Finding energy of gamma ray with spectoscopy

[dhqpwoe]

Hey everyone,
I need to be able to be able to explain how a gamma spectrometer (NaI scintillator) can determine the energy of a given gamma ray (or more particularly, determines the effective dose rate, but that shouldn't be to hard once I figure out how to get the energy). The problem, as far as I can see is that 3 processes govern the device: photoelectric effect, compton effect, and pair production. Only in the photoelectric effect is (close to) the full amount of energy transferred from the gamma ray into the electron. In the compton effect, some particles won't even be measured at all.

Any thoughts?
Thanks!

 

[Bob S]

First, since you mention dose rate, I assume you are interested in radiation surveys, and not in physics determination of gamma ray energies. Second, I assume you have a calibration of the sodium iodide crystal for a specific gamma ray energy, such as cesium-137 (661 KeV). Third, do you have a known incident gamma ray spectrum (or spectra), and do you want to calculate the sodium iodide efficiency for this spectrum? Fourth, will you be operating the sodium iodide in the pulse-height mode, and not the dc cueent mode?

You should read the NIST report on X-ray mass attenualtin coefficients at
http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html
and in particular the mass attenuation coefficient
http://physics.nist.gov/PhysRefData/XrayMassCoef/chap2.html
And the mass-energy attenuation coefficient
http://physics.nist.gov/PhysRefData/XrayMassCoef/chap3.html
Finally, once you have mastered these, look at the table and data table for cesium iodide (sodium iodide is not available)
http://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/cesium.html

Now, you will need to calculate the energy loss for your specific crystal, meaning the leakage of Compton-scattered secondary gamma rays, and for annihilation gammas from pair production. If you are operating in pulse height mode, you should understand the reason for the "backscatter" peak in the cesium-137 spectrum, and be able to calculate the energy and efficiency of this peak relative to the main peak.
I have found chapters 23, 24, and 25 of Evans "The Atomic Nucleus" McGraw Hill (1955) very useful on the cross section theory for photoelectric, Compton, and pair poroduction interactions. Figure 1.6, on page 717, gives a detailed graph of the mass attenuation coefficient in sodium iodide up to 100 MeV, showing the separate contributions of photoproduction, Compton, and pair production.

 

[dhqpwoe]

Thank you Bob. I am about to look over the NIST report and that book. You are correct that I am not interested in the physics determination of gamma ray energies. I am measuring the gamma dose rate of objects (in order to model the gamma dose rate field around them to determine if they are hazardous to health). I have a device that does in situ measurements and displays the dose rate immediately. I need to be able to explain what the device is doing to convert the spectroscopic measurements into dose rate.