Many of today's x-ray detectors rely upon indirect measurements, whereby a scintillator first converts x-ray energy into light. The photons then travel through optical fibers to a camera or photodetector. While this setup is widely used in medicine and industry, the machines are cumbersome and creating a small-scale version is challenging. The quantity of x-rays detected is dependent upon the size of the scintillator and the resulting photons are emitted in all directions. Therefore, a small scintillator produces very few photons and the likelihood of them emitting in the direction of the camera is low.
A nano-optical antenna directs light from a scintillator down a thin optical fiber for indirect x-ray detection. Image courtesy of Miguel Angel Suarez, FEMTO-ST Institute.
To overcome this problem, Thierry Grosjean, PhD, from the University of Burgundy --Franche-Comté in France, and colleagues, incorporated a NOA between a small scintillator cluster and an optical fiber. Analogous to a microwave horn antenna, a NOA can direct light. So, when an x-ray hits the tiny scintillator cluster, the light emitted can be directed down a thin optical fiber to the camera, thereby increasing the amount of photons detected (Optics Letters, Vol. 42, Issue 7, pp. 1361-1364, 2017).
The NOA is a miniature horn antenna -- it contains a flared waveguide that amplifies a dipolar signal and directs it into a linear waveguide. To create the device, the team grew a 38 µm-long polymer microtip (tip radius 1 µm) at the end of a thin optical fiber. The scintillation cluster was then grafted onto the tip. To prevent visible light from entering the system, a thin layer of aluminum and titanium was applied to its surface.
"Such a cluster-to-fiber coupling enhancement enables the realization of x-ray detectors at the end of a narrow single-mode fiber -- 125 µm outer diameter in our study, or less," Grosjean explained.
As well as miniaturizing x-ray detection, a key challenge was to design a device that could be made at low-cost and has the potential to be mass-produced. The researchers point out the resulting detectors are inexpensive to make and do not require clean-room processes to be fabricated.
The researchers have tested their device for soft x-rays (low-energy radiation around 10 keV). They have achieved a spatial resolution in the order of 1 µm, although they hope to improve this to 100 nm in future work to distinguish chemical components during low-energy x-ray scanning microscopy.
"The next step will be to demonstrate our concept with high-energy x-rays for medical applications," Grosjean said. The compact nature of the device means it could be incorporated into endoscopy techniques and used to measure radiation exposure from inside the body during radiotherapy cancer treatment. To proceed with the work, the group has applied for funding from the French National Research Agency and hopes to produce a market-ready prototype within three years.
"The compactness of our sensor is unprecedented," Grosjean concluded. "Our nano-optically driven technology is totally new."
Sarah Tesh is a reporter for physicsworld.com.
© IOP Publishing Limited. Republished with permission from medicalphysicsweb, a community website covering fundamental research and emerging technologies in medical imaging and radiation therapy.
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