Measured Evaporation Residue Research Articles

Effects of isospin and shell on probing nuclear dissipation with survival probabilities of heavy systems

Using the Langevin model, we calculate the survival probability (Peva) of 228U, 234U and 240U as a function of the presaddle dissipation strength (β). It is shown that Peva displays a greater sensitivity to β with increasing the isospin of the U system and with decreasing excitation energy and angular momentum, and that shell effects can significantly enhance the sensitivity. Furthermore, we compare calculated and measured evaporation residue cross sections of heavy compound nuclei 224Th produced in the 16O + 208Pb reaction and find that the best-fit value of β required to satisfactorily describe the measured data has an apparent drop as shell effects are taken into account in the calculation compared to that neglecting the effects, clearly indicating a strong influence of shell effects on the precise determination of β. These results demonstrate that to more stringently constrain the strength of presaddle dissipation with survival probabilities of heavy nuclei, on the experimental side, it is best to produce high-isospin compound systems with low energy and low spin; theoretically, it is important to account for the shell effects in the calculations.

Entrance channel dependent hot fusion reactions for superheavy element synthesis

We have studied the compound nucleus probability, the survival probability, and the evaporation residue cross sections for 6645 projectile-target combinations to synthesize the superheavy nuclei with atomic numbers $Z=110$--126 in the light of the entrance channel effects such as mass asymmetry, charge asymmetry, isospin asymmetry, non-compound nuclear fission probability, and Businaro-Gallone mass asymmetry. The role of quasifission in every reaction has also been considered. Decay chains of various superheavy nuclei produced from the reactions are portrayed as well. The most striking results are that the measured evaporation residue cross sections are of the order of picobarns, whereas the proposed reactions can yield up to microbarn cross sections for the superheavy nuclei with $110\ensuremath{\le}Z\ensuremath{\le}126$. We suggest future experiments may utilize these hot fusion reactions to synthesize new elements ($Zg118$) or to study the properties of the known superheavy nuclei in greater detail. In particular, a proposed reaction containing both projectiles and targets that are naturally abundant, for example, $_{36}^{82}\mathrm{Kr}+_{90}^{232}\mathrm{Th}\ensuremath{\rightarrow}^{314}126+0n$, yields evaporation residue cross sections as high as 31 nb. The presently available experimental setup can test the prediction with ease to make a dream come true by putting a footstep into the eighth row of the periodic table.

Evaporation residue measurements of compound nuclei in the A≈200 region

Evaporation residue measurements of compound nuclei in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>A</mml:mi><mml:mo>≈</mml:mo><mml:mn>200</mml:mn></mml:mrow></mml:math> region

Search for stabilizing effects of the Z=82 shell closure against fission

Presence of closed proton and/or neutron shells causes deviation from macroscopic properties of nuclei which are understood in terms of the liquid drop model. It is important to investigate experimentally the stabilizing effects of shell closure, if any, against fission. This work aims to investigate probable effects of proton shell ($Z = 82$) closure in the compound nucleus, in enhancing survival probability of the evaporation residues formed in heavy ion-induced fusion-fission reactions. Evaporation residue cross sections have been measured for the reactions $^{19}$F+$^{180}$Hf, $^{19}$F+$^{181}$Ta and $^{19}$F+$^{182}$W from $\simeq9\%$ below to $\simeq42\%$ above the Coulomb barrier, leading to formation of compound nuclei with same number of neutrons ($N = 118$) but different number of protons across $Z = 82$. Measured excitation functions have been compared with statistical model calculation, in which reduced dissipation coefficient is the only adjustable parameter. Evaporation residue cross section, normalized by capture cross section, is found to decrease gradually with increasing fissility of the compound nucleus. Measured evaporation residue cross sections require inclusion of nuclear viscosity in the model calculations. Reduced dissipation coefficient in the range of 1\textendash3 $\times$ $10^{21}$ s$^{-1}$ reproduces the data quite well. No abrupt enhancement of evaporation residue cross sections has been observed in the reaction forming compound nucleus with $Z = 82$. Thus, this work does not find enhanced stabilizing effects of $Z = 82$ shell closure against fission in the compound nucleus. One may attempt to measure cross sections of individual exit channels for further confirmation of our observation.

