The design purpose of the high neutron yield anode pulse circuit of the MZ-4 carbon-oxygen ratio energy spectrum logging instrument is to double the neutron yield of the original neutron generator. To achieve this goal, all parts of the neutron generator must work reliably. Changing the oscillator and monostable composed of CC4069 six-inverter that sometimes failed in the past to discrete component transistors has achieved good results in on-site logging in Daqing Oilfield. Since there is no problem with the anode pulse, the ion flow between the anode and cathode of the self-made target ceramic neutron tube ion source uses more than 600μA, and the target voltage can be increased to 125kV×250μA or 125kV×350μA. The detector is a sodium iodide crystal with a size of φ50mm×100mm and a source distance of 55cm. On the carbon energy region (3.17~4.65MeV) counting rate (TTLC) curve on the inelastic scattering gamma spectrum, the number of pulses per second increased from 1400 to 3500. Take out the self-made ceramic neutron tube and replace it with a commercial target neutron tube and use the same parameters to hit neutrons. The TTLC curve also reaches the same count rate, but it only takes half an hour and the count drops from high to zero. Maybe The target burned. However, there is no problem if the self-forming target ceramic neutron tube is used for a long time with high target pressure and large target flow. The circuit diagram and waveform diagram of the high neutron yield anode pulse of the MZ-4 carbon-oxygen ratio energy spectrum logging tool are shown in Figure 2-2-25.
After the MZ-4 carbon-oxygen ratio energy spectrum logging tool achieved high neutron yield, in order to further increase the neutron yield, while maintaining the neutron injection time width of the precursor unchanged, the neutron injection time was The neutron frequency increased from 10kHz to 20kHz. For this reason, the circuit in Figure 2-2-25 has been improved, as shown in Figure 2-2-26.
As can be seen from Figure 2-2-26, the oscillator consists of two CC4069 inverters, the frequency is increased from 10kHz to 20kHz, its period is 50μs, the neutron period is 50μs, and the detector output The period of the inelastic scattering gamma pulse concentration area is 50μs. Because it is an analog linear transmission, surface measurement requires underground instruments to transmit synchronization signals upward. After passing through thousands of meters of cables, the synchronization signal with a width of 5 μs is broadened to 20 to 30 μs. Affects the linear transmission of the detector output pulse on the cable. For this reason, the synchronization signal needs to be divided by sixteen and minus one, that is, neutrons are injected fifteen times and the synchronization signal is transmitted upward once. During the transmission of the synchronization signal, if neutrons are not injected, the detector will not generate or transmit inelastic scattering. , capture gamma pulses. The ground receives the synchronization signal, performs shaping, delaying, and then multiplies the frequency to 20 kHz, which is used as the trigger signal for the inelastic scattering door and capture door.
The 20kHz square wave is input to CC4024 and divided by sixteen. After the frequency division, it is input to the four inverters of CC4069 to form two monostable states. The first monostable state is delayed by 15μs. The second monostable forms a positive pulse with a width of 5μs and an amplitude of 12V, which is transmitted from the cable to the ground as a synchronization signal.
After the other channel is divided by sixteen, the two inverters input to the second chip of CC4069 form a monostable state of sixteen minus one, generating a positive pulse with a width of 8μs and an amplitude of 12V. , input to the monostable state composed of three NAND gates of CC4011, the positive pulse leading edge triggers, forming a negative pulse with a width greater than 50μs and an amplitude of 12V, input to the fourth NAND gate to subtract a positive pulse, that is, 20kHz Sixteen sorted first pulses. After dividing the sixteenth frequency by one and subtracting one, the 20kHz square wave is restored to a positive pulse by the inverter, and is input to a monostable composed of a transistor to generate a positive pulse with a width of 12μs and an amplitude of 12V, which is passed through the TD823 power amplifier and then input to the follower 3DD104E. Then it is input to the anode pulse generator - high-power transistor 3DD104E and transformer T. The secondary winding of transformer T generates a positive pulse with a width of 12μs and an amplitude of 2000V, which is added to the anode of the neutron tube ion source.
The high neutron yield improvement circuit of the MZ-4 carbon-oxygen ratio energy spectrum logging tool not only increases the neutron frequency to 20kHz, but also uses U-16 ferrite cores in the anode pulse transformer. The insulation withstand voltage between primary and secondary windings is 160kV. There is a tin or aluminum foil shielding layer between the two windings to improve the anode pulse waveform, reduce electromagnetic interference, and ensure that ground instruments can measure the gamma ray energy spectrum.