Re: Y-90

[Joel L Lazewatsky <joel.lazewatsky@bms.com>]

I looked up the decay schemes in Lederer, Shirley, et al. Table of the Isotopes, 7th edition (a bit out of date, but I don't think Y-90 has changed

lately).  The story is that the gammas are associated with the Y-90 decay product Zr-90. There are two very tiny branching ratios resulting from Y-90

decay that populate the 2.186 MeV (93 fs) and the 1.76 MeV (61 ns) excited states of the Zr-90 nucleus.  Y-90, as you mentioned,  has that metastable

state (i.e. Y-90m) corresponding to the 0.682 MeV energy level with a 3.19 hour half life (there is also a 25 ns-half life level at a lower energy).

Y-90m does not appear as a result of Sr-90 decay, however, as all such decays are to the ground state of Y-90.  Interestingly, a very small fraction

of Y-90m decays to another Zr-90 excited state (the 2.31 MeV level, 809 ms half life).  The seven gammas are shown in the complete scheme for the

excited levels of Zr-90, but they don't all appear to come from Y-90 or Y-90m decay..



All of these branching ratios are so small that I would expect that the number of gammas produced are unlikely to figure in to most shielding

calculations except when the source is hot enough.  I don't have any personal experience with such sources, though and I don't know what "hot enough"

is, exactly.  It would be interesting to hear from someone who has.



Joel Lazewatsky

Any opinions expressed herein are strictly my own and do not necessarily represent those of my employer.



Jack_Earley@RL.GOV wrote:



> I was "somewhat" surprised today to hear a nuc safety rep say that since

> Sr-90 is pure beta, it doesn't need to be considered in a shielding

> calculation. When I mentioned that it's in equilibrium w/ Y-90, which emits

> some significant gammas, I was even more surprised to hear him say Y-90

> doesn't emit gammas. Rather than address it further in the meeting, I pulled

> up Grove's (Kocher) decay program, which showed only two betas for Y-90; no

> gamma. But it then lists Y-90m w/ seven gammas ranging from about 2 keV to

> 0.7 MeV. My day for surprises, I guess--I've always associated Y-90 gammas

> w/ 2+ MeV. Sure enough, my 15th edition of the chart shows 202 keV and 2.2

> and 2.3 MeV gammas. So, since I'm apparently not the brightest bulb in the

> lamp, can someone tell me why there's such a difference?

>

> Jack Earley

> Radiological Engineer

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