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.
