Cesium Magnetometer Research Articles

Hardware Design and Implementation of a High-Precision Optically Pumped Cesium Magnetometer System Based on the Human-Occupied Vehicle Platform

High-precision magnetometers play a crucial role in ocean exploration, geophysical prospecting, and military and security applications. Installing them on human-occupied vehicle (HOV) platforms can greatly enhance ocean exploration capabilities and efficiency. However, most existing magnetometers suffer from low sensitivity and excessively large size. This study presents a high-sensitivity, miniaturized magnetometer based on cesium optically pumped probes. The designed magnetometer utilizes a three-probe design to eliminate the detection dead zone of the cesium optically pumped probe and enable three-dimensional magnetic detection. The proposed magnetometer uses a flux gate probe to detect the three-axis magnetic field and ensure that the probe does not enter the dead zone. The three probes can automatically switch by measuring the geomagnetic elements and real-time attitude of the HOV platform. This article primarily introduces the cesium three-probe optically pump, flux gate sensor, and automatic switching scheme design of the above-mentioned magnetometer. Moreover, it is proven through testing that the core indicators, such as the accuracy and sensitivity of the cesium three-probe optically pumped and flux gate sensor, reach international standards. Finally, the effectiveness of the automatic switching scheme proposed in this study is demonstrated through drone-mounted experiments.

Using Gradiometric Technique to Prospect Archaeological Features in Tell Al-Deylam, South of Babylon City, Middle of Iraq

An extensive vertical magnetic gradiometry survey was carried out over an area spanning 1,188 square meters in the northwestern section of Dilbat; a recently discovered archaeological city within Tell Al-Deylam located approximately 20 km south of Babylon city, so as to reveal the real image of the archaeological features hidden beneath the earth’s surface. The vertical gradiometric survey was done by the Geometrics-G-858 Cesium magnetometer. The gradiometric results showed three clear longitudinal magnetic anomalies in different places on the magnetic contour map. The main anomaly is located on the edge of the northern part of the study area and extends longitudinally in the northwest and southeast direction. This may indicate the presence of the main outer wall, as it extends lengthwise about 30 m, with a width of 3-4 m, and a depth of about 1 m below the surface of the ground, and may contain the main northern gate of the Dilbat Temple. Besides that, this anomaly displayed approximately a high magnetic value of about + 60 nT, which indicates that the wall was built from baked brick composed of clay minerals rich in iron oxides. Furthermore, the magnetic contour map showed two other magnetic anomalies. The first one is located in the western part of the study area and appears perpendicular to the main magnetic anomaly. Meanwhile, the second anomaly occupied the southern part of the study area and is parallel to the main anomaly. The two anomalies showed approximately lower magnetic strength than the outer wall. So, they may represent the remains of the inner room walls built from mud bricks. The thickness of the two supposed walls is about 2.5 m and they are at a depth one meter from the ground. The findings of this method have been a good guide for new excavation, which led to finding the main wall built from fired bricks.

Between water and land: Connecting and comparing underwater, terrestrial and airborne remote-sensing techniques

Ostrów Lednicki is one of the most important early medieval sites in Poland. Land and underwater archaeological excavations have been carried out there for many decades. This has allowed for a significant reconnaisance of the Lednica settlement complex. However, further research, especially that carried out underwater and in the shore-zone, has required the use of modern methods involving a multibeam echosounder (MBES), sub-bottom profiler (SBP), caesium magnetometer and LiDAR scans. The primary platform used for combining the underwater and aerial survey data was a Geographic Information System (GIS) environment. This software made it possible to compare and correlate acoustic, magnetic, seismic and laser data. As a result, new important archaeological features were discovered, including two previously unknown bridges and extensive shore-zone fortifications.

