Chapter 9: Basic Radiation Protection and Radiobiology

Core Concepts and Key Terminology

• Epigraph and Theoretical Context:

  • Adrienne Rich, "Power," The Dream of a Common Language: Poems 1974–1977: "Today I was reading about Marie Curie / she must have known she suffered from radiation sickness / She died a famous woman denying her wounds / denying her wounds that came from the same source as her power".

• Core Educational Objectives:

  • Identify all natural and human-made sources of ionizing radiation.

  • List standard physical quantities, SI and traditional units used to measure radiation exposure, and their precise clinical applications.

  • Explain the physical mechanisms by which x-rays interact with matter in both diagnostic and therapeutic energy ranges.

  • Enumerate regulatory radiation exposure limits established for occupational workers, trainees, and the general public.

  • Explain the biological factors determining cellular radiosensitivity.

  • Describe the systemic and total-body responses to varying acute and chronic radiation doses.

  • Detail technical practices used to minimize radiation exposure to patients and occupational personnel.

  • Describe the operational mechanisms, sensitivity thresholds, and physical construction of radiation monitoring devices and field survey instruments.

• Key Terms and Verbatim Definitions:

  • Air Kerma: SI quantity used to measure energy transferred from radiation to matter, which may be at the surface of a patient's or radiologic and imaging sciences professional's body.

  • ALARA: Mnemonic meaning to keep all radiation exposure as low as reasonably achievable.

  • Becquerel ($\text{Bq}$): Unit of radioactivity in the International System of Units, equal to one disintegration per second ($1\,\text{d/s} = 1\,\text{Bq}$).

  • Classic Coherent Scattering: Interaction with matter in which a low-energy photon (below $10\,\text{keV}$) is absorbed and released with its same energy, frequency, and wavelength but with a change of direction.

  • Compton Scattering: Interaction with matter in which a higher energy photon strikes a loosely bound outer electron, removing it from its shell, and the remaining energy is released as a scattered photon.

  • Curie ($\text{Ci}$): Unit of radioactivity defined as the quantity of any radioactive nuclide in which the number of disintegrations per second is $3.7 \times 10^{10}$.

  • Exposure ($X$): The amount of radiation delivered to a point. Measured in coulombs per kilogram ($\text{C/kg}$).

  • Germ Cells: Cells of an organism whose function is to reproduce the organism (e.g., ovum, spermatozoa).

  • Gray ($\text{Gy}$): Unit in the International System used to measure the amount of energy absorbed in any medium; $1\,\text{Gy} = 100\,\text{radiation absorbed doses (rad)}$.

  • International System (SI) of Units: System of units based on metric measurement developed in 1948 and having units used to measure radiation (officially adopted in 1985).

  • Kiloelectron Volts ($\text{keV}$): Units of energy equal to $1000\,\text{electron volts}$.

  • Photoelectric Interaction: Interaction with matter in which a photon strikes an inner shell electron, causing its ejection from orbit with the complete absorption of the photon's energy.

  • Radiation: Forms of energy emitted and transferred through matter.

  • Sievert ($\text{Sv}$): Unit in the International System used to measure the dose equivalence, or biologic effectiveness, of differing radiations; $1\,\text{Sv} = 100\,\text{rem}$.

  • Somatic Cells: All of the body's cells except germ cells.

  • X-rays: Form of electromagnetic radiation traveling at the speed of light, possessing the ability to penetrate matter.

Nature and Sources of Ionizing Radiation

• Fundamental Nature of Ionizing Radiation:

  • Ionizing radiation is energy capable of penetrating matter and possesses sufficient energy to eject orbital electrons along its path, thus ionizing atoms.

  • Exposure to radiation always involves a risk for biological changes that cannot be ignored.

  • The primary clinical objective is ensuring that the diagnostic benefits of disease detection outweigh the inherent radiation risks through sound clinical judgment and dose minimization.

• Natural (Background) Radiation Sources:

  • Background sources occur spontaneously in nature and are completely unaffected by human activity.

  • Cosmic Radiation: Extraterrestrial radiation originating from the sun and other planetary bodies.

  • Terrestrial Substances: Naturally occurring radioactive materials present in the earth's crust, such as uranium and radium.

  • Internal Exposure: Ingested or inhaled radionuclides present in food, water, or air (specifically radon gas).

