H. Genz

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.

Lineshape, linewidth and spectral density of parametric x-radiation at low electron energy in diamond

Applying an absorber technique, the experimental shape and width of a parametric x-radiation line has been determined. The 9 keV radiation was produced by bombarding a diamond crystal of 55 μm thickness with electrons of 6.8 MeV. The variance of the spectral line distribution was found to depend on the tilt angle of the crystal and to have a magnitude of σ=51 eV. Simulations based on a Monte Carlo method exhibit that the observed variance is mainly influenced by multiple scattering of electrons passing through the crystal (≈43 eV) and the finite detector opening (≈18 eV), leaving for the intrinsic linewidth a value of the order of 1 eV. The spectral density of the line was found to be J≈10−7 photons/(electron×sr×eV).

Channeling radiation and parametric X-radiation at electron energies below 10 MeV

Abstract The intensity of channeling radiation (CR) was compared for diamond and ruby crystals and also with the intensity of parametric X-radiation (PXR) originating from the same diamond. The investigations were carried out with relativistic electrons provided by the injector of the S-DALINAC at energies between 3.0 and 9.0 MeV. Both types of radiation were observed by means of Si(Li) detectors placed at 0° and 44° with respect to the electron beam axis for CR and PXR, respectively. The highest photon flux Φ = 2 × 108 photons/(smm2) was found to result from CR of diamond crystals. Ruby crystals exhibit somewhat narrower X-ray lines but less intensity. A comparison with PXR reveals that — against predictions — its intensity is three orders of magnitude weaker than CR under the same conditions. The possibility to use the CR and PXR as intense tunable photon sources is discussed.

Low energy channeling radiation experiments in a germanium crystal

Abstract Channeling radiation from relativistic electrons has been measured in germanium at low electron energies (6.00, 9.00 and 10.18 MeV) using a 20 μm thick germanium crystal at the Darmstadt superconducting linear accelerator S-DALINAC. The results of the theoretical calculations (planar case) were compared with experimental data and are in reasonable agreement with the observed results. The channeling radiation energies scale as γ5/3 (γ is the ratio of the electron total energy and its rest mass) for the deeply bound states (states localized in the vicinity of the atomic plane) and γ2 for states localized between the channeling planes. We have also measured the intensity of the 1-0 transition along the (1xa01xa00) plane as a function of the tilt angle (angle between electron beam and plane) and found that channeling radiation occurs even when the tilt angle is larger than the critical angle, due to multiple scattering.

Test of detailed balance at isolated resonances in the reactions 27Al+p⇄24Mg+α and time reversibility

Abstract The principle of detailed balance has been tested in the reactions 27 Al ( p , α 0 ) 24 Mg ( Q = 1.600 MeV ) and 24 Mg (α, p 0 ) 27 Al ( Q = −1.600 MeV ) at bombarding energies Elabp = 1.35−1.46 MeV and Elabα = 3.38−3.52 MeV, respectively. Protons and α-particles were detected at θc.m = 177.7°. The relative strengths of two resonances at Ex = 12.901 MeV Jπ = 2+ and Ex = 12.974 MeV (Jπ = 1−) in 28Si excited in the forward and backward reaction agree within the experimental uncertainty δ = 0.0025 ± 0.0192. This experimental result is converted into a difference of phase angles for reduced widths amplitudes, Δξ = (0.3 ± 3)°, which is consistent with time reversibility.

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.

Elastic scattering of 40Ar on 40Ca at Elab = 191, 236 and 272 MeV☆

Abstract Cross sections for elastic scattering of 40 Ar on 40 Ca have been measured at energies E lab = 191, 236 and 272 MeV employing position-sensitive detectors and the method of kinematical coincidences. The experimental data are first compared with the ordinary and the generalized Fresnel models. Only the generalized Fresnel model describes the experimental data well. An optical model analysis with a Woods-Saxon potential yields an energy independent set of parameters ( V R = −21.76 MeV, r OR = 1.37 fm, a R = 0.45 fm; W 1 = −13.69 MeV, r 01 = 1.40 fm, a 1 = 0.36 fm) very similar to the one found in 40 Ca- 40 Ca scattering at corresponding energies. Values deduced for the total reaction cross sections for the three energies are in good agreement with those predicted by the generalized Fresnel model. The data are also compared with optical model calculations with the real part of the potential replaced by various microscopically determined potentials. The proximity, Fleckner-Mosel and the Krappe-Nix-Sierk potentials like the phenomenological optical model potential reproduce the measured data fairly well over several orders of magnitude.

Orbital electron capture ratios in the decay of 205Pb

The M/L and N/M electron capture ratios in the decay of 205Pb have been determined by introducing the radioactive compound tetramethyllead, 205Pb(CH3)4, into a wall-less multiwire proportional counter as part of the counting gas mixture. The results are: PMPL = 0.525 ± 0.008 and PNPM = 0.271 ± 0.010. These results are compared to the theory of Behrens and Buhring using the relativistic wave functions of Mann and Waber. The comparison, along with an estimate of the overlap-exchange correction factor, gives a value of QEC = 41.4 ± 1.1 keV. The half-life of 205Pb is deduced from the results to be (1.51 ± 0.04) × 107y.

Experimental determination of the linewidth of parametric X-ray radiation at electron energies below 10 MeV

Abstract The linewidth of parametric X-ray radiation has been determined experimentally applying an absorption technique. Using a copper foil of 27.7 μm thickness and tuning the energy of the PXR peak across the K-absorption edge of copper by tilting a 55 μm thick diamond crystal, the variance of the PXR line at 8.98 keV photon energy was found to be σ = 48 eV. The diamond crystal has been bombarded with electrons of 6.8 MeV kinetic energy delivered by the superconducting linear accelerator S-DALINAC at Darmstadt. The experimental yield of the intensity amounts to N = 0.91 × 10 −5 photons/(electron sr). From the measured variance the spectral density in the peak is deduced to be J = 0.95 × 10 −7 photons/(electron sr eV).

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.