Thick Si Crystal Research Articles

Microfocus Bremsstrahlung Source Based on a Narrow Internal Target of a Betatron

The paper addresses the research into the properties of bremsstrahlung generated by interaction of the internal electron beam of the 18 MeV betatron with narrow targets, 50 and $$8 \,\upmu \hbox {m}$$ thick Si crystals and $$13 \,\upmu \hbox {m}$$ thick Ta foil with a length of 4 mm along the electron beam, mounted in the goniometer inside the betatron chamber. The radiation beams generated in the targets were used to obtain the images of a microstructure of thin wire pairs and an object that consisted of four steel bars with $$10\, \upmu \hbox {m}$$ gaps between them. The images indicate high absorption contrast of their components due to a small horizontal size of the radiation source the width of which in the cases of Si crystals and Ta foil was 30-, 187- and 115-fold smaller than the diameter of the electron beam, respectively. The edge phase-contrast was also observed due to high spatial coherency of the radiation with the linear microfocus generated in the source based on the B-18 betatron with a narrow target inside.

Metastable Ar17+ (2 s) production by Stark-assisted resonant coherent excitation

We report on the selective production of the 2s metastable state of 390-MeV u−1 H-like Ar17+ ions by the resonant coherent excitation (RCE). The RCE to the 2s state was achieved by using Stark mixing of n = 2 states under planar channeling conditions in a 1-m thick Si crystal. The population of the 2s state was quantitatively estimated from the ionization probability in a stripper foil installed 50 mm downstream of the crystal. The 2s population was found to be 0.05 of the total number of Ar17+ ions at the stripper. The result is in good agreement with a theoretical calculation performed with the density matrix approach.

Studies of relativistic electron scattering at planar alignment in a thin Si crystal

Abstract Experiments on 255-MeV electron scattering under (220) planar channeling conditions in a Si crystal were carried out at the linac of the SAGA Light Source. The spatial and angular distributions of electrons penetrating through a 20-μm thick Si crystal at different incident angles with respect to the (220) plane were measured, and features characteristic of the planar alignment were identified. The experimental results were compared with computer simulations, and showed a reasonable agreement. A comparison with doughnut scattering at axial channeling in the same crystal was also performed. It was confirmed that the planar alignment effect is weaker than the axial alignment effect. These studies are important for understanding the basic mechanism of electron scattering and radiation processes in a crystal.

Results on the coherent interaction of high energy electrons and photons in oriented single crystals

The CERN-NA-59 experiment examined a wide range of electromagnetic processes for multi-GeV electrons and photons interacting with oriented single crystals. The various types of crystals and their orientations were used for producing photon beams and for converting and measuring their polarisation.The radiation emitted by 178GeV unpolarised electrons incident on a 1.5cm thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised photon beams.A new crystal polarimetry technique was established for measuring the linear polarisation of the photon beam. The polarimeter is based on the dependence of the coherent pair production (CPP) cross section in oriented single crystals on the direction of the photon polarisation with respect to the crystal plane. Both a 1mm thick single crystal of Germanium and a 4mm thick multi-tile set of synthetic Diamond crystals were used as analyzers of the linear polarisation.A birefringence phenomenon, the conversion of the linear polarisation of the photon beam into circular polarisation, was observed. This was achieved by letting the linearly polarised photon beam pass through a 10cm thick Silicon single crystal that acted as a “quarter wave plate” (QWP) as suggested by Cabibbo et al.

Transmitted ion energy loss distributions to detect cluster formation in silicon

The energy loss distribution of ions transmitted through a 5.7±0.2 μm thick Si crystal was measured and simulated with the Monte Carlo channeling simulation code FLUX. A general resemblance between the measured and simulated energy loss distributions was obtained after incorporation of an energy dependent energy loss in the simulation program. The energy loss calculations are used to investigate the feasibility to detect the presence of light element dopant clusters in a host crystal from the shape of the energy loss distribution, with transmission ion channeling. A curved crystal structure is used as a model for a region in the host crystal with clusters. The presence of the curvature does have a large influence on the transmitted energy distribution, which offers the possibility to determine the presence of dopant clusters in a host crystal with transmission ion channeling.

