Nuclear Physics

1
Nuclear Physics

• Chapter 29

2
Fossils

• How are scientists able to determine the age of
  fossils?
• The iceman

3
Nuclear Physics

• The year 1896 marked the birth of nuclear physics
• Henri Becquerel accidentally discovered natural
  radioactivity in uranium compounds.
• Researchers tried to identifythe radiation from
  atomic nuclei.
• 1 Thorite -------gt

4
Rutherfords Discovery

• Rutherford showed that there were three types of
  nuclear radiation.

5

• Alpha (a)
• Helium nuclei
• Least penetrating
• Positively charged

6

• Beta (b)
• Electrons or positrons
• More penetrating than alpha particles
• Negatively or positively charged

7

• Gamma (g)
• High-energy photons
• Most penetrating
• No charge

8
Rutherfords Experiment

• Rutherford bombarded gold foil with alpha
  particles and disproved the Thompson model of the
  atom..

9
The Strong Nuclear Force

• Rutherford and his students discovered the
  nuclear strong force in 1911
• It explained why the protons dont go flying off
  from the nucleus.

10
Milestones in Nuclear Physics

• Nuclear reactions were observed in 1930.
• The neutron was discovered in 1932.
• Artificial radioactivity was produced in 1933.
• Nuclear fission was discovered in 1938.
• Nuclear fission was first controlled in 1942.

11
Atomic Nuclei

• All nuclei are composed of two types of
  particles.
• The atomic number, Z, equals the number of
  protons in the nucleus.
• The neutron number, N, equals the number of
  neutrons n the nucleus.
• The mass number, A, equals the number of
  nucleons. (A Z N)
• A nucleon can be either a proton or a neutron

12
Nuclear Symbols

• The symbol we use to represent nuclei is
• Sometimes Z is not shown when the chemical symbol
  is obvious.

13
Isotopes

• Isotopes have the same number of protons but
  different numbers of neutrons.
• There are four isotopes of carbon.

14

• has a 98.9 natural abundance.

15
Hydrogen Isotopes

• Hydrogen has three isotopes.
• Protium
• Deuterium
• Tritium

16
Symbols for Other Particles

• alpha
• beta or
• gamma
• neutron

17
Artificial Isotopes

• Artificial isotopes do not occur naturally and
  are produced in the laboratory.
• 17-1, 17-2

18
Charge and Mass

• The proton and electron have charges that are
  equal in magnitude but opposite in sign.
• The neutron has no charge.
• The masses of the proton and neutron are nearly
  equal. (See Table 29.1) on pg. 914.
• The masses of selected isotopes are in Appendix B
  on pg. A.14.

19
Unified Mass Units

• Unified mass units (u) are based upon the
  carbon-12 atom which has a mass of exactly 12 u.
• 1 u 1.660540 x 10-27 kg
• The proton and neutron each have a mass of
  approximately 1 u.
• The mass of the electron is much less.

20
Particle Masses

• Proton mass 1.007276 u
• Neutron mass 1.008665 u
• Electron mass 0.000549 u

21
Mass Energy Conversion

• The energy equivalent of one atomic mass unit is
  931.494 MeV.

22
Rutherfords Experiment
23

• Rutherford found that an alpha particle on a
  head-on collision with a nucleus will stop
  instantaneously at a distance d from the nucleus
  because of Coulomb repulsion.
• 29.1

24

• A neutron has the best chance of causing a
  nuclear reaction because it has no charge.

25
Nuclear Radii

• The average radius of most atomic nuclei can be
  found by using
• ro 1.2 x 10-15 m
• A is the mass number
• 285

26
Nuclear Density

• All nuclei have nearly the same density.

27
Nuclear Stability

• Why dont the protons in the nucleus fly apart
  because of the repulsive Coulomb forces?

28
The Strong Nuclear Force

• The strong nuclear force is an attractive force
  between all nuclear particles.
• The strong nuclear force dominates the Coulomb
  force over short distances.

29
Nuclear Stability

• There are over 260 stable nuclei.
• Hundreds more are unstable
• Light nuclei are stable when N Z.
• Heavier nuclei are stable when N gt Z.
• Elements with more than 83 protons are always
  unstable (radioactive).
• 29.3

30
Nuclear Mass

• The total mass of the nucleus is always less than
  the sum of the masses of its nucleons.

31
Nuclear Energy

• The total energy of the bound nucleus is always
  less than the combined energy of the separated
  nucleons.
• The difference is called the binding energy of
  the nucleus.
• 290

32
Splitting a Nucleus

• In order to break apart a nucleus, energy must be
  added to the system.

33
Binding Energy

• Nuclei with an atomic mass near 60 are the most
  stable.
• The average binding energy per nucleon is about
  8 MeV / nucleon.
• 29.4, 291

34
Natural Radioactivity

• Natural radioactivity was accidentally discovered
  by Becquerel in 1896.
• It was later named radioactivity by Marie Curie.

