The beta-decay of95Rb and97Rb

Theβ-decay of 377 ms95Rb and 168ms97Rb has been thoroughly investigated by means ofγ-ray, conversion electron andβ-delayed neutron spectroscopy. More than 97% of theβ-decay to particle-bound states has been placed in level schemes of95Sr and97Sr. High-resolution neutron spectroscopic studies have allowed to extend the knowledge of the excitation spectra up to 9 MeV, respectively 10 MeV. The density of neutron-unbound levels in95Sr and97Sr has been derived from peak stripping analyses of the neutron spectra. Beta-strength functions (S β(E)) have been investigated in detail. They indicate that Gamow-Tellerβ-decay of both precursors is dominated by nuclear structure. The discovered pronounced …

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.

Overlappingβdecay and resonance neutron spectroscopy of levels inKr87

Energy levels in $^{87}\mathrm{Kr}$ have been studied, with special emphasis on the unbound region, using two different methods. The first method comprises neutron capture and transmission measurements on an enriched gas target of $^{86}\mathrm{Kr}$ using neutron time-of-flight techniques. In this way, neutron widths were determined for 39 resonances below 400 keV and capture areas for 14 resonances below 90 keV. The second method is a decay study of 56-s $^{87}\mathrm{Br}$ in which a level scheme for $^{87}\mathrm{Kr}$ has been established that shows 126 levels in the bound and 12 levels in the unbound region. A detailed comparison amongst the neutron resonance, the $\ensuremath{\gamma}$-r…

Study of $^{45}$Ar through (d, p) reaction at SPIRAL

NESTER; International audience; The structure of the neutron-rich nucleus $^{45}$Ar has been investigated through the d($^{44}$Ar,$^{45}$Ar)p transfer reaction. Radioactive beam of $^{44}$Ar at 10 A MeV has been provided by the SPIRAL facility at GANIL. The protons corresponding to a neutron pick-up on bound or unbound states mechanism in $^{45}$Ar nuclei were detected at backward angles by the detector array MUST. The transfer-like ejectiles were detected in the SPEG spectrometer. Level scheme, spin assignments and spectroscopic factors have been deduced for $^{45}$Ar and compared to shell model predictions. These parameters will be subsequently used to infer (n, $\gamma$) cross sections i…

Delayed-Neutron Spectra Following Decay ofAs85andSb135

Frequency tripling for resonance ionization spectroscopy (RIS) of Cd

To explore the nucleosynthesis of heavy elements in the rapid neutron-capture process, investigations on short- lived, very neutron-rich Cd isotopes are planned that require pure samples. These will be produced at the CERN-ISOLDE on-line mass separator in combination with a chemically se- lective laser ion source. The excitation scheme for resonance ionization of Cd was tested in Mainz with a copper-vapor- pumped dye laser system. Due to the high ionization potential of Cd, frequency tripling was required for the first excita- tion step.1D 228: 8n m/. Laser light with an average power of 2m Wwas obtained after frequency tripling and was suc- cessfully used for RIS test measurements on cadmi…

Actinides in the Source of Cosmic Rays and the Present Interstellar Medium

The abundances of the actinide elements in the cosmic rays can provide critical constraints on the major sites of their acceleration. Using recent calculations of the r-process yields in core collapse supernovae, we have determined the actinide abundances averaged over various assumed time intervals for their supernova generation and their cosmic-ray acceleration. Using standard Galactic chemical evolution models, we have also determined the expected actinide abundances in the present interstellar medium. From these two components, we have calculated the U/Th and other actinide abundances expected in the supernova-active cores of superbubbles, as a function of their ages and mean metallicit…

The astrophysical implications of low-energy resonances in 22Ne + α

Abstract The 22 Ne( 6 Li, d) α-transfer reaction has been used to search for α-unbound levels in 26 Mg of importance for resonant α-capture on 22 Ne in stellar helium burning. To determine the resonance strengths of the observed states the 22 Ne(α, n) 25 Mg reaction was investigated in the energy range between 600 and 900 keV. One resonance was identified and its strength determined. The astrophysical implications of the present results are discussed.

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.

On-line yields obtained with the ISOLDE RILIS

The ISOLDE resonance ionization laser ion source (RILIS) allows to ionize efficiently and selectively many metallic elements. In recent yield surveys and on-line experiments with the ISOLDE RILIS we observed 23–34 Mg, 26–34 Al, 98–132 Cd, 149 Tb, 155–177 Yb, 179–200 Tl, 183–215 Pb and 188–218 Bi. The obtained yields are presented together with measured release parameters which allow to extrapolate the release efficiency towards more exotic (short-lived) nuclides of the same elements. 2002 Elsevier Science B.V. All rights reserved.

