0000000000178121
AUTHOR
A. Wöhr
showing 36 related works from this author
Beta decay of neutron-rich cobalt and nickel isotopes
1998
We report on the first β-γ spectroscopy measurements of the neutron-rich 68–70Co and 70–74Ni nuclei, produced in proton-induced fission of 238U and ionized in a laser ion guide coupled to an on-line mass separator. Several γ lines from the decay of these nuclei have been identified, half-lives determined and production cross sections deduced. The derived level schemes for the copper and nickel isotopes show that the occupation of the ν(1g9/2) state has a strong influence on the structure of these neutron-rich nuclei. This may have a clear impact on the predicted structure and decay properties of doubly-magic 78Ni.
Origin of the odd-even staggering in theA?80 solarR-abundance peak
1990
In contrast to the conventional picture, the r-process yields in the A≃80 abundance peak exhibit a pronounced odd-even staggering. It is proposed that this behaviour may be due to strong β-delayed neutron branching from a few odd-mass isotopes located in or close to the r-process path.
Selective laser ionization of N≥82 indium isotopes: The new r-process nuclide 135In
2002
Production yields and beta-decay half-lives of very neutron-rich indium isotopes were determined at CERN/ISOLDE using isobaric selectivity of a resonance-ionization laser ion-source. Beta-delayed neutron multiscaling measurements have yielded improved half-lives for 206(6)~ms $^{132}$In, 165(3)~ms $^{133}$In and 141(5)~ms $^{134}$In. With 92(10)~ms $^{135}$In, a new r-process nuclide has been identified which acts as an important `waiting-point' in the In isotopic chain for neutron densities in the range n$_n \simeq 10^{24}$--10$^{26}$ n/cm$^3$, where the r-matter flow has already passed the ${\rm A} \simeq 130$ abundance-peak region.
First decay study of the very neutron-rich isotope 93Br
2001
The decay of the mass-separated, very neutron-rich isotope 93 Br has been studied by γ spectroscopy. A level scheme of its daughter 93 Kr has been constructed. Level energies, γ-ray branching ratios, and multipolarities suggest spins and parities which are in accord with a smooth systematics of the N = 57 isotones for Z <~ 4 0 , suggesting the N = 56 subshell 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. peerReviewed
Helium burning of22Ne
1989
The22Ne(α, γ)26Mg and22Ne(α, n)25Mg reactions were investigated forE α(lab) from 0.71 to 2.25 MeV. Neon gas enriched to 99% in22Ne was recirculated in a differentially pumped gas target system of the extended type. Theγ-ray transitions were observed with Ge(Li) detectors and the neutrons with3He ionization chambers. A previously known resonance at ER(lab)=2.05 MeV was verified and 15 new resonances were found in the energy range covered, with the lowest at ER(lab)=0.83 MeV. Information on resonance energies, widths, strengths,γ-ray branching ratios, as well asJ π assignments, is reported. The energy range investigated corresponds to the important temperature range ofT 9 from 0.3 to 1.4 (109…
Level lifetimes in neutron-rich Ru isotopes
1995
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…
Structure of exotic 7He and 9He
2004
The heavy helium isotopes 7,9 He were studied via their isobaric analog states (IAS) in 7,9 Li. The IAS were populated via resonance reactions of protons with radioactive beams of 6,8 He. The isospin-conserving neutron decay of T=3/2 resonances in 7 Li and proton decay of T=5/2 resonances in 9 Li were measured. New spectroscopic information on these states were obtained, and compared with the properties of levels in 7,9 He.
Structure And Decay Of Neutron-Rich Nuclides In The 115 ≤ A ≤ 138 Mass Range And r-Process Nucleosynthesis
2005
The structure and decay of neutron‐rich r‐process nuclides has been studied by a variety of means that take advantage of enhanced selectivity to permit identification of exotic nuclides. New level structures are presented for 134,135Sb along with data for Ag isomers and Cd yrast structures. Some of the properties measured play an important role in calculations of the yields of elements and isotopes produced in r‐process nucleosynthesis that takes place at high temperature in the presence of large densities of neutrons.
