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The pre-scission α-particle multiplicity (αpre), measured in $^{16}$O (96 MeV) + $^{232}$Th reaction, is observed to be significantly lower than the global trend reported earlier. Statistical model calculations also show a much shorter fission-time to reproduce the experimental αpre. By assimilating the previous measurements, it is observed that the αpre makes a changeover from high to a very low value while crossing the Businaro Gallone point in mass asymmetry in the entrance channels in heavy-ion induced fission of $^{232}$Th. Similar discontinuous behavior with respect to the entrance channel mass asymmetry was observed earlier in fission fragment angular anisotropy data, and it was attributed to “pre-equilibrium fission”. The present results on αpre provide compelling evidence for an admixture of non-equilibrium fission which depends on the entrance channel dynamics.

Prompt gamma-ray energy distributions and multiplicities released during thermally induced fission of 235U have been measured using STEFF (SpecTrometer for Exotic Fission Fragments). Thermal neutrons are provided by the high-flux reactor at the ILL, Grenoble. STEFF is a unique 2E2v device that uses a coincidence timing method to measure the emission of prompt gamma rays as a function of the fragment mass and energy. Following scission, the fission fragments contain excitation energy that is released via prompt neutron and gamma-ray emission as the fragment decays to the ground state. STEFF contains an array of 11 NaI detectors surrounding the uranium target providing a 6.8% photopeak detection efficiency for gamma rays released within 1 ns of the scission time. STEFF also consists of 7 NE213 detectors, which detect the emission of prompt neutrons, the release of which is associated with reduction of fragment energy and, to a lesser extent, fragment spin. This experiment acts as a direct response to the NEA high priority demand which requires more accurate knowledge of heating caused by gamma emission in the next generation of nuclear reactors.

A large area bi-dimensional position sensitive multi-wire cathode strip detector (MCSD) has been developed for the detection of fission fragments (FFs). The performance of the detector has been characterized with FFs from 252Cf and α particles from 241Am-239Pu sources. The position information is obtained from the measurement of time delay signals of the cathode strips (X-direction) and split-cathode wires (Y-direction) with respect to the timing signal from the anode wires. The position resolution is found to be about 1.0 and 1.5 mm in X-and Y-directions respectively.

A large variety of experimental works has been done since the discovery of nuclear fission, aimed at studying different aspects of the phenomenon. Yet our comprehension of the fission process is not complete. This is, among others, due to a certain lack in multi-parameter experimental data. An example here is the correlation between fractional independent yields of fission products and neutron and gamma-ray multiplicities. Fragment-gamma-neutron measurements, especially if correlated with fission- fragment kinetic energies, give the complete set of observables and are therefore of interest from the point of view of modeling and understanding of the fission process. A two-arm spectrometer of fission products (STEFF) has been recently built at the Manchester University. In addition to the identification of masses from complementary fission products, by the double energy/double velocity measurement, the spectrometer is capable of delivering information on their nuclear charges, on the event-by-event basis. The spectrometer also comprises an array of NaI and may house a further array of neutron detectors. In such configuration, STEFF has been used at the ILL neutron guide at the benchmark experiment 235U(nth, f). Details on the experiment will be presented, results on the identification of atomic numbers in the light group of fission products will be demonstrated and the perspectives discussed.

A prototype 16-tap high impedance discrete delay-line readout circuit has been designed and fabricated for Si-strip detectors suitable for nuclear physics experiments involving moderate count rate applications. The delay-line offers a delay of ∼30 ns per tap with very good linearity. The energy resolution for α-particles from 241 Am 239 Pu source is found to be satisfactory, when 16 strips are connected through the above readout circuit of a 32-strip Si-strip detector having a dead layer of 5 μm. This readout system is very useful where a large number of detectors are integrated in a compact space and accommodating their readout electronics is a cumbersome task.

A large area position-sensitive ionization chamber with a wide dynamic range has been developed to measure the mass, charge and energy of the heavy ions and the fission fragments produced in heavy-ion-induced reactions. The split anode geometry of the detector makes it suitable for both particle identification and energy measurements for heavy ions and fission fragments. The detector has been tested with α particles from 241 Am– 239 Pu source, fission fragments from 252 Cf and the heavy-ion beams from the 14UD Mumbai Pelletron accelerator facility. Using this detector, measurements on mass and total kinetic energy distributions in heavy-ion-induced fusion–fission reactions have been carried out for a wide range of excitation energies. Results on deep inelastic collisions and mass–energy correlations on different systems using this detector setup are discussed.

Electronic energy loss of charged particles in materials is a fundamental process responsible for the unique response of materials in applications of advanced nuclear power, radiation detectors and advanced processing of electronic devices. In this study, stopping powers of 16 O, 19 F and 28 Si heavy ions crossing thin Mylar foils have been determined in transmission geometry. The energy loss was measured over a continuous range of energies from 1.6 to 5.5 MeV/n (MeV per nucleon) using the data that was tagged by a surface barrier detector (SBD) with and without stopping foils. We have compared the obtained stopping values to those predicted by SRIM-2008 computer code, ICRU-73 stopping data tables and MSTAR calculations. The effective charge values of these heavy ions have been also deduced from the experimental set of data.

An annular ionization chamber detector has been developed to study quasi-elastic and transfer reactions in heavy-ion collisions at near-barrier and sub-barrier energies. The important feature of the detector is that it has a near 2π coverage in the azimuthal angle φ for the particles entering in the detector at a given θ direction. This feature makes the detector very useful for measurement of the differential cross-sections at backward angles with respect to the beam direction, involving low cross-section reaction channels. The split anode configuration of the detector makes it capable of both particle identification and energy measurement for heavy ions and fission fragments. The detector has been tested using heavy-ion beams from the 14 MV-pelletron accelerator at Mumbai. Results on quasi-elastic excitation function measurements and barrier distribution studies in many heavy-ion reactions using this detector setup are discussed.

Energy loss of 252 Cf fission fragments in P-10 gas (90% Ar + 10% CH 4 ) at different pressures has been measured using a hybrid ΔE − E R detector telescope. The data were fitted to a polynomial function and the differential or specific energy loss (d E /d x ) along the range of the fission fragments was determined for different fragment kinetic energies. The experimentally measured energy-loss data for most probable light and heavy fragments are compared with calculations using different available energy-loss expressions based on the Bethe energy-loss formula.

Energy loss of fission fragments of specified charges and kinetic energies have been measured with a gas detector telescope. The singles data from the telescope (DE-E R ) and coincidence data with a HP Ge X-ray detector (DE-E R -X) were taken using a 252 Cf fission source. An empirical expression for the dependence of Z eff on charge and energy of fragments has been determined by fitting the observed energy loss behaviour of the fission fragments. The resolution in the measurement of charge from event by event analysis of DE-E R data for the light fragment mass group was found to be about 1.8 charge units (FWHM). The present data were analysed to determine the fragment charge distribution as well as the X-ray yield per fragment in 252 Cf fission.