Radiation Spectroscopy With NaI (TI) Scintillators

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Pages: 8
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Study level: College

Introduction

The use of thallium-activated sodium iodide as a scintillation material marked the beginning of the contemporary epoch of gamma-ray spectroscopy. The introduction of thallium-activated sodium iodide enabled scientists to detect gamma rays. Today, gamma-ray spectroscopy through scintillators has advanced into an established science. Spectroscopy is applied in varied scientific fields. Even though it was practically the initial powerful detection medium utilized in gamma-ray spectroscopy, NaI (TI) is still the most preferred scintillation material for this role. Adams and Dams (2011, p. 4) state that the reasons behind the popularity of NaI (TI) are ‘its outstanding linearity, enormously good light yield, and high amount of iodine constituent’. This paper will discuss the general considerations in gamma-ray spectroscopy, complications in the response function, and the summation effects. Additionally, it will examine the properties of gamma-ray scintillation spectrometers.

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General Considerations in Gamma-Ray Spectroscopy

A gamma or X-ray photon does not carry a charge. Besides, it does not ionize or excite the medium through which it travels. Adams and Dams (2011, p. 5) argue that for one to detect the gamma rays, they require ‘causing the gamma-ray photon to undergo an interaction that transfers all or part of the photon energy to an electron in the absorbing material’. Since the original gamma-ray photons are “imperceptible” to the detector, the fast electrons that result from gamma-ray interactions help to make out the characteristics of the incoming gamma-ray. The electrons have total energy that corresponds to the energy of the photon. For a material to act as a gamma-ray spectrometer, it has to serve two separate roles. The material has to be used as a transformation means through which incoming gamma rays have a logical possibility of interacting to produce one or more fast electrons. Additionally, the material has to serve as an ordinary detector for the resulting secondary electrons.

Gamma Ray Interactions with Matter

Gamma rays can interact with matter in different ways. However, only three interaction techniques are useful for spectroscopy. The techniques are Compton scattering, photoelectric absorption, and pair production. The possibility of any of the three techniques arising depends on the atomic number of the interaction material. When gamma rays enter a crystal, they yield fast electrons. The fast electrons produce scintillations. Gilmore and Hemingway (2008) maintain that the nature of the interaction course of the gamma rays in a medium determines the observed spectral distribution. When a gamma-ray comes into contact with an ion in the interaction medium, it is absorbed, and its total energy reassigned to one of the bound electrons. The bound electron is liberated making it travel fast through the medium. Because the energy of the incident gamma-ray is higher than the one that binds the particles, the released electron assumes energy that corresponds to that of the incoming radiation.

Photoelectric Absorption

Photoelectric absorption is a kind of gamma-ray interaction that leads to the vanishing of the incident photon. The interaction leads to the production of a photoelectron. The electron shell of the atom that absorbs the incoming gamma-ray photon converts into a photoelectron. The kinetic energy of the photoelectron corresponds to that of the incident photon. For the natural gamma-ray energies, the photoelectron typically surfaces from the K shell (Gilmore & Hemingway 2008). Gilmore and Hemingway (2008, p. 32) allege, ‘The room that is created in the electron shell due to the photoelectron emission is speedily covered through electron rearrangement’. The electron rearrangement process results in the release of the binding energy. Gilmore and Hemingway (2008) claim that in a majority of the cases that involve iodine, a characteristic X-ray is released. The Auger electrons have little energy. As a result, they have a tremendously short range. On the contrary, a characteristic X-ray can travel for a short distance before the slackly bound electron shells suck it up through photoelectric interaction.

According to Gilmore and Hemingway (2008), photoelectric absorption results in the release of a photoelectron that absorbs the energy of the incoming gamma-ray. Additionally, it results in the liberation of numerous low-energy electrons due to the assimilation of the energy that initially bound the photoelectron. Knoll (2010, p. 110) posits, ‘Assuming that nothing gets away from the detector, the total kinetic energy of the electrons that result from photoelectric absorption is equal to the initial energy of the incident photon’. Photoelectric absorption helps to measure the energy of gamma-ray. One only requires measuring the total electron kinetic energy. Nevertheless, this only applies to monoenergetic gamma rays.

