7. Fission Products and Yields, ϒ

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1 7. Fission Products and Yields, ϒ 7.1 Nuclear Fission Among the processes of nuclear decay, fission is certainly the most complicated. Spontaneous fission (SF) was discovered by Flerov and Petrzhak in1940, following the discovery of neutron induced fission by Hahn and Strassmann in The process is observed in high mass number nuclides. The ratio of the probability of spontaneous fission to that of alpha decay increases with the atomic number Z and the number of neutrons N in the nucleus. For 238 U the ratio is 1:10 6, whereas in 256 Fm it is approximately 11.5:1. Spontaneous fission can be described by : A Z A Z + A A υ ( Z Z ) + υ n + δe. Figure 7.1 Spontaneous (left) and neutron induced fission (right) of heavy nuclides (figures courtesy of the Atomic Archive website at ) The process is shown schematically in Fig. 7.1 (left). A parent nuclide (A, Z) splits into two daughter nuclides (with coordinates A, Z and A A υ, Z Z respectively) together with the release of υ prompt neutrons and energy δe. Typically υ ranges from 2 4 and δe is approximately 200 MeV. Additional, so-called delayed neutrons may be emitted by the primary fission products. The daughter nuclides or fission products have in general different mass numbers A and atomic numbers Z. Fission can also be induced through the absorption of a neutron as shown in Fig. 7.1 (right).

2 Fission Products and Yields, ϒ The total energy released in fission is in excess of 200 MeV as shown in Table 7.1. About 87% of the total energy is emitted promptly with the fission fragments. Most of the neutrons released are prompt neutrons and are released within s of fissioning. Some neutrons are released on a much longer timescale and are associated with the fission decay chains. Since the discovery of this process, the identification of the mass and the nuclear charge of the fission products has always been an important part of fission investigations and experiments. In the early days of fission product yields exploration, initiated by nuclear physicists such as Anderson and Fermi, radiochemical methods were used. These are being progressively replaced by more sophisticated physical methods for the determination of the yields [1, 2, 3]. Although mass yield distributions have been measured with sufficient precision for most fission reactions of technical importance, the distribution of independent yields is still not completely known. The reason for this is that the mass distribution of fission products does not change with time after the fission reaction, if the small effect due to delayed neutron emission is neglected. Independent yields, however, describe the elemental distribution of fission products and undergo a rapid change after fission because of the predominance of short-lived β -unstable nuclides. Therefore, the methods of measuring independent yields differ from those for the measurement of chain yields, and thus require more advanced technology. On the other hand, only a small fraction of yields, independent and/or cumulative, of approximately 900 primary products have been measured for any fission reaction. About 25% of the fission products have been measured, at best, for the fissioning systems of technical importance and usually about 1% or less for the other fission reactions. Therefore, most of the independent and cumulative yields must be estimated. Fission yield distributions are interesting from two standpoints a) they provide information on the nature of large collective motion of nuclear matter in different energy ranges and b) low energy fission yield distributions are important for the the control of nuclear reactors. In the framework of nuclear technologies (reactor technology, nuclear medicine, safeguards, etc.), there is an even more important need of nuclear data. The cumulative fission product distribution is of great interest for practical purposes such as waste storage, control of nuclear reactors, etc. Independent yields are of importance for the fundamental understanding of the corresponding nuclear reactions, but also for short-term practical purposes in reactor operation. In the fission process, the probability of measuring the production of an isotope or a nuclide is generally expressed as the yield (given in units of production per unit fission or in percent). In the following sections, the different types of yields used in Nuclides.net are described. Table 7.1. Energy released resulting form neutron induced fission of 235 U Products Prompt energy: Fission fragments 168 Fission neutrons 5 γ emission 7 Radioactivity: β decay (electrons) 8 β decay (neutrinos) 12 γ emission 7 Total 207 Emitted energy (MeV)

A chart of such nuclides can be obtained from the Navigation button in the taskbar followed by selection of the Fission parents.

