Examine the thought experiment involving alpha particle transmission depicted in the following drawing.
A source of monoenergetic alpha particles is directed at a particle detector through an absorber. We know the initially monoenergetic spectrum will demonstrate energy straggling, but the direction of the particles should not be altered and they should ALL make it to the detector, provided the thickness of the absorber is less than the thickness required to stop the slowest of the distribution. This is shown as the constant horizontal line in the graph at the bottom of the drawing. Once the absorber thickness is such that some of the slower particles are stopped, the width of the energy straggling curve presents itself in the slope of the fall-off from constant particle number detected down to zero.
The mean range is the range at which the number of particles detected is one-half the original value.
The same experiment can be performed with a beam of electrons (or a beta emitter). In this case, however, the electrons are more vulnerable to wide-angle scattering even in the thinnest of foils - so wide an angle that the scattered electrons will not be detected. The result is an immediate falloff in the number of electrons detected as a function of absorber thickness as demonstrated is this drawing:
For example, a tritium particle with z=1 will have 1/4 the stopping power of a He-3 particle with z=2.
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