Gamma spec and U-235 concentration(enrichment)

[97txaggie]

Is it possible to determine accurately? We have some mysterious samples, and I seem to be getting higher numbers than I would anticipate!
Thanks for any advice. (Just so you know, I'm having mass spec. done as we speak to see just how far off the gamma spec is, if at all.

Any input would be appreciated!

Thanks,
J

[scooter]

What type of system and software are you using?

[97txaggie]

We have an older Ortec Ace MCA ~30% efficient HPGe. Was bought prior to my coming here, so I'm not 100% sure on those stats, but close enough.  I would have thought it was more accurate...but analyzing the output, I'm hoping its not!  

OH yeah, software is GammaVision V4.1

[atomicarcheologist]

Is it possible to determine accurately? We have some mysterious samples, and I seem to be getting higher numbers than I would anticipate!
Thanks for any advice. (Just so you know, I'm having mass spec. done as we speak to see just how far off the gamma spec is, if at all.

What are you trying to determine?  The enrichment concentration?  Whether or not you have 235?  FYI, if you have 235 to military use levels, you will get a big return on your gamma spec as opposed to your field gamma reading.

[tonymartino]

I was in a similar situation using Ortec as well and was having problems. I ended up calling Canberra who has set me up with a new system. The guy from there was very helpful and is just right up the road from me. 

[thenukeman]

We would first check B25 boxes with NaI to see if we got above twice background.  We then would bring in the HPGE to check for U235.  The results would be written with a caveat of + or - 90 percent.  So 10 grams could be 1 or 19 grams of U235. The geometry was a big factor that made this caveat.  I guess if most was on the one side closest to where you shot you would overestimate and if it was further away or shielded in the middle you would have more than your estimate.  We would have to scavenge boxes to get the u235 below acceptable levels and reshoot.

[tonymartino]

It sounds like an ISOCS system would work much better for your application. You can put in the geometry that is being measured and come up with better measurement. These systems are used throughout our industry and several articles have been written backing their use.

http://www.p2pays.org/ref/09/08073.pdf

[raymcginnis]

Well, it really depends on what you are looking at.  If you know that your sample only contains enriched uranium, then gamma spec can be an OK indicator.  Natural uranium or normal uranium has Ra-226 which also has a peak at around 186 keV, so it interferes with the U-235.  If you are looking at soil with higher than expected numbers, then forget it.  There is no way the U-235 number for the elevated 186 keV peak can be a good indicator, unless your background was taken with a clean soil sample of the same type and same sample density, or it ends up being very high levels of enriched uranium above soil background.  Processed uranium has all of the daughters removed.  Most people with experience never use gamma spec when accuracy is required, especially in your case, where you have an unknown.  You did the right thing by sending it out for lab analysis.  Just my opinion though.

If you do have soil or concrete in your sample, once you get your data back from the lab, you can plug your U-234 and U-238 results into the uranium enrichment calculator at http://www.radprocalculator.com/UraniumEnrichment.aspx and that can be an indicator as to whether you have normal, enriched or depleted uranium in your soil or concrete sample.  If you know there is no normal U in your sample, then it can give you the enrichment of the U, if any is found.