One of the most interesting phenomena in Nuclear Physics with far reaching consequences, is the fission of atomic nuclei. Discovered about 63 years ago, it paved the way for the modern nuclear reactor, a bountiful source of energy. From the fundamental point of view it is a beautiful illustration of Einstein’s E=mc2 formula. When a slow moving neutron strikes a heavy nucleus like Uranium it breaks up into two fission fragments. The puzzling feature of this nuclear reaction is how a nucleus consisting of more than 200 protons and neutrons could split into two pieces with so little provocation. This is analogous to breaking a huge rock by a feather touch. The answer was given by Neils Bohr in 1939, in his theory where he likened the nucleus to a liquid drop. While the Bohr-Wheeler theory touched the essence of the fission problem, modern research shows that the fissioning nucleus behaves more like a drop of honey rather than a drop of water. These developments came about as the scientists began measuring fission time scales. Since the time scale is of the order of 10-21 sec, one might be wondering how it can be measured. Nuclear physics provides its own tools and in this case one makes use of the ‘neutron clock’, a method in which the number of neutrons emitted prior to fission are counted as an indicator of how much time has passed before the fission takes place. These measurements, both those carried out at our laboratory and elsewhere, unequivocally demonstrate that fission is a slow process (about 50-100 times slower) as compared to the expected time scale. These measurements are indicative of the time taken for the nucleus to “re-arrange” itself in preparation to dividing into two smaller pieces. So, the rock does break after a gentle feather touch, but it takes its own time to do so. Experiments at our laboratory have revealed further, that in a heavy-ion reaction there is also a considerable formation time (time for the projectile to get assimilated with the target to form the compound nucleus).
Time scales are not all. The fission angular distribution (the direction in which the fission fragments move relative to that of the incoming projectile) is pretty much a blueprint of the mechanism for fission. The fission angular distribution anomaly, the deviation of the experimental observations from the theoretical predictions, has been a puzzle since over a decade. Measurements at our laboratory showed that the gateway to formation (that is the history of how the compound nucleus was formed) was one of the keys in understanding this puzzle. Thus one of the principle tenets of nuclear physics, Bohr’s compound nuclear hypothesis needed to be revised. The role of nuclear deformation and angular momentum on the fission time and angular distributions are topics of current interest under investigation at our laboratory.
We have built position sensitive detectors, a large area deep ionisation chamber and scattering chambers, to detect fission fragments along with associated particles. We have studied pre-fission neutrons, pre-fission light charged particles, fission angular distributions for the same compound nucleus populated through different entrance channels, fission mass distributions and mass-gated neutron multiplicities. Radio-chemical methods have been used in our studies. Some pictures have emerged and further explorations are underway.
The information of temporal evolution in a nuclear reaction is important for theoretical understanding of compound nucleus formation and decay. To measure such short lifetimes (<10-16s) is an experimental challenge. In the case of a crystalline target, scattered charged particles are ‘blocked’ from traveling along the crystal planes. This fact is utilized in the nuclear reaction where enhancement in the yield along the axial and planar directions is measured. This enhancement is directly related to the distance traveled by the compound nucleus before decaying and hence gives the information on its lifetime. For nuclei in A~40 region, lifetimes in the range of 10-18-10-17 sec have been measured employing this technique with good quality thin (~1mm) silicon single crystal targets. The picture shows a blocking pattern obtained in heavy ion elastic scattering with a X-Y position sensitive detector, where the crystal axis and planes of are clearly seen.