Forest Scintillators

Scintillation

the process by which atoms or molecules of a material are given enough energy by an incident particle of radiation to "excite" the system where upon its relaxation to a lower energy state is accomplished through the emission of light.

In general, most materials are capable of scintillating.

It takes about 10^{-9} s ( 1 ns) for an atom to de-excite by giving off light. Atoms can , however, loose there energy by colliding with other atoms. Atoms which collide on time scales less than 1 ns could loose their energy via the collision instead of through the emmission of light.

At standard temperature and pressure, air has a velocity of about

[math]v_{air}^{STP} = 500 m/s[/math]

and a mean free path (distance between collisions) of 0.3 nm

this means the time between air molecule collision is about

[math]t= \frac{0.3 nm}{500 m/s} = 15 \times 10^{-12} sec \lt \lt 1 ns[/math]

This air can loose its energy via collisions instead of through the emmision of light.

The situation for your average solid isn't much better

[math]t_{collision}^{solid} = 0.1 ns[/math]

An atom in a solid can collide with 10 other atoms before loosing energy by emmitting a photon.

Benzene/Toluene

Benzene, and it's less toxic cousin Toluene, is the magic material for producing scintillation light. The electrons on a Benzene ring are so loosely bound (de-localized) that they do not feel atomic collisions.

Vibration state

Having the atom de-excite through the emission of photons is just half of the problem. Getting the photons out of the material to a photon detector is the last half of the problem. Fortunately , there are vibrational states which allows atom to de-excite produce photons which are not able to excite the next Benzene ring the pass by. This makes the material transparent to the scintillation light.