Introduction
Radiation detectors are widely applied in numerous fields for early detection and mitigation of radiation effects within premises. The advantages associated with the application of radiation detectors are tremendous not only in the medical field but also other professional fields where radiation detectors are employed. Owing to these positive benefits of radiation detectors, their application for various functions across different fields has grown at an alarming rate ever since their inception. Radiation detectors come in numerous types and shapes (Owens and Peacock, 2004).
Nuclear medicine is notorious for the application of radiation detectors as compared to other professional fields. Some of the most common types of radiation detectors used in nuclear medicine include GM, solid state radiation detectors, and scintillation. These radiation detectors are employed in nuclear medicine for different purposes such as area logging, area survey, for quality assurance purposes, personal dosimetry, and radiopharmaceutical production.
Similar to any other electronic or mechanical devices, radiation detectors are subjected to failure owing to various reasons and causes. The operating conditions and time for various radiation detectors vary tremendously (Owens and Peacock, 2004). As a consequence, some radiation detectors exhibit machine failure than others under different operating conditions. The application of radiation detectors in nuclear medicine presents different points of failures for these detectors. In most cases, some of the radiation detectors are operated in concentrated magnetic fields while some are operated in high gamma radiation environments which may incorporate neurons (Hine and Brownell, 2013).
Over a long period, the operation of these detectors is challenged by these environments. Apart from the harsh operating environments, there are other points of failures to these radiation detectors within the field of nuclear medicine as well as other professional fields where radiation detectors are employed.
Operation of radiation detectors
The introduction of transistors led to the innovation of electronic radiation detectors. Mechanical radiation detectors exhibit a different point of failure from electronic radiation detectors (Cooper, 2011). Scintillator-kind of radiation detectors employs the use of vacuum tubes in performing the original light to electrical pulse conversion. The invention and application of transistors in the development of electronic radiation detectors allowed for the amplification and storage of data collector by these detectors (Bolotnikov et al, 2007). Different advancements in technology, including advancements in material and miniaturization in electronics, have resulted in the production of novel and better radiation detectors. These new electronic radiation detectors exhibit various points of failures.
Evaluation of performance
As the need for enhanced efficiency, accuracy, and sensitivity increases, so does the convolution of the radiation detectors and their operation. Despite the fact that various kinds of radiation detectors differ in numerous aspects, there are many common criteria employed in the evaluation of the performance of any radiation detector (Flammang, Seidel, and Ruddy, 2007). Some of the evaluation criteria employed in assessing the performance of radiation detectors include the sensitivity of the detector, the detector efficiency, the time resolution of the detector, and the energy resolution of the detector. Some of the radiation detector failures lie within some of the criteria for their assessment (Mangoubi, 2012).
While assessing the performance of a radiation detector using the sensitivity of the detector criteria, it is imperative to determine the type of radiation the detector detects. For instance, solid scintillation detectors are not sensitive to alpha particles are given that this type of radiation particles cannot penetrate the detector covering. As a consequence, the sensitivity of a detector can be considered as a point of failure in case it is not sensitive to the type of radiation present at the time (Bolotnikov et al, 2007).
Apart from the sensitivity of the detector, another performance assessment criterion is based on the energy resolution of the detector. It is imperative to ensure that the selected detector is capable of measuring the energy of the radiation in its presence. The precision of the radiation to measure the energy of the radiation is crucial (Ruddy and Seidel, 2007). It is also important for the detector to be able to distinguish between the different energy of the radiation striking the detectors. Failure to effectively measure and distinguish different types of radiation energies can result in failure of the radiation detector.
Additionally, the third criterion employed in assessing the performance of radiation detectors is based on the pulse-resolving time of the detector. The time resolution of the detector is an imperative factor in the operation of radiation detectors. The time resolution of the radiation detector entails the measurement of counting rate of the detector without error. Accuracy and precision are significant factors in attaining effective pulse resolving time (Chung, Bouchard, and Seuntjens, 2010). The detector should be able to precisely and accurately measure the time of arrival of radiation particle at the detector. The efficiency of the detector pertains to the amount of radiation particle that the radiation detector will sense in case radiation particles hit the detector (Ramilli, 2008).
