Design and performance of the micro-dose calibrator

Abstract

A new class of dose calibrator called a micro-dose calibrator, designed to measure radioactivity rates in the range between 1 kBq up to 370 kBq, has been designed and an 8 NaI(Tl)/photo multiplier tube (PMT) module prototype built to evaluate its performance. The design of the micro-dose calibrator is based on the concept of a segmented well counter. By segmenting a single crystal well counter into multiple crystals or segments, with each segment operated and read out independently, one can extend the operational range of a standard single crystal well counter while maintaining its accuracy and spectrographic capabilities. The micro-dose calibrator is operated in N-fold coincidence mode to ensure that a single radioactive decay resulting in multiple segments detecting gamma ray events are counted as a single decay. The N fold coincidence detection is achieved using a pipeline coincidence processing algorithm implemented in software as part of a multi-threaded acquisition system. A tri-functional model pulse pileup dead time correction is applied to extend the operational range of the micro-dose calibrator. The micro-dose calibrator achieved an operation range between 1 kBq up to 3.4 MBq for 18F with a 1% linearity precision using a 60 s integration time window. Over a 9 d period, a daily count rate measure of a 137Cs point source had a coefficient of variation of 0.17% with only one high voltage calibration performed on day 0. The central 20 mm up to 35 mm axial length in the sample well showed less than 1% change in count rate depending on the radionuclide. Using a 0.5 ml insulin syringe, a volume of dose between 0.13 ml up to 0.50 ml showed less than 1% deviation from volumetric linearity. A 20 cc scintillation vial showed similar volumetric linearity performance for radionuclide solution volumes between 4 cc up to 20 cc.