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Exploring phase contrast imaging with a laser-based K α x-ray source up to relativistic laser intensity

Abstract : This study explores the ability of a hard K α x-ray source (17.48 keV) produced by a 10 TW class laser system operated at high temporal contrast ratio and high repetition rate for phase contrast imaging. For demonstration, a parametric study based on a known object (PET films) shows clear evidence of feasibility of phase contrast imaging over a large range of laser intensity on target (from ~10 17 W/cm 2 to 7.0 × 10 18 W/cm 2). To highlight this result, a comparison of raw phase contrast and retrieved phase images of a biological object (a wasp) is done at different laser intensities below the relativistic intensity regime and up to 1.3 × 10 19 W/cm 2. this brings out attractive imaging strategies by selecting suitable laser intensity for optimizing either high spatial resolution and high quality of image or short acquisition time. The interest of developing new x-ray sources and/or improving their performances in terms of brightness, stability , and compactness is still growing since decades. This is strongly motivated by applications of x-ray sources for imaging and related applied developments to biology, medicine and material science. In particular, the advent of synchrotron radiation sources in the seventies as well as the development of optical components for x-rays, definitively allowed to transfer the phase contrast imaging (PCI) techniques from the visible spectral range to the x-ray one. Phase contrast x-ray imaging is sensitive to phase shift induced by an object placed in the x-ray path and does not rely on its absorption. Thus, it can image weakly absorbing materials, such as carbon-based materials and biological objects. In addition, it should be noted that the sensitivity of absorption contrast decreases as the photon energy (E) increases 1 as E-3 , whereas that of phase contrast methods decreases only as E-2. Therefore, phase contrast methods are more sensitive at high photon energies (E = 10 to 100 keV), compared to absorption methods. In that case, for comparable image quality, the absorbed x-ray dose is smaller than with conventional radiography. Challenges addressed by hard x-ray PCI are numerous such as the detection of complex damages in composite materials 2-4 or of the apparition of microcalcifications around a hundred micrometers of diameter at an early stage of breast cancer 5-7. X-ray sources with high spatial coherence and photon flux required for PCI 8 are a difficult technological realization. On one hand, conventional microfocus 9-11 and liquid-metal-jet 12 x-ray tubes are very compact and inexpensive sources. Even if liquid-metal-jet x-ray tubes tend to increase their photon flux, both sources are continuous excluding time-resolved studies. On the other hand, synchrotrons 13-16 offer today the highest brightness available and are suitable for imaging applications. However, they are very large infrastructures with limited access, high cost and currently not scalable to civil environment such as hospitals. Between these two alternatives, ultrafast x-ray laser plasma sources appear as good candidates for PCI at a laboratory scale. Among them, x-ray sources provided by laser plasma acceleration such as Betatron 17-23 and inverse Compton scattering 24-27 offer potential alternatives thanks to their high brightness and very small source size, making possible the acquisition of an x-ray image in single shot mode. However, they are until now based on laser driven systems with high peak power (>30 TW) 23 difficult to scale up to high repetition rate. Moreover, these sources provide collimated x-ray beams at low divergence limiting the field of view. In this context, an attractive solution is the K α x-ray table-top source driven by femtosecond laser systems 28,29. Hard K α x-ray source is generated by interaction between an intense femtosecond laser pulse (I ≥ 10 16 W/cm 2) with a high Z solid target. The spectrum is composed of a large x-ray Bremsstrahlung emission, dominated by K α line (up to ~50%) 30,31 , characteristic of the solid target material. In addition, it is spectrally tunable by changing the target material. This can give access to high energetic K α photons (>20 keV) for high Z material like silver, tantalum or tungsten, difficult to reach by other kind of x-ray sources while combining a high flux. Finally, this
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O. Utéza, A. Ferré, M. Gambari, R. Clady, A. Stolidi, et al.. Exploring phase contrast imaging with a laser-based K α x-ray source up to relativistic laser intensity. Scientific Reports, Nature Publishing Group, 2020, 10 (1), ⟨10.1038/s41598-020-63614-3⟩. ⟨hal-02982310⟩

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