Abstract: Evaporation residue cross sections for the reaction 16O+194Pt were measured at beam energies in the range 75.4–103.1 MeV using the gas-filled separator Hybrid Recoil Mass Analyzer at the Inter University Accelerator Centre, New Delhi. The transmission efficiency of the separator was obtained using the calibration system 16O+184W and a Monte Carlo simulation code. Measured evaporation residue cross sections were fitted with statistical model calculations using Kramers' formula. The present measurement provides further evidence for the onset of dissipative forces in fission in the mass ∼200 region.
Abstract: The fission fragment mass angle correlations and mass ratio distributions have been investigated for the two systems 16O+197Au and 27Al+186W, leading to the same compound nucleus 213Fr around the Coulomb barrier energies. Systematic analysis of the variance of the mass distributions as a function of temperature and angular momentum suggests true compound nuclear fission for both the reactions, indicating the absence of nonequilibrium fission processes.
Abstract: The influence of 100 MeV Ag7+ ion irradiation on current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Au/CdTe and Au/Cd0.9Zn0.1Te Schottky barrier diodes as a function of fluence are investigated. The irradiation fluence was varied from 1×1010 to View the MathML source. The current transport across the metal–semiconductor junction for pure and irradiated Schottky barrier diodes has been described by the thermionic-field emission process. Also, there are several mechanisms such as barrier tunneling, carrier compensation and generation–recombination mechanism, which may account for the I–V and C–V characteristics after irradiation. This variation in various diode parameters such as ideality factor (n), Schottky barrier height (Φb) and saturation current (Is) has been studied as a function of irradiation fluence. The energy loss mechanism of swift heavy ions at the metal–semiconductor interface is used to explain the change in Schottky barrier diode parameters. The observed modification at interface states of Schottky barrier height over a wide fluence range are mainly due to intense electronic energy loss mechanism and latent ion track formation created by swift heavy ions.
Keywords:
Abstract: The charge comparison method for n–γ discrimination has been in practice for more than two decades; technically it can be implemented in variety of ways. We present explicitly, in detail, two simple ways of implementing this technique. The first one is the conventional way of comparing the fractional or slow charge in the tail with the full charge of photomultiplier tubes (PMTs) anode pulse fed to a charge integrating ADC or QDC, but has been electronically implemented in a new fashion. The second one is a novel technique of comparing shaped-dynode pulse, fed to a peak-sensing ADC, with the fractional slow charge integrated in QDC. This technique reduces the complexity of individual gate generation, and thus reduces the number of gate and delay generators, compared with the first case. To implement these techniques, we have designed and fabricated a six-channel, single-width NIM module for use with QDC for n–γ discrimination. The module fabrication is very simple and inexpensive, and can be easily integrated with commercially available high-density NIM and CAMAC modules in experimental setups involving a large number of neutron detectors. Commercially available passive analog delay line chips have been used for delaying analog signals fed to the QDC. To simplify the setup for high-density applications we have replaced the constant fraction discriminator (CFD) by a fast leading-edge discriminator. The quality of n–γ discrimination obtained is comparable with that reported earlier in the literature.
Abstract:
Angular distribution measurements of 2H(7Be,7Be)2H and 2H(7Be,8B)n reactions at Ec.m.~4.5 MeV were performed to extract the astrophysical S17(0) factor using the asymptotic normalization coefficient (ANC) method. For this purpose a pure, low emittance 7Be beam was separated from the primary 7Li beam by a recoil mass spectrometer operated in a novel mode. A beam stopper at 0° allowed for the use of a higher 7Be beam intensity. Measurement of the elastic scattering in the entrance channel using kinematic coincidence, facilitated the determination of the optical model parameters needed for the analysis of the transfer data. The present measurement significantly reduces errors in the extracted 7Be(p,γ) cross section using the ANC method. We get S17 (0) = 20.7 ± 2.4 eV b.
Abstract: 7Be radioactive ion beam (RIB) has been successfully extracted by the in-flight separation technique (J. Phys. G. Nucl. Part. Phys. 24 (1998) 1371) [1] using the recoil mass spectrometer (RMS) HIRA (Nucl. Instr. and Meth. A 339 (1994) 543) [2] at NSC. Many experiments measuring elastic scattering and transfer reactions using low intensity 7Be beam have been performed. To compensate for low beam intensity, a new, compact geometry, large-area, high-efficiency detector system has been developed. The detector setup consists of two position-sensitive silicon annular strip detectors, a large-area two-dimensional position-sensitive silicon detector and a transmission-type gas ionization detector. The silicon detectors give the energy as well as the position of the implanted particles and the gas ionization chamber gives the differential energy loss for particle identification.
