A convenient, inexpensive X-ray unit can be constructed suitable for irradiation of thin, flat samples in the laboratory at dose rates between 104 and 105 rad per minute, depending on sample thickness. Sample holders can be altered as needed, and the X-ray beams can be either horizontal or vertical. Gas can flow over or through the sample during irradiation. Calibration is carried out by a combination of ionization and chemical dosimetry.