31.9

Radiation Detectors

The physics of radiation detectors. There are a number of devices that can be used to detect the particles and photons ( rays) emitted when a radioactive nucleus decays. Such devices detect the ionization that these particles and photons cause as they pass through matter.
The most familiar detector is the Geiger counter, which Figure 31-17 illustrates. The Geiger counter consists of a gas-filled metal cylinder. The , , or rays enter the cylinder through a thin window at one end. rays can also penetrate directly through the metal. A wire electrode runs along the center of the tube and is kept at a high positive voltage (1000–3000 V) relative to the outer cylinder. When a high-energy particle or photon enters the cylinder, it collides with and ionizes a gas molecule. The electron produced from the gas molecule accelerates toward the positive wire, ionizing other molecules in its path. Additional electrons are formed, and an avalanche of electrons rushes toward the wire, leading to a pulse of current through the resistor R. This pulse can be counted or made to produce a “click ” in a loudspeaker. The number of counts or clicks is related to the number of disintegrations that produced the particles or photons.

Figure 31-17 A Geiger counter.

The scintillation counter is another important radiation detector. As Figure 31-18 indicates, this device consists of a scintillator mounted on a photomultiplier tube. Often the scintillator is a crystal (e.g., cesium iodide) containing a small amount of impurity (thallium), but plastic, liquid, and gaseous scintillators are also used. In response to ionizing radiation, the scintillator emits a flash of visible light. The photons of the flash then strike the photocathode of the photomultiplier tube. The photocathode is made of a material that emits electrons because of the photoelectric effect. These photoelectrons are then attracted to a special electrode kept at a voltage of about relative to the photocathode. The electrode is coated with a substance that emits several additional electrons for every electron striking it. The additional electrons are attracted to a second similar electrode (voltage ) where they generate even more electrons. Commercial photomultiplier tubes contain as many as 15 of these special electrodes, so photoelectrons resulting from the light flash of the scintillator lead to a cascade of electrons and a pulse of current. As in a Geiger tube, the current pulses can be counted.

Figure 31-18 A scintillation counter.

Ionizing radiation can also be detected with several types of semiconductor detectors. Such devices utilize n- and p- type materials (see Section 23.5), and their operation depends on the electrons and holes formed in the materials as a result of the radiation. One of the main advantages of semiconductor detectors is their ability to discriminate between two particles with only slightly different energies.
A number of instruments provide a pictorial representation of the path that high-energy particles follow after they are emitted from unstable nuclei. In a cloud chamber, a gas is cooled just to the point where it will condense into droplets, provided nucleating agents are available on which the droplets can form. When a high-energy particle, such as an particle or a particle, passes through the gas, the ions it leaves behind serve as nucleating agents, and droplets form along the path of the particle. A bubble chamber works in a similar fashion, except it contains a liquid that is just at the point of boiling. Tiny bubbles form along the trail of a high-energy particle passing through the liquid. Paths revealed in a cloud or bubble chamber can be photographed to provide a permanent record of the event. Figure 31-19 shows a photograph of tracks in a bubble chamber. A photographic emulsion also can be used directly to produce a record of the path taken by a particle of ionizing radiation. Ions formed as the particle passes through the emulsion cause silver to be deposited along the track when the emulsion is developed.

Figure 31-19 A photograph showing particle tracks in a bubble chamber. (CERN/Photo Researchers, Inc.)

Test your understanding of the material in Sections 31.6, 31.7, 31.8 and 31.9 :

· | Radioactive Decay and Activity
· | Radioactive Dating
· | Radioactive Decay Series
· | Radiation Detectors

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