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Scintillation Detectors and Materials Scintillation Detectors and Materials
Published in Douglas S. McGregor, J. Kenneth Shultis, Radiation Detection, 2020
Douglas S. McGregor, J. Kenneth Shultis
There have been some limited successes, which include those scintillators listed in Table 13.1. For instance, CsI:Na is similar in performance to NaI:Tl, but has a longer decay time. CsI:Tl has much higher light output than NaI:Tl, but the emission spectrum has a maximum at 560 nm, a wavelength that does not couple well to PMTs. However, CsI:Tl has been coupled to Si photodiode sensors quite successfully. Bismuth germanate (BGO) has lower light output, but is much denser and a better absorber of gamma rays. As a result, BGO has been used for medical imaging systems to reduce the overall radiation dose that a patient receives during the imaging procedure. In recent years, LSO:Ce has become an effective and useful alternative to BGO. LSO:Ce has higher light yield than BGO while also having comparable gamma-ray sensitivity. LiI:Eu is a scintillator that is primarily used for neutron detection because of the 6Li content in the crystal. In recent years, LaBr3:Ce, a relatively new scintillator with exceptional properties for gamma-ray spectroscopy, has become available. LaBr3:Ce has a much higher light yield and a much shorter decay time constant than NaI:Tl. Further, it is composed of higher Z elements and, hence, is a better gamma-ray absorber than NaI:Tl. However, it is extremely hygroscopic and fragile and, thus, it is difficult to produce and handle. Although it has become commercially available, it is presently much more expensive than NaI:Tl because of production and fabrication problems.
Advanced technologies for beam’s eye view imaging
Published in Ross I. Berbeco, Beam’s Eye View Imaging in Radiation Oncology, 2017
Sawant et al. (2006) built a prototype 16 × 16 cm2 pixelated CsI imager that was 40 mm thick and had a pitch of 1 mm. DQE(0) was measured to be 22% at 6 MV. Compared to a Cu-GOS screen, large improvements in contrast detail were observed. However, as with the Seppi imager, spatial resolution was degraded by optical crosstalk. By comparing the performance of 120 × 60 pixel 1 mm-pitch bismuth germanate (BGO) and CsI imagers, Wang et al. (2009) and El-Mohri et al. (2011) showed that BGO may be a preferred scintillation material due to its higher density and higher index of refraction. For a given quantum efficiency, the increase in density allows the pixel height to be reduced, thus ameliorating beam divergence effects, and the higher index of refraction increases the probability that optical photons undergo total internal reflection at the pixel-glue boundaries, thus reducing crosstalk. However, preirradiation of the BGO scintillator with 2000 cGy was required to achieve acceptable output stability. Figure 12.3 shows two of the detectors built and several associated CBCT images.
PET Systems
Published in Michael Ljungberg, Handbook of Nuclear Medicine and Molecular Imaging for Physicists, 2022
A scintillator in PET should, with a high probability, stop the incoming gamma ray and produce a light signal that allows for determining the energy, time, and position. During the early years of PET, several systems were based on NaI but, due to its low stopping power, this scintillator was suboptimal for PET. The crystal thickness needed to be 4–5 cm to achieve a good sensitivity. NaI was used as a continuous scintillator, but the current PET systems all make use of pixelated scintillators. Most systems during the 1980s and 1990s were using pixelated BGO (Bismuth Germanate) as a scintillator. BGO has excellent stopping power but releases a limited amount of light. This makes it difficult to identify crystals based on their light output when the crystals become quite small (below 6 mm). The limited amount of light also results in relatively poor energy resolution (20–25%). Therefore, most companies started using brighter (more light per 511 keV) scintillators like GSO or LSO in more recent higher resolution systems. LSO (or LYSO) has now become the new standard PET scintillator as it generates a lot of light, is quite fast, and has good stopping power. New scintillators are still being explored, and one of the most promising for PET is LaBr3 due to its very high light output and fast response. Table 18.1 contains the performance values of the most common scintillators used in PET.
