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instructions_unfold_spectrum.md

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Instructions for unfold_spectrum.exe

This application is used to unfold measurements obtained with a Nested Neutron Spectrometer to obtain a neutron fluence spectrum.

Table of Contents

Input files

Measurements file

  • This file contains the measured NNS data to be unfolded.
  • Default file: input/measurements.txt
  • Can specify an alternative measurements file within the settings file via the path_measurements parameter (described below).
  • The required format of the file is shown in input/template_measurements.txt.
    • Start with 4 header lines to: (1) describe the measurements, (2) specify delivered dose per moderator shell [MU], (3) specify dose rate used [MU/min], (4) specify the irradiation duration [seconds]
    • Structure of the data portion is such that measured data can be readily copy/pasted from Google Sheets.
  • Input values should already have noise and photon contamination removed.
  • If multiple measurements were obtained for the same moderator shell, input them sequentially on subsequent lines and adjust the num_meas_per_shell setting accordingly.

Settings file

  • This file contains all of the user-configurable settings for the application.
  • Default file: input/unfold_spectrum.cfg
  • Can specify alternative settings file at runtime via:
./unfold_spectrum.exe --configuration <file_name>
  • The description of each setting is provided in the Settings Table below.
  • Default values are indicated where applicable.
    • To use default values, do not delete settings, simply leave the value blank.

Energy bins

  • This file contains the energies over which the NNS response functions are defined, and thus the energy bins at which unfolded neutron fluence spectra are quantified.
  • Units: [MeV]
  • The first value is the lowest energy.
  • Values are placed on subsequent lines (no commas).
  • File is set via the path_energy_bins setting.

NNS response functions

  • This file contains the values of the NNS response functions for all eight moderator configurations over the energy range specified in the energy bins file.
  • The response of a given moderator is presented on a single line, with values separated by commas and from low to high energy.
  • Units: [cm2]
  • The response of the bare detector is on the first line.
  • Subsequent lines have the response for the next moderator configuration.
  • File is set via the path_system_response setting.

Guess spectrum

  • This file contains a guess neutron fluence spectrum used as the starting point of MLEM unfolding, defined over the energy range specified in the energy bins file.
  • Units: [neutrons cm-2 s-1]
  • The first value corresponds to the lowest energy bin.
  • Values are placed on subsequent lines (no commas).
  • File is set via the path_input_spectrum setting.

Ambient dose equivalent conversion factors

  • This file contains the conversion coefficients used to convert from neutron fluence to ambient dose equivalent.
  • Units: [pSv cm2]
  • Values were obtained at the desired energies by linearly interpolating between the tabulated coefficients in Table A.42 of the ICRP 74 report: Conversion Coefficients for use in Radiological Protection against External Radiation.
  • The first value corresponds to the lowest energy bin.
  • Values are placed on subsequent lines (no commas).
  • File is set via the path_icrp_factors setting.

Output files

Unfolded spectrum CSV file

  • This file contains the unfolded neutron fluence spectrum in CSV form that was generated by the unfolding process.
  • Units: [neutrons cm-2 s-1]
  • The first line of the file contains the comma-separated energy bins [MeV].
  • The second line contains the comma-separated values of the unfolded spectrum.
  • The third line contains the lower uncertainty values.
  • The fourth line contains the upper uncertainty values
  • If multiple unfoldings are performed using the same output file, the output spectra and their uncertainties will be appended on new lines to the existing file.
  • File is set via the path_output_spectra setting.

Unfolded spectrum figure

  • This file contains a plot of the unfolded neutron fluence spectrum (PNG image file).
  • Generation of this figure can be toggled off using the generate_figure setting.
  • Advanced plotting of the spectrum can be performed using plot_spectra.exe and the spectrum CSV file.
  • File is set via the path_figure setting.

Unfolding report

  • This file contains a text summary of the unfolding process.
  • Includes all pertinent info, including all inputs and outputs.
  • Can be used to check and archive previous unfoldings.
  • File is set via the path_report setting.

Settings

Name Default value description
algorithm mlem Specify which unfolding algorithm to use.
mlem: Standard MLEM with a specified mlem_max_error and mlem_cutoff.
mlemstop: use the modified MLEM-STOP criterion (link to paper).
map: use maximum a priori with a specified beta and prior.
beta 0 Beta value used in map unfolding.
cps_crossover 30000 Crossover (optimal) CPS value used in MLEM-STOP. Default value to be used for linac spectra (link to paper).
f_factor 7.2 Conversion coefficient between neutron current and CPS for NNS [fA/cps].
generate_figure 1 1 = generate figure, 0 = no figure.
generate_report 1 1 = generate report, 0 = no report.
meas_units nc Specify units of measured values {nc,cps}.
mlem_cutoff 15000 Maximum # of MLEM iterations.
mlem_max_error 0 Maximum (target) relative error between measured and reconstructed values, below which MLEM terminates. To unfold for a fixed # of iterations, set algorithm=mlem and mlem_max_error=0, then set mlem_cutoff accordingly.
nns_normalization 1.14 NNS-dependent normalization factor.
num_meas_per_shell 1 # of measured values input per moderator shell.
num_uncertainty_samples 50 # of samples generated to determine spectral uncertainty (if uncertainty_type=poisson or gaussian).
path_energy_bins input/energy_bins.csv Pathname to energy bins file.
path_figure output/figure_<name> Pathname to output unfolded spectrum figure file. name determined from measurements file header.
path_icrp_factors input/
icrp_conversion_coefficients		 Pathname to file containing ambient dose equivalent conversion coefficients [pSv cm^2].
path_input_spectrum input/spectrum_step.csv Pathname to input (guess) spectrum file.
path_measurements input/measurements.txt Pathname to NNS measurements file.
path_output_spectra output/output_spectra.csv Pathname to output unfolded spectrum CSV file.
path_report output/report_<name> Pathname to output unfolding report file. name determined from measurements file header.
path_system_response input/response_nns_he3.csv Pathname to NNS response functions file.
prior mrp Type of prior calculation to be done if algorithm=map.
quadratic: smoothing, no edge preservation.
mrp: median root prior; preserves edges by not penalizing regions of monotonic increase or decrease.
medianrp: mean root prior; custom written; similar to mrp but based on mean of neighbours.
uncertainty_type poisson Method used to calculate uncertainty region around the unfolded spectrum {poisson,gaussian,j_bounds}.