0000000000049586
AUTHOR
G. Lhersonneau
Beta Decay of 111Tc to 111Ru
The β-decay of the very neutron-rich 111Tc nucleus has been observed. A partial decay scheme of 111Ru is presented, complementing the high-spin levels earlier reported in spontaneous fission work.
Neutron yield from a thick target irradiated by 90MeV protons
In the context or the design of an intense source of low and intermediate energy, neutrons, the angular and energy distributions of neutrons produced in the interaction of 90 MeV protons in a C-12 target, in which the protons are stopped, have been measured by time-of-flight and activation techniques, As compared to C-12 the yield is less than a factor two higher, while it is somewhat less than for a Be-9 target. (c) 2005 Elsevier B.V. All rights reserved.
First decay scheme of 113Tc and identification of 113Ru m
Very neutron–rich fission products of the mass chain A=113 obtained from the IGISOL on–line mass separator have been investigated by γγ coincidence techniques and γ-spectra multiscaling. Gamma–rays following β–decay of 113Tc have been observed for the first time and a new 0.5 s isomeric state has been found in 113Ru.
Structure of the neutron-rich nucleus 37 94 rb57
A level scheme has been established for 37 94 Rb57 based on studies at the fission-product separator JOSEF ofγγ coincidences following theβ- decay of94Kr. For the 190.7 keV level a half-life of 2.5(8) ns has been determined through the centroid-shift method. The majorβ-decay branch of94Kr proceeds with log ft=3.7 into the 988.4 keV state in94Rb. This transition must be of the 0+→1+ GT type involving the transitionνg7/2→πg9/2. IBFFM calculations have been performed for94Rb, starting from the parametrization used in the previous IBM investigations for the isotope93Rb and the isotone96Y. A theoretical interpretation of the low-lying levels including propositions for the spins is given in a com…
Status report of the Jyvaskyla ion guide isotope separator on-line facility
The ion guide isotope separator facility IGISOL of the University of Jyvaskyla has been moved to the new K-130 heavy ion cyclotron laboratory. The totally reconstructed facility is described in detail. The primary beams and targets, helium pumping, separator beam line construction and separator beam diagnostics are discussed. The spectroscopy stations are introduced with illustrative examples from the research program, including beta-delayed proton and neutron spectroscopy, gamma-ray spectroscopy with and without arrays, conversion electron spectroscopy, collinear laser spectroscopy and nuclear level lifetime spectroscopy.
βdecay of the5+isomer of the odd-odd nucleus100Nb
The decay of the $3s$ isomer of ${}^{100}\mathrm{Nb}$ has been investigated by $\ensuremath{\gamma}$ spectroscopy of online mass separated ${}^{100}\mathrm{Nb}.$ Several new transitions in the daughter nucleus ${}^{100}\mathrm{Mo}$ have been observed. This results in a more consistent decay pattern leading to a firm ${5}^{+}$ assignment to the ${}^{100}\mathrm{Nb}$ isomer. A QRPA based calculation reproduces the decay features best if the $\ensuremath{\pi}{g}_{9/2}\ensuremath{-}\ensuremath{\pi}{p}_{1/2}$ gap is enlarged, confirming the importance of the subshell gap at $Z=40$ previously introduced to account for low-lying ${0}^{+}$ states in other $N=58$ isotones.
Production of refractory elements close to the Z=N line using the ion-guide technique
Production of neutron-deficient isotopes of refractory elements in the A = 80-88 region was studied using the IGISOL technique and the 165 MeV Si-32 + Ni-nat reaction. Radioactive isotopes of Y through Mo could be produced up to the M-T = + 1 line. New information on the decay of the A = 82 and 85 nuclei, including a more detailed decay scheme and more accurate half-life for Y-82, was obtained. (C) 1998 Elsevier Science B.V. All rights reserved.
First observation of β decay ofNb108toMo108
The \ensuremath{\beta}-delayed \ensuremath{\gamma} decay of $^{108}\mathrm{Nb}$ has been studied using the on-line mass-separator technique. The measured \ensuremath{\beta}-decay half-life 0.20\ifmmode\pm\else\textpm\fi{}0.03 s agrees with the previous measurement based on \ensuremath{\beta}-delayed neutron detection. The lifetime of the first ${2}^{+}$ state in $^{108}\mathrm{Mo}$ was measured to be 0.5\ifmmode\pm\else\textpm\fi{}0.3 ns, which is consistent with a deformation \ensuremath{\beta}\ensuremath{\approxeq}0.4. Spin and parity of ${2}^{+}$ are suggested for the ground state of $^{108}\mathrm{Nb}$. \textcopyright{} 1996 The American Physical Society.
