Detecting radiation It is always possible to detect charged particles moving through matter because they rip electrons out of, or ionize, material in their paths as they pass. Such radiation is known as ionizing radiation for that reason. If a living cell is in the way of ionizing radiation, the radiation can rip electrons away from the atoms it passes. Ionizing radiation is dangerous to living things because these ionized atoms can cause cell processes to go haywire. Detection of ionizing radiation almost always involves a tradeoff between taking enough energy from the radiation to see the effect of its passage, and not stopping it altogether. For example, a very thick piece of lead or uranium will be able to stop alpha (α), beta (β), and gamma (γ) radiation. If we note that we simply stopped the radiation, we do not have much information about what sort of radiation it might have been was it an α, a β, or a γ? Most detectors are variations on this theme of sampling but not stopping the passage of ionizing radiation. The most important characteristics of detectors are, therefore, to take enough energy from the particle to be detected to allow it to signal its presence (but not too much), and to amplify the result so that it is visible somehow to humans with human senses. This might mean having the detector make the signal audible or visible directly, or it might mean that the signal is processed and stored in a computer, with the computer making the information available in some fashion. We discuss just a few detectors here: the GM counter, the cloud chamber, the bubble chamber, the scintillator, and the calorimeter.
Energy, Ch. 18, extension 5 Detecting radiation 2 The Geiger counter Geiger-Müller tubes consist of a central wire anode at high voltage and a cylindrical metal cathode sealed in a glass tube containing a gas (Fig. E18.5.1). A charged particle passing through ionizes the gas in the tube, and the large potential difference between the wire and the cylindrical outer casing (high voltage) pulls the ripped-off electrons toward the wire, ionizing even more gas molecules as they collide with them on their ways to the anode. An avalanche of millions of electrons hits the anode. This makes an electrical signal that can be amplified and made to cause a speaker to turn on momentarily and make a click. Fig. E18.5.1 The Geiger-Müller counter. One electron or other particle leads to an avalanche. The resulting pulse can be detected and amplified. The cloud chamber and bubble chamber In cloud chambers, a saturated vapor is in a container (Fig. E18.5.2). As ionizing particles traverse the container, they leave a trail of ions behind, which causes little clouds to form along the track. The heavier the tracks, the greater the ionization. Hence, one would expect alphas to leave heavier tracks than betas in cloud chambers. Permanent records of the tracks could be made by taking photographs.
Energy, Ch. 18, extension 5 Detecting radiation 3 Fig. E18.5.2 The cloud chamber. A particle ionizes alcohol molecules in its path. The ions are cloud condensation nuclei, and little droplets of alcohol form around the ions, making the track of the particle visible. This particle came in through the back wall and exited the side wall. Because it is so straight, it has a lot of kinetic energy and so is going an appreciable fraction of the speed of light. If the path made random wiggles, it would have indicated that the speed was relatively slow. Bubble chambers work in a similar way (Fig. E18.5.3), but they are filled with a liquid that is made supersaturated when the volume of the container expands. As in the cloud chamber, ions left behind cause bubbles to form along the ion path. The tracks can be photographed to make permanent records for later analysis. Fig. E18.5.3 The bubble chamber. It is surrounded by a magnet to help identify particles by their bending. a. The chamber is filled with liquid hydrogen. A diaphragm is visible on the lower left. b. The diaphragm is pulled, decreasing the pressure on the liquid hydrogen so that ionizing particles that ionize the hydrogen cause a bubble as the liquid begins to boil at the ion s location. The thick track is a highly ionizing particle at high energy (because it is so straight); the thin track is lower in energy (it curves) and ionizes less. It may be an electron.
Energy, Ch. 18, extension 5 Detecting radiation 4 The scintillator Scintillator is a crystal or plastic material that produces light when particles traveling through it deposit energy. The light, though very dim, may be seen directly in some cases, or the light is brought through light pipes (plastics that operate using total internal reflection similar to that used for fiber optics) to a photomultiplier tube, which amplifies light and produces an electrical pulse that allows the particle s detection. The first scintillators were sodium iodide crystals. Sodium iodide crystals flash as particles travel through and transfer their energy to the crystal. Modern scintillators are made of a special plastic. The calorimeter This is one detector that actually does sop up all a particle s energy. It gives a way to measure the total energy in a particle or shower of particles. The detector works by providing sufficient mass for all particles to interact and produce secondary ions. This is commonly accomplished by use of depleted uranium plates (see Extension 19.1, Enrichment) sandwiched between plates of scintillator. The showers make light in the scintillator, and one can determine the energy deposited by finding out how far the shower penetrates into the calorimeter. Calorimeters provide one of the few ways to measure neutral particles. Also, the energy loss measurements can be more accurate at high energy in establishing particle identification than from bending in magnetic fields. In addition, these massive detectors can be built to cover most of the volume around the interaction region. Furthermore, these
Energy, Ch. 18, extension 5 Detecting radiation 5 detectors are very fast, and allow electronic vetoing (setting the detector not to detect when right conditions are not present) of events not meeting specific criteria. Many other observation devices are used: acoustic hodoscopes and multiwire proportional chambers, which read out electrical signals from electrodes; spark chambers that make visible a series of sparks to mark a particle s passage; nuclear emulsion film, and so on. Fig. E18.5.4 The Mark I detector at SLAC.
Energy, Ch. 18, extension 5 Detecting radiation 6 Most detectors in use in nuclear and particle physics experiments use many or all of the detectors discussed, as well as some not discussed. The Mark I detector from the Stanford Linear Accelerator Center is shown in Fig. E18.5.4 above. Most detectors such as these used in high-energy physics research nowadays record directly onto computer storage rather than making photographic records.