Recoil Mass Spectrometer Research Articles

Fusion barrier distribution from measurement of quasielastic scattering at θ c.m. = 180°

Distribution of fusion barriers reveals the effect of structure of the collision partners on the dynamics of fusion between two heavy ions. Experimental barrier distribution can be extracted from precisely measured fusion excitation function. Quasielastic excitation function is related to fusion excitation function by conservation of incident flux. Hence, barrier distribution can also be extracted from quasielastic excitation function, which is usually measured at large angles, by detecting the back-scattered projectile-like ions. However, the reflectance coefficient is exactly complementary to the transmittance coefficient at θ c.m. = 180°, which corresponds to orbital angular momentum, ℓ = 0. We report here extraction of fusion barrier distribution for 16O+142 Ce from measurement of quasielastic scattering at θ c.m. = 180°, by detecting the target-like ions in the forward angles using a recoil mass spectrometer. Quasielastic excitation functions were also measured by detecting the projectile-like ions at two large angles. Barrier distributions extracted from both fusion and quasielastic measurements were found to be nearly identical. This work, thus, provided the first experimental verification of validity of the scaling property and the iso-centrifugal approximation in extracting barrier distribution from quasielastic excitation functions measured at large angles.

Measurement of the 86Kr(α,n)89Sr cross section at energies relevant for the weak r-process

The r-process has been shown to be robust in reproducing the abundance distributions of heavy elements, such as europium, seen in ultra-metal poor stars. In contrast, observations of elements 26 < Z < 47 display overabundances relative to r-process model predictions. A proposed additional source of early nucleosynthesis is the weak r-process in neutrino-driven winds of core-collapse supernovae. It has been shown that in this site (α,n) reactions are both crucial to nucleosynthesis and the main source of uncertainty in model-based abundance predictions. Aiming to improve the certainty of nucleosynthesis predictions, the cross section of the important reaction 86Kr(α,n)89Sr has been measured at an energy relevant to the weak r-process. This experiment was conducted in inverse kinematics at TRIUMF with the EMMA recoil mass spectrometer and the TIGRESS gamma-ray spectrometer. A novel type of solid helium target was used.

Fabrication and characterization of thin 116Cd target films for fusion measurements

Fabrication and characterization of thin target films of 116Cd by physical vapour deposition technique are reported. The target films were used in the study of nuclear fusion dynamics near the Coulomb barrier with a beam of 16O. Because of the low melting point of cadmium and its re-evaporation after deposition, coupled with availability of a very small quantity of isotopically enriched material, an unconventional method was employed to fabricate the targets. The prepared target films were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffractometry (XRD) and Rutherford Back-scattering Spectroscopy (RBS) to obtain their surface morphology, elemental composition, chemical composition and thickness, respectively. Thickness of the enriched target films were in the range of 20 –50 μg/cm2. Isotopic composition of one of the target films was ascertained in an in-beam experiment using a recoil mass spectrometer. It was observed that abundance of 116Cd in the fabricated target film was significantly lower than the value quoted by the supplier of the source material.

Fusion and back-angle quasielastic measurements in Si30+Gd156 near the Coulomb barrier

