Radiation with ionization chamber

Brede HJ, Greif KD, Hecker O, Heeg P, Heese J, Jones DTL, Kluge H, Schardt D. (2006) Absorbed dose to water determination with ionization chamber dosimetry and calorimetry in restricted neutron, photon, proton and heavy-ion radiation fields. Phys in Med and Biol 51(15) 3667-3682. 

C. Proportional Counters. Proportional counters are used to detect one type of radiation in the presence of other types of radiation or to obtain output signals greater than those obtainable with ionization chambers of equivalent size. Proportional counters may be used to either detect events or to measure absorbed energy (dose), because the output pulse is directly proportional to the energy released in the sensitive volume of the counter. Proportional counters are most widely used for the detection of alpha particles, beta, neutrons, and protons. 

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. 

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

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... 

All methods of radiometric analysis involve, of course, the use. of various radiation detection devices, The devices available for measuring radioactivity will vary with the types of radiations emitted by the radioisotope and the kinds of radioactive material. Ionization chambers are used for gases Geiger-Miiller and proportional counters for solids liquid scintillation counters for liquids and solutions and solid crystal or semi-conductor detector scintillation counters for liquids and solids emitting high-energy radiations. Each device can be adopted to detect and measure radioactive material in another state, e.g., solids can be assayed in an ionization chamber. The radiations interact with the detector to produce a signal,... 

Ionization chambers and Geiger counters can be used for detecting extreme ultraviolet radiation. Knowledge of the ionization efficiency of the gas makes it possible to use them for measurement of absolute energy. Ultraviolet radiation also can be detected with a thermocouple, thermopile, or bolometer. 

Table I contains our new values for the L2 and Ls absorption limits of Pt, Au and Bi. These measurements were taken with almost the same arrangement of apparatus as before. The X-ray tube was provided with an arm which reached nearly to the first slit of the spectrometer, thus reducing the absorption of X-rays by air. At the end of this arm and also on the front of the ionization chamber thin mica windows were employed which still further reduced the absorption of X-radiation. Finally the sensitivity of our electrometer had been increased many times. We were thus supplied with a much more sensitive detector of X-ray spectra than in our earlier measurements. This enabled us to use narrower slits and to obtain at the same time greater drops in our ionization currents for the absorption limits.

The intensity and penetrating power of the mercury radiation projected in the direction of motion of the electrons and also perpendicular to it have been measured by means of the ionizing effects produced inside of the ionization chamber. These experiments have been made in order to determine whether or not the beams of rays are homogeneous, and, also, to compare the penetration with that to be expected according to certain theoretical distributions of energy in the spectrum of radiation due to the impacts of electrons of given velocity. The material used to de-... 

To determine the penetration of the radiation through the sheet of aluminum a number of measmrements of the ionization current due to the radiation from the mercury vapor with and without the test sheet of aluminum in the path of the rays were made. The test sheet was placed close to the pin-hole, K, so that a negligible amount of the scattered radiation from it entered the ionization chamber. 

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