Optimization of Hight-Sensitivity SQUID Gradiometer for ARIADNE at CAPP

Abstract

ARIADNE (Axion Resonant InterAction Detection Experiment) is a table-top experiment that intends to search for QCD axions from exotic spin-dependent interactions mediated by axion between nuclei at sub-mm range. This experiment has a potential to probe deep within the theoretically interesting regime for the QCD axion in the mass range of 0.1~10meV, independently from the cosmological assumptions of dark matter axion. This experiment includes a non-magnetic mass to source the axion field, and a dense ensemble of hyper-polarized 3He nuclei to detect the axion field with nuclear-magnetic-resonance (NMR) based method. With employing a Superconducting Quantum Interference Device (SQUID) magnetometer, however, the expected NMR signal from the interaction could be easily buried in the noise spectrum of the magnetometer, especially in a frequency range (~ 100 Hz) where the interaction signal is supposed to exist, but at the same time, acoustic noise from the environment becomes dominant source of the background noise. In ARIADNE, attempts are made to overcome these problems with SQUID-based co-centric planar gradiometer leading to significantly increased sensitivity. In this work, we report a comprehensive study of the noise spectrum for the SQUID magnetometers configuration in order to optimize the gradiometer design parameters. The magnetic flux has been estimated as a function of the pickup loop radius of magnetometer and gradiometer which allows us to achieve an optimum system design. A series of measurements for the characterization of the SQUID gradiometer are also presented. The obtained noise level of the gradiometer may enable us to reach the sensitivity required for the ARIADNE.