Background: Advancement in accelerator facilities has opened the door to dig deep to understand the interplay between nuclear reactions and structures. Although the influence of inelastic excitations on nuclear scattering and sub-barrier fusion is somewhat established, a clear understanding of nucleon transfer with a positive-$Q$ value is yet to achieve.Purpose: The objective of this paper is to examine the role of the $2n$-transfer channel with a positive $Q$ value on sub-barrier fusion and back-angle quasielastic (QE) scattering in the $^{30}\mathrm{Si}+^{156}\mathrm{Gd}$ reaction. Furthermore, extraction of barrier distributions (BDs) from fusion and QE scattering to infer their shapes is also a prime goal.Method: The excitation functions (EFs) of fusion and back-angle QE scattering have been measured over a wide range of incident beam energy around the Coulomb barrier using a recoil mass spectrometer. Furthermore, BDs have been extracted using the measured fusion and back-scattered QE data. The underlying findings have been analyzed within the framework of coupled-channel (CC) formalism using ccfull and ecc programs.Results: Fusion enhancement has been observed compared to those predicted from the one-dimensional barrier penetration model at sub-barrier energies. Fusion enhancement and QE EFs are explained by CC predictions considering the collective excitations among the colliding nuclei. The inclusion of $2n$ transfer and collective excitations in ccfull improves the fit to the experimental fusion data in a short span of energy window around the Coulomb barrier, whereas no significant effect has been observed at the sub-barrier region. However, no such effect of $2n$-pickup transfer has been observed from ecc model calculations. Thus, no firm conclusion can be made on the role of $2n$-pickup transfer with a positive $Q$ value in present measurements.Conclusion: Fusion EFs have been successfully explained by the CC calculations using ccfull and ecc model codes. No significant effect of the $2n$-pickup channel with a positive $Q$ value was observed on sub-barrier fusion enhancement. However, QE EFs are reproduced by considering the collective excitations and $2n$-transfer channel couplings. Fusion and QE BDs are similar in shape within the experimental uncertainty. One-dimensional barrier parameters extracted from the measured data agree with the different theoretical models. Also, the present system obeys the systematic based on deformation values after transfer at the exit.

Investigation of isotopic dependence on the O + Ni fusion cross section near barrier energies

Fusion excitation functions have been measured for $^{16}\mathrm{O}+^{61}\mathrm{Ni}$ and $^{18}\mathrm{O}+^{61,62}\mathrm{Ni}$ systems around the Coulomb barrier ($\ensuremath{\approx}0.7{V}_{B}$--$1.3{V}_{B}$) using the recoil mass spectrometer, heavy ion reaction analyzer. The ground state $Q$ value for two neutron stripping is positive for both systems with $^{18}\mathrm{O}$ as the projectile. Strong enhancement of the experimental fusion cross sections were observed below the barrier for all the systems compared to that of the predictions of the one-dimensional barrier penetration model. To understand such enhancement, a coupled-channels formalism has been used. A comparative study of these systems indicated that the coupling of two neutron transfer channels with the collective excitations could play the role behind the sub-barrier fusion enhancement for $^{18}\mathrm{O}$ induced reactions. However, the sub-barrier enhancement for $^{16}\mathrm{O} + ^{61}\mathrm{Ni}$ is found to be due to the coupling of quadrupole vibrations of both the interacting nuclei. Also after comparing these systems with other systems of different Ni isotopes, we have found that the signature of the role of coupling to neutron transfer channels due to ground state positive $Q$ value for neutron transfer is ambiguous.

Fabrication of thin [formula omitted]Ce target foils for study of nuclear reaction dynamics

We report fabrication of thin target foils of 140,142Ce by the physical vapour deposition technique. The targets are to be used in the study of nuclear reaction dynamics with heavy ion beams. Owing to the highly reactive nature of cerium, the thin films can be oxidized during fabrication. The films were sandwiched between thin layers of natC to impart structural stability and prevent degradation over long periods of time. The parameters for vapour deposition were frozen after several trials with natural cerium. To obtain surface morphology, elemental composition, chemical composition and thickness of the prepared targets, those were characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffractometry (XRD) and Rutherford Back-scattering Spectroscopy (RBS). The thickness of the isotopic targets was in the range of 100–200 μg/cm2. XRD and RBS spectra were used to determine the stoichiometry of oxide present in the films. They were observed to be in the form of CeO2. The isotopic composition of one of the targets was determined in an in-beam experiment using a recoil mass spectrometer. Results from the experiment matched quite well with the data provided by the supplier of the isotopic material.

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.

