Ionizing radiation ionization chamber

The modem ionization chamber, called a dose caUbrator in this appHcation, is capable of linear measurements of radioactivity having a precision in the range of several percent coefficient of variation over a range of 370 kBq (10 -lCi) to at least 370 GBq (10 Ci). This extraordinary range is the chief advantage of this instmment. It may only be used when the sample is known to have only a single isotope. It has no capacity to distinguish radiation from different isotopes. 

The total energy, E, is obtained from the total charge accumulated in both sections of the anode. The second part of the ionization chamber, which measures the energy E - AE, can be replaced by an SBD [3.167], and the first part, which measures the energy loss AE, by a transmission SBD [3.168, 3.169]. When SBDs are used to measure heavy ions, radiation damage of the detector by the ions must be taken into account. 

Ion-Molecule Reactions. The list of ion-molecule reactions observed by mass spectrometry is highly impressive at this time we can easily count several hundred. Most of these were observed at relatively low pressures and in the presence of a draw-out or pusher field in the ionization chamber. Using this information to account for the radiation chemical product distribution requires one to recognize its restrictions to this use. 

Since 1925, The International Commission on Radiation Units and Measurements at Bethesda, Maryland has been publishing reports updating the definitions and units for measurements of various radiation-related quantities. Of these ICRU Reports, special mention may be made of reports no. 19 (1971) [radiation quantities and units], 33 (1980) [radiation quantities and units], 36 (1983) [microdosimetry], 47 (1992) [thermoluminiscent dosimetry], and 51 (1993) [radiation protection dosimetry]. A succinct description of various devices used in dosimetry, such as ionization chambers, chemical and solid-state dosimeters, and personnel (pocket) dosimeters, will be found in Spinks and Woods (1990). In this section, we will only consider some chemical dosimeters in a little detail. For a survey of the field the reader is referred to Kase et at, (1985, 1987), McLaughlin (1982), and to the International Atomic Energy Agency (1977). Of the earlier publications, many useful information can still be gleaned from Hine and Brownell (1956), Holm and Berry (1970), and Shapiro (1972). 

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

Flat plates or concentric cylinders may be utilized in the construction of an ionization chamber. The flat plate design is preferred because it has a well-defined active volume and ensures that ions will not collect on the insulators and cause a distortion of the electric field. The concentric cylinder design does not have a well-defined active volume because of the variation in the electric field as the insulator is approached. Ionization chamber construction differs from the proportional counter (flat plates or concentric cylinders vice a cylinder and central electrode) to allow for the integration of pulses produced by the incident radiation. The proportional counter would require such exact control of the electric field between the electrodes that it would not be practical. 

When radiation enters an ionization chamber, the detector gas at the point of incident radiation becomes ionized. 

Radiation detection circuit currents or pulse rates vary over a wide range of values. The current output of an ionization chamber may vary by 8 orders of magnitude. For example, the range may be from 10"13 amps to 10"5 amps. The most accurate method to display this range would be to utilize a linear current meter with several scales, and the capability to switch those scales. This is not practical. A single scale which covers the entire range of values is used. This scale is referred to as logarithmic. 

In order to measure the absorption of the beam of rays coming from the mercury vapor an ionization chamber with a thin mica window in it and containing methyliodide was set up opposite the window, F, and lead plates with holes in them were placed in the line of the beam so that only the radiation coming from the impacts of the electrons against the mercury entered the chamber with sufficient intensity to be detected. That this was the case in the actual experiments is indicated by the fact that no perceptible ionization current could be observed when the mercury pump was not running. A quadrant electrometer measured the ionization current. 

Knowing the ionization potential of a contaminant is required to determine the appropriate photoionization lamp for detecting that contaminant or family of contaminants in air. Photoionization instruments are equipped with a radiation sonrce (UV lamp), pump, ionization chamber, an amplifier (detector), and a recorder (either digital or meter). Generally, compounds with ionization potentials less than the radiation source (UV lamp rating) being used will readUy ionize and will be detected by the instrument. Conversely, compounds with ionization potentials more than the lamp rating will not ionize and will not be detected by the instrument. 

Figure 2 The photoabsorption (c), photoionization (o-,-), and photodissociation (cr

In Fig. 7 [7], we compared the photoionization quantum yields (t/,) of CH4, C2H6, and C3H8, which were measured by our group using the double ionization chamber and synchrotron radiation, as described in Section 2.1. The photon energies are considered in two ranges, as follows, in terms of the behavior of the t/, curves as a function of the incident photon energy [7] ... 

Calibration of the intensities of the radiation flelds is traceable to the NIST. The ionization chambers and electrometers used by the service laboratories to quantify the intensity of the radiation fields must be calibrated by the NIST or an accredited secondary standards laboratory. The intensity of the field is assessed in terms of air kerma or exposure (free-in-air), with the field collimated to minimize unwanted scatter. Conversion coefficients relate the air kerma or exposure (free-in-air) to the dose equivalent at a specified depth in a material of specified geometry and composition when the material is placed in the radiation field. The conversion coefficients vary as a function of photon energy, angle of incidence, and size and shape of backscatter mediiun. 

Since the ionic states formed by high-energy radiation seem to be the chemically important ones, let us consider their reactions. The reactions between ions and neutral molecules in the gas phase can be studied directly in a mass spectrometer. Under ordinary operating conditions the pressure in the ionizing chamber of the mass spectrometer is about 10 6 mm. and the ions formed have little chance to collide with a molecule during their brief lifetime (10-5 sec.) before collection. Therefore, mainly unimolecular decomposition reactions occur and it is the products of these that are detected. The intensity of these primary ions increases with the first power of the pressure in the ionization chamber. However, when the pressure becomes great enough so that ion molecule collisions can occur readily, additional secondary ions which are the products of these ion molecule Collisions appear. The intensity of these secondary product ions depends on the concentrations of both the molecules and the primary ions, and thus on the square of the pressure. 

Under ordinary mass spcctrometric conditions only unimolecular reactions of excited ions occur, but at higher ionization chamber pressures bimolecular ion molecule reactions are observed in which both the parent ions and their unimolecular dissociation product ions are reactants. Since it requires a time of 10 5 sec. to analyze and collect the ions after their formation all of the ions in the complete mass spectrum of the parent molecule are possible reactants. However, in radiation chemistry we are concerned with the ion distribution at the time between molecular collisions which is much shorter than 10 5 sec. For example, in the gas phase at 1 atm. the time between collisions is 10 10 sec. and in considering the ion molecule reactions that can occur one must know the amount of unimolecular decomposition within that time. By utilizing the quasi-equilibrium theory of mass spectra6 it is possible to calculate the ion distribution at any time. This has been done for propane at a time of 10 10 sec.,24 and although the parent ion is increased by a factor of 2 the relative ratios of the other ions are about the same as in the mass spectrum observed in 10 r> sec. Thus for gas phase radiolysis the observed mass spectrum is a fair first approximation to the ion distribution. In... 

Another method of radiation detection that has value in biochemistry is the use of gas ionization chambers. The most common device that uses this technique is the Geiger-Miiller tube (G-M tube). When (S particles pass through a gas, they collide with atoms and may cause ejection of an electron from a gas atom. This results in the formation of an ion pair made up... 

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