Copyright (c) 1978 Robert Kraus

Neutron is played on a 5x5 square board, with the following setup:

SOLDIERS - A soldier moves in a straight line (orthogonal or diagonal). 
It must move as far as it can; so it continues until it finds an edge or another stone. 
There is no capturing.
NEUTRON - The neutron moves like the soldier.
TURNS - Each player first moves the neutron, then one of his soldiers.
On the first turn, the first player does not move the neutron.
GOAL - Wins the player that gets the neutron on his first row (i.e., the row where his own soldiers start) or stalemate the opponent, so that he cannot finish his turn.

An example

Black has a winning move. By playing Ne3-e4, c5-e3, White will loose in two moves. The Neutron can only go to b4 or c2, and will be unable to defend the Black neutron move to both a5 and c5.

This game is included on the game package of Zillions. According to it, the game was analyzed and considered a win for the 1st player. A possible solution is to increase the board size (perhaps 7x7).  

Mark Owen and Julian Richardson suggested a reversed version of Neutron, called CoNeutron, in which the object is to get the Neutron to your opponent's first row. Another variants implemented by Robert Kraus, focus on restricting the mobility of the soldiers, by not allowing them to move to the opponent's first row (restricted variant) or not allowing them to move to both first rows (double restricted variant).

[Jan 2011] There's a new application for iPhone and iPad by Doug Sartori to play Neutron (check the webpage).