Maximum spot size: 100 MeV electron beam, ~20×20 mm2, uniform beam spot:

 

Intensity diagnostics

Several detectors are generally used for monitoring the particle flux: Medipix-based Silicon pixel imaging device (FitPIX), Cherenkov calorimeters, and integrating current toroid (ICT) monitors (only for high-energy, high-intensity primary beam).
FitPIX sensor has a 1.45×1.45 cm2 active area and is cross-calibrated with the BTF Cherenkov counter. Saturation effects on the 2.1 cm2 area start around 105 particles/pulse (corresponding to 106 cm-2 s-1 at the maximum rate of 49 Hz)

100 MeV electron beam FitPIX calibration vs. Cherenkov counter:

 

In order to produce a variable energy beam, the primary Linac beam is intercepted by a target (Copper, 1.7 X0) and the secondary particles, spread-out in energy and angle, are focussed and energy selected by means of a magnetic (quadrupoles and sector dipole) and collimation system.
The secondary particles energy distribution has a significant dependence on the chosen energy, so that the intensity decreases sharply when approaching the primary beam energy (at the higher end of the spectrum), and close to the critical energy.

For a 510 MeV primary energy, the maximum intensity is reached around 150 MeV.

Flux vs energy on secondary beam:

 

This work was supported by ASI-INFN ASIF implementation agreement no. 2017-15-H.0.