Page 206 - Radiochemistry and nuclear chemistry

190 Radiochemistry and Nuclear Chemistry

An ionization instrument for the analysis of gas has been developed in which the gas
passes through a small chamber where it is irradiated by a small radioactive source. For
a constant source of radiation, the ions produced in the gas depend on the flow velocity of
the gas and on its temperature, pressure and atomic composition. The dependence of the
ionization on the atomic composition is a consequence of the different ionization potentials
of the different types of atoms of the gas and the different probabilities for electron capture
and collision. The ion current is collected on an electrode and measured. This current is a
function of the gas pressure and velocity since the higher the pressure, the more ions form,
while at higher velocity, the fewer ions are collected as more ions are removed by the gas
prior to collection. Such ionization instruments are used in gas chromatographs and other
instruments as well as in smoke detection systems (the normal radiation source is 24] Am,
usually < 40 kBq), where secondary electrons condense on smoke particles, leading to
lower mobility for the electrons and a decreased ion current.
Both c~- and/3-emitters are used in luminescent paint. The fluorescent material is usually
ZnS. T and 14C are preferred sources since their/3-energies are low, but 85Kr, 9~ and
147pro are also used. The amount or radioactivity varies, depending on the nee~ (watches,
aircraft instruments, etc.) but it is usually < 400 MBq (< 10 mCi), although larger light
panels may require > 50 GBq (several Curies). For such high activities only T or 85Kr are
acceptable because of their relatively low radiotoxicity.

7.13. Exercises

7.1. How many ion pairs are produced in 10 m of air of STP by one (a) 5 MeV a-particle, (b) 1 MeV ~particle,
and (c) 1 MeV 7-quantum (/tin(air) ~ /Zm(Water))?
7.2. Estimate the fraction of energy lost through bremsstrahlung for a ~emission of Em, x = 2.3 MeV, when
absorbed in aluminum. The effective/3-energy must be taken into account.
7.3. A freshly prepared small source of 24No had a measured decay rate of I GBq 1 s after its preparation. The
source is shielded by 10 cm ofPb. 24Ne emits 7-rays; 8% with 0.878 MeV and 100% with 0.472 MeV. Estimate
the total integrated dose received at 2 m distance during its life-time after preparation. Neglect build-up factors.
7.4. An acidic aqueous solution is irradiated by a-particles from dissolved ~39pu at a concentration of 0.03 M.
The plutonium is originally in its hexavalent state, but is reduced to the tetravalent state by the reaction Pu(VI) +
2H.-- Pu(IV) + 2H +. How much of the plutonium can be reduced in one week?
7.5. An acid solution of fresh fission products contains 0.8 g 1 -l cerium as Ce 4+. The "y-flux in the solution
corresponds to 520 GBq 1 -i of an average energy of 0.7 MeV. If half of the 7-flux is absorbed in the solution,
what fraction of cerium is reduced to Ce 3+ in 24 h? Assume the same G-value as in Figure 7.7.
7.6. Estimate the LET value in water for B-particles from 9~ and ~Y, and in aluminum for T.
7.7. A ~~ irradiation source is calibrated by the Fricke dosimeter for which the G-value is assumed to be 1.62
#mol/J. Before the irradiation the optical density D of the solution at 305 nm was 0.049 in a 1 cm euvette. After
exactly 2 h the D had changed to 0.213. Calculate the dose rate when the molar extinction of Fe 3+ is 217.5 m 2
mol- I.
7.8. A direct reading condenser chamber (pen dosimeter) is charged from a battery pack so that full scale (100)
is obtained at 20 V. When completely discharged, the accumulated dose is 5.00 mGy. The gas volume is 4 cm -3
air (STP). What is the capacitance of the condenser chamber?

7.14. Literature

S. C. LIND, Radiation Chemistry of Gases, Reinhold, 1961.
A. O. ALLEN, The Radiation Chemistry of Water and Aqueous Solutions, D. van Nostrand, 1961.