Search

The Hoyle analogue state in $$^{16}$$ O was explored by inelastic scattering of 45 MeV $$\alpha $$ s on a Mylar target. The break up 4 $$\alpha $$ s of $$^{16}$$ O have been detected in coincidence with the inelastically scattered $$\alpha $$ beam particle to probe the Hoyle analogue state of $$^{16}$$ O in complete kinematics, for the first time. The data have been analysed for all possible configurations and the excitation function of $$^{16}$$ O has been reconstructed directly from 4 $$\alpha $$ as well as for specific decay channels like $$^{12}$$ C(0 $$^{+}_{2}$$ ) + $$\alpha $$ , $$^{12}$$ C(3 $$^{-}_{1}$$ ) + $$\alpha $$ and $$^{8}$$ Be + $$^{8}$$ Be. Several previously known states have been observed above the $$4\alpha $$ break-up threshold (14.44 MeV) in the above mentioned decay channels. However, the signature of the 15.1 MeV state, most preferable to be the Hoyle analogue state according to the theoretical prediction, remains inconclusive.

Background: The excitation energy dependence of the fission fragment mass and folding angle distributions for reactions with loosely bound projectiles are reported to be significantly different compared to the reactions with tightly bound nuclei. Extensive study to understand the reaction mechanism of weakly bound projectiles is required as it can simulate the mechanism of fission induced with radioactive ion beams.Purpose: The effect of loosely bound nuclei on fission fragment folding angle and mass distributions in the $^{9}\mathrm{Be}+^{238}\mathrm{U}$ reaction near the Coulomb barrier energies is investigated and the contributions of incomplete fusion fission in the total fission cross section are studied.Methods: Mass and folding angle distributions of the fission fragments have been measured near the Coulomb barrier energies using two large area multiwire proportional counters. Fragment masses were determined from the time-of-flight difference method. To understand the effect of breakup of loosely bound nuclei, the results are compared with tightly bound proton-, $\ensuremath{\alpha}$-particle-, and carbon-induced reactions, as well as with the predictions from semiempirical calculations.Results: The width of folding angle distributions is found to increase with lowering excitation energy in the $^{9}\mathrm{Be}+^{238}\mathrm{U}$ reaction. This is in contrast to the reaction $^{12}\mathrm{C}+^{232}\mathrm{Th}$ involving tightly bound projectiles, populating similar compound nuclei. The peak-to-valley ratios of the fission fragment mass distributions for all events are found to sharply increase with lowering excitation energy in the $^{9}\mathrm{Be}+^{238}\mathrm{U}$ reaction. The possible contribution of incomplete fusion into the total fission cross section could be extracted. The measured mass distributions were found to be consistent with the semiempirical calculation gef and macro-microscopic two-center shell model calculations considering the admixture of incomplete fusion fission.Conclusion: For actinide targets, mass distributions of inclusive fission are influenced by the breakup/transfer channels of the weakly bound projectiles. At the above-barrier energies, the contribution of incomplete fusion is found to be $\ensuremath{\approx}25\ifmmode\pm\else\textpm\fi{}4%$ which is similar to the reported value of the complete fusion suppression factor in reactions involving nuclei with low binding energies. The increase in the peak-to-valley ratio with lowering excitation energy in the fission fragment mass distribution could be explained by the combined analysis of folding angle and mass distributions.

The fragment emission mechanism in the binary decay of composites formed in the reactions $^{20}\mathrm{Ne}+^{56}\mathrm{Fe}$ and $^{16}\mathrm{O}+^{58}\mathrm{Ni}$ has been studied at two different excitation energies. The inclusive energy and angular distributions of the emitted fragments $^{6,7}\text{Li}$, $^{7,8,9}\text{Be}$, $^{10,11}\text{B}$, and $^{11,12,13,14}\text{C}$ have been measured in the laboratory angles ranging from ${15}^{\ensuremath{\circ}}$ to ${35}^{\ensuremath{\circ}}$ (corresponding angles in center of mass ranging from ${20}^{\ensuremath{\circ}}$ to ${50}^{\ensuremath{\circ}}$). The energy distributions of the fragments are found to be Gaussian and peaked at energies higher than those expected from fusion-fission-type reactions. The center-of-mass angular distributions of all the fragments have been found to fall faster than $\ensuremath{\approx}1/\mathrm{sin}{\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}$-like dependence and the average $Q$ values of the fragments are found to decrease with increasing the center-of-mass angle of the emitted fragment for both the systems. The above characteristics of fragments signify that they were emitted from nonequilibrium sources, produced in a highly energy damped deep-inelastic-type reaction. The lifetimes of the dinuclear composites estimated from the angular distributions of these fragments are found to be in the range of $\ensuremath{\approx}(0.5--2.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}$ s, which are smaller than the respective compound-nuclear lifetimes $[\ensuremath{\approx}(1.0--2.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21} \mathrm{s}]$. The angular-momentum dissipations estimated from the average kinetic energies of the fragments are found to be, for lighter fragments in particular, greater than those predicted by the empirical sticking limit.