Evaporation residue cross section in the Cl37+Zn68 fusion reaction near the Coulomb barrier

Background: The measured subbarrier fusion cross sections in heavy-ion reactions are found to be significantly large as compared with those expected from the one-dimensional barrier penetration model (1d-BPM). Although attempts have been made to comprehend the enhancement in terms of different intrinsic degrees of freedom of the colliding nuclei, a clear understanding of the same is yet to come.Purpose: The objectives of this study is to understand the interplay of the channel coupling effect on fusion excitation function and explore the decay dynamics of the excited compound nucleus in the sub-and-near barrier region.Method: Fusion excitation function has been measured at energies from 7% below to 38% above the Bass barrier using a recoil mass spectrometer. Furthermore, cross sections of the different evaporation residues, such as 98, 99, 100, and 101 u, have been extracted.Results: The observed enhancement in the subbarrier fusion cross sections over the 1d-BPM predictions has been explained by considering the low-lying inelastic excitations among the interacting partners using ccfull code and by the barrier modification using the dynamical cluster-decay model (DCM). The cross sections of individual residual mass fractions (98, 99, 100, and 101 u) have been compared with the Hauser-Feshbach model.Conclusion: Coupled-channel calculations and DCM have successfully explained the total fusion cross sections data. The measured total fusion cross section ${\ensuremath{\sigma}}_{\mathrm{fus}}$ was found to be approximately equal to the sum of various residual mass fractions ($\mathrm{\ensuremath{\Sigma}}{\ensuremath{\sigma}}_{\mathrm{ER}}$) at energies above the Bass barrier. One-dimensional barrier height and radius parameters extracted from the measured data are in good agreement with the Bass model and DCM parameters.

Role of neutron transfer in the sub-barrier fusion cross section in O18 + Sn116

Background: In heavy-ion-induced fusion reactions, cross sections in the sub-barrier region are enhanced compared to predictions of the one-dimensional barrier penetration model. This enhancement is often understood by invoking deformation and coupling of the relative motion with low-lying inelastic states of the reaction partners. However, effects of nucleon transfer on fusion below the barrier, especially for the systems having positive $Q$ value neutron transfer (PQNT) channels, are yet to be disentangled completely.Purpose: We intend to study the role of the PQNT effect on the sub-barrier fusion of the $^{18}\mathrm{O} + ^{116}\mathrm{Sn}$ system having positive $Q$ value for the two-neutron stripping channel. Also we reflect on the interplay of couplings involved in the system around the Coulomb barrier.Method: The fusion excitation function was measured at energies from $11%$ below to $46%$ above the Coulomb barrier for $^{18}\mathrm{O} + ^{116}\mathrm{Sn}$ using a recoil mass spectrometer, viz., the Heavy-Ion Reaction Analyser (HIRA). Fusion barrier distributions were extracted from the data. Results from the experiment were analyzed within the framework of the coupled-channels model.Results: Fusion cross sections at energies below the Coulomb barrier showed strong enhancement compared to predictions of the one-dimensional barrier penetration model. The fusion process is influenced by couplings to the collective excitations with coupling to single- and two-phonon vibrational states of the target and the projectile respectively. Inclusion of the two-neutron transfer channel in the calculation along with these couplings could reproduce the data satisfactorily.Conclusions: The significant role of PQNT in enhancing the sub-barrier fusion cross section for the chosen system is not observed. It simply reduced the sub-barrier fusion cross section. Therefore, a consistent link between PQNT and sub-barrier fusion enhancement could not be established vividly while comparing the fusion excitation function from this work with the same from other $^{16,18}\mathrm{O}$-induced reactions. This clearly points to the need for more experimental as well as theoretical investigation in this field.

Simulation of a recoil mass spectrometer for measurement of differential quasi-elastic scattering cross sections

We report development of a semi-microscopic Monte Carlo code to calculate the transmission efficiency of a recoil mass spectrometer for quasi-elastic scattering. Realistic distributions of angle, energy and charge state of the forward-moving target-like ions are generated event by event. Particle trajectories are calculated employing first-order ion-optical transfer matrices. We present results for two test reactions, viz.$$^{16}$$O+$$^{116}$$Sn and $$^{58}$$Ni+$$^{116}$$Sn assuming Rutherford scattering at projectile energies near the Coulomb barrier. This work has an important application in determining barrier distributions from quasi-elastic back-scattering measurements employing recoil mass spectrometers. The code can also be utilized to simulate multi-nucleon transfer reactions and adapted for other similar recoil separators because of its modular structure.

