# Optically Switchable MeV Ion/Electron Accelerator
## 4 Discussion  
Several potential applications may benefit from this irradiation scheme. Energy-dispersive x-ray (EDX) [38] spectroscopy and particle-induced x-ray emission (PIXE) [39] are two powerful techniques for material analysis. They are widely used in the semiconductor industry [40; 41] and in biomedical applications [42; 43]. EDX reveals the elemental composition of solid samples, while PIXE resolves -um-deep stratigraphic structures. Both methods rely on measuring x-rays emitted from a sample, following its irradiation with particle beams in the keV-to-MeV energy range. Laser-driven EDX was recently demonstrated [44], using a mixed beam of laser-accelerated electrons and protons emitted from irradiated solid foils. Laser-driven PIXE was demonstrated on the same setup, by sweeping out the electrons with a magnetic field. Our acceleration scheme, which would amount to adding the controlled pre-pulse to this setup, can enable EDX with a 1000 times brighter beam of electrons in the MeV range (compare dashed to solid blue curves in Figure 4a). If necessary, removal of these excess electrons using magnetic deflection may be applied to this setup as well. Proton energies of over 3 MeV, which are the requirement for conducting PIXE, may be obtained using a 100 TW-class laser system.  
Particle beams in the MeV energy range are also used for conducting non-destructive testing (NDT). Example applications include the investigation of trucks and cargo containers [45] for detecting the contraband of explosives [46], narcotics [47], and special nuclear materials [48]. The use of \(\gamma\)-rays has already reached commercial maturity [49], but the applicability of other beam types is limited by the titanic dimensions of conventional particle accelerators.  
With shadowgraphy, the simplest form of NDT, information is revealed by the fraction of particles transmitted through the interrogated sample. Using multiple types of radiation may reveal details that are not obtained by each beam type by itself [50]. We illustrate this idea in Figure 5, which shows the simulated transmission of 10 MeV protons (left) and electrons (right) through a sample of 600 um thick coaxial cylinders made of Al, W, and Cu.  
The simulation was conducted with the FLUKA particle transport simulation code [51]. Figure 5 shows that the Al casts a -50% shadow on the proton beam, whereas the W and Cu shadows are absolute and indistinguishable. The electron beam, however, reveals a difference between the two heavier metals, but the Al is nearly 100% transparent.  
For nuclear physics research, the ability to switch between beams of electrons and ions within >1 Hz may be applied to study \({}^{\mathrm{A}}\)Z(p,x) reactions, on isotopes with O (1 s) lifetimes. A sample of long-lived \({}^{\mathrm{A+1}}\)Z isotope, e.g., \({}^{56}\)Ni, may be irradiated with MeV electrons to induce the \({}^{56}\)Ni(\(\gamma\),n)\({}^{55}\)Ni reaction by bremsstrahlung. The resulting \({}^{55}\)Ni has a half-life of T\({}_{1/2}\) = 204 ms [52]. Measurements of the \({}^{55}\)Ni(p,\(\gamma\)) reaction, which is important  
Figure 5: Particle transport simulation of the transmission of 10 MeV proton (**left**) and electron (**right**) beams passing through a sample of 600 μm thick Al, W, and Cu. The color scale is normalized to the beam fluence.  
for determining whether the rp-process bypasses the \({}^{56}\)Ni waiting point [53], may then be made in situ.