Knowledge base

Glossary

Units, quantities and abbreviations found in radiation instrument data sheets – briefly explained.

28 terms

A

Alpha spectrometry: Measurement of alpha particle energies, usually with silicon detectors in vacuum chambers, to identify and quantify alpha emitters such as uranium, plutonium and americium. Samples normally require radiochemical preparation into thin sources.
Ambient dose equivalentH*(10): Operational quantity for area monitoring: the dose equivalent at 10 mm depth in the ICRU sphere in an expanded and aligned radiation field (ICRU Report 51). Used to estimate effective dose from penetrating radiation.

Calibration source: A radioactive source with certified activity or emission rate, traceable to national measurement standards, used to calibrate instruments and measurement systems.
Compton continuum: The broad background below the full-energy peaks, caused by photons that deposit only part of their energy in the detector through Compton scattering. The continuum ends at the Compton edge, somewhat below the peak energy.
Counts per secondcps, cpm: The number of registered events per second (cps) or minute (cpm) – the instrument’s raw reading. Converting to activity or surface activity requires a calibration for the nuclide and geometry in question.

Dead time: The time after a registered event during which the detector or electronics cannot register the next one. At high count rates the dead time must be corrected for, otherwise the activity is underestimated.
Detection efficiency: The fraction of the emitted particles or photons that the instrument registers. It depends on the radiation energy, the measurement geometry and the detector; gamma spectrometry usually uses the full-energy peak efficiency.
Dose rateµSv/h: Dose per unit time, for example µSv/h. Radiation protection dose rate meters normally display ambient dose equivalent rate, H*(10).

ElectronvolteV, keV, MeV: Energy unit for ionising radiation: 1 eV ≈ 1.602 × 10⁻¹⁹ J. Gamma lines are usually given in keV (thousand eV) or MeV (million eV).
Energy resolutionFWHM: The width of a peak in the spectrum measured at half of its height (full width at half maximum). Given in keV or as a percentage of the energy, it determines how close two gamma lines can be and still be told apart.

Full-energy peak: The peak in a gamma spectrum where the photon’s entire energy has been absorbed in the detector, also called the photopeak. Its position identifies the nuclide and its area is used to determine the activity.

GM tubeGeiger–Müller: A gas-filled tube in which every ionising event produces a pulse of the same size. Rugged and sensitive but without energy information; common in dose rate meters and contamination monitors.
GrayGy: The SI unit of absorbed dose: one joule of absorbed energy per kilogram.

Half-lifeT½: The time it takes for half of the nuclei in a radioactive substance to decay. Cs-137 has a half-life of about 30 years and Co-60 of about 5.27 years.
Helium-3 counter³He: A proportional counter filled with helium-3, which captures thermal neutrons through the ³He(n,p)³H reaction. For fast neutrons the detector is surrounded by a moderator, such as polyethylene, that slows the neutrons down.

Ionisation chamber: A gas-filled detector that measures the total ionisation current without gas amplification. It has a good energy response and copes with pulsed radiation fields, so it is often used to measure X-ray and gamma dose rates.
IP ratingIEC 60529: Degree of protection according to IEC 60529. The first digit states protection against solid objects and dust (6 = dust-tight), the second against water (for example 5 = water jets, 7 = temporary immersion).

Minimum detectable activityMDA: The lowest activity that can be detected in a measurement with a chosen statistical confidence. It depends on background, efficiency, counting time and emission probability. Detection limits are calculated according to ISO 11929.
Multichannel analyserMCA: Electronics that sort detector pulses by pulse height – that is, energy – into channels and thereby build up a spectrum.

Peak-to-Compton ratioP/C: For HPGe: the height of the 1,332.5 keV peak from Co-60 divided by the average level of the Compton continuum between 1,040 and 1,096 keV (IEEE Std 325). A higher value makes weak peaks easier to see.
Personal dose equivalentHp(10), Hp(0.07): Operational quantities for personal dosimetry: the dose equivalent in soft tissue at 10 mm and 0.07 mm depth below the point where the dosimeter is worn. Hp(10) is used to estimate effective dose, Hp(0.07) skin dose.
Proportional counter: A gas-filled detector in which the pulse size is proportional to the deposited energy. Used, among other things, for alpha/beta measurements and as a neutron detector with ³He or BF₃ gas.

Radionuclide identification deviceRIID: A handheld instrument that measures a gamma spectrum and identifies which nuclides are present, often also showing dose rate. Performance requirements are given in IEC 62327 and ANSI N42.34.
Relative efficiency: For HPGe: the full-energy peak efficiency at 1,332.5 keV for a point source 25 cm from the detector, compared with a 3″ × 3″ NaI(Tl) crystal in the same geometry (1.2 × 10⁻³), according to IEEE Std 325. A comparison number – not the efficiency for a particular sample.

Scintillator: A material that emits flashes of light when radiation is absorbed. The light is converted into an electrical pulse by a photomultiplier or photodiode. Examples: NaI(Tl), CsI(Tl), LaBr₃(Ce), plastic and ZnS(Ag).
SievertSv: The unit of equivalent dose and effective dose, which take into account the biological effect of the radiation type and – for effective dose – the sensitivity of the organs. Radiation protection instruments display operational quantities such as H*(10) and Hp(10), also in sieverts.
Surface activityBq/cm²: Activity per unit area of a contaminated surface. Determined from the contamination monitor’s count rate and its efficiency for the nuclide in question (ISO 7503-1).