3 7.2 The Fission Yield Module The Fission Yield Module Before the Fission Yield module can be accessed, a nuclide which undergoes fission must be selected. A chart of such nuclides can be obtained from the Navigation button in the taskbar followed by selection of the Fission parents. The resulting window is shown in Fig Figure 7.2 Selection of the Fission parents chart from the Nuclide Explorer Once a particular nuclide, e.g. Pu240, has been selected, the Fission Yield module can be launched from the taskbar through the Data button followed by Yields as shown below.

4 Fission Products and Yields, ϒ Fission reactions differ according to the mass and nuclear charge of the fissioning nuclide, and according to the excitation energy of the compound nucleus. In general there are four different Excitation Energies for which data is available for each fissioning nuclide (listed in Table 7.2) and three different sources of data (listed in Table 7.3). The resulting window is shown in Fig. 7.3 for fissioning of Pu240 by fast neutrons using the Japanese datafile. Table 7.2. Neutron excitation energies for which fission yield data is available in Nuclides.net Excitation Energy Spontanaeous fission Thermal neutron induced fission Fast neutron induced fission 14 MeV neutron induced fission Table 7.3. Data libraries used in Nuclides.net Sources of data libraries used in Nuclides.net Version Joint evaluated File JEF (European) JEF-2/FPY [4] Japanese Evaluated Nuclear Data Library JENDL-3.2/FPY [5] Evaluated Nuclear Data Files (U.S.) ENDF/B-6 [6] The fission products of Pu240 are listed in the main frame in Fig There are over 1000 fission products grouped into pages with each page containing approximately 50 products. The data for each fission product consists of the half-life, the independent fission yield (IND), the error in IND, the cumulative fission yield (CUMUL), and the error in CUMUL. The fission product data listed in Fig can be re-arranged by clicking on the header labels. By default, the fission products are listed in ascending order of charge Z starting with Z = 25 (Mn) and ending with Z = 71 (Lu). The entries can be re-arranged by clicking once on Nuclide. The entries are then arranged in descending order of charge Z with 71 Lu172 at the top. Clicking on the Halflife arranges the nuclides in descending order with the longest lived nuclides at the top (Zn70, stable). The order can be reversed by clicking again on the header label. Clicking on IND arranges the nuclides with the highest independent yields at the top in the case of Pu240, this is Te134 with IND = Clicking again results in the nuclides with the lowest indepenedent yields at the top (Xe125, IND = ). Clicking on CUMUL arranges the nuclides with the highest cumulative yields at the top (Ba135, Cs135, Xe135 with CUMUL = , , respectively). More detailed information on nuclide yields can be obtained by clicking on the icon to the left of each nuclide in Fig This opens the Fission Product Yield Comparisons window which is discussed in more detail in Sect. 7.3.

5 7.2 The Fission Yield Module 191 Figure 7.3 Main Fission Products Window: In this example, one can see the fission products and their independent and cumulative yields from the fast neutron fission of 240 Pu using the JENDL library Definitions of Fission Yields A fission product 1 is denoted symbolically by the notation (A, Z, I) where A and Z are respectively the mass number and the atomic number, and I indicates the isomeric state (I = 0 for the ground state, I = 1, 2... for the 1st, 2nd,... isomeric states). If a fission product has no isomers, or if one is referring to the sum of yields of all its isomers, the notation (A, Z) is used. Using this terminology, the following fission yield definitions are given [7]: Independent Fission Yield, IND (A, Z, I) The independent fission yield, IND (A, Z, I), is the number of atoms of a specific nuclide produced directly (after emission of prompt neutrons but excluding radioactive decay) per 100 fission reactions. 1 The daughter nuclei showing up right at scission of a fissioning mother nucleus are called primary fission fragments. In the large majority of the cases, the fragments will be sufficiently excited to evaporate neutrons in times less than s. This means that the nuclei detected in experiments are not the primary fragments, but instead secondary fragments having lost a varying number of neutrons. The secondary fragments are called the fission products (i.e. fission fragments after prompt neutron evaporation).