Development of Tracer Particles for Positron Emission Particle Tracking
Published in Nuclear Science and Engineering, 2023
Thomas Leadbeater, Andy Buffler, Michael van Heerden, Ameerah Camroodien, Deon Steyn
At PEPT Cape Town, a modified Siemens HR++ positron camera15–17 is used to record coincidence pairs of photons in list mode acquisition. The device consists of 36 detector modules each hosting 12 blocks of 8×8 segmented bismuth germanate oxide (BGO) scintillator crystals arranged into 48 rings of 576 detector elements (27 648 total) with diameter of 83 cm and axial depth of 23.4 cm. Independent and parallel readout allows events from each module (768 crystals) to be simultaneously read out up to around 1.5-MHz singles event rates per module (dead-time limited). A lossless coincidence processor is used to determine the valid event pairs from the singles event data (60-MHz bandwidth) with a maximum throughput of around 16-MHz coincidence events acquired to disk. Both prompt (true events plus random) and delayed (random events only) coincidences are streamed onto disk storage in list mode for postprocessing at data rates approaching 15 Gbytes/h.
Laboratory investigations on fine aggregates used for concrete pavements due to the risk of ASR
Published in Road Materials and Pavement Design, 2021
Daria Jóźwiak-Niedźwiedzka, Aneta Antolik, Kinga Dziedzic, Katalin Gméling, Karolina Bogusz
The sands were washed, dried, crushed and sieved through a 0.75 mm sieve to unify the specimen. For the PGAA analysis, 3–6 g-portion of the specimen was weighed and heatsealed into Teflon bags. The neutron flux at the specimen position of the PGAA station was about 9.6 × 107 cm2s−1. The cross-section of the neutron beam was adjusted to 20 × 20 mm2 to optimise the count rate. The gamma radiation from the radiative neutron capture was detected with a High-Purity Germanium (HPGe) detector, surrounded by a Bismuth Germanate (BGO) scintillator and 100 mm thick lead shielding for a few hours; the signals were processed with a Canberra AIM 556A multichannel analyzer. The facility was described in an earlier publication by Szentmiklósi et al. (2010). The spectra were evaluated with Hypermet-PC gamma spectroscopy software. The element identification was done with the ProSpeRo program, utilizing our prompt-gamma analysis library (Revay & Belgya, 2004).
Determination of the ash content of coal samples by nuclear techniques with bismuth germanate detectors
Published in International Journal of Coal Preparation and Utilization, 2020
Chompon Khoonthiwong, Phannee Saengkaew, Nares Chankow
As one of the nuclear techniques to determine the coal-ash content, the dual-energy gamma-ray transmission (DEGT) has been developed and applied in the coal industry for more than 30 y (Cheng et al., 2015; Clayton and Wormald 1983; Sowerby and Watt 1990; Vardhana and Giribabu 2015; von Ketelhodt and Bergmann 2010; Watt and Steffner 1985; Yazdia and Esmaeilnia 2003), and there have already been some recent new commercial systems (CSIROpedia 2017). In this research project, the use of a bismuth germanate (BGO) detector instead of the typical sodium iodide-thallium [NaI(Tl)] detector was investigated to measure the DEGT. To develop the linear calibration equation for determining the coal-ash content, the relationship between the natural logarithm ratios of the transmittance of low- and high-EGT (energy gamma-ray transmission), , and the ash contents have been explored previously (Johansen and Jackson 2004). Theoretically, a BGO scintillator has a higher peak detection efficiency than a NaI(Tl) scintillator when measuring high-energy gamma-ray radiation. This is because of the bismuth component of BGO, which has a high-density and large atomic number leading to a large probability of gamma-ray photoelectric absorption (Johansen and Jackson 2004; Knoll 2010; Orion and Wielopolski 2000). The higher detector efficiency could lead to a greater operation safety by allowing the use of a lower level of radioactivity or a shorter measurement time. Therefore, DEGT with a BGO detector has become an interesting topic for a promising approach to determine the ash content of coal samples.