Source energy distribution for neutron of different energy group.

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Alexandre :pray: was just showing me this thread and walked me through how to make a source from a spectrum tally. I refactored the code into a method that could potentially be added to the EnergyFilter class in openmc/filter.py file. This makes it quite convenient when generating Tabular.

    def get_tabular(self, values):
        """Creates a openmc.stats.Tabular distribution using the EnergyFilter
        bins and the provided values. Intended use is to help convert a
        spectrum tally into a source.

        Parameters
        ----------
        values : np.array
            Array of numeric values, typically a tally.mean from a spectrum tally

        Returns
        -------
        openmc.stats.Tabular
            Tabular distribution with histogram interpolation
        """

        spectrum_probability = values / sum(values)

        proberbility_per_ev = spectrum_probability / np.diff(self.bins).flatten()

        return openmc.stats.Tabular(
            x = self.bins,
            p=proberbility_per_ev,
            interpolation='histogram'
        )

example usage with spectrum tally

cell_tally = results.get_tally(name='cell_spectra_tally')

tab = energy_filter.get_tabular(values=cell_tally.mean.flatten())

source = openmc.Source()
source.energy = tab
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