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Chemistry LibreTexts

11.8: Detecting Radiation

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  • Learning Objectives

    • To recognize who the dosimeter and Geiger counter can be use to detect radiation

    Radioactivity is determined by measuring the number of decay processes per unit time. Perhaps the easiest way is simply to determine the number of counts/minute, with each count measuring a single decay process, such as the emission of an \(\alpha\)-particle. A particular isotope may have an activity of 5,000 counts/minute \(\left( \text{cpm} \right)\) while another isotope might only have \(250 \: \text{cpm}\). The amount of activity gives a rough indication of the amount of the radioisotope present - the higher the activity, the more radioactivity isotope in the sample.

    Two images are shown. The first, labeled “Rate of radioactive decay measured in becquerels or curies,” shows a red sphere with ten red squiggly arrows facing away from it in a 360 degree circle. The second image shows the head and torso of a woman wearing medical scrubs with a badge on her chest. The caption to the badge reads “Film badge or dosimeter measures tissue damage exposure in rems or sieverts” while a phrase under this image states “Absorbed dose measured in grays or rads.”
    Figure \(\PageIndex{1}\): Different units are used to measure the rate of emission from a radioactive source, the energy that is absorbed from the source, and the amount of damage the absorbed radiation does. (CC BY 4.0; OpenStax)

    Measurement of exposure to radioactivity is important for anyone who deals with radioactive materials on a regular basis. Perhaps the simplest device is a personal dosimeter - a film badge that will fog up when exposed to radiation (Figure \(\PageIndex{1}\)). The amount of fogging is proportional to the amount of radiation present. These devices are not very sensitive to low levels of radiation. More sensitive systems use crystals that respond in some way to radioactivity by registering the number of emissions in a given time. These systems tend to be more sensitive and more reliable than film badges.

    When alpha, beta or gamma particles collides with a target, some of the energy in the particle is transferred to the target, typically resulting in the promotion of an electron to an “excited state”. In many “targets”, especially gasses, this results in ionization. A Geiger counter (or Geiger-Müller counter) takes advantage of this to detect these particles (Figure \(\PageIndex{2}\)). In a Geiger tube, the electron produced by ionization of a captive gas travels to the anode and the change in voltage is detected by the attached circuitry.

    Figure \(\PageIndex{2}\): (left) Geiger counter with pancake type probe. Public Domain; TimVickers via Wikipedia). (right) Schematic of a Geiger-Müller counter using an "end window" tube for low penetration radiation. A loudspeaker is also used for indication. (CC-BY-SA-3.0 Svjo-2 vai Wikipedia).

    Most counters of this type are designed to emit an audible “click” in response to the change in voltage, and to also show it on a digital or analog meter.

    Contributors and Attributions

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

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