Neutron yield from carbon, light- and heavy-water thick targets irradiated by 40MeV deuterons
Abstract Angular and energy distributions of neutrons produced by the interaction of deuterons of 40 MeV in carbon, light- and heavy-water targets, in which they are stopped, have been measured by the activation method. A discrepancy with a time-of-flight measurement for d + C has been found. The results are compared with a Monte-Carlo calculation and are discussed in the frame of building a deuteron-to-neutron converter for the SPIRAL2 radioactive ion-beam facility.
Structure of the Triplet of low-lying states in101Mo
The properties of the triplet of low-lying states in101Mo have been studied through spectroscopy of theγ radiation following thermal neutron capture in100Mo and β− decay of101Nb and through a measurement of the proton angular distributions in the100Mo(d,p) reaction with 14 MeV deuteron energy. The half-lives of the 13.5 keV state and the 57.0 keV 5/2+ state have been measured as 226(7) and 133(7)ns, respectively. These values and the quadrupole/dipole mixing ratios of the 13.5 keV and 43.5 keV transitions yield spin and parity 3/2+ for the 13.5 keV level. The E2 components in the 13.5 (3/2+ →1/2+) and 43.5 keV (5/2+→3/2+) transitions are ≦ 8·10−4 and 54(9)%, respectively. The possibility of…
Intruder states in even-even neutron-rich Pd isotopes
The decays of 112Rh g and 112Rh m obtained as on-line mass separated fission products have been investigated by standard β and γ spectroscopic techniques and γ-γ angular correlations. Low-lying 0+ and 2+ states are identified in the daughter nucleus 112Pd. Systematics suggests the existence of an intruder band which is the lowest in 110Pd, i.e. two neutrons before the midshell.
About the Reliability of Extrapolation of Nuclear Structure Data for r-process Calculations
Gross decay properties are the nuclear part of the input for calculations of elemental abundances. They depend, sometimes very sensitively, on details of nuclear structure. Models for predictions of nuclear masses and shapes have to be used for isotopes very far from stability. The reliability of extrapolations far from experimentally reachable nuclei is, however, not always granted due to singularities in the nuclear landscape. We review data on the region of the neutron-rich isotopes near A = 100, which is a region of especially dramatic changes.
Production of a 15C radioactive ion beam based on 18O(n, α)
In the context of the SPIRAL2 radioactive beam facility the production rate of the neutron-rich 15C nucleus by 18O(n,α) has been investigated. In a water target of 20 cm3, enriched in 18O and placed behind the neutron converter, a rate of a few 1010 nuclei per second can be reached with 1mA of 40MeV deuterons. A 18O(n,α) cross-section based on the activation method is proposed. It is intermediate between the highest and lowest evaluations available to date. peerReviewed
Level scheme ofZr101and structure of theN=61 Sr, Zr, and Mo isotones
The neutron-rich nucleus $_{40}^{101}\mathrm{Zr}_{61}$ has been studied at the isotope separator ISOLDE via the \ensuremath{\beta} decay of $^{101}\mathrm{Y}$. A detailed level scheme has been obtained from \ensuremath{\gamma}-ray singles, \ensuremath{\gamma}-\ensuremath{\gamma}-t and \ensuremath{\beta}-\ensuremath{\gamma}-t coincidence measurements. The level structure of $^{101}\mathrm{Zr}$ is similar to that of its isotone $^{103}\mathrm{Mo}$ showing that, in contrast to their Sr neighbors, the Zr isotopes do not reach maximum of deformation immediately at its onset. This result is rather well reproduced by particle-rotor coupling calculations. A level systematics for the deformed N=61 i…
A new isomer in 125La
Levels in 125La have been studied via β+/EC decay of on-line mass-separated 125Ce using the HIGISOL technique. A new (390 ± 40) ms isomer is definitely attributed to 125La by conversion electron measurements of the 107 keV E3 isomeric transition.
Level structure of ^100Nb
Levels in the odd-odd nucleus ${}^{100}\mathrm{Nb}$ situated at the edge of a region of especially fast shape transitions have been calculated in the framework of the interacting boson fermion fermion model. Levels observed in decay studies can be interpreted in a spherical basis. Low-lying ${I}^{\ensuremath{\pi}}{=8}^{+}$ and ${10}^{\ensuremath{-}}$ states are predicted. Their relationship with the unplaced levels populated with a $12 \ensuremath{\mu}\mathrm{s}$ delay after fission is discussed.