Nuclear-structure input to r-process calculations

Abstract By comparing results from r-process parameter studies within the classical waiting-point approximation with the observed solar-system r-abundance distribution N r ,⊙ , in-formation on nuclear-structure phenomena on neutron-rich isotopes far off the stability line can be obtained. Such information is — to a large extent — experimentally not accessible. Investigations in the early 1990's, making use of QRPA β-decay properties and nuclear masses from the macroscopic-microscopic FRDM and ETFSI-1 models, showed r-abundance deficiencies around A ⋍115 and 180 , indicating overly strong N=82 and 126 shell strengths far from stability. Recent experiments in the A ⋍110–130 region as well as …

The beta-decay half-life of 48 130 Cd82 and its importance for astrophysicalr-process scenarios

The s-decay half-life of130Cd has been measured and its importance as N=82 ‘waiting-point’ nucleus for astrophysical rapid neutron capture (r−) process scenarios is discussed.

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…

Origin of the odd-even staggering in theA?80 solarR-abundance peak

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.

The Hamburg/ESO R-process Enhanced Star survey (HERES)

We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe] > 1 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (~2A) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (HES) as well as the HK survey to identify sharp-lined stars with [Fe/H] < -2.5dex. For several hundred confirmed metal-poor giants brighter than B~16.5mag (most of them from the HES), ``snapshot'' spectra (R~20,000; S/N~30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2-3% r-II stars expected to be a…

Delayed-neutron spectroscopy with 3He spectrometers

Abstract The characteristics of a 3 He ionization chamber for high-resolution spectroscopy of β-delayed neutrons have been examined. The detector response function is given in the energy range 0.019–2.77 MeV. A considerably improved energy resolution has been achieved by minimizing effects due to pulse summing, detection of scattered neutrons, and microphonics, and is most evident in the low-energy part of the spectra.

Erratum to “Beta decay half-lives of neutron rich Ti–Co isotopes around N=40” [Nucl. Phys. A 660 (1999) 3–19]

Abstract The neutron-rich 57−59 Ti, 59−62 V, 61−64 Cr, 63−66 Mn, 65−68 Fe, 67−70 Co have been produced at GANIL via interactions of a 60.4 MeV/u 86 Kr 34+ with a 58 Ni target. They have been separated by the doubly achromatic spectrometer LISE3. Beta-decay half-lives have been determined for all produced nuclei, and subsequent γ -rays have been observed for the most efficiently produced nuclei. The measured half-lives of vanadium and chromium isotopes are compared to QRPA calculations using ground states deformations from the FRDM or ETFSI models. In an astrophysical context, the short half-lives of 58 Ti and 64 Cr indicate that these nuclei are potential r-process progenitors, after β -dec…

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…

Half-lives andP n values of delayed-neutron precursors in the mass chains 85?87, 92, 135, 136 and 145

Further results are presented on half-lives and neutron emission probabilities (P n ) of short-lived delayed-neutron precursors in the mass chains 85–87, 92, 135, 136 and 145 obtained with the recoil focussing parabola-type mass separator for unslowed fission products LOHENGRIN installed at the Grenoble high-flux reactor. A newP n -value of (44±14)% is given for the (0.73±0.06)s87As; theP n -values of seven precursor nuclides have been redetermined: (1.9±0.1)s85As with (22±8)%, 0.9s86As with (10.5±2.2)%, (0.35±0.04)s92Br with (21±8)%, (1.6±0.15)s135Sb with (14±1)%, (0.9±0.1)s136Sb with (19±9)%, (17.5±0.2)s136Te with (2.0±1.0)% and 0.6s145Cs with (12.5±3.0)%. The results are compared with th…

Selective laser ionization of N≥82 indium isotopes: The new r-process nuclide 135In

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.

N22e(α,n)M25g: The Key Neutron Source in Massive Stars

The excitation function of the reaction (22)Ne(alpha,n)(25)Mg, the key neutron source in the astrophysical s process in massive stars, has been determined from threshold at E(alpha) = 570 up to 1450 keV with an experimental sensitivity of 10(-11) b. For all resonances in this energy range new resonance parameters have been measured. For a possible resonance at about 635 keV a new upper limit omega gamma < 60 neV for the strength was obtained. Based on the new data, improved reaction rates were calculated as a function of temperature. The new uncertainty limits are considerably smaller than in previous determinations, ruling out the large enhancement factors, up to 500, assumed in some stell…

Constraints on r-process conditions from beta-decay properties far off stability and r-abundances.