Calculation of beta-decay properties of neutron-rich nickel isotopes
1989
Gamow-Tellerβ-decay properties of neutron-rich Ni isotopes have been calculated in the RPA with Nilsson model wave functions. On the basis of the results for the known isotopes67–69Ni, half-lives and neutron-emission probabilities for70–82Ni are predicted and compared to earlier model estimates. Possible implications onr-process nucleosynthesis are discussed.
Nuclei Far from Stability and the R-Process Waiting-Point Concept
1992
The nucleosynthesis process by rapid neutron captures (the r-process) is responsible for the formation of about half of the nuclear species in nature beyond Fe. While the astrophysical site for the r-process is not yet unambiguously identified, its association with the cores of low-mass stars undergoing type II supernova (SN) events is strongly suggested (see, e.g., Refs.1,2).
Chlorine loss from polyvinylchloride under neutron irradiation
1997
PVC samples were irradiated for 1 hour with a thermal neutron flux of Φ th =4.71·1011n·cm−2·s−1 and the chlorine lost during irradiation was measured by γ-ray spectrometry. About 15% of loss of chlorine has been observed for untreated samples while samples heated to temperatures of 60 and 80°C for one minute before irradiation have been found to loose about 8% and 3%, respectively. The results indicate an influence of the polymer structure on the release of chlorine.
N=82Shell Quenching of the Classicalr-Process “Waiting-Point” NucleusCd130
2003
First $\ensuremath{\beta}$- and $\ensuremath{\gamma}$-spectroscopic decay studies of the $N=82$ $r$-process ``waiting-point'' nuclide $^{130}\mathrm{C}\mathrm{d}$ have been performed at CERN/ISOLDE using the highest achievable isotopic selectivity. Several nuclear-physics surprises have been discovered. The first one is the unanticipatedly high energy of 2.12 MeV for the [$\ensuremath{\pi}{g}_{9/2}\ensuremath{\bigotimes}\ensuremath{\nu}{g}_{7/2}]$ ${1}^{+}$ level in $^{130}\mathrm{I}\mathrm{n}$, which is fed by the main Gamow-Teller transition. The second surprise is the rather high ${Q}_{\ensuremath{\beta}}$ value of 8.34 MeV, which is in agreement only with recent mass models that include…
Gamow-Teller decay ofZn80: Shell structure and astrophysical implications
1988
Gamow-Teller \ensuremath{\beta} decay of $^{80}\mathrm{Zn}$ to levels in $^{80}\mathrm{Ga}$ has been calculated from a random-phase-approximation shell model with Nilsson model wave functions. The predicted features are compared to existing experimental results. Gross \ensuremath{\beta}-decay properties as well as quasiparticle structure indicate shape coexistence for $^{80}\mathrm{Ga}$. This result implies a rather rapid weakening of the shell strength far from \ensuremath{\beta} stability above $_{28}^{78}\mathrm{Ni}_{50}$. The nuclear properties of $^{80}\mathrm{Zn}$ as an astrophysical ``waiting-point'' nucleus suggest that the r-process scenario which is responsible for the solar r abu…
Single-Neutron States inS133n
1996
The location of several single-neutron states in ${}^{133}\mathrm{Sn}$ has been identified. The ${p}_{3/2}$, ${h}_{9/2}$, and ${f}_{5/2}$ states were found at 853.7, 1560.9, and 2004.6 keV, respectively, by measuring $\ensuremath{\gamma}$ rays in coincidence with delayed neutrons following the decay of ${}^{134}\mathrm{In}$. Crucial for obtaining the new data were the improved yields at the mass-separator facility ISOLDE-PSB at CERN. A semiempirically adjusted Woods-Saxon calculation, based on parameters from the Pb region and normalized on the mass data at ${}^{132}\mathrm{Sn}$, reproduces the new single particle energies with good precision.
Production of radioactive Ag ion beams with a chemically selective laser ion source
1997
Abstract We have developed a chemically selective laser ion source at the CERN-ISOLDE facility in order to study neutron-rich Ag nuclides. A pulsed laser system with high repetition rate has been used based on high-power coppe-vapour pump lasers and dye lasers. With this source significant reductions of the isobaric background has been achieved.
