Help & Methodology Reference
The Space RHA NIEL Conversion Tool is a web-based calculator for proton non-ionizing energy loss (NIEL) in semiconductor materials. It provides NIEL lookup, fluence conversion between proton energies, displacement damage dose (DDD) calculation, and 1 MeV neutron equivalent fluence computation — all within a single browser-based interface requiring no software installation.
The tool is organized into six tabs:
| Tab | Function |
|---|---|
| NIEL Lookup | Look up proton NIEL at a specific energy for a given elemental or compound material |
| Fluence Converter | Convert fluence between different proton energies using NIEL ratios |
| DDD Calculator | Calculate displacement damage dose from fluence and proton energy |
| 1 MeV neq | Calculate 1 MeV neutron equivalent fluence using the standard 95 MeV-mb reference |
| NIEL Plot | Interactive log-log plot of NIEL vs. proton energy for multiple materials simultaneously |
| Data Table | Browse raw NIEL data from Jun et al. Table I |
The tool supports 10 elemental materials (Si, C, Al, P, Ga, Ge, As, In, Cu, Se) and compound materials computed via Bragg's Rule, with presets for common semiconductor compounds including GaAs, InP, SiC, SiGe, InGaAs, and CIGS (CuInGaSe2).
Non-Ionizing Energy Loss (NIEL) is the rate of energy loss by an incident particle due to atomic displacements in the target material, as opposed to ionization. When an energetic particle — such as a proton, neutron, or electron — traverses a crystalline semiconductor, it can transfer enough kinetic energy to a lattice atom to permanently displace it from its site. The displaced atom (the primary knock-on atom, or PKA) and the resulting vacancy form a Frenkel pair. If the PKA has sufficient energy, it produces additional displacements in a collision cascade, creating clusters of defects.
These displacement defects introduce energy levels within the semiconductor bandgap that act as generation-recombination centers, trapping centers, and compensation centers. The practical consequences for space electronics and photonics include:
Increased dark current in photodetectors and CCDs/CMOS image sensors, due to enhanced thermal generation of carriers at defect sites. Reduced minority carrier lifetime (τ), which degrades the gain of bipolar transistors (hFE ∝ τ) and the quantum efficiency of solar cells. Carrier removal in lightly doped regions, which can change the effective doping concentration and shift threshold voltages or depletion widths. Reduced LED and laser output power due to increased non-radiative recombination.
NIEL serves as the displacement damage analog of Linear Energy Transfer (LET) for ionizing dose. Just as LET quantifies ionizing energy deposition per unit path length, NIEL quantifies the non-ionizing (displacement) energy deposition per unit path length (or equivalently per unit areal density). The key utility of NIEL is that it enables the NIEL hypothesis: the assumption that displacement damage effects in a given material scale linearly with the total non-ionizing energy deposited, regardless of the incident particle type or energy. While this hypothesis has important limitations (discussed in Section 10), it remains the foundation of displacement damage dose methodology and enables the comparison of damage produced by different radiation sources.
The NIEL values used in this tool are taken from Jun et al. (2003) [1], which provides proton NIEL data for 10 elemental semiconductor materials (C, Al, Si, P, Cu, Ga, Ge, As, Se, In) over proton energies from below 1 keV to 1000 MeV. The Jun et al. calculations represent one of the most comprehensive and widely cited proton NIEL datasets available for device applications.
Jun et al. computed NIEL values using Monte Carlo simulations with the Geant4 radiation transport code. The total NIEL has two components: Coulomb (elastic) scattering, which dominates at low proton energies, and nuclear (inelastic) reactions, which dominate above approximately 10–20 MeV.
For Coulomb scattering, Jun et al. used analytic ZBL (Ziegler-Biersack-Littmark) screened potential cross-sections with relativistic corrections. For nuclear interactions, Geant4 models the full nuclear interaction cascade initiated when a proton enters a target material, tracking all secondary particles and their subsequent interactions. The total kinetic energy transferred to recoiling target atoms constitutes the displacement damage energy. The Lindhard partition function is applied to separate the nuclear (displacing) and electronic (ionizing) energy loss components of the recoiling atoms.
The resulting NIEL values are expressed as damage energy cross-sections in units of MeV-mb (equivalently, MeV-cm2/g when normalized by atomic mass). Table I of Jun et al. provides NIEL values at discrete proton energies for all 10 elements: C, Al, Si, P, Cu, Ga, Ge, As, Se, and In.
Silicon has a high-resolution 36-point dataset covering the full energy range, providing finer energy resolution than the standard Table I data available for other materials. This extended dataset enables more accurate interpolation for silicon, which is the most commonly used reference material for displacement damage calculations.
The NIEL Lookup tab allows you to find the proton NIEL value at any energy within the tabulated range for a selected material. Enter the proton energy in MeV and select the target material (elemental or compound). The tool returns the NIEL value in your chosen units.
Since NIEL data is tabulated at discrete energies, the tool uses log-log (power-law) interpolation to compute NIEL at intermediate energies. This is the standard interpolation method for NIEL data because the NIEL vs. energy relationship is approximately linear on a log-log scale over most of the energy range.