β-delayed neutron decay of 104Y, 112Tc, 113Tc and 114Tc: test of half-life predictions for neutron-rich isotopes of refractory elements
Abstract Beta-decay gross properties of neutron-rich isotopes 104Y and 112,113,114Tc produced in 25 MeV proton-induced fission of 238U have been measured. Decays of 104Y with a half-life of 180±60 ms and of 114Tc with a half-life of 150±30 ms are reported for the first time. Beta-delayed neutron emission probabilities of 112,113,114Tc are determined as (1.5±0.2)%, (2.1±0.3)% and (1.3±0.4)%, respectively. Comparison of the observed Pn values with a recent theoretical calculation based on deformed quasiparticle random phase approximation (QRPA) shows good overall agreement for Tc isotopes. However, comparison of beta-decay half-lives of neutron-rich isotopes of Y to Rh with the QRPA model and…
The 1+→0+ Gamow–Teller strength of the 58Cug.s.→58Nig.s. transition
The ground-state branch in the beta decay of Cu-58 has been remeasured by using gamma-ray detection combined with ion-guide-based on-line mass separation. The measured value 80.8(7)% is three times more precise and in agreement with the earlier reported value. The deduced Gamow-Teller strength of this transition, to be used for the calibration of the charge-exchange reactions, is 0.0821(7). (C) 2001 Elsevier Science B.V. All rights reserved.
IBFFM predictions for low-lying spherical states in the shape-coexisting odd-odd nucleus98Y
Extrapolating the Interacting Boson-Model treatment from lighter neighbours to98Y we predict the properties of low-lying states of spherical nature in IBFFM. This odd-odd nucleus which presents one of the rare cases of coexisting spherical and deformed shapes is of fundamental importance for understanding nuclear phase transitions.
The ground-state deformation of98Sr
The half-life of the first excited state (21+) of98Sr has been determined through the measurement of delayed γ-γ coincidences at the fission product separator OSTIS to t1/2=2.74(12) ns. This value is smaller than those obtained earlier from β− — conversion electron delayed coincidences and it indicates a stronger collectivity of the 21+→01+ transition in98Sr [B(E2)=98 (3) spu] than in100Sr [B(E2)=79(3) spu] which is normally assumed to be the better rotor.
Neutron yield from a 13C thick target irradiated by protons of intermediate energy
Abstract Angular and energy distributions of neutrons produced by the interaction of protons of 20, 25 and 40 MeV in 13C targets, in which they are stopped, have been measured by time-of-flight and activation methods. Neutron yields are compared with a Monte-Carlo based model prediction and with the experimental neutron yields of proton and deuteron beams impinging on natural carbon thick targets. The gain with respect to p + 12C is only sizeable at the lowest proton energies. The neutron yield curve of p + 13C versus proton energy is definitely lower than the one of d + 12C, in contrast to the suggestion by an earlier published measurement at 30 MeV.
Level lifetimes in neutron-rich Ru isotopes
The neutron-rich Ru isotopes are situated between the strongly deformed Sr-Zr region with axial deformation of f l _ 0.40 [1] and the spherical Cd-Sn region. Evidence for triaxiality of even-even Ru isotopes has been presented in [2-5], and lifetimes of some of their 2 + states have been measured in fission studies [6-9]. It seems, however, that in these experiments the nearly identical energies of the 2 + ---* 0 + transitions in l~ and 11~ could not be separated. Therefore, high-resolution measurements are required in order to perceive the evolution of deformation with increasing neutron number. So far only few measurements by the delayed-coincidence method [10] have been performed on chem…
Decay of the neutron-rich isotope 113Ru to 113Rh
The decay of neutron-rich isotope 113Ru obtained as on-line mass separated product of proton-induced fission has been investigated by γγ coincidence and spectrum multiscaling measurements. Decay schemes for both low- and high-spin isomers of 113Ru have been constructed. The level scheme of 113Rh is considerably extended. Systematics of the lowest-lying rhodium levels is smooth. The picture of shape coexistence established for neutron-rich Rh isotopes near-neutron midshell is confirmed with the observation of a K = 1/2 deformed band, with its 3/2+ state at 600 keV being the lowest-lying level and of probable 7/2+ and 5/2+ band members. A large fraction of β feeding is found to populate high-…
Yields of neutron-rich rubidium and cesium isotopes from fast-neutron induced fission of 238U, obtained by studying their release from a thick ISOL target
Abstract In the framework of the EURISOL project, the production of neutron-rich isotopes of rubidium and cesium has been studied. The intensities of mass-separated beams of rubidium and cesium isotopes generated from a thick 238 UC x target connected to a surface ionizer have been measured. The release properties of 140 Cs and 91 Rb have been investigated. The obtained results allowed us to determine the relative fission yields of rubidium and cesium isotopes with high accuracy.
Lifetime measurements in99Sr and100Zr
The lifetime of the first excited state in99Sr has been determined by the delayed γ-γ coincidence method. The value of t/2=0.58(9) ns corresponds to a deformation parameter s=0.38(4). The deduced value confirms the previously proposed ν[411]3/2 Nilsson assignment for the99Sr ground state. In addition, lifetime values for the 2+ (t1/2=0.40(8) ns) and 0+ (t1/2=5.36(23) ns) states in100Zr are obtained, which deviate from older measurements and indicate a larger deformation of s=0.39(4).
