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Fermi LAT Simulator

Before you begin using the LAT simulator, please make sure you have read the introduction first, and specifically the introduction to the LAT and its components. The instructions on how to start the simulation and tips on understanding the results are below. The instructions you need to operate the simulator are also online in the simulator itself. Enjoy!

Instructions for using the Simulator

Begin by reading the text in the main display window (the large green window on the right). It will tell you what steps to take. If you place your mouse over any of the buttons on the simulator, a popup will appear with helpful information about that button (this feature can be turned off by un-selecting the "help tooltip" box).

The first thing to do is to choose an energy for your incoming gamma ray. To do this, go to the upper left window, which says "Please choose a photon energy range to use in the simulation." Hold your mouse over this window and the text will disappear, revealing nine different energy level options, from 50MeV up to 100GeV. Choose an energy value by clicking on it. If you choose "random" the simulator will pick a random value for you. To choose a different value, simply click on it. When you move your mouse from that window your chosen energy will show in place of the old text.

Next you must choose an angle for the incoming gamma ray at which it hits the LAT. To pick an angle, mouse over the box right under where you chose your energy. There are three different angles to pick from: 0 degrees (straight down; in reality the photon comes in at a very slight angle to prevent numerical errors in the simulator), 30 degrees, and 60 degrees. Again simply click the desired angle or change it by clicking another value. The chosen angle will show up in the angle window just as the energy did in the energy window.

Press the start button to start the simulation! Pictures of the simulation may take a moment to load. Use the 1 - 4 number keys on the top of your keyboard to change camera views (using a number pad may not work). The image displayed is only a piece of the entire image; click and drag the image to see more of it. To run another simulation simply hit the reset button and pick new input parameters. If you would just like to see the structure of the LAT with no gamma ray interaction, you can select the "LAT Structure" box and it will display a wide-field view of the LAT (complete with Fermi's solar panels) with no gamma-ray interaction (see "Understanding the Results" below for a description of the components).

Understanding the Results

The image displayed when you run the simulator shows a wireframe diagram of the LAT, the incoming gamma ray, and any particles created due to interactions with the gamma ray and the LAT.

The LAT is shown in white (this is most easily seen by choosing the side view). The outer cube is the shell of the anti-coincidence detector (ACD; an instrument that helps Fermi determine if an event is really from a gamma ray or not) and the thermal blanket which wraps around the LAT assembly, protecting it from large variations in heating.

The 16 towers of the LAT trackers are inside of the ACD and thermal blanket, and are shown as long rectangular boxes on end in a 4 x 4 array. Below the trackers is the calorimeter, which is comprised of flat brick-shaped boxes.

The incoming gamma ray is colored purple. It passes into the LAT and then down into the calorimeter. Particles created in the interaction are electrons (red lines), positrons (green lines), and neutral particles (white lines).

Things to Note

As you use the simulator, you will see changes in the output as you vary the input. Some things to pay attention to are:
  • How does the output change as you increase the incoming gamma ray energy?
  • How does the output change as you keep the energy the same but change the angle?
  • How does the output change if you vary the angle but keep the energy the same?
  • How does the number of neutral particles compared to electrons/positrons change as the energy is increased?
As you run different cases, keep your eye on the number of particles generated, what kind they are, and where they tend to go after the interaction. You'll get an appreciation for the level of care and understanding Fermi scientists must have to study the high-energy Universe! More information about the simulator software and its input and output can be found on the Stanford Fermi website

Fermi Gamma-ray Space Telescope - Education and Public Outreach - LAT Simulator