Given two adjacent tabulated points (E1, NIEL1) and (E2, NIEL2), the interpolated NIEL at energy E is:
Equivalently, this fits a power law NIEL ∝ Eα between each pair of adjacent data points, where α is the local slope on the log-log plot. This approach preserves the smooth, monotonic behavior of the NIEL curve and avoids the oscillation artifacts that can arise from polynomial interpolation on sparse data.
Many semiconductor devices use compound materials such as GaAs, InP, SiC, and more complex alloys like InGaAs or CIGS (CuInGaSe2). Since the Jun et al. dataset provides NIEL only for individual elements, the tool computes compound NIEL values using Bragg's additivity rule (also known as the Bragg-Kleeman rule).
Bragg's Rule states that the stopping power (or equivalently, the NIEL) of a compound material can be approximated as the mass-weighted sum of the stopping powers of its constituent elements:
where wi is the mass fraction of element i in the compound and NIELi is the NIEL of pure element i at the same proton energy. The mass fraction for a compound AmBnCp… is:
where ni is the number of atoms of element i in the chemical formula and Mi is the atomic mass of element i.
The tool includes presets for commonly used semiconductor compounds:
| Compound | Formula | Constituent Elements |
|---|---|---|
| Gallium Arsenide | GaAs | Ga, As |
| Indium Phosphide | InP | In, P |
| Silicon Carbide | SiC | Si, C |
| Silicon Germanium | SiGe | Si, Ge |
| Indium Gallium Arsenide | InGaAs | In, Ga, As |
| CIGS | CuInGaSe2 | Cu, In, Ga, Se |
You can also define custom compounds by specifying the elements and their stoichiometric coefficients. The tool automatically computes the mass fractions and the compound NIEL at the requested energy.
The Fluence Converter tab enables conversion of proton fluence from one energy to a damage-equivalent fluence at another energy. This is essential when comparing test data obtained at different proton energies, or when translating a space environment spectrum (with a range of proton energies) into an equivalent monoenergetic test fluence.
The conversion is based on the principle that equal displacement damage is produced when the product of fluence and NIEL is the same. Two fluences Φ1 at energy E1 and Φ2 at energy E2 produce the same displacement damage dose if:
Therefore, the damage-equivalent fluence at the target energy is:
Test planning: You need to reproduce the displacement damage from a mission environment (e.g., a spectrum of trapped belt protons) using a monoenergetic proton beam at a test facility. Convert the mission DDD into an equivalent fluence at the available beam energy.
Comparing test results: Two laboratories tested the same device type at different proton energies (e.g., 50 MeV and 200 MeV). Convert both datasets to a common reference energy to compare degradation behavior.
Literature comparison: Published degradation data is reported at a different proton energy than your test conditions. Convert to a common basis for meaningful comparison.
Displacement Damage Dose (DDD) is the total non-ionizing energy deposited per unit mass of the target material by the incident particle fluence. It is the displacement damage analog of total ionizing dose (TID), providing a single metric that quantifies the accumulated displacement damage regardless of particle type or energy (under the NIEL hypothesis).
For a monoenergetic proton fluence Φ at energy E, the DDD is simply:
For a polyenergetic spectrum (as encountered in space), the DDD is the integral over the energy spectrum:
Device parameters that are controlled by minority carrier lifetime τ (such as bipolar transistor gain, solar cell short-circuit current, and photodetector dark current) degrade in proportion to the displacement damage dose through damage coefficients. The classic Messenger-Spratt relationship for bipolar transistor gain degradation is:
where hFE,0 is the pre-irradiation gain and K is the displacement damage coefficient (which depends on the device, technology, and operating conditions but is independent of the radiation source under the NIEL hypothesis). Similarly, for solar cells, the short-circuit current degradation and maximum power degradation can be correlated with DDD.
The power of the DDD approach is that once K is determined from ground testing at any convenient proton energy, the in-orbit degradation can be predicted for any arbitrary proton (or neutron, or electron) environment by computing the mission DDD and applying the same damage coefficient.
The 1 MeV neutron equivalent fluence is a widely used normalization convention that expresses any displacement damage exposure as an equivalent fluence of 1 MeV neutrons that would produce the same displacement damage dose in silicon. It is the most common displacement damage metric in the radiation effects community and is the basis of most device specification limits for displacement damage hardness. Note that this is a silicon-referenced convention, not a material-independent universal metric — it is used by convention even for non-silicon devices.
The standard reference is the NIEL of 1 MeV neutrons in silicon:
The 95 MeV-mb value originates from displacement kerma calculations referenced in ASTM E722 [5]. Converting to mass units via NA/A gives 2.04 × 10−3 MeV-cm2/g. In practice, the community uses a range of slightly different reference values (typically 1.95–2.04 × 10−3 MeV-cm2/g) depending on the source and the exact displacement kerma calculation used. This tool uses 1.95 × 10−3 MeV-cm2/g, which is the value adopted by several widely used radiation transport codes and JPL analyses. The difference is less than 5% and is within the overall uncertainty of displacement damage predictions.
