From: Bill Sommerfeld <sommerfeld@orchard.medford.ma.us>
Date: Tue, 23 Aug 94 07:36:31 PDT
To: perry@imsi.com
Subject: Re: Nuclear Weapons Material
In-Reply-To: <9408231318.AA01904@snark.imsi.com>
Message-ID: <199408231425.KAA00411@orchard.medford.ma.us>
MIME-Version: 1.0
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[this is wandering off the subject of this list, but...]

> Lastly, an ordinary A-bomb is just a way to bring together
> a critical mass of a fissionable material, like U-235 or Plutonium.
> Once a critical mass is in one place the chain reaction will handle
> the rest.

No, not exactly.  See Robert Serber's "The Los Alamos Primer/The First
Lectures on How To Build An Atomic Bomb" (ISBN 0-520-07576-5),
published by the University of California Press.  

I'll quote from the beginning of Chapter 18:

   "To avoid predetonation, we must make sure that there is only a
   small probability of a neutron appearing while the pieces of the
   bomb are being put together.  On the other hand, when the pieces
   reach their best position we want to be very sure that a neutron
   starts the reaction before the pieces have a chance to separate or
   break.  It may be possible to make the projectile seat and stay in
   the desired position.  Failing in this, or in any event as extra
   insurance, another possibility is to provide a strong neutron
   source which becomes active as soon as the pieces come into
   position.   ...
    
Note that this is especially important with the more efficient
implosion-type bombs, where the critical mass just can't "seat" like
on one of the simple gun-type bombs.  Also note that you can't build a
gun-type bomb using P239; it reacts too quickly.

   "Evidently a source of this strength that can be activated within
   about 10**-5 sec and is mechanically rugged enough to stand the
   shocks involved with firing presents a difficult problem."

The text later states that the initiator used in the first two bomb
designs was a mixture of Polonium and Beryllium.

					- Bill