## 10 First Results of the Self-Injection Test Experiment (SITE)  
In the first phase of SITE, accelerated electron bunches were obtained from the interaction of the femtosecond laser pulse with a nitrogen supersonic-gas-jet working with a backing pressure of about 17 bar and a laser-intensity of \(2\times 10^{18}\) W/cm\({}^{2}\). These preliminary data were obtained using an F/15 focusing optics, at fixed pulse duration of about 30 fs and over an energy per pulse ranging from a minimum of about 300 mJ up to 1 J. A preliminary analysis of the laser-accelerated bunches in test laser shots was carried out using the SHEEBA [24] detector [31] consisting of sandwiched radiochromic films (RCF) as shown in details in Figure 14. The active compound of RCFs is a micro-crystalline monomer belonging \begin{table}
\begin{tabular}{c c c c} \hline \hline \(\mathbf{L_{gas\;jet}}\) [mm] & \(\mathbf{n_{e}}\) [cm\({}^{-3}\)] & \(\mathbf{\tau}\) [fs] & \(\mathbf{I_{0}}\) [W/cm\({}^{2}\)] & \(\mathbf{w_{0}}\) [\(\mu\)m] \\ \hline
4 & \(3\times 10^{18}\) & 30 & \(5.2\times 10^{19}\) & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A possible working point of the self-injection test experiment at FLAME for laser-acceleration in the GeV scale. Figure 13: Energy spectrum of the accelerated electrons as obtained from the 3D PIC simulation for the SITE at three different propagation length inside the gas-jet. to the diacetylene molecule family, uniformly distributed in a gelatine matrix. Under exposure to ionising radiation, the crystallised diacetylenes undergoes a solid-state polymerisation reaction producing a dye polymer referred to as polydiacetylene, which results as a change in the Optical Density (OD) in the layer, and it is a localised process. The polymerisation begins during the irradiation and does not stop after exposure, but stabilises to its asymptotic value within 24 h. Because of their sensitivity to ionising radiation, the first layer of SHEEBA is always a thin Aluminium foil, which act as a stop filter for optical (scattered laser) and UV radiation and soft X-rays. The electrons propagate in the detector releasing their energy till they stop into the medium via several physical mechanisms. In order to take into account these effects and the different materials in the stack, a Monte Carlo code was used [31], which is based upon CERN library GEANT 4.2.0 [49]. The code enabled to model the detailed geometry of the detector and to calculate the electron stopping power. According to the calculations, in our configuration the detector is sensitive to electrons in the range between 100 keV and 100 MeV. Retrieval of the actual energy spectrum can be obtained starting from the energy deposited in each layer and convergence of the retrieval process requires that the electrons are completely absorbed in the detector. However, a first direct estimate of the maximum electron energy can be obtained by simply observing the number of layers with signal and comparing this value to the expected energy loss per layer for a given incident electron energy. The key information of these simulations is the penetration depth of the electrons as a function of their initial kinetic energy. By using this function, the electron spectrum can be deconvolved considering the position of each layer in the stack [31] and the energy can be resolved as a function of the angle of emission as demonstrated in a recent experiment [21]. We used HD810 and MD55 types, to obtain charge, angular and spectral information of the full electron bunch. This detector was used in the first phase of the SITE experiment to obtain a first estimate of the properties of accelerated electron bunches. The images of Figure 15 shows four layers of the detector including the first layer and the last layer, all showing electron signal. In this particular circumstance, the presence of signal on all the layers does not allow the full spectrum to be recovered.