Introduction

Fitter, the package for fitting a chosen theoretical multi-parameter function through a set of data points, has been successfully used in small-angle neutron scattering data analysis since 2003 [1]. The several examples of researches using Fitter are given below. This handbook gives a long write-up description of the new version of Fitter.

The small-angle neutron scattering (SANS) method has long history. So, the earliest references to SANS can be traced back to 1948 by D.J.Huges et al. [2]. Starting with classical work of A.Guinier, who suggested the so called ``Guinier approximation'' to powerfull package of D.Svergun team [3,4,5,6,7,8,9], who developed and brogth SANS method up to present moment. At the same time, in spite of the wide SANS method application and already more than 50-years history of its development, the problem of interpreting the data obtained on spectrometers, still remain open. It is related, first of all, to the ill-posedness of the solved problem and, on the other hand, to specifics of existing SANS spectrometers. Practically each spectrometer is provided by not only software on initial processing experimental data, but also by a complex of programs for fitting the curves measured in coordinates scattering intensity/q, where $q =(4\pi/\lambda)\sin(\theta/2)$.

A modernization of the time-of-flight SANS YuMO spectrometer at the high flux pulse IBR-2 reactor (JINR, Dubna) [2] was launched in 2000. One of the goals of the improvement was to increase the dynamic Q-range of the instrument. It was realized by using a two-detector system with central holes at the YuMO instrument. For this purpose ``old'' and ``new'' detectors were used. The main view of the setup of the modernized YuMO instrument as well as an example of a SANS experiment are presented in [10]. The use of the two detectors YuMO set up for the users have clearly demonstrated its high efficiency [11,12,13,14,15,16,17,18,19,20]. That are only several examples. In 2006 will start at the YuMO spectrometer position sensitive detector [21].

The new Fitter version is better suit requirements of new detector setup. The main changes are:

• fitting in a given range;
• improving the robust approach;
• choosing the MINUIT strategy;
• exponential function in STD models;
• new visualization and GUI;

This paper is a long write-up description of the current version of program for fitting a chosen theoretical multi-parameter function through a set of data points. Examples of application to experiments on the small-angle spectrometer YuMO are considered.