- 1. ATOMIC NUCLEUS AND RADIOACTIVITY E.H.ANNEX Medical Physicist
Batra Hospital and Medical Research CentreNew Delhi 62
2. 1896 Henry Becquerel studied phosphorescence with Uranyl
sulfate discovered the Uranium Radioactivity.Nobel Prize in Physics
1903 for discovery of radioactivity Becquerel investigated whether
there was any connection between X-rays and naturally occurring
phosphorescence. He had inherited from his father a supply of
uranium salts, which phosphoresce on exposure to light. When the
salts were placed near to a photographic plate covered with opaque
paper, the plate was discovered to be fogged. The phenomenon was
found to be common to all the uranium salts studied and was
concluded to be a property of the uranium atom. Later, Becquerel
showed that the rays emitted by uranium, which for a long time were
named after their discoverer (Becquerel rays), caused gases to
ionize and that they differed from X-rays in that they could be
deflected by electric or magnetic fields. 3. From 1896 on Marie and
Pierre Curie pursued the study of Becquerel rays)
- They studied radioactive materials, In 1898 they deduced a
logical explanation: that the pitchblende contained traces of some
unknown radioactive component which was far more radioactive than
uranium; thus on December 26 th1898 Marie Curie announced the
existence of this new substance.
- Over several years of unceasing labour they refined several
tons of pitchblende, progressively concentrating the radioactive
components, then two new chemical elements. The first they named
polonium after Marie's native country, and the other was named
radium from its intense radioactivity.
- Used termRADIOACTIVITYfor the first time.
4. Atomic and nuclear structure
- Nucleus contains protons and neutrons.
- Protons have charge Q p= +e, where e is defined as the
magnitude of the electron charge.
- e = 1.6 x 10 -19Coulombs=1.6 x 10 -19C
- Electrons have charge Q e= -e
- Neutrons have charge Q n= 0(zero, exactly)
- The nucleons (protons and neutrons) are bound together by the
strong nuclear force in a small nucleus which has a size of about
10 -15m.
5. Different forms of Atomic nucleus
- Isotopes - Atoms having the same number of protons but
different number of neutrons
- Isobars - Same number of nucleons but different no of
protons
- Isotones - Same number of neutrons but different number of
protons
- Isomers - Contains same number of protons as well as same
number of neutrons but the energy level of the nucleus is
different
6. Size of Nucleus
- Since the time of Rutherford, many experiments have concluded
the following
-
- Most nuclei are approximately spherical
7. Summary of Masses 0.511 5.486x10 -4 9.109 x 10 -31 Electron
939.57 1.008665 1.6750 x 10 -27 Neutron 938.28 1.007276 1.6726 x 10
-27 Proton MeV/c 2 u kg Particle Masses 8. Chemical vs. Nuclear
9. Density of Nuclei
- The volume of the nucleus (assumed to be spherical) is directly
proportional to the total number of nucleons
- This suggests thatall nuclei have nearly the same density
- Nucleons combine to form a nucleus as though they were tightly
packed spheres
10. What is radioactivity
- The spontaneous emission(decay) of particles orrays from a
nucleus
- There is something about the number of protons and neutrons in
the nucleus that makes a nucleus stable or unstable.
11. Natural and artificialradioactivity
- Natural happens by itself.( naturally existing radioactive
elements)
- Artificial is induced in the laboratory( with the help of
cyclotron)
12. Natural Radioactivity
- By unstable nuclei of particles or electromagnetic radiation,
or both
- Resulting in the formation of a stable isotope
13. Artificial radioactivity nuclear transmutation
- Collision of two particles or collision of a particle like
neutron with the atomic nucleus
- May generate the unstable element from a stable one
14. Activity of the radio nuclide
- The number ofdecaying nuclei per unit time is called the
activity of the radio nuclide
- The Systme International (SI) unit of radioactivity is
thebecquerel (Bq) One Bq = 1 desintegration per second
- Non-SI unit of radioactivity is theCurie (Ci)
- Specific activity -It is the activity of a gram of radioactive
material.the unit of specific activity isCi/gram
15. Units
- The unit of activity, is theCurie, (Ci )
- 1 Ci = 3.7 x 10 10decays/second
- The SI unit of activity is theBecquerel , Bq
-
-
- Therefore, 1 Ci = 3.7 x 10 10Bq
- The most commonly used units of activity are the mCi and the
Ci
16. Source of Instabilities
- Too many neutrons for the protons.
- Not enough neutrons for the protons.
17. The Strong Force
- The force that acts to hold the nucleons (protons and neutrons)
in close proximity in the nucleus must be very strong to overcome
the electrostatic repulsion between the protons in the nucleus
- It is about 100 times as strong as the EM force, but is very
short-range, acting only over distances of about 3 x 10 -15meters
(smaller than the nucleus!)
