The Sprytron, otherwise known as the Vacuum Krytron, is a device of very similar performance to the Krytron. Though it generally exhibits a somewhat lower time delay after triggering. The Sprytron is designed for use in environments were high levels of radiation are present. The sprytron is a hard vacuum 'filled' device unlike the krytron which, as noted above contains a low pressure gas.
The Sprytron has only three leads, (no keep alive), but is otherwise very similar in outward construction to the Krytron. The reason for the use of a vacuum filling is almost certainly that there is no medium present for radiation from the external environment to ionize (such ionization could promote spurious triggering effects.)
The Sprytron requires a more powerful trigger pulse than the Krytron, as the device works by forming an arc directly between the trigger and the cathode, which causes the tube to breakdown (go into conduction) by disrupting the field between the anode and cathode.
A Sprytron is triggered in a similar fashion to Krytron, but as mentioned requires a higher energy trigger pulse and therefore a more powerful trigger transformer etc. EG&G makes trigger transformers optimized for use with their various tubes, and also make devices named Krytron-Pacs which incorporate a gas filled krytron and trigger transformer in a single housing.
One final point. It is interesting to note that in application circuits (references 1 and 4) the sprytron is always shown directly switching a load (an Exploding bridge Wire.) and a Krytron is always shown triggering a secondary device such as a triggered spark gap.