Signals from the Subatomic World: How to Build a Proton Precession Magnetometer

The discovery of nuclear magnetic resonance earned Felix Bloch and Ed Purcell the 1952 Nobel Prize in Physics. What their discovery took advantage of, is that protons are the world's smallest magnets. These tiny magnets can also be used to make a magnetometer, of the type described in this book. This book describes how to build a proton precession magnetometer, suitable for measurements of the Earth's magnetic field. This method of measuring magnetic fields offers the theoretically highest possible precision, limited only by the known value of the gyromagnetic ratio of the proton. Uses of the magnetometer include: making precise measurements of the Earth's magnetic field, calibrating low field magnetometers, teaching modern signal processing techniques, demonstrating nuclear magnetism and NMR to students, and measuring nuclear magnetic relaxation in liquids. The Earth's field proton precession magnetometer, called the Magnum, described in this book, was formerly a commercial product, developed and sold by Exstrom Laboratories LLC. It was designed by Stefan Hollos and Richard Hollos.

1 Introduction		1
1.1 The proton precession magnetometer		2
1.2 How PPM's work		5
1.3 Overview of magnetometer		12
2 Polarization Coil and Platform		17
2.1 Materials not allowed		17
2.2 Wire for polarizing coil		18
2.3 Purpose of polarizing coil		19
2.4 Positioning polarizing coil		20
2.5 Inductance of a coil		21
2.6 Magnetic field strength due to coil		22
2.7 Polarizing coil parts list		23
2.8 Specifications of polarizing coil		25
2.9 Construction of polarizing coil		25
2.10 Tiltable platform parts list		26
2.11 Construction of tiltable platform		29
3 Pulse Controller		31
3.1 Purpose of pulse controller		31
3.2 Quenching the current		32
3.3 How a MOSFET works		33
3.4 Advantage of using multiple MOSFETs		34
3.5 Pulse controller circuit description		35
3.6 Microcontroller circuit description		38
3.7 Microcontroller software		39
3.8 Power supply		41
3.9 Assembly and enclosure		42
3.10 Pulse controller parts list		51
4 Sensor Coil		57
4.1 Sensor coil requirements		57
4.2 How the coils are wired together		60
4.3 How to connect cable to coils		61
4.4 Specifications of sensor coil		62
4.5 Sensor coils parts list		62
4.6 Construction of sensor coils		63
5 Amplifier		69
5.1 Why a differential amplifier is best		70
5.2 Why a low noise amplifier is needed		71
5.3 Description of amplifier circuit		71
5.4 Supplying power to the amplifier		73
5.5 Layout of amplifier circuit		74
5.6 Capacitors at amplifier input		75
5.7 Estimating precession frequency		78
5.8 Amplifier parts list		79
6 Data Acquisition and PC Control		87
6.1 What sampling rate to use		88
6.2 What resolution to use		89
6.3 How long to take data		90
6.4 The ADC board used in the Magnum		90
6.5 Pulse controller and data acquisition software		92
7 Data Processing and Analysis		101
7.1 Signal averaging		101
7.1.1 Why it improves signal to noise ratio		101
7.1.2 Limits due to MOSFET slow down		102
7.2 Filtering the data		103
7.3 Spectral analysis using the FFT		108
7.4 Converting bin number to actual frequency		112
7.5 High resolution spectrum for peak location		113
A Magnum Control Program		117
B Microcontroller Program		129