The size of a BF3 tube is not infinite, the effects occur
within the tube and are seen in the produced spectrum. If either
particle (alpha or lithium) strikes the chamber wall a smaller pulse
is produced.
Due to the incoming neutron carrying no appreciable momentum, the
two reaction products must be oppositely directed. Therefore if the
alpha hits the wall the 7Li recoil is likely to deposit its
full energy within the gas, and vice versa. These procedures can be
seen in the inset and compared with points in the graph.
The gas pressure and neutron detector material play a large part in
the extent of the wall effects. The diameter of the tube being
increased also reduces the wall effects.