• Human-Made (Artificial) Radiation Sources:

  • Nuclear Industry: Atmospheric fallout from historical aboveground nuclear weapons testing, releases from nuclear power plant accidents, and nuclear waste disposal.

  • Radionuclides and Consumer Products: Consumer items containing small radioactive quantities, such as ionization smoke detectors, building materials, video monitors, suntan beds, and microwave ovens (which contribute minimally to annual exposure).

  • Radiopharmaceuticals: Unsealed radioactive substances administered for medical diagnosis and therapy.

  • Medical and Dental X-Rays: Diagnostic medical and dental imaging procedures constitute the single largest source of human-made ionizing radiation exposure to the population.

• Physics and Mechanics of X-Ray Production:

  • X-rays are non-particulate bundles of electromagnetic energy (photons) moving as waves through space at the speed of light ($3.00 \times 10^8\,\text{m/s}$) and depositing energy randomly.

  • Three Essential Conditions for X-Ray Generation:

  1. A source of free electrons.

  2. A means to rapidly accelerate the electrons.

  3. A target medium to rapidly decelerate/stop the electron movement.

  • X-Ray Tube Assembly Components:

- Enclosed in a vacuum sealed within a heat-resistant Pyrex glass or metal envelope.
- **Cathode Assembly**: Negative terminal containing a filament made of thoriated tungsten. Application of electric current (milliamperage, $\text{mA}$) heats the filament, boiling off electrons via **thermionic emission**.
- **Anode Assembly**: Positive terminal featuring a molybdenum neck and tungsten rotating disc target.
- **Accelerating Potential**: High voltage (kilovoltage, $\text{kVp}$) applied across the terminals forces free electrons to accelerate instantaneously from cathode to anode.
- **Energy Conversion**: High-speed electrons strike the anode target, converting kinetic energy into thermal energy (> 99\% heat) and electromagnetic energy (< 1\% x-rays).
- **Beam Characteristics**: The emerging primary beam is **heterogeneous** (polychromatic), containing photons of many different energies measured in kiloelectron volts ($\text{keV}$).

• Primary Beam Interactions with Matter:

  1. Complete absorption within the tissue.

  2. Partial energy transfer resulting in scatter radiation.

  3. Direct transmission through tissue completely unaffected.

X-Ray Interactions with Matter

• Overview of Interaction Energy Ranges:

  • X-rays interact with matter in five distinct physical processes: classic coherent scattering, photoelectric absorption, Compton scattering, pair production, and photodisintegration.

  • Diagnostic Energy Range: Classic coherent scattering, photoelectric absorption, and Compton scattering occur within diagnostic radiology energy spectrums.

  • Therapeutic Energy Range: Pair production and photodisintegration occur exclusively at high megavoltage therapeutic energy levels.

  • Clinical Relevance: Photoelectric absorption and Compton scattering dictate both patient dose absorption and occupational worker radiation hazard.

• Classic Coherent Scattering:

• Also termed unmodified, Thomson, or Rayleigh scattering.

• Occurs with low-energy x-ray photons possessing energy levels below $10\,\text{keV}$.

• Mechanism: An incoming x-ray photon strikes an atom and is completely absorbed, causing the atom to become temporarily excited. The atom rapidly releases the excess energy as a new x-ray photon with identical energy, frequency, and wavelength as the incident photon, but traveling in a modified direction.

• Result: The majority of scattered photons travel forward. No energy transfer occurs to the patient, resulting in no absorbed dose from this specific interaction.

  • Photoelectric Absorption:

• Primary interaction responsible for image contrast and patient exposure in diagnostic radiography.

• Mechanism: An incoming x-ray photon strikes an inner-shell electron (typically K-shell) of a target atom and ejects it from orbit, creating an ion pair. The incident photon transfers all of its energy and ceases to exist (complete absorption).

• Photoelectron: The ejected inner-shell electron travels through adjacent matter, causing further ionizations until its kinetic energy is dissipated.

• Characteristic Radiation / Secondary Radiation: As outer-shell electrons (L-shell, M-shell) drop down to fill inner-shell orbital vacancies, they release discrete energy bundles in the form of characteristic x-rays within the patient's body.

• Clinical Impact: Photoelectric absorption represents the single greatest hazard to patients in diagnostic imaging.

  • Compton Scattering:

• Also known as modified scattering.

