UT 1: Introduction to Radiation and Radiation Detection Procedures

Unit Curriculum Overview and Evaluation Criteria

• Educational Context and Unit Weighting:
  • Unit Identifier: UT 1. Introducción al fenómeno de la radiación. Aplicación de procedimientos de detección de la radiación.
  • Learning Outcome: RA1.
  • Final Grade Weight: $15\%$.
  • Academic Timing: 1st Evaluation (1ª EVA).
• Evaluation Criteria, Grade Distribution, and Instrument Types:
  • Criterion a: Magnitudes and units employed in dosimetry and radiation protection have been selected ($2\%$, Instrument: E).
  • Criterion b: Radiation detection has been described based on the processes of interaction between radiation and matter ($2\%$, Instrument: E).
  • Criterion c: Area/environmental dosimetry has been differentiated from personal dosimetry ($1.5\%$, Instrument: E).
  • Criterion d: The most appropriate radiation monitors and dosimeters have been selected to perform dosimetry ($2\%$, Instrument: E).
  • Criterion e: Pre-use verifications of radiation monitors have been performed ($2\%$, Instrument: A).
  • Criterion f: Procedures for the detection and measurement of radioactive contamination have been systematized ($2\%$, Instrument: A).
  • Criterion g: Radiation measurements have been performed ($1.5\%$, Instrument: E).
  • Criterion h: Dosimetric readings have been interpreted ($2\%$, Instrument: A).
• Thematic Content Structure:
  • Radiological Quantities and Units: Definition and operational use of exposure, absorbed dose, equivalent dose, and effective dose.
  • Radiation Detection and Measurement: Physical principles of detection, gas ionization detectors, semiconductor detectors, and scintillation detectors.
  • Radiation Dosimetry: Environmental and personal dosimetry, radiation monitors and dosimeters, neutron detectors, and interpretation of dosimetric readings.

Historical Foundations of Radiation and Radiological Protection

• Sensory Limitation and Initial Ignorance of Danger:
  • The complete lack of human sensory organs capable of detecting ionizing radiation prevented early experimenters from perceiving the acute and chronic biological risks associated with radiation exposure.
• Milestones in Discovery:
  • 1895: Wilhelm Conrad Röntgen discovered X-rays. Within months of this discovery, X-rays were deployed clinically in medicine, first for diagnostic radiographic imaging and subsequently for therapeutic radiation treatments.
  • 1896: Henri Becquerel identified "uranic rays" (spontaneous radiation emitted by uranium salts) while investigating phosphorescence, marking the discovery of natural radioactivity.
  • 1898: Marie Curie and Pierre Curie discovered the radioactive elements polonium ($\text{Po}$) and radium ($\text{Ra}$). They identified the harmful cellular effects that radioactivity exerts on healthy living tissue. In her later years, Marie Curie dedicated her scientific work to quantifying radiation doses absorbed by patients.
• Emergence of Occupational Casualties and Radiation Protection:
  • In the early 20th century, radioactive materials were applied across European and North American hospitals to treat skin diseases and superficial organ pathologies.
  • Medical practitioners and scientists routinely handled open radioactive sources without shielding. Pierre and Marie Curie experienced chronic radiation dermatitis, skin burns, severe pain, ocular cataracts, and pregnancy loss.
  • Among physicians and researchers exposed to X-rays and radium, high incidences of alopecia, cutaneous ulcers, tissue necrosis, and radiation-induced leukemia emerged.
  • These widespread injuries necessitated dose limits, shielding protocols, and systematic exposure monitoring, giving birth to the discipline of Radiological Protection.

• The Hamburg Memorial:
  • The Monumento a los Mártires de los Rayos X y Radio de Todas las Naciones (Monument to the X-ray and Radium Martyrs of All Nations), erected in Hamburg, commemorates the hundreds of physicians, researchers, and technicians worldwide who lost their lives to radiation-induced diseases during the pioneering years of radiology.

Nature, Propagation, and Classification of Radiation

• Fundamental Definition of Radiation:
  • Radiation is the emission and propagation of energy through space or a material medium, occurring either as electromagnetic field waves or as high-velocity subatomic particles carrying kinetic energy.
• Classification Criteria:
  • By Energy Propagation Mode: Electromagnetic radiation versus corpuscular (particulate) radiation.
  • By Ionizing Capability: Ionizing radiation versus non-ionizing radiation.
  • By Source Origin: Natural background radiation versus artificial (man-made) radiation sources.