Types of radiation detectors in Nuclear Medicine
Nuclear medicine employs the use of different types of radiation detectors for diverse purposes. Some of the most common types of radiation detectors used in nuclear medicine include GM, solid state radiation detectors, and scintillation. These radiation detectors are employed in nuclear medicine for different purposes such as area logging, area survey, for quality assurance purposes, personal dosimetry and radiopharmaceutical production (Bolotnikov et al, 2007).
Geiger-Muller counter, commonly known as GM counters, employs the use of gas-filled tube fitted with a high voltage central wire for the collection of the ionization generated by incident radiation. Geiger-Muller counter has the capability to detect beta, alpha and gamma radiations. Although this type of radiation detector is capable of sensing these different types of radiations, it is incapable of differentiating between them. This shortcoming alongside other limitation forces the detector to be limited to demonstrations or use in radiation environments with rough estimates of radioactivity.
Apart from GM counters, Scintillation detectors also commonly used in nuclear medicine for various radiations detection purposes. In most cases, scintillators come in solid form even though there are varieties for both gasses and liquids. This type of radiation detectors employs the use of scintillator solids which produces light when they interact with radiation particles (Tsoulfanidis, 2013). The light produced by the scintillators during interaction with radiation particles is transformed into electrical pulse which is further processed by computers into the more understandable format. The most common used scintillators include bismuth germanate and sodium iodide. The application of these materials is widespread including medical imaging machinery, research, and radiation monitoring.
Solid state gamma-ray and X-ray detectors are also commonly employed in radiation detection in nuclear medicine as well as other fields dealing with radiation. This type of radiation detectors employ the use of germanium and silicon detectors, cooled to an estimated temperature of 77 k, for accurate measurement of gamma-ray and X-ray for intensities and energies. Silicon detectors are suitable for the application in X-ray detection and measurement up to an estimate of 20 KeV (Onodera, Hitomi, and Shoji, 2006). On the other hand, germanium detectors are effective in measuring radiation energy in the range above 10 KeV to a small amount of MeV. This type of radiation detectors is applied in environmental measurement and trace element measurement (Gruner, Eikenberry, and Tate, 2006).
Common source of radiation detector failures
The sources of failures in radiation detectors can be categorized into general problems and specific problems or sources. General problems or points of failure applied to most of the detectors irrespective of the type while the specific points of failures are specific to the type of radiation detectors (Bolotnikov et al, 2007). The point of failure exhibited in Ion chambers lies with the temperature. It is imperative to keep the air inside the ion chamber dry at all times. The background tends to increase either negatively or positively in case the desiccant is exhausted. This increases the level of fluctuation. Drying of the desiccant should be done by the instructions provided by the manufacture (Lisauskas, et al, 2009).
For Geiger-Muller radiation detectors, the point of failure lies in their rigidity. These detectors are extremely rigid and highly susceptible to damage through contact with any stuff that is rigid like swarf, tools, and grass stems. There is a possibility that the detector can implode if not carefully handled. The conductive coating is susceptible to flaking or inadvertently eliminated by solvents. The removal of the conductive coating (DAG) highly affects the performance of the detector (Onodera, Hitomi, and Shoji, 2006).
For thin window beta and alpha scintillation radiation detectors, the point of failure lies in the damage to the instruments. Physical damage to the instrument results in light leaking which in turn leads to elevated whistling noise or background count rates. Additionally, the exposure of these types of radiation detectors to a magnetic field causes them to become faulty. The exposure of photo-multiplier to a magnetic field causes interference with the operation of the detector. Puncturing sealed thin window proportional counter leads to a gradual loss of sensitivity of the radiation detector.
The source of failure for sodium iodide scintillation detectors lies in the window destruction. This leads to the formation of yellow patches or light leakage which reduces the sensitivity of the radiation detector by increasing the energy threshold. Additionally, the impact of the detectors can result to the shattering of the crystal which may lead to the very marked thrashing of sensitivity. Additionally, the tubes employed in this radiation detector are photo-multipliers which are highly susceptible to magnetic interference (Owens and Peacock, 2004). As a consequence, the operation of this type of radiation detector in a magnetic field can result into gradual failure.
Apart from the particular source of radiation detection and monitor failures, there are general points of failures that can lead to malfunction or inaccurate function of various types of radiation detectors. One of the most common points of failure in most electronic radiation detectors lies in pitiable battery contact. It is crucial that one kind of dry cells be used given that the lengths vary tremendously between different battery producers. Placement of a shot lived battery in place of a long-lived battery may lead to poor contact. The design of battery location about flat spring contact present a point of failure about poor terminal contacts in most electronic radiation detectors (Bolotnikov et al, 2007). Apart from the poor battery contact, corrosion of the battery contacts is also another source of radiation detector failures. This is especially common in poorly maintained units and where there is battery leakage.
