arXiv subjects

K. Ramachandran

Publications and source records attributed to K. Ramachandran.

At least 19 recordsLinked to original sources

arXiv2024

Observation of near-scission "polar" and "equatorial" proton emission in heavy-ion induced fission

Proton and $α$-particle energy spectra were measured in coincidence with fission fragments at different relative angles in $^{16}$O (96 MeV) + $^{232}$Th reaction. The multiplicity spectra were analyzed within the framework of a Moving Source Disentangling Analysis (MSDA) to determine contributions from different emission stages. The MSDA conclusively shows ``Near Scission Emission (NSE)" as an essential component in the multiplicity spectra. In contrast to NSE $α$ particles which emit mainly perpendicular (``equatorial emission"), the NSE protons are observed to be emitted perpendicular as well as parallel (``polar emission") to the fission axis with similar intensities ($\sim$20\% for each). Thus, around 40\% of total pre-scission protons are emitted near the scission stage, whereas the same fraction for $α$ particles is only around 10\%. The inevitable presence of ``polar" and ``equatorial" NSE protons in a heavy-ion induced fission has been observed for the first time. Present results open up a new avenue to study the heavy-ion induced fission dynamics.

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arXiv2024

Investigation of reaction and $α$ production cross sections with $^9$Be projectile

In order to investigate the contribution of $α$ production in the reaction cross sections, measurements of elastic scattering and inclusive $α$ particle angular distributions have been carried out with the $^9$Be projectile on $^{89}$Y, $^{124}$Sn, $^{159}$Tb, $^{198}$Pt, and $^{209}$Bi targets over a wide angular range at energies near the Coulomb barrier. The measured elastic scattering angular distributions were fitted with optical model calculations, and reaction cross sections were extracted. The same data were also analysed using both global optical model potentials (Global OMP) and microscopic S$\tilde{a}$o Paulo potentials (SPP), to obtain the reaction cross sections. The data available in the literature for $^9$Be projectile includes the elastic scattering angular distributions, $α$ production cross sections, and complete fusion cross sections on these and other targets at several energies are also utilised for comparative studies. The reaction cross section extracted from the three potentials (Best Fit, Global OMP and SPP) are in reasonable agreement for all the targets except for the energies below the barrier where the results from SPP deviate by 30-50 \%. Inclusive $α$ particle production cross sections were also extracted by integrating the $α$ particle angular distributions. The present data and data available from literature of reaction and $α$-particle production cross sections were utilised to make systematic studies. Systematics of reaction and $α$-particle production cross sections revealed their universal behaviour.

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arXiv2024

Fusion of $^{7}$Li with $^{205}$Tl at near barrier energies

The complete and incomplete fusion cross sections for the $^{7}$Li+$^{205}$Tl reaction were measured at near barrier energies by online characteristic $γ$ ray detection technique. The complete fusion (CF) cross sections at energies above the Coulomb barrier were found to be suppressed by $\sim$ 26 \% compared to the coupled channel calculations. Reduced fusion cross sections for the present system at energies normalised to the Coulomb barrier were also found to be systematically lower than those with strongly bound projectiles forming a similar compound nucleus. The suppression observed in CF cross sections is found to be commensurate with the measured total incomplete fusion (ICF) cross sections. In the ICF cross sections, t capture is found to be dominant than $α$ capture at all the measured energies. The systematic study of available CF, ICF and total fusion (TF) data with $^7$Li projectile is performed.

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arXiv2022

Systematic investigation of channel coupling effects on elastic, inelastic and neutron transfer channels in $^6$Li+$^{159}$Tb

Elastic scattering angular distribution for weakly bound nucleus $^{6}$Li on the deformed rare earth $^{159}$Tb target nucleus has been measured at energies around the Coulomb barrier. The elastic scattering cross sections for this reaction consist of inelastic contributions from low lying excited states of $^{159}$Tb. The pure elastic cross-sections have been extracted from the admixture of elastic and inelastic data. The optical model potential parameters for the system have been obtained from the extracted pure elastic scattering cross sections. Coupled channel calculations have been performed with this set of potential parameters, to compare the theoretical and experimental inelastic scattering cross sections. The work has been extended to obtain the spectroscopic factor for $^{158}$Tb+n configuration from the experimental 1n-pickup data.

