Neutron, pulsed

Hydrocarbon-water contact movement in the reservoir may be determined from the open hole logs of new wells drilled after the beginning of production, or from a thermal decay time (TDT) log run in an existing cased production well. The TDT is able to differentiate between hydrocarbons and saline water by measuring the thermal decay time of neutrons pulsed into the formation from a source in the tool. By running the TDT tool in the same well at intervals of say one or two years (time lapse TDTs), the rate of movement of the hydrocarbon-water contact can be tracked. This is useful in determining the displacement in the reservoir, as well as the encroachment of an aquifer. 

The discriminator excludes passage of pulses that are less than a predetermined level. The function of the discriminator is to exclude noise and gamma pulses that are lower in magnitude than neutron pulses. 

High energy neutron generators are particularly useful in well logging applications. In such applications one important factor is accurate knowledge of the neutron pulses that irradiate the surrounding formation. For example, it is desirable to accurately measure the neutron output, e.g., the number of neutrons emitted by the neutron detector. 

L. Avrami et al, Neutron Pulse Irradiation of Electroexplosive Devices , Defense. Nuclear Agency Rept DNA 4032P, Vol 3, paper 5.4, pp 133-52, Proc 5th Symp, Nuclear Survivability of Propulsion and Ordnance Systems (1975) (SRD) (paper unclassified)... 

The residence time was determined for our neutron counter by measuring the time intervals between beta start signals and neutron stop signals. With a residence half-time of 11 ms and a coincidence resolving time of 40 ms. 92 of the true coincidence events were included. The fraction of true events not detected does not influence the present results because we normalize the Pn measurements to a known Pn value measured under identical conditions. The coincidence rate was measured by a simple overlap coincidence module where the beta pulse Input was stretched to 40 ms by a gate and delay generator. To measure the accidental coincidence rate, the same beta pulse was sent to a second coincidence module and overlapped with neutron pulses which had been delayed 45 ms. After correcting each coincidence rate for deadtime effects, the difference was the true coincidence rate. 

Some coal analyzers use several gamma-ray detectors and operate by the use of neutron pulses. Such analyzers can measure the density and sulfur content of coal along with its heating value, moisture, and volatile matter content. This pulsed fast/thermal neutron analyzer can be self-calibrating and can determine such elements as carbon, oxygen, and sodium. 

In a time-of-flight spectrometer (TOP spectrometer), the energy of the incident neutrons is determined, for example also by diffraction from a monochromator crystal M, but instead of a continuous neutron beam, a neutron pulse or a series of pulses is used, produced by periodic interruption of the neutron beam by a so-called chopper. The energy of the scattered neutron wave then is found from the velocity of the scattered neutrons, which is calculated from the distance and the measured time of flight of the neutrons between the sample crystal and the detector. 

After a short neutron pulse, the prompt neutrons are assumed to decay, asymptotically, as follows ... 

The pulse reactors have neutron spectra, analogous to electron LINACs but one order of magnitude wider thermal neutrons pulse. The best machine in this class is the reactor IBR-2 [14]. Its peak power is Fmax = 8300 MW and average power P 4 MW. The FWHM of thermal neutron pulse is 150/is, a value which satisfies the requirements of noncrystalline solids diffraction studies. 

Fast burst reactor (FBR) Produces neutron pulses about 0.1-1 msec in width, by rapid mechanical assembly of the critical mass in a neutron-free environment. 

A. Velocity distribution following a fast neutron pulse. The physical processes occurring in neutron thermalization are best illustrated by considering the velocity distribution of neutrons as a function of time following a burst of fast neutrons at = 0. We consider an infinite homogeneous medium at a uniform temperature T with an absorption cross section varying inversely as the neutron velocity. The linearized Boltzmann equation describing the neutron distribution in velocity and time is... 

T. F. Wimett and J. D. Orndoff, Applications of Oodiva II, Neutron Pulses, Proceedings of the Second International Congress, vol. 10, Research reactors, 1958, pp. 449-460. 

The energy of the neutrons from a spallation source is selected using time of flight techniques, while a chopper or a crystal monochromator is used to select the neutron energy for neutrons from a reactor. For a reactor, the double chopper inelastic spectrometer is used while for the pulsed spallation source, one chopper is used for matching the neutron pulse duration to that from the moderator. 

Answer Hie reactions are such that neutron-induced pulses are considerably larger than those resulting from gamma rays and as llflers are so arranged that the Icirger neutron pulses are eu cepted and passed on to a scaler whereas the gamma rays are rejected. 

Gamma, thermal neutron, and fast neutron pulses are of different size. The instrument weights the different types of radiation according to the biological... 

Gamma rays are eliminated by a discriminator. Fast neutron pulses are much larger than those from thermal neutrons, so the total response is proportional to the biological damage caused. 

Gamma-induced detector pulses are of insufficient width to generate a significant log-level amplifier output. Neutron pulses are the only ones with sufficient width. No special compensation is necessary. 

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