Superconducting Darmstadt Electron Linear Accelerator Research Articles

Regenerative multibunch beam breakup instabilities and countermeasures for a high-intensity electron accelerator and the superconducting Darmstadt linear electron accelerator

Regenerative multibunch beam breakup instabilities and countermeasures for a high-intensity electron accelerator and the superconducting Darmstadt linear electron accelerator

New data acquisition system of the Lintott magnetic spectrometer at the S-DALINAC

A new data acquisition system for the high-resolution magnetic spectrometer Lintott at the superconducting Darmstadt electron linear accelerator S-DALINAC was developed. It allows inclusive and coincidence electron scattering experiments with event rates up to 10 kHz.

First operation of the superconducting Darmstadt linear electron accelerator as an energy recovery linac

The superconducting Darmstadt linear electron accelerator (S-DALINAC) has been operated as an energy recovery linac (ERL) for the first time. The S-DALINAC is a recirculating superconducting radio-frequency (SRF) accelerator and had been upgraded with an additional recirculation beamline. It features a path length adjustment system that provides a freedom of choice of 360\ifmmode^\circ\else\textdegree\fi{} for the rf phase difference between the electron bunches recirculated through the new beamline and the phase of the accelerating ${\mathrm{TM}}_{010}$ mode of the oscillating electromagnetic field in the SRF cavities of the accelerator. A choice of around 180\ifmmode^\circ\else\textdegree\fi{} for this phase difference results in a deceleration of the recirculated beam and a corresponding transfer of the particles' kinetic energy back to the energy of the electromagnetic field in the cavities. The main components relevant for ERL operation are described and data of the first operation as an SRF-ERL are presented.

Shell evolution and E2 collectivity: new spectroscopic information

We report here on the first evidence for shape coexistence caused by Type II Shell Evolution which is firmly based on the measurement of absolute E2 transition rates. The data have been obtained in high-resolution inelastic electron scattering spectroscopy of the nucleus 96 Zr at the Superconducting Darmstadt Linear electron Accelerator (S-DALINAC). The data are presented and discussed. The neutron sub-shell closure at neutron number N = 56 plays a crucial role for the occurence of shape coexistence in 96,98Zr. We have studied the structure of exotic neutron-rich isotopes near the N = 56 neutron sub-shell closure using γ-ray spectroscopy and radioactive beams. We also present and discuss spectroscopic data on 84,86,88Ge taken recently at RIBF, RIKEN. The new data suggest the hitherto unknown existence of a region of pronounced nuclear triaxiality in neutron-rich Ge isotopes.

Magnetic dipole excitations ofCr50

The low-lying $M1$-strength of the open-shell nucleus $^{50}$Cr has been studied with the method of nuclear resonance fluorescence up to 9.7 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC and Compton backscattered photons at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility between 6 and 9.7 MeV of the initial photon energy. Fifteen $1^{+}$ states have been observed between 3.6 and 9.7 MeV. Following our analysis, the lowest $1^{+}$ state at 3.6 MeV can be considered as an isovector orbital mode with some spin admixture. The obtained results generally match the estimations and trends typical for the scissors-like mode. Detailed calculations within the Skyrme Quasiparticle Random-Phase-Approximation method and the Large-Scale Shell Model justify our conclusions. The calculated distributions of the orbital current for the lowest $1^{+}$-state suggest the schematic view of Lipparini and Stringari (isovector rotation-like oscillations inside the rigid surface) rather than the scissors-like picture of Lo Iudice and Palumbo. The spin M1 resonance is shown to be mainly generated by spin-flip transitions between the orbitals of the $fp$-shell.

Direct determination of ground-state transition widths of low-lying dipole states in 140Ce with the self-absorption technique

The technique of self absorption has been applied for the first time to study the decay pattern of low-lying dipole states of 140Ce. In particular, ground-state transition widths Γ0 and branching ratios Γ0Γ to the ground state have been investigated in the energy domain of the pygmy dipole resonance. Relative self-absorption measurements allow for a model-independent determination of Γ0. Without the need to perform a full spectroscopy of all decay channels, also the branching ratio to the ground state can be determined. The experiment on 140Ce was conducted at the bremsstrahlung facility of the superconducting Darmstadt electron linear accelerator S-DALINAC. In total, the self-absorption and, thus, Γ0 were determined for 104 excited states of 140Ce. The obtained results are presented and discussed with respect to simulations of γ cascades using the DICEBOX code.

