H.-D. Gräf

X-ray polarimetry using the photoeffect in a CCD detector

Abstract A new method of X-ray polarimetry based on the photoeffect in a finely segmented MOS charge coupled device (CCD) has been applied to monochromatic synchrotron radiation at HASYLAB at DESY and to planar channeling radiation at the superconducting electron linac S-DALINAC at Darmstadt. For the smallest pixel dimensions available today (6.8×6.8 μm 2 ) an analyzing power of 10% is observed at an energy of 33 keV. In addition to events from photoeffect in the thin depleted front layer of the CCD also diffusion spread events from much more abundant conversions deeper inside the chip were utilized for polarization analysis.

Influence of radiation damage on BGO scintillation properties

Abstract Aboard INTEGRAL, the next medium-size γ-ray mission of the European Space Agency (ESA), a high-resolution Ge-spectrometer array with a BGO anticoincidence shield and imaging capability will be flown. The influence of the radiation damage on the photoelectron yield of the BGO scintillators due to the radiation environment in the orbit, i.e. γ-rays, electrons and protons from the radiation belts and the cosmic diffuse radiation, was investigated. Irradiation tests with doses equivalent to the orbit conditions were performed and the photoelectron yields of different BGO crystals were measured. It was found that for equal doses the reduction of the photoelectron yield varies strongly for the crystals of different manufacturers. Furthermore, electromagnetic radiation affects the photoelectron yield much stronger than particle radiation. A possible explanation is given by interpreting the effect due to γ-rays primarily as damage of the electronic structure of the BGO, whereas the particle radiation damages mainly the crystal structure.

First experimental evidence for quantum echoes in scattering systems.

A self-pulsing effect termed quantum echoes has been observed in experiments with an open superconducting and a normal conducting microwave billiard whose geometry provides soft chaos, i.e., a mixed phase space portrait with a large stable island. For such systems a periodic response to an incoming pulse has been predicted. Its period has been associated with the degree of development of a horseshoe describing the topology of the classical dynamics. The experiments confirm this picture and reveal the topological information.

The S-DALINAC facility — Operational experience from the accelerator and the experimental installations☆

Abstract The superconducting cw-electron-accelerator S-DALINAC at Darmstadt became operational in January of 1991. It has been used extensively since then to deliver beam to the various experimental installations for nuclear and radiation physics associated with the accelerator. In order to fulfill the requirements of the experiments beam from the nA range up to 40 μA and at energies between 2.5 and 84 MeV could be provided in true cw operation. Energies above 100 MeV could be achieved in accelerator test runs. A special time structure of the electron beam was used for Free Electron Laser (FEL) physics. Spontaneous emission has been observed routinely. The principle of operation of the accelerator is sketched briefly while main emphasis is on specific problems of a superconducting recirculating electron accelerator and their solutions. The operating performance of the superconducting accelerator cavities like the gradients achieved and the amplitude and phase stability of the accelerating field in the presence of microphonic perturbations are discussed in detail including the influence of radiation pressure. For the two recirculating beamlines simple devices were developed to provide variable path lengths for the electron beam in order to optimize the reinjection phase for the successive acceleration in the linac.

Resonances of a superconducting microwave cavity: A test of the Breit-Wigner formula over a large dynamic range

Abstract Resonance data spanning a dynamic range of six decades from a superconducting two-dimensional stadium microwave cavity are investigated. The data set is described by various multi-resonance formulae. A good description spanning the full dynamic range of the data with an overall accuracy of about 10% is obtained. This is presumably the first test of the Breit-Wigner formula over such a large dynamic range

Electroexcitation of isoscalar and isovector magnetic dipole transitions in 12C and isospin mixing

Abstract The excitation of the J π ;T=1 + ;0 and 1 + ;1 states at E x =12.71 and 15.11 MeV in 12 C has been studied at momentum transfers q fm −1 in inelastic electron scattering. The corresponding ground state radiation widths and transition radii for the isoscalar and isovector transition are deduced as Γ γ 0 =0.32(2) eV, R tr =2.95(9) fm and Γ γ 0 =35.9(6) eV, R tr =2.63(4) fm, respectively. For the evaluation of Γ γ 0 and R tr for the transition to the 15.11 state (e,e ′ ), data of the present work have been combined with older data from other low energy electron accelerators. The results support the strong isospin mixing of the two 1 + states, and in the frame of a two-state model a Coulomb matrix element 〈H C 〉=118(8) keV is derived. This value agrees with other experiments using electromagnetic or pionic probes but is much more accurate. Furthermore, the measured difference in the transition radii for the isoscalar and isovector transitions allows for the first time a unique determination of the sign of 〈H C 〉 .

Simulation for a New Polarized Electron Injector (Spin) for the S-DALINAC

The Superconducting DArmstädter 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 con cept 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™.

Precision test of the Breit-Wigner formula on resonances in a superconducting microwave cavity

Abstract Resonance data spanning a dynamic range of six decades from a superconducting microwave cavity in the form of a two-dimensional stadium billiard are described by a multilevel formula. The fitting procedure results in relative errors of about 0.3% in the close vicinity of strongly excited resonances. As compared to a previous experiment the technique for measuring the resonances has been improved distinctly.

The high current injection for the Darmstadt near infrared free electron laser

Abstract Presently an FEL operating in the near infrared (λ = 2.5 5μm) is under construction at the superconducting accelerator which has originally been designed for nuclear physics experiments requiring electron beams of low peak currents. The redesign of the present injection with the purpose of achieving higher peak currents for the operation of the FEL and the resulting requirements for the individual injection components will be described. Furthermore the experimental results obtained with an injection test stand consisting of the electron gun, the chopper-resonator and some electron beam diagnostics are discussed.

First observation of amplification of spontaneous emission achieved with the Darmstadt IR-FEL

Abstract Utilizing electron beams delivered by the new S-DALINAC with energies of 32.8 and 38.4 MeV, the pulse structure, charge per pulse and energy resolution were studied and found to meet or almost reach the requirements needed. Despite the low intensity of this FEL experiment, spectra of spontaneous emission could be recorded for the first and third harmonic. An accumulation of intensity up to twelve times compared to a single photon pulse could clearly be identified as at least twelve round trips within the cavity. A length region with distinct amplification of the spontaneous emission intensity could be established unequivocally.