An apparatus and method for testing scintillator arrays, e.g., crystal arrays for PET imaging. The apparatus includes, a two-sided tray arranged to hold scintillator arrays in either side and slide the arrays into a light-tight box having a radiation source beneath the arrays and photomultiplier tubes (PMTs) above the arrays. When arranged in the testing position with the arrays interposed between the radiation source and the PMTs, ambient light from outside the box is prevented from leaking into the box and high-voltage power is supplied to the PMTs. Otherwise, to prevent PMT damage, the high-voltage is off. The radiation source is an arrangement of sealed low-activity pieces of radioactive elements, thus minimizing requirements for radiation shielding and minimizing safety risks. The method calculates a flood map from scintillation data/counts and performs analysis according to predefined criteria, e.g., the peak-to-valley ratio, to flag arrays exhibiting inferior quality.