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The range of most emitted
β
's is very short. It should be noted that in
β
+
and
β
-
emission, the
emitted electron or positron has a continuous energy spectrum, which ranges from E
max
to
zero, where E
max
is the maximum transition energy.
β
-rays (with the exception of a small
portion of very high energy
β
s) will be stopped in the sample, in the chamber liner, and in the
chamber wall.
As the electron decelerates, it also produces continuous low energy photon emission called
Bremsstrahlung (stopping or braking radiation).
Many radionuclides which decay by
β
emission also emit de-excitation photons (x-rays,
γ
-
rays), which can be detected by the ionization chamber.
Electron Capture
The actual electron capture process cannot be detected since the electron is not emitted but
is captured by the nucleus. The capture of the orbital electron, however, leaves a vacancy in
the atomic orbital shell, resulting in x-rays as the atom de-excites.
The energy of k x-ray is approximately
100
2
Z
E
k
≅
[keV]
where Z is the atomic number of the daughter nucleus.
γ
-rays are also often given off as the daughter nucleus de-excites.
Photon Radiation
Photon radiation is associated with most nuclear transformations. A high-energy photon
interacts with matter very weakly. Photon intensity is therefore, not altered substantially by
the surrounding media, i.e., measurement of activity can be accomplished with a minimum of
disturbance from the sample configuration.
As can be seen from the above, in all cases we are detecting photons. We will therefore,
discuss photons and their interactions with matter in detail.
PHOTONS
Photon is the general term for a quantum of radiation. Photons are classified according to
their method of production.
γ
-Rays
Photons resulting from nuclear transitions, nuclear reaction or annihilation of particles (e.g.,
electron-positron annihilation) are called Gamma-rays (
γ
-rays). Radioisotope sources
January 08
APPENDIX I
A1 - 3
