Position Sensing of Beta Particle Emitters Using Self-Absorption in Plastic Scintillation Fibers

Accurate detection and localization of ionizing particles is critical in medical radiation therapy, and Muon tomography. These applications demand sophisticated positioning techniques that can provide high sensitivity, range, and spatial resolution. A recent study published in the Optics Letters Journal by Yoshinobu Ojima, Takuya Hamasaki, and led by Professor Yasuhiro Tsutsumi from Kindai University in collaboration with Professor Ichiro Fujieda from Ritsumeikan University developed a new technique for precisely detecting the position of beta particle emitters using plastic scintillation fibers (PSFs) in combination with silicon photomultipliers (SiPMs). Traditional methods for position sensing relies on time-of-flight or attenuation of scintillation light during propagation, necessitating photodetectors at both ends of the PSF. However, the authors’ new innovative method allows for position determination by reading the signal from only one end of the PSF, opening up new possibilities in various applications, including monitoring irradiation doses around nuclear reactors and fuel storage facilities or continuously assessing brachytherapy effects.

The research team utilized the phenomenon of self-absorption in luminescent materials which results in redshift in the spectrum of photoluminescence (PL) photons. This phenomenon formed the basis of the position sensing technique developed by the researchers. When a beta particle excites a specific spot on the PSF, PL photons are emitted. These photons follow an isotropic emission pattern, with some getting trapped within the PSF due to total internal reflection. The trapped photons then propagate a distance “z” before exiting the PSF at various angles. The researchers employed large-area SiPMs to collect as many photons as possible.

According to the authors, the spectral flux of PL photons decreases exponentially with distance “z” as it is attenuated by the wavelength-dependent coefficient “μ.” By measuring the number of photoelectrons generated by the SiPMs, this attenuation can be exploited to determine the incident point of radiation along the PSF. In essence, the greater the distance “z,” the larger the redshift, and this redshift can be correlated to the position of the beta particle along the PSF.

The authors conducted a series of experiments to validate their method with a beta particle emitter (90Sr) and a configuration consisting of a PSF, a dichroic mirror, and two SiPMs. The setup allowed the researchers to distinguish between scintillation photons with different wavelengths, directing shorter-wavelength photons to one SiPM and longer-wavelength photons to the other SiPM. The sensitivity of the SiPM-based photodetector was meticulously measured, showing a remarkable sensitivity of 0.59 mV per photoelectron. Subsequently, the authors conducted experiments with the PSF setup at varying distances from the beta particle emitter. The recorded data demonstrated the expected trend: as the distance “z” increased, the number of scintillation photons with shorter wavelengths (detected by SiPM1) decreased, while those with longer wavelengths (detected by SiPM2) remained constant. This differential behavior allowed for the calculation of a position index “E,” a monotonic function of distance “z,” which served as the calibration curve for position determination.

The results of this study are highly promising and offer significant implications for a range of applications, particularly in fields where precise position sensing of ionizing particles is critical. By leveraging self-absorption in luminescent materials, researchers have introduced a method that provides a novel and elegant solution for detecting and localizing beta particle emitters. Moreover, the ability to fine-tune the calibration curve by modifying the properties of the PSF opens up opportunities to customize the new technique to specific applications. For instance, by increasing the concentration of luminescent materials within the PSF, a steeper calibration curve can be achieved, enhancing both spatial resolution and sensitivity. Such versatility ensures that this approach can be adapted to a wide array of scenarios, offering a flexible tool for researchers and engineers in various domains. From an academic perspective, the incorporation of a dichroic mirror by the authors in this position sensing technique is of high interest. It is intriguing to observe that the total number of photoelectrons remains constant even with extremely low photon flux, akin to the wave nature of light explaining interference phenomena. This aspect of the research underscores the fundamental principles of optics and adds a layer of theoretical depth to the practical applications.

In conclusion, the study conducted by Professor Yasuhiro Tsutsumi and his collaborators introduces an innovative and highly promising position sensing technique for beta particle emitters. By harnessing self-absorption in PSFs and leveraging dichroic mirrors, the researchers have opened up new avenues for precision positioning in high-energy physics, medical radiation therapy, and muon imaging. This technique promises to enhance the accuracy and efficiency of a wide range of experiments and applications, with the potential to drive further advancements in the field of optical position sensing.