The measured beta -decay properties of the 'waiting-point' nuclei 130Cd and 80Zn, together with improved shell model predictions of T1/2 and Pn-values for their experimentally unknown N approximately=82 and 50 neighbours, can be directly related to the observed r-abundances in the A approximately=130 and 80 abundance peaks. Based on this result, new constraints are given on the stellar conditions under which the r-process has operated.

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.

Heavy Elements and Age Determinations

The age of the universe, measured from the Big Bang to the present, is at the focus of cosmology. Its determination relies, however, on the use of stellar objects or their products. Stellar explosions, like type Ia supernovae serve as standard(izable) candles to measure the expansion of the universe. Hertzsprung—Russell diagrams of globular clusters can determine the age of such clusters and thus are lower limits of the age of the galaxy and therefore also the universe. Some nuclear isotopes with half-lives comparable to the age of galaxies (and the universe) can serve as clocks (chronometers) for the duration of nucleosynthesis. The isotopes 238U and 232Th with half-lives of 4.5 × l09 and …

Helium burning of22Ne

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…

Study of the β-delayed neutron decay of 17C and 18C

Abstract The β-delayed neutron decays of 17C and 18C have been studied using a time-of-flight array with a high detection efficiency. The 17C and 18C ions were produced by fragmentation of an E A = 69 MeV 22Ne beam. Transitions to several neutron unbound states have been observed for the first time for both decays with total branching ratios of (10.8 ± 2.2)% and (21.4 ± 4.4)%. Half-lives of 193 ± 6 ms and 92 ± 2 ms were found for 17C and 18C, respectively. The results are compared with previous measurements of the β-decays and with shell-model calculations.

Nuclear physics far from stability and explosive nucleosynthesis processes

In this paper, we discuss the astrophysically relevant nuclear-physics input for a selected set of explosive nucleosynthesis scenarios leading to rapid protonand neutron-capture processes. Observables (like,e.g., luminosity curves or abundance distributions) witness the interplay between nuclear-structure aspects far from β-stability and the appropriate astrophysical environments, and can give guidance to and constraints on stellar conditions and/or key features of reaction and decay data for radioactive isotopes.

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…

Nuclear structure theory for the astrophysical rp-process and r-process

Abstract The astrophysical processes of rapid-proton capture and rapid-neutron capture require the knowledge of many nuclear properties which are not known from experiment. I will describe two examples of how theoretical models are used to provide this input. The first of these uses the Hartree-Fock method for displacement energies to obtain the masses of proton-rich nuclei needed for the rp-process. The second uses a model for configuration mixing near 132 Sn to provide Q values and beta-decay lifetimes for the r-process.

β-decay of22O

A mass-separated 12 C 22 O molecular ion beam from the ISOLDE facility was used to study the decay of neutron-rich 22 O. The experimental results were compared with the results from an earlier experiment and predictions by shellmodel calculations using various effective interactions. The mechanism leading to the vanishing decay strength to the first 1 + level of the 22 F nucleus, predicted with the USD effective interaction but not supported by the experimental data, is analysed.

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).

Rotational bands in99Sr

The β-decay of 59 ms99Rb has been studied at OSTIS. As is confirmed by RPA calculations with Nilsson model wave functions, the lowest energy levels in99Sr are consistent with rotational bands built on the [411 3/2], [413 5/2] and [422 3/2] Nilsson neutron configurations at 0, 423 and 1071 keV, respectively. All three bands have similar values of the inertial parameter ħ2/2θ indicating a nearly rigid rotor.

Structure And Decay Of Neutron-Rich Nuclides In The 115 ≤ A ≤ 138 Mass Range And r-Process Nucleosynthesis

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

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.

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.

Nuclei Far from Stability and the R-Process Waiting-Point Concept

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).

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.

Beta-delayed neutron emission from93?100Rb to excited states in the residual Sr isotopes

Beta-delayed neutron emission from the precursors93–100Rb to excited states in the residual nuclei92–99Sr has been measured by means ofγ-ray and neutron spectroscopy. In general, considerable neutron feeding of excited final states is observed. The experimental branching ratios (P ) are compared to predictions from statistical model calculations. It is demonstrated that the main parameter affecting theP -values is the shape of theβ-strength function (S β). For several Rb isotopes hindrance, respectively enhancement of neutron branches to specific final states is observed giving evidence for the persistence of intermediate structure of the neutron-emitting states. By the example of98Rb it is…

Direct neutron capture for magic-shell nuclei.

