemical elements found on Earth and in the Universe.

Potential Impact:
Academic impact:

The present study of the triple-alpha reaction will clarify the impact of the reaction rate on the several stellar scenarios as listed in the following. Through this, the impact on the creation of the chemical elements in stellar burning and stellar explosions will be clarified.

1) 12C enrichment of the stellar envelope in AGB stars and of the interstellar medium: The thermal flash process in AGB (Asymptotic Giant Branch) stars has been shown to be sensitive to the uncertainty in the triple-alpha reaction rate. After the hydrogen and helium burning phases, intermediate mass stars go through the AGB phase. In this phase, the star has a carbon-oxygen core and burns helium and hydrogen in shells surrounding the core. The helium and the hydrogen burning shell are separated by a non-burning inter-shell region. Thermal flashes induced by the helium burning shell dredge up 12C into the inter-shell region from which it can be blown off into the interstellar medium. Studies show that the dredge up can lead to an increased enrichment of 12C in the stellar envelope by a factor as large as two when the triple alpha rate is increased by an amount equal to its current uncertainty [F. Herwig et al., Phys. Rev. C 73(2006)025802].

2) The mass of the pre-supernova ion core: Studies also show that variations within the current uncertainty in the triple-alpha rate can produce changes of about 0.7 solar mass in the determination of the mass of the iron core in core- collapse supernovae [C. Tur et al., Astrophys. J., 671(2007)821; S.E. Woosley et al., Nucl. Phys. A718(2003)3c]. This is a significant variation given that even a 0.1 solar mass variation can make a significant difference to the energetic of the explosion, and to the composition of the material later ejected in the interstellar medium.

3) Further chemical element production: The production of mid-weight A < 40 isotopes [C. Tur et al., APJ, 671(2007)821], weak S - Process elements [C. Tur et al., APJ 702(2009)1068] and gamma emitters [C. Tur et al., APJ. 718(2010)257] are all sensitive to the triple-alpha rate.

Academic beneficiaries in the above include astronomers, astrophysicists, and nuclear physicists.

Societal impact:

The above research will in addition to its high profile in academic circles feed directly into the teaching of A-level physics. The present reaction rate measurement and its results may be used as a direct example of contemporary research covering both the Nuclear Decay and Nuclear Energy headings in the A2 specification. Furthermore, the optional A2 module on Astrophysics includes both red giants and supernovae, both of which are significantly influenced by the described measurement. The incorporation of such high-profile science in the teaching at A-level will aid our efforts to attract the next generation of physicists to higher education.