Detecting Environmental Radiation- Radiation Spectrometer

Team 8l: Radiation Spectrometer Presentation Video  https://mediaspace.carleton.ca/media/CCDP2100I+-+Presentation+of+Findings+-+Team+8A+Radiation+Spectrometer/1_qvgule7i

Overview

Figure 1: Radiation spectrometer device [1]

The radiation spectrometer is used to detect radiation in the environment so that contaminated environments can be safely and efficiently cleaned. The radiation spectrometer works by converting rays of radiation into light photons in a device called a scintillator, reducing interference between types of radiation called crosstalk, converting the light photons into electrons and electrical signals in the photomultiplier tube, and then analyzing the signals in the digital pulse processor to differentiate between different types of radiation. This process is done in about 15 minutes and saves time and money during radiation cleanup over previous methods which took up to half a day for results. The US spent over $100 billion on radiation cleanup in 2018 alone, so any time saving will go a long way in reducing the cost and amount of radiation that can be cleaned.

Figure 2: Inside the radiation spectrometer device [2]

Radiation spectrometer: “New technology to speed cleanup of nuclear-contaminated sites,” Oregon State University, 30-Dec-2010. [Online]. Available: https://today.oregonstate.edu/archives/2010/dec/new-technology-speed-cleanup-nuclear-contaminated-sites-reduce-costs-and-create-jo. [Accessed: 28-Sep-2020].

Scintillator(s): Jacob Williams

Figure 3: α and β particles hit Material A, only the α particle reacts(left). Α and β particles hit Material B, only the β particle reacts(right) [Jacob Williams]

     A scintillator is a transparent material that, when hit by particles, exhibits scintillation. Scintillation is a type of chemical reaction that produces light in the form of a spark. Decay time is how fast a scintillation reaction happens (from start to finish), and it is characteristic of the specific type of scintillation material with respect to the specific type of particle of radiation that is hitting it. The scintillator can be made so that it sparks when it is hit by a specific type of particle of radiation by changing the scintillation material. If the specific type of particle of radiation being looked for is known, then the specific type of scintillation material that would react best with that specific type of particle of radiation can be found (based on decay time characteristics), and used as scintillation material of the scintillator. This allows the scintillator to be configured to react to specific radiation particles in the environment for detection.

Scintillator(s): "Chapter 4 Scintillation Detectors", Med Phys 4R06/6R03, McMaster University, 2020. Available: https://www.science.mcmaster.ca/radgrad/images/6R06CourseResources/4R6Notes4_ScintillationDetectors.pdf. [Accessed: 25-Nov-2020].

Decay-time characteristics: "Radiation measurement - Silicon detectors", Encyclopedia Britannica, 2020. [Online]. Available: https://www.britannica.com/technology/radiation-measurement/Silicon-detectors#ref620888. [Accessed: 25- Nov- 2020].

Crosstalk Reduction: Ethan Bradley

Figure 4: Beta particles vs. Gamma rays through quartz [Ethan Bradley]

Crosstalk is a phenomenon in which signals from two or more sources mix together such that unwanted “noise” is created. To reduce crosstalk between signals, a physical barrier with certain properties is required to isolate the signals. The piezoelectric effect of quartz acts as this barrier in the Radiation Spectrometer. Different kinds of radiation have different physical properties. For example, beta particle radiation has mass, while gamma rays have no mass. If there were a barrier of quartz, the beta particles with mass would be stopped by the barrier, while the massless gamma rays would phase through the quartz, allowing to get to its intended destination without the interference from the beta particles. The quartz acts as a form of energy conversion, the impact of the beta particles, while negligibly small, causes some stress in the quartz. The piezoelectric effect converts this stress into a negligibly small voltage, effectively “removing” the energy introduced by beta particles.

Crosstalk reduction: J. Price and T. Goble, “10.8.7 Crosstalk,” in Telecommunications Engineer's Reference Book, F. Mazda, Ed. Oxford, England: Butterworth-Heinemann, 1993, pp. 10–1, 10–3-10–15.

