Direct mass measurements of exotic nuclei at the FRS-ESR facility at GSI

An overview of direct mass measurements of exotic nuclei at the FRS-ESR facility at GSI is given. The nuclides are produced at relativistic energies by projectile fragmentation and fission, separated in-flight at the fragment separator FRS and injected into the storage ring ESR. Mass measurements are performed using Schottky and Isochronous Mass Spectrometry, which both allow for high precision measurements with single-ion sensitivity. Recent experimental developments are summarized, and examples for measurement results are given, including applications in nuclear structure physics and astrophysics, comparisons with mass predictions, and the search for new isotopes and isomers. Further research potential will be available at next-generation fragment-separator-storage-ring facilities such as the Super-FRS-CR-NESR complex at the future FAIR facility.     

Present and future experiments with stored exotic nuclei at the FRS-ESR facility

High accuracy mass and lifetime measurements are performed with the combination of the in-flight separator FRS and the cooler-storage ring ESR at GSI. Exotic nuclei are produced via projectile fragmentation and fission at (400–900) MeV/u. This energy range is the basis for unique experimental conditions with bare and few-electron fragments and allows for the first time investigations for decay channels which are rare or strictly forbidden in neutral atoms. New experimental developments and data including the discovery of new isotopes are presented. The comparison of the experimental results with theoretical predictions indicates the potential for improvements of nuclear models. A new generation of experiments will be possible with the advent of the proposed international Facility for Antiproton and Ion Research (FAIR).     

Direct mass measurements of exotic nuclei

The present status and recent results from direct mass measurements of exotic nuclei are presented. ISOL, in-flight, and combined facilities provide a wide variety of nuclides far-off stability covering a wide range of half-lives down to the sub-millisecond region. Modern direct mass measurements are carried out using frequency and time-of-flight techniques. The obtained accurate mass data point to nuclear-structure phenomena and serve as a basis for astrophysical and weak-interaction studies. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: c.scheidenberger@gsi.de     

Accurate mass measurements of very short-lived nuclei

Mass measurements of ³⁴Ar, ^73-78Kr, and ^74,76Rb were performed with the Penning-trap mass spectrometer ISOLTRAP. Very accurate Q _EC-values are needed for the investigations of the t-value of 0⁺ → 0⁺ nuclear β-decays used to test the standard model predictions for weak interactions. The necessary accuracy on the Q _EC-value requires the mass of mother and daughter nuclei to be measured with δm/m ⩽ 3 ^. 10^-8. For most of the measured nuclides presented here this has been reached. The ³⁴Ar mass has been measured with a relative accuracy of 1.1 ^. 10^-8. The Q _EC-value of the ³⁴Ar 0⁺ → 0⁺ decay can now be determined with an uncertainty of about 0.01%. Furthermore, ⁷⁴Rb is the shortest-lived nuclide ever investigated in a Penning trap. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: frank.herfurth@cern.ch     

Direct mass measurements of neutron-deficient 152Sm projectile fragments at the FRS-ESR facility

The FRS-ESR facility at GSI provides one of the most efficient methods for direct mass measurements. In the present experiment, exotic nuclei were produced via fragmentation of ¹⁵²Sm projectiles in a thick beryllium target at 500-600 MeV/u, separated in-flight with the fragment separator FRS, and injected into the storage-cooler ring ESR. Time-resolved Schottky Mass Spectrometry was applied for mass measurements of stored and electron-cooled bare and few-electron ions. 373 different nuclides were identified by means of the spectra of their revolution frequencies. Masses for 18 nuclides (⁸⁴Zr, ⁹²Ru, ⁹⁴Rh, ^107,108,110Sb, ^111,112,114I, ¹¹⁸Ba, ^122,123La, ¹²⁴Ce, ¹²⁷Pr, ¹²⁹Nd, ¹³²Pm, ¹³⁴Sm, ¹³⁷Eu) have been determined for the first time. Masses for ^111,112I and ¹¹³Xe have been obtained via known α-decay energies. The experiment and first results will be presented.     

