In real situation, current flows in the detector from more than one interaction at a given time. Detector working modes: Three different detector working modes can be considered: pulse mode, current mode and mean square voltage mode. If the response time of the measuring device connected across the detector output terminals is long compared with the average time spacing between current pulses, then a current is recorded that is given by the mean rate of charge formation averaged over many individual radiation quanta. This mode of operation is called current mode. The average current represents the product of the average event rate and the charge created per event.
However, different currents will result from radiations that have equal interaction rates but deposit a different average energy per interaction. In mixed radiation environments, when the charge amount produced by various radiation types is different, detectors operating in mean square voltage mode are useful. In such devices, the mean square of the time variance of the detector current is recorded which is proportional to the event rate and the square of the charge produced in each event.
In the pulse mode operation, each quantum of radiation that interacts with the detector has to be measured. The output of such type of detector is a sequence of individual signal pulses, each representing the result of a single quantum of radiation within the detector. The rate at which such pulses occur, give a measure of the radiation interactions inside the detector while the amplitude of each pulse is related to the amount of charge generated during each interaction.
Semiconductor Detectors and Principles of Radiation-matter Interaction
Pulse mode operation is the most common choice for different radiation detector applications; in fact it presents several advantages for instance a higher sensitivity in comparison with the other two working modes. Pulse height spectra and energy resolution: In the pulse mode detector, the amplitude of the various pulses is different because of differences in radiation energy or fluctuations in detector response to a monoenergetic radiation. The pulse amplitude distribution gives information about the incident radiation. In Fig. The number of pulses having an amplitude between H a e H b is obtained adopting the following expression:.
In many applications, the radiation detector task is the energy distribution measure of the incident radiation. The capability of detectors to distinguish between two particles or photons with different but close energies is indicated as the energy resolution and denoted with R. A large amount of fluctuations in the number of produced charges is recorded even if radiation beams with equal energy are adopted. There is a high number of potential sources of fluctuations in the response of a detector that result in imperfect energy resolution. They can be produced by drift of the detector operating characteristics, random noise sources of the instrumentation system and statistical noise derived from the fact that the charge generated within the detector by a radiation quantum is not a continuous variable but represents a discrete number of charge carriers.
It is evident that the smaller the value of R, the better the detector will able to distinguish between radiations having energies lie near each other. Assuming that the formation of each charge carrier is a poisson process, it was demonstrated that for detector having a linear response, R results Knoll, :. For some type of detectors, experimental results show that the obtained R-values are lower than the values predicted by Eq. This difference indicates that the processes which give rise to the generation of charge carriers are not independent and therefore the total charge produced cannot described by a simple poisson process.
A corrective parameter, called the Fano factor and indicated with F, is introduced to quantify the difference of the observed values from the number of charge carriers obtained considering a pure Poisson statistic Fano, Introducing the Fano factor, expression Eq. Detection efficiency: Charged radiation interacts with detector producing ionization or excitation of material nuclei as it enters in the active volume.
Therefore, after a typical particle is travelled a small fraction of its range, a lot of pairs is created to ensure the presence of a signal pulse large enough to be detected. In this situation no charged particles reaching the detector active volume are lost. The intrinsic efficiency, which is the most used parameter, depends on the detector material, the radiation energy, the detector thickness in the direction of incident radiation.
Moreover, a slight dependence on the distance between the source and the absorber material is observed as the average path length of the ray inside the detector changes somewhat with this spacing. Dead time: The dead time is the time during which a detector is not able to detect a next coming particle, in fact two events must have a minimum amount of separation in time in order to be recorded as two different pulses.
Solid-State Radiation Detectors: Technology and Applications - CRC Press Book
This minimum recording time can be determined by physical processes in the detector or by associated electronics. For example, when a radioactive sample is counted, the possibility can arise that two interactions in the detector will occur too close together in time to be registered as separate events. Two different models have been proposed to approximate the dead time behaviour of counting systems: paralyzable and nonparalyzable response.
Differences in the two models are illustrated in Fig. The central line represents six events as they occur along the horizontal time axis, and the other two axes show the responses of the two types of detectors. Of the six events in this example, the paralyzable counter registers three and the nonparalyzable, four.
In the paralyzable case, dead periods are not always of fixed length. The average number of events that takes place in a time t is r i t. Plotting r c vs. Increasing the event rate, the measured count rate with a paralyzable system will decrease beyond this maximum and will approach zero, because of the decreasing opportunity to recover between events.
