Gamma Spectroscopy for Estimating Radiological Risk in Rocks

[meshac A]

Am currently doing an undergraduate project work on 'estimation of radiological risk pose by U-238, th-232 and K-40 in rock samples' . The instrument I'll use are NaI(TI) crystal for scintilation were the gamma rays from the source(my rock samples) interact with the crystal through photoelectric effect, compton effect & pair production thus releasing photoelectrons, A photomultiplier tube (PMT) optically couple to the detector- to energize the photoelectrons from the detector, multichannel analyzer (MCA) plus an amplify with A-D converter, and a computer with the software winDAS installed to display the spectrum in histogram with count/second on the vertical axis and channel(energy in MeV) on horizontal axis.
The radionuclides present in the sample is identified if you trace it's energy on the spectrum, eg K-40 who's E is 1.460MeV will be traced.
Now, once the spectra corresponding to K-40 energy is identified how do i CALCULATE the ACTIVITY(Bq) of this nuclide and that of others to be identified?.
Thanks in advance!

 

[mfb]

You (well, your software) can count the number of detected gamma rays.
Together with the geometry of the setup (which fraction of photons reaches your detector?), radiation length calculations (which fraction interacts in your detector?), background rates, effective deadtimes and some additional things I forgot, you can find the activity of the sample.

 

[meshac A]

Ok, can you give me the explicit expression that bind this informations as a whole?- that will allow me compute the activity

 

[mfb]

It depends on your setup, there is no way to do this calibration if you do not have access to the setup itself.
Do you have a source with known activity? This will make the calibration easier.

 

[meshac A]

Thanks, I'll carry out the experiment tommorow, I do not have a source with known activity, is there no other way to caliberate without a source? And how does finding the activity depends on my set-up?

 

[mfb]

And how does finding the activity depends on my set-up?

By the effects I mentioned. The geometry, the efficiency of the detector and so on.

is there no other way to caliberate without a source?

It is possible to get an estimate, but it will need a lot of work to do that properly.

If you work with radioactive samples, it should be possible to get a source with known activity.

 

[meshac A]

Thanks a lot, I'll get back to you when i start the analysis.

 

[meshac A]

i've been delaying the experiment because I've got no standard source, can i go on without it?

 

[mfb]

The activity measurement will get larger uncertainties (and much more work), but it is possible to find some estimate.

 

[meshac A]

ok, what is peak area?peak intensity?efficiency of detector?branching ratio of the source? Peak centroid?and FWHM? How do they relate with the activity concentration of a nuclide at that peak?..i know my questions are plenty ,i just need your help!

 

[mfb]

What do you mean with peak area and peak intensity? Peak with respect to what?

efficiency of detector?

(detected particles) / (particles going though the detector)

branching ratio of the source?

I know branching ratios of particles... the fraction of particles decaying to some specific set of products.

Peak centroid?

Context?

and FWHM?

Wikipedia helps - not only there, but also at the other questions.

 

[meshac A]

i'll try it, thanks a lot

 

[daveb]

A good way to create a makeshift standard for K-40 is to get some of the salt substitute (which is KCl). Measure out a known mass, and with the isotopic abundance value you should be able to get the mass of K-40, which will give you the activity. It isn't exact because of impurities, but it's certainly a better estimate than anything else I can think of.