Neutron dispersion technology
Hydrogen has the unique ability to absorb fast neutron energy much more than other elements. When any substance containing hydrogen is irradiated with fast neutrons, hydrogen atoms absorb their energy and create thermal neutrons (low energy neutrons). The number of neutrons slowed down for thermal energy is directly proportional to the concentration of hydrogen atoms, which means that the density of the substance.
Approximately the same weight of hydrogen nuclei and neutrons is the reason for the backscattering of neutrons, the phenomenon of which is the basis for determining hydrogen-containing substances. As an illustration of the mechanism of the effect of neutrons on hydrogen nuclei, one can consider a bowling ball (in which nuclei of atoms other than hydrogen are present, but other elements), a red ball (hydrogen nuclei) and a white ball (neutrons).
If the white ball “shoots” at a fixed bowling ball, then the white ball will jump over the bowling ball without moving it. The big difference in mass will not allow any significant amount of energy to be transferred to this ball from the white ball.
On the other hand, if the white ball is “fired” on a fixed red ball, the red ball will be displaced in the direction of the force applied by the white ball. This will transfer energy.
After several such strikes, the energy of the white ball or neutron will decrease to a level called the thermal range. Having reached the range corresponding to thermal energy, the neutron can be captured and thus can be detected by a thermal neutron detector. These detectors can only capture or detect thermal neutrons and are not able to detect the presence of fast neutrons emitted by a source.
The source, emitting a stream of slow neutrons, and special gas-filled ionization chambers with a large collection area, are located inside one compact body mounted on the outer side of the coke drum wall. In this position, fast neutrons emitted by the source easily penetrate the steel grid of the vessel.
Any hydrogen-containing substance, such as gas, foam, coke, or water that is no more than 18 inches from the inside of the wall where the sensor is located, will absorb neutron energy in proportion to its density. When the neutron energy is reduced by the hydrogen present in the substance being treated to a warm level, they are backscattered (reflected) through the vessel wall to the detectors. These scattered neutrons, slowed down to thermal energy, are collected by detectors in which current is generated.
The current created by the detector is amplified by a computer-based transmitter, and is scaled to produce a 4–20 mA current signal proportional to the density of the substance in the drum. The output 4–20 mA signal can be used by an alarm system, sent to a recording device or a distributed control system.
Knowing the relationship between the concentration of hydrocarbon molecules present in a substance and the amount of scattered neutrons, the device is able to distinguish gas from foam, coke, or quenching (cooling) water. Monitoring the magnitude of the output signal of the system allows the operator to obtain information not only about the moment when the substance to be treated reaches the detector level, but also about what substance is at this level.
The efficiency of capture of thermal neutrons by detectors is close to 100%. In addition to the high capture rate, the thermal neutron capture mechanism leads to the generation of a signal that is 1000 times larger than that produced by a gamma detector in a similar case. Therefore, a system based on neutron backscattering allows measurements to be made with higher resolution, with a lower value of the time constant, less statistical noise and a weaker radioactive source compared to systems using gamma radiation.