Mass-yield curves

The curves show the general shape of the 235U fission mass-yield curve. The yields at masses 89, 91, and 140 are down because of their gaseous and volatile precursors (see text). The yields at masses 103 and 129 are also low owing to the volatile prop-ties of the elements Ru and Te... 

Just as earlier we were able to observe mass-yield distributions of the fission products from the fissionable nuclide used in the Chinese nuclear device, it is possible to see part of the mass-yield curve from the fission of 244Pu, which was synthesized originally in a supernova. Figure 6 shows the mass-yield distribution of the excess fissiogenic xenon observed in the meteorite Pasamonte (15). 

Th, and by fast neutrons are listed in Table 2.9 and are shown as the familiar double-hump mass-yield curves in Figs. 2.12 and 2.13. 

The value of this branching ratio measured directly from j8 -decay data is 0.30 (11) whereas a direct measure of the fission yields of Kr86 and Rb86 using mass spectrometry and isotope dilution gave a value of 0.28 (13). The assumption that the Kr88 yield falls on a smooth mass yield curve in the case of the thermal neutron fission of U236 therefore appears to be justified. 

The relative yields of the isotopes of the aforementioned (Section V.B.4) twelve elements that can be measured mass spectrometrically give reliable yields of about 43 mass chains. These, taken together with a number of yields determined radiochemically, give essentially the whole mass yield curve, which may be normalized to 100% in order to obtain absolute yields for the various mass chains. This procedure is used when mass chain yields for most of the chains of appreciable yield for either the light or heavy fragments are known. Absolute yields good to 3% have been obtained in this way. 

Interpretation of Mass Yield Curves for U236 + n and Pu239 + n Fission... 

A complete mass yield curve makes it possible to calculate the total... 

Because of the fine structure in the mass yield curve in the 131-136 mass range, ratios of the xenon isotopes in this region vary markedly from... 

FIG. 14.6. Mass yield curve obtained by bombardment of lead with high energy protons. (From Miller and Hudis.)... 

FIG. 14.8. Comparison of mass yield curves for proton interaction with ct per as predicted by Monte Carlo calculations (histogram) with experimental results (curve). (From G. Friedlander.)... 

From the mass yield curve in Figure 14.9 we leant that conqjlementary fission products (i.e. the two products Aj and A2 with identical yield values symmetrically located around the minimum) add up to about 234, not 236. Direct neutron measurements reveal that on the average 2.5 neutrons are emitted in fission of U by thermal neutrons. This number increases as the Z of the tarcet and as the bombarding energy increases. 

In contrast to the above, low energy fission of nuclei in the radium region results in mass yield curves with three peaks symmetric around AH. Furthermore, low energy or spontaneous fission of some heavy actinide isotopes, e.g. produces symmetric mass... 

FIG. 14.11. Mass yield curves for fission products from uranium irradiated with protons ... 

The SF process that results in two nearly equal mass fragments (a process called symmetric fission ) has been observed in Fm (1.5 s). More commonly, SF occurs as asymmetric fission, a split of the parent radionuclide into two unequal large FF. As in neutron-induced fission, many different asymmetric mass (and charge) divisions with varying yields can result, with mass numbers from about 70 to 170, each with many isotopes. Hundreds of different nuclides can be produced. Figure 2.1 displays the predominantly asymmetric mass yields as a function of mass number (dubbed mass-yield curves ) that have been measured for several SF and neutron-induced fission nuclides. 

Rgure 2.1. Mass-yield curves for some fission products, including and Pu. The curve labels refer to the compound nuclei, as in + n = (From Vertes et al. 2003,... 

For a given radionuclide, some fission-fragment pairs are more common than others. This is easily seen in the mass-yield curves of Fig. 2.1, which show the spec-tmm of atomic masses given off by the fission of several radionuclides. Atom fission... 

A model, based on the construction of a potential energy surface from a combination of liquid drop terms for protons and neutrons as a function of deformation and shell corrections, has been published by Wilkins, Steinberg, and Chasman (Wilkins et al. 1976). In the model, a specific excitation energy (nuclear temperature) is assumed at scission and the probability to reach this specific state is calculated. The general trends of mass-yield curves in the fission of very different nuclides from "Po to Fm and for different excitation energies of the fissioning nucleus from 0 MeV (spontaneous fission) to highly excited fission reactions are reproduced correctly. In particular, the transition from the symmetric fission of (the compound nuclei) Po to a triple-humped mass-yield curve for Ra to double-humped yield curves for and Cf and, finally, a partial return to symmetry for Fm is... 

The physical measurements of mass yields based on momentum conservation (double energy, double velocity or energy, and velocity determination) — as opposed to the radiochemical measurements discussed further above - have the advantage of producing a more or less complete mass-yield curve in one experiment using up only small amounts of the possibly exotic fissile material (in one thin target). The mass resolution in the yield curves obtained is, however, not quite as good as that of the radiochemical methods, so that fine details of the mass-yield curve cannot be observed. 

Mass-yield curves for the thermal-neutron-induced fission of Th and from (von Gunten 1969)... 

O Figure 4.17 shows the mass-yield curves determined for the fission fragments as weU as for the fission products of for comparison (Schmitt et al. 1966). The two curves are slightly shifted relative to each other along the mass axis due to the emission of neutrons. Accordingly, the number of neutrons emitted can be obtained from a comparison. 

Mass-yield curves in the 14 MeV-neutron-induced fission of, and from... 

Schematic representation of the known mass-yield curves for the spontaneously fissioning isotopes of the elements with Z = 98 to Z = 104 (Hoffman and Lane 1995)...

So far, essentially mass-yield curves were dealt with. Each point of such a curve represents the formation cross section of isobaric nuclei of mass number A, composed of different combinations of protons and neutrons. Because heavy, fissile nuclei are generally more neutron rich than stable nuclides with about half their mass, fission products are generally also more neutron rich than stable nuclides of the same mass, even after the loss of a few prompt neutrons. (Example The symmetric fission of the compound nucleus (Z - 92, N -144) would form two/raiment nuclei of Pd (Z = 46, iV= 72). Assuming the emission of one prompt neutron, the corresponding primary fission product would be Pd. The stable isobar in mass chain with A = 117 is, however, Sn (Z -50, N- 67). As a consequence, the nucleus Pd would have to undergo a sequence of four P decays to reach stability.) Thus, the products... 

As typical data, the TKE and mass-yield distributions observed in SF of Md (Hulet et al. 1989) are depicted in Fig. 18.14a, b, respectively two components are clearly seen in the TKE distribution. The two-component analysis yielded the fact that the high-TKE events mostly constitute the sharp mass-yield curve around symmetry and the low-TKE ones a broad flat-topped distribution. Some theoretical calculations to understand bimodal fission of heavy actinides have been extensively performed (Warda et al. 2002 Asano et al. 2004 Bonneau 2006 Dubray et al. 2008 Ichikawa et al. 2009). 

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