Multinucleon Transfer Reactions Research Articles

Lifetime measurements in the even-even Cd102–108 isotopes

The heaviest N=Z doubly-magic nucleus, $^{100}$Sn, and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction in extreme conditions. In particular, the Cd isotopes are expected to present features similar to those found in the Sn isotopic chain, since they have only two proton holes in the Z=50 shell. In this manuscript, the lifetime measurements of low-lying states in the even-mass $^{102-108}$Cd is presented. Thanks to the powerful detection capabilities of AGATA array and VAMOS++ spectrometer, the unusual employment of multi-nucleon transfer reactions permitted to investigate the first 2$^+$ and 4$^+$ states in all these nuclei, together with various deformed bands in $^{106}$Cd. The results were interpreted in the context of new state-of-the-art beyond-mean-field calculations, using the symmetry-conserving configuration-mixing approach. Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the Z=48 and Z=50 isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with $\beta_2 \approx 0.2$. At this deformation Z=48 becomes a closed-shell configuration, which is favored with respect to the spherical one.

Possibilities for synthesis of new transfermium isotopes in multinucleon transfer reactions

Multinucleon transfer mechanisms near barrier collisions between very heavy nuclei are investigated in the framework of the improved dinuclear system model by considering the influence of orientation effects combined with statistical model code gemini++. Several transfer reactions for different actinide targets ($^{238}\mathrm{U},^{248}\mathrm{Cm}$, and $^{249}\mathrm{Cf}$) are studied in detail. One notices that the isotopic distributions of the above-target products decrease drastically by a factor of 10 per proton, which is moved from projectile to target. However, the calculated and measured results show that the larger production cross sections above targetlike fragments can be produced by using the heaviest available target. The predicted cross sections and excitation functions of the new transfermium isotopes may be useful for future experiments.

First direct observation of isomeric decay in neutron-rich odd-odd Ta186

De-excitation $\gamma $ rays associated with an isomeric state of $^{186}$Ta were investigated. The isomers were produced in multinucleon transfer reactions between a $^{136}$Xe beam and a natural W target, and were collected and separated by the KEK Isotope Separation System. Two $\gamma $ transitions with energies of 161.1(2) and 186.8(1) keV associated with an isomeric decay were observed for the first time. The half-life of the isomeric state of the neutral atom $^{186 \rm m}$Ta was deduced as 17(2) s. Based on the comparison with the previous measurements of the isomeric state using the ESR storage ring at GSI Darmstadt and the coupling of angular momenta of individual particle orbitals in odd-odd nuclei, a decay scheme of $^{186 \rm m}$Ta was proposed.

Fabrication and characterization of thin 64Zn and 68Zn targets for nuclear reaction measurements

Isotopically enriched targets of 64,68Zn were prepared for heavy ion fusion and multi-nucleon transfer reactions. In view of the fragility of the targets under the ion current and during characterization, multiple targets of each isotope were required. Thin films of 64,68Zn isotopes were fabricated via vapor deposition in the target laboratory at Inter-University Accelerator Center (IUAC), New Delhi. Multiple targets of 64,68Zn in the thickness range 150 μg/cm2–300 μg/cm2, were synthesized on a carbon backing of 10 μg/cm2 using only 30 mg amount of available enriched material for each isotope. The use of a pin source and carbon foils as substrate allowed the fabrication of Zn targets without employing previously suggested methods of substrate cooling and forced condensation for thin film fabrication of volatile elements. The deposition results have been estimated quantitatively. The thickness measurement of the targets was carried out using the α particle energy loss technique and Rutherford Back Scattering (RBS) method. The elemental composition of the targets was examined using RBS and Dispersive Wavelength X-ray Fluorescence (DWXRF) to investigate the contamination. Morphological analysis of the films was performed using Scanning Electron Microscope (SEM) to study homogeneity and the impact of ion bombardment on the targets.

Influence of entrance channel on multinucleon transfer cross sections

Multinucleon transfer cross sections around targetlike fragments of the $^{58}\mathrm{Ni}+^{208}\mathrm{Pb}$ reaction and the $^{64}\mathrm{Ni}+^{207,208}\mathrm{Pb}$ reactions were calculated by the improved dinuclear system $\mathrm{model}+\mathrm{GEMINI}++$. One finds that the $^{64}\mathrm{Ni}+^{208}\mathrm{Pb}$ reaction is more favorable to obtain neutron-rich nuclei as compared to the $^{58}\mathrm{Ni}+^{208}\mathrm{Pb}$ and $^{64}\mathrm{Ni}+^{207}\mathrm{Pb}$ reactions. That is because the difference of injection point position on the potential energy surface has a significant influence on the transfer of protons and neutrons in multinucleon transfer reaction. The effect of the entrance channel on transfer cross sections was predicted and analyzed by $^{58,64,70}\mathrm{Ni}+^{198}\mathrm{Pt}$ and $^{86,91,102}\mathrm{Kr}+^{208}\mathrm{Pb}$ reactions.

