Abstract: The accelerating field in superconducting cavities has to be stabilized in amplitude and phase by a radio-frequency (rf) control system. Because of their high loaded quality factor superconducting cavities are very susceptible for microphonics. To meet the increased requirements with respect to accuracy, availability, and diagnostics, the previous analog rf control system of the superconducting Darmstadt electron linear accelerator S-DALINAC has been replaced by a digital rf control system. The new hardware consists of two components: An rf module that converts the signal from the cavity down to the base-band and a field-programmable gate array board including a soft CPU that carries out the signal processing steps of the control algorithm. Different algorithms are used for normal-conducting and superconducting cavities. To improve the availability of the control system, techniques for automatic firmware and software deployment have been implemented. Extensive diagnostic features provide the operator with additional information. The architecture of the rf control system as well as the functionality of its components will be presented along with measurements that characterize the performance of the system, yielding, e.g., an amplitude stabilization down to (ÎA/A)rms=7Ã10-5 and a phase stabilization of (ÎÏ)rms=0.8° for superconducting cavities.
Abstract: The electronâion scattering experiment ELISe is part of the installations envisaged at the new experimental storage ring at the International Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany. It offers an unique opportunity to use electrons as probe in investigations of the structure of exotic nuclei. The conceptual design and the scientific challenges of ELISe are presented.
Abstract: Atomic-field bremsstrahlung has been studied with a longitudinally polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been measured at the photon high energy tip region. In the time reversal this corresponds to a so-far unobserved phenomenon of production of longitudinally polarized electrons by photoionization of unpolarized atoms with linearly polarized photons. The results confirm the fully relativistic calculations for radiative recombination and suggest a new method for electron beam polarimetry.
Abstract: Fission of 238U and 234U induced by bremsstrahlung of 6.5â9.0 MeV endpoint energy has been investigated at the superconducting Darmstadt electron linear accelerator S-DALINAC. Using a twin Frisch grid ionization chamber, fission-fragment energy and mass distributions have been determined by means of the double kinetic-energy technique. Results on the fission-fragment characteristics from 238U(g,f) are in agreement with results from the literature. In addition fission-fragment mass and energy distributions from 234U(g,f) are presented for the first time in this energy region. An analysis of fission modes within the Brosa model has been performed. The relative yield of the S1 mode was found to be (13±3)% in 234U and (35±2)% in 238U.
Abstract: The consequences of the spontaneous breaking of rotational symmetry are investigated in a field theory model for deformed nuclei, based on simple separable interactions. The crucial role of the Ward-Takahashi identities in describing the rotational states is emphasized. We show explicitly how the rotor picture emerges from the isoscalar Goldstone modes and how the two-rotor model emerges from the isovector scissors modes. As an application of the formalism, we discuss the M1 sum rules in deformed nuclei and make the connection to empirical information.
Abstract: The double kinetic energy measurement of fission fragments with a double-sided Frisch grid ionization chamber allows a careful determination of the emission angle, which is essential in order to apply appropriate energy-loss corrections. We present a drift-time method, which uses the time that free electrons need to drift from the location of their creation, e.g. by a fission fragment in the counting gas, to the grid, before inducing a signal on the anode. Such a measurement leaves energy and angular information fully decoupled. We demonstrate the applicability of the drift-time method for the example of the 234,238U (g, f) reactions performed at the superconducting Darmstadt electron linear accelerator. The angular resolutions achieved with this method are comparable to those obtained with other methods.
Abstract: Spectra of the 235U(γ,γâ²) reaction were measured at the S-DALINAC at bremsstrahlung end-point energies E0=3.5 and 4.4 MeV and scattering angles of 90° and 135° with respect to the beam axis. Discrete transitions are observed at excitation energies below 2.3 MeV only. The deduced cross sections are in rough agreement with the findings of Bertozzi et al. Phys. Rev. C 78 041601 (2008) except for the most prominent transition, where the present result is lower by a factor of about 2. Evidence for unresolved dipole strength is found in the spectra by means of a fluctuation analysis, which was carried out up to an excitation energy of 2.8 MeV. If this unresolved strength exhibits the same ratio of E1/M1 cross sections as observed in the even-mass neighbor 236U, then the energy centroid EÌ x=2.5(3) MeV and total strength âB(M1)â=3.6(1.3)μN2 of the M1 part are in good agreement with the systematics of the scissors mode in even-even actinide nuclei.
Abstract: Results are presented from a one-neutron knockout reaction at relativistic energies on 56Ti using the GSI FRS as a two-stage magnetic spectrometer and the Miniball array for gamma-ray detection. Inclusive and exclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the orbital angular momentum of the populated states. First-time observation of the 955(6) keV ν p 3 / 2 â 1 -hole state in 55Ti is reported. The measured data for the first time proves that the ground state of 55Ti is a 1 / 2 â state, in agreement with shell-model calculations using the GXPF1A interaction that predict a sizable N = 34 gap in 54Ca.
Abstract: The Pb(g,g') photon scattering reaction has been studied with the nearly monochromatic, linearly polarized photon beams at the High Intensity g-ray Source (HIgS) at the DFELL. Azimuthal scattering intensity asymmetries measured with respect to the polarization plane of the beam have been used for the first time to assign both the spin and parity quantum numbers of dipole excited states of 206,207,208Pb at excitation energies in the vicinity of 5.5 MeV. Evidence for dominant particle-core coupling is deduced from these results along with information on excitation energies and electromagnetic transition matrix elements. Implications of the existence of weakly coupled states built on highly excited core states in competition with 1 hbar omega single particle (hole) excitations at comparable energies are discussed.
Abstract: Inclusive inelastic electron scattering off the deuteron under 180° has been studied at the S-DALINAC close to the breakup threshold at momentum transfers q=0.27ââfm-1 and 0.74ââfm-1 with good energy resolution sufficient to map in detail the spin flip M1 response, which governs the starting reaction pnâdγ of big-bang nucleosynthesis over most of the relevant temperature region. Results from potential model calculations and (for q=0.27ââfm-1) from pionless nuclear effective field theory are in excellent agreement with the data.
Abstract: A comparison between the electric quadrupole (E2) strength distributions in 40,48Ca with new results from 52Cr is presented. The deduced E2 strength distributions and the exhaustion of the isoscalar energy-weighted E 2 sum rule are very different. Microscopic approaches fail to reproduce these differences. A survey of the available data shows that the exhaustion of the energy-weighted isoscalar E 2 sum rule in doubly magic nuclei below the isoscalar giant quadrupole resonance is typically more than two times larger than in semi-magic nuclei. On the other hand, the E 2 strength in this energy region exhausts about 50% of the total E 2 strength, independent from shell closures.
Abstract: We report on direct experimental evidence of the population of the 3 / 2 - intruder state in 27Ne in the knockout of a single neutron from the ground state of 28Ne. This low-lying negative parity state is consistent with a narrower shell gap for exotic nuclei with Z < N and N ~ 20 . Monte Carlo shell-model calculations with the modern SDPF-M interaction successfully describe neutron-rich nuclei in the vicinity of N = 20 where normal and intruder configurations coexist at low excitation energy. This observation demonstrates the importance of direct reactions for the study of exotic nuclei and the predictive power of these large-scale shell-model calculations.
