6MV, water
Low energy dependence of radiochromic film
Accurate dosimetry with GafChromic™ EBT film of a 6 MV photon beam in water:
What level is achievable?, LJ van Battum, D Hoffmans, H Piersma, and S Heukelom,
Med. Phys. 2008; 35(2): 704-716