• Electromagnetic Radiation:
  • Physical Mechanism: Propagates via oscillating electric ($\mathbf{E}$) and magnetic ($\mathbf{B}$) fields that are mutually perpendicular to each other and perpendicular to the direction of propagation.
  • Mass and Charge: Possesses zero rest mass and zero electric charge, but transports momentum and energy.
  • Wave Kinematics: Travels in a straight line at the speed of light ($c$ in a vacuum). The frequency ($f$) and wavelength ($\lambda$) satisfy:       \n      f = \frac{c}{\lambda}\n      
  • Quantum Energy Relation: Electromagnetic radiation is quantized into discrete packets called photons or quanta. Photon energy ($E$) depends directly on wave frequency according to the Planck relation:       \n      E = h \cdot f\n             where Planck's constant is defined as:       \n      h = 6.62 \times 10^{-34}\,\text{J}\cdot\text{s}\n      
  • High-Energy Electromagnetic Waves:
    • X-rays: Generated extranuclearly when high-energy kinetic electrons decelerate in matter (bremsstrahlung) or undergo atomic inner-shell transitions (characteristic X-rays).
    • Gamma ($\gamma$) Rays: Generated intranuclearly, emitted during de-excitation of unstable atomic nuclei following radioactive disintegration.
• Physical Nature of Diagnostic X-rays:
  • X-rays are high-energy ionizing electromagnetic waves that penetrate human tissues, producing radiological images.
  • Differential Attenuation in the Human Body:
    • Bone Tissue: Rich in calcium ($Z = 20$), presenting high physical density and high atomic number. Bone absorbs a large fraction of the incident beam, resulting in high attenuation and appearing white on radiographic images.
    • Soft Tissues (Muscle, Fat, Visceral Organs): Intermediate density and effective atomic number; allow partial beam penetration and appear in varying shades of gray.
    • Air/Lungs: Extremely low density; provides minimal beam attenuation, allowing nearly unimpeded passage of photons to reach the receptor, appearing black.
• Corpuscular (Particle) Radiation:
  • Consists of subatomic particles possessing mass and traveling at high velocities. They may be generated artificially in linear accelerators or emitted spontaneously from the nuclei of radioactive atoms seeking energetic stability.
  • Constituent Particles: Alpha ($\alpha$) particles, beta ($\beta$) particles (electrons and positrons), free neutrons, accelerated protons, and heavy stripped ions.
  • Particle Range: The straight-line distance traversed by a charged particle within a given absorbing material before losing all its kinetic energy through interactions.
  • Stopping Power: The rate of kinetic energy lost by a charged particle per unit path length traveled through an absorbing medium ($-\frac{dE}{dx}$.
• Ionizing vs. Non-Ionizing Radiation:
  • Ionizing Radiation: Carries sufficient energy per quantum event to overcome atomic binding energies, ejecting orbital electrons and producing positive ions and free electrons. Examples include X-rays, gamma rays, alpha particles, beta particles, neutrons, and accelerated electrons.
  • Non-Ionizing Radiation: Lacks sufficient quantum energy to strip orbital electrons; causes thermal heating or molecular/electronic excitation. Examples include ultraviolet radiation (near-UV), visible light, infrared radiation, microwaves, and radiofrequency waves.

• Penetration Depth of Different Radiations:
  • Alpha ($\alpha$) Particles: Heavy, doubly charged helium nuclei ($2p + 2n$). High linear energy transfer and low penetrating power. Completely halted by a sheet of paper or the human epidermis.
  • Beta ($\beta$) Particles: High-speed electrons ($\beta^-$) or positrons ($\beta^+$). Intermediate ionizing power and penetration. Halted by a few millimeters of aluminum.
  • X-rays and Gamma ($\gamma$) Rays: Highly penetrating photons. Attenuated exponentially; stopped by dense shielding materials like thick lead or high-density concrete.
  • Neutrons: Electrically neutral particles. High penetrating power; interact via nuclear collisions. Halted by hydrogen-rich moderating substances (water, paraffin) followed by thick concrete barriers.
• Radiation by Source of Origin:
  • Natural Background Radiation: Constant environmental baseline exposure affecting all living organisms:
    • Cosmic Radiation: High-energy atomic nuclei (primarily hydrogen and helium nuclei, with trace heavy elements) originating outside Earth. Radiation flux intensifies with increasing altitude due to thinner atmospheric shielding.
    • Terrestrial Crustal Radiation: Primordial radionuclides embedded in Earth's rocks and soil, notably potassium-40 ($^{40}\text{K}$), rubidium-87 ($^{87}\text{Rb}$), uranium-238 ($^{238}\text{U}$), and thorium-232 ($^{232}\text{Th}$).
    • Atmospheric Radiation: Inhalation of radioactive radon ($\text{Rn}$) gas produced through the radioactive decay series of uranium within Earth's crust. Geological zones containing granite yield elevated radon concentrations.
    • Internal Biological Radionuclides: Radionuclides absorbed through food and water ingestion that incorporate into living tissue, primarily isotopes of potassium ($^{40}\text{K}$) and carbon ($^{14}\text{C}$).
  • Artificial (Anthropogenic) Radiation Sources:
    • Nuclear Energy: Controlled nuclear fission of uranium-235 ($^{235}\text{U}$) within power reactors.
    • Industrial Applications: Radiographic non-destructive testing of structural materials, food preservation, and product sterilization.
    • Medical Applications: Diagnostic imaging (radiography, computed tomography, fluoroscopy), radiopharmaceuticals in nuclear medicine, and therapeutic beams from linear accelerators.
    • Agro-food Applications: Food irradiation indicated by the international Radura certification symbol.