Beta-Decay Half-Lives and Neutron-Emission Probabilities of Very Neutron-Rich Y to Tc Isotopes
Neutron-rich {sub 39}Y to {sub 43}Tc isotopes have been produced by fission of uranium with a 50MeV H{sub 2}{sup +} beam. Beta-decay half-lives, delayed neutron-emission probabilities, and production yields have been measured and compared with theory. Beta decay of 4 new isotopes is reported, and the {beta}-delayed neutron-emission mode has been discovered for 12 isotopes of the elements niobium and technetium. The results compared to quasiparticle random phase approximation predictions indicate the increasing importance of fast {beta} transitions to high-lying states of nuclei with large neutron excess. {copyright} {ital 1996 The American Physical Society.}
Identical transitions in the strongly deformed99Srand100Sr
The decay of the very neutron-rich nucleus ${}^{100}\mathrm{Rb}$ has been studied by \ensuremath{\gamma} spectroscopy of online mass-separated samples. Schemes for \ensuremath{\beta} decay to ${}^{100}\mathrm{Sr}$ and $\ensuremath{\beta}n$ decay to ${}^{99}\mathrm{Sr}$ are presented. New sets of transitions in ${}^{99}\mathrm{Sr}$ and ${}^{100}\mathrm{Sr}$ with identical energies are observed. All identical bands so far observed in neutron-rich Sr isotopes obey a simple energy rule valid for even-even, odd-even, and odd-odd bands.
Structure of theN=59 nucleus97Sr: coexistence of spherical and deformed states
A band with a rotational pattern based on a state at 585.1 keV has been identified in theN=59 neutron-rich nucleus97Sr. Its properties lead to the [422] 3/2 Nilsson-orbital assignment for the band head. There is evidence for a second band with the head at 644.7 keV and the configuration [541] 3/2. Since the ground state and the lowest excited levels are spherical, shape coexistence is established for97Sr. A deformed nature of several levels at 500–600 keV results also from QRPA-model calculations. The structure of the low-lying spherical levels has been studied in the frame of the IBF model. The results of the present investigations lead to a better understanding of theN=59 isotones which c…
Shape coexistence in theN=59 isotone97Sr
Experimental evidence for shape coexistence in the N=59 isotone97Sr is presented. The ground state and the lowest excited levels are confirmed to be spherical. At 585.1 keV, a K=3/2 rotational band built on the v[422 3/2] Nilsson configuration has been identified. Nilsson orbital assignments for three further levels of deformed origin are proposed. The results are compared to RPA shell-model predictions.
Decays of110Rhand112Rhto the near neutron midshell isotopes110Pdand112Pd
The decays ${}^{110}\mathrm{Rh}$ and ${}^{112}\mathrm{Rh}$ have been investigated using on-line mass separation with the ion-guide technique. Extended decay schemes have been constructed for both the low- and high-spin states in ${}^{112}\mathrm{Rh}$. Mixing ratios for the collective transitions from the ${2}_{2}^{+}$ and ${3}_{1}^{+}$ states in ${}^{112}\mathrm{Pd}$ have been measured by $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ angular correlation. The presence of two sets of ${0}^{+}$ and ${2}^{+}$ states in the 1.1\char21{}1.4 MeV range suggests the existence of an intruder band, the energy of which is the lowest in ${}^{110}\mathrm{Pd}$ with two neutrons fewer than the mid…
IBMF and IBFFM Approach to Nuclei in the A≃100 Region
An overview of the calculations in IBFM and IBFFM for nuclei in the A ≃ 100 region is presented. The application to these nuclei with a complex structure including the rapid transition from spherical to deformed nuclear shapes provides a stringent test for the capacities of this theoretical approach. From the result of the studies of the heavy Yttrium isotopes and the N=59 isotones it is concluded that it can account for the basic structure and the phase transition in these nuclei.
The decay scheme of the 3-s Isomer of100Nb and the properties of levels in100Mo
Theβ− decay of the longer-lived isomer in100Nb has been studied at the fission-product separator JOSEF. Measurements ofγ-ray singles spectra, ofγ-γ coincidences and ofγ-γ angular correlations have been performed. A value oft1/2=2.99(11)s has been determined for the isomer which probably hasIπ=4+ or 5+. A scheme of the levels of100Mo which are populated in the decay of this isomer has been established. Information on the spins of several states of100Mo has been obtained. Thus,I=0 levels have been identified at 1,505, 2,038 and 2,087 keV. The mixing ratios have been determined for the 22+→21+ and 23+→21+ transitions. The results provide evidence for a vibrational structure of100Mo with separa…
Medium-spin structure of neutron-rich Pd and Cd isotopes
Cadmium isotopes are traditionally considered as good examples of nearly spherical, vibrational nuclei, which can be described by quadrupole vibrators in the collective model, as well as the U(5) dynamical symmetry in the interacting boson model (IBM). However, the shape coexistence of more deformed intruder states originating from proton 2p–4h excitation across the Z = 50 major shell is another interesting structural aspect, whose presence in Pd-isotopes has also been demonstrated [1,2].