The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited $^{116}\mathrm{Sn}$ and $^{116}\mathrm{Te}$ nuclei. These nuclei are populated via $p+^{115}\mathrm{In}$ and $^{4}\mathrm{He}+^{112}\mathrm{Sn}$ reactions in the excitation energy range of 18--26 MeV. Because of low excitation energy, the neutron spectra are predominantly contributed by the first-chance decay leading to the $\ensuremath{\beta}$-stable $^{115}\mathrm{Sn}$ and neutron-deficient $^{115}\mathrm{Te}$ as residues for the two cases. Theoretical analysis of the experimental spectra has been performed within the Hauser-Feshbach formalism by employing different models of the level-density parameter. It is observed that the data could only be explained by the level-density parameter that decreases monotonically when the proton number deviates from the $\ensuremath{\beta}$-stable value. This is also confirmed by performing a microscopic shell-model calculation with the Woods-Saxon mean field. The results have strong implications on the estimation of the level density of unstable nuclei and calculation of astrophysical reaction rates relevant to $r$ and $rp$ processes.

The nuclear level density of $^{115}$Sn has been measured in an excitation energy range of $\sim $2 - 9 MeV using the experimental neutron evaporation spectra from the $^{115}$In($p,n$)$^{115}$Sn reaction. The experimental level densities were compared with the microscopic Hartree-Fock BCS (HFBCS), Hartree-Fock-Bogoliubov plus combinatorial (HFB+C), and an exact pairing plus independent particle model (EP+IPM) calculations. It is observed that the EP+IPM provides the most accurate description of the experimental data. The thermal properties (entropy and temperature) of $^{115}$Sn have been investigated from the measured level densities. The experimental temperature profile as well as the calculated heat capacity show distinct signatures of a transition from the strongly-paired nucleonic phase to the weakly paired one in this nucleus., This article has been submitted to European Journal of Physics A for publication

Neutron kinetic energy spectra in coincidence with low-energy $\ensuremath{\gamma}$-ray multiplicities have been measured around $A\ensuremath{\approx}110$ in the $^{16}\mathrm{O}+^{93}\mathrm{Nb}$ and $^{20}\mathrm{Ne}+^{93}\mathrm{Nb}$ reactions in a compound nuclear excitation energy range of $\ensuremath{\approx}90--140$ MeV. The excitation energy (temperature) dependence and the angular momentum (spin) dependence of the inverse level density parameter $k$ have been investigated by comparing the experimental data with statistical Hauser-Feshbach calculations. In contrast to the available systematic in this mass region, the inverse level density parameter showed an appreciable increase as a function of the excitation energy. The extracted $k$ values at different angular momentum regions, corresponding to different $\ensuremath{\gamma}$ multiplicities, also showed an overall increase with the average nuclear spins. The experimental results have been compared with a microscopic statistical-model calculation and found to be in reasonable agreement with the data. The results provide useful information to understand the variation of nuclear level density at high temperature and spins.

Background: The recent surprising experimental discovery of asymmetric fission in the nucleus $^{180}\mathrm{Hg}$ at low excitation energy has renewed interest concerning our understanding of the role of nuclear shell effects in the dynamics of the fission process. New theoretical models have been developed that claim to successfully explain the observed asymmetric fission of $^{180}\mathrm{Hg}$ and also predict the nature of mass distributions of several other preactinide nuclei for which experimental data are scarce.Purpose: We investigated fragment-mass distributions in the fission of the preactinide nucleus $^{214}\mathrm{At}$ at different excitation energies near the Coulomb barrier. The present results were compared with the predictions from a recent state-of-the-art theoretical calculation, as well as with our previous measurement for the nearby nucleus $^{210}\mathrm{Po}$.Methods: A pulsed heavy-ion beam was used in the experiment. Fission fragments were detected with two large-area position-sensitive multiwire proportional counters placed at the folding angle. Mass distributions were extracted from time-of-flight differences and position information ($\ensuremath{\theta},\ensuremath{\phi}$) for the fission fragments.Results: An asymmetry in fragment-mass distribution in the fission of $^{214}\mathrm{At}$ was observed at an excitation energy of $\ensuremath{\approx}31$ MeV, where a shell effect is found to influence the potential energy surface. At higher excitation energies, mass distributions are predominantly symmetric indicating the absence of shell effects.Conclusion: This finding is consistent with the prediction of a recent macroscopic-microscopic dynamical model. However, for the preactinides, static models with nascent fragment shell structure are likely to produce similar results.

