The Commissioner of Energy and Environmental Protection shall, by regulation, require registration of devices emitting x-rays used for diagnostic or therapeutic purposes by or under the supervision of a person or persons licensed to practice medicine, surgery, chiropractic, naturopathy, dentistry, podiatry or veterinary medicine and surgery, as authorized by law. The commissioner shall charge a registration fee of one hundred ninety dollars biennially for each such device, except that hospitals operated by the state or a municipality shall be exempt from payment of the fee.