The distribution of the electric potential in the DUNE FD-VD, with a horizontal central cathode and anodes located at the top and bottom of the cryostat, makes the deployment of vertical arrays of RTDs unfeasible. This is because metallic elements would be exposed to high electric potentials, resulting in frequent discharges that could damage the detector. As an alternative, Fiber Bragg Grating (FBG) technology has been selected. However, substantial R&D is required to achieve the necessary precision and stability in a cryogenic environment.
The system is based on a highly precise laser beam, generated by a device called an interrogator, which is sent through an optical fiber that contains periodic modulations (FBGs) of the refractive index in its core. When light reaches one of these FBGs, it is partially reflected at a wavelength that shifts slightly depending on the temperature and strain at that point. This shift is measured by the interrogator. If strain is controlled—for instance, by keeping the fiber straight and under constant tension—the temperature can be accurately inferred from the wavelength shift.
In collaboration with several FBG providers, IFIC has studied the sensitivity of fibers with different coatings using a newly developed setup. This consists of a 30-liter pressure vessel in which LN₂/LAr temperatures can be raised by up to 10 K by increasing the pressure to 3 bar. Sensitivities of up to 20 pm/K have been observed for certain coatings. Combined with an interrogator resolution of 30 fm, this implies that a temperature precision of 1.5 mK could potentially be achieved.
After proving the feasibility of a millikelvin-precision system, resolving interfacing issues with other detector components, and mitigating all associated risks, the system has been officially approved by the Far Detector Construction Executive Board.
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