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Name: Thierry
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Question:
According to Astronomy Collector's Edition (January 2007 p.44), "neutrinos can pass through normal matter several light-years thick"; however According to Astronomy Now (June 2005 p.65), "they usually travel through thousands of kilometers of solid matter without interacting with it"; finally According to Curt Suplee's Physics in the 20th Century, "they interact so weakly with matter that most shoot right through several million miles of lead". Since a light-year is approximately 10 trillion km, there seems to be quite a discordance in what people write, because 10 trillion km >> 1 million km of lead >> 1 thousand km of solid matter. So which is it? And how do scientists calculate the neutrino penetration depth?



Replies:
Thierry,

Actually, all three are correct. You must look very closely at the wording.

First, "neutrinos CAN pass through normal matter several light years thick". Send a large beam of neutrinos through normal matter several light years thick. Most will interact along the way, but some will make it through.

Second, "they interact so weakly with matter that MOST shoot right through several million miles of lead". Some will interact with the lead, but most will get through.

Third, "they USUALLY travel through thousands of kilometers of solid matter without interacting with it". Very few, if any, will interact with this distance.

There is no one penetration depth: it is based on probability and percentages. It is very much like radioactivity half-life, but in terms of distance rather than time. The specific quantities depend on the density of the material of concern. It may even depend on temperature.

Consider this example. If a neutrino has a 90% (0.90) chance of passing through one million miles of a material, then it has an 81% (0.90*0.90) chance of passing through two million miles of that material. The neutrino would have a 72% (0.90*0.90*0.90) chance of passing through three million miles of that same material.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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