RADIOACTIVE DECAY

1
RADIOACTIVE DECAY

• In 1896 Henri Becquerel discovered that film
  would be exposed when Uranium was placed on the
  film and was wrapped to keep light out.
• Somehow there was energy being emitted
• by the uranium that could penetrate the film.
  These were different from X-Rays because they
  were constantly emitted.

2
RADIOACTIVE DECAY

• Marie Curie was studying for her doctorate in
  Paris and her husband Pierre suggested that she
  study Becquerel Rays for her thesis. She found
  both Uranium and Thorium emitted rays. Later she
  found other ore samples that emitted more
  radiation and suspected they were new substances.
  These samples were found to be two new elements
  which she named Polonium and Radium.

3
RADIOACTIVE DECAY

• In 1897 Ernest Rutherford discovered two types
  of radiation, alpha rays (a) and beta rays (ß) .
  These were found to be oppositely charged. The
  alpha rays were charged positive and beta rays
  were negative, later they were found to be
  electrons. Working with Frederick Soddy in 1900
  they found during radioactive decay there was
  transmutation of an element.

4
RADIOACTIVE DECAY

• In 1900 Rutherford found that a rays were
  really the nuclei of helium atoms. Experiments
  carried out in his lab by Geiger and Marsden led
  Rutherford to propose a model of the atom in 1911
  which had a small dense nucleus with electrons
  moving around it. He named the proton in 1919 and
  proposed that the nucleus should contain a
  neutral particle. Chadwick found it in 1932.

5
RADIOACTIVE DECAY

• Gamma Rays (?) (the third type of radiation)
  were found to be electromagnetic waves with high
  energy.
• In 1934 Irene Curie and her husband Frederick
  Joliot bombarded neutrons into certain nuclei and
  found there was artificial transmutation of the
  nuclei.

6
ARTIFICIAL RADIOACTIVE DECAY and FISSION

• Enrico Fermi began using neutrons and
  bombarded all known elements and soon produced 40
  new isotopes by 1938. In 1939 Hahn and Strassman
  discovered that when Uranium was bombarded,
  Barium (a much lighter element) was produced.
  Lisa Meitner and her cousin Otto Frisch explained
  this effect as nuclear fission.

7
FISSION AND THE ATOMIC BOMB

• In 1939 all of the major nuclear scientists
  had moved to the United States when war broke out
  in Europe. They realized that a tremendous amount
  of energy was released during fusion and
  convinced President Roosevelt that an atomic bomb
  should be developed before Hitler developed it.
  Led by Fermi and Oppenheimer the bomb was
  developed in early 1945. The rest is history.

8
NATURAL NUCLEAR DECAYS

• Radioactivity is the decay of naturally
  occurring nuclei. These are unstable nuclei and
  they have either two many protons or electrons.
  To get rid of protons, an alpha particle is
  emitted. (This removes both protons and
  neutrons.) Since there are usually more neutrons
  than protons, this lowers the net ratio of
  protons/neutrons.

9
NUCLEAR NATURAL DECAYS

• To reduce the number of neutrons a strange
  effect occurs. A beta particle is emitted.
• It was found that a neutron is unstable
  outside the nucleus and in a few minutes it
  decays into a proton electron antineutrino.
  Thus the beta particle coming from the nucleus is
  really the transmutation of a neutron to a proton.

10
NUCLEAR NATURAL DECAYS

• Whenever a decay occurs from the nucleus that
  releases an a or ß particle, the nucleus recoils
  and then oscillates. This oscillation is the
  presence of kinetic energy which is eventually
  released. The release of kinetic energy from the
  nucleus occurs when a ? ray is emitted.

11
RADIOACTIVE DECAY

• RADIOACTIVE DECAY. As we have seen
• a rays are helium nuclei
• ß rays are high speed electrons
• ? rays are high energy e m waves
• The nucleus is composed of protons and neutrons
  and each is designated as Az N where N atomic
  name
• A total number of protons and neutrons
  in amu
• Z total number of charge or number of
  protons.

12
NUCLEI AND ISOTOPES

• For example
• The Nitrogen nucleus is 14 7 N thus it
  has 7 protons and 7 neutrons. There are other
  nuclei of nitrogen, these are called isotopes.
  Each has 7 protons, thats what makes nitrogen.
  Other isotopes of nitrogen would be 15 7 N and 13
  7 N . Nitrogen15 has 8 neutrons and nitrogen13
  has 6 neutrons.

13
NUCLEAR NATURAL DECAYS

• a particles are heavy since they contain four
  atomic mass units. A proton and electron are each
  one atomic mass (amu). So the a is 4 2 a .
• a particles can be stopped by a sheet of paper.
• ß particles are electrons so 0 -1 ß. They can
  penetrate paper and some thin metals, a few
  centimeters.
• ? rays are electromagnetic waves and can travel
  very long distances. A meter or two in lead, even
  completely through the earth.

14
NUCLEAR DECAY EQUATIONS

• There are two conservation laws working during
  nuclear decay. Conservation of Mass and
  Conservation of Charge. This can be used to
  determine the products of decay. Consider the
  alpha decay of Uranium
• 23892 U ---gt 23490 Th 42 a
• Note there are 238 amu on each side of the
  arrow and there are 92 charges on each side.

15
How Decay Occurs in Time

• The decay of nuclei is a completely random
  process. It is much like throwing dice. All that
  can be said is that after many throws of the
  dice, a particular number For example 7 will
  occur in 6 chances out of 36. Namely 16, 25,
  34, 43, 52, 61. So in the same way you can
  determine that ratio after a large number of
  throws, say 1000 or more.

16
How Decay Occurs in Time

• One specifies the time it takes for 1/2 of the
  nuclei to lose their radioactivity. That is
  called the half-life. If one waits for two
  half-lives, then 3/4 of the nuclei have decayed
  and 1/4 remain radioactive. In three half-lives
  7/8 of the nuclei have decayed and 1/8 remain
  radioactive. In four half lives 15/16 of the
  radioactive nuclei have decayed and 1/16 remain.
  Thus the fraction of radioactive nuclei has been
  reduced to 1/2, 1/4, 1/8, 1/16 in each of the
  four half-lives.

17
How Decay Occurs in Time

• The radioactive decay of a nucleus is not
  affected by temperature, outside pressure or any
  other natural process and is thus a constant for
  each isotope.
• The number of decays/time is the activity.
• One can determine the future or past time
  based on a knowledge of todays activity. This
  is how one can date the age of the earth. Since
  carbon is radioactive, one can date the age of
  bones since after death no more carbon is added
  to the bones.