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arXiv2022

Reaction mechanism study for multinucleon transfer processes in collisions of spherical and deformed nuclei at energies near and above the Coulomb barrier: The $^{16}$O+$^{154}$Sm reaction

Background: Multinucleon transfer reactions at energies around the Coulomb barrier offer a vital opportunity to study rich physics of nuclear structure and dynamics. Despite the continuous development in the field, we have still limited knowledge about how deformation - one of the representative nuclear structures - affects multinucleon transfer reactions. Purpose: To shed light on the effect of deformation in multinucleon transfer processes, we study the $^{16}$O+$^{154}$Sm reaction at $E_{\rm lab}$=85 MeV (around the Coulomb barrier) and 134 MeV (substantially above the Coulomb barrier), where the target nucleus, $^{154}$Sm, is a well-established, deformed nucleus. Results: Angular distributions for elastic scattering and for various transfer channels were measured over a wide angular range at the BARC-TIFR pelletron-Linac accelerator facility, Mumbai. The $Q$-value- and angle-integrated isotope production cross sections have been extracted from the measured angular distributions. The experimental data are analyzed along with time-dependent Hartree-Fock calculations. For the lower incident energy case, we find a reasonable agreement between the measurements and the TDHF calculations for a-few-nucleon transfer channels; whereas TDHF underestimates cross sections for many-nucleon transfers, consistent with earlier works. On the other side, we find that calculated cross sections for secondary reaction products for the higher incident energy case, qualitatively explains the measured trends of isotopic distributions observed for the lower energy. The latter observation indicates possible underestimation of excitation energies in the present TDHF+GEMINI analysis. Although certain orientation effects were observed in TDHF results, it is difficult to disentangle them from the $Q$-value- and angle-integrated production cross sections. (Shortened due to the arXiv's length limit.)

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arXiv2021

Neutron transfer in $^{9}$Be + $^{159}$Tb system

One neutron stripping cross sections ($σ_{-1n}$) are measured in $^{9}$Be+$^{159}$Tb system in the energy range E$_{cm}$/V$_{B}$ $\sim$ 0.79 - 1.24 using offline gamma counting technique. The CRC model calculations including the ground state and the 2$^{+}$ resonance state of $^{8}$Be, carried out using the FRESCO code, give a reasonable description of the measured data. In addition, comparisons of reduced 1n-stripping cross sections- $σ_{red}$ with $^{9}$Be for different target nuclei (A $\sim$150-200), and $σ_{red}$ for $^{9}$Be, $^{6}$Li with $^{159}$Tb target are presented. While no strong target dependence is observed with $^{9}$Be projectile, $σ_{red}$($^{9}$Be) is significantly larger than that for $^{6}$Li, which is consistent with the Q-value for transfer reactions and breakup threshold energy of projectiles.

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arXiv2021

Unraveling the reaction mechanism for large alpha production and incomplete fusion in reactions involving weakly bound stable nuclei

The origin of the large $α$ particle production and incomplete fusion in reactions involving weakly-bound $α$+$x$ cluster nuclei still remains unresolved. While the (two-step) process of breakup followed by capture of the ``free" complementary fragment ($x$) is widely believed to be responsible, a few recent studies suggest the dominant role of (direct) cluster stripping. To achieve an unambiguous experimental discrimination between these two processes, a coincidence measurement between the outgoing $α$ particles and $γ$ rays from the heavy residues has been performed for the $^7$Li($α$+triton)+$^{93}$Nb system. Proper choice of kinematical conditions allowed for the first time a significant population of the region accessible only to the direct triton stripping process and not to breakup followed by the capture of the ``free'' triton (from the three-body continuum). This result, also supported by a cluster-transfer calculation, clearly establishes the dominance of the direct cluster-stripping mechanism in the large alpha production.

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arXiv2021

Fusion of 16O+165Ho at deep sub-barrier energies

Fusion cross-sections have been measured for the asymmetric system 16O+165Ho at energies near and deep below the Coulomb barrier with an aim to investigate the occurrence of fusion hindrance for the system. Fusion cross sections down to ~ 700 nb have been measured using the off-beam gamma-ray technique. The fusion cross sections have been compared with the coupled channel calculations. Although the onset of fusion hindrance could not be observed experimentally, an indication of a small deviation of the experimental fusion cross-sections with respect to the calculated cross-sections could be observed at the lowest energy measured. However, the energy onset of fusion hindrance has been obtained from the extrapolation technique and is found to be about 2 MeV below the lowest energy of the present measurement.

