Abstract
Neutron-induced fission cross sections of 235U and 238U are widely used as standards for monitoring of neutron beams and fields. An absolute measurement of these cross sections at an absolute scale ...
Highlights
Neutron-induced fission cross sections of 235U and 238U are widely used as standards for monitoring of neutron beams and fields
Because of the presence of the polyethylene target, only one of the fission fragment (FF) will be detected in one Parallel Plate Avalanche Counter (PPAC), and it may reach one of the telescopes, where it will be stopped in the first Si detector
The fission fragments will lose an average of ∼8 MeV per PPAC foil and, despite the energy losses in the target and in the gas (C3 F8 at 3 mbar), only less than 0.1% of them will reach the telescopes with an energy lower than 8 MeV, so that they will be discriminated from α-particles
Summary
Neutron-induced fission cross sections of 235U and 238U are widely used as standards for monitoring of neutron beams and fields. The necessity of improving the quality of the standards has been highlighted [1] because of inconsistencies between experimental information and various evaluations (e.g. ENDF-B/VII, JEFF-3.2, TENDL2015), as it can be seen in Fig. 1 for the case of 238U. In the particular case of the ratio of fission cross sections 238U/235U , recent results from Refs. [6] and references therein), considered as a primary standard; in particular, the results from Ref. The strong anisotropies observed at the opening of the multiple-chance fission channels (n,xn’f) help to determine their thresholds. These anisotropies affect the precision of the cross section measurements when using detectors with a limited angular acceptance, requiring a good knowledge on the angular distribution to correct for this effect [9]. We propose a quasi-absolute measurement of cross sections and angular distributions for 235U(n,f) and 238U(n,f) using recoil protons from the H(n,p) reaction for normalization, linking together three standard cross sections for neutron measurements with good accuracy
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