Page 213 - Radiochemistry and nuclear chemistry

Detection and Measurement Techniques 197

(i) As mentioned SSNTD has been used in cosmic ray experiments at high altitudes and
in space journeys where memory effect and simple construction make them especially
suitable. Many elementary particles have be~n discovered by this technique, notably the 7r-
and #-mesons. Figure 10.2 shows tracks of high energy cosmic ray particles, probably iron
atoms, which have been stopped in Apollo astronaut helmets.
(ii) Nuclear reactions can be studied by SSNTD. The target material may either be regular
atoms of the detector (H, O, Ag, Si, etc.) or material introducexl into the matrix, e.g. thin
threads of target metals or uranium atoms. The former have been used in high energy
physics for hadron-induced reactions, and the latter for studying fission processes. From
experiments with uranium the frequency of spontaneous fission of 238U has been
determined, and also the rate of ternary fission and emission of long range ot's.
(iii) When emulsions are dipped into solution, some of the dissolved material is soaked
up or absorbed in the emulsion. For example, if the solution contained samarium, some
a-tracks of its spontaneous decay (decay rate 127 Bq g-1) appear in the emulsion (cf. Fig.
8.2(a)). Since 147Sin has an isotopic abundance of 15 %, its half-life is calculated to be 1.1
x 1011 y. The lower limit of detection is about 500 tracks cm 2 d-1, so quite low decay
rates can be accurately measured, making this a valuable technique for determination of
long half-lives.
(iv) 222Rn is releasext through the earth's surface from uranium minerals. The amount
released varies not only with the uranium content and mineral type, but also with the time
of the day; variation from 1 - 75 Bq l- 1 has been registered during a 24 h period. To avoid
this variation, cups containing a piece of plastic TD are placed upside down in shallow (0.5
- 1.0 m) holes for about 3 weeks, after which the SSNTD are etched and or-tracks from
radon counted. Mineral bodies hundreds of meters underground can be mapped with this
technique in great detail in a reasonably short time. The US Geological Survey uses the
same technique to predict earthquakes; it has been observed that just before earthquakes the
radon concentration first increases, then suddenly decreases, the minimum being observed
about one week before the earthquake appears.
(v) The average radon concentration in houses can be measured by hanging a plastic film
inside the house over a time period of some weeks. The film is returned to a laboratory
where it is etched and the number of c~-tracks per unit area counted.
(vi) Fission fragments make dense tracks in all solid material. The tracks are short and
thick: in a crystal material like zircon (a common mineral of composition ZrSiO4) they may
not be more than 10 -2 #m (10 nm) in diameter, and 10 - 20 ttm in length. They are
therefore not visible even in the best optical microscopes. Using manning electron
microscopy, it has been found that the hole formed retains the crystal structure or regains
it (Fig. 8.2(d)). On the other hand, if the track is formed in glass, a gas bubble appears
instead of a track (Fig. 8.2(e)); these slightly elongated bubbles can be distinguished from
other completely spherical bubbles formed by other processes. To make the tracks visible,
the specimen is embedded in a resin, then one surface is ground and carefully polished after
which it is dipped in an acid, e.g. HF. Because of defects in the crystal structure along the
fission track, the track and its close surroundings are attacked by the acid, and the diameter
of the track increases a hundredfold to a micron or so. The tracks are then visible under
a microscope with a magnification of 500- 1000 x.
This procedure has been used as an analytical tool for determination of uranium and
plutonium in geological and environmental samples. In this technique, the sample (either