β-decay measurements ofA≃ 70 − 110 r-process nuclei at the National Superconducting Cyclotron Laboratory
2011
The present paper reports on several r-process motivated β-decay experiments undertaken at the National Superconducting Cyclotron Laboratory. β-decay half-lives and β-delayed neutron-emission probabilities were measured for nuclei around the r-process A = 70–80 and A = 90 – 110 mass regions. The data are discussed on the basis of quasi-random phase approximation calculations. The emphasis is made on the impact of these data upon calculations of r-process abundances.
Beta Decay of68–74Niand Level Structure of Neutron-Rich Cu Isotopes
1998
The isotopes ${}^{68--74}\mathrm{Ni}$, of interest both for nuclear physics and astrophysics, have been produced in proton-induced fission of ${}^{238}\mathrm{U}$ and ionized in a laser ion guide coupled to an on-line mass separator. Their $\ensuremath{\beta}$ decay was studied by means of $\ensuremath{\beta}$- $\ensuremath{\gamma}$ and $\ensuremath{\gamma}$- $\ensuremath{\gamma}$ spectroscopy. Half-lives have been determined and production cross sections extracted. A partial level scheme is presented for ${}^{73}\mathrm{Cu}$ and additional levels for ${}^{71}\mathrm{Cu}$, providing evidence for a sharply lowered position of the $\ensuremath{\pi}{1f}_{5/2}$ orbital as occupancy of the $\ens…
Beta decay of neutron-rich Co: Probing single-particle states at and above the N=40 subshell closure
1999
Neutron-rich Co nuclei with A=66–70 were produced by the laser-ionization isotope-separation on-line method. The β decay from these nuclei has been studied. A case example is given by reporting on the observed decay scheme of 68Co. The half life of the ground-state decay of this nucleus was measured to be 0.21(3) seconds. In addition, a new β decaying isomer half life of 1.16(25) seconds was discovered. The level scheme of 68Ni has been significantly extended, and an interpretation of the observed levels is made by assuming that the N=40 gap has the characteristics of a shell closure.
New states in heavy Cd isotopes and evidence for weakening of the N = 82 shell structure
2000
A chemically selective laser ion source has been used in a β-decay study of heavy Ag isotopes into even-even Cd nuclides. Gamma-spectroscopic techniques in time-resolving event-by-event and multiscaling modes have permitted the identification of the first 2+ and 4+ levels in 126Cd78, 128Cd80, and tentatively the 2+ state in 130Cd82. From a comparison of these new states in 48Cd with the E(2+) and E(4+)/E(2+) level systematics of 46Pd and 52Te isotopes and several recent model predictions, possible evidence for a weakening of the spherical N = 82 neutron-shell below double-magic 132Sn is obtained.
Selective laser ionization of radioactive Ni-isotopes
1997
Abstract A chemically selective laser ion source based on resonance ionization of atoms in a hot cavity has been applied in the study of Ni-isotopes at the CERN-ISOLDE on-line isotope separator. Laser ionization enhanced the yields of long-lived Ni-isotopes almost four orders of magnitude when compared to the yields obtained with the surface ionization mode of the source. As a result, high yields of long-lived Ni-isotopes were obtained. Separation efficiencies of 0.3 and 0.8% were obtained for Ni produced in uranium-carbide, produced from uranium-di-pthalocyanine, and Ta-foil targets, respectively. Ni was found to be released very slowly from the present target and ion source combination.
β-decay studies of r-process nuclei at NSCL
2008
Abstract Observed neutron-capture elemental abundances in metal-poor stars, along with ongoing analysis of the extremely metal-poor Eu-enriched sub-class provide new guidance for astrophysical models aimed at finding the r-process sites. The present paper emphasizes the importance of nuclear physics parameters entering in these models, particularly β -decay properties of neutron-rich nuclei. In this context, several r-process motivated β -decay experiments performed at the National Superconducting Cyclotron Laboratory (NSCL) are presented, including a summary of results and impact on model calculations.