• Mechanism: A high-energy incoming x-ray photon strikes a loosely bound outer-shell electron of an atom. Part of the photon's energy ejects the outer electron (creating a Compton or recoil electron). The remaining photon energy continues in a altered direction as a scattered photon with reduced energy and longer wavelength.

• Fate of Products: The recoil electron produces secondary ionizations until stopped. The scattered photon can exit the patient, striking the image receptor (causing image fog) or striking clinical personnel.

• Clinical Impact: Compton scattering is responsible for the vast majority of occupational radiation exposure received by radiologic technologists and physicians.

Radiation Quantities, Units, and Regulatory Standards

• Definitions of Physical Radiation Units:

  • Exposure ($X$): Quantification of ionization in dry air produced by x-rays or gamma rays.

  • Quantitatively defined as the amount of radiation that generates $2.08 \times 10^9\,\text{ion pairs per cubic centimeter (cc)}$ of air at standard temperature and pressure, representing a total electrical charge of $2.58 \times 10^{-4}\,\text{coulombs per kilogram (C/kg)}$.

  • Air Kerma: Kinetic energy transferred from a radiation beam to a specific mass of air or tissue. Expressed in joules per kilogram ($\text{J/kg}$), where $1\,\text{J/kg} = 1\,\text{Gray (Gy}_a)$.

  • Absorbed Dose ($D$ / $\text{EFD}$): The amount of energy absorbed per unit mass of matter (e.g., patient tissue). Measured in Gray ($\text{Gy}_t$).

  • $\text{Gy}_a$ designates radiation absorbed dose in air; $\text{Gy}_t$ designates absorbed dose in biological tissue.

  • Effective Dose ($\text{EfD}$) & Dose Equivalent ($\text{EqD}$):

  • Effective Dose: Best indicator of overall biological risk to humans from partial-body or whole-body ionizing radiation exposure.

  • Dose Equivalent: Product of the average absorbed dose in a specific tissue/organ and its corresponding radiation Quality Factor ($Q$):       $\text{EqD (Sv)} = \text{Absorbed Dose (Gy)} \times Q$

  • Quality Factors ($Q$):

    • X-rays and Gamma rays: $Q = 1$

    • Beta particles: $Q = 1$

    • Fast Neutrons: $Q = 10$ (indicates neutrons are 10 times more biologically destructive than x-rays for an equivalent absorbed dose)

    • Alpha particles: $Q = 20$

  • Radioactivity ($\text{Bq}$ / $\text{Ci}$): Quantity of radioactive material undergoing nuclear disintegrations per unit time.

  • $1\,\text{Becquerel (Bq)} = 1\,\text{disintegration per second (d/s)}$.

  • $1\,\text{Curie (Ci)} = 3.7 \times 10^{10}\,\text{Bq} = 3.7 \times 10^{10}\,\text{d/s}$.

• Summary Table of Radiation Quantities and Units:

• Regulatory Framework and Agencies:

  • CDRH (Center for Devices and Radiological Health): Operating under the US Food and Drug Administration (FDA), sets and enforces manufacturing and safety performance standards for radiation-producing equipment.

  • NCRP (National Council on Radiation Protection and Measurements): Non-profit advisory organization chartered by US Congress in 1964. Collects, analyzes, and publishes authoritative radiation protection recommendations (non-enforcing body).

  • NRC (Nuclear Regulatory Commission): Federal government agency possessing direct statutory enforcement authority over radiation safety standards, radioisotope licensing, and nuclear regulations.

• Dose-Response Relationships and Protection Philosophy:

• Nonthreshold Model: Assumes that no safe radiation dose threshold exists below which biological damage is zero. Any radiation dose carries a incremental risk.

• Threshold Model: Assumes a minimum dose level must be reached before observable biological damage occurs.

• Risk Versus Benefit: Radiographic procedures must yield a diagnostic benefit to the patient that clearly outweighs the potential risks of biological harm.

• ALARA Principle: Guideline requiring that occupational and patient exposures be maintained As Low As Reasonably Achievable.

  • Recommended Effective Dose Limits:

• Cumulative Dose Calculation: A 40-year-old technologist has a lifetime cumulative whole-body effective dose limit calculated as:     $\text{Cumulative Limit} = 10\,\text{mSv} \times 40 = 400\,\text{mSv} \quad (0.4\,\text{Sv})$

Radiobiology and Cellular Response to Radiation

• Basic Cellular Structure and Architecture:

• Cells represent the fundamental functional unit of organic protoplasm capable of independent life.

