N. Kornilov

The NIFFTE project

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) is a double-sided Time Projection Chamber (TPC) with micromegas readout designed to measure the energy-dependent neutron-induced fission cross sections of the major and minor actinides with unprecedented accuracy. The NIFFTE project addresses the challenge of minimizing major sources of systematic uncertainties from previous fission chamber measurements such as: target and beam non-uniformities, misidentification of alpha and light charged particles as fission fragments, and uncertainties inherent to the reference standards used. In-beam tests of the NIFFTE TPC at the Los Alamos Neutron Science Center (LANSCE) started in 2010 and have continued in 2011, 2012 and 2013. An overview of the NIFFTE TPC status and performance at LANSCE will be presented.

Achievements and Still Open Problems

One may conclude that there are several points which may be demonstrated as real achievements. First of all, the Prompt Fission Neutron Spectra (PFNS) of 252Cf(sf) are proclaimed as “standard spectrum” with rather small uncertainties in the energy range < 10MeV. Many years’ efforts addressed to “create” dosimetry reactions, cross sections, and experimental investigations of average reaction cross sections in this neutron field confirmed this conclusion. The ratio R = C/E calculated to experimental average cross sections is 1.004 ± 0.020 in the energy range 1–15 MeV. It was confirmed (see previous chapters) that the average energy of the PFNS 235U(th) is known with high accuracy E = 1.974 ± 0.002 MeV. Recent theoretical investigations gave very interesting result: neutrons may have strong angular distribution relative to fission fragments (FFs) due to fast, non-adiabatic rupture of the fissile system. So, the orientation of the fission neutron along the FF direction is not the sign that they are emitted from moving fragments. However, many problems are still open: contradiction between macroscopic and microscopic experimental data for 235U(th) (mic–mac problem), left–right and angular anisotropy for 0.5 MeV neutrons, and the contribution of different mechanisms of neutron emission in fission. New additional experimental and theoretical efforts are urgently necessary. Some proposals are given in this chapter.

Microscopic Spectra Evaluation. Semiempirical Modeling

A rather realistic theoretical model has been in the hand of researchers since the 1950s, the so named “traditional assumptions” (see Introduction); however, the spectrum shape calculated on the basis of this assumption contradicts the experimental data, and this “physical” approach cannot help with the main request—to provide any useful function for data analysis. Therefore, prompt fission neutron spectra (PFNS) are analyzed with semi-empirical formulas: Maxwellian distribution, single-Watt or two-Watt spectra approaches, “scale method,” and so on. All these approaches do not have any strong physical basis. However, the spectrum shape, predicted with these approaches and adjusted parameters, allows us to normalize experimental data, extrapolate the calculated PFNS to different input energies, and prepare the data library. The main attention is paid to the discussion of the evaluation procedure for two very important PFNS, 252Cf(sf) and 235U(th), and to the uncertainties for evaluated spectra and average energy of fission neutrons for these isotopes. In addition, peculiarities connected with pre-fission neutron emission are also discussed.

New Experimental Proposal for 235 U PFNS Measurement to Answer a Fifty Year Old Question

The Prompt Fission Neutron Spectrum (PFNS) from 235U(n,f) is very important for various nuclear applications. It has been investigated in different experiments. In spite of ∼50 years of experimental efforts, a continuing conflict exists at thermal neutron energy. Microscopic experimental PFNS cannot describe macroscopic data. In this report we discuss the current status of this problem and suggest a new experiment, which could possibly resolve this problem.