503 DR Hydroprobe Moisture Neutron Depth Gauge


The neutron probe is used by researchers, consultants, and large landowners with high cash crops for water conservation and to measure soil moisture. The radioactive device is expensive and its use requires an Atomic Energy Commission (AEC) permit. Also, the operator has to wear/use a radiation detection film badge to determine exposure levels which is monitored by the AEC.

The Model 503 DR Hydroprobe Neutron Depth Moisture Gauge manufactured by Campbell Pacific Nuclear International Inc. /Boart Longyear Corporation (CPN) uses 50mCi Americium-241-beryllium as the source of fast neutrons. The hydrogen atom has the same mass as the high energy neutrons and the majority of hydrogen in soil is associated with the soil water; therefore, when the fast neutrons collide with the hydrogen, the fast neutrons lose enough energy to become slow neutrons, and the slow neutrons rebound back towards the probe and are absorbed by the nucleus of the gas in the probe. Boron tri-fluoride or helium-3 gas can be used to absorb the rebounding slow neutrons. The 503 DR Hydroprobe uses the helium-3 gas. When the slow neutrons enter the nucleus of the gas, a higher energy state results, and emitted photons can then be detected as electrical pulses with an electronic counting device. There is a proportional number of thermalized neutrons related to the soil water content. See Theory Figure I.

Organic material in soils also have hydrogen associated with them. In soils with high organic material, the counting device will register the hydrogen from both sources. Also, the neutron probe user will relate the location of the majority of the organic material (ie. at the surface? or deep plowed under the surface?) to the counting device readings.

The number obtained from the counting device is considered the count ratio and is used in a calculation to obtain the soil moisture: water content (volume %) = (a) multiplied by the (count ratio) plus (b). The a and b coefficients must be determined for the specific soil type. This requires a lengthy process involving obtaining soil samples with a core sampler, calculating soil moisture before and after drying, and calculating bulk density so a linear function is obtained to normalize the calibrations. To learn more about calibrations click here calibration. At sites were research is ongoing, the calibration coefficients have been calculated and remain constant enough over time to make recalculation of coefficients unnecessary every time the probe is used. To learn more about calibration coefficients click here. coefficients

Stoppers on the cable determine the depth of the probe in the aluminum access tube. Always put a dummy probe of similar size down the tube to check for obstructions before lowering the neutron probe. Calibrate the probe before the daily use by obtaining a standard count and recording it for reference. To learn more about cable depth click here. cable

Bonnie Homola MS RD bhomola@nmsu.edu
and Haiying Dong MS hdong@nmsu.edu

Agronomy and Horticulture Dept.
Box 30003/Dept. 3Q
Las Cruces, NM 88003
PHONE: 505-646-3405
FAX: 505-646-6041

Special thanks and acknowledgements to Dr. Sorenson, Dr. Sammis, Dr. Frias-Ramirez, and Mr. Douglass Wilmer from CPN CORP.

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