Lifetime in heavy ion reactions studied by crystal blocking

The crystal blocking technique is used for measuring compound nuclear lifetimes and reaction times in the range 10-15-10-19 sec., provided charge particles are emitted in the process. The lifetime is extracted from the changes in the shape of blocking dip as compared to the prompt. The parameters that characterize the blocking dip, namely, the minimum yield (χmin) and the dip volume (Ω) are in general a function of energy (E), atomic number (Z) and the crystal thickness. A 2-D position sensitive solid detector (24 x 24 mm2) was used to record the blocking patterns of elastically scattered projectiles (prompt) and various reaction products. The detector gives both X and Y position along with energy information. A time of flight discriminator method in conjunction with blocking using the pulsed beam from the pelletron accelerator for separating the reaction products was used for the first time. Different mass groups are identified from energy (E) vs time of flight (T) spectrum and their blocking dips are then analyzed to extract the lifetime. The details of the measurements of lifetimes for the reactions 16O + 28Si and 12C + 28Si are presented. The dependence of lifetime of the incident energy are also discussed. 4 refs., 1 tab., 3 figs