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Overhauser Magnetometer

Instrument Details:- Make : GEM SYSTEMS, Canada, Model : GSM 90
Specification:- Sensitivity: 0.022 nT / √ Hz, Resolution: 0.01 nT (gamma), 
Absolute Accuracy: 0.2 nT, Dynamic range: 20,000 - 120,000 nT, 
Long term stability: <0.05 nT/year, Sampling - Model GSM-90F1: 1 sample / 1 sec 
Sensor size: 70mm dia. X 150mm, Power: 12V 200mA maximum,40mA average, 
RS232C parameters: programmable

The GSM-90 is a scalar magnetometer of high absolute accuracy (0.2nT) and low long term drift (0.05nT / year). It is optimized for use in magnetic observatories. 

Working Principle: 
Overhauser sensor has a free radical added in contrast to standard proton magnetometer sensors which only use a proton-rich liquid to produce precession signals. The free radical contributes free, un-bound electrons that couple with protons producing a two-spin system. A strong RF magnetic field is used to disturb the electron-proton coupling. By saturating free electron resonance (ESR) lines, the polarization of protons in the sensor liquid is strongly increased. Therefore, the Overhauser Effect offers a superior method of proton polarization delivering stronger signals from smaller Sensors and with less power (i.e. 2Ws per reading or about 0.5W average power consumption for 1 reading per 5 seconds) .
Applications :
For continuous geomagnetic field measurements, magnetic field surveys, to detect magnetic anomalies due to ferromagnetic objects in the environment, to calibrate low field magnetometers.