Charged particle beams
Charged particle beams
Unidirectional streams of charged particles traveling at high velocities. Charged particles can be accelerated to high velocities by electromagnetic fields. They are then able to travel through matter (termed an absorber), interacting with it, losing energy, and causing various effects important in many applications. Examples of charged particles are electrons, positrons, protons, antiprotons, alpha particles, and any ions (atoms with one or several electrons removed or added). In addition, some particles are produced artificially and may be short-lived (pions, muons).
In traveling through matter, charged particles interact with nuclei, producing nuclear reactions and elastic and inelastic collisions with the electrons (electronic collisions) and with entire atoms of the absorber (atomic collisions). Usually, in its travel through matter a charged particle makes few or no nuclear reactions or inelastic nuclear collisions, but many electronic and atomic collisions. The average distance between successive collisions is called the mean free path, λ. In solids, it is of the order of 10 cm (4 in.) for nuclear reactions. It ranges from the diameter of the atoms (about 10-10 m) to about 10-7 m for electronic collisions. The mean free path, λ, depends on the properties of the particle and, most importantly, on its velocity.
If a charged particle is accelerated, it can emit photons called bremsstrahlung. This process is of great importance for electrons as well as for heavy ions whose kinetic energies are much greater than their rest energies. It is used extensively for the production of x-rays in radiology. See Bremsstrahlung
In gases, all electrons are bound to individual atoms or molecules in well-defined orbits. These electrons can be moved into other bound orbits (excitation) requiring a well-defined energy. Another possibility is the complete removal of the electron from the atom (ionization), requiring an energy equal to or greater than the ionization energy for the particular electron. In both processes, the charged particle will lose energy and will be deflected very slightly.
There are some major differences between electron beams and beams of heavier particles. In general, the path of an electron will be a zigzag. Angular deflections in the collisions will frequently be large. Electron beams therefore tend to spread out laterally, and the number of primary electrons in the beam at a depth x in the absorber decreases rapidly.
In general, for the same dose (the energy deposited per gram along the beam line) heavy charged particles will produce, because of their higher local ionization, larger biological effects than electrons (which frequently are produced by x-rays).
Electron beams are used in the preservation of food. In medicine, electron beams are used extensively to produce x-rays for both diagnostic and therapeutic (cancer irradiation) purposes. Also, in radiation therapy, deuteron beams incident on Be and 3H targets are used to produce beams of fast neutrons, which in turn produce fast protons, alpha particles, and carbon, nitrogen, and oxygen ions in the irradiated tissue. Energetic pion, proton, alpha, and heavier ion beams can possibly be used for cancer therapy.
Charged particle beams are used in many methods of chemical and solid-state analysis. Nuclear activation analysis can be performed with heavy ions. See Electron diffraction
Beams of nuclei with lifetimes as short as 10-6 s are used for studies in nuclear physics, astrophysics, biology, and materials science. Nuclear beams (or heavy-ion beams) are usually produced by accelerating naturally available stable isotopes. However, radioactive nuclei, most of which do not occur naturally on Earth, must be produced as required in nuclear reactions by using various accelerated beams. Because these radioactive nuclei are produced by the nuclear reactions of primary beams, they are called secondary particles and beams of such nuclei are called radioactive secondary beams. See Radioactivity
Radioactive secondary beams have made possible the study of the structure of nuclei far from stability. Another important application occurs in the study of nuclear reactions of importance in hot stars and in supernovae, which are crucial for understanding nucleosynthesis in the universe. See Nuclear structure