Key Points (~400 words): Applications

Medical Imaging and Treatment:

PET scans use β^{+} emitters (e.g., ^{18}F-FDG, t_{1}/_{2} = 110 min). Positron annihilates with electron to produce two 511 keV gamma rays detected in coincidence.
Gamma therapy (cobalt-60, ^{60}Co): gamma rays used to irradiate tumors. t_{1}/_{2}(Co-60) = 5.27 years.
Iodine-131 (^{131}I, t_{1}/_{2} = 8 days) for thyroid cancer treatment — preferentially absorbed by thyroid.
Technetium-99m (^{99}ᵐTc, t_{1}/_{2} = 6 hours) — most widely used diagnostic radioisotope (bone, brain, heart scans). Short half-life minimizes radiation dose.

Radiocarbon Dating (^{14}C):

^{14}C continuously produced in atmosphere by cosmic rays: ^{14}N + n → ^{14}C + p.
Living organisms maintain constant ^{14}C/^{12}C ratio. After death, ^{14}C decays (t_{1}/_{2} = 5730 years).
NEET formula: measure current activity, compare to expected activity for living sample, calculate t using A = $A_{0}$ (1/2)^(t/t_{1}/_{2}).
Effective range: up to ~50,000 years. For longer timescales, use U-Pb or K-Ar dating.

Nuclear Power (Fission):

^{235}U + n → fission products + 2–3 neutrons + ~200 MeV (per fission).
Chain reaction: neutrons from one fission trigger more. Control rods (boron/cadmium) absorb neutrons.
1 kg of U-235 fission releases energy equivalent to ~20,000 tonnes of TNT.
Nuclear density concept: fuel rods contain uranium oxide at densities unattainable in chemistry.

Stellar Fusion:

Stars fuse hydrogen to helium via the proton-proton chain: net reaction 4p → ^{4}He + 2 $e^{+}$ + 2ν + 26.7 MeV.
Sun converts ~ $4.3 \times 10^{6}$ tonnes of mass to energy per second via E = $mc^{2}$ .
Future fusion reactors (ITER): D + T → ^{4}He + n + 17.6 MeV.

Hydrogen Spectrum in Technology:

Spectroscopy: chemical composition of stars, galaxies, and interstellar gas determined via spectral lines.
Laser technology: stimulated emission from atomic transitions.
LED and display: quantum well transitions emit photons of specific wavelengths.

Nuclear Weapons:

Atomic bomb: uncontrolled U-235 or Pu-239 fission chain reaction.
Hydrogen bomb: fission trigger ignites D-T fusion (uses same physics as stellar interiors).
Application of BE/A curve and mass-energy equivalence (E = $mc^{2}$ ) directly predicts energy yield.

Radioactive Tracers:

Small amounts of radioactive isotopes used to track biological processes, fluid flow, or chemical reactions.
Example: ^{24}Na (t_{1}/{2}=15h) to trace blood circulation; ^{32}P (t{1}/_{2}=14.3 days) in plant research.