This invention relates to pulsed neutron logging systems and more particularly to a method and apparatus for controlling the operation of an accelerator-type pulsed neutron source.
Accelerator-type pulsed neutron sources are employed in many applications. A well-known application is in radioactivity logging of wells penetrating subterranean formation. For example, in the art of radioactive assay well logging, an assay tool is lowered into the well to the level of a formation to be assayed. The assay operation is then carried out by cyclically operating a neutron source in the tool in order to irradiate the formation with repetitive bursts of neutrons. In one assay procedure disclosed in U.S. Pat. No. 3,686,503 to Givens et al, the time between each neutron burst is sufficient to allow the neutrons from the source to disappear and to allow delayed fission neutrons emitted by uranium within the formation to arrive at and be detected by a neutron detector. Another procedure, disclosed in U.S. Pat. No. 4,180,730 to Givens et al, involves the detection of prompt fission neutrons emitted from uranium in the formation. In this procedure both thermal and epithermal neutron fluxes are detected at time intervals within 50 to several hundred microseconds subsequent to each neutron burst. In this case, the neutron source may be operated at a significantly higher rate, typically on the order of one or two thousand neutron bursts per second.
A pulsed neutron generator for systems such as those disclosed in the above-mentioned patents to Givens et al commonly take the form of a three-element, linear accelerator tube. This tube includes a replenisher element which is electrically heated to boil off deuterium gas adsorbed by the filament. The deuterium molecules are ionized by an ionizing section which commonly includes plates to which a positive ionization pulse is applied. The deuterium ions are then accelerated and bombard a target which included tritium molecules. The bombardment of the deuterium ions on the tritium molecules yields helium plus a supply of neutrons. One commercially available tube which is capable of such operation is the Kaman Nuclear Model A-801 Neutron Generator.
In operating such a tube it is important that the power supplied to the replenisher be correctly adjusted so that the proper amount of accelerator gas, deuterium, as described above, boils off the replenisher element. If the replenisher is overheated, too much accelerator gas boils off. In this case, ion recombination takes place in the tube. Also, arcing in the tube shortens the tube life and neutron output falls off. If too little power is supplied to the replenisher, there is not enough accelerator gas available in the tube to provide a good neutron output.
The adjustment of the power supply to the replenisher is complicated by the fact that the characteristics of the tube change as the tube ages. That is, after the tube has been in use, a greater amount of power must be supplied to the replenisher to boil off the same amount of accelerator gas. U.S. Pat. No. 3,719,827 to Charles L. Dennis describes a system in which the power supply to the replenisher element in a linear accelerator tube is automatically controlled. In this system, the time duration of the ionization pulse is compared to a reference pulse, and a control signal generated. The control signal is applied to a stepping motor. Each time the accelerator tube is ionized, the motor is advanced in one direction or the other, depending upon the comparison of the ionization pulse to the reference pulse. This motor increments a variable autotransformer which supplies power to the replenisher. In this manner the replenisher power is adjusted to supply the correct amount of accelerator gas to the tube.
U.S. Pat. No. 3,984,694 to Dennis describes another system for adjusting the power supply to the replenisher section of an accelerator-type neutron tube. In this system, first and second reference pulses are generated in response to the ionization pulse in order to delineate a time window within which acceptable operation of the tube is achieved. When the ionization pulse falls outside of the time window, the power supply is increased or decreased as necessary; for example, by operating a stepping motor to drive a variable autotransformer applying power to the replenisher as described above.