arXiv subjects
Publications and source records attributed to C. Samanta.
We discuss the usefulness of the double charge-exchange reactions (DCX) for the production of the neutron-rich Lambda-hypernuclei. We believe the (pi-,K+) reaction is one of the most promising DCX reactions, and propose to produce the neutron-rich Lambda-hypernuclei, 6(Lambda)H and 9(Lambda)He, at the J-PARC 50 GeV PS by the reaction (J-PARC E10 experiment). The design of the experiment is presented.
Magic islands for extra-stable nuclei in the midst of the sea of fission-instability were predicted to be around Z=114, 124 or, 126 with N=184, and Z=120, with N=172. Whether these fission-survived superheavy nuclei with high Z and N would live long enough for detection or, undergo alpha-decay in a very short time remains an open question. Alpha-decay half lives of nuclei with 130 < Z < 100 have been calculated in a WKB framework using density-dependent M3Y interaction with Q-values from different mass formulae. The results are in excellent agreement with the experimental data. Fission survived Sg nuclei with Z=106, N=162 is predicted to have the highest alpha-decay half life ~ 3.2 hrs in the Z=106-108, N=160-164 region called, small island/peninsula. Superheavy nuclei with Z > 118 are found to have alpha-decay half lives of the order of microseconds or, less.
Gamow-Teller transitions from 24Mg to 24Na were studied via the (t,3He) reaction at 115 AMeV using a secondary triton beam produced via fast fragmentation of 150 AMeV 16O ions. Compared to previous (t,3He) experiments at this energy that employed a primary alpha beam, the secondary beam intensity is improved by about a factor of five. Despite the large emittance of the secondary beam, an excitation-energy resolution of ~200 keV is achieved. A good correspondence is found between the extracted Gamow-Teller strength distribution and those available from other charge-exchange probes. Theoretical calculations using the newly developed USDA and USDB sd-shell model interactions reproduce the data well.
A mean field calculation is carried out to obtain the equation of state (EoS) of nuclear matter from a density dependent M3Y interaction (DDM3Y). The energy per nucleon is minimized to obtain ground state of the symmetric nuclear matter (SNM). The constants of density dependence of the effective interaction are obtained by reproducing the saturation energy per nucleon and the saturation density of SNM. The energy variation of the exchange potential is treated properly in the negative energy domain of nuclear matter. The EoS of SNM, thus obtained, is not only free from the superluminosity problem but also provides excellent estimate of nuclear incompressibility. The EoS of asymmetric nuclear matter is calculated by adding to the isoscalar part, the isovector component of M3Y interaction. The SNM and pure neutron matter EoS are used to calculate the nuclear symmetry energy which is found to be consistent with that extracted from the isospin diffusion in heavy-ion collisions at intermediate energies. The $β$ equilibrium proton fraction calculated from the symmetry energy and related theoretical findings are consistent with the constraints derived from the observations on compact stars.
Theoretical estimates for the half lives of about 1700 isotopes of heavy elements with Z from 100 to 130 are tabulated using theoretical Q-values. The quantum mechanical tunneling probabilities are calculated within a WKB framework using microscopic nuclear potentials. The microscopic nucleus-nucleus potentials are obtained by folding the densities of interacting nuclei with a density dependent M3Y (DDM3Y) effective nucleon-nucleon interaction. The alpha-decay half lives calculated in this formalism using the experimental Q-values were found to be in good agreement over a wide range of experimental data spanning about twenty orders of magnitude. The theoretical Q-values used for the present calculations are extracted from three different mass estimates viz. Myers-Swiatecki [MS], Muntian-Hofmann-Patyk-Sobiczewski [M] and Koura-Tachibana-Uno-Yamada [KUTY].
The existence of long lived superheavy nuclei (SHN) is controlled mainly by spontaneous fission and $α$-decay processes. According to microscopic nuclear theory, spherical shell effects at Z=114, 120, 126 and N=184 provide the extra stability to such SHN to have long enough lifetime to be observed. To investigate whether the so-called "stability island" could really exist around the above Z, N values, the $α$-decay half lives along with the spontaneous fission and $β$-decay half lives of such nuclei are studied. The $α$-decay half lives of SHN with Z=102-120 are calculated in a quantum tunneling model with DDM3Y effective nuclear interaction using $Q_α$ values from three different mass formulae prescribed by Koura, Uno, Tachibana, Yamada (KUTY), Myers, Swiatecki (MS) and Muntian, Hofmann, Patyk, Sobiczewski (MMM). Calculation of spontaneous fission (SF) half lives for the same SHN are carried out using a phenomenological formula and compared with SF half lives predicted by Smolanczuk {\it et al}. Possible source of discrepancy between the calculated $α$-decay half lives of some nuclei and the experimental data of GSI, JINR-FLNR, RIKEN are discussed. In the region of Z=106-108 with N$\sim$ 160-164, the $β$-stable SHN $^{268}_{106}Sg_{162}$ is predicted to have highest $α$-decay half life ($T_α\sim 3.2hrs$) using $Q_α$ value from MMM. Interestingly, it is much greater than the recently measured $T_α$ ($\sim 22s$) of deformed doubly magic $^{270}_{108}Hs_{162}$ nucleus. A few fission-survived long-lived SHN which are either $β$-stable or having large $β$-decay half lives are predicted to exist near $^{294}110_{184}$, $^{293}110_{183}$, $^{296}112_{184}$ and $^{298}114_{184}$. These nuclei might decay predominantly through $α$-particle emission.
