RE: Gamma Level for Cs-137 (second try)

[Mike Bohan <mike.bohan@yale.edu>]

Don,

I tried sending this thru RadSafe to you but I didn't see it posted yet.  I'm
sending it directly to you this time and giving RadSafe a second try.

Later,

Mike Bohan

Mike Bohan wrote:
> 
> Hoy! Hoy! RadSafers,
> 
> I'd like to know the manufacturer of the questionable Cs-137 source.  3M used
> to be one of the primary U.S. suppliers of Cs-137 sources and they changed
> their source calibration procedures back in 1979.  This may be the reason that
> the calibration is off.
> 
> In a 3M memo dated Sept. 27, 1982, 3M stated that sources produced between
> 1966 to August 31, 1979 were calibrated by direct comparison to a Ra-226
> standard, using a Mediac (TM) dose calibrator/well ion chamber.  The apparent
> Cs-137 mCi (with 0.5 mm Platinum filtration) was computed by using the
> respective gamma factors ratio between mgRaeq and Cs-137 mCi.
> 
> In 1979, 3M obtained a Cs-137 standard from NBS and switched to a direct
> comparison method.  They also changed to a CRC-22NB Well ionization dose
> calibration system.  This NBS source was calibrated using the Gamma Factor
> published in NCRP 41 (0.328 mR-m^2/mCi-hr).  (This is the value I currently
> use for my medical use inventory.)
> 
> A detailed description of the 3M calibration procedure is available in AAPM
> monograph, Recent Advances in Brachytherapy Physics Ed.,  D.R. Shearer, Pub.
> Amer. Institute of Physics, NY, 1981, pp. 57-69.
> 
> The 3M memo goes on to state "It may be appropriate to revise the certified
> source strength of any 3M Cs-137 tube sources and needles manufactured and
> shipped prior to Sept. 1, 1979.  If these sources are recalibrated using the
> new procedure, the source strengths would be 5-9% lower."  ... snip ...
> "The change in calibration cannot be stated exactly as either a 5% or 9%
> reduction.  Recent communications from CRP groups indicate that corrections
> range from 5 to 9%.  The most suitable correction can be established by
> returning one or more sources to 3M for recalibration.  For details of
> recalibration fees, shipping procedures and turn-around time, call (800) 328-1671."
> 
> Don't call that number, 3M got out of the source business several years ago.
> They sold the Cs-137 source line to Amersham/Medi-Physics.  You might be able
> to get some further info from them.  Their technical info number is (800) 554-0157.
> 
> If the source is not a 3M source, you might investigate it's original
> calibration with the manufacturer.  They  probably did the same thing,
> switching from a Ra-226 to a Cs-137 standard, around the same time frame.
> 
> Regards to all, still lurking, mostly...
> 
> <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
> Michael J. Bohan, RSO   |  e-mail: mike.bohan@yale.edu
> Yale-New Haven Hospital |    Tele: (203) 688-2950
> Radiological Physics    |     FAX: (203) 737-4252
> 20 York St. - WWW 204   |    As usual, everything I say may be plausibly
> New Haven, CT    06504  |    denied at my employer's convenience ...
> ><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><
> 
> 
> Date: Wed, 17 Feb 1999 12:59:39 -0800
> From: "Mercado, Don" <don.mercado@lmco.com>
> To: "'radsafe@romulus.ehs.uiuc.edu'" <radsafe@romulus.ehs.uiuc.edu>
> Subject: RE: Gamma Level for Cs-137
> Message-ID: <18DDB29BFB91D211A9220000F807E86C8E4E42@emss01m02.ems.lmco.com>
> 
> 7.4 mCi @ 4-1-72. 3.985 mCi today. Dosimeters at 10 cm. 132 mR/hr.
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