Nucleus fission

State whether the following statements are true or false. If false, explain why. (a) The dose equivalent is lower than the actual dose of radiation because it takes into account the differential effects of different types of radiation, (b) Exposure to 1 X 1 ()x Bq of radiation would be much more hazardous than exposure to 10 Ci of radiation, (c) Spontaneous radioactive decay follows first-order kinetics, (d) Fissile nuclei can undergo fission when struck with slow neutrons, whereas fast neutrons are required to split fissionable nuclei. 

These fissioning nuclei (such as 8tP°i2-211> formed by reaction of Bi209 and a deuteron) have a nearly spherical normal-state structure, resembling that of the doubly magic nucleus seP m208, with an outer core of 16 spherons and an inner core of 4 spherons, shown in Fig. 6. The nucleus is excited, with vibrational energy about 25 Mev (for bismuth bombard-... 

The greater stability of the core than of the mantle requires that fission occur along a plane between layers of the core. The number of layers is odd (five) accordingly the fission is not symmetric, as for the lighter fissionable nuclei (with four layers in the core), but is asymmetric. 

The product nuclei as initially formed are highly unstable isotopes and emit delayed neutrons as well as electrons and gamma photons while settling down into their stable configurations, which ate usually isotopes of different elements from those first formed. The neutrons, both prompt and delayed, continue the reaction by encountering other fissionable nuclei... 

The element uranium is the element used for almost all fission processes. It has two natural isotopes. One of them is 238CI which, constitutes 99.3% of uranium ore, and the other is 235CJ, which constitutes 0.7% of uranium ore. Fissionable nuclei such as 235CJ and 239Pu are called fissile. Nuclear fission reactions occur... 

Nuclei with closed shells of nucleons are stabilized, particularly against spontaneous fission. Nuclei where the numbers of protons and neutrons are both magic numbers ( doublemagic nuclei) are particularly stable, such as He, Ca, and ° Pb. After ° Pb, the... 

The mass distribution obtained by fission of and with thermal neutrons (Fig. 8.14) is similar to that observed for Whereas the maximum for heavy fission products is nearly at the same place in the case of and Pu, the maximum for light fission products is shifted to the right in the case of Pu. This tendency continues with increasing mass of the fissioning nuclei, and in thermal-neutron fission of Fm the two maxima merge into one another. 

Fission by thermal neutrons proceeds also via a double-humped barrier as in spontaneous fission (Fig. 5.19). The excitation energy acquired by the uptake of an additional neutron enables easily fissionable nuclei like and Pu to sur-... 

MeV) and of prompt y-ray photons. The number of prompt neutrons emitted by the primary fission fragments depends mainly on their excitation energy. It increases with the mass number of the fissioning nuclei (Table 8.2). In Fig. 8.17 this number is plotted as a function of the mass of the fission fragments. It is relatively low for fragments with filled neutron shells N = 50, = 82). 

The fuel elements in a reactor core consist of cylindrical pellets about 0.6 in (1.5 cm) thick and 0.4 in (1.0 cm) in diameter. These pellets are stacked one on top of another in a hollow cylindrical tube known as the fuel rod and then inserted into the reactor core. Fuel rods tend to be about 12 ft (3.7 m) long and about 0.5 in (1.3 cm) in diameter. They are arranged in a grid pattern containing more than 200 rods each at the center of the reactor. The materials that fuel these pellets are made of must be replaced on a regular basis as the proportion of fissionable nuclei within them decreases. 

Fissionable nuclei are nuclei that can undergo induced fission. 

Fissile nuclei are fissionable nuclei that can undergo induced fission with slow-moving neutrons. Examples of fissile nuclei ... 

We harness the energy of nuclear fission, much of which appears as heat, by means of a chain reaction, illustrated in Figure 23.14 the two to three neutrons that are released by the fission of one nucleus collide with other fissionable nuclei and cause them to split, releasing more neutrons, which then collide with other nuclei, and so on, in a self-sustaining process. In this manner, the energy released increases rapidly because each fission event in a chain reaction releases two to three times as much energy as the preceding one. 

The most probable charge Zp for the mass chains 128 and 130 for the various fissioning nuclei may be calculated using either the Glendenin or Pappas equation. 

In nuclear reactors the fission rate is controlled to generate a constant power. The reactor core consists of fuel elements containing fissionable nuclei, control rods, a moderator, and a primary coolant. A nuclear power plant resembles a conventional power plant except that the reactor core replaces the fuel burner. There is concern about the disposal of highly radioactive nuclear wastes that are generated in nuclear power plants. 

The probability that a neutron will trigger fission of a nucleus depends on the speed of the neutron. The neutrons produced by fission have high speeds (typically in excess of 10,000 km/s). The function of the moderator is to slow down the neutrons (to speeds of a few kilometers per second) so that they can be captured more readily by the fissionable nuclei. The moderator is typically either water or graphite. 

Spontaneous fission Nuclei f fission products Mainly for very high atomic numbers (atomic mass >240)... 

We assume throughout that the resonances are so widely spaced and so narrow that the absorption a a can be assumed to be zero between resonances, so that there is also no interference between different resonances. This excludes, of course, the treatment of fissionable nuclei. These assumptions are not essential in the treatment proposed in 4. An estimate of interference effects was recently given by Schermer and Corngold [7] using squareshaped line forms. We also do not include the Ijv part of the absorption, since this is best treated by modifications of the effective thermal absorption cross sections. 

The second possibility (Weinstein et al. 1993, 1994) is based on adding small amounts of fissionable nuclei, such as to the superconductor prior to processing, and to expose the final product to a beam of thermal neutrons, thus inducing fission of the added uranium and creating fission tracks in much the same wry as discussed in the last paragraph. Thermal neutrons have a very large penetration range in all superconductors... 

The fact that the lines have the same slope for various fissioning nuclei (from to Cf) shows that not only the two spherical parts of the dumb-bell configuration are unchanged but also the neck diameter remains the same. Certainly, the neck for the heavier system ( Cf) will be slightly longer and neutron emission will reach further into the mass region near A = 120 and 160 (see O Fig. 4.19). 

It is interesting to note that the spectra of ternary particles extend to different atomic numbers, which coincide nearly with the size of the neck that results from the postulate extracted from the mass yield and nuclear charge distributions. To correlate the data, it has been postulated that the two spheres of the dumbbell configuration shown in O Fig. 4.11 for the fission of are practically the same for all asymmetrically fissioning nuclei from Z= 90 to about 99 and, consequently, that the variation in the neutron/proton numbers of the different compound nuclei must be connected with the size of the neck. O Fig. 4.29 is the direct experimental proof for this assumption in the fission of uranium, the neck size is (92 — 82 =) 10 protons in the fission of californium, the neck size is (98 — 82 =) 16 protons. The situation is similar for neutrons and for the total mass. This is, however, less convincing due to prompt neutron emission. 

In the seven decades since the discovery of nuclear fission, experimental studies on low-energy fission have been restricted to about 80 fissionable nuclei. They represent only about 15% of all known nuclei with Z > 82. However, recently a novel experimental technique has been introduced. The fission of relativistic secondary projectiles has now been studied in flight. The benefit of the radioactive beams for studying the fission process is clear, but no fission probabilities below the fission barrier have been determined so far (Schmidt et al. 1994). 

Successful production of fissionable nuclei, such as transuranium nuclides in nuclear reactions, depends mainly on two factors fusion cross section, fTftision> and survival probability,... 

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