Re: How are elements above uranium formed?

There are several ways, depending on how far past uranium you are trying to 
go.  The first is extremely rare and probably undetectable.  The earth is 
constantly being bombarded by high energy particles from the sun and deep 
space.  There is definitely a very slim chance that on occasion, one of 
these particles will scatter all the way down to the surface, hit a uranium 
atom and increase its mass in such a way as to make it go above uranium.  
(We call elements that are greater than atomic number 92 transuranic, by 
the way.)  The likelyhood of this event is so small however, that it really 
doesn't count.  

The next way involves a standard nuclear reactor.  In the process of making 
the heat to boil water to make steam to turn turbines to generate 
electricity, one particular isotope of uranium, U-235, is split in two.  
This is called fission.  This is done by hitting the U-235 atom with a slow 
or "thermal" neutron.  When that happens, the U-235 atom splits into two 
parts, give of heat, and usually from 1 to 3 neutrons with the average 
being about 2.43.  To keep the reactor going steady, for every atom split, 
one neutron has to go off and split another atom of U-235.  So, what 
happens to the other 1.43?  That's where your question comes in.  For the 
sake of running a reactor, the isotope U-235 is the good stuff.  
Unfortunately there isn't much around, the fuel used in American light 
water reactors is only about 4% U-235 and pretty much all the rest of it is 
U-238.  U-238 doesn't realy care for the thermal neutrons that U-235 uses 
to keep the reactor going.  What it does like are the fast neutrons that 
come out of the U-235 atom that just split up (remember the extra 1.43 
neutrons).  A fraction of those extra neutrons that aren't used up in some 
other way are absorbed by the U-238 and it temporarily becomes U-239.  Now 
the fun starts.  

U-239 will decay by ejecting an electron from its nucleus (beta particle).  
This converts a neutron into a proton, increasing the atomic number by 1 
giving you: Np-239 (neptunium).  From here the Np-239 can decay by beta 
particle again and increase to Pu-239 (plutonium) or it can absorb another 
neutron, become Np-240 which can decay by beta particle into Pu-240.  In 
nuclear reactors, the process primarily stops at plutonium, but some 
americium is produced.  So what about elements 96 and up?  Welcome to the 
world of:

Particle accelerators, particularly cyclotrons, can be used to take medium 
weight elements and smash them into heavier elements to produce a few atoms 
at a time of much heavier elements.  Obviously there aren't many places 
that this can be done.  The three major labs are Lawrence Berkley Labs in 
California, the Dubna Joint Institute for Nuclear Research in Russia, and 
the GSI heavy-ion accelerator group in Germany.  Making the choice as to 
which lighter atoms you use to bombard which heavier atoms are rather 
complicated but the idea is that there are certain combinations of numbers 
of neutrons and protons in the nucleus that make "islands of stability" 
from a nuclear standpoint (the forces inside the nucleus don't rip the atom 
apart right away) so you basicly want to target those combinations.  

To get a better look at how nuclear engineers and nuclear physicists look 
at elements, you need a chart of the nuclides.  One is available on line 
from Barnwell National Labs at http://www.dne.bnl.gov/CoN/index.html  

For a biography of Glen Seaborg, one of the scientists responsible for 
finding many transuranic elements (he even had an element named after him), 
go to Lawrence Berkley Labs web site at: 
http://teamweb.lbl.gov/seaborg/bio.htm 

You or your teacher can also order a book or really big poster of the chart 
of the nuclides (about $10 each) from General Electric that operates the US 
Navy/Department of Energy Knolls Atomic Power Laboratory.  The address is: 

General Electric
Nuclear Energy Operations
175 Curtner Ave., M/C 397
San Jose, CA 95125

(I don't work for GE, they just make the best chart.)  

If you have any other Nuclear related questions or, if I was horribly 
unclear, please email me at skniffin@pop300.gsfc.nasa.gov
Thanks for a good question!

[Moderator note:  Take a look at this URL for the news on the newest super-heavy 
element:]


Science News 2/6/99