K-isomeric state in even-even100Sr
A t1/2=85 ns isomer has been identified at 1619 keV in100Sr. The halflife is attributed to a ΔK=4 hindered decay. The configuration is proposed to be built on the [411]3/2 and the [532]5/2 neutron quasiparticle orbitals.
Nuclear structure of97Y in the interacting boson-fermion model
Unique-parity states of97Y are studied in the framework of the interacting boson-fermion model (IBFM). Spin assignments for the levels of the unique-parity yrast band are proposed on the basis of nuclear systematics and the IBFM band pattern associated with coupling of a particle-type quasiparticle (g9/2) to the SU(5) core3896Sr58. The good fit of the experimental data with the IBFM calculations confirms the single particle character of this immediate neighbour of the doubly sub-magic nucleus96Zr.
Decay spectroscopy of neutron-rich nuclei with A $$ \simeq $$ 100
We review structure data obtained by decay spectroscopy of neutron-rich nuclei of mass close to 100. Emphasis is put on the contribution of experiments at IGISOL in the nineties. They confirmed the earlier postulated shape coexistence in the fast shape-transition region between N = 58 (spherical ground states and low collectivity) and N = 60 (strong axial deformation). A detailed spectroscopic study of the A = 99 chain established the upper-Z limit of the N = 56 shell closure region with 99Nb, owing to striking similarities with 97Y. A consequence of the N = 56 closure is that the s 1/2 odd-neutron becomes the ground state of the most neutron-rich N = 57 isotones, starting with 99Mo, instea…
Level structure of 99Nb
The β decay of 97Sr to 97Y has been investigated using ion-guide on-line mass separation and a 10 Ge-detector array to record γ−γ coincidences to a detection limit well below that of former studies. Similarities are found in the β-decay patterns of 99Zr and of its isotone 97Sr and also in the γ-ray decay rates and branchings of the corresponding levels in their respective daughters 99Nb and 97Y. This indicates a persisting influence of the d5/2 neutron shell closure for 99Nb. The level structure of 99Nb and the β-feeding pattern are discussed in the frame of the interacting boson-fermion plus broken pair model and the microscopic quasiparticle phonon model.
Shape coexistence near the double-midshell nucleus 111Rh
The decay of 111Ru obtained from fast on-line chemical and mass separation has been investigated by β-γ-t and γ-γ coincidence techniques. Earlier spin and parity assignments of 111Rh levels based on extrapolations of level systematics are confirmed. In particular, the K=1/2 intruder band is supported by the hindrance of E2 transitions between deformed and spherical states and enhancement of intraband E2 transitions. The excitation energies of intruder band members in Rh isotopes show a minimum at 109Rh64, with two neutrons less than 111Rh at the N=66 midshell. This trend, which differs from the one in the higher-Z neighbouring elements Ag and Cd with minima at N=66, follows the evolution of…
Band Structures in 96Zr and 98Zr Studied Through the High-Spin Beta Decays of the Y Parents
The isotopes 96Zr and 98Zr are presently subject to intensive studies since their properties play a key role in the understanding of the rapid transition from spherical to deformed nuclear shapes at N ∼ 60. An interesting question is the strength of the subshell closures at Z = 38,40 and N = 56 and to which extent the structure of 96Zr, 98Zr and of their immediate neighbours is influenced by these shell effects. There is evidence, although limited, for closed- shell-or individual-particle character of these nuclei, but more information on the nature of the excited states, especially those with sizeable spins is desired. This is expected to help clarifying the situation with respect to the p…
Production of neutron-rich nuclei in fission induced by neutrons generated by the p + 13C reaction at 55 MeV
Cross-sections for the production of neutron-rich nuclei obtained by neutron-induced fission of natural uranium have been measured. The neutrons were generated by bombarding a 13C target with 55 MeV protons. The results, position of the maximum in the (Z, A)-plane, width and magnitude, are very comparable with those where the neutrons are generated by bombardment of natural 12C graphite with 50 MeV deuterons. Depending on the geometry of the converter/target assembly the isotope yields, however, are a factor of 2-3 lower due to less efficient production of neutrons per primary projectile, especially at small forward angles.
