layer 23 and a thermal neutron absorbing layer 26 is adhesively bonded to the x ray and gamma ray absorbing material 24 such that the aluminum layer 23 forms the inside surface of the collimator walls the x ray and gamma ray absorbing material the intermediate layer and the thermal neutron absorbing layer the outer surface of the walls in thermal neutron radiography applications suitable reduction of background x ray gamma ray and thermal neutral radiation is possible by sandwiching the x ray and gamma ray absorbing material 24 between the aluminum layer 23 and the thermal neutron absorbing layer 26 a layer of lead approximately 0 3125 5 16 inches thick interposed between the 0 063 inch aluminum layer and a 0 04 inch cadmium layer provides optimal reduction of background radiation for up to about 14 mev neutron sources and yields a high quality neutron radiograph on standard x ray film while the use of collimator tube 10 is not restricted to thermal neutral radiography applications its greatest utility so far has been found in thermal neutron radiography particularly in portable neutron ray inspection heads such a portable on off neutron ray inspection system is disclosed in a copending application of william e dance and harry n bumgardner jr ser no 118 150 filed feb 4 1980 now u s pat no 4 300 054 entitled 34 directionally positionable neutron beam 34 fig2 shows schematically the use of the collimator tube 10 in such a system an ion accelerator neutron tube 30 is energized by selectively.