Abstract: Results of a photon scattering experiment on 112Sn using bremsstrahlung with an endpoint energy of E0=3.8 MeV are reported. A J=1 state at Ex=3434(1) keV has been excited. Its decay width into the ground state amounts to Î0=151(17) meV, making it a candidate for a [2+â3-]1- two-phonon state. The results for 112Sn are compared with quasiparticle-phonon model calculations as well as the systematics of the lowest lying 1- states established in other even-mass tin isotopes. In contrast to findings in the heavier stable even-mass Sn isotopes, no 2+ states between 2 and 3.5 MeV excitation energy have been detected in the present experiment.
Abstract: The single-particle structure of 57Ni and level structure of 56Ni were investigated with the 9Be (57Ni,56Ni+γ)X reaction at 73 MeV/nucleon. An inclusive cross section of 41.4(12) mb was obtained for the reaction, compared to a theoretical prediction of 85.4 mb, hence only 48(2)% of the theoretical cross section is exhausted. This reduction in the observed spectroscopic strength is consistent with that found for lighter well-bound nuclei. One-neutron removal spectroscopic factors of 0.58(11) to the ground state and 3.7(2) to all excited states of 56Ni were deduced.
Abstract: The energy of the first excited state in the neutron-rich N=28 nucleus 45Cl has been established via in-beam γ-ray spectroscopy following proton removal. This energy value completes the systematics of the E(1/21+)-E(3/21+) level spacing in odd-mass K, Cl, and P isotopes for N=20-28. The results are discussed in the framework of shell-model calculations in the sd-fp model space. The contribution of the central, spin-orbit, and tensor components is discussed from a calculation based on a proton single-hole spectrum from G-matrix and Ï+Ï meson exchange potentials. A composite model for the proton 0d3/2-1s1/2 single-particle energy shift is presented.
Abstract: From isobaric analog symmetry arguments and nuclear shell model calculations, we evaluate the temperature dependence of the beta-decay rates of the ground state and first excited state at an excitation energy of 96 keV in 19 O. It is shown that the decay from the excited level exceeds that of the ground state for T 9 > 1. This feature may be of significance in the r -process and explosive nucleosynthesis.
Abstract: The 9Be(46Ar,45Ar+γ)X one-neutron removal reaction has been studied in inverse kinematics at 70 MeV/nucleon. Coincidences with γ rays served to disentangle knockout events leading to the 45Ar ground state. The measured partial cross section corresponds to a spectroscopic factor of 4.9(7). The residue momentum distribution is compared with new calculations based on eikonal theory and represents the first case of an â=3 neutron removal, as is expected for populating a 0f7/2 hole in the N=28 projectile. However, the measured 45Ar momentum distribution has a marked low-momentum tail suggestive of dissipative effects whereas the eikonal model predictions are symmetric. The angular distribution of the residues confirms that there is a deviation from the model.
Abstract: The degree of collectivity in the neutron-rich nuclei 32Mg and 34Mg has been determined via intermediate-energy Coulomb excitation in inverse kinematics. Measured energies of the first excited 2+ states and reduced electric quadrupole transition probabilities B(E2;0g.s.+â21+) are presented for 32Mg and 34Mg. The results agree with previous measurements and confirm the placement of both nuclei within the âisland of inversion.â
Abstract: Low-lying excited states of 46Ar have been studied via inverse-kinematics proton scattering with a thick target. Coupled-channels calculations have been used to extract the deformation length of the 21+ state. This result, combined with existing Coulomb excitation data, yields a ratio of the neutron-to-proton transition matrix elements of Mn/Mp=1.19(25)N/Z, showing a departure from the proton dominance observed in the N=28 isotones above 48Ca. The status of the N=28 shell below 48Ca is discussed.
Abstract: The low-lying orbital magnetic dipole strength in even-even nuclei is discussed using a sum-rule approach. It is shown that both the mean excitation energies as well as the summed excitation strengths from the experiment can be described well in heavy nuclei if the moments of inertia and the g factors are substituted by the parameters of the ground-state rotational bands. The influence of the high-lying scissors mode is taken into account explicitly, leading to a successful description of the low-lying mode in heavy deformed nuclei with no free parameters. A quantitative estimate of the gross features of the high-lying mode is deduced. The application of the sum-rule approach to medium-mass nuclei is presented, and the discrepancies with the experimental data are discussed.
Abstract: An accurate determination of the partial cross sections to four final states in the 9Be(15C,14C)X reaction has been carried out at 103 MeVânucleon. The cross section of 101(5) mb to the 14C 0+ ground state is of special interest. Relative to the theoretical cross section calculated on the basis of the spectroscopic factor from effective-interaction theory, this amounts to a quenching factor Rs=0.90(4)(5). Here the first number in parentheses is the experimental error, and the second is the error on the theoretical unit (reaction) cross section. The result gives support to the idea that weakly bound halolike states have quenching factors that approach unity, in contrast to factors of 0.5â0.6 characteristic of well-bound states in nuclei near stability.
Abstract: One-neutron knockout reactions from the deeply bound N=16 isotones with Z=16,17, and 18 have been studied in inverse kinematics with intermediate-energy beams. γ-ray spectroscopy in coincidence with the detection of knockout residues allowed for an investigation of the one-neutron removal leading to individual excited states. Spectroscopic factors are deduced in the framework of the sudden and eikonal approximations and are compared to USD shell-model predictions. The momentum distributions observed in the experiment are used to identify the angular momentum l carried by the knockedout neutron by comparing with calculations based on a black-disk reaction model. The systematics of reduced single-particle occupancies attributed to the effect of short-range correlations, observed so far for stable and near-magic nuclei in (e,eâ²p) and (d,3He) reactions and in one-nucleon knockout on light deeply bound systems, are extended in this work.
Abstract: Dipole and quadrupole excitations in the semimagic N=50 nucleus 88Sr were investigated at the superconducting Darmstadt electron linear accelerator S-DALINAC with bremsstrahlung of an end-point energy of 6.8 MeV. Many new dipole excitations could be identified, and their reduced excitation probabilities were determined. The experimental findings are discussed in the context of quasiparticle-phonon-model and shell-model calculations. A breaking of the N=50 core is essential to describe the structure of the observed excitations. The two-phonon quadrupole-octupole JÏ=1â state exhibits unusual features which are presently not understood.
Abstract: We study the statistical properties of the electric pygmy dipole resonance in four different isotones with neutron number N=82. These nuclei are 138Ba, 140Ce, 142Nd, 144Sm. The data set comprises 184 levels with spin and parity JÏ=1â and their ground state B(E1) transition strengths. The statistics are found to be âmixedâ, i.e., in between the predictions of random matrix theory for correlated and uncorrelated spectra. Moreover, we calculate spectra and transition strengths in the quasiparticle phonon model (QPM). We compare experimental and theoretical findings. The incompleteness of the data sets and its impact on the results is discussed. A consistent picture emerges which yields an improved understanding of the statistical properties of the pygmy resonance.
Abstract: The 9Be(32Ar,31Ar)X reaction, leading to the 5/2+ ground state of a nucleus at the proton drip line, has a cross section of 10.4(13) mb at a beam energy of 65.1ââMeV/nucleon. This translates into a spectroscopic factor that is only 24(3)% of that predicted by the many-body shell-model theory. We introduce refinements to the eikonal reaction theory used to extract the spectroscopic factor to clarify that this very strong reduction represents an effect of nuclear structure. We suggest that it reflects correlation effects linked to the high neutron separation energy (22.0 MeV) for this state.
