Radioactive Decay and Nuclear Fission

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Radioactive Decay and Nuclear Fission

• Fall 2009, Lecture 15

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Atoms

• Atoms are composed of protons (1) and neutrons
  (0) in the nucleus of the atom
• Electrons (-1) surround the nucleus in the
  electron cloud, which makes up the bulk of the
  atomic volume
• The number of electrons must equal the number of
  protons to maintain charge neutrality

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Atomic Nucleus

• The nucleus has almost the entire mass of the
  atom, because protons and neutrons are nearly two
  thousand times as massive as electrons
• The nucleus is about 100,000 smaller than the
  whole atom

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Elements

• All atoms of the same element have the same
  number of protons
• Hydrogen has 1 proton 1H
• Oxygen has eight protons - 8O
• Iron has twenty-six protons - 26Fe
• The number of protons is called the atomic number
• If desired, it may be written as a lower left
  subscript preceding the chemical symbol, as shown
  above, although this is redundant

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Atomic Weight

• The combined number of protons and neutrons in a
  nucleus is called the atomic weight
• Atomic weights are written as upper left
  superscripts preceding the chemical symbol
• Ordinary hydrogen has one proton and no neutrons,
  so 1H

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Isotopes

• Many elements have atoms with different atomic
  weights
• Since the number of protons is fixed, this can
  only mean the number of neutrons varies
• These varieties are known as isotopes
• The isotopes of oxygen are 16O, 17O, and 18O

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Stability

• Isotopes may be stable or unstable
• Unstable isotopes undergo radioactive decay,
  transforming themselves into another isotope of a
  different element, or into a less energetic form
  of the same isotope
• The daughter isotopes produced by radioactive
  decay may themselves be unstable

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Radiation Types

• There are three types of radiation, known as
  alpha, beta, and gamma
• Each has its own characteristics, and dangers
• Each type of radioactive decay has an associated
  half-life

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Half-Life

• The half-life is the interval of time required
  for one-half of the atomic nuclei of a
  radioactive sample to decay (change spontaneously
  into other nuclear species by emitting particles
  and energy)
• Equivalently, the time interval required for the
  number of disintegrations per second of a
  radioactive material to decrease by one-half

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Example 60Co

• The radioactive isotope cobalt-60, which is used
  for radiotherapy, has, for example, a half-life
  of 5.26 years
• After that interval, a sample originally
  containing 8 g of cobalt-60 would contain only 4
  g of cobalt-60 and would emit only half as much
  radiation
• After another interval of 5.26 years, the sample
  would contain only 2 g of cobalt-60

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60Co Decay Product

• Neither the volume nor the mass of the original
  sample visibly decreases, however, because the
  unstable cobalt-60 nuclei decay into stable
  nickel-60 nuclei, which remain with the
  still-undecayed cobalt
• Ni60 is an example of a decay product

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Decay Product

• Decay products may be either radioactive or
  stable
• If the decay product is unstable, decay continues
  until a stable product is reached
• This is called a decay chain

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Half-Life is a Characteristic Property

• Half-lives are characteristic properties of the
  various unstable atomic nuclei and the particular
  way in which they decay
• Alpha and beta decay are generally slower
  processes than gamma decay

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Half-Life Ranges

• Half-lives for beta decay range upward from
  one-hundredth of a second
• For alpha decay, upward from about one
  one-millionth of a second
• Half-lives for gamma decay may be too short to
  measure (around 10-14 second), though a wide
  range of half-lives for gamma emission has been
  reported

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Half-Life and Danger

• The length of the half-life is an important
  property when dealing with radioactive waste
• Isotopes with short half-lives may produce
  intense radiation for short periods
• Isotopes with long half-lives, although producing
  less radioactivity per unit time, are potentially
  dangerous for geologic times

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Alpha Decay

• A type of radioactive disintegration in which
  some unstable atomic nuclei dissipate excess
  energy by spontaneously ejecting an alpha
  particle

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Alpha Decay Process

• Because alpha particles have two positive
  charges and a mass of four units, their emission
  from nuclei produces daughter nuclei having a
  positive nuclear charge or atomic number two
  units less than their parents and a mass of four
  units less