New interpretation of shape coexistence in99Zr
Levels in ${}^{99}\mathrm{Zr}$ populated by $\ensuremath{\beta}$ decay of on-line mass separated ${}^{99}$Y have been studied by $\ensuremath{\gamma}$-ray spectroscopic methods, including $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidences recorded with an array of ten Ge detectors and level-lifetime measurements. The formerly reported strongly collective character of the 53-keV transition turns out to be questionable. This implies a revision of the experimental evidence for shape coexistence in ${}^{99}\mathrm{Zr}.$ Transition rates and $\ensuremath{\gamma}$-ray branching ratios make a new level at 679 keV with ${t}_{1/2}=9$ ns another candidate for a deformed state.
Nature of93Rb levels studied in IBFM
The structure of the low-lying states with negative parity of3793Rb56 has been studied in the framework of the Interacting Boson-Fermion Model (IBFM) using the even-even neighbour3894Sr56 as a core. A good description of the properties of the levels has been obtained. A sensitive test of the quality of the theoretical studies is the decay pattern of the second excited state at 267 keV with a half-life of 2.0(2) ns. The calculations givet1/2=2.26 ns and they predict the existence of a 13 keV transition into the first excited state, the existence of which has been confirmed afterwards by experiments.
High-resolution studies of beta-delayed proton emitters at IGISOL facility
Beta-decays of23Al and41Ti have been studied by applying ion guide techniques, gamma detection and a gas-Si telescope for charged-particle detection. The experimental beta-decay strength of41Ti was found to be quenched by a factor ofq2 = 0.64 compared to our shell model calculations below 8 MeV excitation energy in41Sc. This result is in line with the generally accepted value ofq = 0.77. The isobaric analogue state of the41Ti ground state was measured to contain 10(8)% isospin impurity. This relatively high value could be well reproduced by the shell model calculation in thesdfp space. Gamma-decay of theT = 3/2 isobaric analogue state in23Mg was observed together with its decay by proton em…
Independent yields of neutron-rich nuclei in charged-particle induced fission
Abstract Yields of fission products have been studied at the recently upgraded IGISOL facility in Jyvaskyla using 50 MeV H2+ beam (Ep = 25 MeV) and thin natTh and natU targets. The independent yields of fission products in the mass regions A = 99−112 (elements Y, Nb and Tc) and A = 127−134 (elements Sn, Sb and Te) have been obtained. Comparisons of the experimental isotopic distributions with theoretical ones are presented. Enhancement of the production rate of the doubly magic 132Sn is observed in the experimental yield curve.
Decays of theY97isomers to the single neutron nucleusZr97
The decays of the 9/2(+) and of the three-quasiparticle high spin isomer in Y-97 have been investigated following on-line mass separation with the ion-guide technique. Spins and parities for several of the lowest-lying levels in the daughter nucleus Zr-97 are revised and configurations are proposed. In particular, the 2264 keV level is identified as the h(11/2) single-neutron level. The population of a cascade on top of the h(11/2) level provides evidence for Gamow-Teller beta decay of the high-spin Y-97 isomer. Levels and transition rates are qualitatively well reproduced by regarding Zr-97 as a single-neutron nucleus with respect to the doubly closed Zr-96 core.
AK=3two-quasiparticle isomer in98Sr
The decay of on-line mass-separated ${}^{98}\mathrm{Rb}$ to ${}^{98}\mathrm{Sr}$ is studied by $\ensuremath{\gamma}$ spectroscopy. The revised decay scheme adds further evidence of the coexistence of very different shapes in ${}^{98}\mathrm{Sr}.$ A set of levels is proposed to originate from particle-hole pair excitations across the $Z=40$ spherical gap in analogy with ${}^{96}\mathrm{Sr}.$ A deformed $K=3$ band with probable even parity is built on a 7.1-ns isomer at 1838 keV. It is interpreted as a two-quasineutron excitation in accordance with a quantum Monte Carlo pairing calculation based on a deformed shell model. Configurations of the calculated lowest-lying two-quasiparticle levels …
Structure of the highly deformed nucleus101Sr63 and evidence for identical K=3/2 bands
The low-energy level scheme of theN=63 nucleus101Sr has been obtained from a β-decay study of101Rb. The ν[532]5/2 and ν[411]3/2 orbitals are well established as the ground state and a band head at 271.2keV, respectively. The general properties of the level scheme indicate a quadrupole deformation ofβ ≃ 0.4, confirming the unique feature of saturation of deformation inN ≥ 60 Sr isotopes. The energies of theK=3/2 intraband transitions in the odd-neutron neighbours99Sr61 and101Sr63 are very similar. More generally, the identical bands at low spin in98–101Sr nuclei are correlated with the mass-independent moments of inertia in this region.
