الجزئية الي لازم حفظ وتركيز CH2

Radioactivity

• Radionuclides are unstable and decay by emitting particles or gamma radiation to achieve a stable configuration of protons and neutrons in the nucleus.
• Radionuclides can decay through several modes:
  • Spontaneous fission (SF)
  • Isomeric transition (IT)
  • Alpha decay
  • Beta decay (β-)
  • Positron decay (β+)
  • Electron capture (EC) decay

Spontaneous Fission (SF)

• Fission is a process where a heavy nucleus breaks into two fragments, accompanied by the emission of two or three neutrons.
• The nuclear binding energy of elements reaches a maximum at an atomic mass number of approximately 58.
• Spontaneous breakdown into smaller nuclei and a few isolated nuclear particles becomes possible at higher atomic mass numbers.

Alpha Decay (α)

• Alpha decay primarily occurs in heavy nuclides like uranium and radon.
• Alpha particles are helium ions with two protons and two neutrons.
• After alpha decay, the atomic number is reduced by 2 and the mass number by 4.
• Example: 222Rn86 → 218Po84 + α
• Alpha particle energy ranges from 1 to 10 MeV.
• Alpha particles have a very short range in matter.

Beta Decay (β−)

• Atoms undergo radioactivity to achieve stability.
• Atoms with too many neutrons and not enough protons can stabilize by lowering the number of neutrons and increasing the number of protons.
• A neutron is converted into a proton, an electron (β−), and an antineutrino (ν).
• Example: n → p + β− + ν
• Beta emission increases the atomic number by one (Z → Z + 1), while the atomic mass number remains the same.
• This nuclear transformation changes an atom from one element to another.
• Example: 131I53 → 131Xe54 + β−

Positron Decay (β+)

• When a radionuclide is proton-rich, a proton converts to a neutron, emitting a positron (β+) and a neutrino (ν).
• The atomic number decreases by 1 (Z → Z – 1).
• Example: p → n + β+ + ν
• Positron decay only occurs when the energy difference between the parent and daughter nuclides is greater than 1.02 MeV.
• Positrons lose energy while passing through matter.
• When losing almost all their energy, positrons combine with an atomic electron, and annihilation occurs, resulting in two photons of 511 keV each. These are called annihilation radiations.

Electron Capture (EC)

• Electron capture is an alternative to β+ decay for proton-rich radionuclides.
• An electron from an inner electron shell (particularly the K shell) is captured by a proton in the nucleus, forming a neutron and a neutrino.
• Example: p + e− → n + ν
• Electron capture lowers the atomic number of the daughter nuclide by 1 (Z → Z – 1).
• Electron capture usually occurs in nuclides with excitation energies less than 1.02 MeV.

Radioactivity

• Radioactivity (nuclear decay) is the process by which an unstable atomic nucleus loses energy by emitting radiation (e.g., alpha particles, beta particles, gamma rays).
• A material containing unstable nuclei is considered radioactive.

Radioactive Decay Laws

• A = A₀ * e^(-λt), where:
  • A = activity after time t
  • A₀ = initial activity
  • λ = decay constant
  • t = time
• Activity is also A = λN, where N is the number of radioactive atoms.

Half-life (T½)

• The half-life is the time needed for half of the radioactive nuclei to decay.
• T½ = 0.693/λ

Average (Mean) Lifetime (T)

• The average time required for all radioactive atoms to decay.
• T = 1/λ, or T = 1.44 T½

Units of Radioactivity

• Curie (Ci) = 3.7 × 10¹⁰ disintegrations/sec
• Becquerel (Bq) = 1 disintegration/sec
• 1 Ci = 3.7 × 10¹⁰ Bq

Half-value Layer

• Half-life is essential to radiologic science and has a parallel in x-ray terminology. Using half-life, you understand the meaning of half-value layer.
• 3.3 half-lives = 1 tenth life

Radiopharmaceutical Loss

• Loss of radiopharmaceutical in a biological system is due to both radioactive decay and biologic elimination.