ABMP Medical Health Physics flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Half-life

Time required for the activity of a radionuclide to decrease to half its initial value; T½ = ln2/λ, where λ is the decay constant.

Becquerel

SI unit of radioactivity equal to one disintegration per second; 1 Ci = 3.7×10^10 Bq.

Photoelectric effect

Photon interaction in which the photon transfers all its energy to an inner-shell electron, which is ejected; dominant at low energies and high atomic number.

Geiger-Mueller counter

Gas-filled detector operating in the Geiger region; highly sensitive but cannot discriminate photon energy and has significant dead time.

Ionization chamber

Gas-filled detector operating in the ionization region; used as the reference instrument for radiation protection surveys and dose rate measurements.

HPGe detector

High-purity germanium semiconductor detector providing excellent energy resolution for gamma spectroscopy; requires cryogenic cooling.

Absorbed dose

Energy deposited by ionizing radiation per unit mass of material; SI unit is the gray (Gy), equal to 1 J/kg.

Equivalent dose

Absorbed dose averaged over a tissue or organ multiplied by the radiation weighting factor w_R; SI unit is the sievert (Sv).

Effective dose

Sum over tissues of equivalent dose multiplied by tissue weighting factor w_T; used to estimate stochastic risk; unit is the sievert.

Kerma

Kinetic energy released per unit mass in a medium by indirectly ionizing radiation; air kerma is the basis of many calibration protocols.

Stochastic effect

Radiation effect, such as cancer, for which probability increases with dose and for which no threshold is assumed.

Deterministic effect

Tissue reaction that has a threshold dose and whose severity increases with dose, such as erythema or cataracts.

Linear-quadratic model

Cell survival model S = e^(−αD−βD²); α/β ratio distinguishes early-responding (~10 Gy) from late-responding (~3 Gy) tissues.

ALARA

Principle of keeping radiation doses as low as reasonably achievable, considering economic and social factors, through time, distance, and shielding.

Dose limit

Regulatory upper bound on dose to workers or the public; ICRP occupational limit is 20 mSv/y averaged over 5 years.

Medical event

Administration of byproduct material differing from the prescribed dose by 20% or more and exceeding 0.5 Sv to an organ or 50 mSv effective dose.

Bremsstrahlung

Continuous X-ray spectrum produced when electrons decelerate in the anode target; maximum photon energy equals the applied kVp.

Half-value layer

Thickness of material that reduces photon intensity to half its original value; HVL = ln2/μ, typically 2.5–4 mm Al in diagnostic beams.

CTDIvol

Volume computed tomography dose index, a standardized measure of CT radiation output per unit volume; DLP equals CTDIvol times scan length.

Tc-99m

Technetium-99m, a 6-hour half-life radionuclide emitting 140 keV gamma rays; the most widely used diagnostic radiopharmaceutical.

PET annihilation

Positron emission tomography process in which a positron annihilates with an electron, producing two 511 keV photons emitted approximately 180 degrees apart.

Dose calibrator

Re-entrant ionization chamber used to measure the activity of radiopharmaceuticals before administration; geometry and volume affect readings.

Percentage depth dose

Ratio of absorbed dose at a given depth to the maximum dose along the central axis of a radiation beam, expressed as a percentage.

Brachytherapy TG-43

Dosimetry formalism for brachytherapy sources that calculates dose to water using radial dose and anisotropy functions.

Primary barrier

Shielding designed to attenuate the primary radiation beam; its thickness depends on workload, use factor, occupancy, and distance.

Occupancy factor

Factor T used in shielding calculations reflecting the fraction of time an area is occupied; full 1, partial 1/4, occasional 1/16.

Tenth-value layer

Thickness of material that reduces photon intensity to one tenth; number of TVLs equals log10 of the inverse transmission.

Personal dosimeter

Device such as a TLD, OSL, or film badge worn to measure occupational dose; ring badges measure extremity dose.

Investigation level

Dose threshold, typically 10%, 30%, or 100% of the regulatory limit, that triggers review, investigation, or reporting.

Public dose limit

Regulatory limit on dose to members of the public from licensed activities, generally 1 mSv per year.

Secular equilibrium

Condition in a decay chain where the parent half-life greatly exceeds the daughter half-life, so daughter activity approaches parent activity.

Radiation weighting factor

Factor w_R that adjusts absorbed dose for the biological effectiveness of the radiation type; 1 for photons, 20 for alpha particles.