Fermi Spacecraft and Instruments
The Fermi Gamma-ray Space Telescope (formerly called GLAST) is an international and multi-agency space observatory that will study the cosmos in the photon energy range of 8,000 electronvolts (8 keV) to greater than 300 billion electronvolts (300 GeV). An electronvolt is a unit of energy close to that of visible light, so Fermi will catch photons with energies thousands to hundreds of billions of times greater than those we see with our eyes (1 keV = 1,000 eV, 1 MeV = 1,000,000 eV, 1 GeV = 1,000,000,000 eV).
Fermi carries two instruments: the Large Area Telescope (LAT)
and the GLAST Burst Monitor (GBM)
. The LAT is Fermi’s primary instrument, and the GBM is the complementary instrument.
REQUIREMENTS FOR THE LAT AND GBM
Based on our knowledge of the gamma-ray sky from previous missions, scientists defined the following requirements for the Fermi instruments:
The Large Area Telescope (LAT)
The GLAST Burst Monitor (GBM)
- Because the sky at gamma-ray energies has so many variable sources, the LAT must have a large field of view, over 2 steradians (one-fifth of the entire sky).
- To identify and study sources accurately, the LAT must be able to measure the locations of bright sources to within 1 arcminute (about 1/30 of the diameter of the full Moon).
- The study of gamma rays covers a broad energy range, so the LAT must catch photons with energies from 30 MeV to greater than 300 GeV. In particular, the LAT will have high sensitivity above 10 GeV, because almost nothing is known about cosmic objects at these energies.
- Since gamma-ray bursts can release a torrent of gamma rays within a fraction of a second, the LAT must be able to measure gamma rays over short time intervals.
- Because scientists need long observations to understand many types of sources, the LAT should be able to operate for many years without degradation.
- Because of the high flux of cosmic rays, which can mask the much smaller flux of gamma rays, the LAT must be able to reject 99.999% of signals generated by cosmic rays.
Large Area Telescope (LAT)
- Gamma-ray bursts (GRBs) come from random directions of the sky, so the GBM must watch as much of the entire sky as possible at all times.
- To gain the most information about GRBs, the GBM should be able to measure photon energies over a wide range, down to 8 keV and up to energies that overlap the LAT energy range.
- Since GRBs last from mere microseconds to thousands of seconds, the GBM must be able to detect GRBs over a wide range of timescales.
The LAT has four subsystems that work together to detect gamma rays and to reject signals from the intense bombardment of cosmic rays. For every gamma ray that enters the LAT, it will have to filter out 100,000 to one million cosmic rays, charged particles that resemble the particles produced by gamma rays. The four main subsystems are:
Image right: The LAT (silver box at the top) was integrated on the spacecraft at General Dynamics Advanced Information Systems in December 2006. (NASA/General Dynamics Advanced Information Systems)
- Anticoincidence Detector
- Data Acquisition System
With its very large field of view, the LAT sees about 20% of the sky at any given moment. In sky-survey mode, which is the primary observing mode, the LAT will cover the entire sky every three hours. The observatory can also be pointed at targets of opportunity, and can slew autonomously when either instrument detects sufficiently bright gamma-ray bursts (GRBs). The LAT is at least 30 times more sensitive than any previous gamma-ray instrument flown in space, and will detect thousands of new sources during GLAST’s five-year primary mission.
The LAT was assembled at the Stanford Linear Accelerator Center (SLAC), but with substantial hardware contributions from partners in France, Italy, Japan, Sweden, and the U.S. SLAC also manages the collaboration. The Principal Investigator is Peter Michelson of Stanford University/SLAC.
How the LAT Detects Gamma Rays and Rejects Cosmic Rays
Components of the LAT
Image right: The LAT has 16 towers of particle detectors, seen here before the installation of the Anticoincidence Detector. Each tower contains a Tracker module and a Calorimeter module. The Data Acquisition System is located underneath the towers. Credit: SLAC
- A gamma ray enters the LAT. It first passes through the Anticoincidence Detector without producing a signal.
- The gamma ray interacts in one of 16 thin tungsten sheets. This interaction converts the gamma ray into an electron and a positron via pair production (governed by Einstein’s equation E=mc2).
- The Tracker uses silicon strips to measure the paths of the electron and positron, allowing the LAT to determine the arrival direction of the gamma ray.
- The electron and positron enter the Calorimeter, which measures the energies of the particles, and therefore the energy of the original gamma ray.
- Unwanted cosmic-ray particles produce a signal in the Anticoincidence Detector, which tells the Data Acquisition System to reject the signal. The Anticoincidence Detector rejects 99.97% of unwanted signals produced by cosmic rays that enter the LAT.