18. Nuclear Forces
- As the nuclei get bigger, some of the nucleons get so far
apart, the strong nuclear force isnt effective due to its short
range
- But the electrostatic repulsions between the protons is a long
range force and keeps pushing the protons apart
19. Nuclear Energy Our everyday life units for energy and mass
are not suitable for atoms. The atomic mass unit (unified mass
unit): 1u = 1.66 x10 27kg Mass of a hydrogen atom is 1.0078 uThe
energy unit is the electronvolt (eV). 1eV = 1.60 10 19J 1Mev = 1.60
10 13J E (1 u) = mc 2= 931 MeV 20. Which type of nuclei is more
stable
- There are three major factors:
- 1. Nuclear Binding Energy ( binding energy per nucleon )
- 2. Band of Stability (n/p ratios)
- 3. Magic Numbers ( 2, 8, 20, 28, 50, 82, and 126- are
calledmagic numbers .) mass number of the nucleus
21. Mass Defect and Nuclear Stability
- the amount of energy you need to add to the nucleus to break it
apart into separated protons and neutrons.
22. Binding Energy Einsteins famous equationE = m c 2
Deuteron:mc 2= 1875.6MeV Difference isBinding energy , 2.2MeV
Proton:mc 2= 938.3MeV Neutron: mc 2 = 939.5MeV Adding these, get
1877.8MeV 23. Decay General Rules
- When one element changes into another element, the process is
calledspontaneous decayortransmutation
- The sum of the mass numbers,A , must be the same on both sides
of the equation
- The sum of the atomic numbers,Z , must be the same on both
sides of the equation
- Conservation of mass-energy and conservation of momentum must
hold
24. Rate of decay
- The amount of decay of a radioactive material depends only
ontwo things :the amount of radioactive materialand thetype of
radioactive material(the particular isotope).
- The rate of decay doesNOTdepend on temperature, pressure,
chemical composition, etc.
25. Nuclear Transformation
- When the atomic nucleus undergoes spontaneous transformation,
calledradioactive decay , radiation is emitted
-
- If the daughter nucleus is stable, this spontaneous
transformation ends
-
- If the daughter is unstable, the process continues until a
stable nuclide is reached
- Most radio nuclides decay in one or more of the following ways:
(a) alpha decay, (b) beta-minus emission, (c) beta-plus (positron)
emission, (d) electron capture, or (e) isomeric transition.(f)
internal conversion
26. Types of Radioactivity particles:electrons :photons(more
energetic than x-rays) penetrate! Easily Stopped Stopped by metal
particles:nucleii Radioactive sources B field into screen detector
27. Alpha Decay
- Alpha ( ) decay is the spontaneous emission of an alpha
particle (identical to a helium nucleus) from the nucleus
- Typically occurs with heavy nuclides (A > 150) and is often
followed by gamma and characteristic x-ray emission
28. -decay Emission of an -particle or4 He nucleus(2 neutrons, 2
protons) This is the preferred decay mode of nuclei heavier than209
Bi with a proton/neutron ratio along the valley of stability The
parent decreases its mass number by 4, atomic number by 2 29.
-decay Emission of an electron (and an antineutrino) during
conversion of a neutron into a proton The mass number does not
change, the atomic numberincreasesby 1. Example:87 Rb ->87 Sr +
e + This is the preferred decay mode of nuclei with excess neutrons
compared to the valley of stability 30. -decay and electron capture
Emission of a positron (and a neutrino)orcapture of an inner-shell
electron during conversion of a proton into a neutron The mass
number does not change, the atomic numberdecreasesby 1. Examples:40
K ->40 Ar + e ++ 50 V+ e ->50 Ti ++ These are the preferred
decay modes of nuclei with excess protons compared to the valley of
stability 31. Beta-Plus Decay (Positron Emission)
- Beta-plus ( + ) decay characteristically occurs with
radionuclides that are neutron poor (i.e., low N/Z ratio)
- Eventual fate of positron is to annihilate with its
antiparticle (an electron), yielding two 511-keV photons emitted in
opposite directions
32. Gamma transition Excited state 33. Electron Capture
Decay
- Alternative to positron decay for neutron-deficient
radionuclides
- Nucleus captures an orbital (usually K- or L-shell)
electron
- Electron capture radionuclides used in medical imaging decay to
atoms in excited states that subsequently emit detectable gamma
rays
34. Electron capture p ++ e -n + A Z XA Z-1Y125 53I125 52Te 35.
Internal conversion 36. Isomeric Transition
- During radioactive decay, a daughter may be formed in an
excited state
- Gamma rays are emitted as the daughter nucleus transitions from
the excited state to a lower-energy state
- Some excited states may have a half-lives ranging up to more
than 600 years
37. Nuclear reactions
- Threshold energy for each reaction is from the rest energy of
the target nucleus + incident particle and residual nucleus +
emitted particle
38. What is a decay series
- Sometimes when a nucleus decays, the productis not stable
either(radioactive isotope) and it will decay.The series of
disintegration until a stable nuclide is reached is called a decay
series.235 U decaying into207 Pb is a well-known one another is
thorium series that starts with232 Th and ends with208 Pb.