• Nucleus: Enclosed by a porous double-walled nuclear envelope. Houses chromosomes composed of genes. Genes consist of deoxyribonucleic acid (DNA), structured as a double-stranded helical spiral staircase. DNA is the primary target molecule vulnerable to radiation damage.

• Cytoplasm: Composes the bulk of cellular volume, composed of approximately $80\%$ water. Contains cellular organelles:

  • Mitochondria: Cellular energy production.

  • Ribosomes: Protein synthesis.

  • Endoplasmic Reticulum: Intracellular transport network.

  • Lysosomes: Intracellular waste breakdown.

  • Centrosome: Coordinates mitotic spindle creation during cell division.

  • Cell Types and Division Mechanisms:

• Somatic Cells: Comprise all body tissues except reproductive cells. Contain 46 paired chromosomes ($23\,\text{pairs}$). Divide via mitosis.

• Germ Cells: Reproductive cells (ova and spermatozoa). Contain 23 single unpaired chromosomes ($23\,\text{total}$). Divide via meiosis.

  • Theories of Cellular Radiation Action:

• Direct Hit Theory: Ionizing radiation transfers its energy directly to critical cellular macromolecules (DNA). Direct breaks in nucleotide bases or hydrogen/phosphate bonds alter genetic coding, risking cell mutation or lethality.

• Indirect Hit Theory: Radiation interacts with abundant water molecules ($\text{H}_2\text{O}$) surrounding cellular components (radiolysis of water). Ionization creates free radicals and toxic chemical compounds (such as hydrogen peroxide) that alter the internal cellular environment, damaging DNA.

• Dominant Mechanism: The vast majority of cellular damage caused by diagnostic x-rays and gamma radiation results from indirect hits due to high cellular water content.

  • Target Theory of Absorption:

• Molecules in abundant supply within the cell (such as water or specific enzymes) can sustain radiation damage without compromising immediate cell survival.

• Molecules in limited supply with no immediate replacement are critical target molecules.

• The designated target molecule essential for cell survival and reproduction is DNA in the cell nucleus.

  • Cellular Radiosensitivity Determinants:

• Law of Bergonie and Tribondeau (1906): Cellular radiosensitivity is directly proportional to mitotic activity and inversely proportional to degree of differentiation/specialization.

  • High Radiosensitivity (Stem/Primitive/Unspecialized Cells): Basal cells of skin, crypt cells of small intestine, germ cells, erythroblasts.

  • High Radioresistance (Highly Specialized/Non-dividing Cells): Nerve cells, muscle cells, brain tissue.

• Ancel and Vitemberger Modification: All biological cells possess equal intrinsic sensitivity to radiation; the observed difference lies in the time of expression of radiation injury. Rapidly dividing cells express injury much faster than slowly dividing cells.

  • Total-Body Responses: Acute Radiation Syndromes (ARS):

• ARS occurs only when an organism receives a large total-body exposure from an external radiation source delivered over a short timeframe (a few minutes).

• Three Sequential Stages of ARS:

  1. Prodromal Stage: Immediate onset of nausea, vomiting, and diarrhea (NVD).

  2. Latent Period: Symptom-free period during which biological damage progresses internally.

  3. Manifest Stage: Full clinical expression of systemic illness leading either to recovery or death.

• Acute Radiation Syndrome Categories:

• Late Effects of Radiation Exposure:

  • Long-term biological somatic or genetic alterations developing months or years after radiation exposure.

  • Somatic Late Effects (Exposed Individual):

  • Cataractogenesis: Lens of the eye is highly radiosensitive; high doses induce opacity and blindness.

  • Carcinogenesis: Radiation-induced cancer. Historical cases include: radiographers' hand skin cancer (1902); early radiologists developing leukemia; watch-dial painters ingesting radium-containing paint developing osteosarcoma; underground miners inhaling radioactive radon gas developing lung cancer.

  • Genetic Late Effects (Future Generations):

  • Radiation damage to germ cell DNA induces gene mutations.

  • Radiation mutations are universally recessive; expressed only if a mutated germ cell pairs with another germ cell carrying the identical genetic mutation.

