  Beyond prompt irradiation, Cs-137 is likely to be the most biologically
important agent released in many accidental (or malicious) radiation exposure
scenarios. Cs-137 either can enter the food chain or be consumed or if present
in the environment (e.g. fallout) can provide external irradiation. In either
case, due to the high penetration of the 662 keV gamma rays emitted by Cs-137,
the individual will be exposed to a uniform, whole body, irradiation at low
dose rates.
  The VADER (VAriable Dose-rate External Cs-137 irradiatoR) allows modeling
these exposures, bypassing many of the problems inherent in internal emitter
studies. Making use of discarded Cs-137 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
Cs-137 brachytherapy seeds are mounted above and below the cage and can be
moved under computer control to provide constant low dose rate or a varying
dose rate mimicking Cs biokinetics in mouse or man. We present the VADER design
and characterization of its performance.