Abstract: A nuclear resonance fluorescence experiment with two highly efficient EUROBALL Cluster detectors has been performed on the γ-soft nucleus 194Pt. Dipole excitations were observed between 2 and 4 MeV excitation energy. They are tentatively interpreted as the main fragments of the scissors mode based on the measured excitation strengths and a comparison to microscopic calculations in the framework of the quasiparticle random phase approximation (QRPA). The data indicate large differences to the neighbouring isotope 196Pt: a doubling of the observed dipole strength and a shift of the energy centroid by about 600 keV. None of the currently available models is able to reproduce these features consistently in both nuclei.
Abstract: Dipole and electric quadrupole excitations in 204,206,207,208Pb have been measured up to 6.75 MeV in resonant photon scattering experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC using two Euroball-Cluster detector modules. In 208Pb, 14 excited states have been populated; in 206Pb, the decays of 41 states have been detected. Information about 45 heretofore unknown excited states in 204Pb could be measured as well as eleven known levels in 207Pb. The extracted dipole strength distributions are discussed within phenomenological (âpygmy resonanceâ) and microscopic models (quasiparticle-phonon model). A strong fragmentation and a small shift of the detected E1 strength towards higher energies is observed with the opening of the neutron shell closure.
Abstract: The knockout of protons from 8B and 9C on a carbon target has been studied at average energies of 76 and 78 MeV/nucleon, respectively, with beams from the A1900 fragment separator incident on a stack of silicon detectors. The following cross sections were obtained: Ï-1p(8B-7Be)=130(11)mb, Ï-1p(9C-8B)=54(4)mb, and Ï-2p(9C-7Be)=98(7)mb. The results are discussed within the framework of an eikonal approach and compared with measurements performed at higher energies. From this analysis, a consistent picture emerges that gives evidence for the validity of the eikonal approach at energies below 100 MeV/nucleon. Knockout reactions at intermediate energy can thus be used to deduce absolute shell occupancies. We find the spectroscopic factors to be reduced by Rs of 0.86(7) and 0.82(6) for 8B and 9C, respectively, relative to shell-model predictions. The 9C result provides an accurate measurement of the asymptotic normalization coefficient of 1.27(10)fm-1. A new technique is reported for determining separately the contributions from stripping and diffractive breakup.
Abstract: The reaction 9Be(28Mg,26Ne+γ)X has been studied at 82ââMeV/nucleon together with two similar cases, 30Mg and 34Si. Strong evidence that the reactions are direct is offered by the parallel-momentum distributions of the reaction residues and by the inclusive cross sections. The pattern of the partial cross sections for 28Mg suggests the presence of correlations. A preliminary theoretical discussion based on eikonal reaction theory and the many-body shell model is presented. The reaction holds great promise for the study of neutron-rich nuclei.
Abstract: Intermediate-energy Coulomb excitation is a key method to investigate collectivity in exotic nuclei far from β stability. We report on the measurement of the absolute B(E2;01+â21+) excitation strength in 46Ar for five different minimum impact parameters. Our findings underline the validity, feasibility, and perspective of this technique for the study of exotic nuclei also in the regime of higher beam energy.
Abstract: The cross sections of the single proton knock-out reactions from 24F, 25F, and 26F on a 12C target were measured at energies of about 50 MeVânucleon. Ground state populations of 6.6±0.9 mb, 3.8±0.6 mb for the reactions 12C(24F,23O) and 12C(25F,24O) were extracted, respectively. The data were compared to calculations based on the many-body shell model and the eikonal theory. In the reaction 12C(26F,25O) the particle instability of 25O was confirmed.
Abstract: Nuclear resonance fluorescence experiments were performed on the rare-earth nuclei 151,153Eu, and with considerably increased sensitivity on 163Dy and 165Ho to study the fragmentation of the M1 scissors mode in odd-mass nuclei, and to clarify the puzzle of the missing total M1 strength observed for odd-mass nuclei so far. Using the bremsstrahlung photon beam of the Stuttgart Dynamitron (end point energy 4.05 MeV) and high-resolution Ge γ-ray spectrometers, detailed information was obtained on excitation energies, decay widths, transition probabilities, and branching ratios. Whereas in 151Eu only 11 weak excitations were observed, 161 and 138 excitations could be detected in the heavier nuclei 163Dy and 165Ho, respectively. The results are compared to those observed recently at the Stuttgart facility for the neighboring odd-mass nuclei 161Dy, 155,157Gd, and 159Tb. The measured total strengths increase with the mass number A. Ascribing the same portion of the dipole strength to M1 excitations as measured in the neighboring even-even nuclei, the total M1 strength deduced from the most sensitive experiment on 163Dy is comparable to those found in the neighboring even-even nuclei. The results for 163Dy and 165Ho are compared with a fluctuation analysis of the photon scattering spectra to estimate the amount of still unresolved strength eventually hidden in the background due to the extreme fragmentation of the M1 scissors mode in odd-mass rare-earth nuclei. For 165Ho, the total derived strength of B(M1)â=2.9(5)μN2 agrees within error bars with an earlier analysis of a different measurement of the 165Ho(γ,γâ²) reaction. In 163Dy the method leads to an unphysical background shape, underlining the experimental observation of a significantly reduced fragmentation pattern of the dipole modes in this nucleus, which must be traced back to structure features of the Dy isotopes.
Abstract: Results of single-neutron knockout experiments from 34,35Si at 73AâMeV and from 37S at 69AâMeV on a Be target are reported. The final states in the reaction residues have been identified by measuring γ rays in coincidence with the (A-1) fragments. The results are compared with predictions based on the many-body shell model combined with eikonal and transfer-to-the-continuum reaction models. For the 9Be(34Si,33Si+γ)X reaction, the measured parallel-momentum distributions of the reaction residues demonstrate the orbital-angular-momentum assignments l=2, 0, 2 to levels at 0, 1.010, and 4.32 MeV (adopted values). The measured absolute spectroscopic factors exhaust a large fraction of the sum rules for a full (N=20)âsd shell. We also give results for the absolute and relative positions of the parallel-momentum distributions and present experimental and theoretical results for the complex spectra of the 9Be(35Si,34Si+γ)X reaction. The measured inclusive cross section of 106(19) mb agrees well with the theoretical value of 98 mb, but more detailed conclusions are not possible. For the inclusive reaction 9Be(37S,36S)X a cross section of 99(12) mb is found compared to the theoretical prediction of 85 mb where only final states with spectroscopic factors greater than 0.03 have been included.
Abstract: The photo-induced depopulation of the quasistable isomer (t1/2>~1.2Ã1015âyr) in 180Ta with angular momentum and parity JÏ=9- at an excitation energy Ex=75âkeV was studied at the new bremsstrahlung irradiation facility installed at the Stuttgart 4.3 MV DYNAMITRON accelerator in the energy range of bremsstrahlung end point energies between E0=0.8â3.1âMeV. The onset of the isomer depopulation could be observed starting at an end point energy of E0â1âMeV, i.e., at an intermediate state of 180Ta at or below that energy. Higher-lying intermediate states were found at 1.22, 1.43, 1.55, 1.85, 2.16, 2.40, 2.64, and 2.80 MeV. The extracted integrated cross sections show a remarkably strong depopulation of the 180Tam isomer by photoexcitation. The results are compared with previous experiments and recent quasiparticle-phonon model calculations. Implications of the results for a possible nucleosynthesis of 180Ta in the s process and the neutrino process are discussed.