• Clinical Side Effects of Radiation Therapy:

  • Early (Acute) Side Effects: Short-term, treatable reactions occurring during or immediately following treatment. Includes generalized fatigue, nausea/vomiting, skin reactions (erythema, dryness, itching, peeling, blistering). Head and neck irradiation damages salivary glands and oral mucosa, causing xerostomia and dysphagia.

  • Late Side Effects: Long-term complications emerging months or years later. Includes lymphedema (lymphatic fluid obstruction causing severe swelling following lymph node surgery/irradiation), tissue fibrosis, organ failure, limb loss in soft-tissue sarcoma treatments, and permanent infertility from pelvic/abdominal fields.

Radiation Protection Procedures for Patients and Personnel

• Cardinal Principles of Radiation Protection:

  • Applies to both patient dose reduction and occupational safety: Time, Distance, Shielding.

• Patient Protection Strategies:

  • Time: Minimize exposure duration. Use accurate technical technique charts, short exposure times, and proper patient positioning to eliminate repeat exposures.

  • Distance: Maintain maximal distance between radiation source and patient. Standard Source-to-Image Receptor Distance (SID) must be at least 40 inches ($100\,\text{cm}$).

  • Gonadal and Fetal Shielding Policy: Updated clinical guidelines discontinue routine patient gonadal and fetal shielding due to evidence showing negligible dose reduction benefits and risk of obscuring diagnostic anatomy or artifactually increasing automatic exposure control (AEC) dose.

  • Targeted Shielding: Lead shields ($Z = 82$) absorb x-ray photons via photoelectric interaction. Shielding is applied to specific non-targeted organs outside the primary beam area of interest: lens of eyes, thyroid gland, and breast tissue.

  • Beam Restriction (Collimation): Strictly collimate primary x-ray beam to anatomical area of interest, never exceeding image receptor dimensions.

  • Image Receptor Speed: Utilize high-speed image receptor systems to minimize required exposure factors.

  • Technical Factor Selection: Utilize high kilovoltage ($\text{kVp}$) and low milliamperage-seconds ($\text{mAs}$) techniques. High $\text{kVp}$ shifts interaction probabilities toward Compton scattering, decreasing photoelectric absorption within patient tissues.

  • X-Ray Beam Filtration: Insert aluminum filters in x-ray beam path to remove low-energy non-penetrating photons that increase entrance skin dose without contributing to image formation.

• Occupational Worker Protection Strategies:

  • Time: Minimize duration spent in active exposure room. Radiographic rooms retain zero residual radiation once exposure terminates. Fluoroscopic equipment features a mandatory 5-minute cumulative timer alarm.

  • Distance: Distance represents the single most effective protection measure for occupational personnel. Governed by the Inverse Square Law:     $\frac{I_1}{I_2} = \frac{(d_2)^2}{(d_1)^2}$

  • Physical Law: Radiation intensity varies inversely with the square of the distance from the point source.

  • Worked Mathematical Example: If a radiographer receives an exposure intensity of $20\,\text{Gy}$ standing $1\,\text{m}$ from the tube, calculate the intensity at a distance of $2\,\text{m}$:       $\frac{\text{New intensity}}{20\,\text{Gy}} = \frac{1^2}{2^2}$       $\text{New intensity} = \frac{1 \times 20\,\text{Gy}}{4} = 5\,\text{Gy}$

  • Rule of Thumb: Doubling distance from radiation source reduces exposure by a factor of 4 ($1/4$ original intensity). Halving distance increases exposure 4-fold.

  • Patient Restraint: Radiologic technologists must never hold patients during exposures. Utilize mechanical immobilization devices (sandbags, restraint straps). If mechanical restraints fail, recruit non-occupational personnel (e.g., patient relative, nurse) wearing lead protective apparel.

  • Protective Shielding & Apparel:

  • Standard lead aprons contain $0.25$ to $1.0\,\text{mm}$ lead equivalency. Aprons worn during fluoroscopy must contain at least $0.5\,\text{mm}$ lead equivalency. Minimum lead equivalency for aprons operated at $100\,\text{kVp}$ is $0.25\,\text{mm}$.

  • Apparel Storage and Testing: Aprons and gloves must be hung on specialized racks; folding causes lead lining cracks. Protective apparel must undergo mandatory annual fluoroscopic inspection for integrity checks.

  • Structural Barriers:

  • Primary Barriers: Structural surfaces capable of being struck directly by the primary useful beam.

