Scintillation light read-out by low-gain thin avalanche photodiodes in silicon wells

We have proposed a new type of gamma-ray camera, which takes advantage of micromachining technology [1]. It consists of an array of scintillator crystals encapsulated in well-type silicon sensors. The light created by the interaction of an X-ray or a gamma ray with the crystal material is confined by vertical silicon sidewalls and collected onto the avalanche photodiode at the bottom of the well. Several parameters of the photodiode need to be optimised: uniformity and efficiency of the light detection, gain, electronic noise and breakdown voltage. In order to evaluate these parameters we have processed 3*3 arrays of 1.8 mm(2), similar to 10 mu m thick photodiodes using (100) wafers etched in a potassium hydroxide (KOH) solution. Their optical response at 675 nm is comparable to that of a 500 mu m thick silicon PIN diode. The low light detection efficiency is compensated by internal amplification. Several scintillator materials have been positioned in the wells on top of the thin photodiodes, i.e. a layer of structured CsI(TI) and single crystals of CsI(Tl) and Lu2S3(Ce3+). First experiments of gamma-ray detection have been performed.