A generalized mass formula is used to calculate the neutron and proton drip lines of normal and lambda hypernuclei treating non-strange and strange nuclei on the same footing. Calculations suggest existence of several bound hypernuclei whose normal cores are unbound. Addition of Lambda or, Lambda-Lambda hyperon(s) to a normal nucleus is found to cause shifts of the neutron and proton driplines from their conventional limits.
The $α$-decay half lives of nuclei in the decay from element $^{277}112$ are calculated in a WKB framework using DDM3Y interaction and experimental Q-values. Theoretical estimation of half lives in the same quantum tunneling model, using Q-values from the mass formula of Muntian-Hofmann-Patyk-Sobiczewski, are also presented. Calculated results furnish corroborating evidence for the experimental findings at RIKEN and GSI. Certain discrepancies indicate necessity of a better mass formula. Further experimental data with higher statistics would also be useful.
Theoretical estimates for the lifetimes of several isotopes of heavy elements with Z=102-120 are presented by calculating the quantum mechanical tunneling probability in a WKB framework and using microscopic nucleus-nucleus potential obtained by folding the densities of interacting nuclei with the DDM3Y effective nuclear interaction. The $α$-decay half lives calculated in this formalism using the experimental $Q$-values are in good agreement over a wide range of experimental data. Half lives are also calculated using $Q$-values extracted from two mass formulae. The Viola-Seaborg-Sobiczewski (VSS) estimates of $α$-decay half lives with the same $Q$-values are presented for comparison. The half life calculations are found to be quite sensitive to the choice of $Q$-values. Comparison with the experimental data delineates the inadequacies of older mass predictions in the domain of heavy and superheavy elements as compared to the newer one by Muntian-Hofmann-Patyk-Sobiczewski, and highlights necessity of a more accurate mass formula which can predict $Q$-values with even higher precision.
Theoretical estimates of $α$-decay half lives of several nuclei in the decay from element 113 are presented. Calculations in a WKB framework using DDM3Y interaction and experimental Q-values are in good agreement with the experimental data. Half life calculations are found to be quite sensitive to the Q-values and angular momenta transfers. Calculated decay lifetime decreases, owing to more penetrability as well as thinner barrier, as Q-value increases. Deviations to this predominant behaviour observed in some recent experimental data may be attributed to non zero spin-parities in some cases.
An axial gas ionization chamber has been fabricated for use as a $ΔE$ detector in heavy ion induced nuclear reactions. Different operating parameters such as gas type, pressure, anode voltage and anode structures have been optimized. The transparency of the anode structure is observed to play an important role in improving the energy resolution of the detector.
A mean field calculation for obtaining the equation of state (EOS) for symmetric nuclear matter from a density dependent M3Y interaction supplemented by a zero-range potential is described. The energy per nucleon is minimized to obtain the ground state of symmetric nuclear matter. The saturation energy per nucleon used for nuclear matter calculations is determined from the co-efficient of the volume term of Bethe-Weizsäcker mass formula which is evaluated by fitting the recent experimental and estimated atomic mass excesses from Audi-Wapstra-Thibault atomic mass table by minimizing the mean square deviation. The constants of density dependence of the effective interaction are obtained by reproducing the saturation energy per nucleon and the saturation density of spin and isospin symmetric cold infinite nuclear matter. The EOS of symmetric nuclear matter, thus obtained, provide reasonably good estimate of nuclear incompressibility. Once the consants of density dependence are determined, EOS for asymmetric nuclear matter is calculated by adding to the isoscalar part, the isovector component of the M3Y interaction that do not contribute to the EOS of symmetric nuclear matter. These EOS are then used to calculate the pressure, the energy density and the velocity of sound in symmetric as well as isospin asymmetric nuclear matter.
