interspec-isotopics

name	interspec-isotopics
description	Performs isotopic ratio analysis (enrichment, burn-up) for Plutonium and Uranium from gamma-ray spectroscopy using InterSpec MCP tools. Covers preset selection, configuration, relative efficiency fitting, ROI management, and result evaluation. Use when determining U-235 enrichment, Pu isotopic composition, and non-U/Pu isotopic relative activity analysis.
metadata	{"author":"InterSpec","version":"0.1"}

Isotopics Analysis Workflow

Determine isotopic composition (mass fractions) of nuclear materials from gamma-ray spectroscopy, usually Plutonium and Uranium, but can be used for any nuclide. For non-U/Pu spectra, can be useful to get relative activity ratios without knowing detector efficiency or source geometry/distance/shielding.

Method Overview

Isotopics analysis uses a relative efficiency approach: the efficiency as a function of energy, and activities of each nuclide, are fit by comparing observed peak areas to predictions from a given set of activities, efficiency parameters, and optionally shielding thicknesses. Once optimized, relative activities yield isotopic mass fractions. Distance and absolute detector efficiency are not required.

Workflow

Step 1: Analyze Request

Determine the analysis type:

• Plutonium: Pu-239, Pu-240, Pu-241, Am-241, etc. Look for peaks in the ~100-780 keV range.
• Uranium: U-235/U-238 enrichment. Look for peaks near 186 keV (U-235), 1001 keV (Pa-234m from U-238).
• Custom: Other isotopic ratios. Start from blank configuration.

Check prerequisites:

• get_spectrum_info - Verify foreground spectrum is loaded
• get_peaks - Check for relevant peaks to confirm material type

Step 2: Select and Load Configuration

Use list_isotopics_presets to see available presets, then load_isotopics_preset.

For Plutonium:

Preset	When to use
HPGe Pu (120-780 keV).xml	Default. Wide energy range, most comprehensive.
HPGe Pu (120-420 keV).xml	Low-energy region only. Use when low-energy peaks are clean.
HPGe Pu (610-775 keV).xml	High-energy only. Use when Am-241 interference at 59.5 keV is severe.

For Uranium:

Preset	When to use
HPGe U (120-1001 keV).xml	Default for U-235/U-238 enrichment analysis.

For other sources:

• reset_isotopics_config to start blank
• modify_isotopics_rois to add ROIs covering the major gamma lines (consider restricting to >~120 keV if sufficient high-energy lines exist)
• modify_isotopics_nuclides to add the nuclides of interest

Step 3: Validate Configuration

Call get_isotopics_config to review:

• Verify expected nuclides are present
• Verify ROIs cover relevant peaks
• If source ages are known, use modify_isotopics_nuclides to set them

Step 4: Check for Interferences

Review with get_peaks (filter 'analysis') for non-target nuclides with peaks in the analysis energy ranges.

Common interferences:

• Pu analysis: I-131, Cs-137, other fission products
• Mixed samples: U and Pu both present
• Background: K-40, Ra-226, Th-232 (usually outside ROI ranges)

If significant interference is found, add the interfering nuclide to the configuration with modify_isotopics_nuclides, or adjust ROIs with modify_isotopics_rois.

Step 5: Execute Calculation

Call perform_isotopics_calculation (no parameters needed - uses current configuration).

Returns:

• Mass fractions with uncertainties for each isotope
• Relative activities
• Ages (if fitted)
• Quality metrics (chi-squared per DOF)
• Pu-242 corrected values (for Pu analysis, if available)

Step 6: Evaluate Results

Overall chi2/DOF:

Value	Assessment
0.7 - 1.5	Good fit
< 5.0	Acceptable
5.0 - 10.0	Possibly acceptable (especially high-statistics spectra)
> 10.0	Poor - solution likely not acceptable

Per-ROI evaluation:

• Check roi_info -> chi2_per_dof for individual ROIs to find problem regions
• High chi2 in a single ROI may indicate missing source, wrong continuum type, or interference
• For high-statistics ROIs, try changing continuum type (e.g., to stepped continuum)

Observed efficiencies:

• Check observed_efficiencies -> deviation_from_solution_sigma
• Under-prediction (negative deviation) may indicate missing source or too much shielding
• Over-prediction (positive deviation) may indicate shielding needed, especially if low-energy peaks over-predict and high-energy under-predict

If results are poor:

• Try different peak skew model via modify_isotopics_options
• Adjust ROI extents, remove minor ROIs, or add ROIs with more significant peaks
• Try different relative efficiency curve settings (modify_isotopics_curve_settings)

Step 7: Report Results

For U/Pu, report mass fractions with uncertainties:

Uranium Isotopics Results:
  U-235: 20.0 +/- 1.0 %
  U-238: 78.1 +/- 1.5 %
  U-234: 0.8 +/- 0.01 %

Quality: chi2/DOF = 1.2 (good fit)

For non-U/Pu, report relative activities with uncertainties.

Include:

• Quality metrics (chi2/DOF overall and any problematic ROIs)
• Age information if fitted
• Pu-242 corrections if applicable
• Caveats about interferences or limitations

Configuration Modification Tools

• modify_isotopics_nuclides - Add/remove/update nuclides. Set ages, age fitting bounds, activity constraints.
• modify_isotopics_rois - Add/remove/update energy ROIs. Set continuum type, range behavior.
• modify_isotopics_curve_settings - Change relative efficiency equation type (LnXLnY, LnX, LnY, FramPhysicalModel), order, self-attenuation, external shielding.
• modify_isotopics_options - Global options: energy cal fit mode, FWHM form, peak skew type.
• modify_isotopics_constraints - Activity ratio and mass fraction constraints between nuclides.
• get_isotopics_config_schema - Full schema with valid values for all configuration fields.

Glossary

• ROI: Energy range used for isotopics analysis (distinct from peak ROI).
• Mass fraction: Percentage of total mass from each isotope.
• Relative activity: Activity ratio between isotopes (not absolute activity).
• Age: Time since purification of U or Pu (for Pu: when Am-241 was separated from Pu-241).
• Pu-242 correction: Empirical correction for Pu-242, which has no usable gamma lines.