We work on the world’s first real-time in-garment nuclear radiation detection system that is nearly imperceptible to the user or the surroundings. This proposal builds on two breakthroughs achieved in this program: the first the construction of a sensitive in-fiber radiation detector realized in a flexible textile fiber, the second is a fiber computer and fabric network that is realized in textile fabrics and worn garments. This can be achieved by integrating the light collection and hit counting electronics with the scintillating fibers, resulting in a novel type of functional fibers and functional fabrics. Given the wide difference in Compton electron and recoil proton ranges, it has been shown through Monte Carlo simulations that one can achieve a particle ID by observing the number of triggered fibers. Furthermore, experimental work over the last two years has yielded promising results: fibers with built-in photodetectors have been drawn, and their sensitivity to radiation has been demonstrated. The effort for the next two years will involve the continuation of the study of the mechanics of fibers’ thermal draw from a variety of scintillator materials, the various fiber architectures, the development of specific electronics for light readout, as well as algorithmic development for inference of particle type. The effort is led by MIT Prof. Areg Danagoulian (Nuclear Science and Engineering) and Prof. Yoel Fink (Department of Materials Science and Engineering). This multidisciplinary collaboration has shown that it can provide for a synergy that can enable novel engineering and scientific developments.
Throughout last two years, our group has been fully committed to exploring innovative preform designs to address the challenge of short light attenuation length of the fiber. We focused on a material combination utilizing polyvinyltoluene (PVT) as the scintillator and poly methyl methacrylate (PMMA) as the cladding material. This combination, which can be co-drawn with an embedded 1mm SiPM, demonstrated a better refractive index match for the optical waveguide. Our measurements revealed notable light attenuation in this configuration, necessitating the exploration of a new design to enhance light transmission within the fiber. To overcome this challenge, we opted to replace PMMA with elastomeric cyclic olefin copolymer (COCe) as the optical waveguide cladding, considering its favorable optical and mechanical properties. Specifically, COCe possesses a lower refractive index compared to PVT, allowing for a better light transport. Additionally, its toughness helps prevent fiber breakage during the integration of photodetectors in thermal drawing processes. For increased stability during drawing and to ensure clean cuts of COCe during post-processing, PMMA was co-drawn alongside the other polymers.
In this new design, the production of an integrated radiation sensing fiber begins with the preparation of a macroscale preform using three thermoplastic polymers. The preform takes on a rectangular shape with an 8.6 × 17.2 mm² PVT core adjacent to a 4.0 × 17.2 mm² hollow channel, both of which are enclosed by a 1.2 mm COCe layer and a 3 mm PMMA cladding. To incorporate multiple commercial photodetectors into a single fiber, SiPMs are pre-connected to twisted wires at intervals of 10 cm and spooled before the fiber drawing process. Subsequently, the prepared preform is vertically placed in the furnace of the draw tower, where it undergoes necking and reduction to fiber dimensions due to the tension from a pulling capstan. The consolidation of the layers occurs at a temperature of 120°C and a pressure of 5 psi for 1 hour in a hot press. During the thermal drawing process, the preform is fed at a speed of 1 mm/min, while the capstan pulls the fibers at a speed of 0.15 m/min. The wire spool, along with the SiPMs, is integrated into the fibers by capstan pulling through the hollow channel. To ensure proper alignment, the SiPMs are positioned with their photo-detecting array facing the PVT using a rectangular guiding tube, which prevents their rotation. As a result of the feeding and pulling speed ratio, the fiber is reduced to a size of approximately 2.1 × 1.7 mm². Notches with a 45° angle were milled in the PVT at the positions of each SiPM, in order to reflect the light onto the SiPM. To enhance the efficiency of reflection, two types of reflectors were incorporated at the notches using commercial mirrors. Direct measurements of light reflectivity were performed, using a photodetector and a light source, which showed that ~ 50% reflectivity is achievable using the silvered notch method.
For the schematic of the drawing process and the resulting fiber structures see Figure 1.
To verify the capability of SiPM-integrated fiber for radiation detection, an experiment was conducted. The experiment employed a well collimated beta source, Sr-90, with radioactivity of 66 nCi. The source was positioned on the fiber, and the signal was measured using an oscilloscope in data acquisition (DAQ) mode. The results showed that the configuration could reliably detect optical light from the ionization event at the distances of 10-20cm. The analysis was done using SiPM electric signal which was calibrated to single photon data, allowing for determination of the signal in units of collected optical photons.
The analysis shows that while the fiber-SiPM architecture is feasible, there is significant light loss past 5 cm. Our studies have shown that this is primarily due to the quality of the core-cladding interface. Our team has consulted specialists at Eljen Technologies, who have indicated that interface preparation and polishing is key to good light transport. We believe that by improving the quality of the optical interfaces the light transport can be improved by at least 5x. Among other things the next phase of the project will focus on these improvements.
Research team:
PIs:
Prof. Yoel Fink (MSE)
Prof. Areg Danagoulian (NSE)
Dr. Wenzhao Wei
Vik Ohstrom
Nikhil Gupta
Youngbin Lee
David Seda