Why Does Aluminum Cause Significant Deflection of Alpha Radiation?

[wiss]

In air, high angle scattering of alpha radiation from Am241 is almost
non existent. When a sample is pulled away from a detector the
detection rate drops as 1/r^2.

But if a thin aluminium foil is inserted in the path almost half of
the radiation disappears somewhere else, it do not reach the detector
even thou the energy spectrum is not below the detection threshold.

In 1 cm of air the nuclei take up approximately 0.1% of the area faced
by the alpha particles, In the case of 0.1 mm aluminium the area would
also be about 0.1% of the area (using a nuclear radius of 10^-14 m).

Why do 50% of the radiation get deflected very much by aluminium? I
would expect 0.1%. Can someone explain this to me?

Jonas

 

[Bob S]

I am attaching a thumbnail plot of the alpha particle range in air. Alpha particle lose energy by collisions with electrons, not with nuclei. The theory is called the Bethe-Bloch energy loss formula, which applies to just about every charged particle except electrons. The range of particles scales something like grams per cm² for different materials. In particular, note that a 2 MeV alpha has a range of about 1 cm in air. Because the range is actually proportional to grams per cm², I have to multiply this by the density of air, 0.0012 grams per cm³, so the range of a 2 MeV alpha is about 0.0012 grams per cm². Because the density of aluminum is about 2.7 grams per cm³, if I divide 0.0012 g/cm² by 2.7 g/cm³, I get 0.00044 cm or 4.4 microns for the range in aluminum. How thick was your aluminum foil?