## 3 Experimental Results
### X-ray Emission  
The X-ray emissions from the laser-foil interactions were measured with two diagnostics: a set of dosimeters with broad spectral sensitivity [55] and a spatially resolved spectrometer working in the keV range [56]. Twenty GD-351 radio-photo-luminescence dosimeters [55] were placed at various angles and positions inside the interaction chamber, at distances ranging from 87 cm and 115 cm from the target. The dosimeters were encapsulated in a 1.5 mm thick tin holder, making them sensitive to X-rays in an energy range from around 30 keV to a few MeV, with an energy-independent response. The tin holder also served to protect the dosimeters from protons up to 23 MeV and electrons up to 1.5 MeV, by absorbing these particles, preventing them from depositing their energy into the dosimeters. However, in the absence of comparative measurements of the X-ray and electron energy spectra, we cannot exclude the possibility that higher-energy electrons contributed to the dosimeter signal. Taking advantage of the high repetition rate of the Apollon laser, the dosimeter signals were accumulated over several shots (typically 20), enabling comparisons between different sessions by averaging out laser fluctuations. Figure 8 depicts two representative dose distributions obtained with direct and DPM shots, revealing strong differences between the two configurations. For direct shots (left panel), the doses are preferentially delivered in a broad lobe that extends between the target normal and the laser specular reflection axis. By contrast, the DPM shots (right panel) result in a much narrower emission cone centered along the laser specular direction. We also observe significant dose emission along the rear target normal in direct shots, which is absent with the DPM. Specifically, a factor of up to 130 is measured between the doses emitted in the backward direction for direct and DPM shots, with approximately 1670 \(\upmu\)Gy and 12.8 \(\upmu\)Gy per shot, respectively. The broad angular distribution and high doses observed in the direct shots are consistent with dominant Bremsstrahlung emission. This is likely due to the presence of a significant preplasma at the target front, where laser-driven electrons can recirculate and radiate [57; 58]. In contrast, the significant reduction in dose and the directional nature of the X-ray emission in the DPM case suggest a different radiation generation mechanism. The thinner targets used with the DPM reduce the hot-electron path length in matter,  
Figure 8: Spatial dose distribution recorded inside the interaction chamber by a set of dosimeters (details in text). **Left panel**: results for direct shots on Al foils of 0.8 \(\upmu\)m, 1.5 \(\upmu\)m and 2 \(\upmu\)m thicknesses. **Right panel**: results for DPM shots on Si/Al foils of 150 nm, 200 nm and 250 nm thicknesses. Each circle represents a dosimeter, and its color corresponds to the measured dose. The outer circle in both graphs represents the actual diameter of the target chamber. The gray bar representing the DPM in the right panel is to scale. thereby decreasing the probability of Bremsstrahlung. Furthermore, in high-contrast, oblique laser-solid interactions such as those with the DPM, electrons can be accelerated along the target surface to high energies, radiating predominantly in the near-surface direction [59; 60]. In addition, the laser beam reflected off the solid surface can directly accelerate some electrons [61], possibly leading to X-ray emission through betatron oscillations. Both mechanisms align well with the observed narrow cone of emission in the DPM case (Figure 8). We now consider the spatially resolved measurements of the X-ray source. The focusing spectrometer [56], directed at the target front along the target normal, measured the K-shell emission of the plasma in the 1500-2000 eV range (wavelengths \(\lambda\) = 6.4-8 A).