New levels in 118Pd observed in the beta-decay of very neutron-rich 118Rh isotope
We investigate the β decay of very neutron-rich 118Rh isotope using on-line mass-separated sources which are produced by applying 25 MeV proton induced symmetric fission of natural uranium at the IGISOL facility. The β–γ and γ–γ coincidence spectroscopy is employed to establish the level scheme of daughter nucleus 118Pd. Five low-lying new levels are identified for the first time with tentative spin and parity assignments based on the even-mass Pd systematics.
High-resolution study of the beta decay of 41Ti
Abstract An improved high-resolution study of the beta decay of 41 Ti, produced in the 40 Ca( 3 He, 2n) reaction at 40 MeV, has been performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility. The beta-delayed radiation was detected by a low-energy charged-particle detector and a large Ge detector for gamma rays. The experimental beta-decay strength and its distribution, extracted from delayed-proton data, are compared with results of shell-model calculations in the sdfp space. The lowest J π = 3 2 + , T = 3 2 state in 41 Sc, the isobaric analogue state of the 41 Ti ground state, is estimated to contain 10% isospin impurity.
Improved ion guide for heavy-ion fusion-evaporation reactions
The ion guide for heavy-ion-induced reactions developed originally for the SARA facility in Grenoble has been implemented at the Jyvaskyla IGISOL facility. For the Cd-116(Ar-40, 6n)Dy-150 reaction an efficiency of 0.5% relative to the number of reaction products entering the stopping chamber was obtained. This is 3.5 times higher than previously obtained at SARA and corresponds to a yield of about 100 ions/(p mu C mb). Mass-separated yields for the Ar-36 + Mo-92 and Ar-36 + Mo-94 reactions were measured. (C) 1998 Elsevier Science B.V. All rights reserved.
First observation of nonyrast levels inZr103and level systematics ofN= 63 Sr, Zr, and Mo isotones
The {beta} decay of the very-neutron-rich nucleus {sup 103}Y has been studied at the isotope separator IGISOL, allowing for the first time the observation of nonyrast levels in its daughter {sub 40}{sup 103}Zr{sub 63}. The level structure is similar to that of its isotones {sup 101}Sr and {sup 105}Mo, suggesting a large ground-state deformation of {beta}{approx_equal} 0.4. Level systematics allows for new level assignments in both {sup 103}Zr and {sup 105}Mo. {copyright} {ital 1996 The American Physical Society.}
Evidence for rotational bands in103Nb
The nucleus103Nb62 has been studied through the s− decay of the fission product103Zr at LOHENGRIN and JOSEF. The energies of the lowlying levels and the γ transitions indicate rotational bands based on the Nilsson proton configurations [422 5/2+] [303 5/2−] and [301 3/2−] at 0, 164 and 248 keV, respectively. The measured half-lives of (4.7±0.5) ns and (2.0±0.6) ns of the levels at 164 and 248 keV are consistent with the Nilsson-model estimates including pairing.
Saturation of deformation atN=60 in the Sr isotopes
From the halflife oft 1/2=3.91 (16) ns for the 21 + state in100Sr a deformation parameter ofβ=0.40(1) has been derived. Within the given uncertainties, this value is equal to the recently determinedβ-parameters for98Sr and99Sr, indicating that in the Sr isotopes saturation of deformation is reached immediately at its onset well before neutron midshell. This behaviour is reflected by the “twin” character of98Sr and100Sr with identical features of their unperturbed ground-state rotational bands. These observations are discussed in terms of strong two-nucleon interactions.
Nuclear structure of97Yin the interacting boson fermion plus broken pair model and the nature of the 3.523 MeV high-spin isomer
Nuclear structure of 97Y is described in the interacting boson fermion plus broken pair model, including quasiproton and quasiproton-two-quasineutron configurations in the basis states. In particular, the yrast bands and the decay of the 27/2- high-spin isomer are accounted for in this approach.
Identical transitions in the strongly deformed Sr-99 and Sr-100
The decay of the very neutron-rich nucleus Rb-100 has been studied by gamma-spectroscopy of on-line mass-separated samples. Schemes for beta-decay to Sr-100 and beta-n-decay to Sr-99 are presented. New sets of transitions in Sr-99 and Sr-100 with identical energies are observed. All identical bands so far observed in neutron-rich Sr isotopes obey a simple energy rule valid for even-even, odd-even and odd-odd bands.