In neutron capture for magic--shell nuclei the direct reaction mechanism can be important and may even dominate. As an example we investigated the reaction $^{48}$Ca(n,$\gamma)^{49}$Ca for projectile energies below 250\,keV in a direct capture model using the folding procedure for optical and bound state potentials. The obtained theoretical cross sections are in agreement with the experimental data showing the dominance of the direct reaction mechanism in this case. The above method was also used to calculate the cross section for $^{50}$Ca(n,$\gamma)^{51}$Ca.

New region of deformation in the neutron-rich 60 24Cr36 and 62 24Cr38

The neutron-rich nuclei 23 60-63V have been produced at GANIL via interactions of a 61.8A·MeV 76Ge beam with a 58Ni target. Beta-decay to 24 60-63Cr has been investigated using combined β- and γ-ray spectroscopy. Half-lives of the 60-63V nuclei have been determined, and the existence of a beta-decay isomer in the 60V nucleus is strongly supported. The observation of low-energy 2+ states in 60Cr (646keV) and 62Cr (446keV) suggests that these isotopes are strongly deformed with β2 ∼ 0.3. This is confirmed by shell model calculations which show the dominant influence of the intruder g and d orbitals to obtain low 2+ energies in the neutron-rich Cr isotopes.

First beta-decay studies of the neutron-rich isotopes 53-55Sc and 56-59V

The neutron-rich isotopes Sc53-55 and V56-59 have been produced at GANIL in interactions of a 64.5 MeV/u Cu-65 beam with a Be-9 target. They were separated by the doubly achromatic spectrometer LISE3. Beta-decay half-lives and subsequent low-energy gamma-rays were observed for the first time. The present results are compared to QRPA model predictions. The quick drop of the half-life observed at N = 33 for Ca-53(20)33 is water V-56(23)33 and absent for Sc-54(21)33, indicating a vanishing of the N = 32 subshell north to Ca-52(32). In an astrophysical context, these neutron-rich isotopes represent r-process progenitors which, after beta-decay, would produce the correlated isotopic over-abundan…

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.

N=82Shell Quenching of the Classicalr-Process “Waiting-Point” NucleusCd130

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…

Evaluation of Nanoparticles as Contrast Agent for Photoacoustic Imaging in Living Cells

Gamow-Teller decay ofZn80: Shell structure and astrophysical implications

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…

Evidence for excited 0+ states in94Sr

Beta-delayed neutron measurements on95Rb have indicated evidence for two heretofore unknown excited O+ states in94Sr which are not fed by s -decay from the Jπ = 3− ground state of94Rb.

The Hamburg/ESO R-process Enhanced Star survey (HERES): Project Overview, and New r-II Stars

Interpretation of the SolarCa48/Ca46Abundance Ratio and the Correlated Ca-Ti Isotopic Anomalies in the EK-1-4-1 Inclusion of the Allende Meteorite

$\ensuremath{\beta}$-delayed neutron-emission probabilities of neutron-rich S to K isotopes are calculated with nuclear-structure effects taken into account. These results strongly affect predictions made in high-neutron-density astrophysical scenarios for isotopic abundances of several elements. In particular, it is demonstrated that the solar abundance ratio $^{48}\mathrm{Ca}$/$^{46}\mathrm{Ca}$ as well as the correlated Ca and Ti isotopic anomalies can be explained by the same nucleosynthesis process.

Single-Neutron States inS133n

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.

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…

Magicity of theN68iSemidouble-Closed-Shell Nucleus Probed by Gamow-Teller Decay of the Odd-ANeighbors

The particle-hole excitations through the $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}40$ subshell around ${}^{68}\mathrm{Ni}$ have been studied by the $\ensuremath{\beta}$ decay of ${}^{69}\mathrm{Co}$ and ${}^{69}\mathrm{Ni}$. The half-life of ${}^{69}\mathrm{Co}$ was measured to be 0.22(2) s, and a new $\ensuremath{\beta}$-decaying isomer with a half-life of 3.5(5) s was identified in ${}^{69}\mathrm{Ni}$. From the decay of the ${}^{69}\mathrm{Ni}$ isomer a 9(4)% mixing of the $\ensuremath{\pi}{p}_{3/2}^{+1}\ensuremath{\nu}{p}_{1/2}^{\ensuremath{-}2}\ensuremath{\nu}{g}_{9/2}^{+2}$ configuration into the ground state of ${}^{69}\mathrm{Cu}$ can be deduced. Significant polarizatio…