Abstract: A large area telescopic detector set-up (DeltaE-E) has been developed for elastic scattering and transfer reaction measurements involving low intensity radioactive ion beam (RIB) at Nuclear Science Centre (NSC), New Delhi. The detector system consists of a transmission type gas ionization chamber (DeltaE) with axial field geometry followed by E-twotwo-dimensional position sensitive silicon detectors (2D-PSSD) placed side by side (50 mm x 50 mm each) for the residualenergy measurements. The gas ionization chamber (IC) gives the energy loss of the particles for Z-identification whereas the silicon detectors give their energy as well as the position. The responses of the IC with alpha and fission sources, and of the entire detector system with 7Be beam are presented which demonstrate the overall performance.
Abstract: Fusion cross sections were measured for the systems 16O and 28Si+120Sn in order to understand the role of specific channels involved in sub-barrier fusion. Fusion cross sections were measured by direct detection of the evaporation residues. Extracted fusion barrier distributions were analyzed using the exact coupled-channel code CCFULL as well as the simplified code CCMOD. These calculations show that the observed fusion barrier distribution for the Si+Sn system is reproduced only if the rotational excitation in 28Si is taken into account.
Abstract:
Gamma ray spectra in the range of ∼8–35 MeV have been measured at 45°, 90°, and 135° in the reaction p+197Au at Ep=27 MeV. The statistical and the nucleon-nucleon bremsstrahlung models fail to describe the data. The direct-semidirect capture model works well for Eγ>20 MeV. The possibility of extracting information on the single particle strength distribution in the final nucleus has been illustrated.
Abstract: We have measured α-particle spectra at different laboratory angles from the fusion reaction 28Si+51V at 140 MeV. These spectra deviate at higher energies from the statistical model calculations using rotating liquid drop model values of the moment of inertia. In order to explain the experimental spectra, changes were required in the moments of inertia corresponding to deformation, which suggests dynamical effects on the deexcitation process for charged-particle emission. The comparison of decay times with formation times implies that light charged particles are preferentially emitted prior to the full relaxation of the compound nucleus. Statistical models and dynamical calculations have been employed in an attempt to interpret the experimental data. The results were compared with the 28Si+27Al system studied earlier in order to understand the symmetric and asymmetric entrance channel effects in the formation of the compound system.
Abstract: Baby, I. Mazumdar, R. Singh, C. V. Baba, Y. K. Agarwal, A. M. Vinodkumar, and K. M. Varier
Excitation functions for one- and two-nucleon transfer in 28Si + 68Zn system have been measured at energies below the Coulomb barrier. The experiment was carried out by detecting the forward recoiling targetlike nuclei using the recoil mass separator, HIRA. With a pulsed beam, the time-of-flight of the recoils was measured and used to resolve the M/q ambiguity. This enabled the determination of the two-nucleon transfer yields. The role of one- and two-nucleon transfer in the sub-barrier fusion cross-section enhancement has been investigated. It turns out that the coupling of the positive Q-value two-neutron transfer channel results in a significant contribution to the enhancement. Coupling to both the transfer and the inelastic channels is able to explain the observed enhancement.
Abstract: Measurements have been carried out for the fusion excitation function and one and two nucleon transfer probabilities at energies near and below the Coulomb barrier for the system 28Si+93Nb. The recoil mass separator HIRA at the Nuclear Science Center, New Delhi, has been used for these measurements. A coupled-channels formalism has been employed for explaining the observed enhancement in the sub-barrier fusion cross section. Present results support the significance of positive Q-value transfer channel in the sub-barrier enhancement. The possible role of higher-order multiphonon coupling effects in the reaction mechanism have also been investigated.
Abstract:
According to Stelson’s neutron flow model the threshold barrier is correlated to the onset of neutron flow. A systematic study of a large set of systems indicates that there exists a very good correlation between the threshold barrier and the intersurface distance after including the zero point motion of collective surface vibrations of the colliding nuclei in the prescription of Stelson.
Abstract:
Fusion process in the near and sub-barrier region has been investigated for the systems 48Ti+58,60,64Ni using the heavy-ion reaction analyzer (HIRA). Fusion excitation functions and the mean angular momenta are obtained from the measured evaporation residue cross sections. Significant enhancements both in the cross section and mean angular momentum data are seen with respect to the predictions of the one-dimensional barrier penetration model. Simplified coupled channel calculations incorporating linear coupling to the inelastic channels (lowest 2+ and 3- states of both the projectile and the target) are not able to explain the