Low-energy dipole strength inSn112,120

The $^{112,120}\mathrm{Sn}(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reactions below the neutron separation energies have been studied at the superconducting Darmstadt electron linear accelerator S-DALINAC for different endpoint energies of the incident bremsstrahlung spectrum. Dipole strength distributions are extracted for $^{112}\mathrm{Sn}$ up to 9.5 MeV and for $^{120}\mathrm{Sn}$ up to 9.1 MeV. A concentration of dipole excitations is observed between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays to excited states is estimated in a statistical model. A fluctuation analysis is applied to the photon scattering spectra to extract the amount of the unresolved strength hidden in the background due to fragmentation. The strength distributions are discussed within different model approaches such as the quasiparticle-phonon model and the relativistic time blocking approximation, allowing for an inclusion of complex configurations beyond the initial particle-hole states. While a satisfactory description of the fragmentation can be achieved for sufficiently large model spaces, the predicted centroids and total electric dipole strengths for stable tin isotopes strongly depend on the assumptions about the underlying mean field.

Low-lying dipole strength in theN=28shell-closure nucleus52Cr

The low-lying dipole strength of the $N=28$ closed-shell nucleus ${}^{52}$Cr was studied with nuclear resonance fluorescence up to 9.9 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC. Twenty-eight spin-1 states were observed between 5.0 and 9.5 MeV excitation energy, 14 of which for the first time. Both electric dipole excitations ($E1$, around 8 MeV) and magnetic dipole excitations ($M1$, around 9 MeV) were detected. Microscopic calculations within the quasiparticle-phonon nuclear model were performed and show good agreement with experimental results. The structure of $E1$ and $M1$ excitations, respectively, is discussed.

Low-lying dipole strength of the open-shell nucleus94Mo

The low-lying dipole strength of the open-shell nucleus ${}^{94}$Mo was studied via the nuclear resonance fluorescence technique up to 8.7 MeV excitation energy at the bremsstrahlung facility at the Superconducting Darmstadt Electron Linear Accelerator (S-DALINAC), and with Compton backscattered photons at the High Intensity $\ensuremath{\gamma}$-ray Source (HI$\ensuremath{\gamma}$S) facility. In total, 83 excited states were identified. Exploiting polarized quasi-monoenergetic photons at HI$\ensuremath{\gamma}$S, parity quantum numbers were assigned to 41 states excited by dipole transitions. The electric dipole-strength distribution was determined up to 8.7 MeV and compared to microscopic calculations within the quasiparticle phonon model. Calculations and experimental data are in good agreement for the fragmentation, as well as for the integrated strength. The average decay pattern of the excited states was investigated exploiting the HI$\ensuremath{\gamma}$S measurements at five energy settings. Mean branching ratios to the ground state and first excited ${2}_{1}^{+}$ state were extracted from the measurements with quasi-monoenergetic photons and compared to $\ensuremath{\gamma}$-cascade simulations within the statistical model. The experimentally deduced mean branching ratios exhibit a resonance-like maximum at 6.4 MeV which cannot be reproduced within the statistical model. This indicates a nonstatistical structure in the energy range between 5.5 and 7.5 MeV.

Digital base-band rf control system for the superconducting Darmstadt electron linear accelerator

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 $(\ensuremath{\Delta}A/A{)}_{\mathrm{rms}}=7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and a phase stabilization of $(\ensuremath{\Delta}\ensuremath{\phi}{)}_{\mathrm{rms}}=0.8\ifmmode^\circ\else\textdegree\fi{}$ for superconducting cavities.

Fragment properties from fission of actinide nuclei induced by 6–10 MeV bremsstrahlungI

Abstract Experiments to investigate the photon-induced fission of actinide nuclei at excitation energies in the vicinity of the fission barrier are carried out at the superconducting Darmstadt linear electron accelerator S-DALINAC. A twin–Frisch-grid ionization chamber is used to deduce mass, total kinetic energy, and angular distributions of the fission fragments. First experiments on 238 U and 234 U have shown that the experimental setup provides excellent conditions for investigating low-energy bremsstrahlung induced fission. Further experiments on 234 U and 232 Th are currently in progress. In this contribution results from the first experiment on fission fragment mass and total kinetic energy distributions from 234,238 U are presented along with preliminary data from an on-going investigation of angular distributions from 234 U(γ, f)

Reactions with polarized electrons and photons at low momentum transfers at the superconducting Darmstadt electron linear accelerator S–DALINAC

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 and reactions.