35
Radioactive Elements

• Marie and Pierre Curie discovered radium and
  polonium after years of separating the
  radioactive elements from tons of pitchblende, a
  radioactive ore.
• Experiments indicated that the radiation was the
  result of nuclear decay.
• Marie Curie died of leukemia in 1934.

36
The Weak Nuclear Force

• The weak nuclear force is responsible for
  radioactivity.

37
Anti Matter

• What is a positron?
• It is an antiparticle with the mass of an
  electron and the charge of a proton.
• Symbol

38
Identifying Nuclear Particles

• A magnetic field can be used to identify the
  particles involved in nuclear radiation.
• 29.5, 292

39
Rutherfords Mousetrap

• Rutherfords mousetrap experimentally proved that
  alpha particles were composed of helium nuclei.
• T-41

40
The Decay Constant

• The decay constant (l) determines the rate at
  which isotopes decay
• A large value for l indicates a rapid rate of
  decay.

41
Activity

• The activity (R) is defined as the number of
  decays per second
• 1 Bq 1 decay / second
• 1 Ci 3.7 x 1010 decays / second 3.7 x 1010
  Bq
• This is the approximate activity of 1 gram of
  radium.
• The mCi and the mCi are most commonly used.

42
Remaining Nuclei

• The number of nuclei (N) remaining after a given
  amount of time can be found by using

43
Decay Rate

• The decay rate (Ro) can be found by using

44
Activity

• The activity (R) after a given amount of time can
  be found by using

45
Half Life

• Halflife
• The time it takes or half of a given number of
  radioactive nuclei to decay is given by
• 6, 294, 29.6

46
Questions

• 1, 12
• Pg. 933

47
The Decay Processes

• Alpha decay
• The parent nucleus emits a helium nucleus.
• The remaining nucleus is called the daughter.
• The daughter nucleus has two less protons and two
  less neutrons.
• Examples

48
Transmutation

• Transmutation is the spontaneous decay of of one
  element into another.
• In the decay process, excess mass is converted
  into energy of other forms, mostly into the KE of
  the nuclei.
• 39-1

49
Artificial Transmutation

• Rutherford accomplished the first artificial
  transmutation.
• He bombarded nitrogen-14 with alpha particles and
  produced Oxygen-17.

50

• Beta decay
• The parent nucleus emits an electron or a
  positron. How are these particles produced?
• The daughter nucleus has the same number of
  nucleons as the parent nucleus.
• The atomic number increases by 1 or decreases by
  1.
• Examples
• 32-1

51
The Decay Chain

• Radioisotopes undergo a series of decays which
  eventually result in the formation of a stable
  isotope of lead.
• Radioisotopes may decay by either alpha or beta
  emission.
• Various half-lives are involved. 295, 74

52

• The neutrino
• No charge
• Little if any mass
• A spin of 1/2
• Very little interaction with matter
• There is also an antineutrino.

53

• Gamma decay
• The parent nucleus emits a gamma ray photon.
• This results when the nucleus is in an excited
  state () after a collision or previous decay.
• The atomic number and the mass number do not
  change.
• Example

54
Summary of the Three Decay Processes
55
Penetrating Ability

• Which type of particle would have the best chance
  of reaching the nucleus?

56

• A gamma ray has the best chance of reaching the
  nucleus because it has no charge.

57
Applications Involving Gamma Rays

• Baggage scanners at airports
• 294

58
Natural Radioactivity

• There are two groups of radioactive nuclei
• Natural
• Found in nature
• Nature continuously resupplies us with
  radioactive isotopes with short half-lives
• Artificial
• Produced in the laboratory

59
Radon Hazzard

• Radon is produced by the decay of radium in the
  soil.
• Which type of decay isresponsible?
• Health concerns?
• 13

60
The Carbon 14 Cycle
61
Carbon Dating

• Carbon dating depends upon the beta decay of
  carbon-14
• Carbon-14 is produced when cosmic rays bombard
  nitrogen-14 in the atmosphere.
• The ratio of carbon-14 to carbon-12 in the
  atmosphere remains constant.
• All living things have the same ratio.
• When organisms die, the ratio begins to change
  because of the beta decay of carbon-14.

62
The Shroud of Turin

• The Shroud of Turin is a centuries old linen
  cloth that bears the image of a crucified man. A
  man that millions believe to be Jesus of
  Nazareth. Is it really the cloth that wrapped his
  crucified body, or is it simply a medieval
  forgery, a hoax perpetrated by some clever
  artist?

63
Positive Image of the Shroud
64
Negative Image of the Shroud
65
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66
Biological Damage

• Radiation damage in matter
• The amount of damage depends upon the type of
  radiation and upon the absorbing material.
• Cancer or death may result
• In cells, ionization is the primarycause of
  damage.
• Ions and free radicals may beformed.

