W. Lauth

Channeling and Radiation of Electrons in Silicon Single Crystals and Si1−xGex Crystalline Undulators

The phenomenon of channeling and the basic features of channeling radiation emission are introduced in a pedestrian way. Both, radiation spectra as well as dechanneling length measurements at electron beam energies between 195 and 855 MeV feature quantum state phenomena for the (110) planar potential of the silicon single crystals. Radiation from a crystalline undulator, produced at the Aarhus University (UAAR), has been investigated at the Mainz Microtron electron accelerator facility MAMI. The 4-period epitaxially grown strained layer Si1−xGex undulator had a period length λu = 9.9 μm. At a beam energy of 375 MeV a broad excess yield around the theoretically expected photon energy of 0.132 MeV has been observed. Model calculations on the basis of synchrotron-like radiation emission suggest that evidence for a weak undulator effect has been observed.

Novel digital K-edge imaging system with transition radiation from an 855-MeV electron beam

A novel K-edge imaging method has been developed at the Mainz Microtron MAMI aiming at a very efficient use of the transition radiation (TR) flux generated by the external 855-MeV electron beam in a foil stack. A fan-like quasi-monochromatic hard X-ray beam is produced from the /spl plusmn/1-mrad-wide TR cone with a highly oriented pyrolytic graphite (HOPG) crystal. The absorption of the object in front of a 30 mm/spl times/10 mm pn charge-coupled device (pn-CCD) photon detector is measured at every pixel by a broad-band energy scan around the K-absorption edge. This is accomplished by a synchronous variation of the lateral crystal position and the electron beam direction which defines also the direction of the TR cone. The system has been checked with a phantom consisting of a 2.5-/spl mu/m thick molybdenum sample embedded in a 136- or 272-/spl mu/m-thick copper bulk foil. A numerical analysis of the energy spectrum for every pixel demonstrates that data as far as /spl plusmn/0.75 keV away from the K edge of molybdenum at 20 keV still improve the signal-to-noise ratio (SNR). Prospects are discussed to investigate the human lungs with xenon as a contrast agent at the available total primary photon flux of 2/spl times/10/sup 10//(s/spl middot/0.1% bandwidth (BW)) only.

Tests and developments of the PANDA Endcap Disc DIRC

The PANDA experiment at the future Facility for Antiproton and Ion Research (FAIR) requires excellent particle identification. Two different DIRC detectors will utilize internally reflected Cherenkov light of charged particles to enable the separation of pions and kaons up to momenta of 4 GeV/c. The Endcap Disc DIRC will be placed in the forward endcap of PANDAs central spectrometer covering polar angles between 5° and 22°. Its final design is based on MCP-PMTs for the photon detection and an optical system made of fused silica. A new prototype has been investigated during a test beam at CERN in May 2015 and first results will be presented. In addition a new synthetic fused silica material by Nikon has been tested and was found to be radiation hard.

Transition radiation in the x-ray region from a low emittance 855 MeV electron beam

The interference of transition radiation coherently produced from a periodic stack of four polyimide foils of 7.2 μm thickness and a separation of 162 μm was investigated. This stack has been brought into the low emittance (3 π nm rad) electron beam of the 855 MeV Mainz Microtron MAMI. Transition radiation was observed in the energy range from 2 to 15 keV with a LN2-cooled pin photodiode. A good energy resolution of 0.8 keV and angular resolution of 0.15 mrad was achieved simultaneously allowing for the first time to quantitatively study the interference pattern. Good agreement with theoretical calculations is found. Prospects to exploit transition radiation in the x-ray region from a low emittance electron beam as a high brilliant radiation source are discussed.

INVESTIGATION OF FAR-INFRARED SMITH-PURCELL RADIATION AT THE 3.41 MEV ELECTRON INJECTOR LINAC OF THE MAINZ MICROTRON MAMI

An experiment has been set up at the injector LINAC of the Mainz Microtron MAMI to investigate far-infrared Smith-Purcell radiation in the THz gap (30 μm ≤ λ ≤ 300μm). The essential components are a superconductive magnet with a magnetic induction of 5 Tesla in which 200 mm long gratings of various periods between 1.4 mm and 14 mm are located, and a composite silicon bolometer as radiation detector. First experiments were performed in the wavelength region between 100 μm and 1 mm with a bunched 1.44 MeV electron beam.