Initial operation of the recoil mass spectrometer EMMA at the ISAC-II facility of TRIUMF

The Electromagnetic Mass Analyser (EMMA) is a new vacuum-mode recoil mass spectrometer currently undergoing the final stages of commissioning at the ISAC-II facility of TRIUMF. EMMA employs a symmetric configuration of electrostatic and magnetic deflectors to separate the products of nuclear reactions from the beam, focus them in both energy and angle, and disperse them in a focal plane according to their mass/charge (m/q) ratios. The spectrometer was designed to accommodate the gamma-ray detector array TIGRESS around the target position in order to provide spectroscopic information from electromagnetic transitions. EMMA is intended to be used in the measurement of fusion evaporation, radiative capture, and transfer reactions for the study of nuclear structure and astrophysics. Its complement of focal plane detectors facilitates the identification of recoiling nuclei and subsequent recoil decay spectroscopy. Here we describe the facility and report on commissioning efforts.

Sub-barrier fusion in the Cl37+Te130 system

Background: In heavy-ion induced reactions, the sub-barrier fusion cross sections are found to be higher as compared to the predictions of the one-dimensional barrier penetration model. Attempts have been made to explain sub-barrier fusion enhancement by including the static deformations, the couplings to inelastic excitations, and non-fusion channels.Purpose: To investigate factors which influence the sub-barrier fusion in the $^{37}\mathrm{Cl}+^{130}\mathrm{Te}$ system and to understand the interplay of couplings, the fusion excitation function was measured at energies from $10%$ below to $15%$ above the Bass barrier.Method: The fusion excitation function was measured by employing a recoil mass spectrometer, the Heavy-Ion Reaction Analyser (HIRA), at the Inter-University Accelerator Centre, New Delhi. To study the behavior of the fusion excitation function and the effect of couplings at sub-barrier energies, the excitation function was analyzed in the framework of the coupled-channels code ccfull.Results: In the present work, the fusion cross section was measured down to 1 $\ensuremath{\mu}\mathrm{b}$ at the lowest measured energy, i.e., $10%$ below the barrier. It was found that the inclusion of couplings of low-lying excited states along with the modified barrier between interacting nuclei satisfactorily reproduces the fusion excitation function of the $^{37}\mathrm{Cl}+^{130}\mathrm{Te}$ system. For better insight into the sub-barrier fusion, the fusion barrier distribution, the logarithmic derivative L(E) factor, and the astrophysical S factor were extracted from the analysis of the experimentally measured fusion excitation function.Conclusions: The analysis of the fusion excitation function in terms of the astrophysical S factor and the L(E) factor suggests the absence of fusion hindrance in the $^{37}\mathrm{Cl}+^{130}\mathrm{Te}$ system down to a 1 $\ensuremath{\mu}\mathrm{b}$ cross section achieved at the lowest measured energy. The excitation function of the present system is compared with the existing measurements in which $^{37}\mathrm{Cl}$ has been used as a projectile to understand the interplay of entrance-channel parameters in sub-barrier fusion enhancement.

Fission fragment yield distribution in the heavy-mass region from the Pu239 ( nth,f ) reaction

The fission fragment yield distribution has been measured in the $^{239}\mathrm{Pu}\left({n}_{\mathrm{th}},f\right)$ reaction in the mass region of $A=126$ to 150 using the Lohengrin recoil-mass spectrometer. Three independent experimental campaigns were performed, allowing a significant reduction of the uncertainties compared to evaluated nuclear data libraries. The long-standing discrepancy of around 10% for the relative yield of $A=134$ reported in JEF-2.2 and JEFF-3.1.1 data libraries is finally solved. Moreover, the measured mass distribution in thermal neutron-induced fission does not show any significant dip around the shell closure ($A=136$) as seen in heavy-ion fission data of $^{208}\mathrm{Pb}$($^{18}\mathrm{O}$, $f$) and $^{238}\mathrm{U}$($^{18}\mathrm{O}$, $f$) reactions. Lastly, comparisons between our experimental data and the predictions from Monte Carlo codes (gef and fifrelin) are presented and discussed.