The phenomenon of collective enhancement in nuclear level density and its fadeout has been probed using neutron evaporation study of two strongly deformed ( 173 Lu, 185 Re), and one spherical ( 201 Tl) compound nuclei over the excitation energy ( E ⁎ ) range of ∼ 22 – 56 MeV . Clear signature of the fadeout of collective enhancement in nuclear level density was observed for the first time in both the deformed evaporation residues 172 Lu and 184 Re at an excitation energy range ∼ 14 – 21 MeV . Calculations based on finite temperature density functional theory, as well as macroscopic–microscopic shape transition model, have strongly established a close correlation between the observed fadeout of collective enhancement and a deformed to spherical nuclear shape transition in these nuclei occurring in the same excitation energy zone. Interestingly, a weak signature of fadeout has also been observed for the spherical residue 200 Tl. This is due to a similar shape transition of the deformed excited state configuration of 200 Tl.

Efficiency corrected single ratios of neutron and proton spectra in central Sn112+112Sn and Sn124+124Sn collisions at 120 MeV/u are combined with double ratios to provide constraints on the density and momentum dependencies of the isovector mean-field potential. Bayesian analyses of these data reveal that the isoscalar and isovector nucleon effective masses, ms⁎−mv⁎ are strongly correlated. The linear correlation observed in ms⁎−mv⁎ yields a nearly independent constraint on the effective mass splitting Δmnp⁎=(mn⁎−mp⁎)/mN=−0.05−0.09+0.09δ. The correlated constraint on the standard symmetry energy, S0 and the slope, L at saturation density yields the values of symmetry energy S(ρs)=16.8−1.2+1.2 MeV at a sensitive density of ρs/ρ0=0.43−0.05+0.05.

The excited states of 169Tm have been studied via the 169Tm(32S,32S′)169Tm* reaction at the beam energy of 164MeV. The $\gamma$-rays were detected using the Indian National Gamma Array (INGA) setup, composed of 19 Compton-suppressed clover HPGe detectors. A new level scheme of 169Tm with 11 newly placed $ \gamma$-rays has been proposed. A band crossing in the $ \pi [541]1/2^{-}$ band and several interband E1 transitions between this and the $ \pi [411]1/2^{+}$ ground-state band have been observed for the first time in this nucleus. The role of the $N=98$ deformed shell gap has been discussed by comparing the band crossing parameters of the negative parity bands in Tm and other neighboring nuclei. The origin of the interband E1 transitions has been investigated in terms of coupling to octupole degrees of freedom. The shape evolution of the Tm isotopes around $ N=98$ have been studied in the projected and cranked shell model approaches, both of which predict a change in shape from an axial prolate to a triaxial one after band crossing in these nuclei. The new data and the calculations help to understand the unusual structural phenomena reported for the nuclei with $ N=98$.

Fission fragment mass distributions have been measured for relatively neutron-deficient compound nuclei, $^{188}\mathrm{Pt}$ and $^{190}\mathrm{Pt}$, formed in the fusion reactions, $^{28}\mathrm{Si}+^{160}\mathrm{Gd}$ and $^{12}\mathrm{C}+^{178}\mathrm{Hf}$, respectively. The data were obtained for a similar initial excitation energy range of 49--68 MeV for both the systems. The fragment mass distributions for both the Pt isotopes were found to be single peaked and no appreciable change in the mass symmetry was observed throughout the measured excitation energy range, for both the reactions. However, relatively broader mass distributions observed in the fission of $^{188}\mathrm{Pt}$ in the studied energy domain indicates the presence of fission events originating from a nonequilibrated source as well. This signifies that the mass equilibrium has not been fully achieved in the $^{28}\mathrm{Si}+^{160}\mathrm{Gd}$ system as compared to the $^{12}\mathrm{C}+^{178}\mathrm{Hf}$ system, indicating the presence of quasifission events in the former reaction.

The excited states of $^{197}\mathrm{Tl}$ have been studied via $^{197}\mathrm{Au}(^{4}\mathrm{He}$, $4\mathrm{n})^{197}\mathrm{Tl}$ reaction at a beam energy of 50 MeV from the K-130 cyclotron at the Variable Energy Cyclotron Centre (VECC). The $\ensuremath{\gamma}$ rays were detected using the VECC array for Nuclear Spectroscopy (VENUS) with six Compton-suppressed clover HPGe detectors. An improved level scheme of $^{197}\mathrm{Tl}$ has been proposed from this work, which has been extended to 5.1 MeV of excitation energy and 39/2 $\ensuremath{\hbar}$ of spin from the placement of 28 new $\ensuremath{\gamma}$-ray transitions. Band crossings in the known one- and three-quasiparticle (qp) bands have been identified for the first time in this work. Two new bands, based on 3-qp and 5-qp configurations, have been observed for the first time in this nucleus; both of which are identified as the magnetic rotational (MR) in nature. The excitation energies of these bands are similar to that of the doubly degenerate bands observed for the similar 3-qp and 5-qp configurations in $^{195}\mathrm{Tl}$. These indicate a transition from an aplanar geometry of the neutron, proton, and the core angular momentum vectors to a planar one for neutron number $N\ensuremath{\ge}116$ in Tl isotopes. The total Routhian surface calculations suggest a change in shape from oblate for the 1-qp configuration to a near-spherical one for the 3- and 5-qp configurations. This is consistent with the observed MR nature of the bands with multi-qp configuration. The MR bands are well reproduced by a model calculations in the frame work of the shears mechanism with the principal axis cranking.

