Introduction to Radiation Detctors and Electronics, 22-apr-99 Xi. Photodiodes Principle of an Apd

Although photomultiplier tubes still dominate in scintillation detectors, silicon photodiodes are becoming increasingly popular. Scintillation light: λ = 200 – 500 nm (E= 6.2 – 2.5 eV) Semiconductor photodiodes offer a) high quantum efficiency (70-90% instead of 30% for PMTs) b) insensitivity to magnetic fields c) small size Introduction to Radiation Detctors and Electronics, 22-Apr-99 Helmuth Spieler XI. Photodiodes LBNL 2 Although all semiconductor diodes are light sensitive, for high quantum efficiency they must be designed to avoid significant dead layers at the surface, as most of the photons in the visible range are absorbed within about 1 µm of the surface. The number of absorbed photons If the absorption coeficient α= 10 4 cm, dead layers must be < 0.1 µm to avoid significant losses (<10%). ∫ − = dx e N N x abs α 0 Introduction to Radiation Detctors and Electronics, 22-Apr-99 Helmuth Spieler XI. Photodiodes LBNL 3 Quantum efficiency of well-designed photodiodes is 2 – 3 times better than of PMTs. Measured data of photodiodes fabricated in LBNL Microsystems Lab (used for medical imaging, N. Wang + S. Holland) Photomultiplier tubes provide high gain without introducing significant electronic noise, whereas photodiode systems depend critically on low noise. Unlike PMT systems, photodiode readouts must be very carefully optimized. Example: Photodiode coupled to a NaI(Tl) scintillator crystal. PMT: The photon yield for 511 keV gammas is about 15000 at the photocathode. The resolution obtained in a PMT system is about 2% rms, i.e. 10 keV. For the photodiode assume a quantum efficiency of 80%, i.e. 80% of the incident scintillation photons create an electron-hole pair. The signal charge Q s = 0.8 x 15000 el = 12000 el If the photodiode is to replace a standard phototube, its diameter must be 2 " , i.e. the area is 2000 mm 2. If we use p-type Si with 10 kΩcm resistivity, a 1 mm thick diode requires a depletion voltage of 1000 V, which is possible. The capacitance of a 1 mm thick diode with an area of 2000 mm 2 is about 200 pF. For an optimally matched input transistor and a shaping time of 1 µs the voltage noise would be about 60 el rms, or 210 eV. The reverse bias current of the photodiode, assuming 1 nA/cm 2 , is 20 nA, leading to a noise contribution of 500 el, or 1.8 keV. Helmuth Spieler XI. …