On the Line Shape of Backward Emitted Parametric X-Radiation

Parametric X radiation, emitted in opposite direction of an electron which traverses a semi infinite single crystal on a hypothetical straight trajectory, features a Lorentzian line shape with extremely narrow width. However, small angle scattering of the electron in the Coulomb potential of the crystal atoms results in a stochastic change of the electron direction. The latter leads to a line broadening which can be understood essentially as a stochastic frequency modulation of the exponentially damped wave train. The line shape has been calculated analytically by well known probabilistic methods. Results are quoted for various reflections of a silicon single crystal at an electron beam energy of 855 MeV.

Resolution changes of MCP-PMTs in magnetic fields

Micro-channel plate photomultiplier tubes (MCP-PMTs) are chosen in many applications that have to cope with strong magnetic fields. The DIRC detectors of the PANDA experiment plan to employ them as they show excellent timing characteristics, radiation hardness, relatively low dark count rates and sufficient lifetime. This article mainly focuses on the performance of the position reconstruction of detected photons. Two different MCP-PMTs with segmented anode geometries have been tested in magnetic fields of different strengths. The variation of their performance has been studied. The measurements show improved position resolution and image shifts with increasing magnetic field strength.

The experimental setup of the Interaction in Crystals for Emission of RADiation collaboration at Mainzer Mikrotron: Design, commissioning, and tests

Silicon/germanium flat/bent crystals are thin devices able to efficiently deflect charged particle GeV-energy beams up to a few hundreds of μrad; moreover, high intensity photons can be efficiently produced in the so-called Multi-Volume Reflection (MVR) and Multiple Volume Reflections in One Crystal (MVROC) conditions. In the last years, the research interest in this field has moved to the dynamic studies of light negative leptons in the low energy range: the possibility to deflect negative particles and to produce high intensity γ sources via the coherent interactions with crystals in the sub-GeV energy range has been proved by the ICE-RAD (Interaction in Crystals for Emission of RADiation) Collaboration at the MAinzer MIkrotron (MAMI, Germany). This paper describes the setup used by the ICE-RAD experiment for the crystals characterization (both in terms of deflection and radiation emission properties): a high precision goniometer is used to align the crystals with the incoming beam, while a silicon based profilometer and an inorganic scintillator reconstruct, respectively, the particle position and the photon spectra after the samples. The crystals manufacturing process and their characterization, the silicon profilometer commissioning at the CERN PS T9 beamline, and the commissioning of the whole setup installed at MAMI are presented.

A Disc-DIRC Cherenkov detector with high resolution micro channel plate photomultiplier tubes

The upcoming PANDA Experiment at FAIR in Germany will be equipped with a novel Cherenkov detector type for high-energy particle identification. This very compact Disc-DIRC detector uses a large disc-shaped fused silica plate of 2 cm thickness as its Cherenkov radiator. The internally reflected Cherenkov light is transported to the rim of the disc where it is focused by quartz light guides onto microchannel plate photomultiplier tubes (MCP-PMTs) with high spatial resolution (pitch 0.5 mm) and high time resolution (σ ≈ 100 ps). The device has an active area of about 3 m2 and will be able to identify pions and kaons with a separation power of more than 3σ in the momentum range up to 4 GeV/c. It has 32400 individual pixels and each can handle a mean photon rate of up to about 100 kHz. The presented design deals with numerous challenges that come with the very hostile environment in which the detector has to function properly, caused by the presence of high magnetic fields of up to 2 Tesla, high levels of radiation, high particle and background rates and a tight spatial volume. First test measurements have shown the performance of the design.

A novel type of x-ray interferometer

Novel types of interferometers have been developed at the Mainz Microtron MAMI with which the complex index of refraction of thin self-supporting foils can be measured. For the vacuum ultraviolet and soft x-ray region the interferometer consists of two collinear undulators, between which a foil can be placed, and a grating spectrometer. For the hard x-ray region, up to an energy of about 40 keV, it consists of two foils in which the electron beam produces transition radiation, and a single crystal spectrometer in Bragg geometry. Taking advantage of the low emittance 855 MeV electron beam distinct intensity oscillations have been observed as a function of the distance between the undulators and foils, respectively. The complex index of refraction has been investigated at the K- and L-absorption edges of carbon and nickel.