Induced Fusion-evaporation Reactions Research Articles

Single-particle configurations of low- and medium-spin states in Cu63

The excitation scheme of the $^{63}\mathrm{Cu}$ nucleus has been probed following its population in a light-ion induced fusion-evaporation reaction and using an array of Compton suppressed HPGe clover detectors as the detection system. Apart from ascribing $\ensuremath{\gamma}$-ray transitions to the level scheme of the nucleus, through $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence measurements, and identifying the new transitions in the process, measurements of angular correlation and linear polarization of the $\ensuremath{\gamma}$ rays have also been carried out towards assigning their multipolarities and electric and magnetic characters. The observed level scheme has been interpreted through shell model calculations in two different model spaces. One of these was the $f5pg9$ space outside the $^{56}\mathrm{Ni}$ core while the other included the ${f}_{7/2}$ orbital for the protons outside the $^{48}\mathrm{Ca}$ core and was directed at probing the influence of the ${f}_{7/2}$ excitations in the regime of spins being investigated. The latter has been found to be of rather modest impact, in general, and the observed level structure can largely be interpreted through particle excitations in the $f5pg9$ model space.

Timing characteristics of the R13089-100 photomultiplier tube coupled with 1.5" × 1.5" CeBr3 scintillators and its application in lifetime measurements

The fast-timing performance of the Hamamatsu Photomultiplier Tube (PMT) model R13089-100 is investigated using 1.5" × 1.5" CeBr3 scintillators. The time walk was measured using the Mirror-Symmetric Centroid Difference (MSCD) method for various PMT Bias voltages and CFD delays, to optimize the parameters for measurement of the lifetime of nuclear excited states via the γ-γ fast-timing technique. The 152Eu and 133Ba sources were used for investigation on timing characteristics. Lifetimes of 161 and 383 keV states of 133Cs from the decay of 133Ba radioactive source have been determined for different bias voltages of PMT and CFD delays, with two 1.5" × 1.5" CeBr3 detector setup. The lifetime of 1522 keV state of 209Po from the decay of 209At, produced from α induced fusion evaporation reaction, has also been determined. The results reported in the present paper show that the time resolution and time-walk characteristics of the Hamamatsu PMT R13089-100 can be improved in low-energy region using higher PMT bias voltage and shorter CFD delay.

High Spin Study of 104Ag Nucleus

High spin states in 104Ag were populated via heavy-ion (32S) induced fusion evaporation reaction at a beam energy of 110 MeV. The de-exciting γ-rays were detected by 18 Compton suppressed HPGe clover detectors, placed in different (θ, φ) angles. Spin of several excited states were assigned firmly from the present angular correlation measurement.

Fabrication of [formula omitted]As targets for heavy ion nuclear reactions

Thin uniform targets of 75As isotope with Pb/Au backing layer were fabricated for nuclear structure studies and sub-picosecond lifetime measurements of nuclear excited states using the Doppler shift attenuation method (DSAM) for nuclei populated in the heavy-ion induced fusion-evaporation reactions. The 75As targets of thickness 2.8–4.0 mg/cm2 (4.9–7.0μm) were prepared by in-vacuo sublimation of the As metal powder on to the backing (∼10 mg/cm2) of Pb (9.4μm) or Au (5.2μm), which were prepared by rolling method. The prepared targets were further rolled to improve their uniformity and avoid any interlayer gaps. The amount 75As material required for target preparation could be significantly reduced to ∼80 mg by increasing vapour flux by replacing the open dimpled tantalum boat with the pinhole tubular form. The uniformity, purity and thickness of As and Pb layers in the fabricated targets were assessed using a non-destructive Energy dispersive X-ray fluorescence (EDXRF) technique set-up. The measured thickness values of As and Pb/Au layers were found to agree within ∼3% with those deduced from the weighing method. The prepared targets have been successfully used with 28Si and 31P ion beams and the in-beam γ-ray spectroscopic studies in excited nuclear states in 98,99Rh, 99,100,101Pd and 102Ag isotopes.

