The present invention is related to process and an apparatus for irradiating products by means of high energy X-ray beam source (1) in an installation having an irradiation chamber (2), said process comprising the following steps, controlled by controlling means: determining the density of the products to be irradiated, in order to irradiate said products as a stack predetermining, on the basis of said density, the optimal size of the product stack able to optimize the throughput of the installation and/or the dose uniformity ratio (DUR), in the irradiation chamber (2), loading products as a stack onto rotation means (3) located in front of the X-ray beam source (1), while rotating the rotation means around a rotation axis (10), irradiating said products from a lateral side of said product stack.