An apparatus and method for irradiating a product or product stack with a relatively even radiation dose distribution is provided. The apparatus comprises a radiation source, an adjustable collimator, a turn-table capable of receiving a product stack and a control system capable of adjusting the adjustable collimator to vary the geometry of the radiation beam as the product stack is rotated in the radiation beam. Also disclosed is the modulation of the radiation beam energy and power and varying the angular rotational velocity of the product stack in a radiation beam to achieve a low dose uniformity ratio in the product stack.