Spectral and spatial shaping of a laser-produced ion beam for radiation-biology experiments

The study of radiation biology on laser-based accelerators is most interesting due to the unique irradiation conditions they can produce, in terms of peak current and duration of the irradiation. In this paper we present the implementation of a beam transport system to transport and shape the proton beam generated by laser-target interaction for in vitro irradiation of biological samples. A set of four permanent magnet quadrupoles is used to transport and focus the beam, efficiently shaping the spectrum and providing a large and relatively uniform irradiation surface. Real time, absolutely calibrated, dosimetry is installed on the beam line, to enable shot-to-shot control of dose deposition in the irradiated volume. Preliminary results of cell sample irradiation are presented to validate the robustness of the full system.

Figure 1

(a) Energy dependent divergence of the proton beam source. The contours drawn are the isodose lines at 50% of the maximum dose, and the labels indicate the minimum energy required for a proton to reach the considered radiochromic film. (b) Average divergence of the proton beam, calculated from (a) as function of its energy.

Figure 2

Sketch of the SAPHIR beam line setup used for proton beam transport studies. The first PMQ is set at the minimal distance of 5 cm from the source. The plus and minus symbols indicate the relative polarity of each PMQ. The $75\mu \mathrm{m}$ Mylar film for vacuum/air separation and the transmission ionization chamber are also depicted.

Figure 3

5 MeV proton beam simulations performed with the tracewin multiparticle tool in the described PMQ setup. The top pictures show the calculated envelope in the $X$ and $Y$ directions, with PMQs represented by the red rectangles, the actual beam envelope in blue, and the rms aperture-free beam envelope in green. The particle losses are plotted in the bottom graph, where the shaded area represents the PMQs.

Figure 4

(a) Transmission efficiency of the PMQ system as a function of the proton energy. (b) Relative energy spectrum of the proton beam reaching the $10\times 15{\mathrm{mm}}^2$ output sample area. The bottom scale shows the initial proton energy, while the top scale shows the energy of these same particles when they reach the sample layer after crossing some absorbing elements.

Figure 5

Schematic of the TIC calibration setup.

Figure 6

Dose deposition profile of the CPO proton beam recorded on a stack of RCF EBT2 placed behind the collimator. The graph on the right shows an averaged horizontal lineout of the dose map.

Figure 7

Graph depicting the energy decrease (blue curve, vertical scale on the left) of an instance proton with a 5 MeV initial energy, as it propagates along the beam line. The energy deposited in the various absorbing elements encountered on its path is represented by the bar plot (vertical scale on the right). Each bar is centered in its energy deposition longitudinal range to preserve proportionality between the energy deposition value and the area of the corresponding bar.

Figure 8

Transverse profile of the proton beam dose deposition at the transport line output, recorded on an imaging plate with 28 mm diameter. The IP was placed 1 m away from the target, at the biological sample location in air. The isodose curve at 8 PSL (i.e. 20% of the maximum recorded signal) is represented by the black line, and the white rectangle marks the area where the biological sample response was studied.

Figure 9

Number of $\gamma \mathrm{H}2\mathrm{AX}$ foci generated in HCT116 WT and $\mathrm{p}53^{-/-}$ cells irradiated with an increasing dose provided by four or eight shots of laser-accelerated protons ($n=3$). For each data point, foci were determined from at least 200 nuclei.

Figure 10

Survival curves of HCT116-WT and $\mathrm{p}53^{-/-}$ cells after ${}^{137}\mathrm{Cs}$ (a, mean $\pm$ SD of three independent experiments), 20 MeV proton (b, mean $\pm$ SD of two independent experiments) or laser-driven irradiations (c, mean $\pm$ SD of three independent experiments). (d) Calculated D10 ratio between HCT116-WT and $\mathrm{p}53^{-/-}$ cells for each irradiation condition.