Doppler Broadening

BROADR adds temperature dependence to the pointwise cross sections generated by the RECONR module (see UNRESR and PURR for temperature dependence in the unresolved energy range). It can also be used to rebroaden the results of a previous BROADR run to a higher temperature.

In a material at temperature T, the target atoms and molecules are moving around randomly with a distribution of velocities given by the Maxwell-Boltzman function:

Maxwell-Boltzman Equation

where α=M/(2kT). When a neutron moves through the material, it is the relative velocity between the neutron and the target nucleus that determines the proper cross section to use. We can define a single effective cross section for neutrons with velocity v by conserving the reaction rate:

Effective Doppler Broadened Cross Section

BROADR uses methods based on early versions of Cullen's SIGMA1 code to compute these effective cross sections for linear-linear representations of the basic reaction cross sections. The basic cross sections are obtained from a PENDF tape generated by RECONR or produced in a previous BROADR run. The Doppler-broadened effective cross sections are written out as a multi-temperature PENDF tape.

The following is an example of a simple BROADR run to Doppler broaden the results of a previous RECONR run (on tape21) for the H-1 evaluation (on tape20):

          20 21 22
          125 1/

To understand this deck, study the input instructions. Hint: use the right-hand button with your browser to bring the instructions up in a separate window for easy comparison with the lines in the example.


23 January 2013 T-2 Nuclear Information Service