Deformation effects on sub-barrier fusion cross sections inO16+Yb174,176

Background: Couplings with various reaction channels are known to enhance sub-barrier fusion cross sections by several orders in magnitude. However, a few open questions still remain. For example, the influence of higher order static deformations on sub-barrier fusion cross sections is yet to be comprehensively understood.Purpose: We study the role of hexadecapole nuclear deformation effect on sub-barrier fusion cross sections. Also, this work aims to extract hexadecapole deformation $({\ensuremath{\beta}}_{4})$ in nuclei in the lanthanide region.Method: The evaporation residue (ER) excitation functions for $^{16}\mathrm{O}+^{\phantom{\rule{0.16em}{0ex}}174,176}\mathrm{Yb}$ were measured at laboratory beam energies $({E}_{\mathrm{lab}})$ in the range of 64.6--103.6 MeV. Measurements were carried out by employing the recoil mass spectrometer Heavy Ion Reaction Analyzer (HIRA) at IUAC, New Delhi. Fusion barrier distributions (BDs) were extracted from data. Results from the experiment were subjected to coupled-channels analysis, in which ${\ensuremath{\beta}}_{4}$ was varied as a free parameter.Results: Experimental fusion cross sections at energies below the barrier expectedly showed strong enhancement compared to the predictions from the one-dimensional barrier penetration model. Data were satisfactorily reproduced after inclusion of negative ${\ensuremath{\beta}}_{4}$ for both the targets in the coupled-channels calculation.Conclusions: The significant role of hexadecapole deformation was observed in the sub-barrier fusion of $^{16}\mathrm{O}+^{\phantom{\rule{0.16em}{0ex}}174,176}\mathrm{Yb}$. The proposed value of ${\ensuremath{\beta}}_{4}$ reproduced the measured fusion excitation function reasonably well. The BDs from these data were also extracted but no definitive conclusions could be drawn from them.

Measurements of the Mass and Isotopic Yields of the 233U(nth,f) Reaction by the Lohengrin Spectrometer

Analysis and results of the measurements of the independent isotopic and mass fission yields for the 233U(nth,f) reaction are presented, which were performed at the Lohengrin recoil mass spectrometer in ILL. The spectrometer separates the fission products according to their mass to ion charge ratio and kinetic energy to ion charge ratio. Mass yields were obtained by means of an ionisation chamber after separation by the spectrometer. Isotopic yields were determined by placing two high-purity Germanium clover detectors downstream the spectrometer. An innovative analysis method developed for the Lohengrin spectrometer is presented, from which the final results are independent from the previous evaluations, which is usually not the case.

The electromagnetic mass analyser EMMA

EMMA is a recoil mass spectrometer for ISAC-II at TRIUMF that will be used to separate the recoils of nuclear reactions from the beam and to disperse them according to their mass-to-charge ratios. EMMA has been designed for both efficiency and selectivity, possessing large acceptances in angle, mass, and energy without sacrificing the necessary beam suppression and mass resolving power.

Strength of theEc.m.=1113keV resonance in20Ne(p,γ)21Na

The $^{20}$Ne$(p, \gamma)^{21}$Na reaction is the starting point of the NeNa cycle, which is an important process for the production of intermediate mass elements. The $E_{\text{cm}} = 1113$ keV resonance plays an important role in the determination of stellar rates for this reaction since it is used to normalize experimental direct capture yields at lower energies. The commonly accepted strength of this resonance, $\omega \gamma = 1.13 \pm 0.07$ eV, has been misinterpreted as the strength in the center-of-mass frame when it is actually the strength in the laboratory frame. This has motivated a new measurement of the $E_{\text{cm}} = 1113$ keV resonance strength in $^{20}$Ne$(p, \gamma)^{21}$Na using the DRAGON recoil mass spectrometer. The DRAGON result, $0.972 \pm 0.11$ eV, is in good agreement with the accepted value when both are calculated in the same frame of reference.

