Field Plot Sampling - Neutron Scattering Technique 



A recently developed method to obtain ttie soil moisture 

 data, referred to as the neutron scattering technique. Is based 

 upon the principle that high energy or "fast" neutrons are mod- 

 erated, or "slowed down," in soils almost exclusively by hydrogen 

 atoms contained in soil moisture. The instrument consists of a 

 source of "fast" neutrons, a detector tube which Is sensitive 

 only to "slow" neutrons, and a slow neutron counter. Both source 

 and detector are combined in a cylindrical probe 1.5 Inches in 

 diameter by l4 Inches long. The probe is lowered Into the soil 

 through a small-diameter, cased hole to the desired depth, sus- 

 pended by its electrical cable. The cable is connected to the 

 counting device which counts pulses produced by slow neutrons 

 returning to the detector. Since the "fast" neutron output of 

 the source is essentially constant, the count recorded in a fixed 

 time period may be used with a suitable calibration to determine 

 the moisture content In the soil surrounding the probe. 



The neutron scattering technique has certain advantages 

 over the gravimetric technique. In addition to the ease of making 

 deeper measurements, the neutron measurements take less time, re- 

 peatedly represent approximately the same soil mass, and are gen- 

 erally more precise than gravimetric measurements. Measurement of 

 the same soil mass is particularly important, since soil moisture 

 distribution and extraction patterns appear to be nonuniform. It 

 must be noted, however, that overirrlgation and resulting moisture 

 movement remain a problem with this method. Also, for greater 



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