Ground state and excited state multinucleon transfer channels in interactions of 28Si with 90,94Zr in near barrier region

Multinucleon transfer reactions were measured in interactions of 28Si with 90,94Zr at a near barrier energy using recoil mass separator, Heavy Ion Reaction Analyzer (HIRA) at Inter University Accelerator Centre (IUAC), New Delhi. The effects of shell closure, positive Q-value transfer channels on the relative strengths of ground state and excited state transfer in multinucleon transfer processes were investigated. The kinematic coincidence technique was employed to reduce the background. The m/q ambiguity was resolved by time of flight (TOF) technique. Based on the Q-value considerations, it was observed that pickup channels were predominantly of neutron transfer, whereas, stripping channels were mainly of proton transfer. A 14-element BGO array was used to extract the relative strength of excited state transfer. In interactions of 28Si with 90Zr, it was observed that the ground state and excited states competes for populating all the transfer channels. In reactions of 28Si with 94Zr, nearly similar behavior was observed except for two-neutron pickup channel, where excited state transfer was dominant and four-neutron pickup channel where ground state transfer was dominant.

β−γ spectroscopy of the Os195 nucleus

The $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy of $^{195}\mathrm{Os}$ nucleus was performed at the KEK isotope separation system (KISS). The radioactive isotope of $^{195}\mathrm{Os}$ was produced by the multinucleon transfer reactions of $^{136}\mathrm{Xe}$ beams and $^{198}\mathrm{Pt}$ target. The isotope of interest was separated and extracted by using KISS, which is an argon-gas-cell based laser ion source with a mass separator. Those extracted isotopes are implanted into an aluminized mylar tape, which is surrounded by the multisegmented proportional gas counter (MSPGC) and four clover-type high-purity germanium (HPGe) detectors for the $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy. 27 $\ensuremath{\gamma}$-ray peaks were observed in coincidence with internal conversion electrons and x rays detected by the MSPGC. 21 of them were identified as $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays of the $^{195}\mathrm{Os}$ ground state, where the energies of 19 $\ensuremath{\gamma}$-ray peaks agree with the literature values of transitions observed in $^{195}\mathrm{Ir}(n,\ensuremath{\gamma})$ reactions and two $\ensuremath{\gamma}$-ray peaks were newly found. The $\ensuremath{\beta}$-decay branching ratios of the $^{195}\mathrm{Os}$ ground state were deduced. The obtained log-$ft$ values range from $5.99(4)$ to $7.56(22)$, indicating all $\ensuremath{\beta}$-decay channels are the first forbidden transitions. It suggests that the spin-parity of the $^{195}\mathrm{Os}$ ground state is $3/{2}^{\ensuremath{-}}$, which is consistent with the systematical expectation. The $\ensuremath{\beta}$-decay half-life of the $^{195}\mathrm{Os}$ ground state was evaluated to be 6.5(4) min, which agrees well to the previously known value 6.5(11) min.

Multinucleon transfer reactions in the nearly symmetric reaction systems 204Hg+208Pb and 204Hg+198Pt

In order to test the reliability of calculated transfer cross section for the nearly symmetric reaction systems within the improved dinuclear system (DNS) model, the reactions 204Hg+208Pb and 204Hg+198Pt are investigated with one set of parameters and with the same assumptions. The transfer cross sections are calculated and compared with existing experimental data, we find that the exception of the yield fragments of small transfers for 204Hg+208Pb, the improved DNS model is unable to reproduce the data. We also discuss the influence of the uncertainty of model parameters on the theoretical results.

Shell inhibition on production of N = 126 isotones in multinucleon transfer reactions

The multinucleon transfer process is one promising approach to produce exotic nuclei around the last “waiting point” in the r-process. However, the reliable reaction combinations are still ambiguous so far. We quantitatively evaluate the relative shell inhibition on production cross sections for producing N=126 isotones for the first time with definition of a relative shell inhibition factor. The results suggest that the combinations characterized by double closed shell structure are unfavorable for producing unknown N=126 isotones. It is demonstrated that the reactions 136Xe + 208Pb and 136Xe + 198Pt are not the good candidates. By investigating the yield contributions in main evaporation channels, we find that for producing 200W the effects of shell structure on production cross sections are mainly from de-excitation process. Also, the weak incident energy dependence of cross sections for producing N=126 isotones is revealed with relevance between evaporation probabilities and excitation energy distributions of primary fragments.

