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I'm taking a course in radiation physics and I've come across the following problem:

A thin beam of alpha particles of intensity $I_0$ and energy $E_0$ impacts in a material. What is the intensity and the spectrum of energy after having travelled a distance $d$ into the material?

The part I am interested in is the one regarding the intensity. I was expecting some kind of attenuation depending on the properties of the material but my professor sent us the solution by email and states that the solution is that the intensity at any point in the interior of the material is $I_0$ and then $0$ outside the material.

Why is this so? Physically I can't understand it. The energy is a function of the position and I though that it should be the same for the intensity.

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  • $\begingroup$ Question that might lead you to the answer: what are the units of intensity used here? Several different definitions are possible, and the correct answer depends critically on which is chosen. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Feb 9, 2016 at 0:50

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Actually, i would think you are rigth, too. Usually, such a Problem is described via the Bethe-Bloch Formula. If you do an alpha-particle experiment, you can even measure the attenuation of alpha-particles in air, for example. So maybe he meant something else and just put it queery?

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  • $\begingroup$ Yes, he used the Bethe-Bloch formula to explain that there is a decrease in energy when the particle travels along the material. But, how can I connect this with the intensity? $\endgroup$
    – S -
    Commented Feb 8, 2016 at 22:40
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    $\begingroup$ And seems to assume no scattering. Rutherford would be upset... $\endgroup$
    – Jon Custer
    Commented Feb 9, 2016 at 0:03
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Intensity in this context often refers to only the number of alpha particles incident on a unit area per unit time. If you assume that the alpha particles only slow down and none of them are stopped completely, then the number passing through any area does not change with depth and the intensity in this sense is unchanged. Naturally the intensity in terms of energy per unit area per unit time would drop as the alpha particles lose energy.

This is one of those unfortunate areas in physics where the same word can mean very different things depending on context.

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  • $\begingroup$ Thanks, it must be so. With that definition I see why it is the case. $\endgroup$
    – S -
    Commented Feb 9, 2016 at 21:12

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