Gamma and Cosmic Ray Astrophysics

 
 
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3-Compton Telescope

The Compton scattering process can be exploited to compute the energy of a gamma ray, without the need to absorb the full energy.  This provides a significant increase in detector efficiency vs. a standard calorimeter for energies above a few hundred keV.  A fraction of the gamma rays incident upon a standard calorimeter are only partially absorbed, providing an incomplete measurement of the energy.  This is often a large fraction in the energy range above a few 100 keV.  The 3-Compton technique can recover many of these partial events by computing the correct energy by observing the positions and energy losses of each interaction that a gamma ray undergoes.

Applications of this principle are many, including more sensitive instruments designed for future astrophysics missions, where source fluxes are low, backgrounds are high, and size and weight are limited. 

The process is illustrated below...

Consider a gamma ray that enters an imaging detector, shown here as a stack of six detectors.  Each detector is capable of measuring the position and energy of an interaction.  A gamma ray enters (yellow arrow) and undergoes two Compton scatters, followed by a third interaction of any type.  What happens to the gamma ray after that is immaterial in this example.  We call this the three-Compton technique, representing the need for three interactions. 

Directly measured quantities in the figure (shown in blue) are the energy losses in the detectors, L1 , L2 , L3 , and the second scatter angle, Q2 , determined from the positions of the first three interactions.  Quantities that will be derived (shown in red) are the incident energy, E1, and the first scatter angle, Q1 .

The energy of this gamma ray is uniquely determined by the energy losses of the first two interactions, L1 and L2 respectively, Q2 , through the application of one form of the Compton formula.  In essence, the Compton scattering formula specifies the energy of a gamma ray before a scatter, given the energy lost in the scattering event, and the angle of scatter.   The result is expressed by,

where the term mec2 = 511 keV.  The term on the right is the Compton expression for E2. L1 is added to E2 to find the incident energy, E1. 

Precise position and energy measurements are essential for an accurate energy determination.   Uncertainty in the reconstructed energy, E1, and first scatter angle, Q1 , a function of uncertainties in the measured quantities (link to expression for uncertainties).

Details are available in a few of our online publications.

Links to:  Last revised: 7 November 2000