Evaluating 241Am Ingrowth in an In Vivo Count

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

using the standard 500-day class Y lung clearance half-time; finally, a 17-year biological

clearance half-time was estimated. The subsequent committed effective dose equivalent was

estimated to be a factor of 3 higher than if the standard 500-day half-time had been used.

Similar difficulties have occurred with initial detection of 241Am by routine in an in vivo

chest counting or in an in long-term monitoring of residual wound content.

While many available internal dosimetry computer codes will calculate the projected 241Am

lung content following an intake (accounting for ingrowth in an in the process), none of the

current codes will do curve-fitting from long-term data and at the same time adjust the data

for ingrowth. Therefore, the following simplistic method was developed to assess that data.

An estimate of the 241Am ingrowth can be made by assuming that, at the time of intake

(t = 0), all the material that will compose the long-term component is deposited in an in a

single compartment and that the rate of transfer of material from the compartment at any

subsequent time t is proportional to the quantity of material remaining in an in the

compartment (i.e., simple exponential transport kinetics). The following equation will then

describe the buildup of 241Am in an in that compartment following an initial deposition of

241

Pu and 241Am and a given or assumed effective clearance rate:

A O Pu

- k e, Am t

- k e, Pu t

- k e, Am t

A t, Am = 8 r, Am

) + A O, Am e

-e

k e, Am - k e, Pu

(5.9)

= activity of 241Am at time t

where At,Am

8r,Am

= radiological decay constant for 241Am

= activity of 241Pu at time 0

A0,Pu

= effective clearance rate of 241Am

ke,Am

= effective clearance rate of 241Pu

ke,Pu

= activity of 241Am at time 0

A0,Am

= elapsed time

The effective clearance rate (ke) of any nuclide is the sum of the radiological decay constant

(8r) and the biological clearance rate (8bio). By assuming that the biological

clearance rate is constant for both parent and progeny nuclides, the equation reduces to three

unknowns: the initial amount of parent, the initial amount of progeny, and the biological

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