67
Cancer Treatment

• Radiation can be used to kill cancer cells.
• Cancer cells are more vulnerable to damage from
  radiation than are normal cells.
• 293

68
Cyberknife

69
Biological Damage

• Somatic damage
• Radiation damage to any cells but the
  reproductive organs
• Genetic damage
• Damage to the reproductive cells
• Birth defects may result

70
Natural Radiation

• Low level radiation from natural sources
• Cosmic rays
• Air
• Soil
• Food
• Water
• Building materials

71
Biological Units of Radiation

• Roentgen (R)
• The amount of radiation that deposits 8.6 x 10-3
  J of energy into 1 kg of air
• Rad
• The amount of radiation that deposits 10-2 J of
  energy into 1 kg of absorbing material

72
RBE

• Relative Biological Effectiveness is the number
  of rad of x-radiation or gamma radiation that
  will produce the same biological damage as 1 rad
  of the radiation being used. (Table 29.3) on pg.
  930.

73
rem

• A rem (roentgen equivalent man) is the product of
  the dose in rad and the RBE factor.

74
Radiation Exposure Standards

• Government limits
• 0.5 rem/year for the general population
• Roentgen Equivalent Man
• 5.0 rem/year for those in occupations involving
  higher levels of exposure
• Higher limits are allowed for certain parts of
  the body
• Hands and forearms

75
Lethal Dose

• 400 to 500 rem results in a 50 mortality rate.
• This is called lethal dose 50 (LD 50).

76
Radiation Exposure

• Ingestion is the most dangerous form of exposure.
• Strontium-90 is present in the radioactive
  fallout from above ground nuclear testing.
• Behaves like calcium chemically
• Iodine-131(also in radioactive fallout)
• Affects the thyroid gland

77
The Chernobyl Disaster

• Radioactive isotopes were released into the
  environment.

78
Chernobyl (after the fire)
79

• Applications
• Involving
• Radiation

80

• Sterilization of food by radiation exposure
  kills
• Bacteria
• Worms
• Insects and their eggs
• Exposing food to radiation is very controversial.

81

• Sterilization in medicine reduces the chance of
  infection.
• Surgical equipment
• Grafts
• Bone
• Cartilage
• Skin

82

• Chromium-53 is used as a radioactive tracer to
  locate hemorrhages.
• Cholangiogram, radithor capsules

83

• Radioactive tracers in medicine must have a short
  half-life!
• Examples
• Iodine-131
• Evaluating theperformanceof the thyroid

84

• Radioactive tracers in agriculture are used to
  evaluate the effectiveness of fertilizers
• Nitrogen use is tracked

85

• Smoke detectors use an isotope of americiumto
  ionize air molecules
• Smoke particles interfere with this ionization.

86

• Radioactive tracers in industryare used to check
  for pistonring wear.

87

• Computerized Axial Tomography (CAT Scan) images
  have greater clarity and detail than a normal
  x-ray picture.
• X-rays enter the body from different directions
  and the results are evaluated by a computer.
• A brain scan can be made in about 2 seconds.
• A full body scan requires about 6 seconds
• X-rays do present health risks to the patient.
• CAT scan

88
CAT Scan Images
89
Magnetic Resonance Imaging

• Very strong magnetic fields are used to affect
  proton spin for 30 min.
• Changes can be detectedand used to form a
  highlydetailed image.
• MRI is much less damaging to cells.

90
MRI Image

• Magnetic ResonanceImaging (MRI) of thespine

91
PET Scans

• A PET scan involves injecting a very small dose
  of a radioactive chemical, called a radiotracer,
  into the vein of your arm. The tracer travels
  through the body and is absorbed by the organs
  and tissues being studied. 299

92

• Next, you will be asked to lie down on a flat
  examination table that is moved into the center
  of a PET scannera doughnut-like shaped machine.
  This machine detects and records the energy given
  off by the tracer substance and, with the aid of
  a computer, this energy is converted into
  three-dimensional pictures.

93

• A physician can then look at cross-sectional
  images of the body organ from any angle in order
  to detect any functional problems.

94
Particle Counters

• The Geiger counter
• Radiation ionizes a confined gas which has a
  large potential difference applied.
• 292

95

• The semiconductor diode detector
• Uses a reverse biased p-n junction

96

• The scintillation counter
• Counts flashes of light produced by impacts

97
Particle Track Detectors

• Photographic emulsion
• Uses photographic film

98

• Cloud chamber
• Uses a super cooled gas
• Particles form vapor tracks

99

• Bubble chamber
• Uses liquid hydrogen orliquid helium.
• Particles form tracks inthe liquid.

100
Your Annual Exposure

• Calculate your own yearly exposure.

101
Questions

• 2 - 4, 6 - 9, 11
• Pg. 933