Compact laser accelerators for X-ray phase-contrast imaging

Abstract

Advances in X-ray imaging techniques have been driven by advances in novel X-ray sources. The latest fourth-generation X-ray sources can boast large photon fluxes at unprecedented brightness. However, the large size of these facilities means that these sources are not available for everyday applications. With advances in laser plasma acceleration, electron beams can now be generated at energies comparable to those used in light sources, but in university-sized laboratories. By making use of the strong transverse focusing of plasma accelerators, bright sources of betatron radiation have been produced. Here, we demonstrate phase-contrast imaging of a biological sample for the first time by radiation generated by GeV electron beams produced by a laser accelerator. The work was performed using a greater than 300 TW laser, which allowed the energy of the synchrotron source to be extended to the 10-100 keV range.

Keywords: X-ray phase contrast imaging; laser wakefield acceleration; plasma acceleration.

Figure 1.

Cartoon of a ‘bubble’ accelerator. A region of space is evacuated of electrons (white region) by a high intensity laser creating a force that points inward on an electron (arrows). For a sufficiently large bubble, an electron inside the electron sheath is accelerated forward, as well as gaining transverse momentum, from the wakefield (black trajectory). If the acceleration is sufficient, it can become trapped, performing betatron oscillations of amplitude r_β as it moves forward in the wave frame. At each turn of the oscillation, the acceleration (which points towards the axis) is primarily normal to the particle motion, leading to synchrotron-like radiation in the forward direction (shown by cones). (Online version in colour.)

Figure 2.

Raw images of characteristic (left) electron spectra and (right) corresponding X-ray beams produced in Astra Gemini experiment. For this particular shot, , and the transmission through Al wedges placed in front of the beam indicates a photon energy E_p>20 keV. (Online version in colour.)

Figure 3.

Raw (untreated) phase contrast images of a cricket taken with the Gemini betatron source. Distance from source to sample was u=475 mm, and from sample to detector v=1325 mm. Images show minimal absorption, indicative of high flux of photons at energies greater than 20 keV, for which the phase-shift cross section greatly exceeds (more than 100 times) that for absorption. Each image is taken in a single shot of the τ∼30 fs source.