Neutron Probe (Troxler 3220 Series)

Troxler Electronic Laboratories. P.O. Box 12057 Research Triangle Park, NC., 27709 USA Telpehone 919 549-8661

The radio active source in this instrument is americium 241: beryllium from which fast neutrons are emitted. When the fast neutrons encounter hydrogen in the soil, they lose energy and are slowed down or thermalized. Most of the hydrogen in the soil is associated with soil water. The neutrons, having no electric charge, cannot be detected directly. Therefore, a kind of gas is needed, which can cause neutron absorption. Usually, boron triflouride or helium-3 is used. The slow neutrons enter the nucleus of the gas, the nucleus is raised to a high energy state and photons are emitted from the nucleus. The resulting photons can be easily detected using an electronic device. The electronic counting device is used to measure the number of thermalized neutrons which is proportional to the soil water content.

Use of neutron probe

II. Procedures of operation

1. Turn on the POWER/TIME switch to SLOW and place DISPLAY switch on STD COUNT.
Depress START button, and the gauge begins standard count. While standard count is
taken, be sure the probe is secured inside the gauge body.

2. A bell sound indicates that the standard count is over. Write down the value
on the display. The value will be remain until another standard count is taken
or the gauge is turned off.

3. Unlock the probe lock located on the top back of the gauge body to allow the probe to drop
into the access tube. Lower the probe into the desired depth using the stops on the
cable to control the depth..

4. Set the POWER/TIME switch on NORM and DISPLAY switch on COUNT RATIO, then
depress START. When the count is over, a bell sound will be heard.

5. If more than one depth is needed to measure in the same location, simply lower
the probe to the desired depth, and depress START again. The flashing on and off the
displayed value indicates that the instrument is accumulation another measurement.

6. Calculation of where to put the first stop or curb on the cable for 1st desired depth
(distance of first curb above the shield on the probe cable)=1st depth

c= depth of soil from surface where first measurement desired
a= length of source/ gauge body

b= length of access tube above soil surface

1st depth=A*.33+B-3.8cm+C
if c is 30.5 cm a=25.4 cm and b=15cm then
1st depth= 25.4*.33 +(15-3.8)+ 30.5=50cm

Calculation of soil moisture
     1. Calculation:     Water content ( vol. %)= a* count ratio + b
             For sandy soil, b= -2.6257;   a=46.9789 
       
            example 
             cr = 0.6
          water content=   46.97* 0.6 -2.62  = 25.56               

      2. The a and b coefficient must be determined for each soil type  by experimentation
          measuring the soil moisture at different water contents with the neutron probe
          and measuring it also by taking a soil sample. Weight the soil sample then dry it at 
          104 F for 24 hr and weight the sample again to calculate the dry weight moisture content
          The volume water content   determined by the neutron probe is also determined 
          from the dry weight content of the soil  times the apparent specific
          gravity of the soil or bulk density . The bulk density is determined by measuring
          the volume of dry soil and the dry weight of that volume.

Safety use of neutron probe

  Safety

The operator of neutron probe should wear a film badge which is a dose measurement device.  
If the operator has any doubt about use of neutron probe, call the Safety office on the  campus.
The probe should have a wipe test ever 3 months to check for leakage.
Transport requires securing the neutron probe in a trunk of a car or back of a pick up using a 
tie strap.You must have in the vehical a trasport certicifation . 
A wipe test must be conducted .
Specific safty requirenment are specified for people using the neutron probe at New Mexico State University.

Also, read the safty discussion by Troxler

tsammis@nmsu.edu
Last modified 01/15/96