| Table
salt ( NaCl ) when irradiated will turn brown but will not be
radioactive. When some of this salt is dropped onto a heated
hot plate, flashes of visible light will be given off and the salt
will return to its normal white color. Use a black and white
camera to show this in a dark classroom. The salt will glow
for several seconds after it is placed on the hot plate.
Caution: DO NOT place the camera above or too
close to the hot plate. The explanation
is: The table salt exposed to gamma radiation from a
cobalt 60 source excites electrons in the salt to a higher energy
level. In this excited state, electrons in the salt can absorb
certain wavelengths of light, resulting in the observed brown
color. Heat enables the excited electrons to fall back to the
ground state with the emission of a flash of visible light and the
return of the white color. The
technical explanation is: When white
NaCl is exposed to gamma radiation from a Co 60 source, a
number of defects are formed in its crystalline lattice resulting in
a color change to orange brown.
Chloride ions are ionized into chlorine
atoms and electrons:
energy + Cl 1- ®
Cl 0 + e 1-.
Chlorine atoms migrate to form molecular
chlorine, Cl2 :
Cl 0 + Cl 0
=
Cl2 + energy
Electrons combine with sodium ions, Na+1,
leading to the formation of colloidal clusters of metalic
sodium, Na0:
Na 1+ + e 1- ®
Na 0 + energy
When heat is added, the molecular chlorine,
Cl2, and the sodium atoms, Na 0, recombine to
form the white salt of sodium chloride, NaCl, and the release of
light energy.
Salt substitute ( KCl ) is naturally radioactive due to the
Potassium content. (Potassium 40 is the most abundant
radioactive source on earth ). Counting rates observed with
the geiger counter should be 3 to 4 times background.
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