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The Discovery of the Neutron
The diagram below represents the apparatus used by James Chadwick (in 1932) which led to his discovery of the neutron.  
   
 
   
Alpha particles from the polonium hit the beryllium and caused it to emit a very penetrating, uncharged radiation.  
The "penetrating power" was measured by finding the thickness of metal needed to absorb the radiation.  
 
He found that when the radiation hit paraffin wax (or any other substance containing hydrogen), protons were emitted  
The maximum energy of the protons was estimated using the absorber sheets and found to correspond to a speed, u = 3.3107ms-1.  
   
To explain these observations, Chadwick proposed the following reaction  
 
That is: a beryllium nucleus absorbs an alpha particle (the same as a helium nucleus) and this results in a carbon nucleus and...  something else.  
Looking at the mass and charge numbers, we see that this "something else" has no charge but 1 unit of mass, so, let's call it a neutron.  
   
In further investigations, Chadwick arranged for the neutrons to collide with nitrogen atoms.  
He found that, after the collisions, the nitrogen atoms moved with a maximum speed, V = 4.7106ms-1.  
   
When a particle of mass m moving at speed u has a "head-on" elastic collision with a stationary particle of mass M, it can be shown that the particle of mass M has a speed V, after the collision, given by  
 
see here for proof... if you're too lazy to do it yourself!  
   
From the collision between neutrons and protons  
 
   
and from the collision between neutrons and nitrogen atoms  
 
This gives the mass of a neutron, m to be about 1.15 times the mass of a proton.  
   
For this explanation of the experimental results, Chadwick is considered to be the discoverer of the neutron.  
 
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