FW: Y-90

[Jack_Earley@RL.GOV]

Forwarded w/ concurrence from David.



Jack Earley

Radiological Engineer





-----Original Message-----

From: David W Lee [mailto:lee_david_w@lanl.gov]

Sent: Tuesday, February 12, 2002 1:36 PM

To: Jack_Earley@RL.gov

Subject: Y-90





Jack:



I believe both your nuc safety rep's and your own confusion can be solved 

by looking at page 90-1 and page 90-4 of Browne, Firestone, & Shirley Table 

of Isotopes (Wiley-Interscience 1986).



Sr-90 (28.5 year half-life) is a pure beta emitter, but it beta decays 

99.9979% of the time to Y-90m (3.19 hour half-life).  This is the Yttrium 

that you correctly refer to below that has numerous gamma emissions.  Its 

predominant gamma emissions are two gammas each about 15 keV (a total of 

6+% of the time), a 202 keV (96.6% of the time), and a 480 keV gamma (91% 

of the time) with a 2.18 MeV gamma occurring 8.7 E-8% of the time and a 

2.32 MeV gamma occurring 0.00173% of the time.   A vanishingly small 

percentage of the time (1 - .999979), Sr-90 beta decays directly to Y-90 

(2.671 days half-life).  This Y-90 has virtually no gamma emissions and 

beta emissions of 79 keV, 401 keV, and 432 keV.  While your nuc safety rep 

was technically correct, I conclude that he/she did not realize that 

his/her favorite Y-90 results from Sr-90 decay such a vanishingly small 

percentage of the time.  The Y-90m that you prefer is indeed the 

predominant result of the beta decay Sr-90.  Most of the time, depending on 

the effective Z number of the container in which the Sr-Y-90 is stored, a 

large portion of any measured radiation field external to the source 

container is comprised of bremsstrahlung caused by the betas 

de-accelerating in the source encapsulation material.  Best regards David











At 12:19 PM 2/12/2002 -0800, you 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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DAVID W. LEE, CHP

Los Alamos National Laboratory

Radiation Protection Services

Radiological Engineering Team Leader

ESH-12, MS K483

Los Alamos, NM  87545

PH:   (505) 667-8085

FAX:  (505) 667-9726

lee_david_w@lanl.gov

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