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…

β decay of67Co

The \ensuremath{\beta}-decay properties of ${}^{67}\mathrm{Co}$ produced in proton-induced fission of ${}^{238}\mathrm{U}$ were measured by the detection of \ensuremath{\beta}-delayed \ensuremath{\gamma} rays emitted from an isotopically pure mass-separated source obtained by laser ionization. The measured half-life of 0.425(20) s is more accurate than previous values. New \ensuremath{\gamma} transitions were observed, and corresponding branching ratios and $\mathrm{log}\mathrm{ft}$ values were deduced. The ${}^{67}\mathrm{Co}$ decay scheme is discussed in terms of the single-particle shell model.

Beta-decay half-lives at the N = 28 shell closure

Abstract Measurements of the beta-decay half-lives of neutron-rich nuclei (MgAr) in the vicinity of the N =28 shell closure are reported. Some 22 half-lives have been determined, 12 of which for the first time. Particular emphasis is placed on the results for the Si isotopes, the half-lives of which have been extended from N =25 to 28. Comparison with QRPA calculations suggests that 42 Si is strongly deformed. This is discussed in the light of a possible weakening of the spin–orbit potential.

Nuclear Structure Properties of Neutron Rich Ge-Br Isotopes in the Astrophysical r-Process

The astrophysical r-process is responsible for synthesis of roughly half of the elements heavier than iron. In spite of this significance, there are many uncertainties regarding the site of the r-process and the neutron-rich nuclei involved. Studying these nuclei presents a challenge, as they lie far from the valley of stability. Nuclear properties such as β decay half-lives and βdelayed neutron emission probabilities are critical inputs for r-process models. The neutron rich Ge-Br isotopes are in the region just after the N=50 bottle neck in the “classical” r-process, or may serve as seed material for the high entropy neutrino-wind r-process. Neutron rich nuclei play an important role in b…

Beta Decay of68–74Niand Level Structure of Neutron-Rich Cu Isotopes

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…

Half-lives and delayed neutron emission probabilities of short-lived Rb and Cs precursors

Half-lives and delayed-neutron emission probabilities (P n ) of short-lived Rb and Cs precursors in the mass chains 94–98 and 143–147 were measured. Sources of isotope separated nuclides were obtained from the on-line mass-separator OSTIS installed at the Grenoble high-flux reactor.

Selective laser ionization of very neutron-rich cadmium isotopes: Decay properties of131Cd83and132Cd84

A chemically selective laser ion source has been applied in a decay study of the very neutron-rich isotopes ${}^{131}\mathrm{Cd}$ and ${}^{132}\mathrm{Cd}$ at CERN/ISOLDE. For the ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of the $N=83$ nuclide ${}^{131}\mathrm{Cd}$ a surprisingly short half-life of $(68\ifmmode\pm\else\textpm\fi{}3)$ ms and a weak delayed-neutron branch of ${P}_{n}=(3.5\ifmmode\pm\else\textpm\fi{}1.0)%$ were observed. For the $N=84$ nuclide ${}^{132}\mathrm{Cd}$ a half-life of $(97\ifmmode\pm\else\textpm\fi{}10)$ ms and a ${P}_{n}$ value of $(60\ifmmode\pm\else\textpm\fi{}15)%$ were obtained. Schematic features of both decay schemes are developed. We find that our new d…

Beta decay studies of neutron-rich $_{21}$Sc-$_{27}$Co nuclei at GANIL

Abstract The neutron-rich nuclei 2157,58Sc, 2258–60Ti, 2258–60Ti, 2360–62V, 2464–66Cr, 2564–68Mn, 2667–70Fe and 2769–71Co have been produced at Ganil via interactions of a 61.8 A.MeV 76Ge beam with a 58Ni target. Beta-decay studies have been achieved, bringing new half-lives and a better nuclear structure knowledge in this region. These results could help for understanding the overabundances of the neutron-rich stable 58Fe, 64Ni in certain inclusions of meteorites.

