Page 212 - Radiochemistry and nuclear chemistry

196 Radiochemistry and Nuclear Chemistry

While the memory effect of the developed film is almost infinite, this is not the case for
the latent image which slowly fades, depending on the number of originally activated silver
atoms, the film type and external conditions like temperature, humidity, etc. When stored
under ambient conditions, about 80% of the latent image disappears in half a year.
The developed grains form an interrupted track along the original path of the energetic
particle (Fig. 8.2(a)). The specific energy loss of the particle, dE/dx (i.e. the stopping
power of the absorber), depends on the mass, charge, and velocity of the particle, and on
the composition of the absorber. From the track length, grain density (i.e. grains per track
length), and gap length between the grains, the particle and its energy can be determined
(cf. w167 and 7.1). For a given particle, the range ~ is proportional to the energy as shown
in Table 8.1. The range decreases with increasing mass of the particle and density of the
absorber. The grain density depends on the specific ionization of the particle which does
not vary linearly with the particle energy (or velocity), as seen from Figure 6.7; thus the
grain density changes along the track.
Other solid material may be used as SSNTD instead of AgBr emulsions: plastics (cellulose
nitrate and polycarbonate films), glass, crystals, etc. In order to make the tracks visible in
the microscope the surface of the SSNTD must be polished and etched, usually with alkali.
Because of the natural radiation background, every SSNTD has a memory of past nuclear
events, which must be erased as far as possible before a new exposure. In nuclear
emulsions an c~-radiation background of 20- 60 tracks cm -2 per day is normal. The
technique of background eradication prior to exposure may consist of treating an emulsion
with chromic acid, H202-vapor or heating (annealing) a glass plate. Because this technique
more easily removes weak images, it may also be used after exposure, e.g. to remove
fainter or-tracks from heavier fission tracks.
Let us consider some examples of uses of SSNTD. Tracks obtained under various
conditions are shown in Figure 8.2.

Table 8.1. Range of energetic high-ionizing particles in various solids

Particle Energy Absorber Range
0VIeV) (density) (pan)
IH 10 llford C2 0.8) (-) 540
3H 10 Ilford C2 (3.8) (=) 230
4He 10 Ilford C2 (3.8) (=) 57
4He (214Bi) 7.7 Eastman NTA (3.6) 38
Mica (3.1) 36
Glass (2.5) 41
Water (1.0) 60
4He (238U) 4.2 Mica (3.1) 13
4He (U-series) Pitchblende (7.0) 23 (b)
Camotite (4.1) 32 (b)
235U{ Light fiss. fragm. } -150 Eastman NTC (-3.4) 14 }-25
Heavy fiss. fragm. Eastman NTC (-3.4) 10.5
23SU. Both fiss. fragm. - 160 Leopoldite (-4) -20

(') Density of AgBr 6.47; of gelatin 1.31.
(b) Range of the predominating c=-particles.