Fermi: Orbiting Earth in search of answers

© The GLAST (Fermi) Spacecraft in Orbit*

An unknown view of the sky


We are all familiar with looking at the sky by night, but only some of us know that the visible spectrum is just a tiny part of the electromagnetic spectrum in which our sky shines. Many effects remain hidden from our human eye, although they are much brighter than what we can observe. Gamma-ray bursts (GRB) are such effects. In just a few seconds, a single GRB can release the same amount of energy that our sun radiates over its 10-billion-year lifespan (1). Still, we cannot see them as their wavelength is too short, and the light is absorbed in the upper layers of the Earth’s atmosphere.

 

As GRBs unveil the violent side of the universe, where stars collide or burst near black holes, the scientific community’s keen interest in these events led to the creation of a dedicated instrument for such high-energy electromagnetic radiation: The Fermi Gamma-ray Space Telescope (FGST)

 

Launched into space in 2008, the FGST orbits around the Earth every 90 minutes. Its two main sensors on board, the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT) scan the sky for intriguing events, picking up X-rays and gamma rays within a wide energy range (1). The GBM captures radiation with energies from 8 keV to 30 MeV, while the LAT extends the range to gamma rays ranging from 20 MeV to 300 GeV (1). Since existing instruments in space were limited to detecting light with energies up to 30 GeV, the new sensor marks a significant advancement in the detectable energy spectrum and helps to learn more about GRBs. However, developing and building such a sensor for a spacecraft proved to be a significant challenge.


*This beauty shot begins with the earth in full view, then spans to reveal the spacecraft and the gamma ray sky it will observe. Courtesy of NASA/Goddard Space Flight Center Conceptual Image Lab. Watch the video

© Tower module used in the FGST detector

Detecting high-energy gamma rays in space


The Fermi’s LAT is a modular instrument designed to detect gamma rays using pair production: When a gamma ray collides with a metal layer, it can convert them into an electron-positron pair. The incoming gamma ray’s direction is determined by tracing the path of these subatomic particles back to their source, facilitated by several layers of high-precision silicon tracking detectors. Subsequently, a calorimeter absorbs and measures the energy of the particles, allowing the cumulative energy to be calculated, revealing the energy of the initial gamma ray (1).

 

The Fermi’s LAT comprises 4x4 identical detection towers, each equipped with a tracker, a calorimeter, and a customized readout electronics module. These trackers consist of 36 silicon sensor layers, each containing arrays of 4x4 silicon strip sensors, combined with 18 tungsten layers.

 

In total, the Fermi LAT boasts an impressive 83m² of silicon sensor surface and almost 1 million individual channels, which makes it the largest silicon-strip detector ever built for space applications.

© Fermi’s LAT Tower assembly and alignment

Crucial importance of sensor quality for space applications


However, developing a large-scale sensor for space applications resulted in challenging requirements, especially on the Si strip sensors tracking the subatomic particles (2). Due to the large detection area of the instrument, the Fermi scientists were prompted to seek a larger sensor design which could significantly reduce the number of required sensors. Additionally, the power consumption of the strip sensor had to be minimal since, in space, all energy must be harnessed by the spacecraft through solar panels. The lower the power consumption of each of the 10,000 strip sensors, the better (2). Finding a manufacturer capable of rapid and reliable production meeting the highest quality standards became paramount (2). After all, the team only had 5 years to build the complete sensor!

 

In response to these demanding requirements, the team approached Hamamatsu Photonics in 2001. At this time, Hamamatsu was already producing Si strip sensors on 4” wafers. However, the larger sensor required a new production line for 6” wafers (2), which presented a significant investment challenge. Additionally, the initial power consumption of the sensors was not suitable for this type of space application, necessitating a redesign. Yet, Hamamatsu rose to the occasion, and the results were extraordinary. The optimized sensor including its electronics consumes only a few micro-watts per channel (2). Remarkably, the LAT, weighing 3 tons with almost a million channels of electronics, uses less than half the power of an ordinary hair dryer (1). The production line operated seamlessly, with Hamamatsu producing, testing, and delivering 12,500 newly designed 6” Si strip sensors, over the following 3 years, achieving an impressive rejection rate of 0.6% (2).

 

This exceptionally high production quality proved to be crucial for the project timeline. Instead of measuring each strip individually (approximately 1 million measurements), the scientists could now transition to a more streamlined global testing approach at the sensor level with approximately 10,000 measurements. This led to a significantly faster testing and production phase for the LAT sensor, culminating in an on-time launch into space in 2008.

 

The robustness and high quality of the sensors also became evident on Fermi’s 15th anniversary. Spare part sensors, left untouched on shelves for over 15 years, were employed by scientists to construct a small particle tracker. Impressively, these sensors performed flawlessly, attesting to their enduring quality.



© Brighter colors indicate greater numbers of gamma rays. Labels show the highest-energy sources, all located within our galaxy. Gamma-ray Vision, Courtesy of NASA's Goddard Space Flight Center.

Insights and new questions about our universe


So far Fermi's LAT sensor has enabled a multitude of discoveries, reshaping our understanding of the universe. It has unveiled new pulsars, provided insights into massive gamma-ray bursts, and probed the nature of mysterious dark matter. One of the most significant findings was the discovery of numerous new gamma-ray sources, many of which were previously unknown. These discoveries have not only expanded our cosmic map but have also challenged existing theories and models of cosmic phenomena (4).

 

While originally designed for a minimum operation time of 5 years extendible to 10 years, the Fermi Gamma-ray Space Telescope remains operational even after more than 15 years, with an enduring relevance for the scientific community. Particularly, the burgeoning field of multi-messenger astronomy benefits from Fermi’s observations (4,5). This new approach is based on the coordinated observation of signals from different sources or “messengers”, including gravitational waves, neutrinos, cosmic rays, or electromagnetic radiation. As each of them is created by different astrophysical processes, they reveal different information about their sources and together provide new insights into the universe (4,5).

 

In this way, as the Fermi Gamma-ray Space Telescope persists in its mission, it holds the promise of unearthing further discoveries from the mostly unseen high-energy sky. The convergence of insights from different messengers enhances our ability to decipher the intricacies of the cosmos, fueling anticipation for new revelations in the ever-evolving narrative of our universe.

 

“Looking back, it was more than what we could expect when dealing with the technical success in the quality of this device. In terms of robustness, low power, and capacity in the production, it was great, especially because we needed at the end 2,000 sensors per year, so it was a really high-speed production.”  Ronaldo Bellazzini, former Research Director and Professor at Istituto Nazionale di Fisica Nucleare


REFERENCES

 

1. GLAST SCIENCE WRITER’S GUIDE “Exploring the Extreme Universe”

2. Presentation of Ronaldo Bellazzini at the Fermi-LAT Collaboration Meeting in 2018

3. Hamamatsu Technical Note - Si detectors for high energy particles

4. Fermi: Monitoring the Gamma-Ray Universe, David J. Thompson, Galaxies 2018, 6(4), 117

5. The Fermi Sky in a Multimessenger Context, F. Krauss on behalf of the Fermi-LAT Collaboration, Proceedings of Science of the Neutrino Oscillation Workshop NOW2016 (2016)


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