Selection of the optimal condition for the production of light neutron-rich isotopes in multinucleon transfer reactions

Light neutron-rich isotopes are populated as products of the multinucleon transfer reaction following the collisions of $^{64}\mathrm{Ni}$ beams onto $^{130}\mathrm{Te}, ^{208}\mathrm{Pb}$, and $^{238}\mathrm{U}$ targets and $^{40}\mathrm{Ar}$ beams onto $^{197}\mathrm{Au}, ^{208}\mathrm{Pb}$, and $^{238}\mathrm{U}$ targets at bombarding energies slightly above the Coulomb barrier. Distribution characteristics of projectile-like fragments are studied in the framework of dinuclear system concept where dynamic deformation is taken into account. The odd-even effect in the diffusion process is discussed, and the distribution of final isotopes can be well reproduced by the effective local excitation energies excluding pairing energies. Compared with the experimental data, it is found that the heavier (i.e., larger $N/Z$ ratio) the target is, the more likely it is to produce light neutron-rich isotopes.

Formation of heavy neutron-rich nuclei by Ca48 -induced multinucleon transfer reactions

The reaction systems $^{48}\mathrm{Ca}+^{238}\mathrm{U}$ and $^{48}\mathrm{Ca}+^{248}\mathrm{Cm}$ are investigated within the improved dinuclear system model+GEMINI++. The theoretical results show that the distributions of isotopes in the $^{48}\mathrm{Ca}+^{238}\mathrm{U}$ reaction are well described with the model. For the $^{48}\mathrm{Ca}+^{248}\mathrm{Cm}$ reaction, the model partly describes well the experimental production cross sections. Experimental and theoretical data from multinucleon transfer reactions indicate that the isotopic distributions of the $^{48}\mathrm{Ca}+^{238}\mathrm{U}$ reaction system above Pb are shifted toward the neutron-rich side as compared to the $^{136}\mathrm{Xe}+^{208}\mathrm{Pb}$ reaction except the isotopic distribution of Bi.

Production of neutron-rich N = 126 nuclei in multinucleon transfer reactions: Comparison between 136Xe + 198Pt and 238U + 198Pt reactions

The production cross sections for primary and residual fragments produced in two reaction systems 136Xe + 198Pt and 238U + 198Pt at incident energy 8 MeV/nucleon are calculated by improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model (HIVAP code). It is found that the cross sections for n-rich N=126 residual fragments are several times larger in 238U+198Pt than in 136Xe+198Pt due to the larger cross sections and lower excitation energies of primary fragments in 238U induced reactions. In both systems, N=126 semi-magic residues are produced via neutron emission in the decay of primary fragments, which survive in the semi-central collisions and have lower excitation energies.

Excited states of 90,92,94Y populated in 90Zr+208Pb multinucleon transfer reaction

Multinucleon transfer reactions in 90Zr+208Pb have been studied via fragment-γ coincidences, employing the PRISMA magnetic spectrometer coupled to the CLARA γ-array. An analysis on Y isotopes has been carried out incorporating spectroscopic as well as reaction mechanism aspects. New γ transitions have been observed in 94Y, confirming the findings of recent studies where nuclei were produced via fission of uranium, and a comparison with near-by 90,92Y isotopes populated in the same reaction has been discussed. Experimental cross sections have been extracted and compared with the GRAZING calculations, showing a fair agreement along the neutron pick-up side. The results confirm how multinucleon transfer reactions are a suitable mechanism for the study of neutron-rich nuclei.

Nanosecond isomers and the evolution of collectivity in stable, even- A Hg isotopes

Isomeric states and associated collective structures have been studied up to high spin in $^{198,200,202}\mathrm{Hg}$ using multinucleon transfer reactions and the Gammasphere array. A coupled rotational band, with possible four-quasiparticle character, is established in $^{198}\mathrm{Hg}$. Sequences built on two-quasiparticle, positive- and negative-parity levels are assigned to $^{202}\mathrm{Hg}$. New isomers in $^{202}\mathrm{Hg}$ with ${I}^{\ensuremath{\pi}}=({7}^{\ensuremath{-}})$ and $({9}^{\ensuremath{-}})$, and ${T}_{1/2}$ = 10.4(4) ns and 1.4(3) ns, respectively, have been identified. A half-life of 1.0(3) ns is established for the ${I}^{\ensuremath{\pi}}={12}^{+}$ state in $^{200}\mathrm{Hg}$. $B(E2)$ values deduced from isomeric transitions in Hg isotopes indicate that, while collectivity near the ground state gradually diminishes from $N$ = 112 to $N$ = 124, it is found to increase for the ${12}^{+}$ and ${9}^{\ensuremath{-}}$ states up to $N$ = 118, followed by a reduction for higher neutron numbers. Calculations using the ultimate cranker code provide insight into the variation of deformation with spin and allow for an understanding of observed band crossings. The evolution of collectivity with spin, and along the isotopic chain, is described.