Abstract: Photon scattering experiments have been performed to investigate the structure of the two doubly magic nuclei 40,48Ca. The method is highly selective to induce low-order multipole transitions, i.e., E1, M1, and E2 from the ground state. We determined the energies and spins of excited states and the absolute strengths of the γ decays in a model independent way. We find the summed electric dipole strengths below 10 MeV to exhaust the energy weighted sum rule by 0.023 and 0.27 %, respectively. The summed electric quadrupole strengths are âB(E2)â=332e2fm4 and 407e2fm4 for 40Ca and 48Ca, respectively. In order to explain the difference in the E1 strengths of the two isotopes several theoretical models are discussed.
Abstract: At the Stuttgart 4.3 MV DYNAMITRON accelerator a new bremsstrahlung irradiation facility was installed for photoactivation experiments to complement the well-established arrangement for systematic photon-scattering studies taking advantage of the high photon fluxes achievable at this high-current accelerator. The population or depopulation of isomeric states in low-energy photon-induced reactions (Eγ=0.5â4.0 MeV) can be studied with sensitivities far beyond previous limits. The setup of this facility, its performance, and the absolute calibrations are described in detail. First exemplary results for a test case, the reaction 115In(g,g')115mIn, are presented and analyzed in combination with data from low-energy photon-scattering experiments (115In(g,g')115In). The quality of the results demonstrates the powerful capability of this new installation.
Abstract: The nucleosynthesis of heavy neutron-deficient nuclei in a stellar photon bath at the temperatures relevant for the astrophysical γ process was investigated. In order to derive (γ,n) cross sections and reaction rates, the stellar photon bath was simulated by the superposition of several bremsstrahlung spectra with different end-point energies. As a first test for this method, the (γ,n) reaction rates of the platinum isotopes 190Pt, 192Pt, and 198Pt were derived. The results are compared to other experimental data and theoretical calculations.
Abstract: A nuclear resonance fluorescence experiment on 88 Sr has been performed with bremsstrahlung of 6.7 MeV endpoint energy. The g-ray linear polarisation has been measured with a EUROBALL CLUSTER detector used as a Compton polarimeter. The results indicate positive parity for the J= 1 state at 4.742 MeV in 88 Sr, in contrast to the previous interpretation as a 1- two-phonon (2 + 1 x 3 - 1 ) state and in conflict with the predictions of the quasiparticle-phonon model. On the basis of such calculations the 1 + state at 3.486 MeV may be considered as the 1 + 1 one-phonon state and the very strong 1 + 1 -> 0 + 1 deexcitation as proton spin-flip 2p 1/2 -> 2p 3/2 transition.
Abstract: The doubly magic nuclei 40Ca and 48Ca have been studied in high resolution photon scattering experiments. We have derived absolute dipole and quadrupole excitation strengths up to 10 MeV. Evidence was found for a two-phonon quadrupole-octupole state in 48Ca. At higher energies in contrast to 40Ca, a concentration of dipole strength is observed in 48Ca which is discussed in terms of a pygmy resonance originating from the large neutron excess.
Notes: Erratum: T. Hartmann, J. Enders, P. Mohr, K. Vogt, S. Volz, and A. Zilges, Measurement of the Dipole and Electric Quadrupole Strength Distributions up to 10 MeV in the Doubly Magic Nuclei 40Ca and 48Ca [Phys. Rev. Lett. 85, 274 (2000)], Phys. Rev. Lett. 86, 4981 (2001).
URL: http://link.aps.org/doi/10.1103/PhysRevLett.86.4981
DOI: 10.1103/PhysRevLett.86.4981
Abstract: The nucleosynthesis of heavy proton-rich nuclei in a stellar photon bath at temperatures of the astrophysical γ-process was investigated where the photon bath was simulated by the superposition of bremsstrahlung spectra with different endpoint energies. The method was applied to derive (γ,n) cross sections and reaction rates for several platinum isotopes.
Abstract: Results of a 208Pb (g,g') experiment are presented aiming at an identification of the 2+ source member of the long-sought two-octupole phonon multiplet. Four E2 excitations have been observed below 6.5 MeV excitation energy, two of them for the first time. However, in contrast to new results of calculations within the quasiparticleâphonon nuclear model (QPM), no obvious candidate for the two-octupole phonon vibration could be found in the present study. We discuss the J^pi = 2+ states detected in this as well as previous experiments with respect to their possible two-octupole phonon structure.
Abstract: We analyse the level spacing distributions of scissors mode states in heavy deformed nuclei. To this end, we use an ensemble of photon scattering data of 13 nuclei. The results are consistent with a Poissonian behaviour which indicates that the correlations between the states are weak only. The significance of this finding is critically investigated. With numerical simulations we study the possible influence of detection thresholds in the photon scattering experiments. A similar analysis of other dipole modes suggests that the collective nature shown for the scissors mode may be a general phenomenon of low-lying states in deformed nuclei.
Abstract: The electric dipole strength distributions below threshold in stable even-mass Pb isotopes have been extracted from photon scattering experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC. Between 4 and 6.5 MeV excitation energy a resonance-like clustering of strength is observed. A strong evolution of a fragmentation of the dipole strength with opening the neutron shell is found. The fine structure of the strength is compared to microscopic quasiparticle-phonon model calculations that quantitatively reproduce the data quite well and explain the fragmentation of the dipole strength. Models suggesting the oscillation of excess neutrons with respect to an NâZ core as a possible origin of low-lying E1 strength are critically examined.
Abstract: The half-lives of the platinum isotopes 189,191,197 Pt and of the 4-day isomer in 195m Pt have been measured with high precision using the photoactivation technique. The g-ray activity was counted over several half-lives with a high-purity germanium detector. The measured half-lives are: T 1/2 ( 189 Pt) = 12.1 ± 1.5 h; T 1/2 ( 191 Pt) = 2.862 ± 0.007 d; T 1/2 ( 197 Pt) = 19.96 ± 0.05 h; T 1/2 ( 195m Pt) = 4.0104 ± 0.0047 d.
Abstract: The electromagnetic dipole response of 138Ba was measured up to 6.7 MeV excitation energy in a photon scattering experiment. Two Euroball Cluster detectors were used to detect the scattered photons under 94 and 132 degrees. The Cluster at 94 degrees served as a Compton polarimeter. The total observed dipole cross section is in good agreement with previous tagged photon data, but in the present experiment the transition strength and the electric character of most of the transitions has been determined on a state-by-state basis. The data show a concentration of E1 strength around 6 MeV. For one of the strongest excitations M1 character is suggested. The results of model calculations using the quasiparticle phonon model (QPM) agree with the observed electric dipole strength distribution.
Abstract: The depopulation of the quasistable isomer in 180Ta with JÏ = 9- at Ex = 75keV by resonant photoabsorption was investigated with intense bremsstrahlung. The results indicate a dramatic acceleration of the isomer decay to the short-lived JÏ = 1+ ground state under stellar s-process conditions. The consequences for a possible nucleosynthesis of nature's rarest isotope 180Ta within the s process are discussed.
