The electron mobility in a gas counter is thousands of times greater than that of the ions. In fact, the electron mobility in semiconductors is roughly equal that of the holes and both types of carriers contribute to conductivity. The following equation is obtained by using Equations 3 and 4 :. Thus, both terms in the right-hand side of Equation 7 contribute to the conduction in semiconductor detectors.
This generates a big advantage over other detectors and the output pulse provides much better energy resolution. The energy resolution, R, determines the ability of the system to distinguish two energies that are very close to each other, and that constitute an important parameter in the spectral detection of ionizing radiation Figure 8. It is commonly defined as:.
FWHM for a Gaussian distribution. In order for a semiconductor to act as a radiation detector, the active area to radiation must be free of excess electrical charges depleted. PN junctions are obtained when an n-type semiconductor excess of electrons is placed in contact with a p-type semiconductor excess of holes.
Then, electrons and holes diffuse from n-region to p-region and from p-region to n-region, respectively, and they recombine around the interface. The ions, which are left behind by electrons and holes that were recombined, create an electric field that will attract more electrons and holes until there is no more charge carriers to recombine Figure 9. At this moment, if the ionizing radiation interacts with the semiconductor in this depleted region, electrons are raised to the conduction band leaving behind holes in the valence band and producing a large number of electron-hole pairs.
If a voltage is applied across the semiconductor, these carriers are readily attracted to the electrodes and current flows into circuit resulting in a pulse. The size of the pulse is directly proportional to the number of carriers collected, which is proportional to the energy deposited in the material by the incident radiation.
PN junction. In semiconductors, if the temperature increases, electrons can be thermally excited from the valence band to the conduction band. Consequently, some semiconductor detectors must be cooled so as to reduce the number of electron-hole pairs in the crystal in the absence of radiation. Although solid-state detectors can be manufactured much smaller size than those of equivalent gas-filled detectors and they have short response time, seconds compared to the hours of TLD detectors, they are still expensive because they need to be cooled.
Thus, they are used when higher resolution is required; if higher efficiency is necessary, scintillation detectors are used.
Different semiconductor materials and device arrangements are used, depending on the type of radiation to be measured and the aim of application. Commonly, semiconductor detectors are employed for beta particles or gamma radiation because the heavy charged particles cause more radiation damage. They are widely used in nuclear power station electronic dosimeters and portable survey instruments in gamma spectroscopy systems.
The amount of radiation absorbed by the human body can be determined through radiation dosimetry. A dosimeter has to correlate the absorbed radiation with biological effects induced in humans.
The physical quantity that quantifies this relationship is called absorbed dose. The absorbed dose, D, of any ionizing radiation can be considered as the amount of energy given to the medium by ionizing particles or photons per unit of mass dm [ 2 , 14 , 15 ]:.
Thermoluminescent dosimeters TLDs are the foremost used devices for personal dosimetry. They are composed of crystal devices that emit light when are heated. The TLD reading device is able to calculate the amount of light released during heating, which can then be correlated with the absorbed dose received and stored by the TLD dosimeter.
A useful model of the thermoluminescence mechanism is provided in terms of the band model for solids. Thus, when a thermoluminescent crystal is exposed to ionizing radiation, electrons are quickly promoted to their conduction band through direct excitation process.
However, some electrons are trapped by metastable states and when the material is subjected to thermal stimulation, they have enough energy to leave the trap states and recombine with holes that were left in the valence band. The excess of energy in this process is conserved by radiative deactivations emitting light, which is proportional to the absorbed ionized dose [ 16 ]. Figure 10 shows a model of energy bands with electronic transitions in thermoluminescence process.
Model of energy bands in thermoluminescence process. T is the center of the trap, R is the recombination center, E F is the Fermi level, and E g is the bandgap. The heating of the TLD dosimeter to assess the accumulated radiation dose is done in temperature ramps and each temperature value is associated with a value of the light intensity Figure Thus, through thermoluminescence photons emission it is feasible to establish a curve of thermoluminescence intensity versus temperature that is called TL glow curve.
The area under the TL glow curve is directly proportional to the number of emitted photons and, thereby, proportional to radiation dose received.
Thermoluminescent crystals possess good levels of deeper traps that require greater thermal energy to release the carrier, thus they can accumulate energy for a longer period of time. Many materials are purposely doped to create impurity levels; others such as LiF lithium fluoride already have natural impurities and intrinsic defects.
