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Name: Adarsh
Status: student
Grade: other
Country: USA
Date: Summer 2013


Question:
Gamma radiation can easily pass through thin sheets of paper and metal but gets absorbed (I think) when travelling through thick lead walls. So is there any material that can actually reflect gamma radiation?



Replies:
Hello,

Gamma rays (or radiation) are high-energy photons, with energies typically in excess of 100 keV (or 100,000 eV). For comparison, photons that appear bright orange to our eyes have energy of about 2 eV.

Gamma rays, like visible light, can be reflected from a mirror surface but there is a catch: to reflect high-energy photons, the angle of incidence (the angle the rays make with the mirror surface) must be much smaller than a degree. Otherwise they penetrate and get absorbed.

You can estimate this angle - called the critical angle - using this approximate equation: critical angle = (2 x delta)^0.5. Here delta is called the refractive index increment. Its value depends on the material that the mirror surface is made of, as well as the photon energy. For a platinum-coated mirror and 100 keV photons, this angle is on the order of 0.04 degrees. I used platinum as an example here because it is a dense material resulting in relatively larger critical angle. For higher energy photons, or for lighter materials, this angle would be even smaller.

In practice, mirrors are rarely used to reflect photons with energies over ~ 40 keV because one would need an exceedingly long mirror. To give you an example, to reflect a 1 mm x 1 mm beam of 100 keV photons, one would need a 1.4-m-long highly polished super flat mirror, costing well over $10,000.

So, in summary, one can reflect high energy photons including gamma rays by using a mirror but the mirror surface has be very smooth, and the mirror must be held such that the beam strikes it at very close (nearly parallel) to the surface. However, because of the length of the mirror typically needed as well as some other practical reasons, mirrors are rarely used to reflect gamma rays.

Ali Khounsary, Ph.D. Advanced Photon Source


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