Abstract: From a sum rule approach, it is shown that the excitation energy as well as the M1 transition strength of the low-lying scissors mode in heavy even-even nuclei are well reproduced by using the moments of inertia and g factors of the ground state band. Also, a simple correspondence to the high-lying scissors mode is deduced.
Abstract: We report on a nuclear resonance fluorescence experiment on the semi-magic odd-mass fp-shell nucleus 51V. The detected dipole strength distribution is discussed on the basis of modern shell-model calculations employing a model space allowing for excitations of protons and neutrons from the 1f7/2 to the (2p1/2,2p3/2,1f5/2) shells. The calculations indicate that the main body of transitions is of magnetic dipole type, and they are capable of reproducing the observed distribution well. Possible electric dipole and quadrupole contributions are also discussed.
Notes: Erratum I: <a href="http://www.sciencedirect.com/science/article/pii/S037594740000138X">Nucl. Phys. A 669, (3-4), pp. 368-380 (Apr 2000)</a>, doi:10.1016/S0375-9474(00)00138-X
Erratum II: <a href="http://www.sciencedirect.com/science/article/pii/S037594740000453X">Nucl. Phys. A 679, (3-4), p. 869 (Jan 2001)</a>, doi:10.1016/S0375-9474(00)00453-X
Abstract: A new radiator and collimator system has been installed at the photon scattering facility of the superconducting electron accelerator S-DALINAC at the Darmstadt University of Technology. The new set-up allows the high-resolution (g,g') experiments to be extended to the energy range between 7 and 10 MeV without neutron induced g-ray background. First test measurements of 11B targets in combination with Ge(HP) detectors have been performed successfully and show the huge potential of the improved set-up.
Abstract: The low-energy dipole strength distributions in 165Ho and 169Tm have been studied in nuclear resonance fluorescence experiments at the S-DALINAC utilizing a Euroball Cluster detector. In the energy interval between 2.5 and 4.0 MeV, where the scissors mode is expected, 35 ground state transitions could be observed in 165Ho and 53 in 169Tm. Assuming M1 character for all these transitions corresponds to ΣB(M1) â= 1.54(23), μN2 and ΣB(M1) â= 2.15(63), μN2 for 165Ho and 169Tm, respectively. A statistical analysis is applied to the measured spectra which is capable of reconstructing the complete strength, including contributions of transitions below the observation limit, with the ratio between M1 and E1 excitations and their total strengths taken as an average over the even-mass neighbours. This results in summed M1 transition strengths of ΣB(M1) â= 3.54â0.95+0.75, μN2 (165Ho) and ΣB(M1) â= 3.36â0.57+1.00μN2 (169Tm). The large variations of the summed dipole strength and the fragmentation in other deformed odd-mass nuclei can be reproduced by the same statistical ansatz. The total low-lying M1 strength in heavy deformed odd-mass nuclei is in agreement with the findings in the neighbouring even-mass nuclides as well as with sum-rule predictions.
Abstract: A recently proposed fluctuation analysis of resonant photon scattering spectra [J. Enders et al., Phys. Rev. Lett. 79, 2010 (1997)] is applied to the 157Gd(γ,γâ²) reaction. Evidence for unresolved strength in the experimental background is found which amounts to a total dipole excitation strength of âgÎ0red=(30-10+6)meV/MeV3 in the excitation energy range 2.5â4 MeV. Assuming a smooth variation of the M1/E1 strength ratio in the neighboring even-mass nuclei, the M1 part can be estimated to be B(M1)=(2.0-0.7+0.4) μN2. This value is close to the expectation of sum rules. We discuss possible sources of errors which suggests that the obtained value represents a lower limit only.
Abstract: Results of a natCr(γ,γâ²) experiment at the S-DALINAC are reported for energies up to 7 MeV utilizing a Euroball Cluster detector. The excitation of a J = 1 state at 5544 keV is observed which is believed to belong to 52Cr because of the strength of the signal and the large natural abundance. By a variety of empirical arguments a quadrupole-octupole-coupled two-phonon 1â character is suggested for this transition. Quasiparticle random-phase approximation (QRPA) calculations are able to reproduce the energy and B(E1) transition strength remarkably well. The underlying microscopic structure is suggested to be more complex than a pure two-phonon picture. Furthermore, the calculations indicate a pure neutron (2p_3/2^-1 2p_1/2) magnetic dipole structure for an excitation experimentally seen at 5098 keV which would provide a direct measure of ground-state correlations in 52Cr.
Abstract: It is demonstrated by a fluctuation analysis based on the assumption of a Wigner distribution for the nuclear level spacings and of a Porter-Thomas distribution for the transition strengths that significant parts of the dipole strength excited in photon scattering experiments in heavy, deformed odd-mass nuclei are hidden in the background of the experimental spectra. With this additional strength, the heretofore claimed severe reduction of the B(M1) scissors mode strength in odd-mass nuclei compared to the one in neighboring even-even nuclei disappears.
Abstract: Excited states of the nuclei 122,126,130Te were populated via the (g,g') reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at E ~ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1- state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1+ state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.
Abstract: The feasibility of a new experimental method to identify two-phonon octupole vibrations is investigated for the example of 208Pb. The 2+ member of the 3Iâ â 3Iâ multiplet can be excited with the (γ, γâ²) reaction and a sizable E1 decay to the 3Iâ level serves as a signature of the two-phonon character. Numerical estimates within the quasiparticle-phonon model indicate that such an experiment should be possible with state-of-the-art large volume Ge detectors like the EUROBALL cluster module.
Abstract: The semi-magic nucleus 89Y has been investigated in a (γ,γâ²) experiment at an endpoint energy of the bremsstrahlung of E0 = 7 MeV. The scattered photons have been detected with a EUROBALL Cluster detector. The observed excitations are discussed in the framework of the shell model and a coupling of the unpaired proton to multi-phonon structures in the doubly even neighbours. Transitions above 4.7 MeV are considered to have E1 character. Around 6.3 MeV an unusually large concentration of E1 strength is found. Its origin is likely to correspond to similar structures in the 88Sr and 90Zr isotones which can be interpreted to result from the constructive interference of strong two-phonon amplitudes with weak admixtures from the low-energy tail of the giant dipole resonance.
Abstract: The E1 response of the semi-magic nucleus 140Ce below the particle threshold was measured in a (Ãg,g') experiment utilizing the new Euroball Cluster detector at the S-DALINAC. While the energy averaged data are in good agreement with tagged photon results, here they are resolved for the first time into 54 individual transitions. A quasiparticle-phonon model calculation including up to three-phonon configurations compares well to the extracted strength distribution. The interference between one- and two-phonon contributions is essential for a quantitative reproduction.
Abstract: A nuclear resonance fluorescence experiment with one of the newly developed highly efficient EUROBALL cluster detectors has been performed on the γ-soft nucleus 196Pt. Magnetic dipole excitations were observed between 2 and 3.5 MeV excitation energy. They are interpreted as the main fragments of the scissors mode based on the measured excitation strengths and branching ratios. A strong γ decay, which we believe to be an E2 decay of the scissors mode to the 21+ state, is observed. It allows an extraction of the effective boson quadrupole charges in the interacting boson model.
