JJ M Brouwer

NUCLEAR DENSITY PROBE   4.5
 
For many engineering purposes it is essential to obtain accurate
information on the in-situ density and porosity of the soil. This
requirement has resulted in the development of a variety probes that
contain a radioactive source and a detector that can be used in
conjunction with the CPT.

Types of probes
Essentially two types of probe are available:

• A probe that has the radioactive source mounted near the tip of the
  probe and which is inserted into the ground by conventional
  penetration techniques
  
• A probe that is inserted into the ground through a pre-formed hole.
     

In determining the soil density, the gamma radiation emitted by a
suitable source (normally caesium 137) is attenuated by either
absorption or back-scattering.
 
Compton scatter phenomenon
Most available probes have been designed on the Compton scatter
phenomenon. Compton scattering is proportional to the charge-tomass
ratio of the atoms of the substance, this being approximately
0.50 for carbon, oxygen and silicon, and approximately 1.0 for
hydrogen.
 
The amount of back-scattered radiation is related to density with the
rate of radioactive back-scatter increasing linearly with a decrease in
soil density. Readings of radioactive back-scatter are readily converted
to soil density using appropriate calibration curves.
 
Calibration
Calibration of the instrument can be carried out by lowering the probe
into fluids with known densities of between 1.0 and 2.2 Mg/m3. Typical
calibration fluids used would include water, bentonite and aluminium
oxide.
 
Alternatively, suitable materials could be compacted to a known density
in layers into rigid calibration chambers. The calibration chambers are
accurately weighed and the materials’ water content determined. This
exercise is repeated at least three times and the probe inserted into the
centre of the calibration chamber and readings taken to enable a
calibration curve to be determined. A calibration chamber diameter of
1.0 m is recommended since this size should ensure that the sides of
the rigid chamber are beyond the normal range of the probe.
 
With the ever increasing environmental and legislative concerns relating
to nuclear devices, the potential for the penetration probe to suffer
damage – together with a loss of integrity of the source encapsulation
with associated ground contamination – expensive remediation is high.
Consequently these probes have lost favour as a prime method of
investigation.