Gamma and Cosmic Ray Astrophysics

 
 
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Compton Telescopes

The principle of a Compton telescope is illustrated in the Figure above.  A gamma ray from the sky or other target scatters in Detector 1 (top) and is captured in Detector 2 (bottom).  The scatter angle, F, uniquely determined by measuring the energy lost in Detector 1, E1 , and the total energy of the gamma ray, Et, by the Compton formula:

cos(F) = mec2 (  1/(Et-E1)  -  1/E1  )  ,

where me is the mass of the electron, and c is the speed of light ( mec2 = 511 keV).  Presuming the scattered gamma ray is totally absorbed in Detector 2, then the total energy is given by the measurements, Et  = E1 + E2.  We will discuss below how the total energy, Et, may be determined for events that interact 3 or more times in an instrument, without the need to totally absorb the gamma ray (3-Compton principle).

The positions of the interactions in Detectors 1 and 2 determines the direction of the scattered gamma ray (blue line).  Thus, the original gamma ray can be localized to a conical structure about the direction of the scattered gamma ray, and with a cone 1/2 angle equal to the measured gamma ray scattering angle F.  Projecting the cone back onto the sky as shown, the direction of the original gamma ray is determined to lie somewhere on a ring (shown in red). 

In astrophysics, we are interested in targets that are far away.  Thus, the origin of a single gamma ray is determined to be anywhere within a ring projected onto the sky.  It is the superposition of many such rings that allows us to reconstruct an image of the actual gamma ray sky. 

3-Compton Technique

Modern developments in detector and electronics technology now enable a new generation of gamma ray detectors based on recording each and every energy loss associated with an incident gamma ray.   The energy of an incident gamma ray is determined by measuring the positions of the first three interactions, and the energy loss of the first two (i.e. measure the scatter angle of the second interaction).  Subsequent interactions are beneficial but not necessary.  We call this the 3-Compton principle.

The direction of the incident gamma ray is restricted to a conical range of possible directions, the same as in the "traditional" Compton telescope which requires total energy absorption to perform. 

The significance of a 3-Compton detector is at least three-fold: 

  1. A gamma ray need not be totally absorbed in order to measure its full energy.  Thus, relatively thinner detectors are possible.  Detection efficiencies approaching 40% at 1 MeV are possible in a silicon detector system of only 40 g/cm2 thickness (NRL Advanced Compton Telescope). 
  2. These detectors are naturally imaging without the need for a complex aperture or collimator. 
  3. These detectors have little or no Compton shelf, thus Compton rejection or heavy shielding is no longer required.
Our recent emphasis at NRL has been to exploit the measurements of multiple interactions within the detectors, providing dramatic gains in efficiency, background rejection, and ultimately sensitivity. 

NRL Advanced Compton Telescope (ACT)

Science Goals

The nuclear line region of the gamma ray spectrum (roughly a few 100 keV through 10 MeV) is rich with scientific interest, particularly with the direct observation of nucleosynthesis products and dynamics of supernovae and novae.  These observations require new and significantly more sensitive telescopes to advance the science over the current standard set by the Compton Gamma Ray Observatory and the future ESA INTEGRAL missions.  Various configurations of a Compton telescope are believed to be the most promising approach to achieving this ambitious goal.  A minimum of an order of magnitude increased sensitivity, good energy resolution, and good position resolution are all necessary in this next generation instrument.  NASA's Gamma Ray Astro-Physics Working Group (GRAPWG) has established the science objectives addressed by an advanced Compton telescope as the highest priority for the next major mission in gamma ray astrophysics.  The generic name for this mission is the Advanced Compton Telescope (ACT).

The goals of ACT are broad: 

  1. Map the galaxy, for the first time and with good angular resolution, in the line emissions from 26Al, positron annihilation, 60Fe, 44Ti, 12C, 16O, 56Fe, and the positronium continuum.  These maps will reflect the nucleosynthetic contributions of supernovae, novae, and massive stars, discover many sites of galactic supernovae in 44Ti (last 1000 yrs) and 26Al (last 106 yrs) and map interactions of low energy cosmic rays in the interstellar medium and molecular clouds. 
  2. Detect fresh radioactivity from several extragalactic Type Ia supernovae per year, and determine the nature of Type Ia events.
  3. Test the explosive nucleosynthesis models for galactic novae through observations of prompt and long-lived radioactivities.
  4. Provide high resolution spectra for several thousand AGN in the low-energy gamma ray region, study the evolution of AGN in this energy band where they exhibit peak luminosity, and support multi-wavelength campaigns for AGN.
  5. Elucidate the nature of gamma ray bursts with less than arc-minute position determinations, high resolution spectroscopy, and sensitivity to gamma ray polarization.
  6. Determine the surface gravitational fields of neutron stars by redshift measurements of nuclear line emission and thereby constrain the equation of state of neutron star material.

    7. Determine the character and origin of the cosmic gamma ray background.

ACT Mission Concept

NRL is actively developing ACT mission concepts that exploit the new capabilities of highly segmented semiconducting detectors such as germanium and silicon, and their ability to apply the 3-Compton principle to improve efficiency, reject backgrounds, and image.  Preliminary estimates indicate that the NRL baseline ACT concept will exceed the scientifically driven requirements established by the GRAPWG.

Advanced Telescope for High Energy Nuclear Astrophysics (ATHENA) 

An early configuration of an advanced Compton concept called "ATHENA" that has been studied at NRL consists of two large detectors, each made entirely from an array of germanium strip detectors (GSD).  The excellent energy and position resolution offered by germanium strip detectors will, in principle, fully satisfy the demands of the next generation gamma ray telescope.  Sensitivity of the ATHENA concept has been studied through extensive Monte Carlo modeling. 

Near field Compton telescope 

Compton telescopes are also applied to near field imaging problems.  In this case, the reconstruction is a three-dimensional problem.  The principle advantages of a Compton imager is that it is substantially more efficient than a pin-hole or collimated imaging system, and that it has a wide field-of-view.  Thus, a Compton telescope is the ideal instrument to survey a wide area or large target where the sources of radiation are distributed or unknown.  Again, a Compton telescope using germanium strip detectors have the advantage of better telescope reconstruction and good energy resolution to assist in isotope identification. 

A laboratory configuration of a Compton telescope using two germanium strip detectors.  The orientation of the two detectors is limited by the specific devices that were available at the time. A  Laboratory demonstration  imaging point sources in the near field prove that the germanium strip detectors make good upper (D1) and lower (D2) detectors in a simple Compton telescope. 
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last updated: 7 Novemeber 2000