Atomic Structure, Nuclear Physics, and Radioactivity

• Fundamental Architecture of the Atom:
  • Atomic Nucleus: Dense central core housing nucleons (protons and neutrons), accounting for more than $99.9\%$ of total atomic mass.
  • Electron Cloud (Cortex): Surrounding extranuclear region where negatively charged electrons occupy discrete quantum orbitals. A neutral ground-state atom possesses an identical number of orbital electrons and nuclear protons.
• Fundamental Subatomic Particle Properties:
  • Electron ($e^-$): Electric charge of $-1.6 \times 10^{-19}\,\text{C}$, rest mass of $9.1 \times 10^{-31}\,\text{kg}$.
  • Proton ($p^+$): Electric charge of $+1.6 \times 10^{-19}\,\text{C}$, rest mass of $1.6748 \times 10^{-27}\,\text{kg}$.
  • Neutron ($n^0$): Electric charge of $0\,\text{C}$, rest mass of $1.6725 \times 10^{-27}\,\text{kg}$.
• Nuclide Nomenclature and Parameters:
  • Atomic Number ($Z$): Number of nuclear protons; defines the chemical identity of the element.
  • Neutron Number ($N$): Total number of neutrons contained within the nucleus.
  • Mass Number ($A$): Total nucleon count:       \n      A = Z + N\n      
  • Standard Nuclear Notation:       \n      ^A_Z\text{X}\n      
• Classification of Nuclides:
  • Isotopes: Nuclides with identical atomic number ($Z$) but differing mass numbers ($A$) due to varying neutron quantities. Examples include potassium-39 and potassium-40:       \n      ^{39}_{19}\text{K} \quad \text{and} \quad ^{40}_{19}\text{K}\n      
  • Isotones: Nuclides possessing an identical count of neutrons ($N$) despite possessing different atomic numbers ($Z$) and mass numbers ($A$). Examples include sodium-23 ($N = 23 - 11 = 12$) and magnesium-24 ($N = 24 - 12 = 12$):       \n      ^{23}_{11}\text{Na} \quad \text{and} \quad ^{24}_{12}\text{Mg}\n      
  • Isobars: Nuclides possessing identical mass numbers ($A$) but differing atomic numbers ($Z$). Examples include carbon-14 and nitrogen-14:       \n      ^{14}_{6}\text{C} \quad \text{and} \quad ^{14}_{7}\text{N}\n      

• Subatomic Substructure (Quark Model and Fundamental Forces):
  • Protons and neutrons are non-elementary composite hadrons constructed from triplets of quarks bound by strong gluon exchanges.
  • Quark Flavors: Up ($u$), down ($d$), strange ($s$), charm ($c$), bottom ($b$), and top ($t$).
    • Proton Composition: Two up quarks and one down quark ($uud$).
    • Neutron Composition: One up quark and two down quarks ($udd$).
  • Particle Categorization:
    • Leptons: True elementary particles exempt from the strong nuclear force (e.g., electrons, muons, taus, neutrinos).
    • Hadrons: Composite particles subject to the strong force, subdivided into:
      • Mesons: Quark-antiquark bound pairs.
      • Baryons: Tri-quark bound states (protons and neutrons).
    • Antiparticles: Every fundamental particle has an antiparticle sharing identical mass and lifetime but having reversed electric charge and opposite magnetic dipole moment.
  • Subatomic Dimensions and Structural Scale:
    • Quark and electron diameter: $< 10^{-18}\,\text{m}$.
    • Proton and neutron diameter: $\approx 10^{-15}\,\text{m}$.
    • Atomic nucleus diameter: $\approx 10^{-14}\,\text{m}$.
    • Total atomic diameter: $\approx 10^{-10}\,\text{m}$.
    • Scale Analogy: If nucleons were scaled up to $10\,\text{cm}$ in diameter, quarks and electrons would measure $0.1\,\text{mm}$, and the complete atom would span $10\,\text{km}$.
• Nuclear Energetics and Radioactivity:
  • Nuclear Binding Energy: The total energy holding a nucleus together against the Coulombic electrostatic repulsion of positive protons.
  • Mass Defect: The actual mass of a bound atomic nucleus is measurably smaller than the arithmetic sum of the masses of its constituent free nucleons. This difference corresponds directly to nuclear binding energy ($\Delta E = \Delta m \cdot c^2$), released during nuclear formation and nuclear reactions.
  • Nuclear Stability: Regulated by the neutron-to-proton ratio ($N/Z$). Unbalanced ratios yield unstable nuclei that spontaneously emit radiation to reach lower energy configurations.
  • Radioactivity Threshold: All chemical elements with an atomic number $Z \ge 84$ (beginning at polonium, $\text{Po}$) lack stable isotopes and are radioactive.
• Modes of Radioactive Decay:
  • Alpha ($\alpha$) Decay: Emission of a helium nucleus ($^4_2\text{He}^{2+}$, consisting of $2p + 2n$). Characterized by high linear ionization density and very short penetration range.
  • Beta-Minus ($\beta^-$) Decay: Conversion of a nuclear neutron into a proton accompanied by the emission of an electron ($e^-$) and an electron antineutrino ($\bar{\nu}_e$). Features intermediate ionization capacity and moderate penetration.
  • Beta-Plus ($\beta^+$) Decay: Nuclear transformation accompanied by the emission of a positron ($e^+$) and an electron neutrino ($\nu_e$).
  • Gamma ($\gamma$) Decay: Emission of excess nuclear excitation energy via high-energy photons, involving no changes in atomic number ($Z$) or mass number ($A$).

