Interaction of Radiation with Matter | Class 12 Physics | Nuclear Physics | Chapter 21

Radiation is measured using four parameters: activity (becquerelios), intensity/energy (electron volts), absorbed dose (grays), and effective dose (sieverts). Grays measure the amount of ionizing radiation received by material or tissue (absorbed dose). Sieverts measure the biological effect on health (dose effective). A population receives approximately 2.5 millisieverts annually and can safely receive up to 250 millisieverts without harmful effects.

Radiation dose measures energy absorbed by tissue per unit mass. Absorbed dose is energy (joules) divided by mass (kilograms), with units of joules per kilogram (J/kg) or grays (Gy). Equivalent dose accounts for different biological damage: multiply absorbed dose by a weighting factor (1 for beta/gamma/X-rays, 20 for alpha particles). The sievert (Sv) is the unit for equivalent dose. To calculate total radiation energy absorbed: determine number of decays, multiply by energy per decay, and multiply by time duration. This is essential for radiation safety and medical applications.

This lecture covers the fundamental concepts of radiation dose measurement and biological effects, including the relationship between absorbed dose (measured in grays) and biological damage (measured in sieverts using quality factors), the importance of tissue weighting factors for different organs, and practical applications such as proton therapy and various dosimetry methods including ionization chambers, Geiger counters, and thermoluminescent dosimeters.

The sievert (Sv), named after Swedish physicist Rob Sievert, is the standard SI unit for biological radiation dose since 1979. It measures equivalent dose, calculated by multiplying absorbed dose in grays by radiation weighting factors (1 for X-rays/gamma rays, 20 for alpha particles) and tissue weighting factors (0.01 for brain/skin, 0.12 for bone marrow). For example, a 1 gray dose of gamma rays split 50/50 between liver and colon yields an effective dose of 0.31 sieverts. One sievert represents a 5.5% increase in lifetime cancer probability.

Several units measure radiation: (1) Becquerel (Bq) measures activity (decays per second); (2) Gray (Gy) measures absorbed dose (1 Gy = 1 joule per kilogram); (3) Sievert (Sv) measures equivalent dose, which accounts for different radiation types' biological effects. The key difference is that Gray is a universal unit, while Sievert adjusts for radiation quality—for example, alpha particles have a quality factor of 20, meaning 1 Gy of alpha radiation causes 20 times more biological damage than 1 Gy of gamma radiation.

The half-value layer (HVL) or half-value thickness is the thickness of a material required to reduce the intensity of radiation penetrating it by exactly 50%, and this value varies depending on the type of material, the type of radiation (alpha, beta, gamma, or X-rays), and the energy of the radiation; for example, with aluminum shielding, adding multiple layers of HVL thickness progressively halves the remaining radiation intensity, allowing scientists to measure material thickness by detecting the percentage of radiation that penetrates or is absorbed.

The half-value layer (HVL) is the material thickness reducing radiation by half. For 1 MeV gamma rays, lead HVL is 8.61mm. Radiation attenuation follows an exponential relationship: I = I₀ × e^(-μx), where μ is the linear attenuation coefficient. Higher energy gamma rays require thicker shielding. This mathematical framework enables precise calculation of shielding requirements for any radiation source.

The Half-Value Layer (HVL) is the material thickness reducing photon intensity to half. For I = I₀/2, solving I = I₀ × e^(-μx) gives HVL = ln(2)/μ. The number of HVLs needed to reduce intensity to fraction f is n = log₂(I₀/I). For 99% absorption (I = 0.01I₀), n ≈ 6.64 HVLs are required. HVL depends on material density and photon energy, making it essential for radiation shielding design in medical imaging and nuclear applications.

Radiation intensity decreases exponentially through shielding materials, following the half-value layer principle where each layer reduces intensity by half. Different materials have vastly different effectiveness: air requires 60 meters to reduce radiation by half, lead needs less than 0.5 cm, and water requires approximately 7 cm. This exponential reduction means that multiple layers of shielding dramatically decrease radiation exposure. For nuclear pools, 2 meters of water reduces radiation to 1 billionth of original intensity, while 4 meters reduces it to an even more negligible level. The effectiveness depends on both material density and thickness, with denser materials providing superior protection.

Gamma ray intensity decreases exponentially as they pass through homogeneous materials, following the linear attenuation law: I = I₀ × e^(-μx). The half-value layer (HVL) is the material thickness required to reduce intensity to half its original value, calculated as HVL = ln(2)/μ ≈ 0.693/μ. This relationship determines shielding requirements for radiation protection, allowing calculation of appropriate material thickness to achieve desired radiation reduction.

Nuclear medicine is a major application of nuclear technology alongside electricity production, encompassing two main fields: radiodiagnosis and radiation therapy. Diagnostic imaging includes gamma camera imaging using radioisotopes that deposit in organs with varying affinities, X-ray imaging based on differential tissue density absorption, PET scans detecting positron-electron annihilation for 3D tumor visualization, and CT scans creating cross-sectional 3D body images. Radiation therapy uses high-energy beams to destroy tumors, with proton therapy representing cutting-edge technology that deposits energy precisely at tumor depth while minimizing damage to surrounding healthy tissue. Spain has two major proton therapy centers.

Diagnostic nuclear medicine includes PET scanner and gamma scanner (SPECT). Applications are divided into systems: endocrine, hematopoietic and lymphatic, gastrointestinal, musculoskeletal, cardiovascular, respiratory, nervous, and genital urinary. Therapeutic nuclear medicine is used for cancer treatment, radiation treatment, neuroendocrine tumors, and prostate cancer.

