Abstract: A source in which the neutron fluence can be varied makes use of a (.alpha.,n) reaction between a radioactive emitter in powdered form and a target in fluid form. The emitter is immersed in the target fluid between an upper and lower filter within a closed chamber. When the emitter rests on the lower filter in the quiescent state, radiation is limited by self-absorption and the source is in the state of minimum fluence. When the target fluid is circulated by means of a booster, the emitter is fluidized and maximum fluence is achieved. Variable intermediate values can be obtained by regulating the pressure and rate of circulation of the target fluid.