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									Radioactive Decay
    Eric R. Christian

    Elements 2002 Workshop
What is Radioactive Decay?
Some atoms are not stable, which
  means that even if they are
  completely left to themselves, they
  will not last forever.
Radioactive Decay is when an               U238
  unstable atom of one element
  SPONTANEOUSLY changes to
  another element (Wow! Alchemy!).
Because of certain conservation laws
  (you should always conserve!), this
  necessarily includes the release of
  other particles.
                                        Th234     a

Radioactive Decay relates to Isotopes
The type and speed of a radioactive             Element 1: Hydrogen
   decay depends upon the isotope                ALL Hydrogen has
   rather than the element (in other              1 Proton per atom
   words, the number of neutrons          There are 3 Isotopes of Hydrogen:
   determines how stable the nucleus
   is).                                   Hydrogen - No Neutrons - Stable
An element may not have ANY stable
   isotopes (true of all the heaviest                  P
   elements such as Uranium), or it
   may have one, or several, or as         Deuterium - 1 Neutron - Stable
   many as six stable isotopes. This is
   determined by whether it has the
   “right” number of neutrons to make
   it stable.
Every element has radioactive isotopes     Tritium - 2 Neutrons - Unstable
   (the “wrong” number of neutrons).
Line of Stability
Protons are all positively charged and
    therefore electrically repulsive to one
    another. This is compensated by the
    attractive “Strong Nuclear Force.”
    You need enough neutrons so that the
    strong nuclear force balances the
    tendency of the protons to push apart.
If you put the stable isotopes on a plot of
    Number of Protons (Z) vs. the Number
    of Neutrons (N), you find that they
    cluster around a curve that is known
    as the “Line of Stability.”
Light Elements are stable when the
    number of neutrons roughly equals
    the number of protons (N = Z).
Heavy Elements need approximately 1.5
    times as many neutrons as protons in
    order to be stable.
Types of Radioactive Decay
                                               Beta Decay
There are several different types of
   radioactive decay, but they group into                      Little
   two basic varieties:                                        Stuff

Beta Decay
   An electron or anti-electron (positron)
   is emitted or captured. The total
   number of nucleons (protons +
   neutrons) is the same in the old and the   Little Stuff means electrons, positrons,
   new element but either an proton has       and neutrinos

   changed into a neutron or vice versa.       Fission
   The nucleus splits into pieces. The
   total number of nucleons remains the
   same, but they are split into two or                                      Stuff
   more smaller nuclei (elements)
Beta Decay
There are two basic types of Beta Decay:     Electron Emission
Electron emission: an electron (negatively
                                                  P                              P
   charged) and an anti-neutrino are          N
                                                   N            e-                N
                                                  P                          N
                                                N                                P
   released and a neutron is changed into    P      N                       P      P
                                                 N                              N
                                              N    P
   a proton. The total number of nucleons                 Anti-neutrino      N    P

   and the charge is conserved. The          Beryllium-10                  Boron-10
   element moves up one space on the           4 Protons                   5 Protons
   periodic chart (since it now has one       6 Neutrons                  5 Neutrons
   more proton).
Positron emission: a positron (anti-
   electron, positively charged) and a       Positron Emission
   neutrino are released and proton is            P                              P
   changed into a neutron. The total          N
                                             P        P                     P        N
   number of nucleons and the charge is                     Neutrino
                                              N       P                      N       P
   conserved. The element moves down
   one on the periodic chart (since it now   Beryllium-7                   Lithium-7
   has one less proton).                      4 Protons                    3 Protons
                                             3 Neutrons                   4 Neutrons
Electron Capture
There is another way for isotopes that
     decay by positron emission to decay. A
     proton in the nucleus can capture an
                                                 Electron Capture
     electron (usually one of the orbiting
     electrons in the inner shell) and change        P       e-                     P
                                                 N                              N
     into a neutron. The end result is the       P
                                                         P                     P
     same as positron emission.                  N       P        Neutrino      N       P

It is also possible for nuclei that decay via                                 Lithium-7
     electron emission to have positron           4 Protons                   3 Protons
     capture as well. But positrons are much     3 Neutrons                  4 Neutrons
     rarer in the universe than electrons, and
     there are none orbiting close by, so
     positron emission is nearly impossible.
Fission only happens with heavy
                                             Alpha Decay
The simplest type of fission is called
   alpha-decay. A group of two protons                         Little
                                                                        90 Protons
   and two neutrons (called an “alpha                          Stuff
                                                                        144 Neutrons
   particle”, which is basically a helium     Uranium-238
                                              92 Protons         Alpha Particle
   nucleus) splits off and the rest of the
                                              146 Neutrons       (Helium nucleus)
   nucleus remains as a whole.                                   2 Protons
Fission can also result in the nucleus                           2 Neutrons
   splitting into a bunch of fragments of    Little Stuff means electrons, positrons,
   varying sizes.                            neutrons, and neutrinos

Fission is sometimes called                  Spontaneous Fission
   Spontaneous Fission to distinguish it
   from Induced Fission, which is when
   you hit the nucleus with a projectile                       Little   Curium-244
                                                               Stuff    96 Protons
   such as a neutron. Induced fission is                                148 Neutrons
   responsible for most of the reactions     Seaborgium-258
                                             106 Protons
   in nuclear power plants and nuclear                                   Neon-20
                                             152 Neutrons
   bombs.                                                                10 Protons
                                                                         10 Neutrons
Radioactive Decay is a Random Process

You can NEVER tell when an individual
   atom is going to decay. You can
   figure out approximately how many
   atoms in a group are going to decay
   in a certain time, but you can’t tell
   which ones are going to blow.
The timescale for radioactive decay is
   described by the quantity called a
Half-lives can be VERY short (helium-5
   decays in 7.6 x 10-22 seconds), or
   very long (thorium-232 decays in 1.4
   billion years).
What is a Half-Life?
                                           Time (T) = 0          N undecayed atoms
The half-life (t½) is the amount of
   time that it will take half of the
                                                                 N/2 undecayed atoms
                                           T=                    N/2 something else
   atoms to decay. This does not
   mean that in twice that amount of
   time, all the atoms will decay.         T=2x      t½          N/4 undecayed atoms
                                                                 3/4 x N something else
   Since this is a random process,
   there is no history and you have        T=3x      t½           N/8 undecayed atoms
   to start over, so in the second                                7/8 x N something else

   half-life, half of the remaining
   atoms will decay, leaving a
   quarter of the original atoms.                         ...
Note: All the atoms will still be there,
   but the ones that have decayed
   will be a different element.
                                           T = 10 x   t½        N/1024 undecayed atoms
                                                                1023/1024 x N something else
Radioactive Decay is Important for Which Elements?

 During their nucleosynthesis in large stars and supernovae, many of the heavy
    elements (heavier than iron) are actually created as different isotopes that
    decay really quickly to something stable or, at least, less unstable.
 On longer timescales, radioactive decay is important for lead, because it is one
    of the most stable of the heaviest elements and many heavier elements
    decay to it (sometimes via a long chain of radioactive decays: U238Th234
    Pb210Bi210Po210Pb206 which is stable).

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