VADER: a variable dose-rate external (137)Cs irradiator for internal emitter and low dose rate studies

In the long term, (137)Cs is probably the most biologically important agent released in many accidental (or malicious) radiation disasters. It can enter the food chain, and be consumed, or, if present in the environment (e.g. from fallout), can provide external irradiation over prolonged times. In either case, due to the high penetration of the energetic γ rays emitted by (137)Cs, the individual will be exposed to a low dose rate, uniform, whole body, irradiation. The VADER (VAriable Dose-rate External (137)Cs irradiatoR) allows modeling these exposures, bypassing many of the problems inherent in internal emitter studies. Making use of discarded (137)Cs brachytherapy seeds, the VADER can provide varying low dose rate irradiations at dose rates of 0.1 to 1.2 Gy/day. The VADER includes a mouse “hotel”, designed to allow long term simultaneous residency of up to 15 mice. Two source platters containing ~ 250 mCi each of (137)Cs brachytherapy seeds are mounted above and below the “hotel” and can be moved under computer control to provide constant low dose rate or a varying dose rate mimicking (137)Cs biokinetics in mouse or man. We present the VADER design and characterization of its performance over 18 months of use.