Nuclear medicine is a branch of medicine that uses radiation for both diagnostic and therapeutic purposes. Diagnostic applications include X-rays (mammograms, dental X-rays, bone X-rays), CT scans, and PET scans. Therapeutic applications include implantation of radioactive pellets (commonly in the prostate) and radiation therapy to destroy cancer cells. The field also involves administering radioactive substances that patients inhale, inject, or swallow to illuminate specific organs, tissues, or systems for diagnostic imaging.

Radioactivity has two major medical applications: diagnostic imaging and radiotherapy. Diagnostic methods include gamma scintigraphy (using gamma-emitting traceurs like Technetium-99m, Iodine-123, Thallium-201) and PET tomography (using positron emitters like Fluorine-18 with glucose). Gamma cameras use collimators, scintillators, and photomultipliers to capture images, while PET provides superior precision through positron-electron annihilation. PET-CT combines functional metabolic imaging with anatomical CT data. These techniques diagnose neurodegenerative diseases (Alzheimer's, Parkinson's), cardiovascular conditions, and psychiatric disorders.

This segment covers nuclear medicine applications in diagnosis and treatment. Nuclear medicine uses radioactive tracers to image organ function and detect disease. Unlike X-ray which shows structure, nuclear medicine shows function. Applications include: renal function assessment using renogram, cardiac function assessment using perfusion imaging, tumor detection and staging, and metabolic and functional imaging. Nuclear medicine can detect disease at molecular level before structural changes occur, enabling earlier diagnosis. PET (Positron Emission Tomography) uses positron-emitting radioisotopes (F-18, C-11, N-13, O-15, Ga-68, Rb-82) to image metabolic processes. The process involves injection of radiolabeled compounds, positron emission and annihilation with electrons, and detection of gamma photons in opposite directions. PET is superior for oncology (tumor detection and staging), neurology (brain function), and cardiology (myocardial viability).

Neutrons follow a complex life cycle: born from fission at high energies (~1-8 MeV), they undergo multiple scatterings in moderators to slow down to thermal energies (~0.025 eV). During this slowing-down process, neutrons pass through a resonance absorption region where certain uranium-238 isotopes have high absorption probabilities, potentially capturing neutrons before they reach thermal energies. Moderators like water (hydrogen) and graphite (carbon) slow neutrons through elastic scattering; effective moderators require low absorption cross-sections and high scattering efficiency. The neutron multiplication factor k represents the ratio of neutrons produced in one generation to those lost in the previous generation. When k = 1, the reactor is critical with steady neutron population. When k > 1, the reactor is supercritical with growing neutron population. When k < 1, the reactor is subcritical with decreasing neutron population. Maintaining k ≈ 1 is the target condition for controlled nuclear power operations.

Neutron moderators reduce fast neutron speeds to create thermal neutrons capable of sustaining nuclear chain reactions. Moderation transfers kinetic energy through elastic/inelastic collisions with moderator nuclei. The most efficient moderators have low mass, high scattering cross-section, and low absorption cross-section. Hydrogen-based moderators like water and graphite are optimal because their masses match neutrons, enabling efficient energy transfer. The mean logarithmic energy reduction per collision depends only on atomic mass. Heavy water (D2O) has nearly 80 times higher moderating efficiency than light water (H2O) despite slower moderation due to lower absorption. After sufficient collisions, neutrons follow a Maxwell-Boltzmann distribution modified by absorption preferences. In thermal reactors, uranium-235 absorbs slow neutrons, undergoing fission releasing 2-3 MeV neutrons requiring moderation to ~few km/s for efficient further fissions. Fission cross-section depends strongly on neutron speed—thermal neutrons have much higher fission probabilities than fast neutrons. Reactor types vary by thermalization level: CANDU uses heavy water for nearly all thermal fissions, PWRs have significant fast fission contributions, while supercritical water reactors may exceed 50% fast fission, classifying them as fast reactors.

A neutron moderator is a material that slows down fast neutrons to thermal energies, enabling them to sustain nuclear chain reactions in fissile materials like uranium-235; the most efficient moderators are those with low atomic mass, high scattering cross-section, and low absorption cross-section, with light water (75% of reactors), graphite (20%), and heavy water (5%) being the most commonly used, while the number of collisions required for moderation depends on the moderator's atomic mass through the mean logarithmic energy reduction formula ξ = ln[(A+1)/(A-1)]/2.

Neutron moderation uses materials like water, graphite, or heavy water to slow fast neutrons to thermal speeds. The probability of neutron capture depends on the neutron's speed and the target nucleus. At thermal speeds (~0.025 eV), uranium-235 has a very high capture cross-section (probability), while uranium-238 has a very low one. This difference enables controlled fission in moderated reactors.

Neutron moderation slows fast neutrons to thermal energies to enhance fission probability. Water serves as an excellent moderator because hydrogen (A=1) transfers nearly all neutron kinetic energy during elastic collisions. The macroscopic cross-section equals microscopic cross-section multiplied by number density, representing total interaction probability per unit path length. Every nuclear reaction has its own energy-dependent cross-section: fission, capture, and multi-neutron reactions each contribute differently. Forward scattering preserves neutron energy, while all other reactions cause energy loss and angular deflection. Understanding these relationships enables prediction of reactor behavior and development of neutron transport equations.