Low-Energy Nuclear Excitations along the α-Decay Chains of Superheavy ^{292}Lv and ^{294}Og.

We present the first triaxial beyond-mean-field study of the excitation spectra of even-even superheavy nuclei. As representative examples, we have chosen the members of the α-decay chains of ^{292}Lv and ^{294}Og, the heaviest even-even nuclei which have been synthesized so far using ^{48}Ca-induced fusion-evaporation reactions. In our calculations, the effective finite-range density-dependent Gogny force is used and the angular-momentum and particle-number symmetries are restored. Configuration-mixing calculations are performed to determine ground- and excited-state deformations and to establish the collective band structures of these nuclei. Rapidly varying characteristics are predicted for the members of both decay chains, which are further accentuated when compared to the predictions of simple collective models. Based on the present calculations, the prospect of observing α-decay fine structures in future experiments is discussed.

α-decay study of 218Ac and 221Th in 40Ar+186W reaction * *Supported by the National Natural Science Foundation of China (11805289, 11805291, 11875329), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34010000), and the National Key R&D Program of China (2018YFA0404402)

In this study, 218Ac and 221Th nuclides were produced via the heavy-ion induced fusion evaporation reaction 40Ar + 186W. Their decay properties were studied with the help of the gas-filled recoil spectrometer SHANS and a digital data acquisition system. The cross section ratio between 222Pa and 218Ac was extracted experimentally, with measured value 0.69(9). Two new possible α decay branches to 221Th are suggested. The valence neutron configurations for the daughter 217Ra are discussed in terms of the hindrance factors.

Fine structure of α decay in 222Pa * *Supported by the National Natural Science Foundation of China (11805289, 11775316), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34010000) and the National Key R&D Program of China (2018YFA0404402)

With the help of the gas-filled recoil spectrometer SHANS and a digital data acquisition system, the fine structure of the decay for Pa was studied. The nuclides were produced through the 1p3n evaporation channel via the heavy-ion induced fusion evaporation reaction Ar + W. Based on the ER- - - and - correlation measurement, three new decays were observed in addition to the three branches known previously. The one with the largest decay energy was regarded as the ground state to ground state transition. The newly measured decay properties of Pa were examined in a framework of reduced width.

Revealing multiple band structures in Xe131 from α -induced reactions

The excited states in the transitional nucleus $^{131}\mathrm{Xe}$ have been populated by using $\ensuremath{\alpha}$-induced fusion-evaporation reaction and the de-exciting $\ensuremath{\gamma}$ rays were detected with the Compton suppressed clover detector setup of the Indian National Gamma Array coupled to digital data acquisition system. The existing level structure of $^{131}\mathrm{Xe}$ has been significantly extended with the observation and placement of 72 new $\ensuremath{\gamma}$-ray transitions. The use of light-ion ($\ensuremath{\alpha}$) beam helped to identify several new band structures in $^{131}\mathrm{Xe}$ with different quasiparticle (qp) configurations. The multipolarities of the observed $\ensuremath{\gamma}$ rays have been determined on the basis of the directional correlation from oriented states ratio and polarization asymmetry measurements. The yrast negative-parity band has been confirmed up to ${35/2}^{\ensuremath{-}}$ spin and the highly nonyrast signature partner of this band has been identified for the first time. The positive-parity band, based on the ${3/2}^{+}$ ground state, has been extended up to ${23/2}^{+}$ with the observation of a signature inversion, which signifies a pair of particle alignment around the spin of 15/2 $\ensuremath{\hbar}$. A dipole band, consisting of $M1$ transitions has been identified and assigned a 5-qp configuration. A new band structure built on a 3-qp ${23/2}^{+}$ state has been observed with a large signature splitting. A comparison in the isotopic and isotonic chains reveals the transitional nature of the $N=77$ nuclei. Total Routhian surface calculations have been performed to understand the structure of $^{131}\mathrm{Xe}$ associated with different configurations.

