Radioactivity

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Radioactivity
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Radiation
Radiation The process of emitting energy in the
form of waves or particles. Where does
radiation come from? Radiation is generally
produced when particles interact or decay.A
large contribution of the radiationon earth is
from the sun (solar) or from radioactive
isotopes of the elements (terrestrial). Radiati
on is going through you atthis very moment!
http//www.atral.com/U238.html
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Isotopes
Whats an isotope? Two or more varieties of an
element having the same number of protons but
different number of neutrons. Certain isotopes
are unstable and decay to lighter isotopes or
elements.Deuterium and tritium are isotopes of
hydrogen. In addition to the 1 proton, they have
1 and 2 additional neutrons in the nucleus
respectively. Another prime example is Uranium
238, or just 238U.
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Radioactivity

• By the end of the 1800s, it was known that
  certain isotopes emit penetrating rays. Three
  types of radiation were known
• Alpha particles (a)
• Beta particles (b)
• Gamma-rays (g)

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Where do these particles come from ?

• These particles generally come from the nuclei
  of atomic isotopes which are not stable.
• The decay chain of Uranium produces all three
  of these formsof radiation.
• Lets look at them in more detail

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Alpha Particles (a)
Note This is theatomic weight, whichis the
number ofprotons plus neutrons
Radium R226
Radon Rn222
p

n
n
p
a (4He)
88 protons 138 neutrons
86 protons 136 neutrons
2 protons 2 neutrons
The alpha-particle (a) is a Helium nucleus.
Its the same as the element Helium, with the
electrons stripped off !
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Beta Particles (b)
Carbon C14
Nitrogen N14

e-
electron (beta-particle)
6 protons 8 neutrons
7 protons 7 neutrons
We see that one of the neutrons from the C14
nucleus converted into a proton, and an
electron was ejected. The remaining nucleus
contains 7p and 7n, which is a nitrogen nucleus.
In symbolic notation, the following process
occurred n ? p e ( n )
Yes, the same neutrino we saw previously
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Gamma particles (g)
In much the same way that electrons in atoms can
be in an excited state, so can a nucleus.
Neon Ne20
Neon Ne20

10 protons 10 neutrons(in excited state)
10 protons 10 neutrons(lowest energy state)
gamma
A gamma is a high energy light particle. It is
NOT visible by your naked eye because it is not
in the visible part of the EM spectrum.
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Gamma Rays
Neon Ne20
Neon Ne20

The gamma from nuclear decayis in the X-ray/
Gamma ray part of the EM spectrum(very
energetic!)
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How do these particles differ ?
m E / c2
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Rate of Decay

• Beyond knowing the types of particles which are
  emittedwhen an isotope decays, we also are
  interested in how frequentlyone of the atoms
  emits this radiation.
• A very important point here is that we cannot
  predict when aparticular entity will decay.
• We do know though, that if we had a large sample
  of a radioactive substance, some number will
  decay after a given amount of time.
• Some radioactive substances have a very high
  rate of decay,while others have a very low
  decay rate.
• To differentiate different radioactive
  substances, we look toquantify this idea of
  decay rate

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Half-Life

• The half-life (h) is the time it takes for
  half the atoms of a radioactive substance to
  decay.
• For example, suppose we had 20,000 atoms of a
  radioactive substance. If the half-life is 1
  hour, how many atoms of that substance would be
  left after

10,000 (50)
1 hour (one lifetime) ?
5,000 (25)
2 hours (two lifetimes) ?
2,500 (12.5)
3 hours (three lifetimes) ?
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Lifetime (t)

• The lifetime of a particle is an alternate
  definition ofthe rate of decay, one which we
  prefer.
• It is just another way of expressing how fast
  the substancedecays..
• It is simply 1.44 x h, and one often associates
  the letter t to it.
• The lifetime of a free neutron is 14.7 minutes
  t (neutron)14.7 min.
• Lets use this a bit to become comfortable with
  it

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Lifetime (I)

• The lifetime of a free neutron is 14.7 minutes.
• If I had 1000 free neutrons in a box, after 14.7
  minutes some number of them will have decayed.
• The number remaining after some time is given by
  the radioactive decay law

N0 starting number of particlest
particles lifetime
This is the exponential. Its value is 2.718,
and is a very usefulnumber. Can you find it on
yourcalculator?
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Lifetime (II)
Note by slight rearrangement of this formula
Fraction of particles which did not decay N
/ N0 e-t/t
After 4-5 lifetimes, almost all of the unstable
particles have decayed away!
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Lifetime (III)

• Not all particles have the same lifetime.
• Uranium-238 has a lifetime of about 6 billion
  (6x109) years !
• Some subatomic particles have lifetimes that are
  less than 1x10-12 sec !
• Given a batch of unstable particles, we
  cannotsay which one will decay.
• The process of decay is statistical. That is, we
  can only talk about either, 1) the lifetime of
  a radioactive substance, or 2) the
  probability that a given particle will decay.

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Lifetime (IV)

• Given a batch of 1 species of particles, some
  will decay within 1 lifetime (1t), some within
  2t, some within 3t, and so on
• We CANNOT say Particle 44 will decay at t 22
  min. You just cant !
• All we can say is that
• After 1 lifetime, there will be (37) remaining
• After 2 lifetimes, there will be (14) remaining
• After 3 lifetimes, there will be (5) remaining
• After 4 lifetimes, there will be (2) remaining,
  etc

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Lifetime (V)

• If the particles lifetime is very short, the
  particles decay away very quickly.
• When we get to subatomic particles, the
  lifetimesare typically only a small fraction of
  a second!
• If the lifetime is long (like 238U) it will hang
  around for a very long time!

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Lifetime (IV)
What if we only have 1 particle before us? What
can we sayabout it? Survival Probability
N / N0 e-t/t Decay Probability
1.0 (Survival Probability)
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Summary

• Certain particles are radioactive and undergo
  decay.
• Radiation in nuclear decay consists of a, b, and
  g particles
• The rate of decay is give by the radioactive
  decay law Survival Probability (N/N0)e-t/t
• After 5 lifetimes more than 99 of the initial
  particles have decayed away.
• Some elements have lifetimes billions of years.
  
• Subatomic particles usually have lifetimes which
  are fractions of a second Well come back to
  this!