Speaker
Description
The construction of 4th generation synchrotron sources will make it possible to obtain a fully coherent high-energy x-ray beam with extremely low emittance and high brightness. The use of effective refractive optics already at the radiation output (front-end) is necessary to realize all the possibilities and fully reveal the potential of new diffraction-limited sources.
X-ray refractive lenses (hereinafter CRL) [1] satisfy the above requirements. Monocrystalline diamond is considered to be an ideal material for CRL, mainly because of its stability in flow and high reflectivity in the hard X-ray range. Being universal for a wide range of applications, diamond СRL, nevertheless, have a feature for application in spectroscopic experiments, which was noted in a recently published article [2], which reported on the observation of intensity modulation at certain energies in the transmission spectrum.
This issue is well known in X-ray spectroscopy and is called “diffraction loss” or “glitch” [3]. In paper [2, 4] X-rays were propagating through lenses, while the transmitted intensity was measured at different energies. Use of compound refractive lenses, that were perfectly aligned by stacking in a single plate, gave us strong diffraction losses, reducing the outgoing signal by maximal value of 40%. The magnitude of the effect was then minimized down to ~ 10% by use of CRLs compiled from individual lenses with different crystallographic orientation. At the same time, X-ray glitches did not affect any focal spot’s size or shape while only arousing the darkening of the focal spot at exact energies of X-ray glitches [4]. Despite X-ray glitches, monocrystalline X-ray optical materials are preferable than polycrystalline ones. While single crystals only uniformly reduce the outgoing intensity (i.e., gain factor of the lens), polycrystalline materials introduce speckles and distortions to the transmitted wave front. Such negative influence of the inner material limits the resolution and optical efficiency of X-ray lenses.
In [5-6], it was found that the spatial position of the incident X-ray beam (relative to the lens aperture) most strongly affects the intensity and position of the glitches. The value of the glitch intensity strongly depends on the thickness of the material of the test sample, which is associated with the effect of attenuation (extinction) i.e. as the thickness of the material increases, the fraction of incident radiation diffracts, which causes a decrease in the intensity passing through the lens. Glitches become more pronounced with increasing thickness of the material through which the x-rays pass.
Although glitches are always present in the transmission spectrum of single-crystal materials, based on the data obtained above, attempts were made to develop approaches to minimize or, ideally, get rid of glitches in the spectrum of single-crystal optical elements.
An important result of the proposed work is a theoretical model, which was confirmed by the experimental results and which allows predicting the position and strength (intensity) of glitches if the crystallographic orientation relative to the x-ray beam is known with sufficient accuracy. Therefore, performing calculations for a precisely positioned single-crystal lens with a known crystallographic orientation, it is possible to choose specific energy intervals where the effect of glitches will have a minimal effect.
The results and assumptions made it possible to start developing a method of getting rid of glitches at each energy using a small lens flap, which is calculated on the basis of a program that can determine the lens orientation solely from the spectrum of glitches and on the basis of an analytical approach that allows you to refine the orientation of the crystal and, probably , will allow the use of crystalline lenses for experiments in which it is necessary to change the energy of the incident beam.
How to avoid glitches (practical guidance). First current orientation and cell parameter of the lens has to be determined. For this energy of several glitches (at least 3) have to be measured with high precision. Then the found energies can be fitted using proposed approach (by tilting possible K0 directions and comparing the resulting spectrum to the measured one). This will allow to assign hkl indexes to the observed glitches. Next step is to calculate analytically the exact direction of the incident beam and cell parameter
This will allow to predict with high precision all glitches for current orientation. Next multiple glitches spectra for different small tilts can be calculated, so for each energy a tilt with no glitches at that energy can be selected. During the energy scan the lens has to be tilt into precomputed orientations.
The glitches are rather sharp, so the tilt required to shift a glitch is small. Therefore the efficiency of the lens shouldn’t suffer. The method might require small increase in the lenses thickness to have possibility to tilt the lenses.
Thus, by performing the sequence of actions indicated in the developed approach, the negative impact of glitches can be avoided.
This research was supported by the Russian Science Foundation (Project No. 19-72-30009).
References:
[1] Snigirev A. V. Kohn, I. Snigireva and B. Lengeler, A compound refractive lens for focusing high-energy X-rays //Nature. – 1996. – Т. 384. – №. 6604. – С. 49.
[2] M. Polikarpov, H. Emerich, N. Klimova. I. Snigireva and A. Snigirev Diffraction losses in monocrystalline X-ray refractive lenses // Proceedings of SPIE – 2017. – Vol.10235, – P. 102350H-2.
[3] G. Turiyanskii, S. S. Gizha, V. M. Senkov, I. V. Pirshin, Y. M. Stanishevskii, Band reject filtration of the excitation spectrum at energy dispersive X-ray spectroscopy of weak signals // JETP Lett. – 2016, – Vol. 104, – P. 417.
[4] M. Polikarpov, H. Emerich, N. Klimova, I. Snigireva, V. Savin and A. Snigirev Spectral X-ray glitches in monocrystalline diamond refractive lenses // Physica Status Solidi B – 2018. – Vol.255 – P. 1700229.
[5] Q. Zhang, M. Polikarpov, N. Klimova, H. B. Larsen, R. Mathiesen, Hermann Emerich, G. Thorkildsen, I. Snigireva and A. Snigirev, Investigation of glitches induced by single-crystal diamond compound refractive lenses based on crystal orientation//AIP Conference Proceedings. – 2019. - Vol. 2054. - P. 060007
[6] Q. Zhang, M. Polikarpov, N. Klimova, H. B. Larsen, R. Mathiesen, H.Emerich, G. Thorkildsen, I. Snigireva and A. Snigirev, Investigation of ‘glitches’ in the energy spectrum induced by single-crystal diamond compound X-ray refractive lenses // Journal of Synchrotron Radiation. - 2019. – Vol. 26(1). – P. 109-118.