A facility for fast-neutron irradiations at Jyväskylä and its use for nuclide cross-section measurements in fission
Abstract An efficient and reliable transport system for fast-neutron irradiations has been built at the Physics Department, Jyvaskyla, Finland. It is constructed from commercial bicycle components and is driven by a computer-controlled stepping motor. It can be operated in single or cyclic mode. The neutron irradiated targets are moved within 1.2 s (full stop to full stop) to a well-shielded position 3 m away where they can be removed or directly investigated by γ spectroscopy. The system has been built with the aim to experimentally verify the calculated production rates of neutron-rich nuclei in the SPIRAL2 uranium target. However, the facility can be used for various kinds of fast-neutro…
Beta decay of very neutron-rich110Mo studied at the new IGISOL facility
The decay of the new activity110Mo (t1/2=0.30(4).s) has been observed at the new IGISOL separator. Multiscaled singles,β-γ-t and γ-y-t coincidences were recorded. The decay scheme suggests Iπ=2+ for the ground state of the daughter nucleus110Tc. Three 1+ levels are fed with logft values below 5, indicating no drastic change among Mo and Ru decays at the middle of the neutron shell. This experiment confirms the expectation that the new IGISOL facility will allow the identification of one or two new more neutron-rich isotopes per element in this region.
First decay study of the very neutron-rich isotope Br-93
The decay of the mass-separated, very neutron-rich isotope Br-93 has been studied by gamma-spectroscopy. A level scheme of its daughter Kr-93 has been constructed. Level energies, gamma-ray branching ratios and multipolarities suggest spins and parities which are in accord with a smooth systematics of the N=57 isotones for Z less-equal 40, suggesting the N=56 shell closure still to be effective in Kr isotopes. So far, there is no indication of a progressive onset of deformation in neutron-rich Kr isotopes.
First evidence for the 2+ level in the very neutron-rich nucleus102Sr
Evidence for the 2+ level in102Sr is obtained from theβ-decay of102Rb, mass separated at the ISOLDE facility. The energy of 126 keV and the collectivity of the 2+→0+ transition indicate that102Sr is a strongly deformed rigid rotor. In addition, a sizeable lifetime is measured for the 94 keV transition in102Y.
Experimental Evidence for Shape Coexistence in 97Sr59 and Implications for the Structure of the Odd-Odd Isotone 98Y
Experimental evidence for shape coexistence in the odd-mass N=59 isotone 97Sr is presented. The ground state and the lowest excited levels of 97Sr are shown to be spherical, whereas a rotational band based on a state at 585 keV has been identified. Three further levels of deformed origin are proposed. Nilsson-orbital assignments are supported by RPA shell-model calculations. These findings lead to a better understanding of the structure of the odd-odd N=59 isotone 98Y.
Superasymmetric fission at intermediate energy and production of neutron-rich nuclei with A < 80
Abstract The yields of neutron-rich Ni, Cu, Zn, Ga and Ge-isotopes were measured in 25 MeV proton induced fission of 238 U using the ion guide-based isotope separator technique. The results indicate enhancement for superasymmetric mass division at intermediate excitation energy of the fissioning nucleus and show the potential of this reaction for the production of neutron-rich exotic nuclei around Z = 28.
Evolution of deformation in the neutron-rich Zr region from excited intruder state to the ground state.
Strong evidence for the existence of previously postulated deformed excited states in $^{98}\mathrm{Zr}_{58}$ has been obtained by an accurate measurement of the ${\mathrm{\ensuremath{\rho}}}^{2}$(${0}_{3}^{+}$\ensuremath{\rightarrow}${0}_{2}^{+}$) value for the decay of the second excited ${0}^{+}$ state at 1436 keV. In the neighboring isotope $^{99}\mathrm{Zr}_{59}$, at the edge of the region of strong ground-state deformation, evidence is found for a rotational band built on the state at 614 keV. Systematics of the excitation energies of the deformed states in Zr nuclei and their Sr neighbors show that the apparent sudden onset of deformation is explained by the steady lowering of a stro…
Production of neutron-rich surface-ionized nuclides at PARRNe
Yields of neutron-rich isotopes produced by fast neutron induced fission of 238 U were measured at the ISOL set-up Production dAtomes Radioactifs Riches en Neutrons. A surface ion source was used to ionize selectively elements with low ionization potentials. In order to observe also the most n-rich isotopes, the identification was achieved by a combined measurement of b- and c-rays and b-delayed neutrons. The flux of fast neutrons inducing fission was generated by a 1 lA beam of 26 MeV deuterons stopped in a graphite converter. The target was a standard ISOLDE type 238 UC
Production of neutron-rich isotopes in fission of uranium induced by neutrons of 20 MeV average energy
In the context of a parameter study conducted by several laboratories for future European radioactive beam facilities based on fast-neutron induced fission, in particular for the SPIRAL-II project at GANIL, we have measured the yields of neutron-rich isotopes in the mass range of 88 to 144. These nuclei were obtained as fission products of natural uranium bombarded by neutrons of 20 MeV average energy emitted by a thick carbon target irradiated by 50 MeV deuterons. Yields have been measured using on-line mass separation with the ion-guide method. Compared with proton-induced fission at 25 MeV the magnitude of cross-sections, except for the symmetric region, is similar. Z-distributions of is…