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3-04 Transient Structures and Imaging with X-Rays
Project coordinator(s): H. Stiel, M. Wörner
Subproject: Instrumentation, generation and application of x-rays from laser-based sources

H. Stiel, H. Legall

Hard x-ray spectroscopy

Funding: This project was founded by the Land Berlin (ProFit) and EFRE.

Collaborations: Optigraph GmbH, TU-Berlin, IAP, IfG, HZB-BESSY and PTB Berlin

 

Investigations of the spectral properties of Highly Oriented Pyrolytic Graphite (HOPG)
Von Hamos specrometer based on thin films of Pyrolytic Graphite

 

 

Investigations of the spectral properties of Highly Oriented Pyrolytic Graphite (HOPG)

For x-ray spectroscopy in the keV range using ultrashort fs-pulses emitting laser plasma sources or microfocus x-ray tubes efficient collecting x-ray optics with high integral reflectivity and high energy resolution are required. For this special purpose the diffraction properties of thin Highly Oriented Pyrolytic Graphite (HOPG) layers were studied.

Thin crystals of HOPG are of particular interest for the use as dispersive x-ray optics because their unique structure enables them to be highly efficient in x-ray diffraction in an energy range between 2 keV up to several 10 keV. HOPG is a mosaic crystal, which consists of a large number of small crystallites (Fig.1). The angular distribution of the crystallites, with plane orientations off the normal axis to the surface, is called mosaic spread. Mosaicity makes it possible that even for a fixed angle of incidence to the crystal surface, an energetic distribution of photons can be reflected, because each photon of this energetic distribution can find a crystallite plane at the right Bragg angle. On the other side, if the radiation is monochromatic large accepting angle can be obtained. Therefore the mosaicity is responsible for the dramatic increase of integral reflectivity for mosaic crystals in comparison to perfect crystals. The mosaicity also gives rise to mosaic focusing (parafocusing), which further enhances the intensity in the image plane. In addition thin crystals of HOPG give the opportunity to realize crystal optics with arbitrary geometry by mounting them adhesive on a polished mould of any shape. This enables the design of crystal optics with high collecting efficiency.

 

 

Fig. 1: Experimental setup.

For investigation of the spectral properties a micro-focus x-ray tube (IfG) with a source size of 50 µm was used and for recording the reflected radiation a 16-bit deep depletion CCD camera (Roper Scientific model PI-LCX 1300).

Fig.2: The CCD-images show the Cu Ka emission of a micro focus x-ray tube after reflection by HOPG crystals in different distances and reflection orders.

 

 

The spectral resolution of HOPG crystal foils were investigated in two reflection orders of (002) and (004). Images of the reflected Cu Ka radiation are shown in Fig.2 for different distances. As can be seen in Fig. 2 the smearing of the Ka lines in the images remaines nearly constant whereas the seperation of the lines increases. Consequently the energy resolution increases with distance. Due to increasing dispersion with distance constant contributions to a smearing in the image plane, as e.g. source size and penetartion depth can be neglected and the energy resolution is then limited by the intrinsic broadening (darwin width for perfect crystals) of the small crystallites in the mosaic crystal. For the largest measured distance source-crystal of 310 mm in (002)-reflection an energy resolution of E/deltaE=1800 was found at 8keV and in (004)-reflection a spectral resolution of E/deltaE=2900. By rocking curve measurements the integral reflectivity was determined to be 0.7e-3 rad in (002)-reflection, which is about 10 times higher than that of Ge(111), while in (004)-reflection an integral reflectivity of 0.08e-3 rad was estimated from the counts on the CCD.

Further enhancement of energy resolution can be obtained if the intrinsic reflection broadening is reduced. This is the case for a new kind of pyrolytic graphite crystal provided by Optigraph GmbH which is called Highly Annealed Pyrolytic Graphite (HAPG). In a comparison HAPG shows clearly better results in spectral resolution. By rocking curve measurements an intrinsic width of 27arcsec was obtained for HAPG crystals. On the other side in the same manner as the intrinsic broadening was reduced the intgral reflectivity of the HAPG crystals was reduced. Measurements performed with different crystal thickness indicate a reduction in mosaic spread for HAPG crystals. Latter is favorable for high energy resolution because nearby a reduced penetration depth an distance independent focusing error which arises due to crystallites not aligned on the Rowland circle is reduced too. Therefore with lower mosaic spread higher spectral resolution limits can be reached with HAPG. Furthermore, we found that the mosaic spread increases with crystal thickness as well as the integral reflectivity on the cost of spectral resolution (cp. Fig 4).

 

 

 

Fig.3: Cross section of the CCD-images measured for a thin HOPG crystal and for spectroscopic application optimized HAPG crystal a distance of F= 400mm..

As mentioned above thin crystals of pyrolytic graphite can be bent easily by mounting them on bent surfaces. That we have done to investigate the influence of bending on the spectral resolution. By bending crystals large collecting angles can be realized. Latter is important if the signal is emitted in a large solid angle as it is the case in laser plasma experiments. In Fig. 4 the measured spectra for a 40 µm thick HAPG are shown. It can be seen that bending seems not to affect the spectral resolution of the thin pyrolytic graphite crystals.

Fig..4: CCD-images measured for a flat HAPG crystals and and a bent HAPG film at a distance of F=400 mm and a bending radius of 150mm.

 

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Von Hamos spectrometer based on thin crystals of pyrolytic graphite

Spectrometer design

The spectrometer design can be different depending on the desired field of application. If a broad energy range should be covered and/or high collecting efficiency should be reached the von Hamos geometry can be used. In this geometry a cylindrically bent crystal focuses rays with different energy emitted from a point source into different points on the cylinder axis, while in the dispersion plane the curved crystal works like a flat crystal. The position of the focal points on the cylinder axis depends on the Bragg angle respectively on the photon energy of the radiation, which is focused. Placing a CCD linear array in the cylinder axis, with this scheme very compact spectrometer can be realized.

Fig..5:Von Hamos spectrometer.

Reported energy resolution with von Hamos spectrometer ranges from 200 to 800 for HOPG in the first order reflection. However there are some disadvantages of the von Hamos geometry. High spectral resolution in a large spectral window results in spatially extended image planes, exceeding the dimensions of commonly used electronic detector systems. Additionally it is difficult to place a detector, as e.g. commercially available cooled CCD-chips, on the axis of the cylinder. Therefore we used a modified von Hamos geometry, in which the CCD was placed perpendicular to the reflected radiation. In this setup only a single photon energy is optimal focused on the CCD. On the other side, test measurements have shown an only sligthly reduction in spectral resolution for the used distances F in our measurements. A picture of the spectrometer setup is shown in Fig. 6. As can be seen, the design of the spectrometer allows to adapt the spectrometer to an ultrafast laser plasma X-ray source.

Fig. 6: Experimental setup (for photography the shielding around the spectrometer was removed).

 

 

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H. Stiel
stiel@mbi-berlin.de
Letzte Änderung dieser Seite: 06.04.2011
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