Overall, a radiation detector can detect all radiation types described above (alpha, beta, photons).
Various radiation detectors utilize different radiation interaction mechanisms and deduce information about the radiation type and energy. For the scope of this dissertation, semiconductors and scintillators will be the only radiation detectors discussed further. Semiconductors typically provide the best energy resolution, whereas scintillators are less expensive and more versatile. For imaging, the optimum sensor depends on the application and other parameters such as uniformity, efficiency, spatial and energy resolution, and count-rate capability [20]. Based on the desired application, some of these parameters can have dominating contributions in performance tradeoffs. These parameters will be characterized further in Chapter 3 below to use performance metrics to determine a detector’s suitability for other application spaces such as national security and international safeguards.
1.2.1. Semiconductors
Semiconductor diodes operate as thermalized electrons and holes migrate with an electric field's assistance to produce an electronic pulse proportional to the incident photon's energy. The solid crystalline material of the semiconductor creates a periodic lattice with energy bands for electrons. Energy bands represent the electron's energy. These energy bands are defined as a forbidden range of energies and specify the type of material. Semiconductors typically have a smaller energy gap than insulator materials.
As shown in Figure 4, the semiconductors' energy gap is greater than or equal to 1 eV. The lower band, the valence band, represents the outer-shell electrons bound to specific lattice sites within the crystal.
The next upper band is called the conduction band and represents the electrons free to migrate through the crystalline material under an applied electric field. The electrons within the conduction band correspond to the electrical conductivity of the material [16].
Figure 4: Insulators and semiconductor band structure for electron energies [16].
When a charged particle interacts within a semiconductor, the particle deposits its energy and produces multiple electron-hole pairs along the particle’s track (i.e., ionization track). As the incoming charged particle enters and traverses the semiconductor, the particle generates high-energy electrons.
High-energy electrons generate more electron-hole pairs along the incident particle’s track. The particle’s energy deposition leads to the formation of conduction electrons and valence holes within picoseconds along the ionization track. Once a valence electron gains significant thermal energy, the covalently bonded electron is excited across the bandgap into the conduction band (i.e., excitation process). The excited electron leaves a vacancy (i.e., a hole) in the valence band, representing a new positive charge.
The number of created electrons and holes are equal despite whether the semiconductor is pure, intrinsic, doped p-type, or n-type. The amount of average energy expended by the incident charged particle to produce one electron-hole pair is called ionization energy ∈. Ionization energy is independent of energy and type of incident radiation.
Within the detector's active volume, as the particle comes to a full halt, the number of electron- hole pairs produced is relative to the incoming radiation's incident energy. The active volume of the semiconductor has an applied electric field. Electrostatic forces act on the electron-hole pairs or charge carriers, which cause them to drift in opposite directions. The drifting motion of electrons or holes creates an electric current until the charge carriers arrive at their respective nodes. Electrons travel in the opposite direction towards the electric field vector, whereas the holes will move in the same direction as the electric field. Equation 11 below represents the probability per unit time of thermally generated electron-hole pair. T is absolute temperature, Eg is the bandgap energy, k is Boltzmann constant, and C is continuous proportionality characteristics of the material.
𝑝(𝑇) = 𝐶𝑇32𝑒𝑥𝑝(− 𝐸𝑔
2𝑘𝑇) Equation 11
The time it takes the charge carriers to be collected is based on drift distances and charge carrier mobilities. The mobility 𝜇 for both electrons and holes is defined by Equation 12 below where v is the drift velocity proportional to the applied electric field, and 𝜀 is the electric field's magnitude. The rising pulse produced by a preamplifier signifies the charge carriers’ current integrated on a measuring circuit.
A preamplifier processes the counts from the semiconductor detector [16].
𝑣ℎ = 𝜇ℎ𝜀 Equation 12 𝑣𝑒 = 𝜇𝑒𝜀
One prominent semiconductor detector class is high purity germanium (HPGe), which possesses advantageous properties useful for radiation spectroscopy such as excellent energy resolution. These semiconductor detectors are commonly manufactured in a four-inch right cylinder, can be expensive, and must be operated at low temperatures, making scintillators more appealing as radiation detectors depending on application [16]. Different types of semiconductors include solid-state, silicon-based detectors, charged-particle, gamma-ray spectrometers, and visible light imaging detectors. This dissertation will focus on semiconductors as imaging detectors. Semiconductors used as digital imaging technology/cameras are typically either charge-coupled devices (CCD) or complementary metal-oxide- semiconductor (CMOS). Traditionally, CCD sensors create high-quality, low noise images with high power consumption, high fill factor, quantum efficiency, and dynamic range. CMOS's imaging technology provides low cost and power consumption with a high capability for radiation hardness, integration, and high-speed readout but generally have more noise in the pixel readout. This research will discuss CMOS sensors in greater detail in the hardware and characterization sections.
1.2.2. Scintillators
There are two main types of scintillators, organic and inorganic, based on their composition. The desired application of the detector determines the kind of scintillator. Beta spectroscopy and fast neutron detection applications prefer organic scintillators. In contrast, inorganic scintillators are favorable for gamma-ray spectroscopy due to the high Z-value of the constituents and high density to stop gamma-rays
The ideal scintillator consists of specific properties: high scintillation efficiency or the ability to convert electron kinetic energy into visible light, high light output linearity to the incident energy (high defined as greater than 25,000 photons/MeV), transparent to scintillation light for excellent light collection, short decay time, ability to manufacture at large sizes, and index of refraction near glass (~1.5) to enhance the coupling of scintillation light to the readout device [16].