Bent-crystal monochromator for 150 keV synchrotron radiation

The 220 reflectivity curve of a 4 mm thick Si crystal is measured in asymmetric transmission (Laue) geometry. The crystal is bent to a radius varying from 24 to 138 m, and the analyzer is a nearly perfect Si(220) crystal in non-dispersive (+, −) arrangement. The full width at half-maximum of the reflectivity curves varies from 61 to 10 μrad, while that of a flat perfect crystal is 1.2 μrad, and the observed maximum reflectivity is about 60%. The calculated value is even higher, exceeding 80%. The bent crystal is used as an inverse-Cauchois monochromator, where the source of radiation is on the focusing circle. The effect of the beam divergence on the energy bandpass is eliminated, leaving only the contribution of the reflectivity curve. This can be matched to the desired resolution of the experiment by the choice of the thickness and asymmetry angle of the crystal. Compton-profile measurements from Ag and Au foils are used for a comparison of different monochromators. The bent-crystal monochromator provides 50 times more flux than the perfect Si(220) crystal and a better energy resolution than heat-treated Si(220) or mosaic Ge(220) crystals.

Rainbow effect in ion channeling.

The problem of ion channeling in very thin crystals is considered with use of a method of classical ion-molecule scattering theory complemented with the Monte Carlo method. Special attention is devoted to the rainbow effect. It is shown that this effect is a consequence of the fact that contributions of the atomic strings of the crystal to the probability of ion transmission interfere. In the calculations the projectiles were 7-MeV ${\mathrm{H}}^{+}$ ions, and they were being transmitted through the 〈110〉 channel of a apeq21500-A\r{}-thick Si crystal; the case in which the incident energy is 10 MeV and crystal thickness apeq21000 A\r{} is also analyzed. The effect of thermal vibrations of the atoms of the crystal and the effect of collisions of the ion with the electrons of the crystal are taken into account.

Trapping of negatively-charged particles GeV-energies into a channel

Abstract The work deals with the kinetic-approximation treatment of the problem of the trapping of ultrarelativistic electrons in the I-55 GeV range into the <III> axial channels of thick Si crystals at the incidence angles exceeding the Lindhard critical angle.

Directional Effects on High Energy Electrons and Positrons Showering into a Silicon Crystal

A directional effect on showering of 20 to 100 GeV electrons and positrons in a thick Si crystal has been found when the particle flight line is parallel to the ⟨111⟩ crystal axis within a few milliradians. In these conditions an average of 12 or more photons are emitted in the first tenths of radiation length of crystal. It is shown, using crystals, that it is possible to realize a compact detector to distinguish high energy electrons aligned in a well defined direction from a random incoming electron.

Channeling at the crystal-crystal interface: Al on GaAs (001)

Abstract The channeling behaviour of 1.8 MeV He+ ions scattered from the interface region of an epitaxially grown 2300 A, Al single crystal film on a GaAs (001) oriented substrate was examined. The high channeled scattering yield from the GaAs substrate was caused by disorder at the crystal-crystal interface. The type of channeling behaviour in a substrate for the case of an ideal crystalline overlayer was experimentally modeled by scattering an ion beam that had channeled through a 5000 A thick Si crystal from a clean GaAs (001) surface.

Textured silicon: A selective absorber for solar thermal conversion

We have used reactive sputter etching to texture the surface of Si wafers. The texturing was in the form of pillars whose diameters and spacing were small compared with the useful solar wavelengths and whose heights were comparable with or larger than these wavelengths. The normal and hemispherical reflectances of textured wafers were measured. The solar absorptance was found to be 0.99 for wavelengths below 1.0 μm. Because of the sharp drop in absorptance for photon energies less than the energy gap, the overall solar absorptance was about 0.85. The calculated thermal emittance was about 0.25 and was primarily due to multiphoton absorption processes normally observed in thick Si crystals. Much smaller values of thermal emittance would be obtained from thin textured films.