126 
Now to return to the highly significant discovery of the emission of 
several neutrons whenever a fission process occurs. This makes pos- 
sible a chain reaction. Suppose you had a piece of uranium the size of 
pea and you bombard it with neutrons from an outside source. Each of 
the neutrons which hits the nucleus of uranium atom causes fission, re- 
leases a tremendous amoimt of energy in the fission process and pro- 
duces say three new neutrons. Now as the neutrons, on the average, 
travel a few inches in uranium before making a collission, it is clear 
that practically all will escape from the pea -sized piece of uranium. 
So when the external supply of neutrons is shut off, fission ceases in 
the uranium. But what happens when you have a piece of uranium say 
the size of this room? Suppose we introduce some neutrons somehow 
in the center of this large piece of uranium. Some of them will pro- 
duce fission, releasing energy and each will produce say three new neu- 
trons. Now because the chunk of uranium is so large, the neutrons will 
not escape. Suppose that each of these three new neutrons produces 
three fission events releasing energy and altogether nine new neutrons. 
These are still well inside and so we may suppose that they, in turn, 
will produce nine additional fission events with energy release and 27 
additional neutrons. This business multiplies itself with tremendous 
speed and the energy release will blow the thing apart. In principle, 
a big piece of uranium could never exist because there are always a 
the 
So we 
arrive at the notion of a critical mass below which the uranium is per- 
, but above which the uranium cannot exist without exploding 
have 
the chain reaction. There 
competition for these neutrons which can be used up in several ways. 
1. escape to the outside. 
2. capture by uranium nuclei leading to non-fission transmutations. 
3. capture by impurities. 
4. capture by uranium leading to fission. 
sufficiently 
piece of uranium. Capture by impurites can be avoided by purifying tne 
uranium as much as possible. There remains the capture of neutrons 
by uranium leading to non -fission changes. How can this be avoided? 
Well 
J 
^oo dna ^00, wmcn are present in natural uranium in the ratio oi i.-*^ 
to 1. Neutrons stopped by the U-238 do not lead to fission and so are 
wasted so far as sustaining the chain reaction. Neutrons stopped by the 
U-235 lead to fission and so to a self -sustained chain reaction. To maK^ 
an atomic bomb we therefore need the U-235 as free as possible from 
U-238. To separate isotopes is a fantastically difficult problem. How- 
ever by a tremendous effort it was done and adequate amounts of U-Z-J^ 
were ultimately obtained. The 
xplosive 
but when put together add up to more than the critical mass and is ex 
plosive. So. the reniiirpr«or,f ic +1,^* i.u_ * : 
shall 
an explosion is needed, thev shall 
producing the condition for the almost instantaneous 
ich results in the explosion. 