ISLA: An Isochronous Spectrometer with Large Acceptances

A novel type of recoil mass spectrometer and separator is proposed for the future secondary radioactive beams of the ReA12 accelerator at NSCL/FRIB, inspired from the TOFI spectrometer developed at the Los Alamos National Laboratory for online mass measurements. The Isochronous Spectrometer with Large Acceptances (ISLA) is able to achieve superior characteristics without the compromises that usually plague the design of large acceptance spectrometers. ISLA can provide mass-to-charge ratio (m/q) measurements to better than 1 part in 1000 by using an optically isochronous time-of-flight independent of the momentum vector of the recoiling ions, despite large acceptances of 20% in momentum and 64msr in solid angle. The characteristics of this unique design are shown, including requirements for auxiliary detectors around the target and the various types of reactions to be used with the re-accelerated radioactive beams of the future ReA12 accelerator.

Isotopic yield measurement in the heavy mass region for239Pu thermal neutron induced fission

Despite the huge number of fission yield data available in the different evaluated nuclear data libraries, such as JEFF-3.1.1, ENDF/B-VII.0, and JENDL-4.0, more accurate data are still needed both for nuclear energy applications and for our understanding of the fission process itself. It is within the framework of this that measurements on the recoil mass spectrometer Lohengrin (at the Institut Laue-Langevin, Grenoble, France) was undertaken, to determine isotopic yields for the heavy fission products from the ${}^{239}$Pu(n${}_{th}$,f) reaction. In order to do this, a new experimental method based on $\ensuremath{\gamma}$-ray spectrometry was developed and validated by comparing our results with those performed in the light mass region with completely different setups. Hence, about 65 fission product yields were measured with an uncertainty that has been reduced on average by a factor of 2 compared to that previously available in the nuclear data libraries. In addition, for some fission products, a strongly deformed ionic charge distribution compared to a normal Gaussian shape was found, which was interpreted as being caused by the presence of a nanosecond isomeric state. Finally, a nuclear charge polarization has been observed in agreement, with the one described on other close fissioning systems.

Channel coupling effects on the fusion excitation functions forSi28+Zr90,94in sub- and near-barrier regions

Fusion excitation functions and angular distributions of evaporation residues (ERs) have been measured for $^{28}\mathrm{Si}+^{90,94}\mathrm{Zr}$ systems around the Coulomb barrier using the recoil mass spectrometer, Heavy Ion Reaction Analyzer (HIRA). For both systems, the experimental fusion cross sections are strongly enhanced compared to the predictions of the one-dimensional barrier penetration model (1-d BPM) below the barrier. Coupled channels formalism has been employed to theoretically explain the observed sub-barrier fusion cross section enhancement. The enhancement could be explained by considering the coupling of the low-lying inelastic states of the projectile and target in the $^{28}\mathrm{Si}+^{90}\mathrm{Zr}$ system. In the sub-barrier region, the measured fusion cross sections for $^{28}\mathrm{Si}+^{94}\mathrm{Zr}$ turned out to be about an order of magnitude higher than the ones for the $^{28}\mathrm{Si}+^{90}\mathrm{Zr}$ system, which could not be explained by coupling to inelastic states alone. This observation indicates the importance of multinucleon transfer reaction channels with positive $Q$ values in the sub-barrier fusion cross section enhancement, because $^{90,94}\mathrm{Zr}$ are believed to have similar collective strengths. This implies that no strong isotopic dependence of fusion cross sections is expected as far as the couplings to collective inelastic states are concerned. In addition, the role of projectile and multiphonon couplings in the enhancement has been explored.