Patent GB-1193003-A (GB)
Title: Improvements in or relating to Devices for Measuring the Moisture Content of a Volume of Material
IPC: G01N23/09
Filed: 1968-03-18
Abstract: 1,193,003. Detecting neutrons. ELECTRICITE DE FRANCE SERVICE NATIONAL. 18 March, 1968 [23 March, 1967], No. 13054/68. Addition to 1,125,017. Heading G6P. [Also in Division G1] A modification of the moisture content measuring apparatus of Specification 1,125,017, which also includes a density measuring device, involves the containing of the neutron detector (for the moisture content measurement) and the gamma-ray detector (for the density measurement) in the same scattering and slowing down medium. This increases the accuracy of the overall measurements of moisture content by weight since the volume of material under investigation is rendered the same for the density measurement as for the moisture content measurement. Further accuracy in the density measurement itself is obtained by use of a shield surrounding the scattering medium to absorb γ-rays of low energy caused by scattering in the material under investigation. In a described embodiment the constructional set-up is similar to that of the above mentioned Patent, with a radio-active source (2) Fig. 1 (not shown), of Caesium 137 and a mixture of Americium Beryllium mounted at the end of probe 6, directing neutrons and γ-rays through the material under test e.g. soil to two casings (7 and 8). Each includes a detector arrangement of Fig. 2, comprising a cylindrical block of scattering medium 15 in which are contained two gamma-ray detectors 13, 14 spaced from the cylinder axis, and the neutron detector 12. The medium 15 can be liquid, solid or gaseous medium of high hydrogen content e.g. water, paraffin, saturated hydrocarbons such as hexane, plastics materials etc, or hydrogen gas under pressure could itself be used. The medium 15 is surrounded by two shields. One shield 16 is of a material such as lead, uranium, tungsten or zinc etc., which has a high photo-electric cross-section for low-energy gamma radiation, and thus prevents transmission of γ-rays scattered by the soil. The γ-rays passing through the shield being those transmitted directly by the soil, are scattered in the medium 15 and thereby directed towards the γ-detectors 13, 14. The medium 15 should be free of elements such as iodine, cadmium, cobalt, boron lithium and beryllium capable of capturing neutrons and producing extraneous γ-radiation or of elements with a high absorption cross-section which would reduce the detector sensitivity. The other shield 17 is of cadmium, indium, etc., and has a high absorption cross-section for neutrons. This shield, as in the case of the above mentioned patent, reverses thermal neutrons caused by scattering in the soil and passes only directly transmitted neutrons. The medium 15 has a slowing down action on these neutrons which can increase the sensitivity curve of the detector 12, which may be of the helium or boron fluoride type. The γ-detectors may be ionization chambers, geiger counters, scintillation counters or junction devices. The γ-ray shield 16 is placed between medium 15 and shield 17 to prevent partially the transmission of γ-rays produced on absorption by shield 16 of thermal neutrons. A further feature of the apparatus is that background γ-radiation is eliminated. In alternative arrangements, the number of detectors may be changed. The source and detector arrangements may be in two separate probes or externally positioned with respect to the volume of material under test. The apparatus may be used for measurement of moisture content of seeds and other bulk materials.