Sources of Charged Particles

Cyclotrons and accelerators are sources of charged particles (i.e., protons, deuterons, a particles, etc.), and the radionuclides produced are generally proton rich and decay by positron emission and/or electron capture. A positive ion beam is eventually extracted from the cyclotron, regardless of whether positive or negative ions were accelerated. The isotope of interest is separated from the target for use in chemical syntheses. Accelerator- or cyclotron-produced radioisotopes tend to be the most expensive as only one radionuclide is produced at a time. 

Dose from a Source of Charged Particles Inside the Body... 

Polonium is a radioactive, low-melting metalloid. It is a useful source of a particles (helium-4 nuclei they are described in more detail in Section 15.11) and is used in antistatic devices in textile mills the a particles reduce static by counteracting the negative charges that tend to build up on the fast-moving fabric. 

One can compensate for charging by using a so-called flood gun, which sprays low-energy electrons onto the sample. Charging can also be minimized by using a beam of atoms instead of ions as primary particles. In this case, kinetic emission of electrons is the only source of charging, if we ignore the low yields of secondary ions. 

This can be accomplished by means of two different processes (1) an electrodeposition process in which z electrons (e) are provided by an external power supply, and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electrodeposition and electroless deposition, constitute the electrochemical deposition. In this book we treat both of these processes. In either case our interest is in a metal electrode in contact with an aqueous ionic solution. Deposition reaction presented by Eq. (1.1) is a reaction of charged particles at the interface between a solid metal electrode and a liquid solution. The two types of charged particles, a metal ion and an electron, can cross the interface. 

All of these ion sources emit beams of positive ions at relatively low velocities, the ions drift or are pulled out from the ionization region with relatively small electrostatic potentials (U 20 kV). These beams of charged particles can be focused and transported in vacuum to the main accelerating machines. 

Spark and arc spectrochemical sources rely on the ability of charged particles to heat the sample surface and ultimately desorb atoms from the solid matrix... 

In addition to activation by thermal ( 0.04 eV), epithermal (>0.1 eV), or fast neutrons (14MeV), a variety of charged particles (p, d, a, etc.) and y-rays have been used when special sources are available. For example, 1-5 MeV protons from a van de Graaff electrostatic generator produce X rays from the excited intermediate z+ B in a technique designated... 

Colloidal dispersions owe their stability to a surface charge and the resultant electrical repulsion of charged particles. This charge is acquired by adsorption of cations or anions on the surface. For example, an ionic precipitate placed in pure water will reach solubility equilibrium as determined by its solubility product, but the solid may not have the same attraction for both its ions. Solid silver iodide has greater attraction for iodide than for silver ions, so that the zero point of charge (the isoelectric point) corresponds to a silver ion concentration much greater than iodide, rather than to equal concentrations of the two ions. The isoelectric points of the three silver halides are ° silver chloride, pAg = 4, pCl = 5.7 silver bromide, pAg = 5.4, pBr = 6.9 silver iodide, pAg = 5.5, pi = 10.6. For barium sulfate the isoelectric point seems to be dependent on the source of the product and its de ee of perfection. ... 

The large amount of energy released in charged-particle bombardment tends to exclude this method where thermally labile organic or biological materials are employed as targets. However, Odeblad (54) was able to determine sodium in urine and other elements in biological materials by means of the weak source of alpha particles (160 millicuries) derived from the decay of Po . A similar source of alpha particles has been used to determine aluminum by the reaction AP (a,n)P —half-life 2.5 min (66). 

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