Radioactive ion beams in the region of 100Sn and 78Ni at the NSCL

The regions around the doubly magic nuclei 100 Sn and 78 Ni are of great interest from a nuclear structure standpoint. These nuclei also play a key role in the astrophysical rp- and r-processes, respectively. Recently, nuclei in these regions were studied at the Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University.

New results with stored exotic nuclei at relativistic energies

Recently, much progress has been made with stored exotic nuclei at relativistic velocities ( v c = 0.7 ) . Fragments of 208Pb and 209Bi projectiles and fission products from 238U ions were produced, separated in flight with the fragment separator FRS, and injected into the storage-cooler ring ESR for precision measurements. 114 new masses of neutron-deficient isotopes in the lead region have been measured with time-resolved Schottky Mass Spectrometry (SMS). A new isospin dependence of the pairing energy was observed due to the improved mass accuracy of typically 1.5×10-7 (30 keV). New masses of short-lived neutron-rich fission fragments have been obtained with Isochronous Mass Spectrometry …

Study of Basic Nuclear Properties of Highly-Charged, Unstable Nuclei at the SIS-FRS-ESR Complex

Recent progress in experiments with exotic nuclear beams at the SIS-FRS-ESR facility is summarized. New results on gross properties of exotic nuclei like binding energy, half-lives, and decay modes are presented. A brief outlook to future experiments is given.

Investigation of beta strength functions by neutron and gamma-ray spectroscopy

Abstract Neutron spectra from the decay of the β− delayed neutron precursors 55.6 sec 87Br, 24.5 sec 137I, 2.05 sec 85As and 1.71 sec 135Sb have been studied with high resolution 3He ionization chambers. By γ-ray measurements, the partial neutron emission probabilities to excited states in 84Se and 134Te have been determined independently. For the neutron-emitter nuclei 87Kr and 137Xe, from which neutron decay proceeds only to the ground states of the final nuclei, 86Kr and 136Xe, unique information on the shape of the β− strength function Sβ(E) is obtained. For the emitter nuclei 85Se and 135Te, correlations between neutron transition energies and differences in level energies in 84Se and …

Selective laser ionization of radioactive Ni-isotopes

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.

The key reactions in Stellar helium burning: 12C(α,γ)16O and 22Ne(α,n)25Mg

Abstract The excitation functions of 12C(α,γ)16O and 22Ne(α,n)25Mg have been determined with a yet unequaled experimental sensitivity of 10−11 b. From γ-angular distributions the SE1-and SE2-factors for 12C(α,γ)16O have been deduced and extrapolated into the range of helium burning temperatures using the R-matrix method. An improved reaction rate has been calculated: NA〈σν〉=(7.9±2.5)x10−15cm3(mols)−1 at T9 = 0.2. For a more precise extrapolation the excitation function has to be measured with better statistics. Therefore a new experiment on 12C(α,γ)16O with a better suited setup has been started recently. The accuracy of the 22Ne(α,n)25Mg reaction rate could be improved considerably compare…

Neutron capture cross sections for neutron-rich isotopes

Average continuum (Hauser-Feshbach) and resonance (Breit-Wigner) neutron capture rates for neutron-rich isotopes, determined on the basis of recent experimental nuclear physics input parameters, are compared to earlier theoretical reaction rates commonly used in astrophysical model computations.

Beta decay half-lives of neutron rich Ti–Co isotopes around

Abstract The neutron-rich 57 – 59 Ti, 59 – 62 V, 61 – 64 Cr, 63 – 66 Mn, 65 – 68 Fe, 67 – 70 Co have been produced at GANIL via interactions of a 60.4 MeV/u 86 Kr 34+ with a 58 Ni target. They have been separated by the doubly achromatic spectrometer LISE3. Beta-decay half-lives have been determined for all produced nuclei, and subsequent γ -rays have been observed for the most efficiently produced nuclei. The measured half-lives of vanadium and chromium isotopes are compared to QRPA calculations using ground states deformations from the FRDM or ETFSI models. In an astrophysical context, the short half-lives of 58 Ti and 64 Cr indicate that these nuclei are potential r-process progenitors, …

Beta-decay properties of the neutron-rich $^{94-99}$Kr and $^{142-147}$Xe isotopes

Beta-decay half-lives and delayed-neutron emission probabilities of the neutron-rich noble-gas isotopes Kr94-99 and Xe142-147 have been measured at the PSB-ISOLDE facility at CERN. The results are compared to QRPA shell-model predictions and are used in dynamic calculations of r-process abundances of Kr and Xe isotopes. (C) 2002 Elsevier Science B.V. All rights reserved.

