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GSI
FRS

Longitudinal-momentum distributions of relativistic projectile fragments and EOS of nuclear matter

In peripheral heavy-ion collisions at relativistic energies, part of the projectile survives as an excited nucleus, named spectator. The interaction with the target results in a characteristic momentum distribution: the mean momentum is shifted, and the width is increased.

In a second reaction stage, the excited spectator cools down by emitting nucleons or light fragments. The observed longitudinal-momentum distribution reflects both, the initial nuclear collision and the deexcitation stage. While the initial nuclear collision changes both the mean value and the width of the longitudinal momentum, the deexcitation stage is expected to only change the width, since one assumes that the particles are emitted isotropically.

The mean longitudinal momentum of the heavy residues has been found to decrease with increasing mass loss, while its width increases. E. Hanelt et al. found that neutron-deficient residues show narrower distributions, because the contribution of the deexcitation stage is more important. The width of extremely neutron-rich residues which emit no or only few nucleons in the deexcitation phase is a direct measure of the Fermi momentum of the nucleons removed from the projectile.

For very large mass losses, the heavy residues have been found first by J. Benlliure and T. Enqvist et al. to become faster again, finally they are even faster than the projectiles. This finding would be equivalent to the following observation: Imagine, you shoot on a target so that the projectile penetrates the target. After the interaction, the residue of the projectile becomes faster than the projectile, and the target moves backward! This is a very surprising finding which is very important for our understanding of nuclear dynamics in mid-peripheral heavy-ion collisions.

The origin of the reacceleration phenomenon  observed in the above experiment is connected with the momentum dependence of the nuclear mean field. In fact, during the interaction between a target and a projectile, a hot and compressed participant zone is formed.  The pressure created will cause the explosion of this zone. In the case of the momentum-dependent nuclear mean field, the explosion is so strong that it will influence the spectator by giving it additional momentum. This was confirmed by the theoretical calculations performed by Shi et al. 

 Fig 1. Schematic picture of the relativistic heavy-ion collisions.

This, new, experimental approach, based on the use of a high-resolution magnetic spectrometer, exploits the direct impact of the participants expansion on the kinematical properties of the surviving heavy spectator remnants. The small change of the spectator velocity is an early signature, giving a direct access to the non-local features of the nuclear EOS.

More details on this topic can be found in the publication of M. V. Ricciardi et al.

References:

E. Hanelt et al. (1993)

T. Enqvist et al. (1999)

Shi et al. (2001)