With a paralyzable system, there are generally two possible event rates that correspond to a given count rate. Radiation detection system is composed of a detector, signal processor electronics and data output display device such as counter or multichannel analyzer. Detector physical properties and characteristics control detection system features. Radiation detectors and detection systems can be classified differently according to the particular parameter taken into account.
In fact, they can distinguish into. Devices employing semiconductors as detection medium have become very diffused in many radiation detection applications. The large number of generated carriers for a given incident radiation beam, produces detectors with good energy resolution. Moreover, other features can be obtained adopting semiconductor as absorber material such as compact sizes and relatively fast timing characteristics. Semiconductor properties: The periodic structure of the particular crystalline material establishes allowed energy bands for electrons which are present inside the material.
The lower band, indicated with valence band, is composed of electrons which are bounded to lattice sites inside the crystal while the upper band, denoted with conduction band, is characterized by the presence of free electrons which contribute to the material conductivity. These two bands are separated by a band of forbidden states named bandgap.
The bandgap dimension defines the classification of material as conductor, semiconductor or insulator. In absence of thermal excitation, semiconductor exhibits a valence band fill of electrons while a conduction band totally empty. If an energy exceeding the bangap energy is imparted to an electron located in the valence band, this electron can be transferred to the conduction band and a vacancy called hole is created inside the valence band.
The average displacement of a movable charge carrier due to random motion is zero but owing to the application of an electrical field, both the electron within the conduction band than the hole inside the valence band which represents a positive charge can move contributing to the material conductivity. Because of electrons is drawn in a direction opposite to the electric field, holes move in the same direction as the electric field with the following net drift velocity:.
Relations Eq. Alternatively, if the field is high enough, such that electron and hole energies become appreciably larger than the thermal energies, strong deviations from linearity are observed and drift velocities become independent of the electric field, reaching a saturation value.
An other way to obtain free electrons is the alteration of semiconductor lattice structure adding a small amount of other elements with a different atomic structure called impurities. This procedure, which can be performed either during crystal growth or later in selected crystal regions, is called doping.
In this way, the electrical properties of pure semiconductor materials can be modified and controlled. When, impurity elements, also called dopants, are added to semiconductor material, impurity atoms take the place of semiconductor atoms in the lattice structure. When impurity atoms have one more valence electron than the semiconductor atom, this extra electron cannot form an electron pair bond because no adjacent valence electron is available and it requires only slight excitation to break away.
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Consequently, the presence of such excess electrons increases the semiconductor conductivity. The resulting material is called n-type because the excess free electrons have a negative charge and the impurities of this type are referred to as donor atoms. In this case, electrons are called majority carriers and holes minority carriers.
A different effect is produced when impurity atoms having one less valence electron than the semiconductor atom is substituted in the lattice structure. Although all the valence electrons of the impurity atoms form electron pair bonds with electrons of neighbouring semiconductor atoms, one of the bonds in the lattice structure cannot be completed because the impurity atom lacks the final valence electron.
As a result, a vacancy hole exists in the lattice and an electron from an adjacent electron pair bond may absorb enough energy to break its bond and to fill the hole. As in the case of excess electrons, the presence of holes encourages the flow of electrons in the semiconductor material with a consequently increasing of the material conductivity.
Semiconductor of this type having an excess of holes is called p-type material and the adopted impurities are indicated as acceptor atoms Fig. Radiation interaction in semiconductor: Radiation interaction with semiconductor materials produces the creation of electron-hole pairs that can be detected as electric signal. As it is shown in the previous sections, when the incident ray is composed of charged particles, ionization may occur along the path of flight by many collisions with the electrons.
In presence of uncharged radiations, such as X-ray, photons have first to undergo an interaction with either a target electron according to photoelectric or compton effect or with the semiconductor nucleus. Moreover, it is weakly depended on the type and energy of the incident radiation except for low energies which are comparable with the semiconductor band gap.
The mean number of generated pairs N is obtained with the following expression:. The fluctuation in the number of carriers generated can be evaluated introducing the Fano factor F:. Their motion will go on until they are collected at the electrodes or they recombine because of the presence of impurities or structural imperfections of the detector material.
Related Silicon Solid State Devices and Radiation Detection
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