HVL of water for Ir-192 ? -Reply

[Charles Willis <CAW@nrc.gov>]

Leif,

Use the "half-value layer" concept with caution.  Usually the HVL is calculated
neglecting buildup and buildup can be important.  For example, for 5 mean free
paths of water with 0.5 MeV gammas, the buildup factor is 18.4.  Also, Ir-192 has a
number of gammas so the HVL is a strong function of how the dose is determined
as well as how the source is configured.  Tap water is another source of
uncertainty; its density is largely determined by temperature and its other shielding
properties can be altered by the impurities in the water.  With these qualifiers,
McGuire (NUREG/BR-0024)  gives a HVL for Ir-192 as 1.75 inches on concrete,
which is about equivalent to 5 inches of water.  On the other hand, the bulk of the
gamma energy is in the 300 to 470 keV range, for which the mass attenuation
coefficient (NUREG/CR-5740) is about 0.1 cm^2/gm, which corresponds to a HVL
of about 3 inches of water neglecting buildup or about 6 inches including buildup. 
Thus your 6 inches of water value is in the right range, but caution in its use is
suggested.

Charlie Willis
caw@nrc.gov