Re: Co-57 Gamma Constant

[daleboyce@charter.net]

Roy,



The answer to your question is not simple, and it seems to crop up every

couple of years. There was a long thread back in the mid to late 90's, and

another around 2001.  I tried to find it in the archives, but got about 6000

hits.  To save time I'll summarize from memory.



The question to answer your question is which dose constant do you want? Do

you want a dose constant that gives you:



Entrance Dose (Calculate from mass energy absorption coefficients in tissue,

and branches)

    Igamma = 16.98 mu-en-rho * E(MeV) * BR (sum over all gammas)



Mean Absorbed Dose



Deep Dose Equivalent



Shallow Dose Equivalent



Peak Dose at Depth  ORNL/RSIC-45 1981 and ANSI/ANSI-6.1.1 (1977?)

(Withdrawn)

(I don't have a copy handy to check the date) that was used for the Second

Edition of the RHH



Whole Body Dose Equivalent or simply DOSE Equivalent ANSI/ANS-6.1.1 (1989?

ditto)



Air Kerma



Exposure



(Place your favorite unit of measure here)



and so on.



These quantities diverge at low energies with factors of two common, and

factors of four not uncommon depending on the quantities being compared. I

personally like to use the entrance dose.  It gives pretty good agreement

with Deep Dose Equivalent (the regulatory important quantity), and converges

quickly at higher energies with Dose Equivalent. It also can easily be

corrected to DDE and SDE at low energies using point kernel absorption

corrections.



Above 70 kev it is the source of the "rule of thumb" in the RHH which I

round slightly to: sum the energies of the gamma's times their branching

ratios, and then divide by two.  This gives the dose constant in rad/hr/Ci @

1 meter if you use mev and  mrad/hr/Ci @ 1 meter if you use kev.



Below 70 kev "mu-en-rho * E" is not approximately constant and the ROT falls

apart.  Mu-en-rho rises sharply at lower energies.  It is at low energies

that the the different quantities diverge most.  Also, tables tend to use 10

kev as a cut off.  If they don't results vary even more. Also use caution

with tables as they may or may not include equilibrium daughter decay.



If I use this to calculate for Co57 I get 0.082 rad/hr/Ci @ 1 meter. I used

the formula and not the ROT and used only the 14, 122, and 136 lines.  Note

that there are 6-7 kev x-rays that sum to about a 54% branch.  If these were

included the constant would be much higher.  Also, I did not correct for

absorption to get a true DDE. However I suspect the difference for the

constant you find for Co-57 stems from the ORNL/RSIC-45 using peak dose at

depth, unless 45-R1 was changed substantially from the 1981 version.



Dale





----- Original Message ----- 

From: "Roy A. Parker" <royaparker@COMPUSERVE.COM>

To: "RADSAFE" <radsafe@list.Vanderbilt.Edu>

Sent: Monday, May 17, 2004 12:44 PM

Subject: Co-57 Gamma Constant





> There seems to be two prevailing Gamma Constants in the literature for

> Co-57: 0.15 RHM/Ci and 0.09 RHM/Ci.  The first Gamma Constant, 0.15

RHM/Ci,

> seems to be derived from ORNL/RSIC-45/R1 Specific Gamma-Ray Dose Constants

> for Nuclides Import to Dosimetry and Radiological Assessment.  The second,

> 0.09 RHM/Ci, seems to go back to Jaeger, R. G., et al., Engineering

> Compendium on Radiation Shielding, Vol. 1, (New York: Springer-Verlag,

> 1968), pp. 21-30.  I am trying to understand the difference between the

two

> values.  The ORNL/RSIC value of 0.15 RHM/Ci is strictly computational.  I

do

> not have a copy of Jaeger.  Is his value based on actual measurement and

is

> it a lower value because of source size (self absorption) and/or

attenuation

> because of encapsulation or covering on the source?

>

> Roy A. Parker, Ph.D.

> roy@royparker.org

>

>

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