Distinguishing Features of Radioactive Compound Nucleus Decays within the Dynamical Cluster Decay Model

In this paper, we are interested to study the distinguishing features of the decaying radioactive compound nuclei 246Bk* and 220Th*, using the Dynamical Cluster-decay Model (DCM) with deformation β and non-coplanar degree-of-freedom Φ. 246Bk* and 220Th* have so-far been studied within the DCM, using quadrupole deformations (β2i), “optimum” orientations (θopt) of the two nuclei lying in the same plane (Φ=0o), which shows that there is a non-compound nucleus (nCN) content in the observed data. The first turning point Ra (equivalently, the neck-length ∆R in Ra=R1+R2+∆R), which fixes both the preformation and penetration paths, is used to best fit the measured evaporation residue (ER) and fusion-fission (ff) cross sections, σER, σff, respectively, in 220Th* and 246Bk*, formed via different entrance channels. In this work, we subsequently add higher multipole deformations, the octupole and hexadecupole (β3i, β4i), `compact’ orientations θci and Φ≠00, and look for their effects on the nCN contribution predicted by the DCM calculations referenced above.

Survival-mediated capture and fusion cross sections for heavy-element synthesis

The cross section for producing a heavy evaporation residue ${\ensuremath{\sigma}}_{\mathrm{EVR}}$ in a fusion reaction can be written as a product of three nonseparable factors, i.e., the capture cross section, the fusion probability ${P}_{\mathrm{CN}}$, and the survival probability ${W}_{\text{sur}}$. Each of these factors is dependent on the spin. However, one must remember that the ${W}_{\text{sur}}$ term is zero or very small for higher spin values, thus effectively limiting the capture and fusion terms. For a series of $\ensuremath{\sim}287$ reactions leading to heavy evaporation residues with ${Z}_{\text{CN}}\ensuremath{\le}110$, we point out the implications of this fact for capture cross sections for heavy element formation reactions. From a comparison of calculated and measured evaporation residue cross sections we deduce values of the fusion probability ${P}_{\text{CN}}$ for some of these reactions.

Fusion and quasifission studies in reactions forming Rn via evaporation residue measurements

Background: Formation of the compound nucleus (CN) is highly suppressed by quasifission in heavy-ion collisions involving massive nuclei. Though considerable progress has been made in the understanding of fusion-fission and quasifission, the exact dependence of fusion probability on various entrance channel variables is not completely clear, which is very important for the synthesis of new heavy and superheavy elements.Purpose: To study the interplay between fusion and quasifission in reactions forming CN in the boundary region where the fusion probability starts to deviate from unity.Methods: Fusion evaporation residue cross sections were measured for the $^{28,30}\mathrm{Si}+^{180}\mathrm{Hf}$ reactions using the Hybrid Recoil Mass Analyser at IUAC, New Delhi. Experimental data were compared with data from other reactions forming the same CN or isotopes of the CN. Theoretical calculations were performed using the dinuclear system and statistical models.Results: Reduced evaporation residue cross sections were observed for the reactions studied compared with the asymmetric reaction forming the same CN, indicating fusion suppression in more symmetric systems. The observations are consistent with fission fragment measurements performed in the same or similar systems. Larger ER cross sections are observed with increase in mass in the isotopic chain of the CN.Conclusions: Fusion probability varies significantly with the entrance channels in reactions forming the same CN. While complete fusion occurs for the $^{16}\mathrm{O}+^{194}\mathrm{Pt}$ reaction, the fusion probability drops to approximately $60--70%$ for the $^{30}\mathrm{Si}+^{180}\mathrm{Hf}$ and less than $20%$ for the $^{50}\mathrm{Ti}+^{160}\mathrm{Gd}$ reactions, respectively, forming the same CN at similar excitation energies.

Fusion of neutron-rich oxygen nuclei

Measurement of the fusion excitation function for 18 O + 12 C and 19 O + 12 C is described. The fusion cross-section is extracted through the direct measurement of evaporation residues resulting from the fusion process. At near barrier energies, the single additional neutron present in 19 O results in an enhancement in the fusion cross-section by a factor of over three as compared to 18 O.

