Flickr member scenery_and_fish found a Kowa 65mm f/0.75 x-ray lens, and mounted it to a Nikon D90 by using macro extension tubes and epoxy. The lens is fixed focus, lacks an iris, and is one beastly piece of glass.
http://www.petapixel.com/2010/11/27/kowa-65mm-f0-75-x-ray-lens-mounted-on-a-nikon-d90/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed:+PetaPixel+(PetaPixel)
that's a beast, alright!