CAPINTEC, INC CRC
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-25R
β
-
Decay
The nucleus emits an electron (β
-
particle), and a neutrino.
β
+
decay
The nucleus emits a positron (β
+
particle) and a neutrino.
Nuclear Transition
A photon (electromagnetic radiation, γ-decay), electron (Internal Conversion Electron
Emission, CE) or electron-positron pair (Internal-pair emission, e±) is emitted by a
nucleus in a transition from a higher to lower energy state.
No nuclear transformation occurs if there is no change in the atomic number or the
mass number. The de-excitation of a nucleus in its unstable state (metastable state)
is, however, included in the definition of activity.
MEASUREMENT OF ACTIVITY
A Nuclear Transformation is always associated with one or more of the following types of
radiation:
α, β
+
, β
-
and γ Photons
We can, therefore, measure activity by detecting one or more of the above radiations.
α-Particle Radiation
The most energetic α-particle emitted by a radionuclide has an energy of less than 10MeV,
which corresponds to a range of about 10mg/cm
2
(8cm in air). Because of its short range, an
α-particle from a radionuclide cannot penetrate to the ionization chamber's sensitive volume
and therefore, cannot be detected.
All α-decays, however, are accompanied by photon radiation as the daughter nucleus
decays to its ground state. The activity of a nuclide that decays through α radiation can
therefore, be measured by detecting the associated photon radiation.
β
+
Radiation
β
+
particle (positron) emitted from a nucleus comes to rest in the media by losing its kinetic
energy mainly by direct ionization processes and then annihilates with an electron to produce
two photons of 511keV each. These photons are easily detected by the ionization chamber.
De-excitation photons are also associated with β
+
decay.
A1 - 2 APPENDIX I June 09
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