Drone-based magnetometer prospection for archaeology

Magnetometry is one of the most efficient and successful methods of archaeological prospection. Drone-based prospecting is increasingly being used in many fields of remote sensing but with respect to magnetometry, with rather poor results. For magnetic surveys, drone prospecting comes with the problem that magnetic and mechanical disturbances originating from the aircraft decrease the quality of the measurements.Here we present the adaptation of a commercial three-axis fluxgate magnetometer setup, which can be suitable for archaeological prospection, after applying appropriate filter methods and minimising the flying altitude and speed. For approval of the system, we performed drone-based surveys in high spatial resolution (50 cm line spacing) at constant, ultra-low sensor altitudes of 45 ± 10 cm and 75 ± 10 cm. We chose an archaeological site from the roman period as survey site (3.8 ha), where high-quality ground-based caesium magnetometer data was available. This allows us to demonstrate the first detailed comparison of drone-based and ground-based magnetic survey data for archaeology. For further evaluation, the influence of the drone on the measurements is assessed, drift and sensor resolution checks are carried out, and suitable data filtering methods are evaluated.Our results show that we can detect main archaeological features such as ditches, pits, fireplaces and remnants of stone fundaments with the drone-based setup, which are the relevant structures for the majority of archaeological prospections worldwide. Further, we can prove that the data quality can significantly be increased by lowering the flight altitude and speed. We conclude that drone-based magnetometry serves above all to cover large and inaccessible areas in short time. Our findings are a first step towards the development of standards for drone-based archaeological prospection approaches.

A highly drift-stable atomic magnetometer for fundamental physics experiments

We report the design and performance of a nonmagnetic drift stable optically pumped cesium magnetometer with a measured sensitivity of 35 fT at 200 s integration time and stability below 50 fT between 70 and 600 s. The sensor is based on the nonlinear magneto-optical rotation effect: in a Bell–Bloom configuration, a higher order polarization moment (alignment) of Cs atoms is created with a pump laser beam in an anti-relaxation coated Pyrex cell under vacuum, filled with Cs vapor at room temperature. The polarization plane of light passing through the cell is modulated due the precession of the atoms in an external magnetic field of 2.1 μT, used to optically determine the Larmor precession frequency. Operation is based on a sequence of optical pumping and observation of freely precessing spins at a repetition rate of 8 Hz. This free precession decay readout scheme separates optical pumping and probing and, thus, ensures a systematically highly clean measurement. Due to the residual offset of the sensor of <15 pT together with negligible crosstalk of adjacent sensors, this device is uniquely suitable for a variety of experiments in low-energy particle physics with extreme precision, here as a highly stable and systematically clean reference probe in search for time-reversal symmetry violating electric dipole moments.

On‐site non‐destructive determination of the remanent magnetization of archaeological finds using field magnetometers

AbstractThe determination of the natural remanent magnetization (NRM) of archaeological features can be used for magnetic modelling, joining of shards, archaeomagnetic dating or the investigation of the firing–cooling–collapsing order of ancient buildings. The measurement of NRM is normally conducted on cylindrical or cubic samples in the laboratory. Nevertheless, archaeological finds should preferably not be destroyed, and laboratory instruments are high in costs. Therefore, we propose a lightweight and portable measurement set‐up including already available field magnetometers (preferably caesium magnetometers) in which the archaeological sample of arbitrary shape, in our case a piece of daub, is mounted inside a gimbal to be rotated in all directions. The magnetic field of the sample is measured at a large number of rotational positions with the magnetometer kept at fixed position. In these measurements, the unknown direction of the NRM vector of the sample is rotated, whereas the average magnetic susceptibility of the sample and the ambient magnetic field are constant and known. Hence, the vector of NRM can be determined through least‐squares inversion. For the inversion computation, the sample volume is discretized either as voxel model or approximated as an equivalent sphere. Under certain conditions depending on sample–sensor distance, dipole moment and radius of the sample, the approximation by a sphere is valid without effect on the accuracy of results. Empirically determined functions quantifying these conditions for different sensor sensitivities and noise levels are provided. Validation with laboratory measurements on palaeomagnetic subsamples from the destroyed daub samples indicate that the NRM can be determined by our proposed method with a maximum error in inclination of 2°, in declination of 20° and in magnetization of ±0.6 A/m. This is accurate enough, for example, to determine from daub pieces of burnt house remains whether the building was burnt and cooled before or after it collapsed.

