Page 214 - Radiochemistry and nuclear chemistry

198 Radiochemistry and Nuclear Chemistry

a ground and polished surface of a mineral, or a dust sample on tape) is firmly pressed
against a photographic film, and the package is irradiated by slow neutrons. From the
fission track count of the developed film the uranium or plutonium content can be
calculated. Thus a Swedish shale was found to contain 4 + 1 ppm U, and a bottom sediment
in a Nagasaki water reservoir 0.44 5:0.04 Bq 239pu per kg sediment. In the latter case, the
ratio between the number of fission tracks Nft and c~-tracks Nat from 239pu is

Nft/Nat = 0rfO/~k a (8.1)

This technique is very useful for routine measurements of fissionable material in very low
concentrations. Figure 8.2(c) shows fission tracks in uranium-containing mineral which has
been exposed to neutrons.
Fission track counting is also important for dating of geological samples (Ch. 5) and for
estimation of the maximum temperature experienced by sedimentary rocks. The latter is
important in oil prospecting operations as the maximum temperature seems to be a useful
indicator on whether to expect oil, natural gas or nothing. For too low temperatures neither
oil nor gas is expected, for intermediate temperatures oil may be present and for high
temperatures only natural gas. Temperature history information can obtained from a
combination of age (estimated by other radioactive methods, of. Ch. 5), uranium content
of crystals of several minerals in the rock, their fission track count and track-length
distribution caused by thermal annealing. Zircon, titanite and apatite are examples of three
such minerals in order of increasing annealing temperature.

8.2. General properties of detectors

A nuclear particle entering a detector produces excitation and ionization, both of which
can be used for detection. When the excitation is followed by fluorescent de-excitation
(w167 and 7.5.3) the light emitted can be registered by light-sensitive devices, e.g. the
photomultiplier tube (PMT) which transforms the light into an electric current. Scintillation
and (~erenkov detectors are based on light emission. A similar current is generated when
production of charge carriers (i.e. ions, electrons and holes) takes place between the
charged electrodes of a detector. Detectors based on the production of charge carriers are
either gas-filled (ion chambers, proportional and Geiger-Mfiller tubes in which charge
carriers are produced by ionization of a gas) or solid, usually semiconductor crystals. In
the latter case electrons and holes are produced in pairs (w
An ionizing particle or photon will produce a collectable charge A Q in the detector

AQ = 1.60 • 10 -19 Eloss ~/w- 1 (8.2)

where Eloss is the total energy lost by the particle to the detector, ~/ is the collection
efficiency, w is the energy required for the formation of a pair of charge carriers in the
detector medium, while the constant is the charge (Coulomb) of a single charge carrier (the
pairs must be regarded as singles because they move in opposite direction in the electric
field due to their opposite charges).