Abstract: Triggered by the need to control the superconducting cavities of the S-DALINAC, which have a high loaded quality factor and are thus very susceptible to microphonics, the development of a digital low level RF control system was started. The chosen design proved to be very flexible since other frequencies than the original 3 GHz may be adapted easily: The system converts the RF signal coming from the cavity (e. g. 3 GHz) down to the base band using a hardware I/Q demodulator. The base band signals are digitized by ADCs and fed into a FPGA where the control algorithm is implemented. The resulting signals are I/Q modulated before they are sent back to the cavity. The superconducting cavities are operatedwith a self-excited loop algorithm whereas a generator-driven algorithm is used for the low Q normal-conducting bunching cavities. A 6 GHz RF front end allows the synchronous operation of a new 2f buncher at the S-DALINAC. Meanwhile, a 325 MHz version has been built to control a pulsed prototype test stand for the p-LINAC at FAIR.
We will present the architecture of the RF control system as well as results obtained during operation.
Abstract: Es werden E-Learning-Elemente vorgestellt, die im Rahmen der Vorlesung Physik IV für Studierende im Bachelor-Studiengang Physik im Sommer 2010 an der TU Darmstadt erprobt wurden.
Zu dem Konzept gehörte auch das Erstellen von Wiki-Beiträgen in Kleingruppen, was auf einer Moodle-Lernplattform realisiert wurde. Neben Beispielen für die von den Studierenden erstellten Beiträge werden auch Evaluationsergebnisse präsentiert, die eine kritische Haltung der Studierenden gegenüber E-Learning-Elementen in der Hochschullehre im Fach Physik aufzeigen
Abstract: Polarimetry is a key field of interest if an experimental program with polarized electron and photon beams is planned. The new photo injector recently installed at the superconducting 130 MeV Darmstadt electron linac S-DALINAC provides polarized electrons from a GaAs cathode for nuclear structure studies at low momentum transfers. The degree of polarization needs to be measured at different positions close to the experimental sites and at different energies ranging from 100 keV up to 130 MeV. A 100 keV Mott polarimeter had been set up and commissioned at a test stand of the polarized source prior to implementation, while a 5-10 MeV Mott polarimeter is currently under construction and simulations for a 50-130 MeV Møller polarimeter are going on. Compton transmission polarimeters are foreseen to monitor polarization during experiments.
Abstract: The linear polarization of bremsstrahlung radiation emitted in collisions of spin-polarized and unpolarized electrons with carbon and gold targets has been measured for an incident kinetic energy of 100 keV. We present preliminary results for the degree of linear polarization for incident unpolarized electrons as a function of bremsstrahlung photon energy.
Abstract: A source of polarized electrons has been installed at the superconducting Darmstadt electron linear accelerator S-DALINAC. Experiments with polarized electrons from 100 keV to about 80 MeV are expected to commence in early 2011. This contribution summarizes the status of the polarized source as well as ongoing preparations for the experimental program with polarized beams. In particular, we present results on unpolarized test experiments of the, 234,238U(γ,f) reactions and considerations for the 2D(e,e'p) and 3He(e,e'p) reactions.
Abstract: At the superconducting 130 MeV Darmstadt electron linac S-DALINAC the new source of polarized electrons uses a GaAs cathode illuminated with pulsed Ti:Sapphire and diode laser light. The electron source had been set up and commissioned at a test stand with a beam line where a Wien filter for spin manipulation, a Mott polarimeter for polarization measurement and a chopper-prebuncher system were part of the system. Upon completion of the tests, test stand and beam line were dismantled and re-installed at the S-DALINAC. The new photo injector opens up the potential for experiments with polarized electron and photon beams for nuclear structure studies at low momentum transfers. Various polarimeters will be installed at all experimental sites to monitor the beam polarization. We report on the S-DALINAC, the results from the teststand performance, the implementation of the polarized source and the polarimeter research and development.
Abstract: For new experiments with polarized electron and positron beams the precise and quick measurement of the beam polarization is required. The relative electron polarization has been monitored on-line using a compact transmission Compton polarimeter for a wide range of average electron currents up to 100/μA and electron beam energy of 3.5 MeV. The asymmetry induced by the beam polarization can be monitored precisely with ÎA/A < 0.2 %. Even small relative variations of the beam polarization during a beam time may be detected using this polarimeter, such as an increase ~ 1 % and subsequent decrease of the beam polarization.
Abstract: At the superconducting Darmstadt electron linac a 100 keV source of polarized electrons has been installed. Major components had been tested prior to installation at an offline teststand. Commissioning of the new source at the S-DALINAC will take place early in 2011. We report on the performance of the teststand, simulations, developments on the laser systems, new radio-frequency components for the S-DALINAC injector, and the status of the implementation of the source.
Abstract: At the superconducting 130 MeV Darmstadt electron linac S-DALINAC a source of polarized electrons is being installed, extending the experimental capabilities with polarized electrons and photons for nuclear structure studies at low momentum transfers. The polarized source generates electrons by irradiating a GaAs cathode with pulsed Ti:Sapphire and diode lasers and has been set up and commissioned at a test stand including electrostatic preacceleration to 100 keV, a Wien filter for spin manipulation, a Mott polarimeter for polarization measurement and a choppe-rprebuncher system. Various polarimeters will be installed to monitor the beam polarization at all experimental sites. We report on the S-DALINAC, the results from the operation of the source at an offline teststand, the implementation of the polarized source and the polarimeter research and development.
Abstract: A spin polarized electron source will be installed at the S-DALINAC to extend the existing experimental capabilities polarization. In order to manipulate and monitor the beam polarization a Wien filter and different polarimeters will be installed. The already installed Wien filter and 100 keV Mott polarimeter and the planned 5 MeV Mott, 30â130 MeV Møller and Compton transmission polarimeters are described.
Abstract: Experiments on photo-induced fission of 238U and 234U using bremsstrahlung with endpoint energies between 6 MeV and 9 MeV have been carried out at the superconducting Darmstadt electron linear accelerator S-DALINAC. A twin Frisch grid ionization chamber has been used to determine energy and mass distributions via the 2E-technique. The fission fragment emission angle, which is used to correct for energy loss in the target and backing material for a correct determination of fragment energy and mass, is deduced from the time difference between cathode and anode signals. The research program including the foreseen search for parity violation in photo-induced fission and first experimental results will be presented.
Abstract: The V-Code simulation tool implemented at TEMF is a fast tracking code applying the VLASOV equation. Instead of directly solving the partial differential equation, the considered particle distribution function is represented by a discrete set of characteristic moments. To describe the time evolution of these moments ordinary differential equations can be deduced, wherein the external forces caused by the electric and magnetic fields within the accelerator components are considered. The entire beam line is represented in the code as a successive alignment of separate independent beam line elements. The proximity of some beam forming elements may lead to overlapping fringe fields between consecutive elements. In order to simulate even such beam lines with the V-Code its database of disjunctive beam line elements has to be enhanced to deal also with superposed fields. In this paper a summary of issues regarding the implementation complemented with simulation results is provided.
Abstract: The injector of the superconducting Darmstadt electron linear accelerator S-DALINAC will soon be extended by a source of polarized electrons. A test set-up of the future source consists of a 100 keV GaAs polarized gun and associated beamline including an assembly for polarization manipulation and determination. We give a short overview of the existing and planned polarimeters at the different accelerator sites, introduce briefly the future implementation of the polarized source at the S-DALINAC and list experiments that are planned with polarized electron and photon beams.
