Bent Germanium Crystals Research Articles

Investigation on radiation generated by sub-GeV electrons in ultrashort silicon and germanium bent crystals

We report on the measurements of the spectra of gamma radiation generated by 855 MeV electrons in bent silicon and germanium crystals at MAMI (MAinzer MIkrotron). The crystals were 15 upmu text {m} thick along the beam direction to ensure high deflection efficiency. Their (111) crystalline planes were bent by means of a piezo-actuated mechanical holder, which allowed to remotely change the crystal curvature. In such a way it was possible to investigate the radiation emitted under planar channeling and volume reflection as a function of the curvature of the crystalline planes. We showed that, using volume reflection, intense gamma radiation can be produced – with intensity comparable to that obtained in channeling but with higher angular acceptance. We studied the trade-off between radiation intensity and angular acceptance at different values of the crystal curvature. The measurements of radiation spectra have been carried out for the first time in bent germanium crystals. In particular, the intensity of radiation in the germanium crystal is higher than in the silicon one due to the higher atomic number, which is important for the development of the X-ray and gamma radiation sources based on higher-Z deformed crystals, such as crystalline undulators.

High-efficiency focusing of hard X-rays exploiting the quasi-mosaic effect in a bent germanium crystal

A germanium crystal was bent through a grid of superficial grooves, manufactured on the sample surface. The resulting diffraction planes were bent thanks to quasi-mosaicity, which is an effect of mechanical anisotropy in crystals. High integrated diffraction efficiency was achieved in symmetric Laue geometry with a monochromatic X-ray beam set at 150 and 300 keV. It is demonstrated that the sample is capable of efficiently focusing X-rays. Such crystals can be used as optical components to focalize X- and γ-rays in a high-resolution Laue lens.

Steering of an ultrarelativistic proton beam by a bent germanium crystal

Curved crystals, thanks to the electrostatic potential generated by the coherent atomic structure, may deflect ultrarelativistic charged particles by means of channeling and volume reflection effects. Most of the experimental knowledge about these phenomena was gathered with Si crystals, though the performance could be improved by using materials with a larger atomic number. In this letter, we investigate planar and axial channeling and volume reflection in a high quality Ge short strip crystal. All the effects are demonstrated to occur in agreement with theoretical expectations, which take into account the stronger confinement potential for an ideal Ge crystal.

Differential energy measurement between He- and Li-like uranium intra-shell transitions

We present the first clear identification and highly accurate measurement of the intra-shell transition 1s2p 3P2→1s2s 3S1 of He-like uranium performed via x-ray spectroscopy. The present experiment was conducted at the gas-jet target of the ESR storage ring in GSI (Darmstadt, Germany), where a Bragg spectrometer, with a bent germanium crystal, and a Ge(i) detector were mounted. Using the ESR deceleration capabilities, we performed a differential measurement between the 1s2p 3P2→1s2s 3S1 He-like U transition energy, at 4510 eV, and the 1s22p 2P3/2→1s22s 2S1/2 Li-like U transition energy, at 4460 eV. By a proper choice of the ion velocities, the x-ray energies from the He- and Li-like ions could be measured, in the laboratory frame, at the same photon energy. This allowed for a drastic reduction of experimental systematic uncertainties, principally due to the Doppler effect, and for a comparison with theory without the uncertainties arising from one-photon quantum electrodynamics predictions and nuclear size corrections.

Simulation of the deflection of 200- and 450-GeV protons by a bent germanium crystal

It is shown that the result obtained by numerically solving the kinetic Fokker-Planck equation in the transverse-energy space on the basis of computer simulations of channeled-particle trajectories describes well the results of an experiment that studied the deflection of 200- and 450-GeV protons by a bent germanium crystal.

Deflection of 200 GeV/c and 450 GeV/c positively charged particles in a bent germanium crystal

Experimental results on high-energy beam deflection by means of a bent germanium crystal are presented. Record deflection efficiencies of 60% have been found for 450 GeV/c protons at small angles. The results obtained with 200 and 450 GeV/c positively charged particles are well described by a classical model, giving confidence in predictions for other crystals and different beam momenta.

The Bragg Crystal Spectrometer for SOLAR-A

The Bragg Crystal Spectrometer (BCS) is one of the instruments which makes up the scientific payload of the SOLAR-A mission. The spectrometer employs four bent germanium crystals, views the whole Sun and observes the resonance line complexes of H-like FexxvI and He-like Fexxv, Caxix, and Sxv in four narrow wavelength ranges with a resolving power (λ/ △λ) of between 3000 and 6000. The spectrometer has approaching ten times better sensitivity than that of previous instruments thus permitting a time resolution of better than 1 s to be achieved. The principal aim is the measurement of the properties of the 10 to 50 million K plasma created in solar flares with special emphasis on the heating and dynamics of the plasma during the impulsive phase. This paper summarizes the scientific objectives of the BCS and describes the design, characteristics, and performance of the spectrometers.

Overcoming of the [formula omitted] reflectivity limit in Laue neutron diffraction

Experimental evidence for the overcoming of the 50% peak reflectivity limit in nuetron diffraction in the Laue geometry is presented. Peak reflectivity up to 75%, in spite of the neutron absorption, has been obtained with a bent germanium crystal. This effect has been explained by a simple model based on the dynamical theory of neutron diffraction and appears as a consequence of the elimination, due to a proper curvature, of the Pendellösung effect.

Esaki-Diodes with Bending-Introduced Dislocations

Characteristics of Esaki-diodes made on plastically bent germanium crystals have been compared with those made on as-grown crystals. Etch pits density on (111) surface of the bent crystals gives the dislocation density of the order of 107/cm2. Two sets of alloyed Esaki-diodes made on (111) faces of bent crystals and as-grown crystals indicate no significant difference between them so far as current ratio Ip/Iv and peak tunneling current density are concerned.

A germanium bent-crystal monochromator for nuclear spectroscopy

A precision bent crystal gamma-ray monochromator with stationary source has been built. The instrument consists of three physically independent units: The line source (a radioactive source or the anode of an X-ray tube), the 2 meter-radius bent diffraction-crystal with its pivot and sine-motion mechanism, and the heavy-duty curved track framework which supports the collimator, detector and shielding. The motions of the crystal-pivot unit and of the detector-carriage unit are linked together in such a way that the reflection condition is satisfied. Results are presented showing that the precision of the monochromator for measurement of γ-ray wavelenghts is 0.003 x-units. The line width at half maximum, Δλ, observed when the (800) planes of a bent germanium crystal are used is 0.08 x-units. The resolution which has been attained with the germanium crystal is illustrated by a measurement of the 244.264 keV, 246.056 keV, and the previously unobserved 245.237 keV gamma line in the decay of W 183. Finally, experimental results obtained using the monochromator to observe nuclear resonance excitation in F 19 and Mn 55 are described.

Dislocations in Plastically Bent Germanium Crystals

Densities and distributions of dislocations in plastically bent germanium crystals before and after annealing were studied. In the bent and annealed crystals, the theoretical relationship between radius of curvature and density of dislocations \(\left( {\rho = \frac{1}{{r\overrightarrow b }}} \right)\) is confirmed. Before annealing, however, more dislocations are present than required, and these are distributed with a minimum at the neutral axis and maxima at the top and bottom surfaces. On annealing, three significant changes occur in the bent bars: 1) the average dislocation density is reduced, presumably by the annihilation of opposite signs; 2) dislocations migrate from the high density outside regions toward the low density neutral axis, thus producing the equilibrium distribution of dislocations; and 3) a polygonized structure is formed by movement of the dislocations into walls normal to the slip plane.