Evidence of the octupole correlation between the shears bands in 142Eu

Two opposite parity dipole sequences (DB I and DB II) in 142Eu have been investigated using the Indian National Gamma Array (INGA), and heavy-ion induced fusion-evaporation reaction, 116Cd(31P, 5n) at ELab = 148 MeV. The Configurations of these bands are assigned on the basis of tilted axis cranking covariant density functional theory (TAC-CDFT) calculations. The measured B(M1) and B(E2) rates of the DB I and DB II, have been well reproduced within the shears mechanism with the principal axis cranking model (SPAC) calculations leading to the conclusion that these bands have been based on the shears mechanism. The large B(E1) transition rates between DB I and DB II indicate the existence of octupole correlation in 142Eu that has also been corroborated through microscopic multidimensionally-constrained covariant density functional theory (MDC-CDFT) calculations. Thus, the present investigation leads to the novel observation of the presence of octupole correlation between two shears band.

Extremely asymmetric shears band in Sm143

A dipole sequence has been observed and investigated in the 143 Sm nucleus populated through the heavy-ion induced fusion-evaporation reaction and studied using the Indian National Gamma Array (INGA) as the detection system. The sequence has been established as a Magnetic Rotation (MR) band primarily from lifetime measurements of the band members using the Doppler Shift Attenuation Method (DSAM). A configuration based on nine quasiparticles, with highly asymmetric angular momentum blades, has been assigned to the shears band in the light of the theoretical calculations within the framework of Shears mechanism with the Principal Axis Cranking (SPAC) model. This is hitherto the maximum number of quasiparticles along with the highest asymmetricity associated with a MR band. Further, as it has followed from the SPAC calculations, the contribution of the core rotation to the angular momentum of this shears band is substantial and greater than in any other similar sequence, at least in the neighbouring nuclei. This band can thus be perceived as a unique phenomenon of shears mechanism in operation at the limits of quasiparticle excitations, as manifested in MR band-like phenomena, evolving into collectivity.

Possible onset of multifaceted excitation modes in Al29

This paper reports on an investigation on the level structure of the odd-mass $^{29}\mathrm{Al}$ nucleus populated via heavy-ion induced fusion evaporation reaction. Rotational sequences with fast magnetic dipole transitions have been identified. These sequences exhibit the essential characteristic features of magnetic-rotation-like behavior, which has been corroborated through lifetime measurements using the Doppler shift attenuation method. The sequences also show features exhibited by axially deformed nuclei. Thus, the level structure presents the possibility of existence of multifaceted excitation modes in $^{29}\mathrm{Al}$, which are believed to be the first of its kind observed in the $A\ensuremath{\approx}$ 30 region. Shell model calculations, carried out within the $sd$-model space, qualitatively reproduce the experimental observations.

Two-Neutron Alignment in 127Xe

High spin states in 127Xe have been investigated via 9Be induced fusion-evaporation reaction at 48 MeV. Spin and parity of excited states up to \( \sim \frac {47}{2} \hbar \) have been confirmed from angular correlation and linear polarization results. Rotational alignment of the second pair of h 11/2 neutrons has been observed at \(\hbar \omega \sim \) 0.44 MeV; beyond that, the band is associated with ν[h 11/2]3 configuration. The alignment phenomena has been discussed in comparison with the neighboring 125,129Xe.