Fission fragment mass distribution has been measured for $^{28}\mathrm{Si}+^{197}\mathrm{Au}$ reaction at bombarding energies around the Coulomb barrier (${V}_{b}=126.5\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$). The experimental variance of the mass distribution shows a sudden change from monotonic decrease to a saturation at energies close to the Coulomb barrier and again a decreasing trend at sub-barrier energies. The variance of the mass distribution was also obtained from dynamical and statistical model calculations for the fission of a fully equilibrated compound nucleus. Both of the theoretical calculations show a monotonic change of the mass variance around the Coulomb barrier. The anomalous dependence of fission fragment mass width at energies around the barrier may be due to the dominance of the quasifission process as compared to compound nuclear fission.

A large 4$\pi$ array of charged particle detectors has been developed at Variable Energy Cyclotron Centre to facilitate high resolution charged particle reaction and spectroscopy studies by detecting event-by-event the charged reaction products emitted in heavy ion reactions at energy $\sim$ 10-60 MeV/A. The forward part ($\theta \sim \pm $ $7^{0}$ - $\pm 45^{0}$) of the array consists of 24 highly granular, high resolution charged particle telescopes, each of which is made by three layers [single sided silicon strip($\Delta$E) + double sided silicon strip (E/$\Delta$E) + CsI(Tl)(E)]of detectors. The backward part of the array consists of 112 CsI(Tl) detectors which are capable of detecting primarily the light charged particles (Z $\le$ 2) emitted in the angular range of $\theta \sim \pm $ $45^{0}$ - $\pm 175^{0}$. The extreme forward part of the array ($\theta \sim \pm $ $3^{0}$ - $\pm 7^{0}$) is made up of 32 slow-fast plastic phoswich detectors that are capable of detecting light (Z $\le$2) and heavy charged particles (3 $\le$ Z $\lesssim$ 20) as well as handling high count rates. The design, construction and characterization of the array has been described., Comment: Paper has been submitted in NIMA

Energy spectra of light charged particles emitted in the reaction $p + {}^{27}\mathrm{Al} \rightarrow {}^{28}\mathrm{Si}^{\ast}$ have been studied and compared with statistical model calculation. Unlike 16O + 12C where a large deformation was observed in 28Si*, the energy spectra of $\alpha$ -particles were well explained by the statistical model calculation with standard “deformability parameters” obtained using the rotating liquid drop model. It seems that the $\alpha$ -clustering in the entrance channel causes extra deformation in 28Si* in the case of 16O + 12C, but the reanalysis of other published data shows that there are several cases where extra deformation was observed for composites produced via non- $\alpha$ -cluster entrance channels also. An empirical relation was found between mass-asymmetry in the entrance channel and deformation, which indicates that along with $\alpha$ -clustering, mass-asymmetry may also affect the emission of a light charged particle.

Background: The influence of shell effect on the dynamics of the fusion fission process and its evolution with excitation energy in the preactinide Hg-Pb region in general is a matter of intense research in recent years. In particular, a strong ambiguity remains for the neutron shell closed $^{210}\mathrm{Po}$ nucleus regarding the role of shell effect in fission around $\ensuremath{\approx}30--40\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ of excitation energy.Purpose: We have measured the fission fragment mass distribution of $^{210}\mathrm{Po}$ populated using fusion of $^{4}\mathrm{He}+^{206}\mathrm{Pb}$ at different excitation energies and compare the result with recent theoretical predictions as well as with our previous measurement for the same nucleus populated through a different entrance channel. Mass distribution in the fission of the neighboring nuclei $^{213}\mathrm{At}$ is also studied for comparison.Methods: Two large area multiwire proportional counters (MWPC) were used for complete kinematical measurement of the coincident fission fragments. The time of flight differences of the coincident fission fragments were used to directly extract the fission fragment mass distributions.Results: The measured fragment mass distribution for the reactions $^{4}\mathrm{He}+^{206}\mathrm{Pb}$ and $^{4}\mathrm{He}+^{209}\mathrm{Bi}$ were symmetric and the width of the mass distributions were found to increase monotonically with excitation energy above 36.7 MeV and 32.9 MeV, respectively, indicating the absence of shell effects at the saddle. However, in the fission of $^{210}\mathrm{Po}$, we find minor deviation from symmetric mass distributions at the lowest excitation energy (30.8 MeV).Conclusion: Persistence of shell effect in fission fragment mass distribution of $^{210}\mathrm{Po}$ was observed at the excitation energy $\ensuremath{\approx}31\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ as predicted by the theory; at higher excitation energy, however, the present study reaffirms the absence of any shell correction in the fission of $^{210}\mathrm{Po}$.