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arXiv2021

Investigating neutron transfer in the $^{9}$Be + $^{197}$Au system

In this work $\textit{n}$-transfer and incomplete fusion cross sections for $^{9}$Be + $^{197}$Au system are reported over a wide energy range, E$_{c.m.}$ $\approx$ 29-45 MeV. The experiment was carried out using activation technique and off-line gamma counting. The transfer process is found to be the dominant mode as compared to all other reaction channels. Detailed coupled reaction channel (CRC) calculations have been performed for $\textit{n}$-transfer stripping and pickup cross sections. The measured 1$\textit{n}$-stripping cross sections are explained with CRC calculations by including the ground state and the 2$^{+}$ resonance state (E = 3.03 MeV) of $^{8}$Be. The calculations for 1$\textit{n}$-pickup, including only the ground state of $^{10}$Be agree reasonably well with the measured cross sections, while it overpredicts the data at subbarrier energies. For a better insight into the role of projectile structure in the transfer process, a comprehensive analysis of 1$\textit{n}$-stripping reaction has been carried out for various weakly bound projectiles on $^{197}$Au target nucleus. The transfer cross sections scaled with the square of total radius of interacting nuclei show the expected Q-value dependence of 1$\textit{n}$-stripping channel for weakly bound stable projectiles.

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arXiv2020

Competing quasifission and asymmetric fusion-fission in neutron-deficient sub-lead nuclei

To disentangle the role of shell effects and dynamics, fission fragment mass distributions of $^{191}$Au, a nucleus in the newly identified island of mass asymmetric fission in the sub-lead region, have been measured down to excitation energy of $\approx$20 MeV above the fission barrier via two different entrance channels, viz. $^{16}$O+$^{175}$Lu and $^{37}$Cl+$^{154}$Sm reactions. Apart from having signature of the shell effects in both the cases, clear experimental evidence of quasifission has been observed in the mass distributions of the Cl induced reaction, that has also been substantiated by the theoretical calculations. This crucial evidence along with a systematic analysis of available experimental data has revealed that the dynamics in the entrance channel has significant influence on most of the reactions used earlier to explore the persistence of recently discovered mass asymmetry in $β$-delayed fission at low energy in this mass region, ignoring which might lead to ambiguity in interpreting the heavy-ion data.

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arXiv2019

Fusion of Borromean nucleus $^{9}$Be with $^{197}$Au target at near barrier energies

To probe the role of the intrinsic structure of the projectile on sub-barrier fusion, measurement of fusion cross sections has been carried out in $^{9}$Be + $^{197}$Au system in the energy range E$_{c.m.}$/V$_B$ $\approx$ 0.82 to 1.16 using off-beam gamma counting method. Measured fusion excitation function has been analyzed in the framework of the coupled-channel approach using CCFULL code. It is observed that the coupled-channel calculations, including couplings to the inelastic state of the target and the first two states of the rotational band built on the ground state of the projectile, provide a very good description of the sub-barrier fusion data. At above barrier energies, the fusion cross section is found to be suppressed by $\approx$ 39(2)\% as compared to the coupled-channel prediction. A comparison of reduced excitation function of $^{9}$Be + $^{197}$Au with other $x$ + $^{197}$Au shows a larger enhancement for $^9$Be in the sub-barrier region amongst Z=2-5 weakly and tightly bound projectiles, which indicates the prominent role of the projectile deformation in addition to the weak binding.

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arXiv2019

The closed range property for the $\overline{\partial}$-operator on planar domains

Let $Ω\subset\mathbb{C}$ be an open set. We show that $\overline{\partial}$ has closed range in $L^{2}(Ω)$ if and only if the Poincaré-Dirichlet inequality holds. Moreover, we give necessary and sufficient potential-theoretic conditions for the $\overline{\partial}$-operator to have closed range in $L^{2}(Ω)$. We also give a new necessary and sufficient potential-theoretic condition for the Bergman space of $Ω$ to be infinite dimensional.

math.CV↗

arXiv2019

Interpretation of coincidence data from strip detectors and extraction of absolute cross section using 3-body Monte Carlo simulation method

Accurate knowledge of the response of the detection system is very crucial for unambiguous interpretation of the experimental data. A simulation code has been developed using the Monte Carlo technique involving 3-body kinematics for the analysis of data obtained with segmented large area Si $ΔE-E$ detector telescopes in nuclear reaction measurements. Care was taken in the analysis to maximize the angular coverage and statistics. The emphasis is placed to extract the absolute cross sections of the different reaction processes, for which coincident measurements are unavoidable. The estimated detection efficiency of different coincidence events are found to depend on various parameters, e.g., the relative energy of the breakup fragments, incident beam energy of the projectile, ground state $Q$-value of the reaction, the excitation of the ejectile as well as target like nuclei, mass asymmetry of the breakup fragments, detection threshold, and geometric solid angle of the detection setup. The interpretation of the various observables from the exclusive measurements of breakup and transfer breakup reactions is reported.

physics.ins-det↗

arXiv2018

Particle identification using digital pulse shape discrimination in a nTD silicon detector with a 1 GHz sampling digitizer

