Fission mass distribution

The key to understanding this situation can be seen in Figures 11.13 and 11.14. In these figures, we show that as the mass of the fissioning system increases, the position of the heavy peak in the fission mass distribution remains constant while the position of the light peak increases with increasing fissioning system mass. 

Let us consider first the low-energy fission of the lighter fissionable elements, in the neighborhood of Pb208. These elements (gold, thallium, lead, bismuth), when bombarded with particles such as 20-Mev deuterons, undergo symmetric fission, the distribution function of the products having a half width at half maximum of 8 to 15 mass-number units (20). 

Zysin, Y. A., Fission Product Yields and Their Mass Distribution, p. 63,... 

A knowledge of the size distribution function of the radioactive debris and the specific activity of individual fission product chains as a function of particle size suffice to define many important radiological properties of the land-surface nuclear explosion. If is the function of a radionuclide or fission mass chain distributed between particle sizes Di and D2, then... 

Figure 11.13 Smoothed fragment mass distributions for the thermal neutron-induced fission of 233U, 235U, and 239Pu. [From Seaborg and Loveland (1990).]...

Fission products span a very 9.33 Smoothed mass distribution Tor the ... 

In Fig. 8.13 the yield of fission products obtained by thermal fission of is plotted as a function of the mass number A (mass distribution). The maxima of the yields are in the ranges of mass numbers 90-100 and 133-143. In these ranges the fission yields are about 6%, whereas symmetrical fission occurs with a yield of only about 0.01%. The peaks in the mass distribution curve A = 100 and at 4 = 134 are explained by the fact that formation of even-even nuclei is preferred in the fission of the even-even compound nucleus It should be taken into account that the sum of the fission yields is 200%, because each fission gives two fission products. 

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. 

The influence of the energy of the neutrons on the mass distribution of the fission products is shown in Fig. 8.15 at higher neutron energies, the probabihty of symmetric fission increases strongly. 

Increase of symmetric fission is also observed at lower atomic numbers Z. It prevails at Z < 85, and at Z = 89 ( Ac) symmetric and asymmetric fission have nearly the same probability, which results in three maxima in the mass distribution. Three maxima are also observed in the fission of Ra by 11 MeV protons or by y rays. 

The mass distribution curves in Figs. 8.13 to 8.15 give the total yields of the decay chains of mass numbers A. The independent yields of members of the decay chains, i.e. the yields due to direct formation by the fission process, are more diflicult to determine, because the nuclides must be rapidly separated from their precursors. Only a few so-called shielded nuclides (shielded from production via decay by a stable isobar one unit lower in Z) are unambiguously formed directly as primary... 

As an example, the mass distribution of the products obtained by the bombardment of with " Ar is plotted in Fig. 8.24. The curve is explained by superposition of the processes described above only few nucleons are transferred by quasielastic reactions (a), and many nucleons by deeply inelastic processes (b). Fusion followed by fission of highly excited products leads to a broad distribution of fission products around l/2(y4i + A2), where A and Ai are the mass numbers of and Ar, respectively (c), and asymmetric fission of heavy products of low excitation energy gives two small maxima (d). 

Simultaneous measurement of energy and mass distribution of fission fragments... 

The fission process can be described in terms of the liquid drop model. The drop is capable of undergoing various deformations considered as vibrational modes. As the drop becomes more distorted, it eventually breaks into two primary fragments. The two fragments are usually unequal in mass, with a mass distribution that depends on the manner in which the drop splits. In nuclear fission, absorption of a neutron induces similar oscillations in the target radionuclide that distort its shape until it splits into the two primary FF. A wide array of possible mass combinations exists for the FF. Shown below are three such possibilities for... 

The thus obtained fission half lives are depicted in the lower part of Figure 8.11. Their distribution as a function of the fragment mass A2 resembles quite well the asymmetric mass distribution. Cluster radioactive decays correspond to the broad peaks around A2 = 20, 30 (200, 210). The confrontation of the calculated fission half lives with experiments is depicted in Figure 8.12. One notices "nearly quantitative" agreement over 20 orders of magnitude, which is—for an ab-initio calculation—remarkable ... 

In addition, the physical measurements (after some corrections for prompt neutron emission) allow one to obtain fragment mass distributions (prior to prompt neutron emission). A comparison of these fission fragment yield curves with fission product yield curves makes it possible to extract information on prompt neutron emission. This will be discussed in the next subsection. 

Pre-neutron emission mass distribution, N m ), corrected for resolution and post-neutron emission mass distribution, N m), for the thermal-neutron-induced fission of from (Schmitt et al. 1966)... 

Chain yields in very asymmetric mass distribution. Recently, using the mass separator Lohengrin briefly presented above, it has become possible to measure yields of the light fission fragments down to very low values. Four fission reactions studied are shown in Fig. 4.23. 

Nud Phys A 597 188 Wahl AC (1988) At Data Nucl Data Tables 39 1 Wahl AC (1989) Nuclear charge and mass distributions from fission. In Behrens JW, Carlson AD (eds) Fifty... 

The focus of this chapter is to review the latest results obtained for the excited states in the second and third minima of the potential barrier. In addition to these exotic shapes, it is also an interesting and longstanding question, at which points of the fission path the mass and energy distributions of the fission fragments are determined. Can one get different mass distributions after the fission of the super- and hyperdeformed states as suggested by Cwiok et al. (1994) What kind of clusterization is expected in these exotic nuclear states Does the predicted cold valley exist in the fission barrier These are very interesting questions but the presently available experimental information is still not sufficient to answer them. 

The HD states lying in the third well of the fission barrier may play the role of a doorwaylike state before fission, from which the fission can only occur through a limited number of fission paths, resulting in a sharper mass distribution (Krasznahorkay et al. 2000, 2001a, b, 2003, 2004). 

Schematic representations of all of the measured mass-yield distributions (normalized to 200% fragment yield) for SF of the trans-Bk isotopes are shown in Fig. 18.13 (Hoffinan and Lane 1995). It is interesting to observe rather sudden changes from asymmetric to symmetric fission as reflected by the mass distributions changing from asymmetric to symmetric mass distributions as the neutron number increases toward N 160 for the elements Fm Z = 100), No (Z= 102), andRf(Z= 104).

Isotopes investigated in low-energy fission are indicated on the chart of nuclides. Circles Mass distributions measured for excitation energies less than 10 MeV above the fission barrier and those from SF. Crosses Data obtained using the Coulomb fission by using the relativistic radioactive beams (Schmidt et al. 2000)... 

Experimental fission fragment distributions for the production of Z = 102 and Z = 108 in cold fusion, and Z = 114 in hot fusion with a Ca beam. In the upper panels the total kinetic energy (TKE) is plotted vs. the mass number of the fission fragment. In the lower panels the counts of the mass distributions of fission fragments are displayed (Oganessian 2001]... 

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