3. Recent HRIBF Research - Crystal-blocking lifetime studies of heavy-ion induced fission
(J.S. Forster, Spokesperson RIB-136)
Blocking of elastically scattered ions and fission fragments were measured at HRIBF with a position-sensitive gas counter by bombardment of thin tungsten crystals with 240-255 MeV 48Ti. The purpose is to study the time scale of fission for very heavy compound systems at high temperature. There is strong evidence from measurements of pre-fission neutrons  and giant-dipole gamma rays (GDR)  that fission times are rather long, and it is now generally accepted that the nuclear mass flow in fission at high temperature is very viscous (like molasses rather than water). However, the experimental evidence is indirect and the interpretation is model dependent [1-4]. In contrast, crystal blocking gives direct information on the recoil of the united nucleus before fission and thereby on the time delay of the fission.
In an earlier experiment we measured blocking dips by bombardment of tungsten crystals by 170-180 MeV 32S ions and found no sign of a delay. The fission dips were virtually identical to dips in elastic scattering after an appropriate scaling of angles . In contrast, the new data indicates an average recoil distance of order 0.1 Å and a corresponding delay of about 2x10-18 sec, as seen in the figure Fig. 3-1 showing blocking dips along a <111> axis. The agreement with calculations is good except for an additive contribution from crystal defects to the minimum yield. The dip in elastic scattering should be narrower than a fission dip without recoil by a factor of 1.8 owing to the higher energy and lower atomic number of scattered beam particles, and the elastic dips have been scaled up in angle by this factor. The angular resolution in the experiment is limited by the size of the beam spot on the crystal, and the resolution is poorer for the narrower dip. The fission dip should rather be compared with an elastic dip with the same resolution, as given by the dotted curve.
Figure 3-1: : <111> blocking dip for fission fragments, compared with dip in elastic scattering, scaled in angle by a factor 1.8. The full line is calculated (continuum model) for an exponential distribution of recoil displacements, with average 0.14 Å, while the dashed line is for scattered Ti ions. For fission fragments the angular resolution is better, and the calculated dip for zero displacement is given by the dotted curve.
These results give clear confirmation of the picture of overdamped nuclear mass flow in fission of very heavy nuclei at high temperatures. The lifetime is at least an order of magnitude longer than inferred from measurement of neutron multiplicity and of GDR gamma rays. The null result for 32S bombardment is consistent with observations of very low intensities of GDR gamma rays in this reaction  and could be due to an influence of the magic neutron shell at N=126, as speculated in . It is interesting that the difference between the minimum yields in the elastic and fission dips is less than predicted by the calculations. The increase comes mainly from the exponential tail of long recoils and its absence suggests a more well defined, classical delay, depending on the angular momentum of the united nucleus, rather than an exponential distribution or a sum of exponentials.
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