Interactive Compton Edge Calculator
Enter incident gamma energy to compute the Compton edge (maximum recoil electron kinetic energy), plus the corresponding backscattered photon energy.
What Is the Compton Edge?
In gamma spectroscopy, the Compton edge is the highest-energy end of the Compton continuum. It represents the maximum kinetic energy transferred to an electron when a gamma photon scatters off a nearly free electron and the photon is backscattered by 180 degrees.
This edge matters because real detectors (NaI(Tl), plastic scintillators, etc.) often show large Compton continua. Identifying the edge helps with detector calibration, response modeling, and source identification.
Equations Used in This Calculator
The calculator uses the standard Compton scattering relations with electron rest energy:
E'180 = Eγ / (1 + 2Eγ/(mec2))
Tmax = Eγ - E'180 = 2Eγ2 / (mec2 + 2Eγ)
- Eγ = incident gamma energy
- E'180 = scattered photon energy at 180°
- Tmax = Compton edge energy (max recoil electron energy)
How to Use the Calculator
- Enter your gamma energy (for example, 661.7 keV for Cs-137).
- Select keV or MeV.
- Click Calculate.
- Read the Compton edge value and the backscattered photon energy.
Quick Reference for Common Sources
| Isotope / Line | Gamma Energy (keV) | Approx. Compton Edge (keV) |
|---|---|---|
| Cs-137 | 661.7 | ~477.3 |
| Co-60 (line 1) | 1173.2 | ~963.2 |
| Co-60 (line 2) | 1332.5 | ~1118.1 |
| Na-22 | 1274.5 | ~1061.7 |
Practical Notes for Spectra Interpretation
1) Why the edge can look “rounded”
In ideal kinematics, the Compton edge is sharp. In real instruments, finite energy resolution, noise, and multiple scattering smear the edge into a shoulder.
2) Detector material matters
High-Z materials have stronger photopeak responses, while low-Z materials can show broad Compton features. Geometry and shielding also affect continuum shape.
3) Don’t confuse edge with peak
The Compton edge is not a photopeak. A photopeak corresponds to nearly full energy deposition. The edge is only the maximum single-scatter electron transfer.
FAQ
Is this valid for X-rays too?
Yes, the same Compton formula applies. At low energies, though, photoelectric absorption often dominates, so Compton features may be weaker.
Does the Compton edge depend on detector type?
The theoretical value depends only on incident photon energy and electron mass. Detector type mainly affects how clearly you can observe the edge.
What unit should I use?
Use whatever is convenient. The calculator accepts keV or MeV and reports both where helpful.