Coulomb dissociation: A new tool for nuclear astrophysics

By Jared SagoffJune 1, 2010

The stars shine because of nuclear reactions and nucleosynthesis processes. Such processes more often than not involve exotic nuclei located far from the stable ones on the nuclear landscape because, under stellar conditions, reaction timescales are often such that, once an unstable nucleus is produced, it gets involved in a subsequent nuclear reaction before it can decay.

Hence, an understanding of how the elements were — and continue to be — made in our universe depends on our ability to obtain the reaction rates for their production.

Until recently, astrophysicists had to resort almost exclusively to model calculations to estimate reaction rates. With the advent of radioactive beam facilities, the situation is changing rapidly: there is a hope to provide solid experimental information, at least through indirect methods often called surrogate reactions. For example, the radiative capture of neutrons on unstable nuclei is difficult to measure directly, but because of the increasing availability of radioactive beams, an alternative method — such as the Coulomb dissociation of the end product — can be used as a surrogate to infer the radiative capture rate, provided that the appropriate framework to link the two processes is available. This has recently become possible through theoretical work by Henning Esbensen of the Argonne Physics Division.

Radiative neutron capture on 14C has been measured some time ago, taking advantage of the long half life of this radioisotope. Recently, the Coulomb dissociation of the reaction product 15C on a Pb target at 68 MeV/u was also measured at the radioactive beam facility at RIKEN. This is the first example where the Coulomb dissociation method has been tested against neutron capture data, and the results originally showed a worrisome discrepancy of at least 25%.  Esbensen has now repeated the analysis of the dissociation experiment and included nuclear and higher-order effects in the calculations to very encouraging results, which gives confidence in the dissociation method and provides astrophysicists with reaction rate information of the desired precision.  However, this recent work also shows that the simple first-order analysis, which was commonly used to analyze dissociation experiments, is inaccurate and needs to be replaced by the more involved approach.

Citation Information: H. Esbensen, "Coulomb dissociation of 15C and radiative neutron capture on 14C", Physical Review C. 80, 024608 (2009); 059904 (2009).