6 Fission Products and Yields, ϒ Cumulative Fission Yield, CUMUL (A, Z, I) The cumulative fission yield, CUMUL(A, Z, I), is the number of atoms of a specific nuclide produced directly and via decay of precursors per 100 fission reactions. If the nuclide is stable, the cumulative yield is the total number of atoms of that nuclide remaining per fission after the decay of all precursors (ignoring the effects of other nuclear reactions e.g. nuclear capture). Similarly, for a nuclide with a much longer half-life than any of its precursors, the cumulative yield is very nearly equal to the amount produced at a time short compared to its half-life but long compared to the half-life of its precursors. However, for a radioactive nuclide for which this is not the case, some atoms will have decayed before all have been produced. In such a case, at no time will there actually be present the reported cumulative yield of atoms per fission present. Chain Yield, Y(A) The chain yield is the number of isobars of a specific mass, produced in 100 fission reactions. In other terms, the chain yield Y (A) is equal to the sum of all stable or long-lived cumulative yields for a given mass chain. The cumulative yield of the last (stable or long-lived) chain member is generally 2 identical to the chain yield. These chain yields apply to fission products after emission of prompt neutrons that takes place in a time of s after scission. 7.3 Fission Product Yield Comparisons From the main fission products window in Fig. 7.3, the user can access the fission product yield comparisons window by clicking the icon to the left of the fission product of interest. The available comparisons, which are discussed later in this chapter, are: Parents (see Sect ) Elements (see Sect ) Mass (isobars) (see Sect ) Libraries (see Sect ) Fission Product Yield Comparisons: Parents For the selected fission product, the fission product yield comparison window opens with the Parents tab highlighted. In this mode, the nuclide 2 A special note about β-delayed neutron emission has to be made: some cumulative and, in consequence, chain yields are subject to increases by neutron emission from heavier mass chains and losses to lower mass chains. In consequence, cumulative yields and chain yields are no longer identical to the sum of the independent yields of their precursors (mass yield).

7.3 Fission Product Yield Comparisons 193 Figure 7.4 Fission Product Yield Comparisons: yields of 135 Cs from various parents yields from various fission Parents can be compared.

7 7.3 Fission Product Yield Comparisons 193 Figure 7.4 Fission Product Yield Comparisons: yields of 135 Cs from various parents yields from various fission Parents can be compared. Yields for the selected fission products from up to five fissioning systems (fission parent + neutron energy) can be compared. The quantity Y (A) in Fig. 7.4 is the sum, mass or chain yield, i.e. the sum of the independent yields (before delayed neutron emission). This quantity is calculated in the program by summing all the independent yields. The second column in Fig. 7.4 gives the fission yield from the fissioning system selected from the main fission yield window (Fig. 7.3) and cannot be changed. In the columns (3 6) the user can select other fissioning systems and data libraries for comparison through the use of list boxes Fission Product Yield Comparisons: Elements With the Elements tab highlighted, yields from all isotopes of the same element, in this case Cs, can be compared. The Elements window is shown in Fig Fission Product Yield Comparisons: Masses (Isobars) With the Mass tab highlighted, fission yields from all nuclides with the same mass, i.e. isobars, can be compared. In the example shown in Fig. 7.6, 135 Cs for the 240 Pu(n f,f) reaction has been selected in the main fission yield window. In Fig. 7.6, fission yields from all isobars (mass 135) are displayed. The user can choose other parents (when available), data libraries, and neutron excitation energy from the appropriate drop-down menus.

Search 194 Contents 7. Fission Products and Yields, ϒ Figure 7.5 Fission Product Yield Comparisons: yields of all Cs isotopes from various fission systems Figure 7.

