Uranium glass I presume?
Once I eventually manage to save up enough money to build the cyclotron/synchrotron I have my heart set on, I'm definitely going to have to get hold of some 238U and some beryllium to set it up as a neutron source, using the 'tron to produce an alpha particle beam, make some thin carbon plates for use as a moderator to slow the neutrons liberated by bombarding the beryllium down to thermal energies where they can be then captured by the depleted uranium targets.
(aiming of course for 239Pu and neptunium