GLAST LAT Performance

The top-level science performance requirements for the LAT are given in Table 1 of the Science Requirements Document.  The resulting performance is governed primarily by three things

Thus, although the hardware design is now complete, as the event reconstruction and event selection algorithms evolve the performance must be updated.  These are based on detailed Monte Carlo simulations.  The performance presented at PDR is summarized in the following table:

 

Starting from the front of the instrument, the LAT tracker (TKR) has 12 layers of 3% r.l. tungsten converters (THIN or FRONT section), followed by 4 layers of 18% r.l. tungsten converters (THICK or BACK section).

Note that a number of significant improvements are underway, particularly in the track and energy reconstruction.  Since the PDR, the full instrument simulation and reconstruction have been completely rewritten to support the flight program.  The performance summarized below is the result of studies for Data Challenge 1 and will soon be replaced with the results from preparations for Data Challenge 2 at the end of 2005.

A primary goal of the analysis is to produce full instrument response functions (IRFs), describing the performance as a function of photon energy, incidence angle, conversion point within the instrument, and other important parameters.  The plots below represent the work of many people on the LAT team.

 

 


 

 


 

 

 

 


It is important to remember that the LAT provides a wealth of information about each event, so an analysis can be optimized by science topic.  Different optimizations will lead to different apparent science performance.  For example, if a particular topic requires better energy resolution or PSF, but is not photon limited, it will be advantageous to make more stringent event selections.  The information needed for these selections will be provided by the standard reconstruction packages.

 

Point Source Sensitivity Plots

Using the above instrument performance characterization, we have produced two additional plots related to point source sensitivity.  The first is a single-energy sensitivity plot, showing the 5-sigma sensitivity to a high-latitude source whose spectrum is a delta function at the energy shown on the horizontal axis.  The assumptions are

 

Experiments are often compared using an integral sensitivity plot (5-sigma sensitivity for E>E0), e.g., by Morselli et al, Nucl. Phys. B85(2000), assuming a 1/E2 spectrum source at high latitude.  We show here an update for the GLAST LAT:

 

 

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updated  06 Nov 2005 , maintained by Steve Ritz