Most manmade radioactive isotopes have much shorter half-lives, ranging
from seconds or days up to thousands of years. Plutonium-239 (a manmade
isotope) has a half-life of 24,000 years.

For the most common uranium isotope, U-238, the half-life is 4.5 billion
years, about the age of the solar system. The much scarcer, fissionable
isotope of uranium, U-235, has a half-life of 700 million years, indicating
that its present abundance is only about 1 percent of the amount present
when the solar system was born.



Note 5: Oxygen, Ozone and Ultraviolet Radiation


Oxygen, vital to breathing creatures, constitutes about one-fifth of the
earth's atmosphere. It occasionally occurs as a single atom in the
atmosphere at high temperature, but it usually combines with a second
oxygen atom to form molecular oxygen (O2). The oxygen in the air we
breathe consists primarily of this stable form.

Oxygen has also a third chemical form in which three oxygen atoms are bound
together in a single molecule (03), called ozone. Though less stable and
far more rare than O2, and principally confined to upper levels of the
stratosphere, both molecular oxygen and ozone play a vital role in
shielding the earth from harmful components of solar radiation.

Most harmful radiation is in the "ultraviolet" region of the solar
spectrum, invisible to the eye at short wavelengths (under 3,000 A). (An
angstrom unit--A--is an exceedingly short unit of length--10 billionths of