Radioactivity

• 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

• • Average radius is

• r o=1.2 x 10 -15m

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

• Electronic orbitals

• Bonds between atoms

• Form new molecules

• Changes in nucleus

• Form new elements

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

• Spontaneous emission

• 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

• • 1 Bq = 1 decay / second

• • • 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 big

• Too many neutrons for the protons.

• Not enough neutrons for the protons.

• Too much excess energy

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.

• E=mc 2

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

• ,p reactions

• ,n reactions

• p, reactions

• d,n reactions

• n, reactions

• n, reactions

• , n reactions

• Nuclear fission

• Nuclear fusion

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

• Uranium-235

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)

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