Production cross sections of superheavy elementsZ=119and 120 in hot fusion reactions

The production cross sections of superheavy nuclei in hot fusion reactions are investigated systematically. In hot fusion reactions, the capture cross section can be obtained by calculating the weighted average of the transmission probability for different orientations of deformed colliding nuclei. An analytical formula for calculating the value of the fusion probability is proposed, which is suitable for both hot and cold fusion reactions. The orientation effects are considered empirically in calculating the fusion probability. The method proposed in the present work reproduces the measured evaporation residue (ER) cross sections in hot fusion reactions acceptably well. The formula also gives reasonable results for fusion probability in cold fusion reactions. Using this method the evaporation residue cross sections for synthesizing $Z=119$ and 120 are predicted. It is found that for hot fusion reaction's larger maximal ER cross section of the $4n$ channel corresponds to lower optimal incident energy.

Evaporation residue excitation function measurement for the16O+194Ptreaction

Evaporation residue cross sections for the reaction ${}^{16}\text{O}+{}^{194}\text{Pt}$ 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 ${}^{16}\text{O}+{}^{184}\text{W}$ 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 $\ensuremath{\sim}$200 region.

Experimental study of theU238(S36,3−5n)Hs269−271reaction leading to the observation ofHs270

The deformed doubly magic nucleus (270)Hs has so far only been observed as the four-neutron (4n) evaporation residue of the reaction Mg-26+Cm-248, where a maximum cross section of 3 pb was measured. Theoretical studies on the formation of (270)Hs in the 4n evaporation channel of fusion reactions with different entrance channel asymmetry in the framework of a two-parameter Smoluchowski equation predict that the reactions Ca-48+Ra-226 and S-36+U-238 result in higher cross sections due to lower reaction Q values, in contrast to simple arguments based on the reaction asymmetry, which predict opposite trends. Calculations using HIVAP predict cross sections for the reaction S-36+U-238 that are similar to those of the Mg-26+Cm-248 reaction. Here, we report on the first measurement of evaporation residues formed in the complete nuclear fusion reaction S-36+U-238 and the observation of (270)Hs, which is produced in the 4n evaporation channel, with a measured cross section of 0.8(-0.7)(+2.6) pb at 51-MeV excitation energy. The one-event cross-section limits (68% confidence level) for the 3n, 4n, and 5n evaporation channels at 39-MeV excitation energy are 2.9 pb, while the cross-section limits of the 3n and 5n channel at 51 MeV are 1.5 pb. This is significantly lower than the 5n cross section of the Mg-26+Cm-248 reaction at similar excitation energy.

Dependence of barrier distribution and fusion-fission process on entrance channel

The entrance barrier distributions for the heavy systems 48Ti, 54Cr, 56Fe, 64Ni, 70Zn, 76Ge and 86Kr + 208Pb have been extracted from excitation functions for quasi-elastic scattering at backward angles. The entrance barrier heights for these systems are presented. The fusion probability for the system 64Ni + 154Sm has been obtained from the measurement of evaporation residues. It is shown that the fusion barrier shifts to high energies corresponding to barrier heights at an equatorial side of the prolate nucleus 154Sm.

Extraction of probability of compound-nucleus formation

We propose an empirical barrier distribution for a unified description of heavy-ion fusion and quasi-elastic scattering based on the Skyrme energy-density functional approach. The capture (fusion) cross sections and the quasi-elastic scattering cross sections of a large number of reactions can be well reproduced. Based on these systematic studies and the HIVAP calculations for the survival probability, the probability P CN of the compound-nucleus formation is extracted from the measured evaporation residue cross sections for “cold” and “hot” fusion reactions.

Effects of nuclear orientation on fusion and fission in the reaction using238U target nucleus

Fission fragment mass distributions in the reaction of 30 Si+ 238 U were measured around the Coulomb barrier. At the above-barrier energies, the mass distribution s howed a Gaussian shape. At the sub- barrier energies, triple-humped distribution was observed, which consists of symmetric fission and asymmetric fission peaked at AL/AH≈ 90/178. The asymmetric fission should be attributed to quasifission from the re sults of the measured evaporation residue (ER) cross-sections for 30 Si+ 238 U. The cross-section for 263 Sg at the above- barrier energy agree with the statistical model calculation which assumes th at the measured fission cross-section originates from fusion-fission, whereas the one for 264 Sg measured at the sub-barrier energy is smaller than the calculation, which suggests the presence of quasifission.