Rock magnetic study of grave infill as a key to understanding magnetic anomalies on burial ground

AbstractMagnetic prospecting of burial grounds is still a great challenge in the field of archaeological geophysics. Some graves appear as positive anomalies, whereas others do not generate any traces or even show up by negative anomalies. This study involves ground magnetic survey, rock magnetic and magnetic mineralogy examinations of soils to determine factors responsible for the formation of magnetic anomalies on Late Roman time burial grounds of Chernyakhiv–Sântana de Mureş archaeological culture located in the forest‐steppe of Ukraine. We surveyed these sites by a total magnetic field caesium magnetometer in order to target the archaeological excavations. We sampled infill from graves excavated under positive anomalies and profiles of background soils to analyse the enhancement of induced and remanent magnetization. In the laboratory, we examined magnetic mineralogy and measured various magnetic parameters and ratios characterizing concentration and granulometry of ferrimagnetic phase of soils. For further explanations of the magnetic anomalies, we created 3D synthetic magnetic models of the studied objects and compared the calculated total field anomalies with field measurements by their intensity and size. Here, we show how positive magnetic anomalies of destroyed burials are generated when a destruction pit remains open and later topsoil material gradually refilled it with aid of precipitation water. In this case, the infill layers acquire detrital remanence of the same intensity as the topsoil. Such burials are the best targets for magnetic survey. However, archaeological excavations revealed both intact and destroyed inhumations that caused no magnetic disturbance. Obviously, they were refilled immediately after digging/destruction, so that natural remanence of the infill remains mechanically destroyed. The study of this specific category of archaeological monuments brought us closer in the understanding the formation of magnetic anomalies in soil‐filled features and showed the potential and limitations of magnetic prospection.

Geophysical Investigation of Babylon archeological City, Iraq

A geophysical survey was carried out at old archeological Babylon City, which is located 90.0 km to the south of Baghdad. Three geophysical methods were applied in this survey. The VES resistivity method, for soil conductivity and layering testing. GPR and magnetic methods, for detecting subsurface archeological bodies. The resistivity results showed values lower than 5.0 ohm. m, and subsurface distortion, which are probably related to buried archeological ruins. Due to the low electrical resistivity of the top soil, the GPR method did not reach a good penetrating depth. Wide range of frequencies, 30 MHz, 250 MHz, 500 MHz, and 100MHz were applied. The high signal attenuation of the top soil resulted in distorted radar-grams with faint shallow anomalies. The GPR survey was conducted at different seasons hoping that the dry conditions of the hot summer could change the top soil electrical resistivity. Nevertheless, few GPR radar- grams show point reflections that related to small dense bodies at depths of 0.8 – 2.0, which could be archeological bricks walls. The magnetic survey was carried out in two stages using Cesium magnetometer of high sensitivity. An area of 140.0 m x 240.0 m was surveyed at the first stage, which is covered by twenty-eight N-S traverses. The preliminary results show the effects of the N-S profiling, which can be removed by directional first derivative filter. It showed indications of subsurface archeological features. In the second stage, an area of 40.0 x 40.0 meters was chosen for detailed survey. The survey is done a long 40 traverses in N-S direction and another 40.0 Traverses along W-E directions. The preliminary results of the both surveys show some differences. These differences were removed by using first derivative filter. They both showed subsurface geometric shapes that are probably related to buried archeological walls. The mathematical modeling results showed walls of widths 0.3-0.4 meters at depths around 0.8 meters. These walls continue down to 2.3-2.5 meters in depth. The magnetic survey also showed many circular high magnetic anomalies within an area of 1.0 – 2.0 meters only. The mathematical modeling suggested that these anomalies could be corresponding to building poles made of andesite or basaltic rocks or they are buried metals objects.

A high-accuracy and non-intermittent frequency measurement method for Larmor signal of optically pumped cesium magnetometer

The optically pumped cesium magnetometer is a quantum magnetometer based on the Zeeman effect. The frequency of the output Larmor signal is proportional to the magnetic field intensity.Therefore, improving the measurement accuracy of the Larmor frequency can make the magnetic field measurement more accurate.However, the Larmor signal is easily coupled with various randomly distributed noises, which will generate an unavoidable trigger error.To address the above problem, we propose a multi-gate and equal-precision fusion method. In this method, the trigger error is averaged into the count of each small gate through the multi-gate structure to improve the frequency measurement accuracy and the continuous sampling performance.We implemented this method on the FPGA and conducted indoor and outdoor experiments. The experimental results show that this method not only improves the measurement accuracy by three times of the traditional method, but also enhances the stability and noise suppression ability of the system, and can provide technical and instrumental support for magnetic field measurement projects.

