The Foundations of Chemistry

Detection by Fluorescence

Fluorescentsubstances can absorb high-energy radiation such as gamma rays and subse-

quently emit visible light. As the radiation is absorbed, the absorbing atoms jump to excited

electronic states. The excited electrons return to their ground states through a series of

transitions, some of which emit visible light. This method may be used for the quantita-

tive detection of radiation, using an instrument called a scintillation counter.

Cloud Chambers

The original cloud chamber was devised by C. T. R. Wilson (1869–1959) in 1911. A

chamber contains air saturated with vapor. Particles emitted from a radioactive substance

ionize air molecules in the chamber. Cooling the chamber causes droplets of liquid to

condense on these ions. The paths of the particles can be followed by observing the fog-

like tracks produced. The tracks may be photographed and studied in detail. Figures 26-3

and 26-4 show a cloud chamber and a cloud chamber photograph, respectively.

Gas Ionization Counters

A common gas ionization counter is the Geiger–Müller counter(Figure 26-5). Radia-

tion enters the tube through a thin window. Windows of different stopping powers can

be used to admit only radiation of certain penetrating powers.

1012 CHAPTER 26: Nuclear Chemistry

Dry ice

Figure 26-3 A cloud chamber. The
emitter is glued onto a pin stuck into
a stopper that is mounted on the
chamber wall. The chamber has
some volatile liquid in the bottom
and rests on dry ice. The cool
air near the bottom becomes
supersaturated with vapor. When an
emission speeds through this vapor,
ions are produced. These ions serve
as “seeds” about which the vapor
condenses, forming tiny droplets,
or fog.

The Geiger counter can detect only
and radiation. The -particles
cannot penetrate the walls or window
of the tube.

collision

Figure 26-4 A historic cloud

chamber photograph of alpha

tracks in nitrogen gas. The

forked track was shown to be

due to a speeding proton

(going off to the left) and an

isotope of oxygen (going off to

the right). It is assumed that

the -particle struck the

nucleus of a nitrogen atom at

the point where the track

forks.