Abstract: We present the status of the polarized injector for the superconducting Darmstadt electron linear accelerator S-DALINAC. An offline teststand of the source was built to test all components of the S-DALINAC polarized injector SPIN. The new electron source will deliver a 100 keV polarized beam and complement the present thermionic source operating at 250 kV. Results of the beam properties measurements will be introduced and an outlook on the upcoming installation of the new injector at the S-DALINAC will be given.
Abstract: The superconducting Darmstadt linear electron accelerator S-DALINAC will be expanded by a newly developed polarized injector concept called SPIN. The polarized beam program includes experiments to search for parity violation in photo induced fission, the investigation of the 5th structure function at low momentum transfer and break up reactions of few body systems. These experiments and an overview of the current experiemental program will be covered in this paper.
Abstract: The nuclei 47Ca and 55Ti were populated in one-neutron knock-out reactions at relativistic energies. Momentum distributions of the residual nuclei as well as gamma-ray spectra were measured at the GSI fragment separator (FRS). Preliminary results of the ongoing analysis including cross sections and spin/parity assignments are presented.
Abstract: Fast online beam dynamics simulations can advantageously assist the machine operators at various particle accelerator machines because they provide a more detailed insight into the actual machine status. Based on the moment approach a fast tracking code named V-Code has been developed at TEMF. Within the SFB 634 project the V-Code beam dynamics simulation tool is supposed to be employed at the control system of the Superconducting Darmstadt LINear ACcelerator S-DALINAC. In order to be able to simulate the entire beam line of the re-circulating linear accelerator, an implementation of fringe field dipole magnets is mandatory.
Abstract: The current upgrade for the injector mainly involves the superconducting rf part. In order to increase the maximum current from 60 μA to 150 or 250 μA the power coupler design had to be modified, resulting in major changes in the whole cryo-module. Second, an additional polarized electron source (SPIN) has been set-up at an offline test area. There, the polarized electrons are produced by photoemission at a strained GaAs cathode on a 100 kV platform. The test beamline includes a Wien filter for spin manipulation, a Mott polarimeter for polarization measurement and additional diagnostic elements. We will give an overview to the project, report on the status and present first measurement results including the proof of polarisation.
Abstract: The design process and the operation of particle accelerator machines can advantageously be assisted by fast online beam dynamics simulations because of its flexible parameter variation combined with nearly simultaneous solution responses providing a detailed insight into the actual machine status. Based on the moment approach a fast tracking code named V-Code has been implemented at TEMF. At the Superconducting DArmstädter LINear ACcelerator S-DALINAC the V-Code is used during the design process of the injector for the new 100 keV polarized electron source but is also supposed to be employed at the control system. For these purposes an implementation of fringe field alpha magnets is mandatory. In this paper a summary of issues regarding the implementation complemented with simulation results will be provided.
Abstract: For planned experiments with polarized electron and photon beams at the superconducting Darmstadt electron linear accelerator S-DALINAC, the absolute degree of the electron beam polarization needs to be known. We present the existing and planned polarimeters at the source of polarized electrons and the experimental sites, especially a 100 keV Mott polarimeter and a Möller polarimeter for 30-130 MeV electrons.
Abstract: Aiming at an extension of the experimental possibilities at the superconducting Darmstadt electron linear accelerator S-DALINAC, a polarized gun is going to be constructed at the moment. The new injector will be able to supply polarized electrons with kinetic energy in the 100 keV range and should add to the present unpolarized thermionic 250 keV source. The design requirements include a polarization degree of at least 80 %, a mean current intensity of 60 muA and a 3 GHz cw time structure. The gun part is simulated in MAFIA whereas subsequent beam dynamics simulations are performed in V-Code. Initial conditions for the V-Codeâs moment approach are extracted from CST MAFIA simulations. The injector consists of short triplets, an alpha magnet, a Wien filter, a Mott polarimeter, a chopper/ prebuncher system and beam diagnostic elements. For the simulations, the 3D electromagnetic fields of the beam line elements are used by means of a Taylor series expansion of variable order. All components except the chopper and a collimator are represented in the simulations. Recent beam dynamic results will be presented.
Abstract: At the superconducting Darmstadt linear accelerator S-DALINAC a new injector for polarized electrons (SPIN) is under development. For this purpose an offline-teststand has been constructed. It consists of the source of polarized electrons and a test beamline including a Wien filter for spin manipulation, a Mott polarimeter for polarization measurement and various beam steering and diagnostic elements. The polarized electron beam is produced by photoemission from a strained GaAs cathode. We report on the status of this project and present first results of the measurements of the beam properties. We also give an outlook on the upcoming installation of SPIN at the S-DALINAC.
Abstract: Beam dynamics simulations for computationally large problems are challenging tasks. On the one hand, for the accurate simulation the electromagnetic field distribution within the whole device and the surrounding environment, it is essential to consider all necessary device components including even small geometry details, complicated material distributions and the field excitations. On the other hand, additional computational effort has to be put into precise modeling of the injected particle beam for accurate beam dynamics simulations. Under linear conditions, it is possible to separate the field calculation of the device from the computation of the particles self-field which can result in the proper application of diverse numerical schemes for the individual field contributions. In the paper, a technique is presented by means of which the static electric and magnetic fields of a Wien filter beam line element can be treated as applied external fields within the beam dynamics simulation tool V-Code under the assumption that the interaction of the particle beam with the surrounding materials can be neglected.
Abstract: Aiming at an extension of the experimenting capabilities for nuclear structure physics at low momentum transfer at the superconducting Darmstadt electron linear accelerator SâDALINAC, a polarized electron gun is being constructed. The new injector will be able to supply the SâDALINAC with 100 keV polarized electrons and should complement the present, unpolarized thermionic source. The design requirements are a degree of polarization of at least 80%, a mean current intensity of 60 μA and a 3 GHz cw time structure. We report on the status of the polarized source with its laser and vacuum system, the preparation setup and a test beam line as well as the present status of preparations for implementing the source at the SâDALINAC.
Abstract: A new source of polarized electrons with energy of 100 keV is presently being developed at the superconducting Darmstadt electron linear accelerator S-DALINAC for future nuclear- and radiation-physics experiments. The pulsed electron beam emitted by the photocathode will be cut to 50 ps by a chopper operated at 3 GHz, and further bunch compression down to 5 ps will be achieved by a two-stage prebuncher section. The chopper-prebuncher system is based on similar devices used at the Mainz Mikrotron (MAMI) where the accelerator frequency is slightly smaller (2.45 GHz). For the chopper, a cylindrical resonator operating at TM110 mode is selected to deflect the electron beam onto an ellipse, i.e., both horizontally and vertically. This is simply achieved by particular slits on both ends of the resonator. The prebunching system consists of two cavities. For increasing the longitudinal capture efficiency, the first cavity will be operated at the fundamental accelerator frequency of the S-DALINAC of 3 GHz, and the second cavity at 6 GHz. The cavities are designed to work at the TM010 mode and TM020 mode for the fundamental and first harmonic, respectively.
