Page 200 - Radiochemistry and nuclear chemistry

184 Radiochemistry atut Nuclear Chemistry

7.10. Dose measurements
The amount of radiation energy absorbed in a substance is measured with dose meters (or
dosimeters). These may react via a variety of processes involving (a) the heat evolved in
a calorimeter. (b) the number of ions formed in a gas, (c) the chemical changes in a liquid
or in a photographic emulsion, and (d) the excitation of atoms in a glass or crystal. The
first two ones are primary meters in the sense that they can be used to accurately calculate
the exposure or dose absorbed from a radiation source. They can be used to calibrate the
secondary meters.
In 1925 C. D. Ellis and W. A. Woorter, using RaE (21~ in calorimetric measurements,
obtained the first proof that the maximum energy and the average energy of/~-radiations
were different. A precision of about 1% can be obtained in a calorimeter for an energy
production rate of - 10 -6 J/s which corresponds to approximately 0.7 MeV average energy
for a sample of 40 MBq (1 mCi). If the average energy of an ct- or/3-emitting nuclide is
known, calorimetric measurement of the energy production rate can be used to calculate the
specific activity. This technique is not suitable for 3~-sources.
A more sensitive and general instrument for the measurement of ionizing radiation is the
condenser ion chamber. This is a detector which has a small gas-filled volume between two
charged electrodes. When radiation ionizes the gas between the two electrodes, the cations
travel to the cathode and the electrons to the anode, thus preventing recombination of the
ion pairs. Measurement of the amount of discharge provides a determination of the
ionization and consequently of the dose delivered to the instrument. This type of instrument
is described in more detail in next Chapter. The flexibility and accuracy of this dosimeter
have led to it being widely employed for the exact measurement of -),-dose rates. The most
common version of this type of instrument is the pen dosimeter (Fig. 7.8), which can be
made to provide either a direct reading of the absorbed dose or indirectly via an auxiliary
reading instrument. Instruments with ranges from 0.0002 to 10 Gy (full scale) are available
commercially.
There are numerous chemical dosimeters based on the radiolysis of chemical compounds,
both organic and inorganic. An illustrative example is the CHC13 dosimeter. This is a two-
phase aqueous-organic system. Radiation produces HCI which changes the pH of the almost
neutral aqueous phase as shown by the color change of a pH indicator. This dosimeter is
suitable only for rather high doses, 102 - 105 Gy.
The most common chemical dosimeter is the Fricke dosimeter (w which consists of
an aqueous solution of approximately the following composition: 0.001 M Fe(NH4)2(SO4) 2,
0.001 M NaCI, and 0.4 M H2SO 4. The amount of Fe 3+ formed through irradiation is
determined spectrophotometrically and the dose absorbed in Gy calculated by the equation:

D(Gy) = Al{e x p G(Fe3+)} (tool/J) (7.14)

where A is the change in absorbance, G(Fe 3+) is the yield of Fe 3+ in mol/J (cf. eqn. 7.12),
is the molar extinction coefficient (217.4 m2/mol at 304 nm), x is the length of the cell
(in m), and p is the density of the solution (1024 kg/m 3 at 15 -25~ The G-value
depends somewhat on the LET value of the radiation as seen in Figure 7.7. The Fricke
dosimeter is independent of dose rate up to dose values of about 2 x 106 Gy/s and can be
used in the range of 1 - 500 Gy. In a common modification, the solution also contains