Radiological Quantities, Units, and Dosimetry Principles

• Distinction Between Radiation and Radioactivity:
  • Radiation: The physical process of emitting and propagating energy through an environmental medium or vacuum.
  • Radioactivity: The spontaneous disintegration of unstable atomic nuclei through radiation emission.
• Activity ($A$):
  • The decay rate of a radioactive source, defined as disintegrations per unit time.
  • SI Unit: Becquerel ($\text{Bq}$):       \n      1\,\text{Bq} = 1\,\text{nuclear disintegration per second}\n      
  • Historical Unit: Curie ($\text{Ci}$), originally based on the activity of $1\,\text{g}$ of radium-226 ($^{226}\text{Ra}$):       \n      1\,\text{Ci} = 3.7 \times 10^{10}\,\text{Bq}\n             \n      1\,\text{Bq} = 2.703 \times 10^{-11}\,\text{Ci}\n      
• Exposure ($X$):
  • Measures the ability of a beam of X-rays or gamma rays to produce electrical charge through ionization in a specified volume of air at standard temperature and pressure:       \n      X = \frac{dQ}{dm}\n             where $dQ$ is the absolute sum of all electrical charges of one sign produced in dry air when all electrons liberated by photons in a mass element $dm$ are completely stopped.
  • SI Unit: Coulombs per kilogram ($\text{C/kg}$).
  • Historical Unit: Roentgen ($\text{R}$):       \n      1\,\text{R} = 2.58 \times 10^{-4}\,\text{C/kg}\n             \n      1\,\text{C/kg} = 3876\,\text{R}\n      
  • Exposure Rate ($\dot{X}$): Exposure per unit time:       \n      \dot{X} = \frac{dX}{dt}\n      
• Kerma ($K$ – Kinetic Energy Released per Unit Mass):
  • The quotient of $dE_{tr}$ divided by $dm$, where $dE_{tr}$ represents the initial kinetic energy of all charged ionizing particles liberated by uncharged ionizing radiation (photons or neutrons) in a mass element $dm$ of material:       \n      K = \frac{dE_{tr}}{dm}\n      
  • SI Unit: Gray ($\text{Gy}$), equivalent to $1\,\text{J/kg}$.
  • Historical Unit: Rad ($\text{rad}$):       \n      1\,\text{rad} = 0.01\,\text{Gy}\n             \n      1\,\text{Gy} = 100\,\text{rad}\n      
  • Kerma Rate ($\dot{K}$): Kerma generated per unit time:       \n      \dot{K} = \frac{dK}{dt}\n      
  • Specific Clinical Kerma Quantities:
    • KASE (Kerma Aire en Superficie de Entrada): Air kerma evaluated at the entrance surface of the patient or standard phantom without accounting for backscattered radiation.
    • DSE (Dosis en Superficie de Entrada): Entrance surface dose measured on the central beam axis at the patient's skin, including the backscatter contribution.
• Cema (Converted Energy per Unit Mass):
  • Applied when the incident primary beam consists directly of charged ionizing particles, measuring kinetic energy lost in electronic collisions per unit mass. Measured in Joules per kilogram ($\text{J/kg}$).
• Linear Energy Transfer (LET):
  • Quantifies the density of energy imparted by radiation to a traversed medium per unit track length (\text{keV/\mu m}):
    • High-LET Radiation: Deposits large amounts of energy over short spatial tracks (e.g., alpha particles). Exhibits low penetration but severe local biological toxicity.
    • Low-LET Radiation: Deposits energy sparsely over long spatial tracks (e.g., diagnostic X-rays, gamma rays). Exhibits high penetration with less localized damage per unit length.
• Absorbed Dose ($D$):
  • The mean energy deposited by ionizing radiation into a mass element of absorbing medium:       \n      D = \frac{d\bar{\epsilon}}{dm}\n      
  • SI Unit: Gray ($\text{Gy}$), where $1\,\text{Gy} = 1\,\text{J/kg}$.
• Relative Biological Effectiveness (RBE / EBR):
  • The ratio of reference radiation dose (typically $250\,\text{kVp}$ X-rays) to test radiation dose required to produce an identical biological endpoint:       \n      \text{EBR} = \frac{\text{Dosis de radiación de referencia}}{\text{Dosis de radiación a estudiar}}\n      
  • An $\text{EBR} > 1$ demonstrates that less absorbed dose of the studied radiation is needed to produce the same biological effect, indicating higher relative damage potential.
• Equivalent Dose ($H$):
  • Accounts for differing biological damages produced by different radiation types within a specific tissue or organ by scaling absorbed dose by a radiation weighting factor ($w_R$):       \n      H = D \cdot w_R\n      
  • SI Unit: Sievert ($\text{Sv}$).
  • Selected Radiation Weighting Factors ($w_R$):
    • Photons (X-rays, gamma rays): $w_R = 1$.
    • Electrons, muons: $w_R = 1$.
    • Alpha ($\alpha$) particles, fission fragments, heavy nuclei: $w_R = 20$.
• Effective Dose ($E$):
  • Quantifies the overall stochastic health risk to an individual by weighting the equivalent dose in each exposed tissue by that tissue's radiosensitivity factor ($w_T$):       \n      E = \sum_T H_T \cdot w_T = \sum_T \left( D_{T,R} \cdot w_R \cdot w_T \right)\n      
  • SI Unit: Sievert ($\text{Sv}$).
  • The sum of all tissue weighting factors across the human body equals unity ($\sum w_T = 1$). Examples: lungs ($w_T = 0.12$), salivary glands ($w_T = 0.01$).

