Gas ionization

Opera.tingPrinciples. No collection will occur in a precipitator, and no current will flow in the secondary circuit, until gas ionization starts around the discharge electrode. This process is known as corona formation. The starting voltage for corona in air in a tube precipitator is given as... 

A gas ionization detector consists of a tube filled with a high pressure gas and two electrodes. A tube filled with 2 MPa (20 atm) of xenon is common. The gas in the tube ionizes when x-rays pass through the tube causing a current to flow between a high voltage potential placed across the electrodes. This concept is similar to that used in a Geiger tube detector. Gas ionization detectors are utilized in some CT scanners. 

Ozone produced by the negatively charged discharge electrode during gas ionization... 

Gas ionization may cause dissociation of gas stream constituents and result in creation of toxic byproducts... 

In brief, the method consists of introducing small amounts (partial pressures of 10 3-10 4 torr) of the substance to be investigated into the ionization chamber of a mass spectrometer which contains a high pressure (1 torr) of methane, the reactant gas. Ionization is effected by electron impact, and because the methane is present in such an overwhelming preponderance, all but a negligibly small amount of the initial ionization occurs in the methane. The methane ions then undergo ion-molecule reactions to produce a set of ions which serve as reactant ions in the chemical ionization process. The important reactant ions formed from... 

HCl(aq), a soluble gas, ionizes almost totally into its ions in aqueous solution, whereas glucose, a soluble molecular solid, does not ionize at all. Weak acids, such as HF, are soluble in water but ionize only slightly into ions. 

Gas Ionization Counters A common gas ionization counter is the Geiger-Muller counter where the electronic pulses derived from the ionization process are registered as counts. The instrument can be adjusted to detect only radiation with a desired penetrating power. 

Radiations outside the ultraviolet, visible and infrared regions cannot be detected by conventional photoelectric devices. X-rays and y-rays are detected by gas ionization, solid-state ionization, or scintillation effects in crystals. Non-dispersive scintillation or solid-state detectors combine the functions of monochromator and detector by generating signals which are proportional in size to the energy of the incident radiation. These signals are converted into electrical pulses of directly proportional sizes and thence processed to produce a spectrum. For radiowaves and microwaves, the radiation is essentially monochromatic, and detection is by a radio receiver tuned to the source frequency or by a crystal detector. 

Excitation of sample by bombardment with electrons, radioactive particles or white X-rays. Dispersive crystal analysers dispersing radiation at angles dependent upon energy (wavelength), detection of radiation with gas ionization or scintillation counters. Non-dispersive semiconductor detectors used in conjunction with multichannel pulse height analysers. Electron beam excitation together with scanning electron microscopes. 

Gas ionization, solid scintillation, liquid scintillation and semiconductor detectors, autoradiography. Single and multichannel pulse height analysers. Coincidence and anticoincidence circuits. 

Three main types of detector are in widespread use, namely, gas ionization, scintillation and semiconductor. Although there are a number of common features they are best discussed separately for the sake of clarity. 

An important feature of all gas ionization detectors is their dead or paralysis time, which has a direct bearing on their utility. Once initiated a voltage pulse takes several hundred microseconds to die away, and may be prolonged by the discharge of secondary electrons from the cathode as the argon ions are reduced. Until this first pulse is terminated the tube is dead to further radiations and the recorded count CM will be less than the true count CT. The relation between the two values will depend upon the length of the dead time t... 

Gas ionization detectors are widely used in radiochemistry and X-ray spectrometry. They are simple and robust in construction and may be employed as static or flow detectors. Flow studies have received attention in the interfacing of radioactive detectors with gas chromatographs. A radio-gas chromatograph (Figure 10.9) uses a gas flow proportional counter to monitor the effluent from the gas chromatography column. To achieve... 

The electron capture detector is another type of ionization detector. Specifically, it utilizes the beta emissions of a radioactive source, often nickel-63, to cause the ionization of the carrier gas molecules, thus generating electrons that constitute an electrical current. As an electrophilic component, such as a pesticide, from the separated mixture enters this detector, the electrons from the carrier gas ionization are captured, creating an alteration in the current flow in an external circuit. This alteration is the source of the electrical signal that is amplified and sent on to the recorder. A diagram of this detector is shown in Figure 12.13. The carrier gas for this detector is either pure nitrogen or a mixture of argon and methane. 

As an alternative, exchange of R20 with tritium gas, R2, or tritiated water with R2, can be measnred by radioative counting. R2 is connted with a gas ionization chamber, or R20 can be measnred by solution scintillation counting. 

In INAA, a rock or mineral sample is irradiated in the reactor. The irradiated sample is removed from the reactor, and the dangerous radioactivities are allowed to decay. Then the sample is placed into a counter and the y-rays emitted by each element in the sample are counted. A variety of counters are used, including scintillation counters, gas ionization counters, or semi-conductor counters. For the most precise results, background counts in the detectors produced by electronic noise, cosmic rays, and other radioactive decays must be eliminated. The technique is very sensitive, and samples as small as a few tens of milligrams can be measured. 

Dry air doped with a chemical ionization reagent (such as Cl2 for anions and acetone or NH3 for cations) sweeps the vapor through a tube containing 10 millicuries of 63Ni. Reagent gas ionized by (3-emission from 63Ni reacts with analyte to generate analyte ions. 

---