Absorption of beta particles by lead

[Elbow_Patches]

TL;DR Summary

What thickness of lead would you expect a strontium-90 beta source to penetrate?

Hello everyone,

We conducted an experiment with a strontium-90 source and some different thicknesses of lead.
With 2.1mm of lead the count rate (corrected for the background) was 0.69 counts per second,
3.0mm 19.7cps
6.8mm 15.4cps
13.8mm 10.0 cps

This would indicate that the thicker the lead, the more the absorption, the lower the count. Which makes sense, but shouldn't the 2.1mm have been thick enough to stop all beta particles from passing through? According to some data I've found (figure 1) for a beta particle to penetrate 1mm of lead would require an energy of about 1-2 MeV, when you'd expect an Sr-90 source to emit them with about 0.5MeV.

So, what do you think? Are these results normal and to be expected (not the opinion of others in my department)? Has the strontium decayed into something that emits gamma? Has the strontium decayed into yttrium which then emits gamma with about 2.3MeV (so according to figure 1 it would penetrate through 1-2mm of lead)? Is the beta striking the lead and producing bremsstrahlung radiation? Something else I haven't thought of?

Any help, suggestions or musings are welcome. :)

 

[phyzguy]

Perhaps all of the lead thicknesses stopped all of the beta particles and all of your measurements are just background. What was the count rate with no lead?

 

[jtbell]

Summary:: What thickness of lead would you expect a strontium-90 beta source to penetrate?

With 2.1mm of lead the count rate (corrected for the background) was 0.69 counts per second,
3.0mm 19.7cps

I suspect a typographical error here. Surely increasing the thickness of lead by nearly 50% doesn’t increase the count rate by more than 20x.

 

[mfb]

Bremsstrahlung doesn't get far in lead.

Y-90 has a half life of just 64 hours, if your source isn't brand new then it will be in equilibrium and you get one Y-90 decay per Sr-90 decay. It is mainly a beta emitter at up to 2.3 MeV (to the stable Zr-90), the emission of a photon is very rare.

Something went wrong with your count rates if the weakest shielding produced by far the lowest count rate.

 

[gleem]

Bremsstrahlung? A few mm of lead will not attenuate 0.5 to 2 MeV Brems radiation very much. The attenuation of 0.5 MeV x-rays of 1mm Pb is only about 0.8. There even might be some characteristic x-rays too in the range 70 -90 KeV.

As with any attenuation measurement the geometry of the setup, source to attenuator distance, collimation of the beam and attenuator to detector distance, is important as well as the detector sensitivity to various types of radiation.

 

[Vanadium 50]

Surely increasing the thickness of lead by nearly 50% doesn’t increase the count rate by more than 20x.

I think we should suspend speculation until the OP posts the correct numbers. Explaining the wrong thing may be more confusing than helpful.