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Radium Decay

• Radium-226 (mass number 226 and atomic number 88,
  i.e., a nucleus with 88 protons) decays by alpha
  emission to Radon-222 (atomic number 86)

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Alpha Particles

• The speed and hence the energy of an alpha
  particle ejected from a given nucleus is a
  specific property of the parent nucleus and
  determines the characteristic range or distance
  the alpha particle travels
• Not very penetrating, alpha particles, though
  ejected at speeds of about one-tenth that of
  light, have ranges in air of only about one to
  four inches

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Alpha Emitters

• The principal alpha emitters are found among the
  elements heavier than bismuth (atomic number 83)
  and also among the rare-earth elements from
  neodymium (atomic number 60) to lutetium (atomic
  number 71)
• Half-lives for alpha decay range from about a
  microsecond (10-6 second) to about 1017 seconds

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Beta Decay

• There are three processes of radioactive
  disintegration by which some unstable atomic
  nuclei spontaneously dissipate excess energy and
  undergo a change of one unit of positive charge
  without any change in mass number, which are
  collectively called beta decay

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Beta Decay Process

• The three processes are called
• Electron emission
• Positron (positive electron) emission and
• Electron capture
• Most beta particles are ejected at speeds
  approaching that of light

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Ernest Rutherford

• Beta decay was named (1899) by Ernest Rutherford
  when he observed that radioactivity was not a
  simple phenomenon
• He called the less penetrating rays alpha and the
  more penetrating rays beta

Lord Ernest Rutherford (1871 - 1937)
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Beta Particles

• Beta particles are electrons (negative) or
  positrons (positive electron)
• A neutron can emit a negative beta particle and
  become a proton
• A proton can emit a positron and become a neutron
• In electron capture, an innermost electron is
  captured by a proton in the nucleus, which
  becomes a neutron

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Example of Negative Beta Decay

• Negative beta decay results in a daughter
  nucleus, the proton number (atomic number) of
  which is one more than its parent but the mass
  number (total number of neutrons and protons) of
  which is the same.

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Positron Emission

• A proton in the parent nucleus decays into a
  neutron that remains in the daughter nucleus and
  ejects a positron, which is a positive particle
  like an ordinary electron in mass but of opposite
  charge
• Thus, positive beta decay produces a daughter
  nucleus, the atomic number of which is one less
  than its parent and the mass number of which is
  the same
• Positron emission was first observed by Irène and
  Frédéric Joliot-Curie in 1934

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Positron Emission Example
The general case is illustrated by
A specific example is
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Electron Capture

• In electron capture, an electron orbiting around
  the nucleus combines with a nuclear proton to
  produce a neutron, which remains in the nucleus
• Most commonly the electron is captured from the
  innermost, or K, shell of electrons around the
  atom for this reason, the process often is
  called K-capture

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Electron Capture Example

• As in positron emission, the nuclear positive
  charge and hence the atomic number decreases by
  one unit, and the mass number remains the same

Note The cobalt isotope produced here is the
same as produced by positron emission this is
an example of branching
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Beta Instability

• Chemical elements consist of a set of isotopes,
  the nuclei of which have the same number of
  protons but different numbers of neutrons
• Within each set the isotopes of intermediate mass
  are stable or at least more stable than the rest
• For each element, the lighter isotopes, those
  deficient in neutrons, generally tend toward
  stability by positron emission or electron
  capture
• The heavier isotopes, those rich in neutrons,
  usually approach stability by electron emission

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Gamma Decay

• A type of radioactivity in which some unstable
  atomic nuclei dissipate excess energy by a
  spontaneous electromagnetic process
• In the most common form of gamma decay, known as
  gamma emission, gamma rays (photons, or packets
  of electromagnetic energy, of extremely short
  wavelength) are radiated

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Gamma Decay Process

• The unstable nuclei that undergo gamma decay are
  the products either of other types of
  radioactivity (alpha and beta decay) or of some
  other nuclear process, such as neutron capture in
  a nuclear reactor or nuclear explosion
• These product nuclei have more than their normal
  energy, which they lose in discrete amounts as
  gamma-ray photons until they reach their lowest
  energy level, or ground state

