Quantum Physics

964 Chapter 29 Nuclear Physicsand in the process they ionize other atoms in their path. This sequential ionizationresults in an avalanche of electrons that produces a current pulse. After the pulsehas been amplified, it can either be used to trigger an electronic counter or deliveredto a loudspeaker that clicks each time a particle is detected. Although aGeiger counter reliably detects the presence and quantity of radiation, it cannotbe used to measure the energy of the detected radiation.A semiconductor diode detector is essentially a reverse biased p–n junction. Asan energetic particle passes through the junction, it produces electron–hole pairsthat are separated by the internal electric field. This movement of electrons andholes creates a brief pulse of current that is measured with an electronic counter.In a typical device, the duration of the pulse is 10 8 s.A scintillation counter usually uses a solid or liquid material having atoms thatare easily excited by radiation. The excited atoms then emit photons of visiblelight when they return to their ground state. Common materials used as scintillatorsare transparent crystals of sodium iodide and certain plastics. If the scintillatormaterial is attached to one end of a device called a photomultiplier (PM) tube, asshown in Figure 29.17, the photons emitted by the scintillator can be converted toan electrical signal. The PM tube consists of numerous electrodes, called dynodes,whose electric potentials increase in succession along the length of the tube.Between the top of the tube and the scintillator material is a plate called aphotocathode. When photons leaving the scintillator hit this plate, electrons areemitted because of the photoelectric effect. As one of these emitted electronsstrikes the first dynode, the electron has sufficient kinetic energy to eject severalother electrons from the surface of the dynode. When these electrons areaccelerated to the second dynode, many more electrons are ejected, and amultiplication process occurs. The end result is 1 million or more electronsstriking the last dynode. Hence, one particle striking the scintillator produces asizable electrical pulse at the PM output, and this pulse is sent to an electroniccounter.Both the scintillator and the semiconductor diode detector are much more sensitivethan a Geiger counter, mainly because of the higher mass density of thedetecting medium. Both can also be used to measure particle energy from theheight of the pulses produced.Track detectors are various devices used to view the tracks or paths of chargedparticles directly. High-energy particles produced in particle accelerators may haveenergies ranging from 10 9 to 10 12 eV. The energy of such particles can’t bemeasured with the small detectors already mentioned. Instead, their energy andScintillationcrystalPhotocathode0 VIncomingparticle+200 V+400 V+600 V+800 V+1 000 V+1 200 V+1 400 V+1 600 VVacuumFigure 29.17 Diagram of a scintillation counterconnected to a photomultiplier tube.Outputto counter