(Left) Principle of the crystal blocking technique. Heavy ions bombard a single-crystal target at an angle α relative to the crystal planes, causing a projectile nucleus and crystal nucleus to fuse. Fission fragments from very short lived nuclei (<10-18 s) are emitted in the plane of the target atoms (that is, the angle of emission relative to the crystal axis ψ=0) and are thus blocked from reaching the detector. Fragments emitted from nuclei that survive long enough to move into a channel between the crystal planes (e.g., at a distance d from a plane) are detected with little energy loss. Thermal vibrations in the crystal determine the lower time limit for blocking. (Right) The blocking dip observed for Z=124 for 238U+Ge reactions (at 6.09 MeV per nucleon collision energy) and fragments in the range of 67<Z<85. The width of the dip depends on atomic number and kinetic energy of the fragment.