**1. Introduction**

We examine the nature of the experimental investigations in this paper. Therefore, we should start with some definitions. Every experiment begins with conceptualization. The model of the experiment should be prepared. After this stage, we implement this model in the experimental set-up and in the relations for obtaining physical quantities from the primarily measured data. The important step is an evaluation of uncertainties of physical quantities applying the error propagation law to the uncertainties of primary measured quantities.

Each experiment can contain a Systematic Distortion Factor (SDF). The SDF [1] changes experimental results and should be treated as a real physical effect. Therefore, it is very important to analyze and to compare the results of all experiments. If we can understand the nature of SDF, one can calculate the corrections, and corrected data can be used in the evaluation. If the data set contains substantial SDF leading to extremely outlaying data and they cannot be corrected, these data should be removed from the evaluation procedure. In some cases, the SDF can be estimated from the comparison of experimental results with the results of a Monte Carlo (MC) simulation of the experiment. MC simulation allows us to calculate or justify the introduced corrections.

**Citation:** Kornilov, N.V.; Pronyaev, V.G.; Grimes, S.M. Systematic Distortion Factor and Unrecognized Source of Uncertainties in Nuclear Data Measurements and Evaluations. *Metrology* **2022**, *2*, 1–18. https:// doi.org/10.3390/metrology2010001

Academic Editor: Simona Salicone

Received: 31 August 2021 Accepted: 8 December 2021 Published: 24 December 2021

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However, the best approach for SDF identification is the comparison of experimental results obtained in different measurements. Some examples of the SDF existence in Prompt Fission Neutron Spectrum (PFNS) measurements, measurements of fission cross sections used as standards and in Maxwellian Average Cross Section (MACS) measurements for astrophysical applications are demonstrated in this paper.

The present approach to the nuclear data evaluation for neutron cross section standards, which a priori can contain the SDF, are the following [2]:


To obtain realistically evaluated uncertainties consistent with a spread of experimental data relative to the evaluated values, the component of the Unrecognized Source of Uncertainty (USU) could be added to the covariance matrix of evaluated data. We examine the nature of the experimental investigations in this paper also. The following notations are used in this text: E0 is the neutron incident energy, and E is the energy of outgoing neutrons.
