Evaluation of Avalanche Photodiode Arrays for the Readout of High Granularity Scintillator Matrices

In order to provide sufficient flexibility for patient positioning at the Heidelberg charged hadron tumour treatment facility, γ-ray detectors with large sensitive area and small volume are highly desirable for the positron camera to combine with a rotating beam delivery (gantry). Photon detector technology yields increasingly more compact PET-detectors consisting of inorganic scintillators, e.g. bismuth germanate (BGO) or lutetium oxyorthosilicate (LSO), coupled to either a position sensitive photomultiplier (PSPMT) [1] or to avalanche photodiode arrays (APDAs) [2, 3]. We have shown [4] that LSO is a suitable scintillation material for in-beam PET due to its higher light yield and shorter decay time when compared to BGO. APDs are preferable to PMTs due to their higher compactness. Furthermore, the presence of weak stray fields originating from the bending magnets of the gantry may permanently deteriorate the performance of a PSPMT. Therefore, PET-detectors assembled from LSOcrystals and APDAs are the first choice for an in-beam positron camera. With this goal set, we have evaluated the performance of a recently released large area avalanche photodiode (LAAPD) array from Advanced Photonics, Inc. (API). The device is a monolithic 2 x 2 squared pixels structure with 21 mm total active area and 2.3 mm pitch. Although it presents a high linear fill factor of 90 %, measured in dc-mode with a laser fine scan by the manufacturer, we measured light pulse crosstalk values inferior to 1.7 % between all pixels by sequentially illuminating the center of one pixel with a 1.7 mm diameter light spot and registering the amplitude spectra of all pixels. These good results are due to the fabrication technique used: one single substrate consisting of a p-type epitaxial layer and an n-type neutron transmutation doped layer is etched using deep reactive ion etching, yielding four adjacent diodes with high gain uniformity. We measured an inter-pixel gain variation with X-rays smaller than 1.3 %. Although the APDA allows stable operation with high gains up to 200, we obtained its noise minimum at an excellent low equivalent noise charge of 16 electrons rms with an internal gain of 50 and with 1 μs signal shaping time. A 2 x 2 LSO crystal matrix (2 x 2 x 10 mm each crystal) optically isolated with teflon tape and coupled with silicon oil to the APDA allowed individual crystal identification to be performed, as summarized in Fig. [1]. An energy resolution of 12.3 % was obtained for the FWHM of the 511 keV photopeak, a promising result when compared to the ∼20 % offered by the BGO/PMT solution. The LSO crystals light yield was calibrated with a XP2020Q PMT which allowed the number of electron-hole pairs (e-h) of the APD to be found at 9200 e-h/MeV. Coincidence timing between one LSO/APD channel and a fast scintillator (BaF2) coupled to a fast PMT resulted in a time resolution of 1.9 ns FWHM, as detailed in Fig. 2. This result is one of the best published for APDAs and corresponds to 2.7 ns FWHM if two LSO/APD detectors are used. Energy and time resolution are essential variables for PET in order to effectively suppress Compton scattered photons and random events, respectively.