The 1 MeV neutron equivalent fluence for a proton irradiation at energy E is:
where Φp is the proton fluence and NIELp(E) is the proton NIEL at energy E. The ratio NIELp(E) / NIEL(1 MeV n, Si) is sometimes called the hardness factor or displacement damage equivalence factor.
ASTM E722 [5] defines the standard practice for characterizing neutron fluence spectra in terms of equivalent monoenergetic neutron fluence for radiation-hardness testing of electronics. The 95 MeV-mb reference value originates from this standard and is based on silicon displacement damage cross-section calculations. All device specifications expressed as "1 MeV neq/cm2" implicitly reference this standard.
NIEL is reported in several unit systems throughout the literature. This tool supports three common representations and provides automatic conversion between them.
| Unit | Symbol | Description |
|---|---|---|
| MeV-cm2/g | — | NIEL per unit areal density. This is the standard "mass stopping power" form, independent of material density. Most commonly used for DDD calculations. |
| keV-cm2/g | — | Same as above but in keV. Simply 1000 × the MeV-cm2/g value. Sometimes preferred for tabulation because values are order-unity for typical proton energies. |
| MeV-mb | — | Damage energy cross-section per atom. This is the form reported in Jun et al. Table I and is the natural output of Monte Carlo NIEL calculations. 1 mb = 10−27 cm2. |
The conversion between MeV-mb (per-atom cross-section) and MeV-cm2/g (mass stopping power) involves Avogadro's number and the atomic mass:
For compound materials, use the molecular mass (sum of constituent atomic masses weighted by stoichiometry) in place of A.
Users should be aware of the following limitations when applying NIEL-based displacement damage analysis.
The Jun et al. NIEL data is tabulated from sub-keV energies up to 1000 MeV, with the exact low-energy bound varying by material (e.g., 0.2 keV for Si, 0.08 keV for Ga). The tool does not extrapolate beyond the tabulated range for each material. At the lowest energies, Coulomb scattering dominates NIEL and the energy dependence changes character significantly — NIEL rises steeply and is sensitive to the displacement threshold energy. The tabulated data should be used with caution below ~1 keV. For energies above 1000 MeV, dedicated calculations (e.g., from the NEMO code [2] or Geant4 directly) should be used.
The mass-weighted additivity assumption (Bragg's Rule) treats compound materials as mechanical mixtures of elements. This neglects:
Chemical bonding effects: The threshold displacement energy (the minimum energy to permanently displace an atom) varies between a pure element and a compound. For example, the displacement threshold for Ga in GaAs differs from that in pure Ga.
Crystal structure effects: Defect formation and migration energies depend on the crystal structure, which differs between the compound and the pure elements.
Sub-threshold effects: In compounds, atoms displaced with energies near the threshold may recombine or form different defect configurations than in the pure elements.
At higher proton energies where nuclear interactions dominate over Coulomb scattering and recoil energies are far above displacement thresholds, Bragg's Rule is generally a reasonable approximation. Jun et al. validated the additivity approach against direct compound calculations for GaAs and found good agreement. The approximation becomes less reliable at lower proton energies where Coulomb scattering (which is more sensitive to the atomic number of individual target atoms and the displacement threshold energy) dominates.
The NIEL hypothesis — that displacement damage effects scale linearly with total non-ionizing energy deposited — is the foundation of all NIEL-based analysis. However, this hypothesis has known limitations [4, 6]:
Defect cluster vs. point defect sensitivity: Different device parameters may be sensitive to different defect types. Clustered defects (produced by high-energy PKAs from nuclear reactions) and isolated point defects (produced by low-energy Coulomb scattering) have different electrical properties. Two irradiations producing the same DDD but with different ratios of clustered to point defects may produce different amounts of device degradation. This is particularly relevant when comparing damage from different particle types (e.g., protons vs. electrons) or very different proton energies.
Material dependence: The NIEL hypothesis works best for silicon and becomes less reliable for compound semiconductors where the defect physics is more complex. In GaAs and InP, for example, the relationship between NIEL and device degradation may exhibit sub-linear or super-linear behavior depending on the specific device parameter and fluence regime.
Annealing effects: NIEL does not account for defect annealing, which depends on temperature, time, and injection level. Room-temperature annealing can reduce displacement damage over time, particularly for certain defect types in silicon. The DDD computed by this tool represents the initial damage deposition and does not include annealing corrections.
Enhanced low-dose-rate sensitivity (ELDRS) analog: Some devices show displacement damage rate effects analogous to ELDRS in TID testing, where the degradation per unit DDD depends on the dose rate. This is not captured by the NIEL-based framework.
This tool provides NIEL data for protons only. It does not include neutron, electron, or heavy-ion NIEL values. For electron displacement damage (relevant for solar cells in the Van Allen belts), separate electron NIEL calculations are required. For heavy-ion NIEL (relevant for certain detector and solar cell applications), see Messenger et al. [3].
Even when the NIEL hypothesis holds, damage coefficients (K values) are device-specific and can vary significantly between manufacturers, lots, and even wafers. A DDD value computed by this tool tells you how much non-ionizing energy is deposited, but the actual device degradation depends on the empirically determined damage coefficient for the specific part and parameter of interest.