Re: why gold foil is used in alpha ray scattering experiment?

Hari,

Rutherford Scattering, the bombarding of a target with alpha particles 
and looking for backscattered particles has been done using a gold foil 
to demonstrate backscattering since the first experiments by Rutherford 
around the turn of the 1900's. 

The interaction that will produce a backscatter is like a ping-pong ball 
hitting a bowling ball (a very small mass hitting a much larger mass). 

The Law of the Conservation of Momentum says that the light 
object "bounces back" 
from the heavier object with just a tiny fraction of the momentum being 
transferred to the heavier object (momentum is a vector--has direction as 
well as the mass times velocity (mv)); the amount transferred to 
the "hit" nucleus is the ratio of the masses: mass alpha/mass target or 
4/197 for an alpha particle on a gold nucleus in a head-on collision). 
Integral calculus is needed to calculate the angular distribution of "not-
quite-head-on collisions". 

I am sure you can find the formulae on the net under Rutherford 
Scattering or Elastic Scattering; my reference is the 14th edition 
of "The Atomic Nucleus" by Robley D. Evans, McGraw Hill, New York, 1955 
(first edition) on. When I was in graduate school in 1965, that book was 
referred to as the Nuclear Physics Bible. 

Now to the question of "Why did Rutherford use a gold foil?"

To increase the probability of an alpha particle hitting a nucleus, the 
more you have in a given volume, the better you chances. Gold has a 
density of 19.3 grams per cubic centimeter. 

Since the derivation of the backscatter equation requires the 
determination of the fraction of the atoms scattered, measurements are 
required from 0 degrees (no scattering, the alpha passes through the 
foil) to as close to 180 degrees (backscatter) as possible. 

Gold is capable of being rolled to extremely thin foils (it's one of the 
most malleable metals), so the beam of alphas will not be totally 
absorbed. Alpha particles are easily stopped, especially since naturally 
occurring alpha particle sources (U, Th, Ra to name a few) do not emit 
alpha particles at very high energies (a sheet of paper stops most of the 
particles, thin aluminum will stop them all). Natural sources were all 
that were available when the first experiments were conducted. 

There are elements with higher density than gold: Osmium and Iridium have 
a density near 22.6, and Platinum has a density of 21.5, compared to 
Gold's 19.3 grams per cubic centimeter. Os, Ir, and Pt are much rarer 
elements than gold, much more expensive, and not easily rolled to a very 
thin foil necessary for backscatter experiments when Rutherford did his 
experiments. 

The observation of backscatter confirmed the proposed model of the atom 
having a positively charged nucleus and being composed mostly of space 
(occupied by electrons). Ernest Rutherford won the 1908 Nobel Prize in 
Chemistry for his discoveries.