Re: Alternative isotopes in RTGs?

["Scott D. Kniffin" <Scott.D.Kniffin.1@gsfc.nasa.gov>]

Actually, the half life is the most important factor to consider.  Given the 
length of the mission, the power can't vary much at all or the bird will
become
useless by the time it reaches its destination.  Since the VAST majority of 
parts on board are of the 5.0 Volt variety the power must not drift much 
or the parts will not function as intended.  It's hard enough to get parts
to function is the space radiation environment as it is without having a
tailoff 
in power over the lifetime of the mission.  As most parts, when they degrade 
due to radiation, tend to need more power to function properly, this becomes 
more of a concern as the mission gets toward its end.  (I can go on for about 
30 pages about radiation induced failures in microelectronic circuits, 
but I'll spare the bandwidth for another day.)  

Pu-239 with its 24,100 year half life has a few more advantages over other 
isotopes.  The alpha decay energy is always over 5MeV (more eficient heat
production), no beta emission (less chance for creating a static
electricity problem), the highest gamma energy is of low (weak) probability
(less 
shielding = less weight), and the spontaneous fission probability is 
incredibly small (very very weak).  In fact the mission neutron 
flux for the parts nearest the RTG's is only on the order of 10^9.  
Quite small really.  

Scott Kniffin

Scott.D.Kniffin.1@gsfc.nasa.gov
RSO, Unisys Corp. @ Lanham, MD
CHO, Radiation Effects Facility, GSFC, NASA, Greenbelt, MD

The opinions expressed here are my own.  They do not necessarily represent
the views of Unisys Corporation or NASA.  This information has not been
reviewed by my employer or supervisor.  

At 20:27 10/13/97 -0500, you wrote:
>Mike McNaughton wrote:
>> 
>> When considering alternative nuclides for Cassini, there are several
>> additional questions to consider. For example:
>> 1. is the daughter short-lived or long-lived? U-232 and Ac-227 daughters
are
>> short lived, so each decay is soon followed by a string of decays emitting
>> betas and high-energy gammas. In contrast, the first Pu-238 daughter is
long
>> lived, so the grand-daughters do not grow in.
>> 2. What is the spontaneous fission cross section? (this could be a problem
>> for Cf-250)
>> 3. What is the availability. Po-209 looks like it would work, but is it
>> available?
>> 
>> >Given all the attention to the Pu-238 in the Cassini RTGs
>> >I was wondering what other isotopes might have been suitable
>> >for this application.  Below are those alpha emitters with
>> >half-lifes of 10 to 150 years.
>> >
>> >   Po- 209   103   a
>> >   Ac- 227    21.8 a
>> >   U - 232    72.0 a
>> >   Pu- 238    87.7 a
>> >   Cm- 243    28.5 a
>> >   Cm- 244    18.1 a
>> >   Cf- 250    13.1 a
>> 
>> "Shlala gashle" (Zulu greeting meaning "stay safe") mike
>
>Cm-244 RTG heat sources have been fabricated as an R&D project at ORNL. 
>There is plenty of Cm-244 in the HLW tanks at Hanford but no one seems
>to have a large bankroll to implement that project.
>
>Cf-252 is generally available in mg quantities not the gram or kg
>quantities necessary for these heat sources.  There are strict
>specifications on neutron generation by these heat sources since that
>would be difficult to shield and will have a negative effect on some
>sensors.
>
>There was a nice balanced piece on CNN in the midday about why we have
>Pu-238 heat sources.  Some of the capsule testing and alternatives
>considered were discussed.  However I only saw that piece one time
>during the entire day.  It's a shame they did not run that piece two
>weeks ago when the furor over the Pu-238 was at its peak.
>
>
>