Re: Fe-55

[mem6@juno.com (Milton E McLain)]

On Tue, 19 Aug 97 13:11:38 -0500 Al Castellane <castella@ohsu.edu>
writes:
> ---------, etc.
>How would one detect this. Gamma spec, LSC? Could you use a NaI or G-M 
>survey
>instrument in the lab to survey? Any help would be greatly 
>appreciated.
>
>Al Castellane
>Oregon Health Sciences University
>castella@ohsu.edu
>
---------------------------
Al,

The decay mode for Fe-55 is electron capture which results in emission of
characteristic, "K capture", x-rays of Mn.  My recollection is that the
x-ray photon energy is 5.8 KeV.  A "pre-counting", radiochemical
separation is usually necessary before you can efficiently cont these low
energy photons.  Traditionally, following addition of mg. quantity of
stable iron carrier, a radiochemical separation was performed which
included precipitation and anion exchange separation of iron/chloride
anionic complex.  Finally, the separated iron was electroplated from
solution onto a metal disk.  The low-energy x-ray photons were counted
with a thin (few mm thick) NaI(Tl) scintillator.  If you want more
details for this procedure, I refer you to the manual of the DOE's
Environmental Measurements Laboratory (EML) which goes by the historical
designation of HASL-300, now more correctly as EML-300.

More recently, Dave McCurdy (Yankee Atomic), et al. have improved the
determination of Fe-55 by counting the 5.8 keV x-rays by LSC.  This is
pretty straight forward since these low-energy photons are essentially
absorbed completely in a 20 mL LSC vial and therefore deposit basically
the same energy per event as the 6 keV H-3 beta particle.  Actually, you
deposit significantly more energy in the LSC counting of the Fe-55 photon
since the average H-3 beta energy is about 2 keV.  Anyway, no problem,
LSC easily counts the Fe-55 photons.  No special cocktail is needed.   If
your LSC instrument provides for viewing the LSC pulse height
distributions (seeing the LS spectrum, it will provide a "picture"
showing a single, well-resolved full-energy peak, not unlike that
obtained with a NaI(Tl) scintillation detector.  This allows one to avoid
a significant part of the radiochemical separation needed to electroplate
the separated iron from the sample.

My recollection is that Dave McCurdy was able to have his revised,  LSC
Fe-55 procedure accepted by the ASTM.  If you have access to the ASTM
procedures, you can find the details there.  Otherwise, you might contact
Dave at: mccurdy@yankee.com for additional information.

Milton McLain, Professor Emeritus
Texas A&M Univ.
mem6@juno.com