Shielding depth
Shielding depth is how deep inside a meteoroid a sample sat, which affects how many cosmogenic nuclides it contains.
Cosmic rays do not penetrate a meteoroid evenly. Near the surface, primary cosmic rays dominate. Deeper inside, a cascade of secondary particles, especially neutrons, builds up and then fades. The result is that production of cosmogenic nuclides depends on shielding depth, the distance between a sample and the meteoroid's surface in space.
Scientists measure ratios of nuclides that respond differently to depth, such as neon-22 to neon-21, to estimate where a sample was located. This must be known to calculate accurate exposure ages, because the production rate changes with depth.
Shielding depth also has practical uses. Comparing samples from different parts of a large meteorite, or from many stones in a strewn field, can reconstruct the original size and shape of the meteoroid before it entered the atmosphere. Studies of the Chelyabinsk and Almahata Sitta falls used this approach to understand how large their meteoroids were in space.
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Level: Expert Advanced