Physics and Technical Characteristics of Radiation Detectors

• Universal Detection Principle:
  • Because human biology lacks direct sensory mechanisms for ionizing radiation, instruments must translate physical radiation interactions into readable visual or electrical outputs.
  • Operational Axiom: All radiation detectors deliver a measurable, precise, and unambiguous signal in response to an external radioactive interaction.
• Classification of Detectors:
  • By Output Signal Modality: Electrical pulses/currents versus visual light emissions.
  • Counter Detectors: Tally the absolute count of ionizing particles or photons interacting with the sensing volume per unit time.
  • Spectrometer Detectors: Tally interactions while resolving the precise kinetic energy deposited by each incident event.
• Detector Performance Parameters:
  • Extrinsic Efficiency ($EE$): The geometric probability that a radiation particle emitted by a source will physically enter the active detector volume:       \n      EE = \frac{\text{N.}^º\text{ de partículas que llegan al detector}}{\text{N.}^º\text{ de partículas totales emitidas}}\n      
  • Intrinsic Efficiency ($EI$): The probability that a particle entering the sensitive volume interacts and yields a measurable signal:       \n      EI = \frac{\text{N.}^º\text{ de partículas que interaccionan y dan señal}}{\text{N.}^º\text{ de partículas que llegan al detector}}\n      
  • Dead Time (Temporal Resolution): The minimum recovery time required by a detector after an event during which it cannot record a subsequent event. If incoming event frequency exceeds this limit, incoming events are lost and reading errors increase.
  • Energy Resolution: The ability to resolve and distinguish between radiation events of closely spaced energies.
  • Sensitivity: The minimum radiation field intensity or exposure rate that a detector can reliably register above background.
  • Precision (Reproducibility): The closeness of agreement among repeated measurements of the same radiation field under unchanged operational conditions.
  • Accuracy (Exactitud): The degree of conformity between the measured value indicated by the detector and the true radiation value.

Operating Principles and Typology of Radiation Detectors

• Gas Ionization Detectors:
  • Operating Mechanism: A gas-filled chamber contains two electrodes: an anode (+\) and a cathode (-\). Applying an external voltage creates an electric field. Radiation entering the gas creates ion pairs (free electrons and positive gas ions). Electrons drift toward the positive anode while positive ions migrate toward the negative cathode, producing a measurable signal.

