Abstract: Channeling and blocking of energetic charged particles in crystals was a hot topic when I was a student about 40 years ago. At that time, Jens Lindhard was professor in theoretical physics, and he created the basic theoretical framework for a description of what he called “the string effect” [1]. This effect can be observed, for example, in nuclear fission induced by ion bombardment of a crystal. If a fission fragment moves in the direction of a “string” of atoms it is blocked by collisions with atoms on the string. This leads to strong dips in intensity in directions of major crystallographic axes.

The string effect can be applied to measure lifetimes in nuclear reactions. If a fission reaction proceeds via formation of a compound nucleus, as argued by Bohr and Wheeler in a famous paper from 1939 [2], and this nucleus lives for a time τ, it will be displaced from its crystal site by a distance vτ before fission, where v is the recoil velocity. The blocking is sensitive to very small displacements, vτ~10-100 pm, and for this reason I have called our experimental set-up a picoscope.

I shall report on very recent measurements of fission lifetimes at Oak Ridge Natl. Labs [3]. Our purpose was to study the fission dynamics for hot nuclei with a very high atomic number, Z≥96.  The measured lifetimes of 1-2 attoseconds are surprisingly long and are inconsistent with the Bohr-Wheeler model. They support a picture of strongly damped quasi-fission: the nucleus behaves like a drop of syrup.

Peter Staanum and Nicolai Nygaard