Scintillator energy and flux linearity for RbGd/sub 2/Br/sub 7/:Ce, LaCl/sub 3/:Ce, and LaBr/sub 3/:Ce

The development of scintillator detectors made of dense ionic crystals with high light output and fast response times offers to improve radiation detection systems commonly employed in medical imaging. Three such cerium doped materials, rubidium gadolinium bromide (RbGd/sub 2/Br/sub 7/:Ce), lanthanum chloride (LaCl/sub 3/:Ce), and lanthanum bromide (LaBr/sub 3/:Ce) show promise as bright, fast scintillators with good energy resolution for gamma rays. To be useful as gamma-ray detectors for positron emission tomography (PET) and single photon emission computed tomography (SPECT) applications, the signal from these scintillators should have a linear energy response. To be useful as x-ray detectors in computed tomography (CT) applications, the output of these scintillators must be linear over a wide range of flux rates. We have therefor measured the response of these scintillators to different energy gamma-rays with a detector operated in pulse counting mode. We have also measured the range of linear response to diagnostic x-rays, of these materials, with a detector operated in current mode with a sigma-delta analog to digital converter (ADC). We find a linear energy response for all three materials from 60 keV to 662 keV. We find a linear x-ray flux response to five decades of flux for RbGd/sub 2/Br/sub 7/:Ce and four decades of flux for LaCl/sub 3/:Ce and LaBr/sub 3/:Ce. The current in the photodiode is proportional to the x-ray flux in the scintillators provided that direct interactions between x-rays and the photodiode are subtracted. This is accomplished by measuring the photodiode current with and without optical opaque tape between the scintillator and photodiode. Coupling dense scintillators to radiation hard solid state detectors may allow for configurations useful for both low flux gamma-ray and high flux x-ray detection which could improve dual-modality imaging techniques such as combined CT/SPECT and CT/PET.