Insight into excitation energy and structure effects in fission from isotopic information in fission yields

The use of the inverse kinematics technique and a magnetic spectrometer permits the simultaneous measurement of proton and neutron content of full fission fragment distributions. This paper reports new measurements of five fissioning systems—${}^{238}\mathrm{U},{}^{239}\mathrm{Np},{}^{240}\mathrm{Pu},{}^{244}\mathrm{Cm}$, and ${}^{250}\mathrm{Cf}$—produced in inelastic scattering, transfer, and fusion reactions at different excitation energies. As a result, isotopic, elemental, and isotonic fission yields are presented. The contribution of the different fission modes and the proton even-odd effect are studied. Structure effects are investigated by means of the neutron excess and the total neutron multiplicity as a function of the fragment atomic number.

Figure 1

Measured isotopic fission-fragment yields as a function of proton and neutron numbers. The valley of stability, shown as grey squares, and both the $n$-rich and the $n$-deficient limits of known nuclei, as well as the spherical closed shells $Z=28$,50, and $N=50$,82, are displayed. Dashed lines represent the $N/Z$ of the fissioning systems. The average initial excitation energy of each fissioning system is indicated in parentheses.

Figure 2

Measured isotopic fission-fragment distributions of the elements $Z=48$, 49, and 50, produced in the different fissioning systems. The corresponding average excitation energies are indicated in parentheses.

Figure 3

Isotopic fission-fragment distributions of fission- fragment pairs with $Z=(36,56),(38,54),(40,52)$, and (42,50) of ${}^{238}\mathrm{U}$. Present data (in black) is compared with previous measurements employing Coulomb-induced fission (in red) from Ref. [29]. Insets: relative yield difference between both sets of data, $2(Y^r-Y^b)/(Y^r+Y^b)$.

Figure 4

Elemental fission-fragment yields fitted to the fission modes. Super Long (SL), Standard I (SI), Standard II (SII), and Superasymmetric (SA) fission modes are presented. See the text for details.

Figure 5

Yields of the fission modes as a function of the excitation energy of the fissioning system (top), and as a function of the fissility parameter of the fissioning system (bottom).

Figure 6

Local even-odd staggering as a function of the nuclear-charge ratio. Each color represents one fissioning system with its average excitation energy indicated in parentheses.

Figure 7

Local even-odd staggering, in absolute values, as a function of the fissility parameter at different asymmetries. Present data (in black) are compared with previous measurements of thermal-neutron induced fission (in red) from Refs. [20, 21, 22, 23, 24, 40, 42]. The excitation energy is shown with dashed lines in the central panel.

Figure 8

Fission-fragment neutron distribution. Each color represents one fissioning system with an average excitation energy indicated in parentheses. Dashed lines indicate $N=60$ and $N=82$ (see text for details).

Figure 9

Present isotonic fission yields of ${}^{238}\mathrm{U},{}^{239}\mathrm{Np}$, and ${}^{240}\mathrm{Pu}$ (circles) compared with previous measurements (squares) [23, 29, 48] at different $E_x$, indicated within parentheses.

Figure 10

Fission-fragment neutron excess as a function of the fission-fragment nuclear charge. Spherical shells $Z=50,N=50,82$, and deformed shells $Z=44,N=64,88$ are indicated with dashed lines. The isotonic line $N=60$ is also presented.

Figure 11

Average total neutron multiplicity as a function of the fission-fragment nuclear charge.

Figure 12

Average total neutron multiplicity at symmetry as a function of the fissility parameter of the fissioning system. The excitation energy of the fissioning systems is indicated with a dashed line.