124 
several million does so. 
of an atom. All vou can 
can 
with a huge number 
particles in the hope that occasionally one will make a square hit on a 
nucleus deep down in an atom. It is like shooting with a battery of shot- 
guns in the general direction of a black owl flying high up at midnight 
on a moonless night in the hope that one shot might by chance hit it. 
The accelerators, like the cyclotron, which came on the scene around 
1930 were a great help in that they provided far more high speed parti- 
cles than one could get from all the radium in the world. As a result, 
information came in more rapidly. 
Soon after 1932 our present picture of the nucleus of the atom came 
into being. A nucleus is composed of protons and neutrons packed tight- 
ly together. The proton carries a positive charge; the neutron carries 
no charge at all. The proton and neutron have very nearly the same 
mass. All nuclei are made up of protons and neutrons. The helium nu- 
cleus, for example, is made up of two protons and two neutrons, giving 
a mass of four units. There are two varieties of Li nuclei, one contains 
J protons and 3 neutrons and so has a mass of 6 units, the other con- 
tains 3 protons and 4 neutrons and so has a mass of 7 units. These are 
oiopes Of Li, Li-e and Li-7. Chemically they 
cannot be spnaratoH kw r.«„ „u i ^ l _. 
masses of the isotopes. When 
exact 
raiFnf Jf ''L''^°^^ *° ^ ""^^^' ^^tually it is 6.01682. The 
said of all other isotopes. Their masses are rinse to whol< 
most isotopes. 
true number is known 
made when the 
is a ronvl^- /''^•?* ^""^^^ ^ little below 1 Mev. An "ev" (electron volt) 
Sore suurhw"^'* ^", chemistry; a "Mev" (million electron volts) is 
He nurlPi flv "''^.^^*'' ^°'''^- ^h® result was the appearance of two 
much rnnr^r^ ^^^''^ ^'^ ^ ^"«^gy ^^ 1? Mev. Thus we are getting 
f^r noSSi^ r/tPn''^"^ ^^ P"^ ^' Thi^ looks like getting something 
sotoD^toi *^"y ^* ^^ ^^^ If we take the exact masses of the 
masses of tt ''^;i-''^ *^ ^'-'^ ^^ ^^ Photon which hit it, and theexac 
T?ere is l^lLhf f """^ ^^^1^°^ '^^^l«i' ^e find that they don't balance, 
was f oiSd f n h? ^ °* «iass. The excess energy appearing afterwardf 
cord^iSrto Ein!.'^'"7n*^^ it should be in terms of the mass lost, ac 
f?rme?tie I w."" ^, ^^^^ ^^"^"1^- Experiment after experiment con- 
cS ^t h^P ^°'°'''^^ spectacularly. One might jump to the con- 
indeed tr?e bufu ^T *^PP^g ^^ «^«rgy ^ the nucleus. This is 
one squLre 'h?t on^^^^^^^^ " P^°^l*^We tapping. Remember that to make 
energv nart^l? ^n^cleus, you have to waste a million or more high 
vlluJle^^t!'J'^" "^^ ^^"^ individual event in which the accelerated 
Si the overall nirh^^"^' ** ^^ ^"^^ yo" a^-e getting more than you put w- 
so n^! ° !!! P''^^^ yo" are getting far less than vou put in, because 
uselessly 
- -^tute't^dt?? ^; ^^^^^^^^ e^erimental ph 
?.if,lf.^"'S ^^.^^^^ ^ radioactivity and the 
ar 
transmutat 
in 1937 that 