β-decay half-lives andβ-delayed neutron emission probabilities of nuclei in the regionA≲110, relevant for the r process
2009
Measurements of $\ensuremath{\beta}$-decay properties of $A\ensuremath{\lesssim}110$ r-process nuclei have been completed at the National Superconducting Cyclotron Laboratory at Michigan State University. $\ensuremath{\beta}$-decay half-lives for $^{105}\mathrm{Y}$, $^{106,107}\mathrm{Zr}$, and $^{111}\mathrm{Mo}$, along with $\ensuremath{\beta}$-delayed neutron emission probabilities of $^{104}\mathrm{Y}$, $^{109,110}\mathrm{Mo}$ and upper limits for $^{105}\mathrm{Y}$, $^{103\ensuremath{-}107}\mathrm{Zr}$, and $^{108,111}\mathrm{Mo}$ have been measured for the first time. Studies on the basis of the quasi-random-phase approximation are used to analyze the ground-state deformation of these…
Decay properties of exoticN≃28 S and Cl nuclei and theCa48/46Ca abundance ratio
1993
Beta-decay half-lives and \ensuremath{\beta}-delayed neutron-emission probabilities of the very neutron-rich nuclei $^{44}\mathrm{S}$ and $^{45--47}\mathrm{Cl}$ have been measured. These isotopes, which lie at or close to the N=28 magic shell, were produced in interactions of a 60 MeV/u $^{48}\mathrm{Ca}$ beam from GANIL (Grand Acc\'el\'erateur National d'Ions Lourds) with a $^{64}\mathrm{Ni}$ target, and were separated by the doubly achromatic spectrometer LISE (Ligne d'Ions Super Epluch\'es). Their decay was studied by a \ensuremath{\beta}-n time correlation measurement. The results are compared to recent model predictions and indicate a rapid weakening of the N=28 shell effect below $_{2…
β-Decay Studies Close to the N=82 r-process Path
2005
New half-lives for neutron-rich ruthenium, rhodium and palladium isotopes close to the r-process path along the N=82 closed shell have been measured at the National Superconducting Cyclotron Laboratory at Michigan State University. The studied isotopes are close to the critical A=118-126 mass region in the astrophysical r-process, where incorrect nuclear structure development towards the shell closure may have the most pronounced effect on the abundances of elements produced. Neutron-rich nuclei were produced by fragmentation of a 120-MeV per nucleon 136 Xe beam on Be and were separated by the A1900 fragment separator. The nuclei of interest were implanted into a double-sided Si strip detec…
Decay of48-50Ar isotopes
2011
International audience; Information on β-decay properties of neutron-rich 84-05Ar was obtained at the ISOLDE mass-separator facility at CERN using isobaric selectivity. This was achieved by a combination of a plasma-ion source with a cooled transfer line and subsequent mass-separation. Normally, argon beams cannot be mass-separated from intense multi-charged symmetric fission krypton and xenon. Several techniques were used successfully in order to overcome this problem. Implication of the obtained information for a better understanding of the origin of the 48Ca/46Ca isotopic anomaly discovered in inclusions from the Allende meteorite is discussed.
Decay of Neutron-Rich Mn Nuclides and Deformation of Heavy Fe Isotopes
1998
The use of chemically selective laser ionization combined with beta-delayed neutron counting at CERN/ISOLDE has permitted identification and half-life measurements for 623-ms Mn-61 up through 14-ms Mn-69. The measured half-lives are found to be significantly longer near N=40 than the values calculated with a QRPA shell model using ground-state deformations from the FRDM and ETFSI models. Gamma-ray singles and coincidence spectroscopy has been performed for Mn-64 and Mn-66 decays to levels of Fe-64 and Fe-66, revealing a significant drop in the energy of the first 2+ state in these nuclides that suggests an unanticipated increase in collectivity near N=40.
Neutron-rich isotopes around the r-process ?waiting-point? nuclei 29 79 Cu50 and 30 80 Zn50
1991
Beta-decay half-lives (T1/2) and delayed-neutron emission probabilities (Pn) of very neutron-rich Cu to As nuclei have been measured, among them the new isotopes77Cu48,79Cu50,81Zn51 and84Ga53. With the T1/2 and Pn-values of now four N ∼-50 ”waiting-point” nuclei known, our hypothesis that the r-process has attained a local β-flow equilibrium araound A∼-80 is further strengthened.
