Answer to Question #429 Submitted to "Ask the Experts"

Q

For a high-energy beta (⁹⁰Y), what are the permissible daily exposure limits (mREM), rate of decay, and recommended/required protective equipment for handling?

A

I assume you are talking about the external radiation exposure from using a discrete source (possibly an ⁹⁰Sr/⁹⁰Y source) or the external radiation exposure from handling a solution. Exposure Limits—Exposure
limits are expressed in terms of annual dose. If you wanted to prorate
this limit equally on a daily basis, you could divide the annual limit
by the number of working days of interest (250 days for 50 five-day
weeks). As you state, ⁹⁰Y is a beta emitter, therefore, the
primary target tissues of concern for external exposure are the skin
and the lens of the eye. Since ⁹⁰Y is a high-energy beta
emitter, bremstrahlung (i.e., x rays) may also be produced so that
deeper tissues of the body may also receive some exposure. The
applicable occupational dose limits are:

• Shallow dose equivalent to the skin: 50 rem/yr (50,000 mrem/yr).
  [Equates to 200 mrem/day if equally divided over 250 working days.]
   


• Lens of the eye: 15 rem/yr (15,000 mrem/yr). [Equates to 60 mrem/day if equally divided over 250 working days.]
   


• Deep dose equivalent: 5 rem/yr (5,000 mrem/yr) [Equates to 20 mrem/day if equally divided over 250 working days.]

Rate of Decay—I'm
not sure what you mean by "rate of decay." The rate of decay at any
point in time of a quantity of radioactive material is expressed by the
activity present. For example, if you have one bequerel (1 Bq) the
activity (or rate of decay) is one atom transforming (or decaying) per
second. In the traditional system of units, if you have one microcurie
(1 uCi), the activity (or rate of decay) is 37,000 atoms transforming
(or decaying) per second. If you are interested in the decrease in
activity level with time, the half-life is a convenient descriptor. For
pure ⁹⁰Y, the half life is 2.67 days—the activity of your
source will have decreased by 50 percent in 2.67 days, another 50
percent (to 25 percent) in another 2.67 days, etc. If, on the other
hand, you really have a ⁹⁰Sr + ⁹⁰Y source, the ⁹⁰Y is constantly being replenished by the beta decay of ⁹⁰Sr and the whole system is governed by the half life of the "parent" ⁹⁰Sr (this is known as a case of radioactive equilibrium). ⁹⁰Sr has a half life of 29.1 yrs. The activity of the ⁹⁰Sr/⁹⁰Y
source decreases by 50 percent in 29.1 yrs, another 50 percent in the
next 29.1 years, etc.; in this case, the change in activity over
periods of a few days will not be perceptible.



Protective Equipment for Handling—It is not possible to give
a specific answer because you did give any particulars about the
situation of use. This depends a lot upon the quantity of radioactive
material, the form, and the use. In general, sources emitting
high-energy betas should be shielded with a low atomic number material
(such as Lucite, polyethylene, or other plastic)—on the order of 1 cm
should be adequate for the 2.27 MeV beta from ⁹⁰Y.
Depending upon the activity it may be necessary of follow this shield
with an x-ray shield (such as lead) to attenuate the unavoidable x rays
produced in the container and/or beta shield. The required thickness of
the x-ray shield will depend upon several factors, including the
activity of the source. If the source is a discrete source, and
depending upon the activity level, some form of handling tool may be
indicated to avoid directly handling the source and to increase the
distance from the source. A small submicrocurie check source can be
handled by the edges of the disk on which it is mounted, while a ⁹⁰Sr/⁹⁰Y medical applicator, which is of higher activity level, is mounted on the end of a handling rod.



Other Circumstances—The above discussion was based on the
assumption of a source handling situation in which the potential
exposure was primarily from external radiation. If, on the other hand,
your situation involves handling or processing liquid solutions or
loose radioactive materials, then the primary concern might be for
contamination, intake of radioactivity, and internal dose. In such a
case the operational limits would be the Annual Limits of Intake
(activity) and the Derived Air Concentrations (activity concentration)
that correspond to the dose limits. Protective equipment and procedures
would be those appropriate for handling unsealed radioactive materials
and controlling contamination. The type and amount would be dictated by
the form and quantity of radioactive material and the particular
operations involved. If we have misinterpreted your situation please
let us know.



Charles E. Roessler, CHP, PhD