Re: Y-90

["Franz Schoenhofer" <franz.schoenhofer@CHELLO.AT>]

-----Ursprüngliche Nachricht-----

Von: Jack_Earley@RL.GOV <Jack_Earley@RL.GOV>

An: radsafe@list.vanderbilt.edu <radsafe@list.vanderbilt.edu>

Datum: Dienstag, 12. Februar 2002 21:47

Betreff: Y-90





>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,



I am not the brightest one in nuclear physics either (0.1 W), but I have

worked as a radiochemist a lot with the analysis of Sr-90. Therefore I think

I can answer your question.



First of all: yes, Sr-90 is a pure beta-emitter, therefore it cannot be

measured by gamma-spectrometry like Cs-137, but needs time consuming and

expensive chemical separation and special measurements methods - earlier

Geiger- or proportional counters, nowadays liquid scintillation spectrometry

is the method of choice.



Secondly: Y-90, the relatively short-lived daughter (about 64 hours), which

therefore is very soon in equilibrium, is as well a pure beta-emitter, with

relatively high beta - energy (about 2.3 Mev E-max). Y-90m is to my

knowledge not part of the decay chain, Sr-90 decays directly to Y-90 (or

maybe there is only a branch with an extremely low probability). Anyway

there is no way to measure Sr-90 by gamma-spectrometry.



Thirdly: To say, that you need not consider beta-emitters for shielding is

surprising. You remember the story of the Sr-90 nuclear batteries in Georgia

recently? You cannot handle them with bare hands. Beta-particles have a

certain range in air, if I remember correctly it is for a medium energy

about 10 cm. So they have to be shielded. But when you shield them, there

will be Bremsstrahlung created!  And since this is electromagnetic radiation

it has to be shielded as well!



I hope that some other one will shed more light with his 1000 W bulb on the

problem of shielding beta radiation......



Best regards,



FRanz









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