Re: Alternative isotopes in RTGs?

["Michael C. Baker" <mcbaker@lanl.gov>]

Don't forget that the half life must also be low enough so that the 
isotope produce sufficeint heat without requiring an extremely heavy 
source.  There is a trade off between long and short half lives with
an optimal point somewhere in between.

Mike Baker

>Date: Tue, 14 Oct 1997 10:24:54 -0400
>From: "Scott D. Kniffin" <Scott.D.Kniffin.1@gsfc.nasa.gov>
>To: radsafe@romulus.ehs.uiuc.edu
>Subject: Re: Alternative isotopes in RTGs?
>
>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.