For a photon to excite an electron of an atom it typically has to have the exact right amount of energy for the transition. When the photon energy is to low it is pretty clear that the transition can not take place. But also a photon with to much energy seems to have a very low probability of interacting (e.g. the answer to this Question says so:

  1. the electron [is] in a different atomic orbital (i.e. it's been excited) and a photon with a different energy.

The probability of (3) is generally negligable

But in szintillator materials the incident photon is, at least as far as I understand, doing exactly that. It is exciting one atom after another, always giving away a fraction of its energy and creating a bunch of secondary photons, until the photon is complety absorbed.

Is my understanding of the process wrong? Or where lies the difference between the two described behaviours?


1 Answer 1


Here is a link that explains this : when atoms are bound in a lattice, the available energy levels are multiplied because of the vibrational and rotational levels of the location of the atoms in the lattice.


An incident photon of energy higher than the energy levels can interact with spill over fields of the molecules (similar to compton scattering) so that its energy can excite an atom/molecule, so that the scattered photon can have , within the width of the energy level, a probability of exciting an electron to a higher level, which will then decay back. With high enough photon energy it can excite multiple states through the secondary products of the scattering:

Suppose a photon of high energy hits the scintillator above. The first hit may move an electron which then can also interact into an avalanche of electrons that finally will excite to the level S1 many molecules (through photons generated by their motion in the spill over fields), that will then fall back and generated the scintillation. It is not "exciting one atom after another, always giving away a fraction of its energy" , except in the sense that the avalanche it generates gets degraded enough for the energy levels to be excited by the secondary particles interacting and giving off photons of appropriate energy. That is how I understand it.


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