An apparatus for detecting radiation comprises a radiation converter (101), a photodetector (102), and a light guide (103) for conducting photons emitted by the radiation converter. The radiation converter is a layer in contact with a first surface (105) of the light guide and the photodetector is attached to an opposite side of the light guide. A convex second surface (106) of the light guide receives the photons from the radiation converter and has a reflective coating (104) reflecting the photons backwards to the radiation converter that, in turn, reflects the photons to the photodetector. As the photons are reflected back-and-forth in the above-mentioned way, the photons can be directed to a small area even if the light guide is short in a direction (z) perpendicular to the first surface of the light guide.