Van de Graaff accelerator and experimental beam line at LNF.
This 2 MeV electron Van de Graaff Accelerator permits material irradiation either by electron beam or by Bremsstrahlung induced by stopping the electron beam. In this way, radiation testing that is normally carried out using a Co-60 source can be undertaken more rapidly (producing a larger and better controlled dose rate) while allowing in-situ measurements. Irradiation parameters (temperature, vacuum pressure, gas environment, dose rate and beam energy) are well controlled. Moreover, irradiation of relatively large components or material samples is possible. The accelerator staff can design and develop different irradiation chambers and experimental set-ups depending on irradiation requirements. Such experimental systems permit performing optical, electrical and dielectrical measurements during irradiation (“in-beam”). This makes it a unique experimental radiation facility in which simultaneous optical, electrical and dielectrical measurements can be made in the range of Hz to GHz. For this, systems to measure optical absorption and radioluminescence, electrical conductivity and dielectric properties during irradiation (in-situ) are mounted on the accelerator beam line.
Beam characteristics;
Energy: 0.25 to 2.0 MeV and Current: 10 pA to 150 µA
Samples from 3 mm2 to about 20×20 cm2
At target area unfocussed beam is 1 cm diameter
Beam can be focussed up to 1 mm diameter (for small samples)
Beam can be defocussed up to 3 cm diameter
Beam can be scanned over 20×20 cm2(for large samples)
The facility allows in-beam testing at a controlled temperature of the electrical, dielectric (RF), and optical properties of solid and gas insulators. Irradiation can be performed in high vacuum, air, or controlled atmospheres (such as N or He).
Flexibility. The facility is extremely flexible and has several unique in-beam systems for measuring electrical conductivity, dielectric loss and permittivity (Hz to GHz), and optical absorption and emission during irradiation over a wide range of dose rates and temperatures. Irradiations can be performed in high vacuum, air, or controlled atmosphere such as N or He. Simulation in electron accelerators offers important advantages, namely easy experimental parameter control and high dose rates available up to 105 Gy/s.
In-situ measurement capability and expertise. To date a range of studies have been carried out on fusion candidate insulators for which low displacement per atom (dpa) rates are required. For these studies typical dpa rates range from about 10-12 to 10-8 dpa/s while ionization rates (Bremsstrahlung or direct electron irradiation) up to 104 Gy/s.For instance, electrical, optical as well as hydrogen and helium diffusion properties are measured during irradiation at a controlled temperature, from liquid nitrogen up to 1000 C. Special irradiation chambers and sample holders are designed by the accelerator staff and they are fabricated at the CIEMAT workshops.