Beta-delayed neutrons and high-energy gamma-rays from decay of137I

Beta-delayed neutrons and high-energyγ-rays following decay of137I have been studied in order to characterizeβ-decay to and deexcitation of neutron-unbound levels in137Xe. The totalγ-ray intensity observed accounts for at least 20% of theβ-intensity to levels aboveB n . No evidence was found for neutron andγ-ray emission from the same levels. With the experimentalβ-strength function aboveB n detailed tests were performed on the reproduction of the experimental neutron spectrum, the level density and Porter-Thomas fluctuations of the reducedβ-transition probabilities to highly excited states in137Xe.

The reliability of nuclear model predictions of?-decay properties of nuclei far from stability

In-source laser spectroscopy of75,77,78Cu: Direct evidence for a change in the quasiparticle energy sequence in75,77Cu and an absence of longer-lived isomers in78Cu

This paper describes measurements on the isotopes (75,77,78)Cu by the technique of in-source laser spectroscopy, at the ISOLDE facility, CERN. The role of this technique is briefly discussed in the ...

Decay properties of exoticN≃28 S and Cl nuclei and theCa48/46Ca abundance ratio

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…

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.

β-Decay Studies Close to the N=82 r-process Path

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

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.

The Hamburg/ESO R-process Enhanced Star survey (HERES):XI. The highly r-process-enhanced star CS 29497-004

We report an abundance analysis for the highly r-process-enhanced (r-II) star CS 29497-004, a very metal-poor giant with Teff = 5013K and [Fe/H]=-2.85, whose nature was initially discovered in the course of the HERES project. Our analysis is based on high signal-to-noise, high-resolution (R~75000) VLT/UVES spectra and MARCS model atmospheres under the assumption of local thermodynamic equilibrium, and obtains abundance measurements for a total of 46 elements, 31 of which are neutron-capture elements. As is the case for the other 25 r-II stars currently known, the heavy-element abundance pattern of CS 29497-004 well-matches a scaled Solar System second peak r-process-element abundance patter…

Neutron-rich isotopes around the r-process ?waiting-point? nuclei 29 79 Cu50 and 30 80 Zn50

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.

Onset of deformation in neutron-rich krypton isotopes

Beta-decay properties of neutron-rich35Br isotopes confirm the predicted smooth onset of quadrupole deformation for Z<37 already below N=60. The observed increase of the energy of the first 2+ state in the N=56 nucleus92Kr may indicate octupole softness.

Observation of beta-delayed neutron decay to excited 0+ states in the residual nucleus: The case

Abstract Beta-delayed neutron emission from 97 Rb to excited 0 + states in 96 Sr has been observed. The experimental branching ratios ( P n i ) are compared to predictions from a statistical model. It is demonstrated that, considering the influence of the strength of β -decay preceding neutron emission, the ( P n i ) reflect the different structure of the two excited 0 + states, in agreement with conclusions drawn from the measurement of their lifetimes.

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.

Actinides and the sources of cosmic rays

Abstract The abundances of the actinide elements in the cosmic rays can provide critical constraints on the major sites of their acceleration. Using recent calculations of the r-process yields in core-collapse supernovae (SNe), we have determined the actinide abundances averaged over various assumed time intervals for their supernovae generation and their cosmic-ray acceleration. Using standard Galactic chemical evolution models, we have also determined the expected actinide abundances in the present interstellar medium. From these two components, we have calculated the U/Th and other actinide abundances expected in the SN-active cores of superbubbles, as a function of their ages and mean m…

r‐Process Abundances and Chronometers in Metal‐poor Stars

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (QRPA beta-decay rates and the ETFSI-Q nuclear mass model), have been made. These calculations are compared with ground-based and HST observations of neutron-capture elements in the metal poor halo stars CS 22892--052, HD 115444, HD 122563 and HD 126238. The elemental abundances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z &gt;/= 56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of t…

Interpretation of the Solar 48Ca/46Ca Abundance Ratio and the Correlated Ca-Ti-Cr Isotopic Anomalies in Inclusions of the Allende Meteorite

