destruction of O₃ in our atmosphere and so will destroy the ozone layer. The prompt burst of γ-rays will not be large enough to cause depletion of our ozone layer for supernova farther than tens of light years distant.

2. X-rays From Supernova Blast's Interaction with Surrounding Material

Aside from prompt x-rays produced in the explosion, a later burst of x-rays may arise from when the ejected material collides with surrounding matter or perhaps collides with a surrounding disk. The strong shock that forms superheats the material to high temperatures causing it to radiate x-rays.
These x-rays may also affect the ozone layer. They may also break apart N₂ molecules in the stratosphere of our atmosphere leading to the formation of molecules composed of NO molecules. The NO molecules then act as catalysts for reactions that destroy our O₃.
A recent study by NASA's CHANDRA x-ray observatory of 31 Supernova's in the x-rays has shown that Supernovas will be dangerous to our ozone layer out to distances of 130 light years or so (a more distant limit but Betelgeuse still sits well outside the danger zone).

3. Cosmic Rays

Aside from the %gamma;-rays and x-rays produced by supernova explosions, high energy particles, cosmic rays are produced by supernova when their shocks collide with surrounding matter. The strong collisons acclerate mainly protons (hydrogen) to nearly the speed of light. Some of the particles have energies larger than 15 Joules! This is huge. There is a single particle that carries the weight of a falling shoe. This is amazing because how many particles are there in a shoe? In a falling shoe there are more than 10²⁷ particles. Each particle then carries only 1.5x10^-26 Joules! The bulk of the cosmic rays carry much smaller energies, usually less than a million-th of a Joule.
The high energy cosmic rays may also affect the ozone layer. At high altitudes, the cosmic rays may collide with nitrogen molecules leading to the production of NO that again catalyzes reactions that destroy the ozone layer. Cosmic rays can cause depletion of the ozone layer for supernovas out to distances of 30-50 light years or so.
Recent work suggests that Cosmic rays may also cause trouble when they produce the particles known as muons, μ near the surface of the Earth. It has been proposed that these μs could produce mini-mass extinctions of large marine critters.

4. Was There a Supernova within 150 Light Years from Earth 2.6 Million Years Ago?

This is an interesting question because 2.6 million years ago marks the end of the Pliocence era which saw changes in the climate of the Earth. Recent work has suggested a supernova may, in fact, occurred 150 light years from Earth and triggered climate change and the mass extinction of large marine mammals.
How could we go about showing that a supernova took place near the Earth 2.6 million years ago?
Is there evidence a supernova took place near the Earth around 2.6 million years ago?

We can determine whether or not a supernova happened nearby through examination of the fossil record on the Earth. A quite useful way is to study layers of sedimentary found in ocean basins. An example of such sedimentary rock is the formation found in Utah. One can see that sedimentary rocks are laid down in layers with the youngest layers lying to the top. Ocean basins are young in a geological sense, but still have ages up to 100s of millions of years and so easily hold the record of events that take place within 5 million years.
So, what can we do?
In supernova outbursts radioactive elements are produced, we have already met nickel 56 and how it decays to produce SN lightcurves. There are other kinds of radioactive elements produced. For the problem at hand, a prime example is iron 60 which decays to cobalt 60 and then to nickel 60 (see left) with a half-life of 2.6 million years.

The strategy is clear. Because the Solar System (and Earth) is 4.5 billion years old, any iron 60 that was around when the Solar System was born will have long ago decayed to nickel 60. Therefore if there is any iron 60 left, we know that it must been incorporated into the Earth within the last millions of years, the exact amount of time depending upon how much of it has changed into nickel 60.

Suppose a rock is born with 64 atoms of iron 60
After one-half life, 2.6 million years, one-half of the iron 60 will have changed to nickel 60. That is, we will now have 32 atoms of iron 60 and 30 atoms of nickel 60. Judging from the relative amounts of iron 60 and nickel 60 will allow us to infer the number of half-lives that elapsed and so the age of the rock.
After another half-life, one-half of the remaining iron 60 will decay to nickel 60. That is, 16 of the 32 remaining iron change to nickel 60. After two half-lives, there will then be 16 iron 60 atoms and 32+16 nickel atoms.
We can use this method to determine the age of a rock through comparison of the amount of nickel 60 it contains to the amount of iron 60 it contains. This method is known as Radioactive Age Dating.

Recent work has suggested that perhaps two supernovas took place near the Earth, one 2.6 million years ago and one 7 to 8 million years ago.