  • Secondary Barriers: Structural surfaces struck only by secondary leakage or scatter radiation (e.g., control booth partition, lead aprons).

• Summary of Radiation Protection Measures:

• Pregnant Student and Worker Regulations:

  • Program accreditation by JRCERT requires distribution of NRC pregnancy regulations to female students.

  • Voluntary Written Declaration: Declaration of pregnancy must be voluntary, written, dated, and include estimated month of conception. Oral declarations hold no legal status. Declaration can be revoked in writing at any time.

  • Dose Limits: Declared pregnant student/worker dose limit is $5\,\text{mSv}$ ($0.5\,\text{rem}$) for entire pregnancy, with a monthly limit of $0.5\,\text{mSv}$. Undeclared workers remain under standard occupational limit ($50\,\text{mSv/yr}$).

  • Fetal Dosimeter: Declared workers receive a secondary dosimeter (fetal badge) worn at waist level (underneath lead apron during fluoroscopy).

  • Equal Exposure Distribution: Regulations discourage reassigning pregnant personnel to lower exposure clinical rotations to avoid disproportionate exposure increases to non-pregnant peers.

  • Legal Workplace Standards: US Supreme Court rulings prohibit employers from excluding women of childbearing age from jobs based on potential fetal risk.

Personnel Radiation Dosimetry and Monitoring Instruments

• Monitoring Criteria and Protocol:

  • Required for any worker potentially receiving $10\%$ or more of the annual occupational effective dose limit of $50\,\text{mSv}$ (i.e., $\ge 5\,\text{mSv/yr}$).

  • Primary dosimeters must be worn continuously at collar level, outside of lead protective aprons, facing forward.

• Optically Stimulated Luminescence (OSL) Dosimeter:

• Construction: Manufactured as the Luxel OSL dosimeter (Landauer). Contains a thin strip of aluminum oxide ($\text{Al}_2\text{O}_3$) sensing material, copper filter, open window, tin filter, and imaging filter enclosed in a tamper-proof plastic blister pack.

• Operation & Readout: Radiation excites electrons in aluminum oxide. During processing, laser light strikes the material, causing luminescence proportional to radiation dose received.

• Energy Range & Sensitivity: Detects x-rays and gamma rays from $5\,\text{keV}$ to over $40\,\text{MeV}$. Dose measurement range spans $0.01\,\text{mSv}$ to $1000\,\text{Sv}$. Exposures below $0.01\,\text{mSv}$ reported as Minimal ("M").

• Wear Cycle & Properties: Exchanged monthly or bi-monthly. Sealed against heat, moisture, and pressure. Main drawback: requires mailing to processing lab (no immediate reading).

  • Thermoluminescent Dosimeter (TLD):

• Construction: Plastic casing holding lithium fluoride ($\text{LiF}$) crystal sensing material.

• Operation & Readout: Radiation excites valence electrons into trapped states within the crystal lattice ("forbidden zone"). Processing heats crystals (annealing); trapped electrons return to ground state, emitting light photons proportional to dose.

• Reusability & Sensitivity: Reusable after thermal annealing. Sensitive to minimum doses of $0.05\,\text{mSv}$. Wear time up to 3 months.

  • Direct Ion Storage (DIS) / Pocket Dosimeter:

• Construction: Miniature gas-filled ionization chamber connected to solid-state Electrically Erasable Programmable Read-Only Memory (EEPROM).

• Operation & Readout: Radiation ionizes chamber gas, storing proportional electrical charge in EEPROM memory. Read out instantaneously by connecting via USB to computer software.

• Advantages: Provides immediate exposure access; eliminates institutional mailing requirements.

  • Summary Table of Personnel Dosimeters:

• Field Survey Instruments:

  • Used to detect ambient radiation presence and quantify radiation source intensity.

  • Geiger-Müller (GM) Counter:

  • Gas-filled ionization detector consisting of an electrode suspended in a chamber with negatively charged walls and a positively charged central anode.

  • Mechanism: Radiation passing through chamber ionizes air; freed electrons accelerate toward positive electrode, producing measurable electrical pulses.

  • Meter Readout: Reads out radiation exposure in coulombs per kilogram ($\text{C/kg}$) or milliroentgens per hour ($\text{mR/hr}$). Commonly used for area radiation surveys and detecting radioactive contamination.