Shape evolution with increasing angular momentum in the Ga66 nucleus

The high-spin nuclear structure of the odd-odd $^{66}\mathrm{Ga}$ nucleus was probed using heavy-ion induced fusion-evaporation reaction. The de-exciting $\ensuremath{\gamma}$ rays were detected by using the Indian National Gamma Array (INGA) consisting of high resolution Compton suppressed clover detectors. The level scheme of $^{66}\mathrm{Ga}$ was extended up to an excitation energy $({E}_{x})$ of $\ensuremath{\sim}10.6$ MeV with spin parity of ${21}^{(+)}$. Two new quadrupole bandlike structures, one positive and one negative parity, based on the states having spin parity of ${J}^{\ensuremath{\pi}}={11}^{+}$ and ${10}^{\ensuremath{-}}$, respectively, were found. The experimental energy levels are compared with calculations within the framework of shell model calculation using $jj44bpn$ effective interaction. The agreement between experiment and theory is good. The total Routhian surface calculations were performed to understand the shape of the $^{66}\mathrm{Ga}$ nucleus associated with the configurations $\ensuremath{\nu}({g}_{9/2}){}^{3}\phantom{\rule{0.16em}{0ex}}\ensuremath{\nu}({f}_{5/2}){}^{2}\phantom{\rule{0.16em}{0ex}}\ensuremath{\pi}({g}_{9/2}){}^{1}\phantom{\rule{0.16em}{0ex}}\ensuremath{\pi}({f}_{5/2}){}^{2}$ and $\ensuremath{\nu}({g}_{9/2}){}^{2}\phantom{\rule{0.16em}{0ex}}\ensuremath{\nu}({f}_{5/2}){}^{1}\phantom{\rule{0.16em}{0ex}}\ensuremath{\pi}({g}_{9/2}){}^{1}\phantom{\rule{0.16em}{0ex}}\ensuremath{\pi}({f}_{5/2}){}^{2}$ assigned to the positive and negative parity bands, respectively. In the frequency domain $\ensuremath{\omega}=0.20\text{--}0.30$ MeV, both the negative and positive parity quadrupole structures, favor prolate deformation whereas at $\ensuremath{\omega}=0.50$ MeV there is a sudden change to noncollective oblate shape in both the bands.

<x content-type="symbol">α</x> decay studies of the neutron-deficient uranium isotopes <sup>215<x content-type="symbol">-</x>217</sup>U

The α -decay study is an important tool for obtaining information on the nuclear structure of heavy neutron-deficient nuclei. For the N =126 and 124 isotones with even proton number, an intruder state ( π h 9/2 π f 7/2)8+ based on the attractive interaction of f 7/2 protons and f 5/2 neutrons was observed, which increases the binding energy of this configuration with increasing proton number. Additionally, taking into account the octupole correlations, a large-scale shell model calculations predict that the ( π h 9/2 π f 7/2)8+ state lies close or below the ( π h 9 / 2 2 ) 6+ state in the 216Th and 218U. To extend the systematic of the I π =8+ isomeric state in N =124 isotones and search for the most neutron-deficient uranium isotopes, an experiment for the α -decay studies of 215 - 217U was performed at the gas-filled recoil separator SHANS at the Heavy Ion Research Facility in Lanzhou (HIRFL). Heavy ion induced fusion evaporation reactions of the 180W(40Ar, x n)220 - x U ( x =3 - 5) were used to synthesize these neutron-deficient uranium isotopes. The 40Ar beam was delivered by the sector focusing cyclotron of the HIRFL. The evaporation residues recoiled from the thin target were separated in-flight from the primary beam particles by the gas-filled recoil separator SHANS. After passing through two multi-wire proportional counters, the residues were planted into a focal plane detection system. Based on the energy-position-time correlation measurement, the isotopes 215 - 217U were identified. At a beam energy of 187.2 MeV, two α -decaying states were observed in 216U with E α =8.384(30) MeV, T 1/2=

On the way to unveiling the atomic structure of superheavy elements

Optical spectroscopy of the transfermium elements (atomic number Z > 100) is nowadays one of the most fascinating and simultaneously challenging tasks in atomic physics. On the one hand, key atomic and even nuclear ground-state properties may be obtained by studying the spectral lines of these heaviest elements. On the other hand, these elements have to be produced “online” by heavy-ion induced fusion-evaporation reactions yielding rates on the order of a few atoms per second at most, which renders their optical spectroscopy extremely difficult. Only recently, a first foray of laser spectroscopy into this heaviest element region was reported. Several atomic transitions in the element nobelium ( Z = 102) were observed and characterized, using an ultra-sensitive and highly efficient resonance ionization technique. The findings confirm the predictions and additionally provide a benchmark for theoretical modelling. The work represents an important stepping stone towards experimental studies of the atomic structure of superheavy elements.