Other substances are used as materials for thermoluminescent dosimetry, for example, CaSO4:Dy calcium sulfate doped with dysprosium ; the CaSO4:Mn calcium sulfate doped with manganese ; and CaF2 fluorite.
A thermoluminescent crystal can be used as dosimeter only if it presents high emission efficiency, good stability on temperature ranges of work, high resistance to environmental variations and linear radiation dose-response.
In chemical dosimetry, the ionizing radiation produces chemical changes in the medium that can be measured by using a suitable measuring system. Oxidation, reduction, and chemical dissociation are the principal mechanisms of chemical detectors. G SI unit is mol. The most widely used chemical dosimetry standard is the Fricke dosimeter. The Fricke dosimetry system provides a reliable means for measurement of absorbed dose to water by ferrous ions oxidation.
The ferric ion concentration is determined by spectrophotometry, which measures absorption peaks at wavelengths of nm and nm.
In this case, G-value is defined as the number of moles of ferric ions produced per joule of the energy absorbed in the solution. The usual range of the Fricke dosimeter is from 30Gy to Gy. Calorimetric methods measure the dose of radiation by measuring the temperature increase in a medium. Although the basic principles of calorimeters are very simple, they have technical problems to ionizing radiation sensing and they have been viewed as complex to make and operate [ 18 ].
Small temperature response to low dose of radiation and necessity of extremely thermic isolation are some problems of this type of detector. Therefore, few laboratories use these detectors; however, efforts have been made in order to increase their performance.
Despite the well-established known techniques and detectors for ionizing radiation, the field still has a lack of new materials and sensor devices. The use of ionizing radiation in industrial processes, in clinics, hospitals, universities, and research centers has increased considerably and consistently in the past few years.
In addition, the inspection and monitoring of aircrews is a current concern and should be mandatory to all flights in the near future. Thus, the development of new materials sensitive to ionizing radiation and robust devices, faster and more accurate, is of crucial importance to this research field and its direct applications.
In the last two decades, there was an effort to combine the energy sensitivity found in semiconductor devices with the low cost and flexibility of organic semiconductor-based conjugated polymers. In this fashion, oligomers and polymers such as PPV poly p-phenylene vinylene [ 19 ], MEHPPV poly 2-methoxy, 5- 2 -ethyl-hexoxy -p-phenylene vinylene [ 20 , 21 ], P3HT poly 3-hexylthiophene [ 22 , 23 ], and pentacene [ 24 ] have become the target of research and are potential candidates for new perspectives to ionizing radiation sensing.
Use of these materials, which have known properties and have been studied, have played an important role in the study of ionizing radiation effects on polymers Figure In the interaction of high-energy radiation with semiconductors, primarily there occur excitations and ionizations that generate ions and electrons.
The electrons generated primary electrons will interact again with the environment and generate secondary excitations that will produce electron-hole pairs. Therefore, the efficiency of the material with the highly energetic radiation will depend on its stopping power or absorption efficiency, the limited capacity of producing electron traps and its ability to grow large areas.
Semiconductor polymers generally have high efficiency luminescence and absorption in the UV-Vis region; they can also form films producing large areas, and, hence, they constitute a new alternative in the area of radiation detectors.
In the field of electromagnetic radiation, there are several possible interactions of the most energetic radiation with matter: mainly, the photoelectric effect, Rayleigh scattering, Compton effect, and production of electron-positron pairs. Eventually, these interactions can lead to temporary or permanent modifications. These changes are called effects of degradation. They may be superficial when there is change only in the physical appearance color, transparency, etc.
Polymer degradation effects have been reported such as scission [ 25 ], cross-linking [ 26 , 27 ], and photobleaching [ 28 ]. In scission, there occurs break of the main chain into smaller molecules, reducing its molecular weight. In cross-linking, due to link between two polymer chains or between two big radicals, there is a formation of an insoluble portion with increasing molecular weight. Decrease in viscosity and increase of ductility are effects of scission. Increase of hardness, viscosity, and brittleness are some of the macroscopic effects of cross-linking.
Photobleaching occurs when the fluorescent signal of a fluorophore disappears permanently due to photon-induced chemical damage and covalent modification. Degradation effects are often considered problems such as the oxidation effects of medical implants based on polyethylene after irradiation sterilization, for example[ 29 ]. However, the ionizing radiation degradation effects are not sometimes bad results.
