Absolute measurements of 235U and 239Pu fission cross-sections with photoneutron sources

Abstract

the fission cross-sections of 235U and 239pa for Na Be, La Be, Na-D and Ga D photoneutrons have been measured absolutely. The neutron source strength was measured using a manganese bath to compare the photoneutron yield from the sources with the standard source NBS-II. Fission counts were accumulated with the source positioned symmetrically between two identical fission foils in an experiment package suspended in a low-albedo laboratory. Fission fragments passing through limited solid angle apertures were recorded on polyester track-etch films. The masses of the foil deposits were determined by microbalance weighings and confirmed by thermal neutron fission and alpha counting. After making a correction for the calculated energy distribution of the source neutrons, values of 1.471 + 0.029, 1.274 + 0.026, 1.162 + 0.025 and 1.195 _+ 0.026 barns were obtained for the 235U fission cross-section at the source median energies of 140, 265, 770 and 964keV, respectively. Corresponding values of 1.469 + 0.045. 1.515 _+ 0.038, 1.670 _+ 0.039 and 1.643 + 0.038 barns were determined for 239pu.

Validating nuclear data uncertainties obtained from a statistical analysis of experimental data with the “Physical Uncertainty Bounds” method

“…The variables N 1 or N 1 /N 2 are often determined via α-counting measurements (α-count.) 1 [25,[29][30][31][32][33][37][38][39]43,45,46,49,50,53,60,61,63] independent from fission counting measurements as can be seen in Table 1. A 1% value is assumed as a conservative bound ∆(N 1 ) 1 across all measurements using α-counting to determine N 1 in 239 Pu samples following the literature of individual measurements and reference [24].…”

“…weigh.) [32,60]-and measurements relative to thermal [28,32,41,43,45,52,60] were employed to determine either N 1 or N 1 /N 2 . Weighing techniques were always used to validate N 1 or N 1 /N 2 obtained by α-counting.…”

“…The neutron flux is directly measured with the associated particle technique (ASSOP) [25,30,33,37], recoil particle measurements (REC) [31,33,38,56], relative to manganese baths (MANGB) [32,33] and with indirect Boron-pile measurements (ind. B-pile) [57].…”

“…Hence, the total bound shrinks down to the combined bound of δη = δβδm of ratio measurements considering all techniques to determine δη. Multiple scattering and attenuation effects are usually corrected via Monte Carlo calculations (MC) [25,32,33,41,46,48,53,56,57]. It is stated for many measurements [29][30][31][36][37][38]45,50,59,60,70,71,75] that these effects were "calculated".…”

“…Similarly to δη and η, the background corrections C b1 and C b2 can be reduced by setting the room up accordingly [31,32], by using a shadow-cone (shadow-cone) [31], by careful detector design (det. design) [33,38], by using shielding (Shielding) [28], by using clearing magnets (clear.…”