Scintillation counting system for in-situ measurement of radioactive samples in a multiple-well plate

A method for in-situ measurement of the radioactivity of multiple beta-emitting samples while the samples are in a multiple-well sample tray in which each well is open at its upper end, said method comprising

• providing a scintillator in the lower portion of each sample-containing well where the scintillator is directly exposed to the beta emissions from the respective samples for producing light pulses in response to said beta emissions,
• positioning multiple photomultiplier tubes adjacent to the open upper ends of said sample wells containing said scintillator for simultaneously measuring the radioactivity of multiple samples with only a single photomultiplier tube sensing the scintillations from each well and converting the sensed scintillations into corresponding electrical pulses, the face of each photomultiplier tube covering substantially the entire open upper end of the corresponding sample well,
• processing the electrical pulses from each photomultiplier tube to discriminate between pulses attributable to sample events within said wells and pulses attributable to non-sample events such as photomultiplier tube noise, said discrimination being effected by determining whether a selected number of electrical pulses occurs within a prescribed time interval, the occurrence of said selected number of pulses within said prescribed time interval signifying a sample event, and
• supplying only the electrical pulses attributable to sample events to a pulse analyzer.