"
"
Their role is to convert light energy emitted by the crystal to an electrical signal that can be
exploited in electronic circuits [3, 5]. This is achieved by the combination of several elements,
placed in a vacuum to allow the flow of electrons. The first element, placed in contact with the
crystal is the photocathode, metal foil on which the light photons are able to extract electrons.
These electrons are attracted to the first dynode by the application of a high voltage between
it (positively charged) and the photocathode. The electrons acceleration allows them to extract
a much larger number of electrons from the dynode. Then there are several cascading dynodes,
on which the same phenomenon is repeated. The successive dynodes are submitted to
potentials higher and higher. From a dynode to another, we obtain a cascade of electrons more
intense (amplification phenomenon), which ultimately results in a measurable electric current.
This current is collected by the last element called anode and a real electrical signal is generated
(Figure 4).
Figure 4. PMTs disposition in a Gamma-camera. Generally a hexagonal shape of PTM is preferred then a circular because
it well cover the detection area. Additional very small PMT can also be used between principal PMT for best detection
area covering (CEM, Rennes, France).
ليست هناك تعليقات:
إرسال تعليق