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Nuclear Physics and Supernovae
All present elements, with the
exception of hydrogen, deuterium and helium, have been synthesized in stars.
Elements heavier than iron however cannot be formed through exothermic nuclear
fusion. They are rather formed in massive stars at the end of their life, when
the star first implodes under its own gravitational pressure and then explodes
in a cataclysmic cosmic event - a type IIa supernova.
While the single phases of the stallar life are
well understood, model calculations for supernovae (SN) are still a problem.
The explosive energy of the star is mainly defined through a single parameter
at the critical point just before the collapse: the number of electrons per
baryon. This ratio is defined through the number of electrons which was
previously converted to neutrinos through electron capture (EC) reactions. As
neutrinos can leave the stellar core with almost no further interaction, they
can carry away gravitational energy, and thereby a part of the explosive power,
in form of kinetic energy.
The precise knowledge of weak EC rates for
nuclei of the iron/nickel region is of central importance for the SN model
calculations. This knowledge is still incomplete, and indeed all simulations of
supernovae are not capable of generating an explosion without any additional
"fine-tuning".
The EC rates are however also accessible by
laboratory experiments. Hadronic scattering processes, e.g. proton-nucleus
scattering at energies between 100 and 500 MeV), proceed, under certain
kinematic circumstances, through just the same interaction operators which also
describe the weak EC process. In nuclear physics, we call these processes
"Gamow-Teller"transitions.
In order to elucidate the structure of such
processes, our group conducts experiments at the Kernfysisch Versneller
Institute (KVI) in Groningen (the Netherlands). The EUROSUPERNOVA
collaboration has developed a novel detector system for a magnetic
spectrometer. Utilizing proton and deuteron beams from a cyclotron, a multitude
of projects with extensive experimental programs, have been launched.
Furthermore, EC rates from nuclei of the
nickel-iron region determine the electron-to-baryon ratio in type Ia SN
progenitors, which in turn fixes the composition of the explosion ejecta. In
type Ia SNe, which are interpreted as thermonuclear explosions of accreting
white dwarfs in binary stellar systems, EC occurs behind the burning front.
Links
ESN-collaboration
KVI Groningen
Joint Institute for Nuclear Astrophysics (JINA)
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