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Nuclear Power

• Nuclear power may be produced in two ways
• Nuclear fission involves the splitting of an atom
  into two fragments, particles, and the release of
  energy
• Nuclear fusion involves the combination of two
  nuclei into a single, more massive nuclei, plus
  energy
• Stars are powered by nuclear fusion

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Nuclear Fusion

• Nuclear Fusion has been used since the early
  1950s in Hydrogen bombs
• These are the most powerful type of nuclear
  weapon
• We have not yet devised a method of utilizing the
  power of nuclear fusion in the laboratory, nor in
  any commercial reactor
• Therefore, we will not further consider fusion in
  this course

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Nuclear Fission

• Fission induced by neutron bombardment and capture

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Fission Diagram

• When a heavy nucleus undergoes fission, a variety
  of fragment pairs may be formed, depending on the
  distribution of neutrons and protons between the
  fragments

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Induced Fission Animation

• Animation shows a heavy nucleus splitting when it
  absorbs a neutron
• Note extra neutrons are released

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Chain Reaction Video

• Video simulates a nuclear chain reaction
• In a reactor, rods control the reaction and keep
  it from progressing too fast

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Fission Yield

• This leads to probability distribution of both
  mass and nuclear charge for the fragments
• The probability of formation of a particular
  fragment is called its fission yield and is
  expressed as the percentage of fissions leading
  to it

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Fission Products

• A fission product is any of the lighter atomic
  nuclei formed by splitting heavier nuclei
  (nuclear fission), including both the primary
  nuclei directly produced (fission fragments) and
  the nuclei subsequently generated by their
  radioactive decay

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Fission Fragment Decay

• Fission fragments are highly unstable because of
  their abnormally large number of neutrons
  compared with protons
• Consequently, they undergo successive radioactive
  decays by emitting neutrons, by converting
  neutrons into protons, antineutrinos, and ejected
  electrons (beta decay), and by radiating energy
  (gamma decay)

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Fission of 235U

• One of the many known fission reactions of
  uranium-235 induced by absorbing a neutron
  results, for example, in two extremely unstable
  fission fragments, a barium and a krypton nucleus
• These fragments almost instantaneously release
  three neutrons between themselves, becoming
  barium-144 and krypton-89

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Barium Decay

• By repeated beta decay, the barium-144 in turn is
  converted step by step to other fission products
• Lanthanum-144
• Cerium-144
• Praseodymium-144
• Eventually relatively stable neodymium-144

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Krypton

• Krypton-89 is similarly transformed by repeated
  beta decay to
• Rubidium-89
• Strontium-89
• To stable yttrium-89

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Fission Product Identification

• Fission products are identified by their chemical
  properties and by their radioactive properties,
  such as their half-lives and the kinds of
  particles they emit
• The multiple decays mean fission products are
  highly radioactive and therefore quite dangerous

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Why are Fission Products Radioactive?

• To maintain stability, the neutron-to-proton
  (n/p) ratio in nuclei must increase with
  increasing proton number
• The ratio remains at unity up to the element
  calcium, with 20 protons
• It then gradually increases until it reaches a
  value of about 1.5 for the heaviest elements

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P vs. N

• Types of decay relative to line of stability

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N/P Ratio

• When a heavy nucleus fissions, a few neutrons are
  emitted however, this still leaves too high an
  n/p ratio in the fission fragments to be
  consistent with stability for them

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Successive ß- Decay

• They undergo radioactive decay and reach
  stability by successive conversions of neutrons
  to protons with the emission of a negative
  electron (called a beta particle, ß-) and an
  antineutrino

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Fission-Product Decay Chain

• The mass number of the nucleus remains the same,
  but the nuclear charge (atomic number) increases
  by one, and a new element is formed for each such
  conversion
• The successive beta decays constitute a
  fission-product decay chain for each mass number
• The half-lives for the decay of the radioactive
  species generally increase as they approach the
  stable isobar of the chain

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Decay Chain Example

• Example of the decay chain starting with 235U

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Summary of Decay Events

• The chart shows the effect of various decay events