Dosimetry ionization chamber

Dosimetry. Ion current measurements required for absolute dosimetry were performed with a Cary 31 ionization chamber and vibrating reed electrometer. Dry nitrogen was used as filling gas for the chamber, and a W value of 34.9 e.v./ion pair was assumed for H-3 beta rays in N2 (27). Deuterium pressures in each of the reaction mixtures were great enough to ensure that less than 1% of the H-3 beta rays reached the walls of the reaction vessel (7). 

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). 

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. 

Lithium formate is the most extensively studied new material for ESR-dosimetry [21], It has also been tested for use in clinical applications [53, 54]. Thus, doses in the range 0.2-3.5 Gy due to 6 MeV photons were determined in blind tests shown in Fig. 9.7. A deviation of less than 1.2% from those by ionization chamber measurements was obtained, which is well within the uncertainty of the measurements. It was concluded that no trend could be seen in the ESR dosimeter response, regarding either the dose rate or the beam quality. Each measurement took 15 min, while several hours would be required with the common alanine dosimeter. 

A dose meter or dosimeter is an instrument that measures radiation dose. Personnel dosimetry is accomplished with such devices as the film badge, thermoluminescent dosimeter, or pocket ionization chamber. In this way continuous recording of cumulative radiation dose can be maintained. 

Instruments for measuring ionized radiation typically include a sensing device and a readout device. Some are usefiil for field measurement whereas other combinations come in small packages useful for dosimetry. Sensors are very critical. Different types of radiation require different types of sensors. Sensors include Geiger-MueUer tubes (used in Geiger counters), ionization chambers, luminescent detectors, scintillation detectors, and photographic emulsions. 

Ionization chambers measure beta, gamma, and X-rays using a charge placed on an electrode in a tube. Radiation ionizes the air surrounding the electrode and allows charge to leak away. The amount of charge lost correlates with the amount of radiation arriving at the chamber. A common dosimetry device is a pocket ionization detector. 

The most common physical methods applied in the dosimetry practice of ionizing radiation are calorimetry and ionization methods. Calorimetry is a primary standard method in dosimetry used both to measure dose rate in various radiation fields and to calibrate standard and routine dosimeters. Ionization chambers are discussed in O Chap. 48, Vol. 5, thus only calorimetric systems wfll be shortly described here, with particular respect to those applied mainly in radiation processing. 

Boag, J. W., Ionization chambers, in Radiation Dosimetry, Hine, G. J. and Brownell, G. L., Eds., Academic Press, New York, 1956. 

Homeck, E, Properties of a liquid ionization chamber irradiated with fast neutrons, in Dosimetry in Agriculture, Industry, Biology and Medicine, International Atomic Energy Agency, Vienna, 1973. 

With the advancement in the understanding of the interaction of radiation and liquids and with improvements in manufacture and purification of liquids, the application of liquid ionization chambers for dosimetry and for detection of elementary particles has received increasing interest. Another promising development are imaging chambers for medical physics and materials testing. 

Basically, two modes of operation of a liquid ionization chamber can be distinguished (1) ion current measurement and (2) electron pulse detection and counting. Ion current chambers are used in medical physics for dosimetry and radiation field mapping, while electron pulse chambers are employed in the detection of individual elementary particles and quanta in radiation and high energy physics. 

Wickman, G. and Nystrom, H., The use of liquids in ionization chambers for high precision radiotherapy dosimetry, Phys. Med. Biol, 37,1789,1992. 

Pb(CH3)4 vapor has been used to fill Geiger counters [170 to 174], and was suggested for use in liquid ionization chambers [175]. Short-lived transients are generated by pulse radiolysis of Pb(CH3)4 dosed with naphthalene and toluene, which can be employed for time-resolved dosimetry of low-energy X-rays [176]. 

---