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The traditional proton magnetometer is a scalar quantum magnetometer made based on the Larmor precession effect of hydrogen protons in the geomagnetic field. Because of its advantages of high reliability, simple operation, convenient portability, low price, etc. It has been widely used in many fields such as geological survey [1-3], coal field exploration, oil and gas exploration, archaeology [4,5] and mineral exploration and prediction. The JPM series proton magnetometer developed by this research group has realized functions such as time, multiple measurement modes, multiple sampling rates, automatic tuning, tracking tuning, data storage and transmission on the console, which can meet the needs of field testing. But there are still deficiencies in sensor research. The performance of the proton sensor will have a vital impact on the technical indicators of the proton magnetometer. In view of the current problem, this paper studies the sensor of the proton magnetometer from principle to realization, and introduces in detail the internal coil structure of the proton sensor, the type of internal solution, and the method of shielding interference. Finally, a comparative experiment of sensor performance test was carried out on the JPM proton sensor and the Canadian GSM-19T proton sensor, and the experimental results were analyzed and evaluated.
 Introduction to proton magnetometer sensor
The proton sensor is mainly composed of an internal solution and an internal coil. The types of internal solutions mainly include deionized water, kerosene, methanol, alcohol, glycerin, etc. They provide a large amount of hydrogen protons. Different solutions have different initial amplitudes of test signals and signal decay time constants. According to the structure, the internal coils can be divided into solenoid coils, cylindrical coils, toroidal coils [6] and “8” type coils [7]. The coil model diagrams of different structures are shown in Figure 1.

Figure 1 Different coil structure, diagrams (a) is a solenoid coil, (b) is a cylindrical coil (c) is a toroidal coil, (d) is a “8” type coil

The advantage of the solenoid coil is that it is easy to wind, but the coil structure has omni-directivity. When the direction of the magnetic field generated by the sensor is parallel to the direction of the magnetic field to be measured, the Larmor precession signal will not be generated, so that the magnetic field cannot be measured. That is, there is a measurement dead zone. And it is very sensitive to the external AC magnetic field. If there is a relatively large magnetic field of several tens of Hz on the surface of the earth, an induced electric potential will be generated on the solenoid. This induced electric potential will even be greater than the voltage generated by Larmor precession, thus affecting measuring.

 Cylindrical coils and toroidal coils, because the magnetic field they generate is distributed in a loop inside the coil, they are omnidirectional, there is no measurement dead zone. But the disadvantage is that it is difficult to wind, and it takes a lot of manpower to wind a coil, so it cannot be mass-produced. It can only be used in very professional occasions that require high omnidirectional magnetometers. 

The last “8” type coil structure is convenient for winding. It only needs to wind two identical square coils and connect them together. And this structure does not have a measurement dead zone, but the signal amplitude generated in the worst case can only reach half of the best case. 

The working principle of the proton sensor is the Larmor precession effect. According to atomic physics, protons will continuously spin around their centers. Since protons have positive charges and a closed loop is generated during the spin process, they will have magnetic moments. When the sensor is placed in a non-magnetic environment, the orientation of the magnetic moments of all protons in the solution is disorganized and does not show a uniform magnetic moment to the outside. When the sensor is placed in the geomagnetic field to be measured, because the geomagnetic field is stable, the total magnetic moment of the protons in the solution is oriented parallel to the direction of the geomagnetic field, and there is no Larmor precession effect. In order to generate the Larmor precession effect, in actual measurement, the console needs to generate a polarization current first. The generated polarization current will pass through the internal coil of the sensor to generate a polarization magnetic field in the direction perpendicular to the magnetic field to be measured. In the total magnetic field of the polarizing magnetic field and the geomagnetic field, the magnetic moments of the protons in the solution are oriented to align along the direction of the total magnetic field. The larger the polarizing magnetic field, the closer the angle between the geomagnetic field and the total magnetic field is to 90 degrees. At this time, the console controls to suddenly turn off the polarization current, and the polarization magnetic field disappears, so the polarized proton magnetic moment will make Larmor precession along the direction of the geomagnetic field in the remaining geomagnetic field. The relationship between the magnetic field to be measured and the Larmor precession frequency is

In the formula, Be is the field strength of the magnetic field to be measured, the unit is nT, and f is the Larmor precession frequency, the unit is Hz. The precession of protons will be affected by thermal collisions, causing the induced signal to decay exponentially. The attenuation constant of the signal attenuation is related to the internal solution of the sensor. Then the proton sensor transmits the received Larmor signal to the console, and the console computer obtains the magnetic field value of the magnetic field to be measured by calculation through formula (1).