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I am performing an experiment to measure the half-value thickness of different materials, and the radioactive source is $^{60}\text{Co}$. To the extent of my knowledge, cobalt-60 is a $\beta$- and $\gamma$-emitter, and the $\beta$-radiation should be nearly completely attenuated by aluminium.

In my case, I am using several layers of aluminium foil (of typical thickness $\sim0.1\text{ mm}$) to achieve this effect. A typical total thickness $d=0.3\text{ mm}$.

Before using the shielding, I have also measured the background rate and the count rate of the source alone; these values are 0.25 counts/second, and 1.937 counts/second.

Now, my issue is that the aluminium shielding appears to have no effect whatsoever on the count rate; in fact, upon introducing the foil shielding, the count rate actually increases. I've checked the foil itself, and there appears to be no difference from background radiation.

What could possibly account for this observation?

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  • $\begingroup$ Here's the energy level diagram. My guess is that you're getting bremsstrahlung x-rays from some of those beta particles. This PDF table from the University of Missouri lists 0.04 cm for aluminium beta shielding of Co-60. OTOH, as Roger says, it's unlikely that you're using an unshielded piece of Co-60, so you should clarify that point. $\endgroup$ – PM 2Ring Mar 26 at 10:29
  • $\begingroup$ @PM2Ring, thanks. W.r.t. bremsstrahlung, according to this source, the effect is less noticeable in low-density materials—I presume aluminium is in this category. Is my presumption incorrect? Is there any empirical relationship between the density of materials and the incidence of bremsstrahlung? I'm not sure of the characteristics of my source—it is a sealed metal tube, something like this. $\endgroup$ – SRSR333 Mar 26 at 11:41
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The $\beta$ rays won't escape from the source encapsulation (unless you've got an open source, which is unlikely for health and safety reasons). So you're just counting the $\gamma$ rays.

These will barely be affected by your aluminium foil. You need centimetres of solid aluminium to attenuate a measurable quantity.

There is also the complication that these $\gamma$s will Compton scatter in the aluminium, which lowers their energy. If your detector happens to be more sensitive for low energy $\gamma$s this can give an increased signal count.

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  • $\begingroup$ Thank you very much! I was initially concerned that the detector was faulty, but we tested with Sr-90 as well (which practically inundated the detector with beta-rays), and a thin layer of aluminium all but completely blocked nearly every last particle. We came to a similar conclusion that the Co-60 source was probably beta-shielded already, shortly after I posed this question. I should also note that this increase was not insignificant: we got a count rate that was around 20% higher than with no aluminium in front of the source. $\endgroup$ – SRSR333 Mar 26 at 11:29
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    $\begingroup$ Also: if the gamma efficiency is small (idk: 10%), and the beta is high (100%?), then shielding 1% of the gamma, each of which makes a secondary will double (1.99) your count rate. $\endgroup$ – JEB Mar 26 at 14:24

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