Linear Accelerator S-DALINAC Research Articles

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.

Soft chemical polishing and surface analysis of niobium samples

The Superconducting Darmstadt Linear Accelerator S-DALINAC uses twelve Niobium Cavities with a RRR of 280-300 which are operated at 2 K. The operating frequency is 3 GHz; the design value of the accelerating gradient is 5 MV/m. To achieve the target value of 3 · 109 for the unloaded quality factor Q0, different surface preparation methods were applied and systematically tested using a vertical 2 K cryostat. A well-established technique is the so called Darmstadt Soft Chemical Polishing, which consists of an ultrasonic cleaning of the cavity with ultrapure water (UPW) followed by oxidizing the inner surface with nitric acid (65% HNO3). After rinsing with UPW again, the niobium oxide layer is removed with hydrofluoric acid (40% HF) in a separate second step. Finally, the structure is rinsed with UPW and then dried by a nitrogen flow. Until now each cavity in operation was chemically treated with a proven record of success. In order to understand and to optimize the process on the niobium surface, systematic tests with samples were performed. The samples were analyzed using material science techniques like Scanning Electron Microscopy (SEM), Secondary Ion Mass Spectrometry (SIMS) and Energy Dispersive X-ray Spectroscopy (EDX). This paper will report on the results of our research and we will give a review on our experiences with varied chemical procedures.

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.

Photon scattering off 94Zr and 96Zr

Photon-scattering experiments off the nuclei 94Zr and 96Zr were performed at the 10-MeV bremsstrahlung facility DHIPS at the super-conducting electron linear accelerator S-DALINAC at Darmstadt. The dipole excitation-strength distributions were determined in an energy range between 2 and 8 MeV, including the 1− quadrupole-octupole coupled Two-Phonon-State and parts of the Pygmy Dipole Resonance. In total 74 transitions in 96 Zr were observed and for most of the excited states the spin quantum number could be firmly assigned to J = 1. Calculations within the Quasiparticle Phonon Model show a good agreement.

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)

Feasibility Study for an Active 238UF6 Gas Target for Photo-Fission Experiments

Abstract A series of fission experiments in the actinide region has been started at the superconducting Darmstadt linear accelerator S-DALINAC. For detailed investigations on, e.g., the energy dependence of fission modes, the population of fission isomers, or even the search for parity non-conservation effects (PNC) in the photon-induced fission process of 238 U, high luminosities are needed. Increasing target thickness reduces mass and angular resolutions. One possible solution is the utilization of an active gas target containing UF 6 . In order to test UF6 as an admixture to standard counting gases (e.g. argon) and to study its properties, an ionization chamber has been built at Technische Universitat Darmstadt. After testing the chamber with pure argon as a counting gas to evaluate signal quality and to determine the drift velocity, gaseous UF 6 was filled into the chamber in steps of one mass-percent uranium for each measurement, where both signal quality and drift velocity at different admixtures have been determined. Up to two percent of uranium in the counting gas one finds that the drift velocity increases with UF 6 content, while overall a good signal quality and energy resolution of the ionization chamber is preserved.

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.

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.

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.

Systematics of the pygmy dipole resonance in stable tin isotopes from resonant photon scattering

The 112,120Sn( γ , γ ′ ) reactions were studied for different endpoint energies of the incident bremsstrahlung spectrum below the neutron separation energies at the superconducting Darmstadt electron linear accelerator S-DALINAC. Dipole transition strength distributions have been extracted for 112Sn; the analysis of 120Sn is still in progress. In the case of 112Sn a concentration of dipole excitations is observed between 5 and 8 MeV . Combining with existing data for 116,124Sn provides a systematics of the pygmy dipole resonance (PDR) in stable Sn nuclei which serves as a benchmark test of recent microscopic calculations aiming at a description of the PDR in exotic neutron-rich Sn isotopes.

Dipole and quadrupole excitations inSr88up to6.8MeV

Dipole and quadrupole excitations in the semimagic N=50 nucleus {sup 88}Sr 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 J{sup {pi}}=1{sup -} state exhibits unusual features which are presently not understood.

Nuclear resonance fluorescence experiments on 204,206,207,208Pb up to 6.75 MeV

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 (“pygmy resonance”) 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.

Polarimetry of coherent polarization radiation

Polarization properties of coherent polarization X-radiation produced by a low-emittance electron beam of 72 MeV interacting with a silicon monocrystal have been investigated experimentally at the superconducting Darmstadt linear accelerator S-DALINAC. Spatially and energetically resolved measurements of the direction and the degree of linear polarization show good agreement with expectations based on the kinematical theory of coherent polarization radiation and rule out a quasi-Čerenkov nature of the radiation.

Improved response function calculations for scintillation detectors using an extended version of the MCNP code

The analysis of (e,e′n) experiments at the Darmstadt superconducting electron linear accelerator S-DALINAC required the calculation of neutron response functions for the NE213 liquid scintillation detectors used. In an open geometry, these response functions can be obtained using the Monte Carlo codes NRESP7 and NEFF7. However, for more complex geometries, an extended version of the Monte Carlo code MCNP exists. This extended version of the MCNP code was improved upon by adding individual light-output functions for charged particles. In addition, more than one volume can be defined as a scintillator, thus allowing the simultaneous calculation of the response for multiple detector setups. With the implementation of 12C(n,n′3α) reactions, all relevant reactions for neutron energies E n<20 MeV are now taken into consideration. The results of these calculations were compared to experimental data using monoenergetic neutrons in an open geometry and a 252Cf neutron source in the complex Darmstadt setup, where in both cases excellent agreement was found.