Linear electron accelerator

The RADEF linear electron accelerator is a recommissioned Varian Clinac® medical accelerator. It is located in its own enclosure within the RADEF cave. It produces a vertical, top-down pulsed electron beam (which can be degraded into a pulsed X-ray beam) with the following characteristics:

  • Beam area: the typical maximal beam area is about 25 cm * 25 cm. The irradiation area can be greatly increased (probably over 80 cm x 80 cm) if required, but at the cost of a lowered dose rate and increased uncertainties about the beam parametres
  • Energies:
    • for electrons: 6, 9, 12, 16 and 20 MeV
    • for X-rays, either:
      • continuous spectrum from 0 to 6 MeV, with a peak around 1 MeV
      • continuous spectrum from 0 to 15 MeV, with a peak around 2 MeV  
  •  Dose rates:
    • for electrons: 100 to 1000 rad/min. (in water)
    • for X-rays: 100 to 600 rad/min. (in water)
    • Note: these values refer to the area of peak energy deposition within the target. The actual dose deposition profile varies depending on the target material, the considered depth in the target, and the beam type (electrons or X-rays.) This is due to secondary electrons escaping from the target surface.
    • Typically, for electron beams, the actual energy deposition at the surface is 75%-90% that of the peak value, which is reached at a depth of 1.5-2.5 cm within the target. The energy deposition then gradually decreases according to the physics of beam attenuation.
      For X-ray beams, the energy deposition profile is similar, with the only difference that the energy deposition at the surface is only 50% of the peak value.
  •  Beam duty cycle: At the maximum dose rate, the linac's source electron beam consists in a series of 5 μs pulses with a period of 5 ms (which corresponds to a duty cycle of 0.1%). The beam dose rate is changed by "removing" some of the pulses: for example, at 750 rad/min, every fourth pulse is missing, effectively achieving 3/4 of the maximum dose rate.

During operation, in particular when producing 15 MeV X-rays, the linac may also generate a certain amount of so-called "photoneutrons" with a kinetic energy of a few MeV, which eventually reach thermal equilibrium within the linac chamber. This has to be accounted for when using neutron-sensitive devices; there is, however, no danger of neutron activation.

The DUT may simply be placed on a platform below the beam window, and can be directly accessed without the need for any interface. The DUT can be connected to the user's DAQ/monitoring hardware in the RADEF barracks with about 20 m of cabling. If this distance is too long, then it is possible to leave the DAQ/monitoring hardware inside the RADEF cave, but user access will be impossible during irradiation.