29.5 Natural Radioactivity

1
29.5 Natural Radioactivity

• Classification of nuclei
• Unstable nuclei found in nature
• Give rise to natural radioactivity
• Nuclei produced in the laboratory through nuclear
  reactions
• Exhibit artificial radioactivity
• Three series of natural radioactivity exist
• Uranium
• Actinium
• Thorium

2
Decay Series of 232Th

• Series starts with 232Th
• Processes through a series of alpha and beta
  decays
• Ends with a stable isotope of lead, 208Pb

3
29.6 Nuclear Reactions

• Structure of nuclei can be changed by bombarding
  them with energetic particles
• Such changes are called nuclear reactions
• As with nuclear decays, the atomic numbers and
  mass numbers must balance on both sides of the
  equation

4
Nuclear Reactions, cont.

• Rutherford was the first observing nuclear
  reactions. He found that protons were released
  when alpha particles collide with nitrogen atoms
  

Proton
5
Nuclear Reactions, cont.

• Balancing atomic numbers and mass numbers enables
  us to find the unknown X

18
18
Mass number
Atomic number
9
9
6
Q Values

• Energy must also be conserved in nuclear
  reactions
• The energy required to balance a nuclear reaction
  is called the Q value of the reaction
• An exothermic reaction
• There is a mass loss in the reaction
• There is a release of energy
• Q is positive
• An endothermic reaction
• There is a gain of mass in the reaction
• Energy is needed, in the form of kinetic energy
  of the incoming particles
• Q is negative

7
Q Values, cont.
13.576 MeV are carried away by carbon and helium
nuclei
Exothermic (energy is released)
Mass before reaction is greater than after the
reaction Q is positive
Endothermic (energy is absorbed)
Energy deficit of 1.194 MeV
Mass after reaction is greater than before the
reaction Q is negative
8
Threshold Energy

• To conserve both momentum and energy, incoming
  particles must have a minimum amount of kinetic
  energy, called the threshold energy
• m is the mass of the incoming particle
• M is the mass of the target particle
• If the energy is less than this amount, the
  reaction cannot occur

9
29.7 Radiation Damage in Matter

• Radiation absorbed by matter can cause damage
• The degree and type of damage depend on many
  factors
• Type and energy of the radiation
• Properties of the absorbing matter
• Exposure time
• Radiation damage in biological organisms is
  primarily due to ionization effects in cells
• Ionization disrupts the normal function of the
  cell

10
Types of Damage

• Somatic damage is radiation damage to any cells
  except reproductive ones
• Can lead to cancer at high radiation levels
• Can seriously alter the characteristics of
  specific organisms
• Genetic damage affects only reproductive cells
• Can lead to defective offspring

11
Units of Radiation Exposure

• Roentgen R is defined as
• That amount of ionizing radiation that will
  produce 2.08 x 109 ion pairs in 1 cm3 of air
  under standard conditions
• That amount of radiation that deposits 8.76 x
  10-3 J of energy into 1 kg of air
• Rad (Radiation Absorbed Dose)
• That amount of radiation that deposits 10-2 J of
  energy into 1 kg of absorbing material

12
More Units

• RBE (Relative Biological Effectiveness)
• The number of rad of x-radiation or gamma
  radiation that produces the same biological
  damage as 1 rad of the radiation being used
• Accounts for type of particle which the rad
  itself does not
• Rem (Roentgen Equivalent in Man)
• Defined as the product of the dose in rad and the
  RBE factor
• Dose in rem dose in rad ? RBE

13
RBE Factors
14
Radiation Levels

• Natural sources rocks and soil, cosmic rays
• Background radiation
• About 0.13 rem/yr
• Upper limit suggested by US government
• 0.5 rem/yr
• Excludes background and medical exposures
• Occupational
• 5 rem/yr for whole-body radiation
• Certain body parts can withstand higher levels
• Ingestion or inhalation is most dangerous

15
Applications of Radiation

• Sterilization most bacteria, worms, and insects
  are easily destroyed by exposure to gamma
  radiation from radioactive cobalt
• Radiation has been used to sterilize medical
  equipment
• Used to destroy bacteria, worms and insects in
  food
• Bone, cartilage, and skin used in grafts are
  often irradiated before grafting

16
Applications of Radiation, cont.

• Tracing
• Radioactive particles can be used to trace
  chemicals participating in various reactions
• Example, 131I to test thyroid action
• CAT scans
• Computed Axial Tomography
• Produces pictures with greater clarity and detail
  than traditional x-rays

17
The Principle of Cat Scans

• Operation of a CAT scanner demonstrated by four
  compartments.
• Local absorption changes are resolved by scanning
  the matter under various directions

18
Applications of Radiation, cont.

• MRI
• Magnetic Resonance Imaging
• When a nucleus having a magnetic moment is placed
  in an external magnetic field, its moment
  precesses about the magnetic field with a
  frequency that is proportional to the field

19
MRI, cont.

• The lower-energy state corresponds to the case in
  which the spin is aligned with the field, whereas
  the higher-energy state corresponds to the case
  in which the spin is opposite the field
• Transitions are observed using nuclear magnetic
  resonance
• A DC field is employed to align the spins and a
  second weaker AC field is superimposed to cause
  the spin flip

20
29.8 Radiation Detectors

• A Geiger counter is the most common form of
  device used to detect radiation
• It uses the ionization of a medium as the
  detection process
• When a gamma ray or particle enters the thin
  window, the gas is ionized
• The released electrons trigger a current pulse
• The current is detected and triggers a counter or
  speaker

21
Detectors, 2

• Semiconductor Diode Detector
• A reverse biased p-n junction
• As a particle passes through the junction, a
  brief pulse of current is created and measured
• Scintillation counter
• Uses a solid or liquid material whose atoms are
  easily excited by radiation
• The excited atoms emit visible radiation as they
  return to their ground state

22
Detectors, 3

• Photomultiplier tube (PMT)
• The PMT consists of electrodes (dynodes) whose
  electric potentials increase along the length of
  the tube
• When photons leaving the scintillation crystal
  hit the photocathode, electrons are released
• After releasing more electrons from the first
  dynode a multiplication process occurs

23
Detectors, 4

• Track detectors
• Various devices used to view the tracks or paths
  of charged particles
• Photographic emulsion
• Simplest track detector
• Charged particles ionize the emulsion layer
• When the emulsion is developed, the track becomes
  visible
• Cloud chamber
• Contains a gas cooled to just below its
  condensation level
• The ions serve as centers for condensation
• Particles ionize the gas along their path
• Track can be viewed and photographed

24
Detectors, 5

• Track detectors, cont
• Bubble Chamber
• Contains a liquid near its boiling point
• Ions produced by incoming particles leave tracks
  of bubbles
• The tracks can be photographed
• Wire Chamber
• Contains thousands of closely spaced parallel
  wires
• The wires collect electrons created by the
  passing ionizing particle
• A second grid allows the position of the particle
  to be determined
• Can provide electronic readout to a computer