Design of the self-oscillating loop of the optically pumped cesium magnetometer

The self-oscillating loop is an important part of the optically pumped cesium magnetometer, and its working characteristics directly determine the accurate measurement of external magnetic field. The design of the self-oscillating loop has been discussed in this paper, including a signal conditioning circuit, a phase shifter and a frequency meter. It can be used to precisely improve the accuracy of resonance signal in a wide range of frequencies from 50 kHz to 350 kHz. The relative error of our system is less than 0.5×10−6 and it has a good prospect in the optically pumped cesium magnetometer.

Frequency Response of Optically Pumped Magnetometer with Nonlinear Zeeman Effect

Optically pumped alkali atomic magnetometers based on measuring the Zeeman shifts of the atomic energy levels are widely used in many applications because of their low noise and cryogen-free operation. When alkali atomic magnetometers are operated in an unshielded geomagnetic environment, the nonlinear Zeeman effect may become non-negligible at high latitude and the Zeeman shifts are thus not linear to the strength of the magnetic field. The nonlinear Zeeman effect causes broadening and partial splitting of the magnetic resonant levels, and thus degrades the sensitivity of the alkali atomic magnetometers and causes heading error. In this work, we find that the nonlinear Zeeman effect also influences the frequency response of the alkali atomic magnetometer. We develop a model to quantitatively depict the frequency response of the alkali atomic magnetometer when the nonlinear Zeeman effect is non-negligible and verify the results experimentally in an amplitude-modulated Bell–Bloom cesium magnetometer. The proposed model provides general guidance on analyzing the frequency response of the alkali atomic magnetometer operating in the Earth’s magnetic field. Full and precise knowledge of the frequency response of the atomic magnetometer is important for the optimization of feedback control systems such as the closed-loop magnetometers and the active magnetic field stabilization with magnetometers. This work is thus important for the application of alkali atomic magnetometers in an unshielded geomagnetic environment.

Expanding the Range of Reproduction of the Magnetic Induction of a Constant Field of Get 12–2011, the State Primary Standard of the Unit of Magnetic Induction, Unit of Magnetic Flux, Unit of Magnetic Moment, and Unit of Magnetic Induction Gradient

Results of studies designed to increase the level of uniformity of measurements of the magnetic induction of a constant field, the basic quantity in the field of magnetic measurements, are presented. A complex of instruments for reproduction of the unit of magnetic induction density of a constant field in the range 1–25 mT is constructed and described. Inclusion of the instrument complex into State Primary Standard GET 12–2011 of the unit of magnetic induction, unit of magnetic flux, unit of magnetic moment, and unit of magnetic induction gradient will support reproduction and direct transmission of the unit of magnetic induction of a constant field in the range not only of weak magnetic fields (10–3–1 mT), but also in the range of intermediate fields (1–25 mT) and in the range of strong fields (0.025–1 T). A quantum cesium magnetometer based on the resolved structure of cesium atoms is created to transmit the unit of magnetic induction in the region of intermediate fields. A procedure for calculating the coefficients of conversion of frequency into magnetic induction in the newly designed quantum cesium magnetometer is described. The budget of uncertainty of the reproduction of the unit of magnetic induction of a constant field is determined by means of the new complex of instruments.

Research on expanding the range of reproduction of magnetic flux density of the DC field of the state primary standard GET12-2011

The results of works aimed at increasing the level of uniformity of measurements of the magnetic induction of a constant field – the basic value in the field of magnetic measurements. A set of equipment for reproducing a unit of magnetic induction of a constant field in the range of 1–25 mT was created and described. The inclusion of this complex in the State primary standard of units of magnetic induction, magnetic flux, magnetic moment and magnetic induction gradient GET 12-2011 will ensure the reproduction and direct transmission of the unit of permanent magnetic induction in the ranges of not only weak (10–3–1 mT), but medium (1–25 mT) and strong (0.025–1 T) magnetic fields. A quantum cesium magnetometer based on the resolved structure of cesium atoms was created to transmit the unit of magnetic induction to the region of medium fields. The procedure for calculating the frequency conversion coefficients to magnetic induction of the created quantum cesium magnetometer is described. The uncertainty budget for reproducing a unit of magnetic induction of a constant field using the created complex is estimated.

