There are two ways of doing it that I'm aware of.

Firstly, you can place a big line of NEUT, set the temperature, vx and vy with console, then using PROP or console, set X of all the NEUT to a value (like, say, 200), which will make all of that NEUT overlap at that point.

Secondly, what I consider the lazy but easier way- put some DEUT with a very high life, and some NEUT so it explodes. Pause after a couple frames when it's a big ball of high temperature NEUT, then use the console to delete any remaining DEUT and anything else, and set the vx and vy of the NEUT.

To make it move straight down at one pixel per frame (approximately), vx should be 0, and vy should be 1