8 Search 194 Contents 7. Fission Products and Yields, ϒ Figure 7.5 Fission Product Yield Comparisons: yields of all Cs isotopes from various fission systems Figure 7.6 Fission Product Yield Comparisons: yields of isobar 135 from various fission systems Fission Product Yield Comparisons: Libraries With the Libraries tab highlighted, fission yields for the selected nuclide from different data libraries are compared. In the example shown in Fig. 7.7, 135 Cs for the 240 Pu(nf,f) reaction has been selected in the main fission yield window. In Fig. 7.7, fission yields from the JENDL, JEF, and ENDF libraries are displayed. The differences in the values between the JENDL and JEF data, i.e. D(JENDL,JEF), and the JENDL and ENDF, i.e. D(JENDL,ENDF) are also shown. The user can choose other parents (where available), and neutron excitation energy from the appropriate drop-down menus.

7.3 Fission Product Yield Comparisons 195 Figure 7.7 Library comparisons for a selected nuclide 7.3.5 The Chain Yield Window The Chain Yield Graphic can be selected from Fig.

The graph can be printed directly on the printer. Alternatively the data can be downloaded as an Excel spreadsheet as shown in Fig. 7.9.

9 7.3 Fission Product Yield Comparisons 195 Figure 7.7 Library comparisons for a selected nuclide The Chain Yield Window The Chain Yield Graphic can be selected from Fig The resulting window shown in Fig displays the plot of Y (A) for a selected fissioning system. The graph can be printed directly on the printer. Alternatively the data can be downloaded as an Excel spreadsheet as shown in Fig The Chain Yield data can then be redrawn as required. Figure 7.8 Graphic representation of the Chain Yield for the fast fission of 240 Pu

196 7. Fission Products and Yields, ϒ Figure 7.9 Data download to an Excel spreadsheet Acknowledgements This chapter was compiled with the assistance of Dr.

10 Fission Products and Yields, ϒ Figure 7.9 Data download to an Excel spreadsheet Acknowledgements This chapter was compiled with the assistance of Dr. Jean Galy. Dr. Galy also developed the Fission Yield program module. References 1. J. Galy: Investigation of the fission yields of the fast neutron-induced fission of 233 U. Thesis submitted for the degree of Doctor of Philosophy (September 1999) 2. H. O. Denschlag in: Experimental Techniques in Nuclear Physics, D. N. Poenaru, W. Greiner (eds.). Walter de Gruyter, Berlin F. Gönnenwein in: The Nuclear Fission Process, C. Wagemans (ed.). CRC Press, Boca Raton JEF-2/FPY (ref. AEA-TRS-1015, 1018 AND 1019, and M. F. James et al.: Progress in Nuclear Energy 26,1 1991). The evaluated data have been taken over from UKFY2, the UK fission-product yield data library by M. James and R. Mills. Brief summary: IAEA- NDS-123 Rev JENDL-3.2/FPY The JENDL fission-product yield data library, has been compiled by T. Nakagawa in ENDF-6 format. The evaluated data have been taken from JNDC-FP2, a special format data library documented in the reports JAERI-M (1989) and JAERI (1990) 6. ENDF/B-6 fission-product yield data (ref. LA-UR ENDF ) This is a separate ENDF/B-6 sublibrary which was released in September 1991 and updated in June 1993 and May It has two parts: one part for neutron induced fission, another part for spontaneous fission. Summary see document IAEA-NDS-106 Rev R. W. Mills: Review of fission product yields and delayed neutron data for selected actinides. Report NEANDC-300. July 1990

Nuclear Fission Fission discovered by Otto Hahn and Fritz Strassman, Lisa Meitner in 1938

Nuclear Fission Fission discovered by Otto Hahn and Fritz Strassman, Lisa Meitner in 1938 Fission Readings: Modern Nuclear Chemistry, Chapter 11; Nuclear and Radiochemistry, Chapter 3 General Overview of Fission Energetics The Probability of Fission Fission Product Distributions Total Kinetic