Compton Scattering

Gilmore and Hemingway (2008, p. 34) aver that in Compton scattering, ‘the gamma-ray is not absorbed, but rather scattered through an angle by an electron’. The Compton scattering interaction results in the development of strewn gamma-ray photon and a recoil electron. The scattered gamma-ray and the recoil electron share the energy of the original gamma-ray photon based on the scattering angle. The scintillator does not absorb the strewn gamma-ray. The energy that the recoil electron absorbs corresponds to the energy, which the gamma-ray loses. Compton edge refers to the maximum power that the original gamma-ray can lose. The power may vary depending on the swing of the wavelength.

Pair Production

Pair production takes place in the nucleus of the absorbing medium. Gilmore and Hemingway (2008, p. 41) state that pair production results in the creation of an electron-positron pair at the ‘point of complete disappearance of the incident gamma-ray photon’. The pair production process cannot be energetically feasible without gamma-ray energy of 1.02 MeV (bare minimum energy). If the power of the incoming gamma ray surpasses the least amount, the electron-positron pair shares the additional energy. The pair shares the power in the form of kinetic energy. Hence, the process of pair production entails transforming the incoming gamma-ray photon into positron and electron kinetic energies. For natural powers, both the positron and electron move for a short distance before the absorbing medium consumes all their kinetic energy. The positron is not a stable element. Thus, it complicates the pair production process. The positron exterminates or merges with an ordinary electron of the absorbing material once its kinetic energy decreases.

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Problems in the Response Function

Secondary Electron Escape

Knoll (2010, p. 111) argues, ‘If the detector is small compared to the average secondary electron ranges, there is a possibility of a considerable portion of the particles escaping from its surface’. Collecting the energy of the escaping electrons would be difficult. The effect of secondary electron escape is severe in gamma rays that possess a high amount of energy. Electron escape interferes with the response function. Knoll (2010, p. 112) alleges that secondary electron escape leads to ‘some events moving to lower amplitude than the one that could be observed if the total electron energy is collected’. Additionally, it leads to the modification of the Compton continuum to support the lower amplitudes.

Characteristic X-Ray Escape

In the photoelectric absorption procedure, the atom that assimilates the gamma-ray photon discharges a characteristic X-ray. Mostly, the X-ray energy is reabsorbed reasonably close to the first interaction spot. Nonetheless, the detector may fail to absorb the X-ray energy, particularly if the absorption process takes place close to its surface. According to Knoll (2010, p. 115), ‘the energy deposited in the detector is decreased by an amount equal to the X-ray photon energy. In the absence of the X-ray escape, the initial gamma-ray is absorbed completely. On the other hand, the X-ray escape results in the creation of a unique group of events. A fresh peak arises in the response function. The position of the peak corresponds to the energy of the characteristic X-ray. According to Knoll (2010), the new peak appears below the photopeak. The peaks are usually marked as “X-ray escape peaks” and are quite common at low incoming gamma-ray energies.

Effects of Surrounding Materials

According to Knoll (2010, p. 115), ‘the materials that surround a detector utilized for gamma-ray spectroscopy may have a considerable effect on its response function’. Mostly, the detector is enclosed to shield it from light and moisture. In some cases, it is enclosed in a vacuum. The majority of the detectors work in a shielded enclosure to minimize natural background. The source of the gamma-ray serves as a component of the surrounding material. According to Knoll (2010, p. 117), the surrounding materials are ‘potential sources of secondary radiations, which can result from the interactions of the primary gamma rays. The secondary radiations interfere with the character of the spectrum if they come into contact with the detector.

Summation Effects

Extra peaks that result from the synchronous detection of numerous gamma-ray photons might also emerge in the documented pulse height range. The most frequent incidents happen in experiments involving an isotope that releases many streams of gamma rays in its decay. In a case where two gamma rays come out concurrently, they interact and dump their total energy within a short duration compared with the period that a detector takes to respond. If any of such incidences occur, a “sum peak” appears on the spectrum. Annunziata (2012, p. 79) asserts that the “sum peak” ‘occurs at a pulse height that corresponds to the sum of the two individual gamma-ray energies’. Annunziata (2012, p. 81) claims, ‘A band of the total incidents appears at lower amplitudes because of the fractional energy loss interactions’. Adams and Dams (2011, p. 9) maintain, ‘The relative numbers of events that happen in the sum peak depend on the angular relationship between the two gamma rays, branching ratio of the beams and the angle that the detector subtends’.