• Operating Voltage Regions in Gas Detectors:
  • Sector I (Recombination Region): Electric field is insufficient to prevent ions and electrons from recombining. No practical detector operates here.
  • Sector II (Ionization Chamber Region / Saturation Region): The applied field collects all primary electrons without secondary ionization. The signal is directly proportional to absorbed radiation energy.
  • Sector III (Proportional Region): Increased voltage accelerates primary electrons, triggering secondary ionizations (Townsend avalanches). Pulse height remains directly proportional to initial deposited energy, enabling pulse-height spectrometry.
  • Sector IV (Geiger-Müller Region): Very high electric field causes complete gas discharge across the entire tube length for every event. Large pulses are generated regardless of initial radiation energy, meaning energy information is lost.
  • Sector V (Continuous Discharge Region): Voltage exceeds the dielectric breakdown threshold of the gas, producing continuous electrical arcing. Detectors must never operate here, as it risks destroying the instrument.
• Comparison of Gas Ionization Detectors:
  • Ionization Chamber:
    • Primary Ionization: Yes.
    • Avalanche Amplification: None.
    • Amplification Factor: Very low ($1$).
    • Signal Magnitude: Small.
    • Single-Event Counting: Limited.
    • Energy Measurement: No direct pulse spectrometry.
    • Primary Application: High-accuracy dose and dose-rate measurements; quality control in diagnostic X-ray systems.
  • Proportional Counter:
    • Primary Ionization: Yes.
    • Avalanche Amplification: Yes (controlled).
    • Amplification Factor: Moderate ($10^3\text{--}10^5$).
    • Signal Magnitude: Intermediate.
    • Single-Event Counting: Yes.
    • Energy Measurement: Yes.
    • Primary Application: Alpha and beta contamination monitoring; spectroscopy.
  • Geiger-Müller Counter:
    • Primary Ionization: Yes.
    • Avalanche Amplification: Yes (massive discharge).
    • Amplification Factor: Very high ($10^6\text{--}10^8$).
    • Signal Magnitude: Very large.
    • Single-Event Counting: Yes.
    • Energy Measurement: None.
    • Primary Application: Radiation survey detection and surface contamination checks.

• Semiconductor Detectors:
  • Operating Principle: Radiation deposits energy in a semiconductor lattice, exciting valence electrons across the bandgap into the conduction band, creating electron-hole pairs. An applied bias separates charges toward opposite electrodes, generating an electrical pulse.
  • Advantage over Gas Detectors: Producing an electron-hole pair requires much less energy ($\approx 3\,\text{eV}$ in silicon or germanium) than ionizing a gas ($\approx 30\,\text{eV}$). This produces more charge carriers per unit of absorbed energy, yielding exceptional energy resolution.
  • Types of Semiconductors:
    • Intrinsic: Highly pure single crystals (pure Silicon or Germanium) where free electron count matches hole count.
    • Extrinsic (Doped):
      • P-Type: Doped with trivalent electron acceptors (boron, indium, gallium), creating an excess of mobile holes as majority carriers.
      • N-Type: Doped with pentavalent electron donors (phosphorus, arsenic, antimony), creating an excess of mobile electrons as majority carriers.
      • P-N Junction: Combines both materials to form a depleted active volume with low leakage current.
  • Clinical Applications: High-resolution gamma spectrometry, solid-state personal dosimetry, digital radiographic image receptors, and computed tomography detector arrays.

• Scintillation Detectors:
  • Operating Principle: Incoming radiation excites a scintillating crystal, producing light photons. These photons strike a photocathode to emit photoelectrons, which are amplified down a dynode chain in a photomultiplier tube (PMT) to produce a measurable electronic pulse.
  • Components:
    • Scintillating Crystal: Commonly thallium-activated sodium iodide ($\text{NaI(Tl)}$).
    • Photocathode: Converts scintillation light into photoelectrons via the photoelectric effect.
    • Photomultiplier Tube (PMT): Dynamically multiplies photoelectrons along high-voltage dynodes, amplifying the signal by factors up to $10^6$.
    • Processing Electronics: Pulse-height analyzers and multichannel analyzers.
  • Operational Strengths: Rapid response time, high stopping power for gamma photons, excellent linearity over broad energy ranges, and high sensitivity. Primary applications include nuclear medicine gamma cameras, SPECT scanners, and scintillation well counters.

