Escape line

Escape lines or escape peaks are spurious spectral lines , which in the X-rays - and gamma spectroscopy occur and z. B. can simulate the presence of non-existent radionuclides in the measured sample.

The name (English to escape ) indicates the origin of these lines: these are detection events in which not the entire energy of the photon is converted in the radiation detector , but a certain, always equal part of the energy returns to the detector leaves. This makes an additional peak in the spectrum , an image (a kind of reduced ghost image ) of the actual photo peak visible with a correspondingly lower energy.

causes

X-ray fluorescence

X-ray fluorescence as the cause of escape lines is most likely if the energy E of the incident photons is not too high above the area of ​​the K-lines of the detector material, i.e. H. approximately between 10 and 100 keV . The vacancy that has arisen in the K-shell (rarely in higher shells) through photoionization is reoccupied with the emission of an X-ray photon from the upper shells. If this X-ray photon leaves the detector material, it leads z. B. the energy E _{K of} a quantum with it. The kinetic energy of the electron, on the other hand, is converted into collisions in the detector, and the other, lower-energy photons that are created when electrons of the higher shells "move up" have only a low probability of escape. Therefore, in addition to the photopeak at energy E, an escape peak at E - E _{K is formed} . ${\displaystyle K_{\alpha }}$

X-ray escape lines from other energy levels ( etc.) are rarer according to the probabilities of these transitions. ${\displaystyle K_{\beta }}$

Pair formation

Example: Gamma spectrum of an Am-Be neutron source, recorded with a scintillation detector.

If the energy of the incident photon is above the rest energy of two electrons (1022 keV), pairs can form in the detector . The resulting positron is captured in the detector material and forms a positronium , which is then radiated in two photons of 511 keV each . If one or both of these photons can leave the detector - the easier the smaller the detector crystal - the photopeak ( full energy peak , FEP) of the energy E escape lines at E - 511 keV ( single escape peak ) and E - 1022 are formed keV ( double escape peak ).

In the gamma spectrum shown there is a component with an energy of 4.4 MeV. Single and double escape lines for this FEP are clearly visible.