Properties of Scintillation Gamma-Ray Spectrometers

Response Function

Annunziata (2012, p. 81) maintains, ‘The relative significance of the response function relies on the gamma-ray interaction probabilities in the detector material’. NaI serves as an excellent detector material since its iodine content constitutes a high atomic number. Therefore, the material enhances the photoelectric absorption process. The response function of a scintillation detector facilitates the study of measured spectra. The size of the detector plays a significant role in the simplification of the response function. The bigger the size of the detector material, the simpler the response function. Additionally, the composition and shape of the detector influence the response function. L’Annunziata (2012) holds that the numerical particulars of the irradiation conditions also affect the response function of a detector. Altering the position of the source of the gamma-ray changes the response function. L’Annunziata (2012) alleges that it is hard to predict the response function of a detector without the help of Monte Carlo computations.

Energy Resolution

The energy resolution of a NaI (TI) scintillator detector is approximately 10%. Superior detectors like the high purity germanium have great resolutions. L’Annunziata (2012, p. 85) maintains that when a beam of ‘mono-energetic gamma-ray strikes the scintillator; there is a fluctuation in the height of the voltage pulse from the photomultiplier’. The changes appear as an enlargement of the photopeak. The disparity in the pulse height is a result of the arithmetic variation of electrons that originate from the cathode of the photomultiplier. The disparity may also arise due to the sporadic escape of electrons from the crystal. According to Manohara, Hanagodimath, and Gerard (2009, p. 469), ‘the resolution of a detector is measured by the full width of a peak at half its maximum height (FWHM)’.

Detection Efficiency

Shleien, Slapback, and Birky (2003, p. 134) claim that the detection efficiency is ‘normally measured and quoted as absolute photopeak efficiencies for detection of gamma rays from unattenuated point sources’. The use of thin detectors enhances the detection efficiency when using gamma rays of little energy. On the other hand, thick detectors improve the detection efficiency when using highly penetrating radiations.

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Peak Area Determination

Knoll (2010, p. 119) posits that the peak area is determined by ‘subtracting a straight line or step background drawn between the endpoints of the region of interest set on the peak’. Endpoint averaging helps to minimize the error in circumstances where the background determination is vague. One also computes the ambiguity in the background together with the inaccuracy in the net area. The results of the calculations can give only estimated values for the location of the peak area. Additionally, the results can give the approximate height of the peak as well as its lower and upper limits. The FWHM of the supposed peak is computed and judged against the values projected for the detector in question. If the values coincide, it is thought that a single photon generated the peak.

Conclusion

To detect a gamma-ray photon, one requires transferring the energy of the photon into an electron of the detecting material. Gamma-ray interacts with matter in three significant ways. They are photoelectric absorption, pair production, and Compton scattering. Different complications may arise in the course of interaction. The complications include secondary electron escape and characteristic X-ray escape. The surrounding materials may also interfere with the response function of a detector. The nature of the detecting material influences the detection efficiency. Thus, it is imperative to select a suitable detector based on the nature of the gamma-ray.

Reference List

Adams, F & Dams, R 2011, Applied gamma-ray spectrometry, Pergamon Press, New York.

Gilmore, G & Hemingway, J 2008, Practical gamma-ray spectroscopy, John Wiley & Sons, Chichester.

Knoll, G 2010, Radiation detection and measurement, John Wiley & Sons, New York.

L’Annunziata, M 2012, Handbook of radioactivity analysis, Elsevier Inc., Oxford.

Manohara, S, Hanagodimath, S & Gerward, L 2009, ‘Photon interaction and energy absorption in glass: a transparent gamma ray shield’, Journal of Nuclear Materials, vol. 393, no. 3, pp. 465-472.

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Shleien, B, Slaback, L & Birky, B 2003, Handbook of health physics and radiological health, Williams & Wilkins, Baltimore MD.