D. M. Nadkarni

Use of gridded ionisation chamber for measuring fission fragment angle

Abstract Measurement of the angle of fission fragments with the electric field direction of a gridded ionisation chamber is described and the angular resolutions which can be achieved by this method are experimentally deduced. The present measurements are carried out in thermal neutron fission of 235 U and for each fission event the angle of emergence of the two fission fragments with respect to the electric field direction on either side of the ionisation chamber was determined by recording the coincident collector and grid pulses. The distribution of this difference (θ 1 −θ 2 ) for different fragment mass windows has been obtained and the fwhm of the angular resolution function is estimated to be 8°–11°. Some results on the fragment mass and total kinetic energy distributions obtained when the fragments were electronically collimated into a cone perpendicular to the source foil are given to bring out the improvement that can be achieved by eliminating energy-degraded fragments emerging at large angles in a gridded ionisation chamber.

Near-barrier fission of the system12C+232Th

The fission fragment angular distributions for the system12C +232Th have been measured in the energy range 0.97<Ec.m./VB<1.22. The measured anisotropies have been compared with the predictions of the standard saddle point statistical model, using the second moment of the compound nucleus spin distribution 〈l2〉 deduced from a Wong model fit to the fission excitation function. While the measured anisotropies agree with the standard saddle point statistical model for Ec.m./VB>1.05, they are abnormally large at lower energies. This is also true for the existing measurements of16O and19F +232Th systems. Since for this system pre-equilibrium fission contributions are not expected, this anomaly indicates strong channel coupling effects leading to an increase in 〈l2〉 at sub-barrier energies.

Measurements of fragment mass, charge and kinetic energy distributions in thermal neutron fission of 235U with a back-to-back ΔE-E detector system

Abstract A simultaneous measurement of mass ( M ), charge ( Z ) and kinetic energy ( E k ) distributions of fragments in thermal-neutron fission of 235 U has been carried out using a back-to-back ΔE - E detector system. A pair of gridded ionization chambers filled with P-5 gas measured the energy losses ΔE 1 , ΔE 2 of the complementary fragments in the gas, and the residual fragment energies were measured with a pair of semiconductor detectors. The four-parameter data were analysed to obtain fragment Z -distributions using the mass-momentum conservation relations to obtain M and then using the dependence of ΔE on E / M and Z . The charge resolution for the light-fragment group was determined at the gas pressures of 44, 150, 270 Torr and the best Z -resolution was obtained at 270 Torr. The data at this latter gas pressure were analysed to obtain the variances σ 2 z of the fragment charge distributions for different E k windows and this was compared with the measured variances σ 2 A of the fragment mass distribution as a function of E k as obtained by twoparameter measurements of the correlated fragment energies. The dependence of the measured variances σ 2 A , σ 2 Z on E k is considered with a view to learning about the degree of neutron-proton correlations in the nucleon exchanges taking place during the mass division in fission, in analogy with that used in the case of heavy-ion deep inelastic collisions (DIC). The variation of σ 2 A σ 2 z with E k observed in the present work, which is somewhat similar to that seen in heavy-ion DIC, implies that the fragment mass division in fission is governed by nucleon-exchange processes, where the degree of neutron-proton correlations is dependent on the total kinetic energy E k . While for very large E k the neutron-proton motion in the exchange processes is found to be uncorrelated, for lower E k values the motion is seen to be correlated. Implications of these results regarding the applicability of fission models such as statistical model are pointed out.

Measurement of complete fusion-fission and transfer-fission cross-sections and angular distributions for19F+232Th at sub-barrier energies by fission fragment folding-angle technique

SummaryFission fragment folding-angle technique has been used to separate the complete fusion-fission (CFF) and the transfer-induced fission (TF) components in the reaction of19F on232Th for the first time in sub-barrier energies. The fragment folding-angle distributions were measured using two position-sensitive Breskin-type detectors and were compared with simulations based on the kinematics for complete fusion-fission and transfer-fission events. The results on the excitation function and angular distributions for the CFF, TF and total fission events have been simultaneously measured over the bombarding energy range of 78.5 MeV to 106.5 MeV for the above system.

