Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 69Insulators-TO current. 'measuringdevice___ct \Metal blockTo pumpFig. 50. Faraday cage. From ref. 167.sorbed energies once the value of W is kn~wn'~~.'~~. Inert gases are suitablereference substance^'^ 3* 195. Ionisation chambers are generally used as secondaryinstruments and calibrated against a Faraday cup or cage. This consists of a metalblock thick enough to stop a beam of charged particles (Fig. 50). The block acquiresthe charge of every absorbed particle, and the current flowing from it is adirect measure of the number of particles entering the chamber. If the energy perparticle is known, from a well characterised source or from magnetic deflectionmeasurements, the intensity of the radiation Z is determined from the expressionin equation (K')Z(erg. ern-'. sec-') = N(partic1es. cm-2. sec-') x E(erg. particle-')(K')Ionisation chambers have been described by B~ag'~~~, La~ghlin'~~~ (electronbeams), and Birge et al. 19' (heavy charged-particle beams). Provided the Bragg-Gray principle is valid, the absorbed dose is given by equation (L')where Q is the charge (e.s.u.) produced in 1 cc of dry air at NTP, Smedium isthe stopping power and the value 0.877 is derived assuming Wail = 34 eV per ionpair. This value is valid for X- and y-rays and electrons but for heavier chargedparticles is modified to 0.877 x Wai,/34, where Wail is obtained for the particularsystem. me is el^'^^ has determined W values for the partial absorption of 1 MeVelectrons by organic and inorganic compounds. The stopping powers were usedto calculate relative energy absorption in various gases. Dosimetry has been thesubject of a ~yrnposium'~~.References pp. 104-111