• Thermoluminescent Dosimeters (TLD):
  • Operating Principle: Ionizing radiation traps electrons in metastable energy levels within an inorganic crystal lattice. During readout, heating the crystal to $200\text{--}300^\circ\text{C}$ frees the electrons, emitting light with an intensity proportional to the absorbed radiation dose.
  • Luminescent Materials: Lithium fluoride ($\text{LiF}$), calcium fluoride ($\text{CaF}_2$), or calcium sulfate ($\text{CaSO}_4$), typically doped with magnesium ($\text{Mg}$) to introduce electron traps.
  • Advantages: Compact size, environmental ruggedness (resistant to humidity and light), tissue equivalence (for $\text{LiF}$), high reproducibility, wide dynamic range, and reusability over hundreds of read-anneal cycles.
  • Limitations: Requires calibrated thermal readout systems; lacks permanent physical records; subject to signal fading over time; and undergoes sensitivity shifts after high radiation exposures.
• Neutron Detectors:
  • Neutrons possess no net charge and cannot produce direct atomic ionization. Detection relies on nuclear reactions that generate secondary charged particles.
  • Slow (Thermal) Neutrons: Detected using proportional counters filled with boron trifluoride ($\text{BF}_3$) gas, capturing neutrons via the $^{10}\text{B}(n,\alpha)^7\text{Li}$ reaction.
  • Fast Neutrons: Detected by surrounding a thermal neutron counter with hydrogenous moderating material (such as paraffin wax or polyethylene) to slow down incident neutrons prior to capture.
• Radiographic Film Dosimeters:
  • Based on radiation-induced chemical dissociation of silver halide grains ($\text{AgBr}$) in an emulsion layer. Chemical processing deposits metallic silver, creating optical blackening proportional to the absorbed dose.

Radiation Monitoring, Dosimetric Practice, and Operational Implementation

• Classification of Radiological Surveillance:
  • Environmental / Workplace Surveillance (Indirect):
    • Area Monitoring: Direct survey instrumentation to verify workplace classification boundaries.
    • Environmental Air Monitoring: Sampling devices that measure airborne radionuclide concentrations.
    • Surface Contamination Monitoring: Direct surface probes or wipe/smear tests measuring removable activity on equipment and work surfaces.
  • Individual / Personal Surveillance (Direct):
    • External Dosimetry: Passive personal monitors (TLD badges, film badges) and active personal dosimeters worn on the body.
    • Internal Dosimetry: Whole-body counting chambers or bioassay analyses of urine and feces to detect incorporated radionuclides.
• Design Requirements for Personal Dosimeters:
  • Lightweight, ergonomic, and non-obstructive to work duties.
  • Clear, stable, and error-free dose readings.
  • High sensitivity across diagnostic and therapeutic ranges.
  • Ability to measure both electromagnetic and corpuscular radiations.
  • Tissue-equivalent composition to mirror human biological response.
• Operational Protocol for Personal TLD Badges:
  • Crystal Configuration and Filtering: TLD badges typically incorporate two distinct, shielded chips:
    • Shallow Dose ($H_p(0.07)$: Calibrated to measure equivalent dose to skin surface at a tissue depth of $0.07\,\text{mm}$.
    • Deep Dose ($H_p(10)$: Calibrated to measure whole-body dose at a depth of $10\,\text{mm}$.
  • Badge Placement: Must be worn on the torso/chest at chest level to monitor whole-body exposure.
  • Use with Protective Lead Aprons: When wearing a protective lead apron, the primary legal whole-body personal dosimeter must be worn underneath the lead apron to record actual trunk dose.

Regulatory Framework, Worker Classification, and Dose Recording (RD 1029/2022)

• System of Radiological Protection (ICRP Foundations):
  • Justification: No practice involving exposure to ionizing radiation shall be adopted unless it produces a net positive benefit to exposed individuals or society.
  • Optimization (ALARA Principle): All exposures must be kept As Low As Reasonably Achievable, taking into account economic and societal factors.
  • Dose Limitation: The equivalent and effective doses received by occupational workers and members of the general public must not exceed legal thresholds established by regulatory authorities.
• Regulatory Context in Spain:
  • Codified in Real Decreto 1029/2022, approving the Reglamento sobre protección de la salud contra los riesgos derivados de la exposición a las radiaciones ionizantes.
  • The Consejo de Seguridad Nuclear (CSN) is the sole Spanish regulatory body authorized to supervise, inspect, and enforce radiological protection standards.
• Age Limit for Exposed Workers (Article 21, RD 1029/2022):
  • Individuals younger than 18 years of age cannot be assigned tasks that would classify them as occupationally exposed workers.
• Classification of Occupationally Exposed Workers (Article 22, RD 1029/2022):
  • Category A Workers:
    • Personnel who, under normal working conditions, may receive an annual effective dose exceeding $6\,\text{mSv}$ per official calendar year, or an annual equivalent dose exceeding $15\,\text{mSv}$ to the lens of the eye, or exceeding $150\,\text{mSv}$ to the skin and extremities.
    • Surveillance Requirement: Mandatory individual personal dosimetry with monthly dose tracking.
  • Category B Workers:
    • Exposed personnel unlikely to exceed the Category A thresholds ($\le 6\,\text{mSv/year}$ effective dose, $\le 15\,\text{mSv/year}$ lens of the eye, $\le 150\,\text{mSv/year}$ skin and extremities).
    • Surveillance Requirement: Monitored via workplace area dosimetry or individualized annual dose assessments.