Status and recent developments at the polarized-electron injector of the superconducting Darmstadt electron linear accelerator S-DALINAC

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.

Fragment characteristics from fission of 238U and 234U induced by 6.5–9.0 MeV bremsstrahlung

Fission of 238U and 234U induced by bremsstrahlung of 6.5–9.0 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 U238(γ,f) are in agreement with results from the literature. In addition fission-fragment mass and energy distributions from U234(γ,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.

3 GHz digital rf control at the superconducting Darmstadt electron linear accelerator: First results from the baseband approach and extensions for other frequencies

The low level rf system for the superconducting Darmstadt electron linear accelerator (S-DALINAC) developed 20 years ago and operating since converts the 3 GHz signals from the cavities down to the baseband and not to an intermediate frequency. While designing the new, digital rf control system this concept was kept: the rf module does the $I/Q$ and amplitude modulation/demodulation while the low frequency board, housing an field programmable gate array analyzes and processes the signals. Recently, the flexibility of this concept was realized: By replacing the modulator/demodulators on the rf module, cavities operating at frequencies other than the one of the S-DALINAC can be controlled with only minor modifications: A 6 GHz version, needed for a harmonic bunching system at the S-DALINAC and a 324 MHz solution to be used on a room temperature cavity at GSI, are currently under design. This paper reviews the concept of the digital low level rf control loops in detail and reports on the results gained during first operation with a superconducting cavity.

Particle emission angle determination in Frisch grid ionization chambers by electron drift-time measurements

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 (γ,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.

New experimental method for investigation of the nucleon polarizabilities

At the continuous wave (cw) Superconducting Darmstadt Electron Linear Accelerator S-DALINAC, a new method has been developed for the determination of the electric ( α ¯ ) and magnetic ( β ¯ ) polarizabilities of the proton and the deuteron. For that purpose the energy and angular dependence of the differential cross-section for elastic γ p and γ d scattering of bremsstrahlung photons in the energy range between 20 and 100 MeV is measured by detecting the recoiling proton (deuteron) in coincidence with the scattered bremsstrahlung photon. α ¯ and β ¯ are then found by means of a best fit to a theoretical description of the scattering cross-section with these quantities as open parameters. The experimental setup consists of a bremsstrahlung photon facility, two specially designed high pressure hydrogen (deuterium) ionization chambers which serve as targets and detectors of the recoil proton (deuteron), NaI gamma spectrometers and several additional detectors for beam diagnostics and normalization. The whole setup was tested using bremsstrahlung photon beams with endpoint energies of 60 and 79.3 MeV. The results of the test experiments show that future high-statistics measurements are feasible.

The low-energy photon tagger NEPTUN

A new photon tagging spectrometer was built at the superconducting Darmstadt electron linear accelerator (S-DALINAC). The system is designed for tagging photons in an energy range from 6 to 20 MeV with the emphasis on best possible energy resolution and intensity. The absolute energy resolution of photons at 10 MeV is expected to be about 20 keV. With scintillating fibres as focal-plane detectors a maximum rate of tagged photons of 10 4 keV -1s -1 will be achieved. Detailed design studies including Monte Carlo simulations are presented, as well as results for the measured tagged photon energy profile of the system realized so far. This photon-tagging facility will allow to determine the photon absorption cross-sections as a function of excitation energy and to study the decay patterns of nuclear photo-excitations in great detail.

Investigation of photoneutron reactions onOs192andIr191,193at energies of relevance for the astrophysicalpprocess

We have investigated the photoneutron reactions on the isotopes {sup 192}Os and {sup 191,193}Ir for astrophysically relevant photon energies just above the neutron separation energy. The experiments were performed using the photoactivation technique at the superconducting Darmstadt linear electron accelerator (S-DALINAC). The measurements extend the existing experimental database on photoneutron reactions in this mass region and serve as an important test for the state-of-the-art statistical model calculations commonly used for the prediction of stellar reaction rates.

Investigation of photoneutron reactions close to and above the neutron emission threshold in the rare earth region

We have investigated the photoneutron cross section of the isotopes {sup 148,150}Nd, {sup 154}Sm, and {sup 154,160}Gd close to the neutron emission threshold in photoactivation experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC. Naturally composed targets were activated with a high-intensity bremsstrahlung beam at various energies and the reaction yields have been determined by measuring the activity of the produced radioactive isotopes with HPGe detectors. The results are compared to two different statistical model calculations.