Compton Scattering

The region from about 1 to 30 MeV is a difficult, but interesting, part of the gamma-ray astronomy energy range. This is the region where nuclear emission lines can be detected, and a region where some pulsars and active galaxies are strongly detected. In addition, solar flares and gamma-ray bursts can often be detected in this energy band. It is also the energy range where Compton scattering is the dominant physical interaction. Compton scattering occurs when a photon "hits" an electron with some of the photon energy being transferred to the charged particle. The Compton Scatter Telescope uses this interaction as the basis of its detection scheme.

Compton scattering process

Compton scatter telescopes have been largely experimental in design. The most advanced and successful instrument is the so-called COMPTEL (COMPton TELescope) aboard NASA's Compton Gamma-Ray Observatory.

Basic operating principles

Compton scatter telescopes are typically two-level instruments. In the top level, the cosmic gamma-ray Compton scatters off an electron in a scintillator. The scattered photon then travels down into a second level of scintillator material which completely absorbs the scattered photon. Phototubes viewing the two levels can approximately determine the interaction points at the two layers and the amount of energy deposited in each layer.

COMPTEL instrument showing an incident gamma-ray

As shown in the picture above, the line between these two interaction points does not point back to the direction of the incoming photon. It is possible, however, to determine the angle of incidence the cosmic photon made with respect to this line because the Compton scattering law provides for a definite relationship between this angle and the energy of the scattered photon (measured in the second level) and the scattering electron (measured in the first level). Unfortunately, while this angle is calculable, you cannot determine which azimuthal direction the photon came from. As a result, the gamma-ray could have come from anywhere in a ring on the sky, which makes analyzing Compton telescope data particularly challenging.
COMPTEL operating principle
The operating principle of COMPTEL. An incoming photon enters from above and compton scatters in the first detection layer (blue), then is (partially) absorbed in the second layer (green).
As with other instruments, careful anticoincidence and time-of-flight techniques are needed to reduce the noise caused by cosmic rays.

Detector characteristics

Even large Compton-scatter telescopes have relatively small effective areas. This is because only a small number of the incident gamma-rays actually compton scatter in the top level. So even if an instrument like COMPTEL has a geometric area of several thousand cm2, the effective area (weighted for the probability of an interaction) is a few tens of cm2.

Energy resolution is fairly good for these detectors, typically 5-10%. This is limited by uncertainties in the measurements of the energy deposited in each layer. Compton scatter telescopes have wide fields-of-view and can form images even though the so-called point spread function (the probability that an event came from a certain area on the sky) is a ring.

Future developments

Current research on Compton telescopes is emphasizing ways of tracking the scattered electron. By measuring the direction of the scattered electron in the top level, a complete solution for the incoming trajectory of the cosmic gamma-ray can be found. This would allow Compton telescopes to have more conventional data analysis approaches since the "event circle" would no longer exist.

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