39. Decay Schemes or Decay series
- Each radio nuclides decay process is a unique characteristic of
that radionuclide
- Majority of pertinent information about the decay process and
its associated radiation can be summarized in a line diagram called
adecay scheme
- Decay schemes identify the parent, daughter, mode of decay,
intermediate excited states, energy levels, radiation emissions,
and sometimes physical half-life
40. Where the Radioactive Series ends
- All the series ends with stable isotope of lead of mass number
206,207 and 208 respectively , until the stability of the nucleus
is achieved
41. Half-Life
- The time taken for the number of atoms in a sample of an
element to decay by half
- Half-life is fixed no matter how big the sample, what the
temperature or pressure is, it isalways the same length of
time.
- A sample of a radioisotope will never completely disappear
- its radioactivity always disappears by half, even in the
tiniest amounts.
42. 43. The Decay Constant N/ tN(t) Nnumber of radionuclides at
some moment of time t Nnumber of nuclei that decay in a time
interval t decay constant N 0 initial number of nuclei T 1/2
half-life e = 2.718 N = N t N(t) = N 0e t N 0 /2 = N 0e T 1/2 T
1/2= 0.693/ 44. Effective half-life
- Physical half-life:The length of time required for one half of
the original number of atoms in a given radioactive sample to
disintegrate.
- Biologic half-life:The time required for the body to eliminate
one half of the dose of any substance by the regular process of
elimination
- Effective half-life: T he time required for the body to
eliminate one half of the dose of any radioactive substance.
45. Radioactive equilibrium
- If the half life of the parent is longer than that of the
daughter,then after a certain time a condition of equilibrium will
be achieved ,that is the ratio of the daughter activity to the
parent activity will become constant . In addition the decay rate
of the nuclide is then governed by the half life or disintegration
rate of the parent
46. Radioactive equilibrium
- There are two types of equilibrium
- 1) Transient equilibrium2) Secular equilibrium
47. Transient Equilibrium
- Between elutions, the daughter (Tc-99m) builds up as the parent
(Mo-99) continues to decay
- After approximately 23 hours the Tc-99m activity reaches a
maximum, at which time the production rate and the decay rate are
equal and the parent and daughter are said to be intransient
equilibrium
- Once transient equilibrium has been reached, the daughter
activity decreases, with an apparent half-life equal to the
half-life of the parent
- Transient equilibrium occurs when the half-life of the parent
is greater than that of the daughter by a factor of ~10
48. 49. Secular Equilibrium
- If the half-life of the parent is very much longer than that of
the daughter (I.e., more than about 100 longer),secular
equilibriumoccurs after approximately five to six half-lives of the
daughter
- In secular equilibrium, the activity of the parent and the
daughter are the same if all of the parent atoms decay directly to
the daughter
- Once secular equilibrium is reached, the daughter will have an
apparent half-life equal to that of the parent
50. 51. Nuclear Fission
- A heavy nucleus (mass number >200) divides to form smaller
nuclei of intermediate mass and one or more neutrons.
- Release a large amount of energy
52. What is fission
- A large nucleus splitting into smaller ones. The classic one
is
- 235 U 141 Ba +92 Kr + 31 n
53. Nuclear Fission
- Fission:process in which the nucleus of a large, radioactive
atomsplitsinto 2 or more smaller nuclei
-
- Caused by a collision with a energetic neutron.
Ba + 139 56 Kr + 94 36 3n + energy 1 0 U 235 92 n+1 0 54. A
Fission Chain Reaction 55. What is fusion
- Combination to light nuclei into a heavy one a good example is2
H +2 H 4 He.
- It is not quite that simple.Because the nucleus is very small
and protons repel, A tremendous amount of energy is needed to get
this reaction to go.(about 40,000,000 K)
56. Nuclear Fusion
- Fusion:process in which 2 nuclei of small elements are united
to form one heavier nucleus
- Requires temperatures on the order of tens of millions of
degrees for initiation.
- The mass difference between the small atoms and the heavier
product atom is liberated in the form of energy.
- Responsible for the tremendous energy output of stars (like our
sun) and the devastating power of the hydrogen bomb.
57. Artificial Transmutation
- First accomplished by Rutherford in 1919, even though
alchemists tried for hundreds of years.
- Transmutation of lead into gold was achieved by Glenn Seaborg,
who succeeded in transmuting a small quantity of lead in 1980.He
also first isolated plutonium for the atomic bomb and
discovered/created many elements.(NY Times, Feb 1999)
- There is an earlier report (1972) in which Soviet physicists at
a nuclear research facility in Siberia accidentally discovered a
reaction for turning lead into gold when they found the lead
shielding of an experimental reactor had changed to gold.
- Accomplished with particle accelerators like the Stanford
Linear Accelerator (SLAC)
58. Activation of the nuclide
- Elements can be made radioactive by various nuclear reactions,
the yield of a nuclear reaction depends on the parameters such
as:
-
-
- Number bombarding particle andoccurrence of nuclear
reaction(Number of target nuclei )
-
-
- Probability cross-section of nuclear reaction)High fluxes of
slow neutrons are used for activating nuclides(10 10to 10
14neutrons/cm 2 /sec)
59. THANK YOU