Radiation Dosimetry and Dosimeters

Engineering of Radiation Dosimetry and Dosimeters

• The document provides an overview of radiation dosimetry, dosimeters, and radiation protection

Unit of Absorbed Dose

• A fundamental unit of radiation lacks a definition due to varying interactions and radiation qualities across biological models
• Relate damage mechanisms to the ionizing energy deposited in tissues
• Microscopic energy distribution affects biological response
• Macroscopic assessment of absorbed energy is quantified by the SI unit Gray (Gy).
• Absorbed dose (D) is the quotient of dE by dm, where dE is the mean energy from ionizing radiation in matter of mass dm
• D = de/dm
• 1 gray is equivalent to 1 joule per kilogram (1 Gy = 1 J/kg⁻¹)

Calorimetry

• Calorimetry measures absorbed dose by quantifying minute temperature changes in irradiated samples
• Heat energy absorbed by a sample is proportional to the absorbed dose

Exposure

• Exposure is not always practical
• Other methods exist for less energetic X-rays and electrons
• It is important to specify a unit of ionization compatible with absorbed dose
• Exposure (X) is the quotient of dQ by dm, where dQ is the absolute charge value of ions of one sign
• These ions are produced in air from electrons (negatrons and positrons) liberated by photons in air of mass dm
• Electrons must be completely stopped in air.
• SI unit of exposure: coulombs per kilogram (C/kg)
• Exposure explicitly defines photons interacting with a defined mass of air

External Dosimetry

• External dosimetry measures the dose when the radiation source is outside the body
• External dosimetry focuses on radiation that penetrates the skin, such as beta, photon, and neutron radiation
• Photons and beta interact through electronic forces, while neutrons interact through nuclear forces

Dosimeters

• A dosimeter is a small radiation detection device worn to measure doses from ionizing radiation
• Dosimeters are either passive or active

Passive Dosimeters

• Passive dosimeters produce a radiation-induced signal which is stored
• The dosimeter is processed and the output is analyzed

Active Dosimeters

• Active dosimeters produce a radiation-induced signal
• Active dosimeters display a direct reading of the detected dose or dose rate in real time
• Dosimeters are worn to estimate effective doses, typically between the waist and neck
• Dosimeters are worn on the front of the torso facing the radioactive source
• Whole-body dosimeters are worn on the torso
• Dosimeters may also be worn on extremities or near the eye to measure equivalent dose

Dosimeter Selection Factors

• Energy and angle dependence: A dosimeter's response varies with radiation energy and the angle between the source and detector
• Radiation type detection: Dosimeters vary in detecting alpha, beta, photon, or neutron radiation
• Fading: A dosimeter's signal may fade over time due to temperature, light, and humidity
• Re-read ability: Some dosimeters lose signals upon processing, others retain signals allowing multiple readings
• Minimum measurable dose: This is the lowest detectable dose at a specified confidence level
• Ruggedness and wearability: Dosimeters vary in their ability to withstand environmental conditions, size, and portability

Dosimetry for Photon and Beta Radiation

• Photon radiation has more penetrating power than alpha and beta radiation
• Alpha radiation does not penetrate the outer skin layer and poses no external health hazard
• Beta and photon radiation are hazardous to the skin and eye
• Beta radiation does not pose a significant risk to the organs under the skin
• Penetrating ability and interaction probability of radiation are related to the radiation's energy
• External beta radiation from tritium is not a hazard, only present internal radiation hazard
• A typical dosimeter has a detector in a holder
• Dosimeters measuring photon and beta radiation use a filter/holder to differentiate equivalent dose to the skin/eye and effective dose
• An open window or thin filter measures equivalent dose to skin
• A thicker filter measures effective dose allowing only penetrating radiation to deposit energy
• Dosimeters use filters of varying thickness to discriminate among energy levels
• Types of dosimeters include film, thermoluminescent dosimeters (TLDs), optically stimulated luminescence dosimeters (OSLDs), and direct reading dosimeters (DRDs)

Thermoluminescent Dosimeters (TLDs)

• Thermoluminescence emits light from certain materials
• Energy from radiation is re-emitted as light when heated
• TLD system includes a dosimeter and a reader to determine accurate dose
• Ionizing radiation releases electrons within the detector (chip)
• Electrons are trapped in impurities (doping centers) in an excited state
• The chip is heated in a TLD reader, and the electrons emit photons of visible light, returning to ground state
• The glow curve is analyzed to determine the dose
• TLD types: lithium fluoride, calcium sulphate and lithium borate dosimeters

Direct Reading Dosimeters (DRDs)

• DRDs are active dosimeters that display doses and dose rates while in use
• DRDs give alarms for preset doses and dose rates
• Common DRDs use electronic dose readings with a diode or GM detector
• DRDs provide real-time estimates and are used in work planning and execution
• They are not typically for measuring doses of record

Radiation Protection

• Radiation protection is protecting people/environment from the harmful effects of ionizing radiation
• Includes particle and high-energy electromagnetic radiation
• Ionizing radiation sources present significant health hazards
• Results in skin burns and radiation sickness at high exposure
• Results in cancer, tumor, and genetic damage at low exposure

ALARP/ALARA

• ALARP is a principle in radiation exposure and occupational health risk
• "As Low As Reasonably Practicable" is the main principle
• Aims to minimize radioactive exposure risk
• Some exposure may be acceptable to further a task
• The equivalent term, ALARA: "As Low As Reasonably Achievable" is also used

Reducing Radiation Exposure

• There are four major ways to reduce radiation exposure
• Shielding: Use proper barriers to block or reduce ionizing radiation
• Time: Spend less time in radiation fields
• Distance: Increase distance between radioactive sources and workers/population
• Amount: Reduce the quantity of radioactive material during use

Radiation Safety Concepts

• Time: Radiation exposure increases/decreases with the time spent near the source
• Minimize time for external direct exposure
• Gamma and x-ray are primary concerns for external exposure
• Biological half-life controls time of exposure if material enters the body
• Distance: Exposure decreases further from the radiation source
• Depends on radiation energy/size of the source
• Distance is vital with gamma rays due to long distances
• Alpha and beta particles do not have enough energy to travel
• Shielding: The greater the shielding, the smaller the exposure
• Shielding absorbs radiation between a person and the source

Radiation Types and Shielding

• Effectiveness of shielding varies
• Alpha: Thin materials like paper or dead skin cells provide adequate shielding
• Living tissue offers no protection against inhaled alpha emitters
• Beta: Additional covering like heavy clothing protects against emitted beta particles.
• Gamma: Thick, dense shielding like lead is necessary with gamma rays
• The higher the gamma ray energy, the thicker the lead must be
• X-ray: X-ray technicians provide lead aprons
• Neutron: Not readily absorbed as charged particle radiation
• Absorbed by nuclei in nuclear reaction
• Cosmic: Earth's atmosphere and magnetosphere act as shields
• Ultraviolet (UV): Shielded by thin opaque layers like sunscreen, clothing, and eyewear.