Modification of neutron and proton driplines by the capture of strange hyperon(s) by normal nuclei has been investigated. A generalised mass formula (BWMH) based on the strangeness dependent extended liquid drop model is used to calculate the binding energy of normal nuclei as well as strange hypernuclei. The neutron (Sn) and proton (Sp) separation energies of all hypernuclei with neutral hyperons Lambda, double Lambda or charged hyperons Cascade(-), Theta(+) inside are calculated using BWMH mass formula. The normal neutron and proton driplines get modified due to the addition of the hyperon(s)(Lambda, double Lambda, Cascade(-), Theta(+) etc.) to the core of normal nuclei. The hypernuclei containing the charged hyperon(s) like those with neutral hyperon(s) have similar nucleon separation energies like core nuclei if proton number instead of net charge is used in the symmetry term. Due to the effect of opposite charges present in Theta(+) and Cascade(-), hyperons their corresponding driplines get separated from each other. All the hyperons modify mean field potential due to strong hyperon-nucleon coupling. Addition of a single charged hyperon in normal nuclei affects the entire proton drip line more prominently than that by neutral hyperon. The neutral hyperonic effect on proton dripline is significant for lighter nuclei than for heavier ones whereas both the charged as well as neutral hyperons affect almost the entire neutron dripline.
Simultaneous description of ordinary and hypernuclei masses by a single mass formula has been a great challenge in nuclear physics. Hyperon-separation energies of about forty Lambda($Λ$), three Lambda-Lambda($ΛΛ$), one Sigma($Σ$) and seven Cascade($Ξ$) hypernuclei have been experimentally found. Many of these nuclei are of light masses. We prescribe a new mass formula, called BWMH, which describes the normal and hypernuclei on the same footing. It is based on the modified-Bethe-Weizsäcker mass formula (BWM). BWM is basically an extension of the Bethe-Weizsäcker mass formula (BW) for light nuclei. The parameters of BWM were optimized by fitting about 3000 normal nuclei available recently. The original Bethe-Weizsäcker mass formula (BW) was designed for medium and heavy mass nuclei and it fails for light nuclei. Two earlier works on hypernuclei based on this BW show some limitations. The BWMH gives improved agreement with the experimental data for the line of stability, one-neutron separation energy versus neutron number spectra of normal nuclei, and the hyperon-separation energies from hypernuclei. The drip lines are modified for addition of a $Λ$ hyperon in a normal nucleus.
A simultaneous description of non-strange nuclei and hypernuclei is provided by a single mass formula inspired by the spin-flavour SU(6) symmetry breaking. This formula is used to estimate the hyperon binding energies of Lambda, double Lambda, Sigma, Cascade and Theta hypernuclei. The results are found to be in good agreement with the available experimental data on 'bound' nuclei and relativistic as well as quark mean field calculations. This mass formula is useful to estimate binding energies over a wide range of masses including the light mass nuclei. It is not applicable for repulsive potential.
The lifetimes of $α$ decays of the recently produced isotopes of the elements 112, 114, 116 and the element $^{294}118$ and of some decay products have been calculated theoretically within the WKB approximation using microscopic $α$-nucleus interaction potentials. These nuclear potentials have been obtained by folding the densities of the $α$ and the daughter nuclei with the M3Y effective interaction, supplemented by a zero-range pseudo-potential for exchange along with the density dependence. Spherical charge distributions have been used for calculating the Coulomb interaction potentials. These calculations provide reasonable estimates for the observed $α$ decay lifetimes and thus provide reliable predictions for other superheavies.
The proton radioactivity lifetimes of spherical proton emitters from the ground and the isomeric states are calculated using the microscopic nucleon-nucleus interaction potentials. The daughter nuclei density distributions are folded with a realistic density dependent M3Y effective interaction supplemented by a zero-range pseudo-potential. The density dependence parameters of the interaction are extracted from the nuclear matter calculations. The saturation energy per nucleon used for nuclear matter calculations is determined from the co-efficient of the volume term of Bethe-Weizsacker mass formula which is evaluated by fitting the recent experimental and estimated atomic mass excesses from Audi-Wapstra-Thibault atomic mass table by minimizing the mean square deviation. The quantum mechanical tunneling probability is calculated within the WKB approximation. Spherical charge distributions are used for calculating the Coulomb interaction potentials. These calculations provide good estimates for the observed proton radioactivity lifetimes.
Half lives of the decays of spherical nuclei away from proton drip line by proton emissions are estimated theoretically. The quantum mechanical tunneling probability is calculated within the WKB approximation. Microscopic proton-nucleus interaction potentials are obtained by single folding the densities of the daughter nuclei with M3Y effective interaction supplemented by a zero-range pseudo-potential for exchange along with the density dependence. Strengths of the M3Y interaction are extracted by fitting its matrix elements in an oscillator basis to those elements of the G-matrix obtained with the Reid-Elliott soft-core nucleon-nucleon interaction. Parameters of the density dependence are obtained from the nuclear matter calculations. Spherical charge distributions are used for calculating the Coulomb interaction potentials. These calculations provide reasonable estimates for the observed proton radioactivity lifetimes of proton rich nuclei for proton emissions from 26 ground and isomeric states of spherical proton emitters.