## 2 The Experimental Platform HELL: High-Energy Electrons by Laser Light
### Radioprotection Analysis and Shielding Design  
The electron beams produced by HELL will generate mixed radiation high energy fields, whose strength will increase with the beam energy. Given the aim to accelerate electrons in the GeV range, an adequate assessment of the ionizing radiation hazard is necessary. The studies were performed by means of simulations using the FLUKA Monte Carlo transport code [46; 47; 48], for a conservative worst-case scenario from the radioprotection point of view. Specifically, for a monochromatic 10 GeV electron beam with a conservative 100 pC pulse-charge, 10 mrad divergence, and single shot operation mode. Civil structure of the experimental building was designed in such a way that the shielding provided by the walls, together with a suitable beam dump, allows for operations of electron beams of tens of GeV, while keeping the dose rate in the highly occupied areas, such as control rooms, below 1 mSv per year (which is the legal limit for exposure of general public). Several factors have to be taken into account for the beam dump shielding design: the possible presence of a magnet to bend the electron beam (as in reality the beam will not be monochromatic); the need for the shielding to be modular in order to scale with the experimental modifications and improvements (e.g., beam energy and charge); the need to allocate sufficient space downstream the beam to allow for the installation of additional experimental equipment; the need to minimize the backscattering toward the experimental chamber; and, the presence of another beamline in the same experimental hall. All of these considerations lead to the preliminary design of the shielding shown in Figure 24. The preliminary shielding design is symmetric with respect to the beam height (1.3 m from the ground). The core of the shielding, designed to be hit at its centre by the electron beam, is located 10 m from the experimental chamber and is flanked by two long lateral \"legs\". The shielding core (3 m x 1 m, w \(\times\) h) is made of five layers of polyethylene, two layers of iron, and two layers of concrete, each layer being 20 cm thick. The core is lying over concrete foundations of the same horizontal dimensions. The electron beam will be stopped mainly by the iron, the concrete behind will  
Figure 23: (left) 3D view technical design of the HELL User Station, (right) measured and simulated electron deflections for the 5 cm, 0.8 T permanent magnet dipole. absorb the attenuated radiation, while the polyethylene will reduce the backscattering. The two lateral legs are 10 m long and 0.5 m wide. One shields the other beamline, while the second shields the beam deflected by the spectrometer and protects walls from possible long term irradiation damage. Figure 25 shows the ambient dose equivalent rate results that were obtained from the Monte Carlo simulations, where it is apparent that between the two shielding legs only few shots would lead to integrating a personal dose of 1 mSv. Currently, modifications to the shielding design and experimental setup are being investigated [49]. First, adding a thin window to the chamber wall that was hit by the electron beam. While the beam is generated with a small divergence, its interaction with the downstream experimental chamber panel (2 cm thick aluminium) produces a widespread radiation. Instead, thin window would effectively reduce the beam scattering. Other modifications under investigation include the presence of a spectrometer in the beam and the reduction of the radiation escaping the back of the shielding. Another future research option is to exploit muons present in this escaping radiation for obtaining \"non-collimated muon beam\" that could be used for the testing of muon detectors for high energy physics. The highly penetrating radiation visible in Figure 25 (i.e., the zones in tones of magenta on the right end of the picture) indicate large muon production in the dump already at the considered energy. Figure 24: 3D view cut at beam height of the FLUKA model of the HELL beamline.