Piezoelectric effect: “How Piezoelectricity Works: EAGLE: Blog,” 13-Jul-2018. [Online]. Available: https://www.autodesk.com/products/eagle/blog/piezoelectricity/. [Accessed: 25-Nov-2020].

Photomultiplier Tube: Matt Reid

 Figure 5: Photomultiplier tubes [2]

     The photomultiplier tube intakes light photons from the scintillator and converts them into an electric signal to allow for analysis. The light photons from the scintillator are absorbed by a photocathode, which is a thin metal that converts light photons from the scintillator into electrons using the photoelectric effect. The photoelectric effect occurs when a light photon from the scintillator contacts the photocathode, and it reacts to eject electrons from its surface. Whether or not an electron will eject, and the energy that it ejects with, depends if the light photon has more energy than the metal can absorb. Since this only ejects single electrons at a time, the tube must then multiply the electrons to make the pulses strong enough to be analysed (like using a microphone to talk in a large group). The electrons strike multiple dynodes which multiply the electrons by splitting them 3 to 4 times (up to a total of over a million electrons after 6 dynodes), allowing them to be sensed as an electrical signal so they can be analysed by the Digital Pulse Processor.

Photomultiplier tube: N. Connor, “What is Photomultiplier Tube - PMT - Definition,” Radiation Dosimetry, 14-Dec-2019. [Online]. Available: https://www.radiation-dosimetry.org/what-is-photomultiplier-tube-pmt-definition/. [Accessed: 29-Sep-2020].

Photoelectric effect: The Editors of Encyclopaedia Britannica, “Photoelectric effect,” Encyclopædia Britannica, 17-Jul-2020. [Online]. Available: https://www.britannica.com/science/photoelectric-effect.  [Accessed: 06-Oct-2020].

Digital Pulse Processor: Duncan MacLeod

Figure 6: Pulse analysis [3]

     The Digital Pulse Processor (DPP) is used to analyze the electric pulse from the photomultiplier to find out if the pulse is from a gamma ray or a beta ray. Voltage samples are taken at two key points of the pulse. These samples are used in an algorithm to determine if the pulse comes from a beta ray or a gamma ray. First, the DPP reads the pulse, and takes the voltage samples at the two critical points, S1 and S2. Next, it checks if the voltage at S1 is above a certain point. If so, the pulse is recorded as a beta ray. If not, the DPP checks if the voltage at S2 is above a certain point. If not, the pulse is recorded as a gamma ray. If so, the pulse is deemed ambiguous. The next pulse is read in and the algorithm repeats for all pulses.

Digital Pulse Processor: A. T. Farsoni and D. M. Hamby, “A system for simultaneous beta and gamma spectroscopy,” ScienceDirect, 21-Jun-2007. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0168900207012600. [Accessed: 23-Sep-2020].

Voltage: McGrayne, Sharon Bertsch, Eustace E. Suckling, “Electricity”, Encyclopædia Britannica, Encyclopædia Britannica inc., 3-Feb-2020. [Online].  Available: https://www.britannica.com/science/electricity. [Accessed Oct 6 2020].

References

[1] K. K. Ma, “Ultra-realistic radiation detection training without using radioactive materials,” Phys.org, 14-Jan-2015. [Online]. Available: https://phys.org/news/2015-01-ultra-realistic-radioactive-materials.html. [Accessed: 21-Nov-2020].

[2] N. Connor, “What is Photomultiplier Tube - PMT - Definition,” Radiation Dosimetry, 14-Dec-2019. [Online]. Available: https://www.radiation-dosimetry.org/what-is-photomultiplier-tube-pmt-definition/. [Accessed: 29-Sep-2020].

[3] “Photomultiplier tubes (PMTs),” Hamamatsu Photonics. [Online]. Available: https://www.hamamatsu.com/jp/en/product/optical-sensors/pmt/index.html. [Accessed: 21-Nov-2020].

[4] “Pulse Modulation”, EEEGuide.com. [Online]. Available: https://www.eeeguide.com/pulse-modulation/ [Accessed: 25-Nov-2020]