The transformation of the group neutron spectrum to the differential form or to group spectra with the other scale of energy groups is necessary in many tasks of reactor dosimetry. If the initial spectrum is given in a fine scale of energy groups, e.g., in the scale SAND-2, so for its conversion in a more rough scale and for an evaluation of a differential spectrum, as a rule, there a method of linear interpolation is used. In this case the linear interpolation method ensures an acceptable precision of the transformation results. If the number of groups of initial spectrum energy scale is not large (less than 100), the application of the linear interpolation method can cause a significant error.

In this work a general-purpose algorithm of spectrum transformation developed by the authors is described. Cubic spline is calculated for the initial spectrum in coordinates (Eφ(E),ln(E)), where E—neutron energy, φ(E)—differential function of flux density. As the spline of the spectral function Eφ(E) contains complete information about the differential neutron flux density, it allows to calculate group flux as well as differential values for an arbitrary energy scale.

The algorithm of the corresponding transformation of the spectrum covariance matrix is described. Both algorithms are realized as a computer code for IBM PC.