Probing the halo and cluster structure of exotic nuclei

The Halo and cluster structure at the ground state of unstable nuclei are among the most exciting phenomena of current nuclear physics. Probing these structures requires a careful selection of reaction tools. In the past twenty years, knockout reactions have been used intensively to investigate spectroscopically the structure of unstable nuclei. In this report we have illustrated the latest development of the knockout reaction tool and have emphasized the recoiled proton tagging method. A quantitative criteria is developed to evaluate the quasi-free feature of the knockout process. The newly discovered "towing mode" reaction tool is also outlined and its applicability at transit energies is discussed.     

Giant resonances in exotic spherical nuclei within the RPA approach with the Gogny force

Theoretical results for giant resonances in the three doubly magic exotic nuclei ⁷⁸Ni, ¹⁰⁰Sn and ¹³²Sn are obtained from Hartree-Fock (HF) plus Random Phase Approximation (RPA) calculations using the D1S parameterization of the Gogny two-body effective interaction. Special attention is paid to full consistency between the HF field and the RPA particle-hole residual interaction. The results for the exotic nuclei, on average, appear similar to those of stable ones, especially for quadrupole and octupole states. More exotic systems have to be studied in order to confirm such a trend. The low energy of the monopole resonance in ⁷⁸Ni suggests that the compression modulus in this neutron-rich nucleus is lower than the one of stable ones.     

A new Channeltron-detector setup for precision mass measurements at ISOLTRAP

Recent technical developments at ISOLTRAP include the design, installation and characterization of a new Channeltron-detector setup in order to increase the detection efficiency by about a factor of 3. Since a detection efficiency close to 100% is reached, true single-ion experiments can be performed for the first time and exotic nuclides further away from the valley of stability with lower production yields and/or shorter half-lives as compared to previous investigations are accessible. This publication comprises part of the thesis of C. Yazidjian.     

A new approach to stroboscopic resonance measurements of the nuclear quadrupole interaction

A new stroboscopic resonance method for the measurement of the quadrupole interaction of long-lived nuclear isomers excited by pulsed beams with a fixed repetition frequency is demonstrated. The method utilizes the phenomenon of beats which occurs when a magnetic perturbation is introduced in the presence of a pure quadrupole interaction. The nuclear precession is thus controlled by the beat frequencies which can be varied by the external magnetic field which is oriented parallel to the symmetry axis of the crystal. Stroboscopic resonance can be achieved by varying thebeat frequency to be an integral multiple of the beam pulsing frequency. The method is demonstrated using the 398 keV level of⁶⁹Ge excited and recoil implanted into a single crystal of tin by means of a heavyion reaction.Supported in part by the National Science Foundation.     

A new approach to the retrieval of surface properties from earthshine measurements

Instruments such as the MODIS and MISR radiometers on EOS AM-1, and POLDER on ADEOS have been deployed for the remote sensing retrieval of surface properties. Typically, retrieval algorithms use linear combinations of semi-empirical bidirectional reflectance distribution function (BRDF) kernels to model surface reflectance. The retrieval proceeds in two steps; first, an atmospheric correction relates surface BRDF to top-of-atmosphere (TOA) reflectances, then regression is used to establish the linear coefficients used in the kernel combination. BRDF kernels may also depend on a number of physical or empirical non-linear parameters (e.g. ocean wind speed for a specular BRDF); such parameters are usually assumed known. A major source of error in this retrieval comes from lack of knowledge of planetary boundary layer (PBL) aerosol properties.In this paper, we present a different approach to surface property retrieval. For the radiative transfer simulations, we use the discrete ordinate LIDORT model, which has the capability to generate simultaneous fields of radiances and weighting functions in a multiply scattering multi-layer atmosphere. Surface-atmosphere coupling due to multiple scattering and reflection effects is treated in full; the use of an atmospheric correction is not required. Further, it is shown that sensitivities of TOA reflectances to both linear and non-linear surface BRDF parameters may be established directly by explicit analytic differentiation of the discrete ordinate radiative transfer equations. Surface properties may thus be retrieved directly and conveniently from satellite measurements using standard non-linear fitting methods. In the fitting for BRDF parameters, lower-boundary aerosol properties can either be retrieved as auxiliary parameters, or they can be regarded as forward model parameter errors. We present examples of simulated radiances and surface/aerosol weighting functions for combinations of multi-angle measurements at several different wavelengths, and we perform some examples of self-consistent non-linear fitting to demonstrate feasibility for this kind of surface property retrieval.     

