KAHIBARO
Table of Contents
Origin of escape peaks
In gamma ray spectroscopy, a detector does not always keep all the energy carried by an incoming gamma photon. An escape peak appears when the gamma ray produces an electron-positron pair inside the detector, but one or both of the annihilation photons leave the detector before depositing their energy.
This effect is mainly important for gamma rays with energy above the pair production threshold. Pair production requires at least
$$
E_\gamma \ge 2m_ec^2 = 1.022 \ \text{MeV}
$$
because creating an electron and a positron needs the rest energy of both particles.
After pair production, the positron usually does not survive for long. It slows down and annihilates with an electron, producing two photons, each with energy
$$
0.511 \ \text{MeV}
$$
If both annihilation photons are absorbed in the detector, the full gamma energy is still measured. If one escapes, the recorded energy is smaller by $0.511 \ \text{MeV}$. If both escape, the recorded energy is smaller by $1.022 \ \text{MeV}$.
For a gamma ray of energy $E_\gamma$, the escape peak energies are
$$
E_{\text{single escape}} = E_\gamma - 0.511 \ \text{MeV}
$$
and
$$
E_{\text{double escape}} = E_\gamma - 1.022 \ \text{MeV}
$$
These peaks can occur only when pair production is possible, so typically for $E_\gamma > 1.022 \ \text{MeV}$.
Physical picture inside the detector
The sequence is simple. A high energy gamma ray enters the detector. Near a nucleus, it converts into an electron and a positron. These charged particles lose their kinetic energy in the detector material. Then the positron annihilates, creating two $0.511 \ \text{MeV}$ photons.
If the detector is large and dense, these annihilation photons are more likely to be absorbed. If the detector is smaller, or if the interaction happens near the edge, one or both photons may escape. In that case the pulse produced by the detector corresponds to less than the original gamma energy.
Single escape and double escape peaks
There are two common types of escape peaks.
A single escape peak occurs when one annihilation photon escapes and the other is absorbed. The detector then misses $0.511 \ \text{MeV}$.
A double escape peak occurs when both annihilation photons escape. The detector then misses $1.022 \ \text{MeV}$.
These peaks appear at fixed distances below the full energy peak, which makes them easy to identify when the gamma energy is known.
| Peak type | Energy recorded |
|---|---|
| Full energy peak | $E_\gamma$ |
| Single escape peak | $E_\gamma - 0.511 \ \text{MeV}$ |
| Double escape peak | $E_\gamma - 1.022 \ \text{MeV}$ |
Example
Suppose the source emits a gamma ray of energy
$$
E_\gamma = 2.614 \ \text{MeV}
$$
Then the expected escape peaks are
$$
E_{\text{single}} = 2.614 - 0.511 = 2.103 \ \text{MeV}
$$
and
$$
E_{\text{double}} = 2.614 - 1.022 = 1.592 \ \text{MeV}
$$
So in the spectrum, besides the full energy peak at $2.614 \ \text{MeV}$, smaller peaks may appear at $2.103 \ \text{MeV}$ and $1.592 \ \text{MeV}$.
When escape peaks are important
Escape peaks are especially noticeable in high atomic number, high density detectors such as germanium detectors, because pair production becomes more important at higher gamma energies. They are also influenced by detector size. A larger detector gives annihilation photons more chance to interact before leaving, so escape peaks are reduced. A smaller detector tends to show escape peaks more clearly.
The probability of observing escape peaks increases when the original gamma energy is high enough for pair production and when the annihilation photons can leave the active detector volume.
Escape peaks are not separate gamma rays emitted by the source. They are detector artifacts caused by incomplete energy deposition inside the detector.
Distinguishing escape peaks from other peaks
Escape peaks must not be confused with photopeaks from different nuclides or with Compton features. Their key signature is their location exactly $0.511 \ \text{MeV}$ or $1.022 \ \text{MeV}$ below a higher energy full energy peak.
If a spectrum contains a strong gamma line above $1.022 \ \text{MeV}$, and weaker peaks appear at these precise offsets, they are strong candidates for single and double escape peaks.
Practical significance
Recognizing escape peaks helps avoid incorrect identification of radioactive sources. It also gives information about how gamma rays interact in the detector. In high resolution spectroscopy, especially with high energy gamma rays, escape peaks are a normal and useful part of interpreting the spectrum.
Important identification rule:
If a peak at energy $E$ is suspected to be an escape peak, check whether there is a stronger full energy peak at
$$
E + 0.511 \ \text{MeV}
$$
for a single escape peak, or at
$$
E + 1.022 \ \text{MeV}
$$
for a double escape peak.