Connectors and cables also present sources of failures in radiation detectors. Both the SHV and the PET connectors are not capable of supporting the weight associated with a probe on a regular basis. Cables and connectors can result in high background count rates or intermitted contact. The damage to cables and connector can be as a result of the application of excess force in pulling and cutting.
Apart from connectors and cables convectional meters present a point of failure to radiation detectors. Damage of convectional meters by the impact can result in an incorrect reading of radiation particles by the detectors. Damaged meters can restrict the free movement of the needle thus a failure in reading data associated with radiation particles. In Geiger-Muller detectors, damage of circuit board through impacts can lead to failure of the radiation detector. Additionally, the internal sections of photo-multiplier tubes and the optical coupling with the scintillator are highly susceptible to damage. The damaged internal structure of photo-multiplier tubes may, and the use of inappropriate coupling liquid may result in inaccurate sensitivity.
References
Bolotnikov, A.E., Camarda, G.S., Carini, G.A., Cui, Y., Li, L. and James, R.B., 2007. Cumulative effects of Te precipitates in CdZnTe radiation detectors. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,571(3), pp.687-698.
Bolotnikov, et al , 2007, October. Effects of Te inclusions on the performance of CdZnTe radiation detectors. In Nuclear Science Symposium Conference Record, 2007. NSS'07. IEEE (Vol. 3, pp. 1788-1797). IEEE.
Chung, E., Bouchard, H. and Seuntjens, J., 2010. Investigation of three radiation detectors for accurate measurement of absorbed dose in nonstandard fields. Medical Physics, 37(6), pp.2404-2413.
Cooper, P.N., 2011. Introduction to nuclear radiation detectors. Introduction to Nuclear Radiation Detectors, by PN Cooper, Cambridge, UK: Cambridge University Press, 2011, 1.
Flammang, R.W., Seidel, J.G. and Ruddy, F.H., 2007. Fast neutron detection with a silicon carbide semiconductor radiation detectors. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 579(1), pp.177-179.
Gruner, S.M., Eikenberry, E.F. and Tate, M.W., 2006. Comparison of X-ray detectors. In International Tables for Crystallography Volume F: Crystallography of biological macromolecules (pp. 143-147). Springer Netherlands.
Hine, G.J. and Brownell, G.L. eds., 2013. Radiation dosimetry. Elsevier.
Hitomi, K., Onodera, T. and Shoji, T., 2007. Influence of zone purification process on TlBr crystals for radiation detector fabrication. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 579(1), pp.153-156.
Hofstetter, K.J., Fulghum, C.K., Harpring, L.J., Huffman, R.K. and Varble, D.L., Washington Savannah River Company Llp, 2008. Portable nuclear material detector and process. U.S. Patent 7,351,982.
Kezić, A., 2015. Geiger-Müller detector (Doctoral dissertation, Zdravstveno veleučilište, Zdravstveno Veleučilište).
Lisauskas, A., Pfeiffer, U., Öjefors, E., Bolìvar, P.H., Glaab, D. and Roskos, H.G., 2009. Rational design of high-responsivity detectors of terahertz radiation based on distributed self-mixing in silicon field-effect transistors.Journal of Applied Physics, 105(11), p.114511.
Mangoubi, R.S., 2012. Robust estimation and failure detection: A concise treatment. Springer Science & Business Media.
Onodera, T., Hitomi, K. and Shoji, T., 2006. Spectroscopic performance and long-term stability of thallium bromide radiation detectors. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 568(1), pp.433-436.
Owens, A. and Peacock, A., 2004. Compound semiconductor radiation detectors. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,531(1), pp.18-37.
Ramilli, M., 2008, October. Characterization of SiPM: temperature dependencies. In Nuclear Science Symposium Conference Record, 2008. NSS'08. IEEE (pp. 2467-2470). IEEE.
Ruddy, F.H., and Seidel, J.G., 2007. The effects of intense gamma-irradiation on the alpha-particle response of silicon carbide semiconductor radiation detectors. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 263(1), pp.163-168.
Tsoulfanidis, N., 2013. Measurement and detection of radiation. CRC press.