• Individual Dosimetric History Requirements (Articles 39–43, RD 1029/2022):
  • Employers must maintain an Individual Dosimetric History for every Category A exposed worker.
  • Required Documentation Elements:
    • Monthly recorded doses ($H_p(10)$, $H_p(3)$, $H_p(0.07)$).
    • Cumulative doses accrued over each official calendar year.
    • Cumulative deep doses over five consecutive official calendar years.
    • Doses received during specially authorized emergency exposures.
  • Record Retention Period:
    • Individual exposure records must be preserved until the worker reaches (or would have reached) 75 years of age, and never for less than 30 years following the termination of work involving radiation exposure.
  • National Oversight: All dosimetric records are sent to the Banco Dosimétrico Nacional overseen by the CSN, supporting the radiation passport (carné radiológico).

Practical Activities, Worked Calculations, and Review Questions

• Worked Problems in Radioprotection Quantities:
  • Problem 1 (Equivalent Dose for Alpha Radiation):
    • Input: Patient absorbed dose $D = 5\,\text{Gy}$; radiation beam consists of alpha particles ($w_R = 20$).
    • Calculation:           \n          H = D \cdot w_R = 5\,\text{Gy} \times 20 = 100\,\text{Sv}\n          
    • Answer: $100\,\text{Sv}$.
  • Problem 2 (Equivalent Dose for Photon Radiation):
    • Input: Patient absorbed dose $D = 2\,\text{Gy}$; radiation beam consists of photons ($w_R = 1$).
    • Calculation:           \n          H = D \cdot w_R = 2\,\text{Gy} \times 1 = 2\,\text{Sv}\n          
    • Answer: $2\,\text{Sv}$.
  • Problem 3 (Effective Dose to Lungs):
    • Input: Lung equivalent dose $H = 10\,\text{Sv}$; lung tissue weighting factor $w_T = 0.12$.
    • Calculation:           \n          E = H \cdot w_T = 10\,\text{Sv} \times 0.12 = 1.2\,\text{Sv}\n          
    • Answer: $1.2\,\text{Sv}$.
  • Problem 4 (Effective Dose to Salivary Glands):
    • Input: Salivary gland equivalent dose $H = 50\,\text{Sv}$; salivary gland tissue weighting factor $w_T = 0.01$.
    • Calculation:           \n          E = H \cdot w_T = 50\,\text{Sv} \times 0.01 = 0.5\,\text{Sv}\n          
    • Answer: $0.5\,\text{Sv}$.
  • Problem 5 (Detector Efficiencies):
    • Input: A radioactive source emits $200$ radiation events per second. The detector active area intercepts $50$ of these events, and $30$ of those events generate an output signal.
    • Extrinsic Efficiency Calculation:           \n          EE = \frac{50}{200} = 0.25 \implies 25\%\n          
    • Intrinsic Efficiency Calculation:           \n          EI = \frac{30}{50} = 0.60 \implies 60\%\n          
• Instrument Selection Scenarios:
  • Scenario A: Measuring precise radiation dose rate within a diagnostic X-ray room.
    • Selection: Ionization Chamber (operates in saturation region without avalanche amplification, yielding signals proportional to total charge).
  • Scenario B: Verifying whether a benchtop is contaminated with trace alpha or beta emitting isotopes.
    • Selection: Geiger-Müller Counter or Proportional Counter (delivers high amplification to reliably detect low-level radioactive contamination).
  • Scenario C: Counting individual radioactive particles while resolving their specific kinetic energy spectrum.
    • Selection: Proportional Counter or Semiconductor Detector (avalanche or charge-carrier generation remains proportional to the deposited particle energy).

Questions & Discussion

• Natural Cosmic Radiation Exposure in Commercial Aviation:
  • Question: What physical link exists between natural background radiation and commercial airline pilots? Are aircrews occupationally exposed to radiation?
  • Answer: Commercial aircraft operate at cruising altitudes between $9,000$ and $12,000\,\text{meters}$, where atmospheric shielding against galactic cosmic rays and solar particle events is reduced. Cosmic radiation dose rates increase with higher altitudes and northern latitudes. As a result, airline flight crews receive higher annual natural equivalent doses than ground workers and are recognized as occupationally exposed workers subject to dosimetric modeling and radiation protection guidelines.