Skyrme-Hartree-Fock approach to the change of level occupancy of low mass halo nuclei

With a new parameterization of potential parameters which reproduces the ground state properties of shell closed nuclei fairly accurately, the role of occupancy of 2s _1/2 level in determining the halo structures of ¹⁷O, ¹⁶N, ¹⁵C, ¹⁴B, ¹³Be have been investigated. The results show interesting cross over of level occupancies which may explain the increase in interaction radii.     

A generalization of the algebraic approach for effective interactions in nuclei

We generalize the algebraic approach of Schucan and Weidenmüller for effective interactions in nuclei and show that in general one obtains an effecti to that of Brandow by an arbitrary similarity transformation. We then show how this arbitrariness can be utilized to obtain hermitian effective interac     

Beyond mean field approach to the beta decay of medium mass nuclei relevant for nuclear astrophysics

The Gamow-Teller strength distributions for the β decay of the ground state as well as the lowest excited states of the rp-process waiting point nuclei ⁶⁸Se and ⁷²Kr are obtained within the complex Excited Vampir variational approach using realistic effective interactions and a rather large model space. The shape mixing is consistently described for both the states in the even-even parent and the states in the odd-odd daugther nucleus. The influence of the shape mixing accounted by the different effective interactions used and comparison with the available data are presented. The possible influence of the decay of the lowest excited states of the parent nuclei in the astrophysical environment of X-ray bursts is discussed.Gamow-Teller strength distributions, β-decay half-lives, and β-delayed neutron emission probabilities for neutron-rich Zr nuclei are investigated for the first time within the complex Excited Vampir approach using a large model space. Comparison with available data and predictions relevant for the astrophysical r process are presented.     

A proposed new experimental setup for determination of refractive indices of liquids by mode line measurements

A new experimental method is proposed for measuring the refractive indices of liquids based on mode line measurements. The experimental setup consists of a dielectric substrate in which a rectangular hollow is formed containing the concerned liquid. This is covered by a trapezium-shaped prism. The liquid, confined between both basin walls and the prism, forms a wavegude. Thus, the refractive index of the liquid can be determined by measuring the propagating modes in the structure. Using this technique, the refractive indices of liquids can be measured to an accuracy of the order of 1×10^-4.     

A new experimental approach for obtaining EPR spectra of alkane and alkene radical cations

A new experimental approach for obtaining EPR spectra of alkane and alkene radical cations is described. The method consists in irradiating the hydrocarbon studied (octane and tetramethylethylene in this work) in a pentane matrix at 77 K. A substantial amount of the positive ions formed by irradiation will then be trapped as radical cations of the additive. Fairly pure EPR spectra of these radical cations may subsequently be obtained by taking the difference, by computer, between the EPR spectra before and after selective removal of the radical cation by illumination with light of suitable wavelength.     

A zero-point energy approach for estimating the photon mass

An upper bound for the photon rest-mass is calculated on the basis of zero-point energy considerations within the context of fluctuations in the quantum foam, our results being compared with experimental data. Furthermore, some aspects related to energy spectrum are discussed.     

A new approach to the problem of electron-atom collision cross section measurements in the low-energy range

Momentum-transfer cross sections for He and Ar have been determined in the energy ranges 5×10^-3?3 eV and 5×10^-3?0.6 eV, respectively using a modified effective-range analysis of the shape of experimental ECR-absorption lines, registered in the afterglows. The following values have been obtained for the scattering length: 1.17±0.01a³₀ for He and 1.53±0.01a³₀ for Ar.