Optimizing ROI for Photon Attenuation Experiment

[cxh]

Hi, I'm using a software program called Maestro for an experiment on photon attenuation. We observed that different region of interets (ROI) gives different mass attenuation coefficients. Is there a way to determine the best ROI besides choosing it so that it's closest to the accepted coefficient?

 

[mormonator_rm]

Wish I could help you, but my knowledge in the field of particles passing through barriers and such is limited. I looked at some graphs and noticed that the attenuation lengths do change a great deal with the energy of the photons used and the density of the material (afterall, the coefficient is related to the density of the target). Looks like all the absorption lengths become fairly constant beyond 10 MeV, though, for their respective elements. The PDG has a couple links that show the experimental behavior of attenuation lengths for data between 30 eV to 100 GeV for all of the elements. Check out http://www-cxro.lbl.gov/optical_constants (Courtesy of LBNL; this is data from 30 eV to 1 keV) and http://physics.nist.gov/PhysRefData (Courtesy of NIST; this is data from 1 keV to 100 GeV). I hope this is helpful, although you may already have it.

 

[Andy1978]

Hello. What you have to remember is trust your data. Choosing the correct ROI's to fit what you expect your coefficients to be is a backward way of doing things and it not experimental physics. When you identify your peaks and highlight them, whatever that data provides you with you must take as your value, even if it is wrong.

Now, if it is wrong, or let's say, not close to the accepted value, you need to take into consideration your sources of error, which you haven't mentioned? I presume you calibrated MAESTRO? Check how many sources you used, i normally use 137Cs 60Co and 57Co along with 152Eu which provides a great range of energies and therefore a good calibration. There will be errors on the calibration and there will be signs of non-linearity, either integral or differential and these need to be combined in order for you to get good error bars, as well as systematic and ststistical errors which are a part of any counting experiment. If you have a value, and an error and you can hypothesize WHY your coefficients may be not as close to accepted as you would like, you will not lose marks when having your report assessed. If, however, you fudge your results and define your ROI's to best match what you would like to see, then you will never make it as an experimentalist!

TRUST YOUR DATA AND EXPLAIN SOURCES OF ERROR! that is the best advice i can give you. Good luck. Andy

 

[jtbell]

I guess you didn't notice that the question was more than six years old.

 

[Andy1978]

yes i did but i felt it worth answering in case somebody else was using the forum for information on the subject. Congratulations for being a smart arse, you can be very proud of yourself, now STFU and be helpful. Regards. Andy

 

[Tinrac]

And some actually did find it useful. Thanks

 

[Bob S]

There are actually two distinct things to consider in photon attenuation:

1) The attenuation coefficient depends on photon energy, due to the energy dependence of the Compton, pair production, and deep-core-photoejection cross sections. So each photon has to be included and tracked separately in the attenuation calculation.

2) The attenuation coefficients of the primary incident photons are different than the energy attenuation coefficients, which include tracking the energy of the Compton and pair production secondaries.

Bob S

 

[Rajini]

Hi Bob,
in NIST website you get two attn. coef.s
Coherent and incoherent..
any idea about it?
For e.g., if you want to shield some radioactive source with lead...which coef. one should consider?
thanks

 

[Bob S]

In the NIST section on x-ray attenuation

http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html

Look at the discussion in

http://physics.nist.gov/PhysRefData/XrayMassCoef/chap3.html

Bob S