This questions is about radioactive Isotopes. My current problem is, how can i "pick" isotopes, that only emit a specific type of radiation.

Here is an explanation of my problem:

Cobalt-60 (Co-60) is known as a "gamma" source. It appears on youtube-videos that only emits gamma ray, since beta sensetive Geiger Mueller Tubes do not give a higher count than only gamma sensetive tubes.

So technically Co-60 is a gamma emitter for me

When i look at the table of nucleides here: http://www.nndc.bnl.gov/chart/

And pick Co-60 (1500days Half life)


I displays that there are betas and gammas Beta- : 95.77kev, 99.88% 625.87kev, 0.12%

Gammas 1173,228 99.8% 1332,492 99.8%

So either i'm not understanding the file, or the betas get "lost" anyhow. I tend to the first one.

Further Information: The source is a plastic disk, it should not shield the betas, since a Sr-90 source of the same construction lights up the Geiger Counter pretty much.

I have noticed there are A LOT of isotopes, and A LOT of them look like Co-60 in the files (having both beta/gamma). How can i find/identify the "real" radiation type of the isotopes?


2 Answers 2


A lot of sealed Co-60 gamma sources have a thin sheet of aluminium above the pellet of radioactive material to absorb the betas.
The betas have a lower energy than the gammas.
Sealed Stronium-90 have a plastic cap above them.

enter image description here

The mica window at the front of the GM-tube with a thickness of about $5\,\mu \rm m$ absorbs some of the betas given that the energy of the betas ranges from "zero" up to $0.31 \, \rm MeV$.

The problem with trying to differentiate directly between betas and gammas is that the design of the GM-tube is such that an ionising radiation of any type will produce approximately the same size output pulse.

Even if the betas originate from an uncovered Co-60 source a GM-tube is often used in the "broadside" position to try and make the GM-tube more efficient at detecting gammas and at the same time the betas would be absorbed by the $\thicksim 75 \,\mu\rm m$ steel cylinder.

If you try to resort to seeing if, after the absorption of some of the radiation by aluminium, the count rate from the GM_counter differs as to whether the GM-tube is head on (detecting beta? and gamma from the source) and broadside (detecting only? gamma from the source) there is a significant problem. When betas slow down they emit bremsstrahlung (braking radiation) which the GM-tube will also detect.

The main problem is that gammas are produced when an excited nucleus, produced as a result of alpha or beta emission, loses energy to go (towards) its ground state.
So gamma sources are going to produce alphas and betas and the best you can do is to "eliminate" the alphas or betas; absorption being one way.

  • 1
    $\begingroup$ The 'feature' of running in saturation that makes these devices relatively easy ( don't need very low noise amplifies), is also the one that results in no appreciable PID. Every pulse is saturated, so every pulse is the same. $\endgroup$ Mar 25, 2017 at 14:56
  • $\begingroup$ So this means, even if my Co-60 would emit beta, i could easely fix this with shielding? I want three sources to check the performance of different GM-Tubes, so i need straight gamma beta and alpha sources that only emit one type. All Co-60 Sources are sold as "gamma" so i guess they have aluminium in them.. $\endgroup$
    – sgt_johnny
    Mar 25, 2017 at 20:34

Sr-90 is a solid beta emitter, 100% beta decay into Y-90. Y-90 is also pretty good with only .011% leading to a gamma.

Cs-137 is a solid gamma emitter at 95% with one easy to recognize energy peak. Just use simple shielding to block the betas. No isotope emits only gammas. Gamma radiation is caused by electron de-excitation. The excitation is caused by alpha, beta, proton or neutron decay.

The ideal alpha source for what you want is Pu-239. With 70% at 5.157 MeV it will give you a good knee on the curve to determine sensitivity. But it's a special nuclear material so very difficult to come by. The substitute is Am-241, but it has 84% at 5.476 MeV so your knee is going to sit a little higher and your tube won't be as sensitive to the lower energies. Th-232 is also an option, it has 78% at 4 MeV which would be a better option than Pu-239 but it also has a half life of about 14 billion years so you're going to need a crap ton of it to get anywhere, so not ideal.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.