Nuclear-Charge Distribution in Low-Energy Fission

New information concerning half-lives and fractional independent and fractional cumulative yields of fission products has been obtained from experiments in which adjacent elements in fission-product chains were rapidly separated.

Fractional cumulative yields that have been determined for the thermal-neutron fission of U²³⁵ are: Kr⁹³, 0.075_-0.002^+0.010; Kr⁹⁴, 0.015_-0.002^+0.005; Kr⁹⁵, (1.1_0.1^+0.3)×10^-3; Kr⁹⁷, <10^-5; Xe¹³⁷, 0.978±0.003; Xe¹³⁸, 0.956±0.003. Fractional cumulative yields determined for the spontaneous fission of Cf²⁵² are: Xe¹³⁹, 0.67±0.01; Xe¹⁴⁰, 0.45±0.01; Xe¹⁴¹, 0.172±0.005; Xe¹⁴⁴, <0.007.

Fractional independent yields determined for the thermal-neutron fission of U²³⁵ are: Sr⁹¹, 0.07±0.05; Nb^95m, <4×10^-5; Nb⁹⁶, (1.0±0.2)×10^-4; Nb⁹⁷, (1.7±0.8)×10^-3; Cs¹³⁶, (1.10±0.15) ×10^-3; Ba¹³⁹, 0.012_-0.003^+0.005; Ba¹⁴⁰, 0.066±0.026; Ba¹⁴¹, 0.27 ±0.08; La¹⁴¹, 0.004±0.002; La¹⁴², 0.019±0.005; Ce¹⁴³, (4.4±3.0) ×10^-3. Fractional independent yields determined for the thermal-neutron fission of U²³³ are: Nb^95m, <3×10^-4; Nb⁹⁶, (1.3±0.2) ×10^-3; Nb⁹⁷, 0.011±0.004. Fractional independent yields determined for the thermal-neutron fission of Pu²³⁹ are: Nb^95m, <3×10^-4; Nb⁹⁶, (7.7±1.0)×10^-4; Nb⁹⁷, 0.015±0.004. The fractional independent yield of Cs¹³⁶ from spontaneous fission of Cf²⁵² is <0.01.

The above data together with other published data give direct information about the distribution of nuclear charge among fission products with mass numbers 91, 139, 140, 141, 142, and 143 from thermal-neutron fission of U²³⁵. The variation of fractional yield with Z for constant A can be represented in cumulative form by the area under a Gaussian curve from -∞ to Z+1/2. The standard deviation of the curve which best represents the data for the six mass numbers is σ=0.62±0.06.

The variation with A of Z_P, the value of Z at the maximum in a charge distribution curve, is discussed, and a new empirical Z_P function is derived on the assumption that the Gaussian curve is applicable to all mass numbers. The function and curve correlate quite well the available fractional yield data for low-energy fission processes.

There are indications from the analysis of charge dispersion, and from some other observed fission phenomena, that primary low-energy fission products with Z less than 50 and greater than the complementary charge (>42 for ₉₂U fission) may be formed in very low yield. It is suggested that observed fission products with atomic numbers in this range may be formed predominantly by beta decay processes starting from products complementary to ₅₀Sn (₄₂Mo for ₉₂U fission) or from products of lower Z.

During the course of this work, fission yields of 51-min Nb⁹⁸ from thermal-neutron fission of U²³⁵, U²³³, and Pu²³⁹ were determined to be (0.064±0.012)%, (0.20±0.03)%, and (0.20±0.03)%, respectively.

New half-life values determined are: Rb⁹¹, 72±8 sec; Rb⁹², <11 sec; Nb⁹⁸, 51±3 min; Cs¹⁴¹, 25±3 sec; Cs¹⁴², <8 sec; Ba¹⁴³, 12±2 sec. Kr⁹⁷ and a reported 14-min isomer of Rb⁹¹ were not found among the fission products; we believe that our data refute the published evidence for their existence.