Neutron evaporation spectra have been measured from $^{4}\mathrm{He}+^{208}\mathrm{Pb}$ and $^{4}\mathrm{He}+^{209}\mathrm{Bi}$ reactions by using $^{4}\mathrm{He}$-ion beams of several bombarding energies. Excitation-energy dependence of the level density parameter has been studied for the two systems in the excitation energy range of $\ensuremath{\sim}18$--50 MeV. For both the reactions an overall reduction of the asymptotic level density parameter with increasing excitation energy (temperature) is observed. The trend of the data was compared with the Thomas--Fermi model predictions and found to be in reasonable agreement. The value of the shell damping parameter has been extracted from the lowest-energy data in the case of $^{210,211}\mathrm{Po}$ and $^{211,212}\mathrm{At}$ nuclei close to the $Z=82$ and $N=126$ shell closure, and it was found to be consistent with the recent measurement in the vicinity of doubly magic $^{208}\mathrm{Pb}$ nucleus.

Background: Suppression of compound nucleus formation in the reactions with heavy ions by a quasifission process in dependence on the reaction entrance channel.

The fission fragment mass distributions have been measured in the reactions 16O + 184W and 19F+ 181Ta populating the same compound nucleus 200Pb? at similar excitation energies. It is found that the widths of the mass distribution increases monotonically with excitation energy, indicating the absence of quasi-fission for both reactions. This is contrary to two recent claims of the presence of quasi-fission in the above mentioned reactions., Comment: Accepted for publication as a regular article in Physical Review C

Cluster state formation viz. the population of unbound 8 Be and the Hoyle state of 12 C produced in the reactions 12 C (77 MeV ) + 12 C and 13 C (75 MeV ) + 12 C have been studied using resonance particle spectroscopy. It was observed that there is a large difference in the cluster state formation in these two reactions and the yield of neutron rich isotopes of different fragments is more in the 13 C + 12 C compared to 12 C + 12 C reactions at 6 MeV/u.

The performance of a dual time of flight (TOF) spectrometer using two 24 cm×10 cm area X – Y position sensitive Multi-Wire Proportional Counters (MWPC) is reported. Good separation of compound nuclear fission channel from elastic, quasi-elastic and non-compound fission channels have been achieved. Time calibration of the spectrometer using a technique, dependent on the detector independence of the mass or kinetic energy distributions gave a three times better mass resolutions compared to the conventional calibration technique using elastically scattered projectiles in a heavy ion induced fission reaction. Detail experimental methods in determining fission fragment mass and angular distribution have been presented and discussed.

The excited states of 26 Al have been produced and studied using 27 Al(d,t) reaction with 25 MeV deuteron as projectile. Optical model potential parameters were extracted from the measured elastic scattering angular distribution. Zero range distorted wave Born approximation analysis for the ground and 0.223 MeV states of 26 Al have been done. The spectroscopic factors calculated for these states are found to be in good agreement with the previously reported values.

Neutral pion production in central 158A GeV [sup 208]Pb+ [sup 208] Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The [pi][sup 0] transverse mass spectrum has been analyzed in terms of a thermal model with hydrodynamic expansion. The high accuracy and large kinematic coverage of the measurement allow one to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out. [copyright] [ital 1999] [ital The American Physical Society ]

Transition energies, oscillator strengths and transition probabilities have been reported for the high lying quadrupolar excited states up to principal quantum number n = 7 for the first few members of the Cl isoelectronic sequence from Cl to Cr7+. A linearised version of the time-dependent coupled Hartree–Fock (TDCHF) theory has been adopted to calculate such transition properties. The data are very useful to explain the unidentified lines in the spectrum of the solar corona and flares. Most of the data for oscillator strengths and transition probabilities are new. Transition energies agree well with the existing experimental results wherever available.

Anisotropic transverse flow has been studied with a magnetic spectrometer at midrapidity and the Plastic Ball detector at target rapidity in the WA98 experiment. Our preliminary results show the existence of directed and elliptic flow in semi-central Pb+Pb collisions. The magnitude of the directed flow is found to be significantly smaller than those observed at the AGS energies.

Neutron evaporation spectra alongwith γ -multiplicity has been measured from the 185 Re * compound nucleus at the excitation energies ~27 and 37 MeV. Statistical model analysis of the experimental data has been carried out to extract the value of the inverse level density parameter k at different angular momentum regions ( J ) corresponding to different γ -multiplicity. It is observed that, for the present system the value of k remains almost constant for different J . The present results on the angular momentum dependence of the nuclear level density (NLD) parameter a (=A/k) , for nuclei with A ~180 is quite different from our earlier measurements in case of light and medium mass systems. The present analysis provides useful information to understand the angular momentum dependence of NLD at different nuclear mass regions.