- Software in the LAT Data Acquisition System also rejects, based on arrival direction, unwanted gamma rays that originate in Earth’s atmosphere.
The Tracker consists of a four-by-four array of tower modules. Each tower module consists of layers of silicon-strip particle tracking detectors interleaved with thin tungsten converter foils. The silicon-strip detectors precisely measure the paths of the electron and positron produced from the initial gamma ray. The pair-conversion signature is also used to help reject the much larger background of cosmic rays.
The Calorimeter measures the energy of a particle when it is totally absorbed. The LAT Calorimeter is made of a material called cesium iodide that produces flashes of light whose intensity is proportional to the energies of the incoming particle. The Calorimeter also helps to reject cosmic rays, since their pattern of energy deposition is different from that of gamma rays.
Anticoincidence Detector (ACD)
The Anticoincidence Detector is the first line of defense against cosmic rays. It consists of specially formulated plastic tiles that produce flashes of light when hit by charged-particle cosmic rays (but not by gamma rays, which are electronically neutral). The ACD forms a "hat" that fits over the tracker.
Data Acquisition System (DAQ)
The Data Acquisition System is the brain behind the LAT. It collects information from the Tracker, the Calorimeter, and the Anticoincidence Detector and makes the initial distinction between unwanted signals from cosmic rays and real gamma-ray signals to decide which of the signals should be relayed to the ground. This system also does an on-board search for gamma-ray bursts. The DAQ consists of specialized electronics and microprocessors.
GLAST Burst Monitor Principal Investigator Charles “Chip” Meegan, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Ala., tests the GLAST Burst Monitor. Credit: NASA/MSFC/D. Higginbotham
GLAST Burst Monitor (GBM)
The GBM consists of 12 detectors made of sodium iodide for catching X rays and low-energy gamma rays, and two detectors made of bismuth germanate for high-energy gamma rays. Together, they detect cover X rays and gamma rays in the energy range between 8 keV to 30 MeV, overlapping with the LAT’s lower-energy limit. The GBM detectors will view the entire sky not occulted by Earth, and are expected to pick up about 200 GRBs per year, as well as solar flares and other transient events. The combination of the GBM and the LAT provides a powerful tool for studying GRBs over a very wide range of energies.
The development of the GBM and analysis of its observational data is a collaborative effort between the National Space Science and Technology Center in the U.S. and the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany. The instrument is managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. Charles "Chip" Meegan of NASA Marshall is the Principal Investigator. In July 2007 Jochen Greiner of Max Planck replaced the now-retired Giselher Lichti as Co-P.I.
How the GBM Detects Gamma-Ray Bursts
Components of the GBM
- An X ray or low-energy gamma ray from deep space enters one of the 12 GBM low-energy detectors, which are flat disks made of a sodium-iodide material that produces a faint flash of light when struck. A photomultiplier tube detects the flash.
- The 12 detectors are located on opposite sides of the GLAST satellite, so they face different directions in the sky.
- When gamma rays from a gamma-ray burst reach the GBM, the disk facing the burst will detect more gamma rays than the others.
- By comparing the rate of signals from four or more detectors, the GBM can triangulate the arrival direction of the burst to within several degrees.
- Two high-energy detectors made of bismuth germanate pick up higher-energy gamma rays and measure their energies in much the same way as the low-energy detectors.
The low-energy sodium iodide detectors detect X rays with about 8 keV of energy up to gamma rays with about 1 MeV. They provide the locations of gamma-ray bursts to within several degrees, and they overlap in energy with other missions that detect GRBs, such as NASA’s Swift satellite. The low-energy detectors are mounted in four banks consisting of three detectors each. The 12 detectors are oriented in various directions so they face different parts of the sky. The GBM uses the signals from the low-energy detectors to detect burst locations.
The high-energy detectors are made of bismuth germanate, which is sometimes abbreviated BGO because germanate is a germanium oxide. They cover the energy range from about 150 keV to about 30 MeV, providing a good overlap with the low-energy detectors at the bottom end of the gamma-ray energy range, and with the LAT at the high end. Bismuth germanate is a high-density material that provides better sensitivity at high energies. The two high-energy detectors are positioned on opposite sides of the spacecraft, providing nearly full sky coverage.
Data Processing Unit
The electronics and microprocessors in the data processing unit receive and analyze the data from the low-energy detectors and high-energy detectors. It detects GRBs, determines their energies and arrival directions, and sends data to the GLAST spacecraft for transmission to the ground.