Charged particles ionization losses

Collisions that result in ionization or excitation are called inelastic collisions. A charged particle moving through matter may also have elastic collisions with nuclei or atomic electrons. In such a case, the incident particle loses the energy required for conservation of kinetic energy and linear momentum. Elastic collisions are not important for charged-particle energy loss and detection. 

Accelerated electrons in the applied electric field ionize gas molecules, and in these ionization processes extra electrons are created. In the steady state the loss of charged particles is balanced by their production. Due to their much lower mass, electrons move much faster than ions. As a result, charge separation creates... 

The rate of energy loss will depend on the energy and type of charged particle as discussed in Chapter 17. The a particles will create intense ionization (—104-105 ion pairs/cm of path length), whereas (3 particles will produce 102-103 ion pairs/cm, and the passage of y rays will result in 1-10 ion pairs/cm. 

When the energy of the charged particle beam is too large to easily stop the beam in a Faraday cup, the beam intensity is frequently monitored by a secondary ionization chamber. These ion chambers have thin entrance and exit windows and measure the differential energy loss when the beam traverses them. They must be calibrated to give absolute beam intensities. If the charged particle beam intensity is very low (<106 particles/s), then individual particles can be counted in a plastic scintillator detector mounted on a photomultiplier tube. 

The Influence of the State of Aggregation on the Ionization Losses of Fast Charged Particles... 

First of all, the experimental observation of tracks provides information about the charged particles themselves (their charge and mass) and plays an important role in discovering new elementary particles. As for the characteristics of the track, such studies allow one to measure only the density of ionization along the track in gaseous media and to determine the specific energy losses. 

Following from formula (4.54), the transfer of energy on excitation of molecules has a noticeable probability even in the case where the impact parameter is much greater than their size d. Since the intermolecular spacings in a condensed medium are of order of d, a charged particle interacts with many of its molecules. The polarization of these molecules weakens the field of the particle, which, in its turn, weakens the interaction of the particle with the molecules located far from the track. This results in that the actual ionization losses are smaller than the value we would get by simply summing the losses in collisions with individual molecules given by formula (5.1). This polarization (density) effect was first pointed out by Swann,205 while the principles of calculation of ionization losses in a dense medium were developed by Fermi.206... 

As in the case of fast electrons, the main contribution to energy losses of heavy charged particles (ions) comes from ionization losses (see Section V), which are responsible for most of the radiation-chemical effects. Therefore, we consider here only the structure of that part of an ion s track where the ionization losses are dominating. The role of elastic interaction between ions and atoms of the medium, which becomes essential only at the end of the ion s track, is not considered in this section. 

One of the characteristics of radiation considered in radiation chemistry and in radiobiology is the linear energy transfer (LET). For fast charged particles the LET practically equals the ionization losses (or polarization losses, in condensed media) and is given by the formulas for the stopping power presented in Section V.A. 

Finally, it should be mentioned that channelling effects (the steering of charged particles in open regions in the lattice) could reduce the specific ionization loss. 

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