In beam test experiments have been carried out for particle identification using digital pulse shape analysis in a 500~$μ$m thick Neutron Transmutation Doped (nTD) silicon detector with an indigenously developed FPGA based 12 bit resolution, 1 GHz sampling digitizer. The nTD Si detector was used in a low-field injection setup to detect light heavy-ions produced in reactions of $\sim$ 5 MeV/A $^{7}$Li and $^{12}$C beams on different targets. Pulse height, rise time and current maximum have been obtained from the digitized charge output of a high bandwidth charge and current sensitive pre-amplifier. Good isotopic separation have been achieved using only the digitized charge output in case of light heavy-ions. The setup can be used for charged particle spectroscopy in nuclear reactions involving light heavy-ions around the Coulomb barrier energies.

physics.ins-det↗

arXiv2018

Deep-inelastic multinucleon transfer processes in the $^{16}$O+$^{27}$Al reaction

The reaction mechanism of deep-inelastic multinucleon transfer processes in the $^{16}$O+$^{27}$Al reaction at an incident $^{16}$O energy ($E_{\rm lab}=134$ MeV) substantially above the Coulomb barrier has been studied both experimentally and theoretically. Elastic-scattering angular distribution, total kinetic energy loss spectra and angular distributions for various transfer channels have been measured. The $Q$-value- and angle-integrated isotope production cross sections have been deduced. To obtain deeper insight into the underlying reaction mechanism, we have carried out a detailed analysis based on the time-dependent Hartree-Fock (TDHF) theory. A recently developed method, TDHF+GEMINI, has been applied to evaluate production cross sections for secondary products. From a comparison between the experimental and theoretical cross sections, we find that the theory qualitatively reproduces the experimental data. Significant effects of secondary light-particle emissions are demonstrated. Possible interplay between fusion-fission, deep-inelastic, multinucleon transfer and particle evaporation processes are discussed.

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arXiv2018

Investigation of complete and incomplete fusion in $^{7}$Li+$^{124}$Sn reaction around Coulomb barrier energies

The complete and incomplete fusion cross sections for $^{7}$Li+$^{124}$Sn reaction were measured using online and offline characteristic $γ$-ray detection techniques. The complete fusion (CF) cross sections at energies above the Coulomb barrier were found to be suppressed by $\sim$ 26 \% compared to the coupled channel calculations. This suppression observed in complete fusion cross sections is found to be commensurate with the measured total incomplete fusion (ICF) cross sections. There is a distinct feature observed in the ICF cross sections, i.e., $\textit{t}$-capture is found to be dominant than $α$-capture at all the measured energies. A simultaneous explanation of complete, incomplete and total fusion (TF) data was also obtained from the calculations based on Continuum Discretized Coupled Channel method with short range imaginary potentials. The cross section ratios of CF/TF and ICF/TF obtained from the data as well as the calculations showed the dominance of ICF at below barrier energies and CF at above barrier energies.

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arXiv2018

Convexity of level lines of Martin functions and applications

Let $Ω$ be an unbounded domain in $\mathbb{R}\times\mathbb{R}^{d}.$ A positive harmonic function $u$ on $Ω$ that vanishes on the boundary of $Ω$ is called a Martin function. In this note, we show that, when $Ω$ is convex, the superlevel sets of a Martin function are also convex. As a consequence we obtain that if in addition $Ω$ is symmetric, then the maximum of any Martin function along a slice $Ω\cap (\{t\}\times\mathbb{R}^d)$ is attained at $(t,0).$

math.AP↗

arXiv2017

Exploring Zeptosecond Quantum Equilibration Dynamics: From Deep-Inelastic to Fusion-Fission Outcomes in $^{58}$Ni+$^{60}$Ni Reactions

Energy dissipative processes play a key role in how quantum many-body systems dynamically evolve towards equilibrium. In closed quantum systems, such processes are attributed to the transfer of energy from collective motion to single-particle degrees of freedom; however, the quantum many-body dynamics of this evolutionary process are poorly understood. To explore energy dissipative phenomena and equilibration dynamics in one such system, an experimental investigation of deep-inelastic and fusion-fission outcomes in the $^{58}$Ni+$^{60}$Ni reaction has been carried out. Experimental outcomes have been compared to theoretical predictions using Time Dependent Hartree Fock and Time Dependent Random Phase Approximation approaches, which respectively incorporate one-body energy dissipation and fluctuations. Excellent quantitative agreement has been found between experiment and calculations, indicating that microscopic models incorporating one-body dissipation and fluctuations provide a potential tool for exploring dissipation in low-energy heavy ion collisions.

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