CAPINTEC, INC. CAPTUS
®
3000
When a gamma or x-ray from a radioactive source reaches the NaI crystal, it collides with the
electrons in the crystal matrix and transfers energy, primarily through Compton interaction.
The excited electron releases this energy as visible light in direct proportion to its absorbed
energy. For this reason, NaI detectors are classified as scintillation detectors. The higher the
energy of the incident ray, the more Compton collisions and consequently the more visible
light emissions will be measured before all of the energy is absorbed. These sequential
emissions of light for each incident ray generated from the sequential Compton collisions
appear to the measuring system as a single light pulse. The intensity of the pulse is directly
proportional to the energy of the incident gamma or x-ray. The light is eventually directed
toward the photomultiplier tube, either by the direction in which it was emitted or by reflection
from the interior of the aluminum housing. The photomultiplier tube, which is optically
connected to the sodium iodide crystal, converts the visible light pulses into electrical energy
and amplifies the signal through a series of dynodes. The resultant electrical signal is directly
proportional to the intensity of the incident light pulse. Therefore, the pulse height generated
by the system is directly proportional to the energy of the incident gamma or x-ray detected
by the system.
Multichannel Analyzer
A multichannel analyzer records the number and height of the electrical pulses generated by
the system, divides the pulses into groups based upon pulse height, and provides a visible
picture or spectrum of the pulses as well as count information. The CAPTUS
®
3000 divides
the pulses into 1024 groups or channels. Since pulse height is directly related to the energy
of the incident photon, the spectrum displays the pulse heights as a graph of pulses or
counts on the vertical scale and energy on the horizontal scale. By matching the displayed
spectrum to the decay scheme of an isotope, positive identification of the radioactive source
can be achieved.
Electronics associated with the process include the high voltage power supply, amplifiers,
and gain adjustments. The high voltage power supply provides voltage to the photomultiplier
tube for the light to electrical impulse conversion and amplification process. The amplifiers
linearly amplify the electrical signal produced by the photomultiplier tube before the signal is
passed to the MCA. The pulse height remains proportional to the energy of the incident ray.
The gain controls the amount of amplification that is performed by the card, and is adjustable
by the user.
The spectral display also has adjustments which control the pulse height that include zero
adjust and lower level discriminators (threshold). The zero adjust controls the channel used
for 0 pulse height or 0 energy. The threshold sets the limit for the lowest pulse height to be
analyzed.
July 14 FUNCTIONAL & TECHNICAL DESCRIPTION 2-5
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