Neutrons density

Eig. 2. Radiation-induced dimensional changes in isotropic graphite at various temperatures, nvt = neutron(density)-velocity-time. 

MWD Technology 901. Directional Drilling Parameters 954. Safety Parameters 961. LWD Technology 971. Gamma and Ray Logs 971. Resistivity Logs 974. Neutron-Density Logs 985. 

A problem with the early MWD mud pulse systems was the very slow rate of data transmission. Several minutes were needed to transmit one set of directional data. Anadrill working with a Mobil patent [100] developed in the early 1980s a continuous wave system with a much faster data rate. It became possible to transmit many more drilling data, and also to transmit logging data making LWD possible. Today, as many as 16 parameters can be transmitted in 16 s. The dream of the early pioneers has been more than fulfilled since azimuth, inclination, tool face, downhole weight-on-bit, downhole torque, shocks, caliper, resistivity, gamma ray, neutron, density, Pe, sonic and more can be transmitted in realtime to the rig floor and the main office. 

The physics of the measurements made by the MWD neutron-density tools are similar to those of corresponding wireline sondes. A sketch of principle of the Anadrill tool is shown in Figure 4-281. 

Figure 4-285. Radioactive sources being installed in the neutron-density sub. (Courtesy Anadrill [113].)...

Hydrocarbon/water contact at 9750 ft with curve. Oil to 9696 ft. Gas above with neutron density. 

A typical MWD bottomhole assembly used in rotary drilling is as follows from bottom to top drill bit, stabilizer, resistivity, WOB torque, directional and telemetry system, neutron-density Pe. The typical distances are seen in Figure 4-296a. When a mud motor is inserted between the lower stabilizer and the drill bit, the distances are increased as shown in Figure 4-296b. 

The true porosity <1> is determined with the neutron-density Pe logs. R is generally given by the deep investigation resistivity curve. R equals R, in the water formations. It increases rapidly in hydrocarbon saturated formations. 

Figure 4-301 shows sample neutron-density Pe log over the same interval. Figure 4-302 shows sample MWD resistivity (left) and wireline dual induction (right) for the same interval. 

With the availability of the MWD/LWD drilling parameters, gamma ray resistivity, neutron-density Pe, a global approach of interpretation has been implemented by Anadrill when all measurements have been made in a given strata. A particular strata is first analyzed with the drilling data for a pressure at the bit estimate. Then, it is reanalyzed later when gamma ray resistivity data... 

The origin of chemical elements has been explained by various nuclear synthesis routes, such as hydrogen or helium burning, and a-, e-, s-, r-, p- and x-processes. "Tc is believed to be synthesized by the s (slow)-process in stars. This process involves successive neutron capture and / decay at relatively low neutron densities neutron capture rates in this process are slow as compared to /1-decay rates. The nuclides near the -stability line are formed from the iron group to bismuth. 

Fig. 6.1. Part of the s-process path, showing some s-only nuclei (marked s ) and some branchings between n-capture and p -decay (shaded boxes), which give an idea of relevant neutron densities and temperatures. After Kappeler, Beer and Wisshak (1989). Copyright by IOP Publishing Ltd. Courtesy Franz Kappeler.

More detailed considerations (Kappeler, Beer Wisshak 1989) lead to a temperature range of up to 4 x 108K and neutron densities up to about 10s cm-3. 

Fig. 6.5. Development of the convective region, neutron density from 13C and 22Ne sources and maximum temperature as functions of time during a thermal pulse in a low-mass star with Z Z0/3, which seems to give the best fit to Solar-System abundances from the main s-process. However, more recent models imply that 13C is all used up in the radiative phases. After Kappeler et al. (1990). Courtesy Maurizio Busso and Claudia Raiteri.

A possible path for the r-process is shown in Fig. 6.9. The iron seed nuclei need to capture many neutrons leading to unstable nuclei with very short /f-decay half-lives, demanding a high neutron density ... 

Such neutron densities are presumably associated with very high temperatures, leading to reverse (y, n) reactions. At each Z, neutrons are added up to a so-called waiting point (which defines the r-process path) in the waiting-point approximation , (n, y) and (y, n) reactions balance, so that for given Z, there is a Saha-type equilibrium... 

Anti-protonic atoms. Recently neutron density distributions in a series of nuclei were deduced from anti-protonic atoms [30], The basic method determines the ratio of neutron and proton distributions at large differences by means of a measurement of the annihilation products which indicates whether the antiproton was captured on a neutron or a proton. In the analysis two assumptions are made. First a best fit value for the ratio I / of the imaginary parts of the free space pp and pn scattering lengths equal to unity is adopted. Secondly in order to reduce the density ratio at the annihilation side to a a ratio of rms radii a two-parameter Fermi distribution is assumed. The model dependence introduced by these assumptions is difficult to judge. Since a large number of nuclei have been measured one may argue that the value of Rj is fixed empirically. 

The s process builds up an abundance distribution with peaks at mass numbers (A = Z + N) 87,138 and 208 and pronounced even-odd imbalance. The main component of the s process is associated with thermal pulsations of stars in the asymptotic giant branch (1-3 Mq) which produce neutron densities between 10 and 10 cm (Fig. 5.6). 

The neutron capture rate in the AGB convective shell is primarily dependent upon the neutron density Nn. Higher neutron densities tend to build up the neutron-rich isotopes. The /T decay rate of an unstable isotope can, in some instances, be sensitive to the temperature at the base of the convective shell, Tcsb. Higher temperatures mean greater excitation of low-lying nuclear levels from which /3 decay may proceed much more rapidly than from the nuclear ground state. 

If the time scale of neutron capture reactions is very much less than 3 -decay lifetimes, then rapid neutron capture or the r process occurs. For r-process nucleosynthesis, one needs large neutron densities, 1028/m3, which lead to capture times of the order of fractions of a second. The astrophysical environment where such processes can occur is now thought to be in supernovas. In the r process, a large number of sequential captures will occur until the process is terminated by neutron emission or, in the case of the heavy elements, fission or (3-delayed fission. The lighter seed nuclei capture neutrons until they reach the point where (3 -decay lifetimes have... 

To summarise, the first method involves a fixed wavelength and a variable 0 while the second involves a fixed 0 and a variable X. In either case the purpose is to find the conditions for Bragg reflection. The advantage of the second method is that higher neutron densities can be obtained and faster experiments and higher levels of resolution can be achieved. 

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