Experimental Indications for the Response of the Spectators to the Participant Blast

Precise momentum distributions of identified projectile fragments, formed in the reactions ${}^{238}\mathrm{U}+\mathrm{P}\mathrm{b}$ and ${}^{238}\mathrm{U}+\mathrm{T}\mathrm{i}$ at $1A\mathrm{G}\mathrm{e}\mathrm{V}$, are measured with a high-resolution magnetic spectrometer. With increasing mass loss, the velocities first decrease as expected from previously established systematics, then level off, and finally increase again. Light fragments are on the average even faster than the projectiles. This finding is interpreted as the response of the spectators to the participant blast. The reacceleration of projectile spectators is sensitive to the nuclear mean field and provides a new tool for investigating the equation of state of nuclear matter.

Figure 1

Velocity distributions of the isotopes of eight elements produced in the reaction ${}^{238}\mathrm{U}+\mathrm{P}\mathrm{b}$ at $1A\mathrm{G}\mathrm{e}\mathrm{V}$. The velocities are given in the projectile frame with the projectile velocity taken to be that of the projectile after transversing half of the target thickness. The measured velocities of the residues behind the target, deduced from the magnetic deflection in the first half of the spectrometer, have been corrected for the energy loss of the fragments in the second half of the target. The scales of the cluster plots are given.

Figure 2

Velocity distributions of different nuclides produced in ${}^{238}\mathrm{U}+\mathrm{P}\mathrm{b}$ (dashed lines) and ${}^{238}\mathrm{U}+\mathrm{T}\mathrm{i}$ (solid lines) at $1A\mathrm{G}\mathrm{e}\mathrm{V}$ measured with the fragment separator [12]. The velocities are given in the frame of the projectile. Note the different scales in velocity. The nuclides were selected from the middle of the fragmentation distributions in Fig. 1. The dotted parts of the velocity distributions of the lightest nuclei in the system ${}^{238}\mathrm{U}+\mathrm{T}\mathrm{i}$ are deduced from the data of neighboring isotopes and elements (see text).

Figure 3

Mean values of the velocity distributions of reaction residues, excluding fission, produced in ${}^{238}\mathrm{U}+\mathrm{P}\mathrm{b}$ [16] (solid squares) and ${}^{238}\mathrm{U}+\mathrm{T}\mathrm{i}$ (open circles, this work) at $1A\mathrm{G}\mathrm{e}\mathrm{V}$ in the frame of the projectile. Relative uncertainties are shown. The absolute uncertainty amounts to less than $0.05\mathrm{c}\mathrm{m}/\mathrm{n}\mathrm{s}$ for each system, ${}^{238}\mathrm{U}+\mathrm{P}\mathrm{b}$ and ${}^{238}\mathrm{U}+\mathrm{T}\mathrm{i}$, independently. The dashed line marks the Morrissey systematics [8].