Heavy-ion fusion and scattering with Skyrme energy density functional

In our recent studies, an empirical barrier distribution was proposed for a unified description of the fusion cross sections of light and medium-heavy fusion systems, the capture cross sections of the reactions leading to superheavy nuclei, and the large-angle quasi-elastic scattering cross sections based on the Skyrme energy-density functional approach. In this paper, we first give a brief review of these results. Then, by examining the barrier distributions in detail, we find that the fusion cross sections depend more strongly on the shape of the left side of the barrier distribution while the quasi-elastic scattering cross sections depend more strongly on the right side. Furthermore, by combining these studies and the HIVAP calculations for the survival probability, the formation probability of the compound nucleus is deduced from the measured evaporation residue cross sections for “cold” and “hot” fusion reactions.

Capture cross sections for the near symmetricSn124+Zr96reaction

Capture-fission cross sections were measured for the near symmetric reaction between the massive nuclei {sup 124}Sn and {sup 96}Zr for center of mass energies from 195 to 265 MeV. Coincident fission fragments were detected and separated from elastic and deep inelastic scattering products by angle/energy/mass conditions. The measured capture cross sections agree quite well with calculations using the dinuclear system (DNS) model. The DNS model also predicts the fusion cross section for this reaction with a fusion barrier height of 208.0 MeV. The deduced extra push energy, corresponding to this barrier height, differs from that deduced from evaporation residue measurements.

Fission hindrance studies inPb200: Evaporation residue cross section and spin distribution measurements

Evaporation residue cross sections and spin distributions have been measured for $^{200}\mathrm{Pb}$ compound nucleus formed in $^{16}\mathrm{O}+^{184}\mathrm{W}$ reaction at the laboratory beam energies of 84, 92, 100, 108, 116, and 120 MeV. The evaporation residues have been selected using the recoil mass spectrometer, HIRA and detected using a 2D position sensitive silicon detector. The evaporation residue spin distributions have been measured by detecting gamma rays with 14 element BGO multiplicity filter. Measured evaporation residue cross sections and spin distributions are compared with the values predicted by a standard statistical model code. Comparison shows that, in the energy region studied, the nuclear viscosity parameter $\ensuremath{\gamma}=3$ is required to explain total evaporation residue cross sections and evaporation residue spin distributions.

Shell corrections at the saddle point for mass~200

Statistical model analysis of the measured evaporation residue and fission excitation functions indicates the presence of shell corrections to the liquid drop energy at the saddle point deformations for nuclei of mass {approx}200, if one attempts to fit simultaneously the measured prefission neutron multiplicities.

Measurements of evaporation residue cross sections for the fusion reactionsKr86+Ba134andKr86+Ba138

We measured evaporation residue (ER) cross sections of the fusion reactions $^{86}\mathrm{Kr}+^{134,138}\mathrm{Ba}$ to investigate the effect of the shell structure of the interacting nuclei on the fusion process. The measured ER cross sections for the reaction $^{86}\mathrm{Kr}+^{138}\mathrm{Ba}$ have a sizable component, 37 nb, at the excitation energy (${E}_{\mathrm{ex}}$) of 12.1 MeV. However, there is no evaporation residue for the reaction $^{86}\mathrm{Kr}+^{134}\mathrm{Ba}$ below the energy ${E}_{\mathrm{ex}}=24.7$ MeV. This result suggests that the neutron shell closure ($N=82$) of $^{138}\mathrm{Ba}$ plays a crucial role in the formation of the compound nucleus.

Mass distribution studies in the 19F + 197Au reaction

Mass distribution and evaporation residue measurements have been carried out in the reaction 19F + 197Au using the recoil catcher technique followed by off-line γ-ray spectrometry. The random neck rupture model (RNRM) has been used to compute the variance of the mass distribution ( σ2A) and the average kinetic energy ( ¯) of the fission fragments for the present system. The results of model calculations have been found to be in good agreement with the experimental observations. Measured evaporation residue cross-sections have been compared with the statistical model calculations.