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?

 

[sir-pinski]

, thank you for your question. The scattering of alpha particles is a complex phenomenon that is influenced by various factors such as the material of the target, the energy of the alpha particles, and the angle of incidence. In the case of air, the high angle scattering of alpha particles from Am241 is almost non-existent because the air molecules are relatively small compared to the size of the alpha particles and do not significantly affect their path. However, when a thin aluminum foil is inserted in the path, almost half of the radiation disappears somewhere else. This is because the aluminum foil is a much denser material compared to air and therefore has a greater chance of interacting with the alpha particles.

When an alpha particle passes through a material, it can interact with the nuclei of the atoms in that material. This interaction can result in the alpha particle being deflected in a different direction or even absorbed by the nucleus. In the case of a thin aluminum foil, the alpha particles have a higher chance of interacting with the nuclei of the atoms, leading to a greater deflection or absorption rate compared to air.

The reason why 50% of the radiation gets deflected very much by aluminum is because of the nuclear size and density of the material. As you mentioned, in 1 cm of air the nuclei take up approximately 0.1% of the area faced by the alpha particles. However, in the case of 0.1 mm aluminum, the area is also about 0.1% due to the smaller size of the nuclei. But because the density of aluminum is much higher than air, the chance of interaction between the alpha particles and the nuclei is also higher, resulting in a higher deflection rate.

I hope this explanation helps to clarify why a thin aluminum foil can have a significant impact on the scattering of alpha particles. The intricate interactions between particles and materials in the microscopic world can be quite fascinating and require a deeper understanding of physics and nuclear processes. Keep asking questions and exploring the wonders of the universe!