Lake Shore Model 475 Gaussmeter User’s Manual
7-4
Probes and Accessories
7.3.2
Radiation Effects on Gaussmeter Probes
The HST and HSE probes use a highly doped indium arsenide active material. The HST material is the more highly
doped of the two and therefore will be less affected by radiation. Some general information relating to highly doped
indium arsenide Hall generators is as follows:
• Gamma radiation seems to have little effect on the Hall generators.
• Proton radiation up to 10 Mrad causes sensitivity changes <0.5%.
• Neutron cumulative radiation (>0.1 MeV, 10
15
/sq. cm.) can cause a 3% to 5% decrease in sensitivity.
• In all cases the radiation effects seem to saturate and diminish with length of time exposed.
7.3.3
Probe Accuracy Considerations
The user must consider all the possible contributors to the accuracy of the reading. Both the probe and gaussmeter have
accuracy specifications that may impact the actual reading. The probe should be zeroed before making critical
measurements. The zero probe function is used to null (cancel) out the zero offset of the probe or small magnetic fields.
It is normally used in conjunction with the zero gauss chamber, but may also be used with an open probe (registering
Earth's local magnetic field). Users wishing to cancel out large magnetic fields should use the Relative function. Refer to
Paragraph 4.6.6.
Probe temperature can also affect readings. Refer to the two separate temperature coefficients listed on the specification
sheet. The High Stability (HST) probes exhibit a low temperature coefficient of gain due to the inherent thermal stability
of the materials used in their construction.
Probe readings are dependent on the angle of the sensor (Hall sensor) in relation to the magnetic field. Maximum output
occurs when the flux vector is perpendicular to the plane of the sensor. This is the condition that exists during factory
calibration. The greater the deviation from orthogonality (from right angles in either of three axes), the larger the error of
the reading. For example, a 5° variance on any one axis causes a 0.4% error, a 10° misalignment induces a 1.5% error,
etc. See Figure 7-1.
Tolerance of instrument, probe, and magnet must be considered for making critical measurements. The accuracy of the
gaussmeter reading is typically ±0.05% of reading and ±0.005% of range, but the absolute accuracy readings for
gaussmeters and Hall probes is a difficult specification to give, because all the variables of the measurement are difficult
to reproduce. Differences in alignment and positioning will degrade measurement accuracy and repeatability. Finally, the
best probes have an accuracy of ±0.10%. This implies that the absolute accuracy of a magnetic field measurement will
not reliably be better than ±0.10% of reading, and is likely to be 0.15% or higher.
Figure 7-1. Effect Of Angle On Measurements
