Simple sandwich-type device structures have been fabricated by deposition of metal contacts on opposing faces of polycrystalline diamond films synthesised using chemical vapour deposition. The electrical characteristics of these devices have been investigated during exposure to a 6 megavolt photon beam from a clinical linear accelerator. The photocurrent appears to vary as the dose rate to the power of 0.78-0.85. The angular dependence of the photocurrent is less than 10 per cent. Further study on a range of CVD diamond substrates from different manufacturers is on-going.