An excited state of 12 C having excitation energy (E x ) 9.65 ± 0.02 MeV and width (FWHM) 607 ± 55 keV, which decays to three α particles via Hoyle state (E x = 7.65 MeV), has been directly identified in the exclusive inelastic scattering of 60 MeV 4 He on 12 C, measured in coincidence with the recoiling 12 C Hoyle state (decaying mostly as 12 C* → 8 Be gs → α → α + α + α) by event-by-event kinematic reconstruction of the completely detected (4α) events. This state is likely to be a candidate for 2 2 + first excited of Hoyle state.

The 9Be(28Mg,27Na) one-proton removal reaction with a large proton separation energy of Sp(28Mg)=16.79 MeV is studied at intermediate beam energy. Coincidences of the bound 27Na residues with protons and other light charged particles are measured. These data are analyzed to determine the percentage contributions to the proton removal cross section from the elastic and inelastic nucleon removal mechanisms. These deduced contributions are compared with the eikonal reaction model predictions and with the previously measured data for reactions involving the re- moval of more weakly-bound protons from lighter nuclei. The role of transitions of the proton between different bound single-particle configurations upon the elastic breakup cross section is also quantified in this well-bound case. The measured and calculated elastic breakup fractions are found to be in good agreement., Comment: Phys. Rev. C 2014 (accepted)

Energy levels, dipole oscillator strengths and transition probabilities for some highly stripped ions of Fe-group elements Fe8+, Co9+, Ni10+, Cu11+ and Zn12+ belonging to the astrophysically important Ar-isoelectronic series have been evaluated using the time-dependent coupled Hartree-Fock theory. The data can be employed to the analysis of spectral lines in the solar spectrum, high temperature astrophysical and laboratory plasma diagnostics. Estimated values compare favourably with existing experimental and theoretical data wherever available and many new ones reported here may constitute a useful set for future reference.

The first evidence of directed flow at 158 A GeV Pb + Pb collisions is observed using the Plastic Ball detector in WA98 experiment. The azimuthal angle of the total transverse energy, Phi(o), is determined for each event using the observed protons in the Plastic Ball. A clear azimuthal asymmetry of fragments is observed with respect to Phi(o). The degree of azimuthal asymmetry is found to be largest in mid-central collisions, which is consistent with the picture of the collective flow.The p(t) spectra of particles measured in the mid-rapidity spectrometer has been studied in conjunction with the Plastic Ball, and different p(t) slopes with respect to Phi(o) for proton and pi(+) are observed.

The excited states in the odd-$A$ $^{197}$Tl nucleus have been studied by populating them using the $^{197}$Au($\alpha$, 4$n$)$^{197}$Tl reaction at the beam energy of 48 MeV. The $\gamma-\gamma$ coincidence data were taken using a combination of clover, LEPS and single crystal HPGe detectors. Precise spin and parity assignments of the excited states have been done through the polarization and the DCO measurements. A new band structure has been identified and the evidence for a possible intruder $\pi i_{13/2}$ state has been found for the first time. Possible configurations of the observed bands have been discussed. The total Routhian surface calculations have been performed to study the shape of $^{197}$Tl for different configurations.

Neutron evaporation spectra at backward angles from ${}^{201}$Tl${}^{*}$, ${}^{185}$Re${}^{*}$, and ${}^{169}$Tm${}^{*}$ compound nuclei, having different ground-state deformations, have been measured at two excitation energies (${E}^{*}$ $\ensuremath{\sim}$ 37 and 26 MeV). The values of the inverse level density parameter ($k$), extracted at these excitations using statistical model calculations, are observed to decrease substantially at the lower excitation energy ($\ensuremath{\sim}$26 MeV) for nuclei having large ground-state deformation (residues of ${}^{185}$Re${}^{*}$ and ${}^{169}$Tm${}^{*}$), whereas for near-spherical nuclei (residues of ${}^{201}$Tl${}^{*}$), the $k$ value remains unchanged at the two energies. The decrease in $k$ at the lower excitation energy for the deformed systems amounts to a relative increase in nuclear level density, indicating a collective enhancement. The present observation clearly establishes the existence of a strong correlation between collectivity and ground-state deformation.

Inclusive energy spectra of the complex fragments (3 $\leq$ Z $\leq$ 5) emitted in the reactions $^{12}$C (77 MeV)+ $^{28}$Si, $^{11}$B (64 MeV)+ $^{28}$Si and $^{12}$C (73 MeV)+ $^{27}$Al (all having the same excitation energy of $ \sim$ 67 MeV), have been measured in the angular range of 10$^\circ$ $\lesssim \theta_{lab} \lesssim$ 60$^\circ$. The fully energy damped (fusion-fission) and the partially energy damped (deep inelastic) components of the fragment energy spectra have been extracted. It has been found that the yields of the fully energy damped fragments for all the above reactions are in conformity with the respective statistical model predictions. The time scales of various deep inelastic fragment emissions have been extracted from the angular distribution data. The angular momentum dissipation in deep inelastic collisions has been estimated from the data and it has been found to be close to the corresponding sticking limit value., Comment: 8 pages, 8 figures

