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 m icromachining technology [1]. It consists of an array of scintillator cryst als encapsulated in well-type silicon sensors. The light created by the int eraction of an X-ray or a gamma ray with the crystal material is confined b y vertical silicon sidewalls and collected onto the avalanche photodiode at the bottom of the well. Several parameters of the photodiode need to be op timised: 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 us ing (100) wafers etched in a potassium hydroxide (KOH) solution. Their opti cal response at 675 nm is comparable to that of a 500 mu m thick silicon PI N diode. The low light detection efficiency is compensated by internal ampl ification. Several scintillator materials have been positioned in the wells on top of the thin photodiodes, i.e. a layer of structured CsI(TI) and sin gle crystals of CsI(Tl) and Lu2S3(Ce3+). First experiments of gamma-ray det ection have been performed.