Analysis of Scintillator Characteristics for Detection of Illicit Radioactive Sources
Chapter One
AIMS AND OBJECTIVES OF STUDY
The main objective is to prevent radioactive material from either being unauthorized or accidentally being accepted by steelworks. This will be done by developing and testing detector systems of high sensitivity suitable for use either at the point of entry of the scrap or within the steel plant. Work needs to be done on the types of detector, the long-term reliability of detectors, their robustness, the optimization of deployment of multidetector systems, and the choice of alarm thresholds.
CHAPTER TWO
LITERATURE REVIEW
SCINTILLATION
The process as a whole in which incoming radiation energy is transferred into multiple photons is called scintillation [11]. The basic steps to scintillation that result in light emission are: (1) the neutron-matter interaction (2) subsequent ionization in the material resulting from gammas or recoil of particles (3) the ionization process causing eh pairs in the material (4) the recombination of e-h pairs results in production of visible light photons. The visible light photons are e-h pair recombinations that occur at the luminescence center [11]. The luminescent center is an activator impurity, and according to Awadalla, “the activator creates special sites in the crystal lattice, creating energy states within the forbidden gap through which an excited electron can de-excite back to the valence band” [12]. The valence band is where electrons are bound at lattice sites, and the conduction band is where electrons are free to migrate throughout the crystal [13]. The luminescence center’s band gap is important, Awadalla states, “luminescence centers determine the emission spectrum of the scintillator” [12]. For example, as shown in Figure 6(a), a schematic diagram of the luminescent center, in this case Ce3+ , has energy levels of 4f and 5d, and after an electron has been excited to the conduction band, it can de-excite from D5/2 to F7/2 by emitting a visible photon. For further explanation in Figure 6(b), an exiton (e-h pair traveling to conduction band) will roam freely until it comes in contact with an activator site, and de-excite to the ground configuration ultimately resulting in the creation of scintillation light [12].
If the energy of the excited electron exceeds the forbidden gap energy, thermal releases (phonons) can allow the electron to de-excite to an allowable photon energy release at a recombination site. Note, in some scintillators (i.e., BGO) there are no added activators like in the case of lanthanum halide crystal luminescence center shown in Figure 6, and they are considered self-activated. Often, these luminescence centers are doped with a material like because of its favorable photonic emissivity, due to its lower band gap. This is similar to a p-type semiconductor in that the positive charged ions with a low band gap allow recapture at energy levels where visible light can be emitted [11]. Regardless of emission, the next step of counting these light emissions is a challenge met with PMTs.
CHAPTER THREE
EXPERIMENTAL APPARATUSES
EXPERIMENT GENESIS
A considerable amount of time and effort was made to develop two of the three experiment apparatuses used in this research in order to create the settings required for two distinct results. The first setup, was built from scratch using a decommissioned Scanning Electron Microscope (SEM) because there was no other Naval Postgraduate School locally available Photon Counting Spectrometer (PCS). Although photon counting with a PMT provides useful data, it does not provide the spectral characteristics of the scintillators. The second experimental apparatus was created using a silicon femptowatt photoreceiver in a light tight box, portable and capable of counting photons after exposure to fast neutrons without significant cost or pre-experiment setup. This was necessary in order to perform the measurements rapidly where the neutron sources were available. Regardless, the genesis of both apparatuses began with a necessity to detect scintillator output beyond just photon counts conducted in the first experiment setups described in Chapter IV.
CHAPTER FOUR
EXPERIMENTAL RESULTS
PHOTONIC EMISSION-GAMMA INDUCED
The initial experiments conducted using the heavy oxide inorganic scintillators from Ukraine, were gamma-induced photon counting experiments using a Hamamatsu 7421–40 PMT. As shown in Figure 22, the radiation source was placed directly in contact with the scintillator crystal in front of the Thorlabs MAP104040-M achromatic lens pair.
The 7421-40 Hamamatsu counting head was powered by a C9525 Hamamatsu Power Supply. The signal was sent to a C8855-01 Hamamatsu Photon Counting Unit and then amplified before being sent to the Computer in the adjacent room to be recorded.
CHAPTER FIVE
CONCLUSION
SUMMARY
The preventative methods to avert a catastrophic nuclear event on American soil are limited, to some degree, by detection methods currently in use. Scintillators can be used to increase the efficiency and reduce the cost of current detectors because of their reaction characteristics when exposed to radiation. The current detectors generally have efficiencies well below 10%, while scintillator efficiency is higher than 40% for fast neutrons [27]. A good understanding of photonics and a little nuclear engineering would equip anyone with the knowledge of how scintillators work. An improved understanding of scintillator characteristics, when exposed to different types of radiation, can help vastly improve current detector systems and reduce the risk of nuclear proliferation.
For gamma-induced scintillation, effective atomic number was not the predominant factor in scintillation as one might expect. Temperature, density, and surface area all play a significant role in scintillation light output. One can increase the light yield measured by photo multiplier tubes by focusing light and reducing the temperature for some crystals to get a stronger signal if this is required for a detection scenario. As expected, light yield significantly decreases with distance, but a signal is still generated at long distances beyond the noise level, especially at high energies of incoming radiation. Regardless, gamma-induced scintillation experiments were useful in understanding the basics of scintillator output, and that background helps clarify higher energy experiment results.
Spectroscopy experiments take a considerable amount of time and require a great deal of experimental setup preparation, but the results can be extremely helpful when characterizing specific light output of each crystal. After implementation and calibration of a Photon Counting Spectrometer, the spectral response of the studied scintillators was obtained for different radiation sources like Cobalt-60 and Cesium-137. The most interesting results were from LuAG and LGSO, as the spectra showed dual peak emissions because of their two luminescent centers [29]. More research into this could produce a better understanding of the probability of occurrence and how it affects the crystal’s efficiency. What may be of some value is that the non-doped crystals do not seem to exhibit this dual peak phenomenon, which further corroborates this finding and validates the data from the Photo Counting Spectrometer. Regardless, the purpose of these crystals is to detect fast neutrons for nuclear counter-proliferation activities, and their response to neutrons and how easily one could create such a device is important.
Since the ultimate use of the scintillator would be to detect fast neutrons at a practical distance, a somewhat rudimentary but effective setup was created to analyze scintillator response. The size of the 1ft3 box with a $900 femtowatt photoreceiver portable detection device can be reduced significantly to provide a cheap detector with no more than $1,500.00, including crystal and materials. This device could be theoretically the size of one’s hand if instrumented correctly. At practical distances all the scintillator signals measured were nowhere near the noise level and distinct enough to move further away from the source. Additionally, at practical distances scintillators can be used effectively in detecting fast neutrons without significant exposure if you have a long enough cable. LGSO seems to be the best candidate for both fast neutrons and gamma detection, while CDWO is the best candidate for gamma detection by several orders of magnitude. Furthermore, as shown in the study of fast neutron response, there seems to be an energy level in which the cross section for inelastic scattering becomes more prevalent than at lower energies. Regardless, each heavy oxide inorganic scintillator produced enough light output to be measured quite easily, effectively, and cheaply, albeit the neutron sources were very powerful.
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