Re: How do alpha particles make it past the electron cloud?

Frank,

Thanks for the question.  

An alpha particle is really a Helium nucleus with 2 protons and 2 neutrons.  
Alpha particles usually are moving fairly quickly, compared to the movements of 
atoms and molecules, and so do not have any electrons bond to them in a cloud.  
If an alpha particle slows down or stops and there are electrons around, it can 
grab two electrons and become a normal, neutral Helium atom.

There are several ways that an alpha particle can collide with an atom with an 
electron cloud.  Collisions can be described by how "head-on" they are.  In 
physics this is called the "impact parameter" and is a measure of the closest 
distance of approach of the collider and collidee, if they did not attract or 
repel each other.  
A large impact parameter means that the alpha doesn't get all that close to the 
atom, and so doesn't really touch either the electrons or the nucleus of the 
target atom.  There can still be interaction through electric and magnetic 
fields, so the path of the alpha can be deflected and some energy can be 
transfered to the atom.  This sometimes goes by the name "Coulomb scattering".

An intermediate impact parameter could have an alpha particle going through the 
atom's electron cloud without it getting near the nucleus.  You would think that 
since the electrons have negative charge and the alpha particle has a positive 
charge, they would stick together some how.  Basically there are two ways to do 
this, as an atom, or as a nucleus.  
For an electron to become part of an atom with an alpha particle, the relative 
energy should not be more than about 100 electron volts, or 0.0001 MeV.  A 
typical relative energy for an electron and a 10 Mev alpha might be about 1000 
electron volts or 0.001 Mev, ten times too high.  The relative speed of the 
electon and alpha is too high for them to stick to each other  this way.
For an electron to change the alpha particle in a nuclear fashion, alot of 
energy is needed.  It turns out that the alpha particle is a very stable 
particle that doesn't change easily.  It takes over 10 Mev of energy to make 
much of an impression on the alpha particle.  The relative energy is only 0.001 
Mev, about 1000 times too little to do the job.  
To summarize, there is too much interaction energy for the electrons to stick 
electronically ( as part of an atom ), and not nearly enough energy for the 
electrons to change the nuclear structure of the alpha particle.

If the impact parameter is small, then the collision of the alpha particle with 
the atom is nearly head-on.  The alpha particle can then interact with the 
nucleus of the atom, which is at the center of the atom.  In this case the 
relative energy of the two nuclei might be 9 Mev ( for an atom that is 9 times 
heavier than an alpha).  This is not quite enough energy to excite the alpha, 
but it may be enough to excite the atom's nucleus. 

For most problems in nuclear physics, quantum tunneling and the uncertainty 
principle are almost certain to appear in some form.  These concepts do have 
some bearing on the small impact parameter case,  but have little to do with 
what is going on in the other cases where the electrons are most directly 
involved.

Regards,

Everett Rubel