Interaction position resolution simulations and in-beam measurements of the AGATA HPGe detectors

The interaction position resolution of the segmented HPGe detectors of an AGATA triple cluster detector has been studied through Monte Carlo simulations and in an in-beam experiment. A new method based on measuring the energy resolution of Doppler-corrected γ ‐ ray spectra at two different target to detector distances is described. This gives the two-dimensional position resolution in the plane perpendicular to the direction of the emitted γ ‐ ray . The γ ‐ ray tracking was used to determine the full energy of the γ ‐ rays and the first interaction point, which is needed for the Doppler correction. Five different heavy-ion induced fusion-evaporation reactions and a reference reaction were selected for the simulations. The results of the simulations show that the method works very well and gives a systematic deviation of < 1 mm in the FWHM of the interaction position resolution for the γ ‐ ray energy range from 60 keV to 5 MeV. The method was tested with real data from an in-beam measurement using a 30Si beam at 64 MeV on a thin 12C target. Pulse-shape analysis of the digitized detector waveforms and γ ‐ ray tracking was performed to determine the position of the first interaction point, which was used for the Doppler corrections. Results of the dependency of the interaction position resolution on the γ ‐ ray energy and on the energy, axial location and type of the first interaction point, are presented. The FWHM of the interaction position resolution varies roughly linearly as a function of γ ‐ ray energy from 8.5 mm at 250 keV to 4 mm at 1.5 MeV, and has an approximately constant value of about 4 mm in the γ ‐ ray energy range from 1.5 to 4 MeV.

Light-ion-induced reactions in mass measurements of neutron-deficient nuclides close to A = 100

A survey of neutron-deficient nuclides which can be produced via proton- and 3He -induced fusion-evaporation reactions in the A = 100 region was made using a Penning trap as a high-resolution mass filter. A comparison of the measured isotopic rates with a statistical model calculation for the proton-induced reactions shows the importance of using the precise binding energy values for the final reaction products. In particular, proton separation energies were found to play an important role in the evaporation process. In addition, accurate masses of 12 nuclides, 97-99, 101Pd , 100Ag , 101-105Cd and 102, 104In , were determined with uncertainties of less than 10keV.

A sextupole ion beam guide to improve the efficiency and beam quality at IGISOL

The laser ion source project at the IGISOL facility, Jyväskylä, has motivated the development and construction of an rf sextupole ion beam guide (SPIG) to replace the original skimmer electrode. The SPIG has been tested both off-line and on-line in proton-induced fission, light-ion and heavy-ion induced fusion–evaporation reactions and, in each case, has been directly compared to the skimmer system. For both fission and light-ion induced fusion, the SPIG has improved the mass-separated ion yields by a factor of typically 4–8. Correspondingly, the transmission efficiency of both systems has been studied in simulations with and without space charge effects. The transport capacity of the SPIG has been experimentally determined to be ∼10 12 ions s −1 before space charge effects start to take effect. A direct comparison with the simulation has been made using data obtained via light-ion fusion evaporation. Both experiment and simulation show an encouraging agreement as a function of current extracted from the ion guide.

Correction method for Doppler broadened γ-ray lines using the linear momentum of the evaporated charged particles in heavy-ion induced fusion–evaporation reactions

A method based on the conservation of linear momentum is used to correct the position and the width of the γ-ray lines of the residual nucleus produced in heavy-ion induced fusion–evaporation reactions. This method implies the measurement of the total number of the emitted particles, their masses, energies and directions of emission in coincidence with the prompt γ-rays. It has been applied to the 120 MeV 32 S+ 58 Ni reaction whose data have been collected using the high efficiency and large granularity 4 π light charged particle detector DIAMANT, coupled to the 4 π γ-ray spectrometer EUROGAM II.

Decay properties of N∼Z nuclei

By using heavy-ion induced fusion-evaporation reactions at the on-line mass separator of GSI, the decay properties of neutron-deficient isotopes between 56Ni and 100Sn were investigated. The experimental results will be presented and discussed in comparison with model predictions.