- Flux from rate:
φ = (dN/dt) / AwheredN/dtis neutron rate andAis area. - Flux from totals:
φ = N / (A·t)whereNis total neutrons in timet. - Detector‑based flux:
φ = (C − B) / (ε·A·t)with gross countsC, backgroundB, and detection efficiencyε. - Fluence:
Φ = φ·t(total neutrons per unit area over the interval).
- Select a method that matches your measurements.
- Enter the exposed area and choose its unit.
- Fill the fields shown for the selected method.
- Set output formatting for reports or lab notes.
- Press Calculate; results appear above the form.
- Use Download CSV or Download PDF to save outputs.
| Method | Key inputs | Flux (n/cm²·s) | Flux (n/m²·s) |
|---|---|---|---|
| Rate & area | Rate = 5,000,000 n/s, Area = 0.02 m² | 2.5e4 | 2.5e8 |
| Total, time & area | N = 1.0e9, t = 120 s, Area = 500 cm² | 1.6667e4 | 1.6667e8 |
| Detector counts | C = 25,000, B = 5,000, ε = 0.25, t = 60 s, Area = 10 cm² | 133.33 | 1.3333e6 |
Examples are illustrative. Real detectors may require geometry, dead‑time, and spectrum corrections.
1) Why neutron flux matters
Neutron flux, φ, describes how many neutrons cross a unit area each second. It is central to reactor physics, activation analysis, shielding design, dosimetry, and neutron imaging. A higher flux increases reaction rates such as (n,γ) capture and fission probability, which directly affects power, material damage, and isotope production. This calculator reports flux in n/(cm²·s) and n/(m²·s) to match common laboratory and engineering conventions.
2) Flux versus fluence
Flux is a rate; fluence, Φ, is the time‑integrated total per area. If you expose a sample for time t, then Φ = φ·t. For irradiation experiments, fluence often correlates better with activation yield and displacement damage because it captures the accumulated neutron field. The time‑based methods in this tool compute both values automatically.
3) Units and conversions
Small samples often use cm², while facility maps and beamlines may use m². Since 1 m² = 10,000 cm², the same physical field will have a flux value that differs by a factor of 10,000 depending on the area unit. This calculator converts internally to SI units, then displays both outputs for quick cross‑checks.
4) Typical flux ranges
Background fields near sources can be modest, while research reactors and dedicated beam ports may reach very high flux. Thermal neutron flux in research settings is frequently reported around 108–1014 n/(cm²·s), depending on facility and location. Always confirm whether values are thermal, epithermal, or fast, because energy spectrum strongly affects activation and shielding requirements.
5) Three practical input pathways
If you know the neutron production or incident rate, use φ = (dN/dt)/A to estimate the surface‑averaged field. If you have a measured total N over an exposure t, use φ = N/(A·t) for a direct average. If you use a detector, correct counts for background and efficiency: φ = (C − B)/(ε·A·t). Each pathway represents a different measurement reality.
6) Detector corrections and data quality
Efficiency ε may depend on neutron energy, geometry, moderator configuration, and electronics thresholds. Background B should be measured under similar conditions, ideally with shielding or a shutter closed. If net counts are small, uncertainty can be large; increasing counting time reduces relative statistical error. For precision work, consider dead‑time, pile‑up, and scattering corrections.
7) Interpreting averages and geometry
The calculator assumes a uniform field across area A. In real setups, beams may have profiles and samples may see gradients. When flux varies spatially, define A to match the effective illuminated area and treat results as an average. For shielding calculations, conservative estimates often use peak flux rather than average flux.
8) Reporting and traceability
Engineering reports should include method, units, area definition, time basis, and detector parameters. Use the CSV/PDF exports to preserve inputs, outputs, and a timestamp for audit trails. When sharing results, specify whether values are instantaneous flux or integrated fluence, and document any assumptions about spectrum and geometry.
1) What is the difference between neutron flux and neutron current?
Flux counts neutrons crossing a unit area per second, regardless of direction. Neutron current is directional and accounts for net flow through a surface. Current can be smaller if neutrons move in many directions.
2) When should I use cm² instead of m²?
Use cm² for small detectors, foils, and sample cross‑sections. Use m² for facility‑scale surfaces or beam cross‑sections reported in SI. The calculator shows both outputs to prevent unit confusion.
3) How do I choose detector efficiency ε?
Use the calibrated efficiency for your detector and neutron energy range. If you only have a single ε, treat results as approximate. Efficiency can change with moderator thickness, geometry, and discriminator settings.
4) Why can net counts become negative?
If background exceeds measured counts, net counts become negative, which is non‑physical for this model. Re‑measure background, extend counting time, or verify that the detector and source conditions match between runs.
5) Does this tool account for neutron energy spectrum?
No. It computes geometric, time‑averaged flux. Spectrum matters because activation and shielding depend on energy. For spectrum‑aware work, use energy‑dependent efficiencies and report thermal/fast components separately.
6) How accurate is the rate-and-area method?
It is only as accurate as your neutron rate and the assumption of uniform distribution across the selected area. It is useful for estimates, but measurements with detectors are preferred for site‑specific validation.
7) What should I include in a formal report?
Include method, area definition, time basis, detector type, efficiency, background, units, and uncertainty notes. State whether you report flux or fluence, and record the geometry and spectrum assumptions used.