Isobaric analog states as a tool for spectroscopy of exotic nuclei
2005
Abstract Spectroscopy of neutron rich exotic isotopes via their isobaric analog states (IAS) in less exotic nuclei is discussed. Several different experimental techniques, which can be applied to search for IAS of exotic isotopes, are described. Successful application of these techniques to the studies of heavy helium isotopes 7 He and 9 He led to the observation of unknown IAS in 7 Li and 9 Li. Spectroscopic information for these states were obtained, and implication of these findings to the structure of 7,9 He is considered.
Beta-decay studies of far from stability nuclei near N = 28
1995
Abstract Beta-decay half-lives and β-delayed neutron-emission probabilities of the very neutron-rich nuclei 43 P, 42,44,45 S and 44–46 Cl, 47 Ar, which lie at or close to the N=28 magic shell, have been recently measured through β or β-n time correlation measurement. The results are compared to recent model predictions and indicate a rapid weakening of the N=28 shell effect below 48 Ca. The nuclear structure effects reflected in the decay properties of the exotic S and Cl isotopes may be the clue for the astrophysical understanding of the unusual 48 Ca 46 Ca abundance ratio measured in the solar system.
First decay study of the very neutron-rich isotope Br-93
2000
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
1995
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.
The neutron long counter NERO for studies of neutron emission in the r-process
2010
Abstract The neutron long counter NERO was built at the National Superconducting Cyclotron Laboratory (NSCL), Michigan State University, for measuring β -delayed neutron-emission probabilities. The detector was designed to work in conjunction with a β -delay implantation station, so that β decays and β -delayed neutrons emitted from implanted nuclei can be measured simultaneously. The high efficiency of about 40%, for the range of energies of interest, along with the small background, are crucial for measuring β -delayed neutron emission branchings for neutron-rich r-process nuclei produced as low intensity fragmentation beams in in-flight separator facilities.
R-Process Abundances and Nuclear Properties Far from Stability
1992
Recent measurements of β-decay properties of the ‘waiting-point’ nuclei 79Cu, 80Zn and 130Cd, together with new QRPA shell-model predictions of so far unknown N ≃ 50 and N ≃ 82 isotopes in the r-process path, have allowed to explain the detailed isotopic composition in the A ≃ 80 and A ≃ 130 r-abundance peaks. The correlation between nuclear data far from stability and r-abundances suggests that the r-process involves a high-neutron-density β-flow equilibrium environment. Based on these results, the r-process components of nuclei in the 90 ≤ A ≤ 100 mass range were predicted for freeze-out conditions (nn ≃ 1020, T9 ≃ 1) and compared to the solar-system r-process abundances.
A new pulsed release method for element selective production of neutron rich isotopes near 208Pb
1998
Abstract A new method to reduce the isobaric contamination problem for the production of neutron rich Bi, Pb and Tl nuclei at on-line mass separators, based on the pulsed release of these radioactive species, is presented. The results of a feasibility study are reported.
β-decay studies of135–137Snusing selective resonance laser ionization techniques
2002
The decays of the very neutron rich Sn isotopes Sn135-137 were studied at CERN/ISOLDE using isotopic and isobaric selectivity achieved by the use of a resonance ionization laser ion source and mass spectroscopy, respectively. Neutron decay rates, gamma-ray singles, and gamma-gamma coincidence data were collected as a function of time. The half-life (T-1/2) and delayed neutron emission probability (P-n) values of 135 Sn were measured to be 530(20) ms and 21(3)%, respectively. For Sn-136, a T-1/2 of 250(30) ms was determined along with a P-n value of 30(5)%. For Sn-137, a T-1/2 of 190(60) ms and a P-n value of 58(15)% were deduced. Identification of low-energy transitions in Sb-135 was made p…
Half-Life of the Doubly Magicr-Process NucleusN78i
2005
Nuclei with magic numbers serve as important benchmarks in nuclear theory. In addition, neutron-rich nuclei play an important role in the astrophysical rapid neutron-capture process (r process). 78Ni is the only doubly magic nucleus that is also an important waiting point in the r process, and serves as a major bottleneck in the synthesis of heavier elements. The half-life of 78Ni has been experimentally deduced for the first time at the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University, and was found to be 110(+100)(-60) ms. In the same experiment, a first half-life was deduced for 77Ni of 128(+27)(-33) ms, and more precise half-li…