Beta versus Gamma attenuation


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Message boards : Science : Beta versus Gamma attenuation

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Craig Daniels
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Message 1993 - Posted: 1 Sep 2013 | 4:54:05 UTC

* I think I'm good on gamma --it's like light (photons). If I shine light through a 0.3d (50%) neutral density filter --there are half as many photons on the other side, but the color (energy) doesn't change. By extension, a "half value layer" of lead or concrete will reduce my Geiger CPMs by half, but I could still recognize the source's energy signature with gamma spectrometry --right? (Aside from a bit of line spreading --and that G-M tubes miss 98% of the gammas.)

* So okay: can someone tell me about a beam of beta --plowing through a half value layer (thickness) of aluminum (and aside from the bremsstrahlung/fluorescence effects).

Specific question: do I simply lose half the beta density, or does approximately the same beta population per square centimeter now have half the original KeV energy?

Thanks,

Craig

exsafs
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Message 2000 - Posted: 4 Sep 2013 | 8:03:59 UTC - in response to Message 1993.
Last modified: 4 Sep 2013 | 8:05:40 UTC

Hello Craig,

i will try to answer your question. Unlike gammas, beta and alpha particles have a mass, and therefore, in a classical way, loose energy by collisions with the matrix material. So, a beam of mono-energetic betas would constantly loose energy by traveling through aluminum roughly staying constant in population. There will be also a spreading of the energy and the path around a mean value. Nevertheless, such term as "half value layer" does only apply to gamma radiation.

Certainly, there is a stopping range (complete energy loss) for each particle of specific energy in a desired material. However, the energy loss per traveled distance is not linear (see Bragg peak). For a deeper understanding you could try out a freeware program called SRIM (stopping & range of ions in matter), where simulations of particle behavior in many different matrixes may be performed.

Keep also in mind that beta emitters never emit mono-energetic particles.

Craig Daniels
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Message 2001 - Posted: 4 Sep 2013 | 16:50:55 UTC - in response to Message 2000.

Thank you, "exsafs". Your explanation, has helped me understand what's going on when the beta count unexpectedly falls off to nothing.

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