Multiparameter study of 1H, 3H and 4He in fast neutron induced fission of 235U

Abstract The emission probabilities per fission of α-particles, tritons and protons have been measured in fast neutron induced fission of 235 U. The measurements were carried out at neutron energies of 120, 180, 230 and 550 keV. A ΔE - E semiconductor detector telescope was used to identify different light charged particles and the fission fragments were detected with an ionization chamber. The three-parameter data corresponding to the pulse heights from the ΔE - E detectors and the ion-chamber were recorded event by event on magnetic tape and were analyzed off-line by computer. No significant variation in the most probable energy ( E ) and the standard deviation ( σ E ) of the energy spectra of different light charged particles with incident neutron energy was observed, although E α was seen to have a slightly higher value beyond E n = 230 keV. The yield of α-particles in fission induced by neutrons of E n ∼ 200 keV was found to be higher by about 20 % than that in thermal neutron induced fission. The yields of tritons and protons were found to increase significantly with neutron energy.

Studies of mass and energy correlations in thermal neutron fission of U-235 accompanied by long range alpha particles

Several characteristics of fission accompanied by long range alpha particles (LRA) have been studied in the thermal neutron induced fission of235U. The kinetic energies of fission fragments and the LRA were measured with a back-to-back ionization chamber and semiconductor detectors respectively. The kinetic energies of the two fragments and the LRA in LRA fission, along with the energies of pair fragments in the normal binary fissions, were recorded event by event on a magnetic tape by means of a four-parameter data acquisition system. The data were analysed to study the dependence of different quantities in LRA fission on the fragment mass ratio, LRA energy and the total kinetic energy of the fission fragments. It is seen that the most probable energy of LRA increases significantly for near symmetric mass divisions. The total kinetic energy for all mass ratios in LRA fission is found to be (2.6±0.7) MeV larger than that in binary fission. The difference in the total kinetic energies in LRA and binary fissions is seen to be dependent on mass ratio. This result may suggest that the scission configuration in LRA fission is different for different mass ratios. Correlations between the fission fragment and LRA energies have been studied for several mass ratios. It is seen that the most probable fragment kinetic energyĒk varies nearly linearly with the LRA energyEa for various mass divisions but the variation of the most probable LRA energyĒa with fragment kinetic energyEk is found to deviate from linearity for several mass ratios. From a least square fit to the variation ofĒk withEa it is found that the slope (dĒk/dEa) increases with the increase in mass ratio. The present results are discussed to arrive at a better understanding of the scission configuration in the fission accompanied by LRA emission.

Sub-barrier fission fragment angular distributions for the system19F+232Th

The measurements of fission fragment angular distributions for the system19F+232Th have been extended to the sub-barrier energies of 89.3, 91.5 and 93.6 MeV. The measured anisotropies, within errors are nearly the same over this energy region. However, the deviation of the experimental values of anisotropies from that of standard statistical model predictions increases as the bombarding energy is lowered.

Prompt neutron emission spectra and multiplicities in the thermal neutron induced fission of235U

The emission spectra of prompt fission neutrons from mass and kinetic energy selected fission fragments have been measured in235U(nth,f). Neutron energies were determined from the measurement of the neutron time of flight using a NE213 scintillation detector. The fragment energies were measured by a pair of surface barrier detectors in one set of measurements and by a back-to-back gridded ionization chamber in the second set of measurements. The data were analysed event by event to deduce neutron energy in the rest frame of the emitting fragment for the determination of neutron emission spectra and multiplicities as a function of the fragment mass and total kinetic energy. The results are compared with statistical model calculations using shell and excitation energy dependent level density formulations to deduce the level density parameters of the neutron rich fragment nuclei over a large range of fragment masses.

Measurement of specific energy losses of individual fission fragments with a back-to-back δE-E detector system

Abstract The specific ionization of fission fragments of specified charge and E M values in thermal neutron induced fission of 235 U have been measured employing a back-to-back gridded ionization chamber ΔE -semiconductor detector E setup. The measurements were carried out for the case of a mixture of argon (95%) and methane (5%) gases at two gas pressures, 44 Torr and 270 Torr. The experimental data for the lower gas pressure were used to determine the effective charge parameter γ for the fragments at various velocities. These γ-values have been used to compute the fragment energy losses at the higher gas pressure and are compared with the measured energy loss values.

Deep inelastic collisions of32S +27Al reaction at 130 MeV bombarding energy

The32S +27Al reaction was studied to investigate the deep inelastic collisions at a bombarding energy of 130 MeV which is well above the Coulomb barrier. The energy distributions of the binary decay products of 6⩽Z⩽10 were determined using a large area position sensitive ionization chamber. The average kinetic energies of the reaction products indicate that the exit shapes correspond to highly stretched scission configurations in the deep-inelastic processes.