Studying multi-nucleon transfer reaction in a recoil mass spectrometer

We present simulation of a recoil mass spectrometer for measurement of multi-nucleon transfer reactions, based on Monte Carlo techniques. Target-like ions are generated event by event and transported to the focal plane of the spectrometer by the method of transfer matrices. Measured focal plane spectra for the elastic and six transfer channels of the reaction $$^{28}$$ Si+ $$^{94}$$ Zr are reproduced at two projectile energies near the Coulomb barrier. Excellent reproduction is achieved even with first-order ion-optical calculations, indicating not so significant role of higher order aberrations in conventional recoil separators with smaller acceptance. This is in striking contrast with the large acceptance magnetic separators used for studying transfer reactions, for which complex trajectory reconstruction algorithms involving higher order aberrations are essential. Transmission efficiency of the spectrometer, calculated by the reported code, is crucial to convert measured transfer probabilities to differential and integral transfer cross sections for each channel. We show effectiveness of the proposed methodology by extracting differential elastic scattering cross sections from measured yields for the reaction $$^{28}$$ Si+ $$^{94}$$ Zr. Besides being useful for other recoil mass spectrometers, our methodology can be employed for measurement of quasi-elastic reaction cross sections in velocity filters and gas-filled separators.

Influence of the incident energy of the projectile and of different targets on the production of neutron-rich nuclei

The multinucleon transfer reaction is treated as a diffusion process by solving four-variable master equations within the dinuclear system (DNS) concept. The reaction systems $^{136}\mathrm{Xe}+^{208}\mathrm{Pb}$ and $^{136}\mathrm{Xe}+^{198}\mathrm{Pt}$ are investigated and compared to the experimental data. The calculated results correctly describe the magnitude and maxima of the observed transfer cross sections for a wide range of transfers. One finds that the neutron richness of the reaction products is strongly dependent on the projectile energy near the Coulomb barrier. By comparing the obtained results for the above two reaction systems, the absolute cross sections from the $^{208}\mathrm{Pb}$ target predict smaller production cross sections for neutron-deficient nuclei and moderately neutron-rich nuclei as compared to $^{198}\mathrm{Pt}$.

Fission studies using multinucleon transfer reactions

It is shown that the multinucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions (FFMDs) are measured for each transfer channel, in selected bins of excitation energy. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the high energies that the shell effects are expected to smear out. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission.

Lifetime measurements of excited states in neutron-rich Ti53 : Benchmarking effective shell-model interactions

Level lifetimes of the yrast (5/2(-)) to 13/2(-) states in the neutron-rich nucleus Ti-53, produced in a multinucleon-transfer reaction, have been measured for the first time. The recoil distance Doppler-shift method was employed and lifetimes of the excited states were extracted by a lineshape analysis aided by GEANT4-based Monte-Carlo simulations. The experiment was performed at the Grand Accelerateur National d'Ions Lourds facility in Caen, France, by using the Advanced Gamma Tracking Array for the gamma-ray detection coupled to the large-acceptance variable mode spectrometer for an event-by-event particle identification and the Cologne plunger for deep-inelastic reactions. Reduced transition probabilities, deduced from the lifetimes, give new information on the nuclear structure of Ti-53, and are used to benchmark different shell-model calculations using established interactions in the f p shell.

Properties of ^{187}Ta Revealed through Isomeric Decay.

Mass-separated ^{187}Ta_{114} in a high-spin isomeric state has been produced for the first time by multinucleon transfer reactions, employing an argon gas-stopping cell and laser ionization. Internal γ rays revealed a T_{1/2}=7.3±0.9 s isomer at 1778±1 keV, which decays through a rotational band with perturbations associated with the approach to a prolate-oblate shape transition. Model calculations show less influence from triaxiality compared to heavier elements in the same mass region. The isomer-decay reduced E2 hindrance factor f_{ν}=27±1 supports the interpretation that axial symmetry is approximately conserved.

Multi-nucleon transfer reactions at ion catcher facilities - a new way to produce and study heavy neutron-rich nuclei

The production of very neutron-rich nuclides heavier than fission fragments is an ongoing experimental challenge. Multi-nucleon transfer reactions (MNT) have been suggested as a method to produce these nuclides. By thermalizing the reaction products in gas-filled stopping cells, we can deliver them as cooled high-quality beams to decay, laser and mass spectrometry experiments. High precision mass spectrometry will allow for the first time to universally and unambiguously identify the atomic and proton numbers of the ions produced in MNT reactions. In this way their ground and isomeric state properties can be studied in high-precision measurements. In experiments at IGISOL, Finland and at FRS Ion Catcher, Germany, we have done and will perform broadband measurements of the reaction products, with the aim to improve the understanding of the reaction mechanism and to determine the properties of the ground and isomeric states of the produced nuclides. First results and preparations for upcoming experiments are presented.