Many times they are desirable, as in the creation of integrated circuits, decreasing the molecular weight to make a material compatible with the other, in polysaccharides, for use in health care products, cosmetics, textile and food industry, or even to increase viscosity or resistance materials, for instance [ 30 - 32 ].
Polymer interaction with gamma radiation has been studied since the s and different effects have been observed depending on the chemical structure of the polymer and the energy range used for irradiation process. The mechanisms involved in the interaction of gamma radiation with polymers have not been fully elucidated, but changes in conductivity and optical properties have been reported mainly in polyaniline [ 33 ] and on PPV and its derivatives. The results indicate the feasibility of using semiconductor polymers as gamma radiation detectors.
The interest in the use of conductive polymers in this area is due to the adjustability of its luminescence properties and conductivity, and they also have a lower cost than inorganic semiconductors.
However, the use of polymers as radiation sensors is recent and few studies are reported in the literature. Studies using MEH-PPV have demonstrated that the use of solutions is effectively more sensitive to gamma radiation than solid state. Current knowledge shows that polymeric materials are more sensitive to gamma radiation when solubilized in halogenated solvents [ 34 ]. The halogens are well-known to have large cross section for interaction with gamma radiation. The main results obtained on irradiated P3HT devices were a significant improvement in conductivity with increasing gamma irradiation dose.
Polythiophenes irradiated with gamma radiation go to a polaronic state and then stabilized for a bipolaronic and neutral state of the chain, where they remain in the oxidized state. However, the order of radiation dose used on P3HT kGy is very high for using in personal dosimetry order of ten grays , for example. In contrast, studies of MEH-PPV with gamma radiation at this order of dose have shown significant results compatible with personal dosimetry area.
However, the results were limited to the use of the polymer in solution, due to the effect being dependent on the solvent. In other words, the effect is indirect: the radiation breaks the solvent chain and the radicals derived from solvent attack break the polymer chain. The attack occurs at the vinylene, breaking the double bond and leading to the conjugation break displayed as blue shift in optical measurements. This experimental result has been corroborated by theoretical studies and the attack mechanism on vinylene is well-established [ 35 ].
The principal disadvantage of MEH-PPV in the interaction with the gamma radiation is its limitation of use in optoelectronic devices due to the effect of this range of dose not included the utilization in film. The radioactive source constantly releases alpha particles that knock off the electrons from the surrounding air atoms, thus ionizing the nitrogen and oxygen atoms within the detector chamber.
The positively-charged ions are attracted to the negative plate whereas the negatively-charged iones are attracted to the positive plate, thus creating a small, continuous electric current. This small ionization current that can be easily measured by electronic circuitry which is connected to the plates.
So how do ionisation smoke detertors exactly work in presence of smoke? When smoke molecules enter the ionization chamber, the smoke particles attach to the ions and neutralize them.
Consequently, the total number of ionized particles in the chamber is reduced. This reduction yields a decrease in the chamber current that is sensed by the electronic circuitry. The drop of current between the plates triggers an alarm. An externally visible red LED lights up when the detector alarm state is energised. Ionisation detectors respond more quickly to flaming fires with smaller combustion particles, due to their sensitivity to minute smoke particles.
They are more prone to false alarms compared to photoelectric detectors, which in their turn respond more quickly to smoldering fires. Smoke detectors are common household items that keep you and your family safe by alerting you to smoke in your home. Ionization smoke detectors use a small amount of radioactive material, americium, to detect smoke. Ionization smoke detectors use americium as a source of alpha particles.
Alpha particles from the americium source ionize air molecules. This makes some particles positively charged and some negatively charged. Two charged plates inside of the ionization smoke detector create a flow of positively and negatively charged ions. The smoke alarm triggers when smoke breaks the constant flow of ions. Alpha particles are very heavy and cannot travel very far. They can be shielded by a layer as thin as a layer of dead skin cells.
Ionization smoke detectors have a small americium source encased in a layer of foil and ceramic, which stops the alpha particles from traveling outside of the smoke detector.
Because of this shielding, the smoke detector poses no radiation health risk when they are properly handled. There is no health threat from ionization smoke detectors as long as the detector is not damaged and used as directed.
Do not tamper with your smoke detectors, as it could damage the shielding around the radioactive source inside of them. There are no special disposal instructions for ionization smoke detectors.
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