The bremstrahlung-induced fission of U-234 and Th-232 has been studied at the superconducting Darmstadt linear accelerator (S-DALINAC) in the excitation energy region close to the fission barrier. Fission-fragment mass and total kinetic energy (TKE) distributions from U-234 were studied for the first time in this energy region. The results have been analysed in terms of fission modes, and a dominant yield of the mass-asymmetric standard-2 mode was found in both nuclei. No strong dependence of the fission-mode weights on the excitation energy of the compound nucleus was found. Correlations among mass, TKE, and angular distributions have also been investigated. A correlation in the form of an increased anisotropy for far-asymmetric masses and low TKE were found in both fissioning systems. A possible interpretation of this correlation in terms of fission modes is discussed.

The bremstrahlung induced fission of 234,238U and 232Th has been studied at the superconducting Darmstadt linear accelerator (S- DALINAC) in the excitation energy region close to the fission barrier. In this contribution results on the fission fragment mass, total kinetic energy (TKE) and angular distributions will be presented. Fission fragment mass and TKE distributions from 234U were studied for the first time in this energy region. The results have been analyzed in terms of fission modes and a dominant yield of the mass asymmetric standard-2 mode was found in all studied nuclei. No strong dependence of the fission mode weights on the excitation energy of the compound nucleus were found. Correlations between mass, TKE and angular distributions have been investigated in 234U and 232Th. A correlation in form of an increased anisotropy for far-asymmetric masses and low TKE were found in both fissioning systems. A possible interpretation of this correlation in terms of fission modes is discussed.

Recent developments for an active uranium-hexafluoride-loaded gas target as well as results on the detector gas properties are presented. The gas of choice is a mixture of argon with small amounts of UF6. This contribution presents the experimental setup and focusses on the electron drift velocity with increasing UF6 content. A time-dependent decrease in electron drift velocity is observed in our setup.

Bremsstrahlung with endpoint energies slightly above the fission barrier is produced from stopping an electron beam in a thick copper target. Mass, total kinetic energy (TKE) and angular distribution of the fission fragments were determined by means of the double kinetic energy technique, using a twin Frisch-grid ionization chamber. First results on angular distributions and their correlations with mass and TKE of the fragments as well as an analysis within the fission-mode concept of the multi-modal random neck-rupture model by Brosa et al.[2], are presented [3]. It is shown that the emission angle of the fission fragments can be extracted from an electron drift-time measurement [4] and that the target thickness influences the experimental results [5].

Detailed studies of the fission process, e.g., the search for parity non-conservation (PNC) effects, the energy dependence of fission modes or the population of fission isomers, depend on high quality data, therefore requiring high luminosities. An active gas target containing uranium may overcome the deterioration of energy and angular resolution caused by large solid target thicknesses.A single Frisch-grid ionization chamber has been built to test a mixture of standard counting gases (e.g., argon) with depleted uranium hexafluoride ((UF6)-U-238), utilizing a triple alpha source to evaluate signal quality and drift velocity. For mass fractions of up to 4 percent of U-238 the drift velocity increases with rising UF6 content, while a good signal quality and energy resolution is preserved.

A source of 100 keV spin polarized electrons has been installed at the 130 MeV superconductingDarmstadt linear accelerator S-DALINAC. Circularly polarized laser light excites a GaAs cathode, producing spin polarized electrons in bunches with pulse lengths in the region of 50 ps and smaller at a repetition frequency of 3 GHz. A Wienfilter for spin manipulation and a Mott polarimeter for polarization measurements are installed in the low-energy beam line. Polarizations up to 86% have been shown with strained superlattice GaAs cathodes. Installed wire scanners in the beam line measure beam radius and position and in conjunction with a solenoid with variable focal length a parameter set of beam sizes depending on the focal length can be obtained, allowing for an emittance calculation. The scanning unit, two perpendicular 50 m tungsten wires for x and y measurements mounted on an insulated frame, is installed at an angle of 45 in a plane perpendicular to the beam. Pneumatic as well as electric translation is used while the data read-out is done by a 24-bit ADC with variable readingspeed. Measurementsat the S-DALINAC give an indication of the beam quality of the spin polarized electron source, permit a comparison with the already installed thermionic electron source, and allow the measurement of a possible emittance growth from the Wien-filter that is to be excluded. Furthermore, the knowledge of the beam size renders a slit measurement of the beam pulse length possible.

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 238U, high luminosities are needed. Increasing target thickness reduces mass and angular resolutions. One possible solution is the utilization of an active gas target containing UF6. 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 Universität Darmstadt.After testing the chamber with pure argon as a counting gas to evaluate signal quality and to determine the drift velocity, gaseous UF6 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 UF6 content, while overall a good signal quality and energy resolution of the ionization chamber is preserved.

A source of polarized electrons has been installed at the superconducting 130 MeV Darmstadt electron linear accelerator S-DALINAC. Polarized electrons are generated by irradiating a GaAs cathode with pulsed Ti:Sapphire and diode lasers and preaccelerated to 100 keV. A Wien filter and 100 keV Mott polarimeter are used for spin manipulation and polarization measurement, and various beam diagnostic elements are installed. A chopper as well as a two-stage harmonic prebuncher section modulates the time structure of the beam. Successful operation has been demonstrated. A two-cell capture cavity complements the injector linac for acceleration of 100 keV electron beams. To measure the beam polarization downstream of the superconducting injector linac, a 5-10 MeV Mott polarimeter and a Comptontransmission polarimeter have been developed. We report on the status of the polarized electron source and foreseen experiments.

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 U-238 and U-234 have shown that the experimental setup provides excellent conditions for investigating low-energy bremsstrahlung induced fission. Further experiments on U-234 and Th-232 are currently in progress. In this contribution results from the first experiment on fission fragment mass and total kinetic energy distributions from U-234,U-238 are presented along with preliminary data from an on-going investigation of angular distributions from U-234(gamma, f).

A source of polarized electrons has recently been implemented at the superconducting Darmstadt electron linear accelerator S-DALINAC. We give an overview of the recent performance of the system. Photo-emission from a superlattice-GaAs photo-cathode is obtained from using either a DC diode laser or a short-pulse Ti:Sapphire laser system. For a robust pulsed quasi-cw operation, it is investigated whether a VECSEL system (Vertical-Cavity Surface-Emitting Laser) can be realized with a wavelength of 780 nm and a repetition rate of 3 GHz with pulse widths of a few picoseconds only. To enhance the availability and performance of the polarized source with respect to quantum efficiency, a separate atomic-hydrogen cleaning system for the cathodes is planned which will allow cathode treatment to be tested and optimized.

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 SDALINAC 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 Moller polarimeter are going on. Compton transmission polarimeters are foreseen to monitor polarization during experiments.

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.

Low energy (E-k similar to 100 keV) Mott scattering polarimetry is a widely established technique to measure the polarization of an electron beam. We analyze the feasibility of Mott scattering at energies up to 20 MeV. For further studies of the electron spin dynamics in the scattering process a correlation between the linear polarization of bremsstrahlung radiation and the electron beam polarization has been measured for the first time using a planar HPGe Compton polarimeter at the 100 keV source of polarized electrons at TU Darmstadt.

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.

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.

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 chopperprebuncher 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.