RESUMEN
Lutetium oxyorthosilicate (LSO) or lutetium yttrium oxyorthosilicate (LYSO) are the scintillator materials most widely used today in PET detectors due to their convenient physical properties for the detection of 511 keV annihilation photons. Natural lutetium contains 2.6% of 176Lu which decays beta to excited states of 176Hf producing a constant background signal. Although previous works have studied the background activity from LSO/LYSO, the shape of the spectrum, resulting from ß-particle and γ radiation self-detection, has not been fully explained. The present work examines the contribution of the different ß-particle and γ-ray interactions to provide a fuller comprehension of this background spectrum and to explain the differences observed when using crystals of different sizes. To this purpose we have shifted the continuous ß-particle energy spectrum of 176Lu from zero to the corresponding energy value for all combinations of the isomeric transitions of 176Hf (γ-rays/internal conversion). The area of each shifted ß-spectrum was normalized to reflect the probability of occurrence. To account for the probability of the γ-rays escaping from the crystal, Monte Carlo simulations using PENELOPE were performed in which point-like sources of monoenergetic photons were generated, inside LYSO square base prisms (all 1 cm thick) of different sizes: 1.0 cm to 5.74 cm. The analytic distributions were convolved using a varying Gaussian function to account for the measured energy resolution. The calculated spectra were compared to those obtained experimentally using monolithic crystals of the same dimensions coupled to SiPM arrays. Our results are in very good agreement with the experiment, and even explain the differences observed due to crystal size. This work may prove useful to calibrate and assess detector performance, and to measure energy resolution at different energy values.
RESUMEN
We present the performance evaluation of a large-area detector module based on the ArrayC-60035-64P, an 8×8 array of tileable, 7.2mm pitch, silicon photomultipliers (SiPM) by SensL, covering a total area of 57.4mm×57.4mm. We characterized the ArrayC-60035-64P, operating at room temperature, using LYSO pixelated crystal arrays of different pitch sizes (1.075, 1.430, 1.683, 2.080 and 2.280mm) to determine the resolvable crystal size. After an optimization process, a 7mm thick coupling light guide was used for all crystal pitches. To identify the interaction position a 16-channel (8 columns, 8 rows) symmetric charge division (SCD) readout board together with a center-of-gravity algorithm was used. Based on this, we assembled the detector modules using a 40×40 LYSO, 1.43mm pitch array, covering the total detector area. Calibration was performed using a 137Cs source resulting in excellent crystal maps with minor geometric distortion, a mean 4.1 peak-to-valley ratio and 9.6% mean energy resolution for 662keV photons in the central region. The resolvability index was calculated in the x and y directions with values under 0.42 in all cases. We show that these large area SiPM arrays, combined with a 16-channel SCD readout board, can offer high spatial resolution, without processing a big number of signals, attaining excellent energy resolution and detector uniformity.
