This gamma only occurs once per every 100million decays of y-90 with a vastly different half-life of the normal decay. It is not a very reliable tool for measuring Y-90 activity. At least not by using 6CEN. With a gamma abundance that low, you can't get statistically reliable measurements.
The following is part of the answer to a similar question, which you can find here:
http://www.hps.org/publicinformation/ate/q4307.htmlNot that we are a bunch of dummies here, but the experts who answer the questions on the HPS site are better at this than we are.
"Let us assume that you have made the required measurements at the bag surface and have established a beta dose rate of D mrad/hr at the bag surface; we will now apply the principle of ESE to estimate the 90Sr activity. We shall assume further that the bag wall itself contributes negligible attenuation and that 90Y is in secular equilibrium with the 90Sr so that equal activities of both radionuclides are present. The ESE principle, as applied to beta radiation, states that at any point within a uniform volume source, such point being removed from any source boundary by at least the range of the beta radiation being emitted, the beta energy absorbed per unit volume will be equal to the beta energy emitted per unit volume. At the surface of such a volume source the energy absorption rate will be approximately 1/2 of what it is at the point within the volume, since source material will be present only on one side of the dose point (2 π geometry as opposed to 4 π geometry). This energy absorption rate will likely be somewhat high because of the lack of backscatter material outside the bag. If ASr is the 90Sr activity in the bag, and it is distributed uniformly among a mass, m, of material in the bag, the 90Sr activity mass concentration is given by C = ASr/m. We will assume that the units of C are curies/g. 90Sr emits beta radiation with an average beta energy of 0.196 MeV and 90Y emits beta radiation with an average energy of 0.935 MeV. The expected beta dose rate at the bag surface is then
D = C(3.7x1010 s-1 Ci-1)(0.196 MeV + 0.935 MeV)(1.6x10-6 ergs MeV-1)(1 rad/100 ergs-g-1)(3600 s/hr)/2 .
The division by 2 is to account for the reduced dose rate at the surface compared to internally in the bag. For a determined value of D (in rad/hr), we may solve the above expression for C, and from this we can obtain the 90Sr activity, ASr. There is an implicit assumption here that the dose rate in the material in the bag would not be significantly different from the dose rate in soft tissue if the latter replaced the mass of material in the bag. For the low atomic number material you cite, this assumption is reasonable. As an example, let's assume that you measured a beta dose rate of 1.3 rad/hr at contact. Using the above equation we solve for C and obtain C = 1.08x10-6 Ci/g; since ASr = Cm and, for m = 4.54x103 g (10 pounds), we obtain ASr = 4.9x10-3 Ci = 4.9 mCi.
There are other possible approaches that one might adopt if it is known that the activity is localized at discrete spots in the bag, but they require making assumptions as to where the activity is within the bag. If you have access to the beta dose code Varskin 3 (recent version completed to upgrade/replace the earlier Varskin Mod 2), written by Jim Durham and intended for use in estimating skin doses from beta and gamma sources, you can play some games with various source geometries and source activities to evaluate possible ranges of activity that might be present. The code is intended primarily for doing skin dose estimations when activity is on the skin or on protective clothing, but it can be used for other situations as well. The code may be purchased from the Radiation Safety Information Computational Center at Oak Ridge National Laboratory. The Web site to order the code is
http://www-rsicc.ornl.gov/. Select the letter "V" on the Package index line, and it will lead you to the page for ordering the code. I hope this proves helpful to you."
George Chabot, PhD, CHP
