THE MEASUREMENT OF SOIL MOISTURE AND DENSITY BY NEUTRON AND GAMMA-RAY SCATTERING

A BRIEF REVIEW IS PRESENTED OF METHODS OF MEASURING SOIL MOISTURE CONTENT. THE NEUTRON METHOD OF COUNTING HYDROGEN NUCLEI SEEMS TO HOLD CONSIDERABLE PROMISE BECAUSE THE NEUTRONS DETECT THE PRESENCE OF WATER REGARDLESS OF STATE SO THAT WHEATHER THE WATER WAS IN THE FORM OF LIQUID, VAPOR, OR SOLID, OBSERVATIONS AT ANY TIME OF THE YEAR ARE POSSIBLE. ONE OF THE MOST IMPORTANT LAWS GOVERNING RADIOACTIVITY IS THE RELATIONSHIP BETWEEN THE EXTENT OF GAMMA-RAY PENETRATION IN SOIL AND ITS IN-PLACE DENSITY. THE PHYSICAL PHENOMENA THAT FORM THE BASIS OF THE NEW METHOD FOR THE DETERMINATION OF MOISTURE CONTENT AND DENSITY OF SOILS ARE THE SCATTERING OF NEUTRONS OR GAMMA RAYS AND THE LOSS IN THEIR ENERGY DURING THIS PROCESS. A DEVICE FOR MEASURING MOISTURE CONTENT CONSISTS OF A SOURCE OF FAST NEUTRONS, SUCH AS A MIXTURE OF RADIUM AND BERYLLIUM OR POLONIUM AND BERYLLIUM, A DETECTOR FOR SLOW NEUTRONS, RIGIDLY PLACED NEAR THE SOURCE A SILVER OR RHODIUM FOIL OR A BORON COUNTER, AND A ELECTRONIC DEVICE FOR COUNTING AND RECORDING THE NUMBER OF SLOW NEUTRONS REACHING THE DETECTOR PER SECOND. THE APPARATUS FOR THE DETERMINATION OF THE DENSITY CONSISTS OF A GAMMA-RAY SOURCE, AS A WEAK RADIUM PREPATATION, AND A DETECTOR FOR GAMMA RAYS, AS A GEIGER COUNTER MOUNTED AT A WELL-DEFINED DISTANCE FROM THE SOURCE, AND SHIELDED FROM THE SOURCE BY LEAD IN ORDER TO PREVENT DIRECT GAMMA RAYS FROM THE SOURCE REACHING THE DETECTOR. BOTH ASSEMBLIES CAN BE LOWERED IN A METAL TUBE PLACED IN THE SOIL AND CONNECTED BY CABLE TO AN ELECTRONIC-COUNT-RATE METER. THE NEUTRON-SCATTERING METHOD PROVES TO BE A RELIABLE METHOD FOR DETERMING, WITHOUT DISTURBING THE SOIL MASS, THE MOISTURE CONTENT. BY USING A POLONIUM-BERYLLIUM SOURCE, READINGS CAN BE CONTINOUSLY AND AUTOMATICALLY RECORDED AND TELEMETERED FROM THE TEST SITE TO A CENTRAL STATION. THE GAMMA-RAY SCATTERING METHOD PROVES TO BE A RELIABLE METHOD FOR DETERMINING DENSITY WITHOUT DISTURBING THE SOIL MASS. BOTH METHODS HAVE PROVEN THEMSELVES IN LABORATORY AND FIELD TESTS AND PROMISE TO BE SATISFACTORY AND USEFUL TOOLS FOR THE MEASUREMENT AND CONTINOUS RECORDINGS OF MOISTURE CONTENT AND DENSITY OF SOIL.