fan Annual Report 2000

Postneutron Yield of Bremsstrahlung-Induced Fission of ²³⁸U with an Endpoint Energy of 12 MeV

S. Fan, A. Wagner, E. Grosse

In order to examine the feasibility of studying neutron rich nuclei close to the astrophysical r-process path at ELBE we investigated the production cross sections of fission fragments from photo-induced fission. The isotopic fragment distribution from bremsstrahlung-induced fission of ²³⁸U using bremsstrahlung from incident electron energies E_e can be written as

where the bremsstrahlung spectrum N(E(E_e)) was calculated using EGS4 [1], the photofission cross section s_F was taken from Ref. [2]. Z_p denotes the most probable charge in a Gaussian distribution and was calculated using the Z_p model [3]. The width parameter s_Z of the charge distribution from photofission of ²³⁸U was found to be 0.34 [4], and N_i denotes the normalization parameter. The correction factor F_p(E(E_e)) for the proton odd-even effect is defined [4] as

where d^mono_p(E) was deduced from experimental data[4].

Fig. 1 shows the postneutron mass distribution for the ²³⁸U(g,f) reaction with bremsstrahlung from 12 MeV electrons [5]. Fig.2 depicts the elemental mass distribution and Fig.3 shows the isotopic distribution of postneutron fission fragments (colored), nuclei with known half-lifes are shown as open boxes, stable nuclei are shown as filled boxes. Also drawn is the limit of nuclei with mean lifetimes against b-decay of more than 43 ms from theoretical predictions by Moeller [6].

Fig. 1 Postneutron mass distribution for ²³⁸U(g,f) reactions with bremsstrahlung from 12 MeV electrons [5].

Fig. 2 Elemental mass distribution (conditions as for Fig. 1).

Fig. 3 Isotopic distribution of fission fragments (colored). The contour line for expected half-lifes of 30 ms from theoretical calculations [6] is shown in red.

References

[1] W.R. Nelson et al., SLAC-Report-265 (1985).

[2] B.L. Berman et al., Phys. Rev. C 21 (1980) 1215.

[3] R.K. Gupta et al., Phys. Rev. C 30 (1984) 395.

[4] K. Persyn et al., Nucl. Phys. A 620 (1997) 171.

[5] E. Jacobs et al., Phys. Rev. C 21 (1980) 237.

[6] P. Moeller et al., Atomic Data Nucl. Data Tables 66 (1997) 131-343.