A New cw-NMR Q-meter for Dynamically Polarized Targets for Particle Physics

TL;DR

This work presents a new cw-NMR Q-meter designed to replace aging Liverpool units for dynamically polarized targets in particle physics. The system closely mirrors the Liverpool architecture while adopting a modular, FPGA-based data acquisition and ethernet-enabled control, along with voltage-tunable capacitance and electronic phase tuning. Key contributions include four-board RF electronics with modern amplifiers, cold-tank capability inside the cryostat, and a Python-based software suite for live polarization analysis and TE calibration; the design demonstrates good linearity and agreement with the Liverpool Q-meter across a wide dynamic range, validated by Run Group C data at Hall B. The instrument promises improved reliability, easier maintenance, and faster data throughput, enabling broader deployment and future enhancements such as quadrature detection and ML-assisted signal processing.

Abstract

Polarized solid targets produced via Dynamic Nuclear Polarization rely on Continuous-Wave Nuclear Magnetism Resonance measurements to accurately determine the degree of polarization of bulk samples polarized to nearly 100%. Since the late 1970's phase sensitive detection methods have been utilized to observe the magnetization of a sample as a small change in inductance under RF excitation near the Larmor frequency of the nuclear species of interest, using a device known as a Q-meter. Liverpool Q-meters, produced in the UK in the 80's and 90's, have been the workhorse devices for these targets for decades, however their age and scarcity has meant new systems are needed. We describe a Q-meter system designed and built at Jefferson Lab in the Liverpool style to have comparable electronic performance with several improvements to update and adapt the devices for modern use.

Figures (18)

• Figure 1: A Liverpool Q-meter unit with the cover removed. From left to right are five boards: tank circuit, the mixer, the mixer signal amplification, the diode rectifier, the final amplification.
• Figure 2: Two simplified diagrams of the Liverpool-style Q-meter circuits built around phase sensitive detectors (PSD). At left, the tuning capacitance of the LC tank circuit is located outside the target cryostat and is connected to the inductance (NMR coil) using a resonant cable of length $n\lambda/2$. At right, both the capacitance and inductance are inside the cryostat. In the original Liverpool Q-meter, both the phase and capacitance are adjusted manually. In the JLab Q-meter, these adjustments are done electronically.
• Figure 3: Example analysis steps for a polarized proton signal, showing the raw signal with characteristic "Q-curve" background, the signal after subtracting the baseline "Q-curve" away (both using left y-axis), and the "final" proton polarization signal (using the right y-axis). The area of the shaded green portion is the final output of the signal analysis.
• Figure 4: A schematic layout of the Liverpool RF module configured for use with a resonant cable , orientated left to right as in Figure \ref{['fig:liverpool']}, showing components and signal routing between the 5 printed circuit boards. Parts denoted "G" are attenuators, "A" RF amplifiers, "D" diodes, "S" splitters, and "LF" low frequency amplifiers.
• Figure 5: Schematic of a Q-meter circuit as simulated from the input RF at left to the output at the mixer at right. Four variable length transmission lines are shown. The tank circuit consists of $R_d$, $C$, and $L$, and various impedance measurements points in the simulation are shown as $Z$s. Should there be an output from the mixer?