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Unformatted text preview: Proportional Counters and Tubes Proportional tubes, proportional wire chambers, and drift chambers are in very wide use in high energy physics experiments for particle position measurement and for energy measurement. In these detectors one or more very fine wires are stretched within a gas volume. The wires are at positive potential with respect to the outer conductor so that electrons are attracted. The main component of the filling gas is usually argon but polyatomic additives are essential. If the particle to be detected is a charged particle (we will be using electrons) it ionizes the filling gas on its way through the detector. The electrons that are released drift toward the positive wire. When they approach within a few diameters of the wire the electric fields become very large and secondary ionization takes place. An avalanche develops and a very large number of ion pairs are created very close to the wire. The electrons produced in the avalanche are very quickly collected at the wire while the positive ions move slowly in the opposite direction through a large potential gradient. This motion of positive ions induces a negative pulse on the wire that can be detected as a current flowing through a resistor connected to the wire. If the incident particle is a low energy x-ray (we will use a 55 Fe source which emits a 5.9 keV x-ray) then the initial interaction is photoelectric effect in argon; in most cases the entire x-ray energy is converted to ionization that is collected. The avalanche process near the wire provides an amplification of the initial charge deposited by the primary particle. This is sometimes called the gas gain; it is sufficiently large so that the output pulse can be observed on a scope. For use in logic circuits or pulse height analysis it is usually amplified further with a high gain current or charge amplifier. See the references at the end of these instructions for more information of proportional tube detec- tors and related apparatus. The proportional tube The simplest device we will work with is a tube with a single wire. Replicated by the hundreds or thousands these are very popular in modern large detector systems. They are easy to make and parameters such as wire position are not very critical. The position resolution is, of course, comparable to the size of the tube but the energy resolution can be quite good. The time resolution is much poorer than with scintillators because the charge must drift to the wire and the time it takes to collect the charge (and see a signal) will depend where the energy is deposited by the ionizing radiation: near the wire or near a wall. Construct a proportional tube detector Rectangular cross section conductive plastic tubes are provided along with plastic end caps and small copper tubes for gas flow and to fix (hold) the central wire (0.0025 inch diameter). Ground wires are connected to each end of the plastic tube and are heated with a soldering iron; they melt into the plastic slightly and make good electrical contact. After the wire is strung the end caps areinto the plastic slightly and make good electrical contact....
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- Fall '11