Abstract: Recent research and development for a polarized electron source (SPIN) for the recirculating superconducting electron linear accelerator S-DALINAC will be presented. The polarized electron beam will be produced by photoemission from an InAlGaAs/GaAs superlattice cathode and will be accelerated to 100 kV electrostatically. The results of the beam dynamics simulation will be shown in detail. The start phase space of the electron bunch at the gun exit has been approximated. The transverse focusing system consists of very short quadrupoles. Further main components of the new injector are a Wien filter, a Mott polarimeter, a chopper-prebuncher system (based on devices used at the Mainz Mikrotron MAMI), and diverse beam diagnostic tools. For the approximation of the start phase space MAFIA and for the beam dynamic simulation V-Code is used.
Abstract: Beam dynamics simulations for long structures can be performed effectively if the involved beam line elements (BLE) do not interfere with each other. This condition permits to concentrate on a single BLE while switching only to the immediately following without keeping all nearby parts in memory. Such a procedure automatically avoids the necessity to build up a complete field description in advance. In addition, a time consuming search for contributing elements can be excluded.
Nevertheless, some devices require for a careful handling to meet those favorable properties. A typical example is given by a compact quadrupole triplet with individual magnetic fields reaching far into neighboring yokes. The beneficial assumption can still be maintained in a global sense if the whole physical device is treated as a single software unit with non-interfering fields to adjacent structures. A database which was assembled previously has to be extended according to the given requirements.
Abstract: In future, a polarized gun will extend the experimental possibilities of the superconducting recirculating linear electron accelerator S-DALINAC. Therefore a new injector is being designed where a new 100 keV polarized source SPIN will be addded to the present unpolarized thermionic source. A polarization degree of 80 %, a mean current of 60 μA and a 3 GHz cw structure are required. All features of the new source will be tested and measured at a seperate beam line.
The longitudinal beam dynamics of the injector are studied. The electron bunch length behind the gun is about 50 ps. The electrons have to be bunched to 5 ps for capturing the electrons to the main linac. Therefore a chopper/prebuncher system based on the devices used at MAMI (Mainz Microton) is being designed. The system consists of a harmonic chopper cavity, a collimator, a fundamental and a first harmonic prebuncher. Recent simulation results will be presented here.
Abstract: The Superconducting DArmst¨adter LINear ACcelerator (S-DALINAC) is a 130 MeV recirculating electron accelerator serving several nuclear and radiation physics experiments. For future tasks, the 250 keV thermal electron source should be completed by a 100 keV polarized electron source. Therefore a new low energy injection concept for the S-DALINAC has to be designed. The main components of the injector are a polarized electron source, an alpha magnet, a Wien filter spin-rotator and a Mott polarimeter. In this paper we report about the first simulation and design results. For our simulations we used the TS2 and TS3 modules of the MAFIA programme which are PIC codes for two and three dimensions and the CST PARTICLE STUDIO(TM).
Abstract: The nucleosynthesis of heavy proton-rich nuclei in a stellar photon bath at temperatures of the astrophysical gamma-process was investigated experimentally. The thermal photon bath in supernova explosions was generated by the superposition of bremsstrahlung spectra with different endpoint energies. Reaction rates for the destruction of several platinum isotopes by the (g,n) reaction were determined.
Abstract: A nuclear resonance fluorescence experiment using Euroball Cluster detectors is proposed with the aim to identify the long-sought two-octupole-phonon vibration in 208Pb. Based on quasiparticle-phonon model calculations and countrate estimates from previous measurements the experiment seems feasible.
Abstract: Within this thesis the investigation of low-lying dipole and electric quadrupole modes in the stable lead isotopes 204,206,207,208Pb by resonant photon scattering is presented. The experiments were carried out at the superconducting Darmstadt electron linear accelerator S-DALINAC utilizing two Euroball-Cluster detectors at an electron energy of E0 = 6.75 MeV.
In total, the decays of 14 excited states into the ground state could be detected for the case of 208Pb, four of them for the first time in a photon scattering experiment. Out of the 41 levels, whose decays were detected in the 206Pb(gamma,gamma') reaction, only 18 were known before. The 45 transitions attributed to the isotope 204Pb were not observed in preceding experiments. In 207Pb the decays of eleven states were detected which were known from previous photon scattering work.
The summed E1 excitation strength between 4 MeV and the endpoint energy amounts to B(E1) = 944(76) x10-3 e2 fm2 in 208Pb, B(E1) = 391(67) x10-3 e2 fm2 in 206Pb, and to B(E1) = 235(73) x10-3 e2 fm2 in 204Pb. The results were compared especially with predictions from the quasiparticle-phonon model (QPM). It turned out that the experimental results are well reproduced by the model calculations. Within the QPM the low-lying local E1 strength concentration is not based on a collective effect but rather on the interference between one- and two-phonon contributrions to the wave functions.
From the electric quadrupole strength distribution in 208Pb the possible evidence for the excitation of a two-octupole vibration with Jpi = 2+ is discussed. Notwithstanding the high sensitivity of the nuclear resonance fluorescence experiment presented here, no candidate for such a mode could be identified. Furthermore, from the comparison between candidates suggested by other spectroscopic investigations and the QPM predictions no consistent picture emerges, either.
The scissors mode, a low-lying orbital magnetic dipole mode discovered in Darmstadt in 1984, was studied over a wide mass range as well. A sum-rule approach was used aiming at the study of the collective properties of the scissors mode. An entirely parameter-free description was deduced which is capable of reproducing the mean excitation energy omegaM1 = 3 MeV in heavy nuclei as well as the dependence of the excitation strength on the square of the deformation parameter (the strength amounts to up to B(M1) = 3 µN2 in well-deformed nuclei). From the inverse energy-weighted sum rule it was extracted that for the description of the scissors mode only collective quantities are needed. These quantities are given by the excitation energy and the g-factor of the 2_1^+ state of the ground state rotational band and by the excitation energies of the isovector giant dipole resonance as well as the isoscalar giant quadrupole resonance.
The levels of the scissors in various deformed nuclei were combined to form a nuclear data ensemble. This ensemble was analyzed with respect to its level spacing distribution. The probability distributions extracted from this procedure are consistent with the Poissonian behaviour of uncorrelated levels. This surprising result indicates that the excitation mechanism exhibits regular dynamics which is related to the collectivity of the mode.
Abstract: In der vorliegenden Arbeit wurde der semimagische Kern 52Cr mit der Methode der Kernresonanzfluoreszenz am supraleitenden Darmstädter Elektronenlinearbeschleuniger S-DALINAC mit Bremsstrahlung einer Endpunktenergie von 7 MeV untersucht. Daszu wurde erstmals für Photonenstreuexperimente der im Rahmen des EUROBALL-Projektes entwickelte CLUSTER-Detektor eingesetzt, der sich durch hohe Ansprechwahrscheinlichkeit, gute Energieauflösung und sehr gutes Peak-zu-Untergrund-Verhältnis als hervorragend geeignetes Werkzeug für derartige Experimente erwiesen hat.
Im Rahmen dieser Arbeit konnten erstmals eine Reihe von Quadrupolanregungen und ein starker Dipolübergang identifiziert werden, der mit einer Anregungsenergie von 5544 keV nur wenige 100 keV unterhalb der Summenenergie des ersten 2+ - und 3--Zustandes liegt und daher als ein Kandidat für einen Zwei-Phonon-|2+x3-;1->-Zustand angesehen werden muÃ. Er besitzt - als 1--Zustand interpretiert - eine reduzierte Ãbergangsstärke von 2.1(4) 10^-3 e^2fm^2.