Prospections non invasives sur le site de Mustis/Musti (El Krib) en Tunisie

The objective of the first step of the non-invasive survey carried out on the Mustis site described herein was to verify the possibility of locating archaeological remains and to detect the location of the supposed remains by geophysical measurements (magnetic and electric methods). Magnetic measurements were made with Geometrics G-858 Cesium magnetometer with two probes located on the same horizontal level at 0.5 m distance or on the same vertical level at 0.5 m and 0.75 m above the ground level. The instrument recorded the values of the total vector of the magnetic field strength and made it possible to calculate the pseudo-gradient of its components (horizontal or vertical). Electric measurements were made by means of axial dipole-dipole electrode configurations (parallel) with AB current electrodes with spacing of one metre and electrodes of potential MN (identical spacing) at equal distance D at 4 and 6 metres, which made it possible to record values of apparent resistivity of the subsoil, with the penetration depth of the current of c. 2.0 and 3.5 m, respectively, below the current ground level. At the time of the data interpretation, the suspected locations of the remains causing the anomalies were indicated by means of dashes of different colours and thicknesses (depending on the assumed depth of the structures). These indications, transferred on maps, can serve as a starting point for extensive analyses of the entire site and its surrounding area. The first non-invasive surveys carried out in Mustis have demonstrated the usefulness of magnetic and electric methods in mapping the preserved remains both inside and around the city. The obtained results enabled elaborating a strategy for the work to come. It is quite probable that the magnetic method will prove capable of indicating places where vestiges of constructions are preserved, while the electric method will determine the depth and conditions of deposition of localised structures.

Magnetometry of the Scythian burial ground Katerinovka in the Lower Dnieper region

The magnetometry of the Katerinovka burial ground (Nikopol district, Dnipropet-rovsk region, Ukraine) conducted on the area of 3.65 hectares showed three times more kurgans than they were visually detected or deciphered from satellite images. Archaeological excavations performed throughout the whole territory of geomagnetic survey area showed that most of the kurgans (24) belong to the Scythian epoch (IV century BC). There were also four burial mounds of the Golden Horde time (XIII—XIV centuries) and one — of the Bronze Age (beginning of the III millennium BC). Anomalies of magnetic induction (Bа) on the burial ground were mainly caused by objects deepened into the loess parent rock with magnetic susceptibility k = (0,3 ¸ 0,4) × 10–3 SI units and puddled with dark humus soil with k = (0.7 ¸ 0.9) × 10–3 SI units. The structural elements of the kurgans — pits, catacombs, ditches, and near-kurgan grooves — create weak (2—10 nT) anomalies of geomagnetic induction, which are reliably recorded using cesium magnetometers even under conditions of intense manmade interference from a heavy machinery in a quarry nearby. The fact of the existence of the near-kurgan grooves, from where the soil was taken to pour the Scythian mounds, was discovered for the first time from magnetometry results. The Bronze Age kurgan was identified by the semiring Bа anomaly from the groove 15—20 m wide. Among 10 burials, only two, the deepest ones, have created Bа anomalies with an intensity of 1.5—5.4 nT. Golden Horde kurgans were defined on the Bа map by weak ring anomalies from the ditches, as well as from the burnt bricks heaps covering some burials. The results of geomagnetic survey and archaeological excavations on the anomalies proved the inter-kurgan space of the visible mounds to be full of smaller sized and worse preserved kurgans. It has been shown that there is an urgent need to use high-resolution magnetometry at registering and studying the kurgan monuments of Ukraine, especially in areas of intensive industrial and agricultural land use.

