Measuring Gamma Radiation
Ionizing gamma radiation is measured using electronic or film dosimeters. However, film dosimeters are only helpful for calculating the overall radiation dosage and not tracking gamma radiation activity.
Electronic dosimeters, in contrast, enable continuous gamma radiation activity monitoring and provide up-to-date data on the overall radiation dosage. A suitable scintillation material or semiconductor is often directly linked to the electronic dosimeter component using a photomultiplier or semiconductor detector for ionizing radiation detection.
High doses of ionizing radiation directly destroy any electronics that are not radiation-shielded. High amounts of electromagnetic radiation, high temperatures, and a shortage of space are issues in nuclear waste dumps, hot cells, tokamaks, or particle accelerators.
Indirect film or electronic measurements at safe places for electronic systems address these inaccessible and dangerous sites. The activity at the target location is then calculated using a facility model. Continuous measurement is not possible at such locations with the available measuring methods for these applications.
Using a Scintillation Material
One method of continually measuring ionizing radiation is utilizing a scintillation material that transforms incoming ionizing radiation into the visible portion of the frequency spectrum and then transmits this light through optical fibers to the electronic detector. These come in plastic and silica varieties. Their application in sensor technology or the detection of diverse physical processes has significantly risen in recent years. However, there is still little usage for optical fibers in incident radiation monitoring, particularly for gamma radiation.
How Ionizing Radiation Damages Plastic Optical Fibers
Ionizing radiation damages the internal structure of optic fibers, causing the optical signal to be attenuated further.
In silica fibers, the reaction of O-H or Ge dopant bonds in the optical fiber with ionizing radiation is the principal source of damage. One of the components or compounds in the fiber interacts with ionizing radiation in plastic optical fibers.
Plastic optical fibers may have different materials depending on the manufacturer and the intended use. The commercial selection of these fibers is vast in terms of material composition. In particular, plastic optical fibers are readily available with diameters ranging from hundreds to tens of millimeters. Their primary drawback is greater attenuation than silica fibers.
Aim of the Study
The sensitivity of various silica optical fibers in conjunction with various scintillation materials has been experimentally characterized.
The researchers created sensor housings that allow the scintillation material to be changed and the optical fiber to be mounted so that the fiber's end and the scintillation crystal's front are always in the exact relative location.
Developing Fiber Optic Dosimeter
The researchers developed a fiber optic dosimeter to monitor ionizing gamma radiation in areas with high radiation levels, electromagnetic interference from radio waves, and limited space.
The components of the optical fiber dosimeter include a scintillation crystal-equipped gamma radiation sensor, a silica multimode optical fiber connection, a scintillation radiation detector, and a control PC.
The researchers created a technique for creating an optical fiber connection employing several fiber types, including termination with SMA (SubMiniature version A) connectors and mechanical embedding inside an armored jacket that boosted mechanical durability and blocked ambient light penetration.
The researchers experimented with several combinations of components, including four different silica multimode optical fibers with numerical apertures (NAs) of 0.37 and 0.5, four gamma-ray sensors with scintillation crystals (NaI(Tl), CaF2(Eu), LaBr3(Ce), and LYSO(Ce)) and two different scintillation radiation detectors (a photomultiplier tube and a single photon counter).
Significant Findings of the Study
The researchers validated that the dosimeter can be used with all-ready optical fibers, sensors, and detectors.
A sensor with an LYSO crystal, a photomultiplier, and a 1.5 mm diameter optical fiber with a NA of 0.5 were used to obtain the highest dosimeter measurement sensitivity.
The photomultiplier offers more sensitive scintillation detection, while the single photon counter is often simpler to operate and slightly inferior.
The results of the measurements demonstrated that greater ionizing radiation activity can be measured using silica optical fibers with reduced optical core sizes and NAs. The research also shows that various silica optical fibers work well with all tested scintillation crystals.
Various industrial and safety applications may readily employ the optical fiber dosimeter for gamma radiation measurements. Due to readily replaceable sensors, optical fibers, and detectors, the system may be modified to assess dosage rates across various critical situations.
Reference
Michal Jelinek, Ondrej Cip, Josef Lazar and Bretislav Mikel (2022) Design and Characterisation of an Optical Fibre Dosimeter Based on Silica Optical Fibre and Scintillation Crystal. Sensors. https://www.mdpi.com/1424-8220/22/19/7312/htm
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