We present theoretical results for electron emission in collisions between helium atoms and dressed projectiles at high energies. Double-differential cross sections (DDCSs) as a function of the emitted electron energies and angles are calculated. In our study we have applied the three-body formalism using the three-Coulomb wave (3CW–3B) model. The interaction between the dressed projectile and the active electron in the target has been approximated by a model potential having both a long-range Coulomb potential part and a short-range part. However, the active electron in the target has been treated as hydrogenic. We have also studied the projectile charge state dependence of the DDCS. Our theoretical results are compared with available experimental data as well as other theoretical calculations. The comparison shows a good agreement between the present calculations and the measurements. The obtained results are also compatible with other theoretical findings.

Time-dependent coupled Hartree–Fock theory was applied to estimate the allowed oscillator strengths of the astrophysically important highly stripped ions of the carbon isoelectronic sequence Ne4+, Mg6+, Si8+, S10+, Ar12+, Ca14+, and Ti16+ from their ground and valence-excited states up to principal quantum number n = 6. Oscillator-strength values compare favourably with the existing available data and many of the results are reported for the first time.

The interaction of an E/A=70 - MeV 7Be beam with a Be target was used to populate levels in 6Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the alpha+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in a good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range., 5 pages, 7 figures; Corrected typos

Elastic scattering of alpha particles from 208Pb at higher energy is explained by a microscopic and phenomenological alpha potential. The same complex potential is used to calculate the alpha width of 212Po from the principle of detailed balance. The alpha width of the ground state can be reproduced to within a factor of 3 of the experimental value.

We report final-state-exclusive measurements of the light charged fragments in coincidence with 26Ne residual nuclei following the direct two-proton removal from a neutron-rich 28Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triple- coincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially-correlated pair-removal mechanism. The fraction of such correlated events, 56(12) %, is consistent with the fraction of the calculated cross section, 64 %, arising from spin S = 0 two-proton configurations in the entrance-channel (shell-model) 28Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams., Comment: accepted for publication in Physical Review Letters

Time-dependent perturbation theory has been applied to calculate the doubly excited triplet statesNsns:3Se,Npnp:3De andNdnd:3Ge (N=2, 3, 4,n=N+1, ... ,5) for He, Li+, Be2+ and B3+. A time-dependent harmonic perturbation causes simulataneous excitation of both the electrons with a change of spin state. The doubly excited energy levels have been identified as the poles of an appropriately constructed linearized variational functional with respect to the driving frequency. In addition to the transition energies, effective quantum numbers of these doubly excited states have been calculated and analytic representations of their wave functions are obtained. These are utilized to estimate the Coulomb repulsion term for these states which checks the consistency of the wave functions. These wave functions may also be used for calculating other physical properties of the systems.

Application of the time-dependent coupled Hartree—Fock theory of many-electron systems like the ions in the argon isoelectronic sequence reveals the occurrence of a new resonance, hitherto unreported in the literature. This resonance is observed from K + onwards and falls within the normal dispersion region for the isoelectronic ions having Z ⩾ 20 and is believed to be due to the dynamic correlations present in such systems. In the neighbourhood of this resonance dynamic polarizability values exhibit complex structure. The oscillator strength associated with the transition is found to be low.

The interactions of an E/A=70-MeV {sup 9}C beam with a Be target was used to populate levels in Be, B, and C isotopes, which undergo decay into many-particle exit channels. The decay products were detected in the HiRA array and the level energies were identified from their invariant mass. Correlations between the decay products were examined to deduce the nature of the decays, specifically to what extent all the fragments were created in one prompt step or whether the disintegration proceeded in a sequential fashion through long-lived intermediate states. In the latter case, information on the spin of the level was also obtained. Of particular interest is the five-body decay of the {sup 8}C ground state, which was found to disintegrate in two steps of two-proton decay passing through the {sup 6}Be{sub g.s.} intermediate state. The isobaric analog of {sup 8}C{sub g.s.} in {sup 8}B was also found to undergo two-proton decay to the isobaric analog of {sup 6}Be{sub g.s.} in {sup 6}Li. A 9.69-MeV state in {sup 10}C was found to undergo prompt four-body decay to the 2p + 2{alpha} exit channel. The two protons were found to have a strong enhancement in the diproton region and the relativemore » energies of all four p-{alpha} pairs were consistent with the {sup 5}Li{sub g.s.} resonance.« less

Mass distribution of fission fragments and neutron multiplicity in the ${}^{16}\text{O}+{}^{238}$U reaction were measured at near- and below-barrier energies. A sudden change in the fragment mass width, observed in the present measurement, confirmed the transition to quasifission at below-barrier energies; the same was indicated earlier from the study of fission fragment angular anisotropy. However, the present measurement of prescission neutron multiplicity as well as the earlier measurement of evaporation residue yield did not indicate any significant departure from the respective statistical model predictions throughout the energy range. It is argued that the first two probes are more sensitive for highly asymmetric systems, whereas all probes would be useful and complimentary to each other for study of quasifission in more symmetric systems, where quasifission is more dominant.

