Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

The absorption of a single photon of light transfers a very small amount of
energy to the crystal — only enough energy to free a single electron from a
bromide (Br – ) ion. Several successive light photons are required to make a
single grain developable (to produce within it, or on it, a stable latent image).
The passage of an electron through a grain can transmit hundreds of times
more energy than the absorption of a light photon. Even though this energy is
used inefficiently the amount is enough to make the grain developable. In fact,
a photoelectron or compton electron can have a fairly long path through a film
emulsion and can render many grains developable. The number of grains
exposed per photon interaction varies from one (for X-radiation of about 10
keV) to 50 or more (for a 1 MeV photon). Because a grain is completely
exposed by the passage of an energetic electron, all X-ray exposures are, as
far as the individual grain is concerned, extremely short. The actual time that
an electron is within a grain depends on the electron velocity, the grain
dimensions and the squareness of the hit. A time on the order of 10 –13 s is
representative. (In the case of light, the exposure time for a single grain is the
interval between the arrival of the first photon and the arrival of the last
photon required to produce a stable latent image.)
Charlie Chong/ Fion Zhang