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essentially electrical in nature, meaning that the it to process an event. This is related to the duration
information from the detector is transformed into of the pulse signal, and during this time, the
electrical impulses that can later be digitalized. detector can be insensitive to other events. It can
also generate overlapping pulses, producing
When interacting with detectors, charged distortion of the first signal and loss of information.
particles like muons, transfer their energy to matter To avoid this, the counting rate of the detector must
through collisions with the atomic electrons, be kept sufficiently low, so that the probability of a
inducing excitation or ionization of the atoms. Even second event occurring during the dead time is
if ionization is produced, a certain minimum small.
amount is necessary in order for the signal to be
usable. This lower limit is determined by the noise C. Scintillation Detection
from the detector and the electronics of the
experiment. This noise manifests as a fluctuating Once radiation of charged particles, like the
voltage or current at the detector’s output and is muons, reaches the detector, the form in which the
always present wether there is radiation or not. As converted energy appears depends on the
a consequence, the ionization signal product of detector and its design. Scintillation detectors
the detection of radiation, must be larger than the make use of the property of some materials to emit
average voltage level associated with the noise. a small flash of light when struck by radiation. In
Otherwise, the signal produced by radiation will be scintillation detectors, the excitation and ionization
indistinguishable from the noise of the detector and produced by the radiation, contribute to inducing
the electronics (Leo, 19994). molecular transitions which result in the emission of
light.
The output signal of electrical detectors is in the
form of a charge or voltage pulse. The amount of Scintillation detectors or scintillators, have a very
ionization in the detector is reflected in the short response and recovery times, compared to
electrical charge contained in this signal. Assuming other detectors. Therefore, the time difference
the shape of the pulse doesn’t change from one between two consecutive events can be
detection to the other, the radiation energy is measured with great precision. The materials used
directly proportional to the pulse height. in scintillators can vary from organic ones like
aromatic hydrocarbon compounds, or crystals like
The response time is the time that takes for the anthracene and naphthalene; to solutions of
detector to form a signal after the radiation arrived. organic scintillators but in a plastic solvent like
For a good timing, it is necessary for the signal to be polyvinyltoluene or polystyrene. Plastic scintillators
quickly formed into a sharp pulse with a rising flank and, scintillators in general, are some of the most
as close to vertical as possible, so that the signal used particle detection devices today.
can represent more precisely an instant in time: the
instant in which the radiation arrived to the When coupled to a signal amplifying device,
detector. Of course, the determination of this scintillations in the detector can be converted into
instant will never be 100% accurate. There will electrical signals that can be analyzed and
always be a small interval of time between the counted electronically to get information about
arrival of the radiation, the excitation or ionization the incident radiation. Photomultipliers (PMT) can
of the atoms in the detector, and the emission of do just that. They are electron tube devices that
the signal. Ideally this time interval should be made convert light pulses into a measurable electric
as small as possible. charge.
The duration of the signal produced is also A PMT consists of a cathode made of
important as it determines the ability of the photosensitive material followed by an electron
detector to perceive two consecutive events. If collection system, an electron multiplier section,
every pulse generated has a very long duration, called dynode, and an anode from which the final
then another detection that occurs during that signal can be taken. All these parts are usually
time interval, will be overlapped to the first signal housed in a glass tube, at vacuum. When
and information about both detections will be lost. operating, a high voltage is applied to the
The dead time of a detector is the time required by cathode, dynodes and anode so that there is a
potential difference along the length of the tube.
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Año 10 Núm. 30 septiembre-diciembre 2024 Tlahuizcalli ISSN: 2448-7260