In the past, astrophysical models encountered severe difficulties in explaining the solar 46,48Ca abundances or the correlated Ca-Ti-Cr isotopic anomalies observed in inclusions of the Allende meteorite [1–3]. Among the various attempts. SANDLER et al. [4] suggested the production of neutron-rich stable Ca-Ti-Cr isotopes in a high neutron density environment of ~107 mol/cm3 with a neutron-exposure time of 10 s. Assuming the initial abundances to be solar and applying Hauser-Feshbach neutron-capture crosa sections, the above authors have calculated a 48Ca/46Ca abundance ratio which is only a factor of 2.6 smaller than the observed solar value of 56. However, the predicted isotopic anomalies …

Beta-decay studies of far from stability nuclei near N = 28

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

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.

r-process chronometers

Abstract We compare calculations of r-process abundances with recent astronomical observations from extremely metal-deficient, neutron-capture-rich halo stars. We derive criteria to determine Th and U chronometric ages, and deduce astrophysical conditions under which the observed abundance patterns can be obtained under r-like conditions.

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.

R-Process Abundances and Nuclear Properties Far from Stability

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.

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.

β− Strength function phenomena of exotic nuclei: A critical examination of the significance of nuclear model predictions

Abstract Reliable predictions of β-decay properties of nuclei far from stability require knowledge of the β-strength distribution, S β ( E ). By comparison of experimental strength functions of neutron-rich Rb isotopes in the transitional region around A = 100 with S β ( E ) from different nuclear models, the sensitivity and physical significance of S β ( E ) on various integral and spectral β-decay properties are examined. The reliability of extrapolations and the applicability to different fields in nuclear physics, astrophysics and reactor technology are discussed. It is shown that, although there has been considerable progress in the physical understanding of the decay properties of nuc…

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…

P n-values of short-lived Sr, Y, Ba and La precursors

Beta-delayed neutron emission probabilities (Pn) of short-lived Sr, Y, Ba and La precursors were measured. As expected from systematics, but in disagreement with recent data from the SOLIS-group, only small Pn -values were obtained.

Properties ofNe22(α,n)25Mg resonances

The $^{22}\mathrm{Ne}$(\ensuremath{\alpha},n${)}^{25}$Mg reaction has been investigated in the energy range ${\mathit{E}}_{\mathrm{\ensuremath{\alpha}}}^{\mathrm{lab}}$=0.73--2.10 MeV using high-resolution $^{3}\mathrm{He}$ ionization chambers. Sixteen resonances have been observed in the three neutron channels to the ground state and the first two excited levels of $^{25}\mathrm{Mg}$. From the analysis of neutron angular distributions, spin and parity assignments could be made for eight resonances. Furthermore, partial widths as well as absolute cross sections have been obtained yielding resonance strengths for the observed levels. The role of the $^{22}\mathrm{Ne}$(\ensuremath{\alpha},n) …

Intense mass-separated beams of halogens and beta-delayed neutron emission from heavy bromine isotopes

Improved production yields of short-lived halogens were obtained from a ThO2 target, irradiated with 600 MeV protons, in combination with a negative surface ionization source. Mass-separated samples were studied by decay spectroscopy. Production yields of radioactive isotopes of chlorine, bromine, iodine and astatine are presented. Half-lives and relative neutron emission probabilities were measured for the heavy bromine isotopes89−92Br. Normalizing to earlier publishedPn values for89Br, the results are:89Br (4.30±0.14s,P n =13.6±0.8%),90Br (1.92±0.06s,P n =24.8±1.5%),91Br (0.53 ±0.03 s,P n =30.1 ±2.1%), and92Br (0.31 ±0.02 s,P n =34.6±2.5%). Energy spectra ofβ-delayed neutrons were measure…

Nuclear structure studies for the astrophysical r-process

Abstract The production of the heaviest elements in nature occurs via the r-process, i.e. a combination of rapid neutron captures, the inverse photodisintegrations, and slower β − -decays, β -delayed processes as well as fission and possibly interactions with intense neutrino fluxes. A correct understanding and modeling requires the knowledge of nuclear properties far from stability and a detailed prescription of the astrophysical environment. Experiments at radioactive ion beam facilities have played a pioneering role in exploring the characteristics of nuclear structure in terms of masses and β -decay properties. Initial examinations paid attention to highly unstable nuclei with magic neu…

β-decay studies of135–137Snusing selective resonance laser ionization techniques

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…

TheK=3/2 1 + side-band in99Y and its description by the IBFM/PTQM model

The K=3/21T rotational band has been identified in99Y with a band-head energy of 536 keV. This result is in contradiction with previously reported data, but is in agreement with predictions of the IBFM/PTQM model.

Half-Life of the Doubly Magicr-Process NucleusN78i

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…