In the measurement of

ionising radiation Ionizing (ionising) radiation, including nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionize atoms or molecules by detaching electrons from them. Some par ...

the counting efficiency is the ratio between the number of

particle In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

s or

photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...

counted with a radiation counter and the number of particles or photons of the same type and energy emitted by the radiation source.

Factors

Several factors affect the counting efficiency: * The distance from the source of radiation * The absorption or scattering of particles by the medium (such as air) between the source and the surface of the detector * The detector efficiency in counting all radiation photons and particles that reach the surface of the detector The accompanying diagram shows this graphically.

Scintillation counters

Radiation protection instruments

US Navy 070208-N-9132D-002 Electronics Technician 2nd Class Shea Thompson tests an Alpha Particle Dection Probe

Large area scintillation counters used for surface

radioactive contamination Radioactive contamination, also called radiological pollution, is the deposition of, or presence of Radioactive decay, radioactive substances on surfaces or within solids, liquids, or gases (including the human body), where their presence is uni ...

measurements use plate or planar radioactive sources as calibration standards. The Surface Emission Rate (SER), not the source activity, is used as a measure of the rate of particles emitted from the source of radiation. The SER is the true emission rate from the surface, which is usually different to the activity. This difference is due to self-shielding within the active layer of the source which will reduce the SER, or backscatter which will reflect particles off the backing plate of the active layer and will increase the SER. Beta particle plate sources usually have a significant backscatter, whereas alpha plate sources usually have no backscatter, but are easily self-attenuated if the active layer is made too thick.''Estimation of calibration factors for surface contamination monitoring instruments for different surfaces''. Mike Woods and Stephen Judge. Pub NPL, teddington, U

Liquid scintillation counters

Counting efficiency varies for different

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s, sample compositions and

scintillation counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the Electron excitation, excitation effect of incident radiation on a Scintillation (physics), scintillating material, and detecting the resultant li ...

s. Poor counting efficiency can be caused by an extremely low energy to light conversion rate, (scintillation efficiency) which, even optimally, will be a small value. It has been calculated that only some 4% of the energy from a β emission event is converted to light by even the most efficient scintillation cocktails.

Gaseous counters

Proportional counter The proportional counter is a type of gaseous ionization detector device used to measure Charged particle, particles of ionizing radiation. The key feature is its ability to measure the Electronvolt, energy of incident radiation, by producing a det ...

s and end-window Geiger-Muller tubes have a very high efficiency for all ionising particles that reach the fill gas. Nearly every initial ionising event in the gas will result in

Townsend avalanche In electromagnetism, the Townsend discharge or Townsend avalanche is an ionisation process for gases where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electro ...

s, and thereby an output signal. However the overall detector efficiency is largely affected by attenuation due to the window or tube body through which particles have to pass. In the case of gamma photons the detection efficiency is more dependent upon the fill gas and gamma energy. Low energy photons will interact more with the fill gas than high energy photons.

References