After seeing @petrol's "solid" photons I knew immediately what caused it, so I made a block of it for you guys :3
To make these enter the following into console. Replace elem with phot for solid photons, and neut for slow moving neutrons.
set vx elem 0
set vy elem 0

Of course, you can make them last forever by entering(without quotes) "set life elem 0".

petrol
No problem, in case you haven't figured out though, try pouring water into it, or putting plasma through it... It basically acts like an atmosphere, allowing elements to pass through, while maintaining heat :D MANY FORESEEABLE AND UNFORSEEABLE USES ARE ABOUND!