Reduction of frequency-dependent light shifts in light-narrowing regimes: A study using effective master equations

Alkali-metal-vapor magnetometers, using coherent precession of polarized atomic spins for magnetic-field measurement, have become one of the most sensitive magnetic-field detectors. Their application areas range from practical uses such as NMR signal detection to fundamental physics research such as searches for permanent electric dipole moments. One of the main noise sources of atomic magnetometers comes from the light shift that depends on the frequency of the pump laser. In this work, we theoretically study the light shift, taking into account the relaxation due to the optical pumping and the collision between alkali-metal atoms and between alkali-metal atoms and the buffer gas. Starting from a full master equation containing both the ground and excited states, we adiabatically eliminate the excited states and obtain an effective master equation in the ground-state subspace that shows an intuitive picture and dramatically accelerates the numerical simulation. Solving this effective master equation, we find that in the light-narrowing regime, where the linewidth is reduced while the coherent precession signal is enhanced, the frequency dependence of the light shift is largely reduced, which agrees with experimental observations in cesium magnetometers. Since this effective master equation is general and is easily solved, it can be applied to an extensive parameter regime, and also to study other physical problems in alkali-metal-vapor magnetometers, such as heading errors.

Design and optimization of a high-sensitivity radio-optical cesium magnetometer

A high-sensitivity radio-optical cesium magnetometer is designed and implemented. By studying the dependencies of the performance of the radio-optical cesium magnetometer on some related parameters, the optimal ranges of these parameters are obtained, and the magnetometer performance is optimized and tested. The test results show that the designed radio-optical cesium magnetometer can achieve a magnetometer sensitivity of 82.5 fT/√Hz, a resolution within 1 pT to an external magnetic field change, and a noise fluctuation of 0.2 pT for an integration time of 1 s.

NEDM experiment at PSI: Data-taking strategy and sensitivity of the dataset

We report on the strategy used to optimize the sensitivity of our search for a neutron electric dipole moment at the Paul Scherrer Institute. Measurements were made upon ultracold neutrons stored within a single chamber at the heart of our apparatus. A mercury cohabiting magnetometer together with an array of cesium magnetometers were used to monitor the magnetic field, which was controlled and shaped by a series of precision field coils. In addition to details of the setup itself, we describe the chosen path to realize an appropriate balance between achieving the highest statistical sensitivity alongside the necessary control on systematic effects. The resulting irreducible sensitivity is better than 1 × 10−26e cm. This contribution summarizes in a single coherent picture the results of the most recent publications of the collaboration.

Ultra-Sensitive Atomic Magnetometers for Studying Spin Precessions of Hyperpolarized Noble Gases Based on System Identification

We reported an efficient approach to analyze spin precessions of different hyperpolarized noble gases with atomic magnetometers based on system identification. Spin evolution of the magnetometer can be robustly approximated as the second order system by referring to the Bloch equation under the negligible self-interaction and its transfer function model can be deduced. We measured the dynamic responses of different atomic pairs, such as cesium magnetometer with a partner of xenon-129, potassium magnetometer with a partner of helium-3, and hybrid of cesium and rubidium magnetometer with a partner of neon-21. By referring to the experimental results, corresponding systematic parameters for the transfer function models have been identified based on least-square method, whose outputs were consistent with experimental results. Based on the models, the system stabilities and their variations with different gains have been analyzed with control theory, both of which are described here. Applying control theory to analyze dynamic responses of spin ensembles plays an important role for the future applications.

Magnetic Imaging of Ultramafic Bodies on the Site of the Ohi Nuclear Power Station, Central Japan

The Ohi nuclear power station is located at the northern Oshima Peninsula in the Wakasa Bay on the coast of Japan Sea, central Japan. The geology of the site of the power station is composed mainly of shales, diabases, gabbros and ultramafic rocks of the Palaeozoic Yakuno Ophiolite. Ultramafic rock is a key geology since fracture zones in the study area can be found only in the ultramafic bodies. To map the distribution of ultramafic bodies, we conducted magnetic surveys on ground and at sea around Daibahama beach. A ground magnetic survey was carried out on a grid and along specified lines on a small peninsula and some reefs by using a proton magnetometer. A seaborne magnetic survey was also conducted by a small rubber boat on which a Cesium magnetometer was mounted. Both measured data were merged and IGRF residual magnetic anomalies were reduced onto a smoothed surface at an altitude of 2.5 m above ground and above sea level at sea assuming equivalent anomalies below the observation surface. 3D magnetic imaging has been applied to the magnetic anomalies and the resultant magnetic structure is generally associated with a dipping-dike by a previous 2D modelling. A reversely magnetized body was imaged with a seaward dip below the surface along the 2D profile but has a horizontal limitation. This means the magnetic imaging is helpful to reveal the three-dimensional subsurface structure of the area.