$^{8}\mathrm{C}$ is found to decay to four protons and an $\ensuremath{\alpha}$ particle in two $2p$ emission steps. The correlations between the protons in the first step ($^{8}\mathrm{C}$ to $^{6}\mathrm{Be}$) exhibit a significant enhancement in the region of the decay phase space where the two protons have small relative energy, a region sometimes called the diproton region. The decay of the isobaric analog of $^{8}\mathrm{C}$ in $^{8}\mathrm{B}$ is also found to decay by $2p$ emission. This is the first case of isospin-allowed $2p$ decay between isobaric analog states.

The influence of alpha clustering on nuclear reaction dynamics is investigated using the giant dipole resonance (GDR) lineshape studies in the reactions 20Ne (Elab=145,160 MeV) + 12C and 20Ne (Elab=160 MeV) + 27Al, populating 32S and 47V, respectively. The GDR lineshapes from the two systems are remarkably different from each other. Whereas, the non alpha-like 47V undergoes Jacobi shape transition and matches exceptionally well with the theoretical GDR lineshape estimated under the framework rotating liquid drop model (RLDM) and thermal shape fluctuation model (TSFM) signifying shape equilibration, for alpha cluster 32S an extended prolate kind of shape is observed. This unusual deformation, seen directly via -decay for the first time, is predicted to be due to the formation of orbiting di-nuclear configuration or molecular structure of 16O+ 16O in 32S superdeformed band., 12 Pages, 5 Figures, Accepted for publication as Rapid Communication in Phys. Rev. C

Fission fragment mass distribution has been measured in two reactions to populate compound nu- cleus 246 Bk. Both the target nuclei were deformed. However, entrance channel mass asymmetry of the two sys- tems was on the either side of the Businaro Gallone mass asymmetry parameter. Near the Coulomb barrier, at similar excitation energies, the width of the fission fragment mass distribution was found to be significantly dif- ferent for the 14 N + 232 T h reaction compared to the 11 B + 235 U reaction. The entrance channel mass asymmetry was found to play a significant role in deciding the fusion process.

Fission fragment mass distributions have been measured from the decay of the compound nucleus 246 Bk that has been populated via two entrance channels. These entrance channels have a slight difference in their mass asymmetries that puts them on either side of the Businaro Gallone mass asymmetry parameter. Both target nuclei were deformed. Near the Coulomb barrier, at similar excitation energies, the width of the fission fragment mass distribution was found to be drastically different for the 14 N + 232 Th reaction compared to the 11 B + 235 U reaction. The entrance channel mass asymmetry was found to affect the fusion process sharply.

Fission fragment mass distribution has been measured from the decay of 246Bk nucleus populating via two entrance channels with slight difference in mass asymmetries but belonging on either side of the Businaro Gallone mass asymmetry parameter. Both the target nuclei were deformed. Near the Coulomb barrier, at similar excitation energies the width of the fission fragment mass distribution was found to be drastically different for the 14N+232Th reaction compared to the 11B+235U reaction. The entrance channel mass asymmetry was found to affect the fusion process sharply.

High energy {gamma} rays in coincidence with low energy yrast {gamma} rays have been measured from {sup 113}Sb, at excitation energies of 109 and 122 MeV, formed by bombarding {sup 20}Ne on {sup 93}Nb at projectile energies of 145 and 160 MeV, respectively, to study the role of angular momentum (J) and temperature (T) over giant dipole resonance (GDR) width ({gamma}). The maximum populated angular momenta for fusion were 67({Dirac_h}/2{pi}) and 73({Dirac_h}/2{pi}), respectively, for the above-mentioned beam energies. The high energy photons were detected using a Large Area Modular BaF{sub 2} Detector Array (LAMBDA) along with a 24-element multiplicity filter. After pre-equilibrium corrections, the excitation energy E* was averaged over the decay steps of the compound nucleus (CN). The average values of temperature, angular momentum, CN mass, etc., have been calculated using the statistical model code CASCADE. Using those average values, results show the systematic increase of GDR width with T, which is consistent with Kusnezov parametrization and the thermal shape fluctuation model (TSFM). The rise of GDR width with temperature also supports the assumptions of adiabatic coupling in the TSFM. But the GDR widths and corresponding reduced plots with J are not consistent with those of the theoretical modelmore » at high spins.« less

The reaction 27 Al(d,t) at 25 MeV was utilized to study the excited states of 26 Al. The angular distributions of the observed excited states of 26 Al were analyzed with zero range distorted wave Born approximation as well as by incorporating finite range correction parameters to extract spectroscopic factors. The two sets of extracted spectroscopic factors were compared with each other to see the effect of using finite range correction in the transfer form factor.