GLAST Reaches Orbit, Set To Begin Observations

Btarlinian writes "GLAST (the Gamma-ray Large Area Space Telescope) was launched Wednesday at 1605 GMT. GLAST was built in a rather interesting manner, in that much of the work was funded by the Department of Energy. In fact, the main instrument on GLAST, the Large Area Telescope was assembled at the Stanford Linear Accelerator Center. It can detect gamma rays at energies between 20 MeV and 300 GeV. Researchers will use GLAST to study some of the most massive and energetic objects known to science."

Don't let them fool you...

[naz404]

It's actually pointing towards the Earth and this is a covert op to try to track down Dr. Bruce Banner with Google Maps...

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[LordAlced]

I, for one, welcome our Gamma-ray Large Area Space Telescope masters.

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[camperdave]

When I read the summary, (okay, okay, when I read the story title) I wondered what sort of picture they'd get if they did point it at Earth.

High-energy photon detection

[blind biker]

One of my colleagues makes hihg-energy photon (basically Gamma ray) detectors. He uses high-purity silicon wafers for the fabrication of the devices. These wafers are very effing expensive, as he needs a large bandgap. Still, 300GeV? I don't think his devices are capable of detecting such photons. I think his max is around 10GeV. Probably with high-purity GaAs it would be possible, I guess.

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[Anonymous Coward]

GLAST uses inorganic scintillator for energy measurement (a.k.a. calorimetry). This would be optically transparent crystals made of something like CsI, NaI, or whatever, possibly grown with a dopant. I don't know the details for what they ended up using.

I think it may also use semiconductor detectors (probably a silicon microstrip detector?), but for determining directionality rather than energy measurement.

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[LordAlced]

Regarding your sig, isn't some large-scale military contractor in charge of Gundam?

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[kestasjk]

One of my colleagues makes hihg-energy photon (basically Gamma ray) detectors. He uses high-purity silicon wafers for the fabrication of the devices. These wafers are very effing expensive, as he needs a large bandgap. Still, 300GeV? I don't think his devices are capable of detecting such photons. I think his max is around 10GeV. Probably with high-purity GaAs it would be possible, I guess.

For rays with less than 100 keV (X-rays, really) the ray is absorbed and dislodges an electron (photo-absorption). For rays with 100-1000keV the ray will be scattered at a lower energy and dislodge an electron as well (Compton scattering). From 1MeV to ~8MeV pair production occurs, where an electron and positron are created.

After ~8MeV photo-disintegration starts to occur, where the gamma ray produces particles like neutrons or tears the atom it hits apart. At this point no band gap is going to be large

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[niklask]

My guess is that the secondaries in turn generate photons. Incoming gamma yields neutrons and ions (for example), ions and neutrons make more lower energy gammas, etc etc. As gamma rays at these energies are not too common, it is possible that the detector even can resolve individual "showers" of secondaries. Correct me if I'm wrong, but this is my intuition.

You are almost right. The dominant interaction in the high-energy regime is pair-production (as long as there is some material to interact with). When the gamma ray hits one of the interaction layers in the GLAST tracker, it produces an electron-positron pair. These secondaries will also interact (bremsstrahlung) and produce more secondary gammas and e-p pairs. This is a well known concept called electromagnetic showering in particle physics. By studying the shower one can determine incoming direction etc.

Re:High-energy photon detection

[Bananenrepublik]

I'm currently building a detector for photons (= gamma rays) and pions in the range of ~10MeV to a ~100GeV. Our implementation is different, due to our interest in particles other than photons, but the approach is similar. The important point to realize is that you're not detecting the photons, you're detecting secondary particles created by the photons. That's why they have the Tungsten layers in GLAST (people with a smaller budget usually uses lead). Photons passing through it will undergo pair conversion, producing pairs of electrons and positron. You need a heavy material for this purpose, as the interaction probability strongly increases with the charge of the nucleus (Z^2) and its density (proportionally). These pairs are then detected in the silicon microstrip detectors, not the photons themselves.

Since these electron-positron pairs carry most of the energy of the photon (some of it is transferred to the recoiling heavy core), they will in turn radiate of gamma rays of lower energy in a process called Bremsstrahlung. These Bremsstrahlung photons will undergo pair prodution again until the end of detector or until all energy has been absorbed, whatever comes first. This process is called showering. Since GLAST is inside a space vessel it can't be large enough to contain the whole shower, and this is where the Caesium Iodide calorimeter comes in: the charged shower particles leaking out of the first part of the detector will produce light flashes whose intensity and duration which allow the GLAST people to determine the number of shower particles (and maybe rough estimates of their energy) and in turn this will allow them to estimate the energy of the original incident particle.

The constraint of low mass really works against a precise enrgy measurement, but looking at shower shapes the way GLAST does may reveal enough information to obtain halfway reliable numbers.

I'm definitely looking forward to seeing their results. Go GLAST.

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[niklask]

Finally someone who knows what he is talking about.

Why not osmium then?

[Beryllium Sphere(tm)]

Z**2 is only 5% better than tungsten but it's denser. That or iridium.

They're more expensive than tungsten, but for a space instrument the cost of materials is nothing compared to the cost of launch.

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[Bananenrepublik]

Well, I don't know why they decided the way they did. But it is clear that even if a material were desirable from a physics point of view, it might be impossible to use it, due to chemical instability, mechanical instability, cost, prohibitve security requirements during manufacturing, etc.

BTW is there any slashdot story that attracted fewer comments?

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[niklask]

Z**2 is only 5% better than tungsten but it's denser. That or iridium. They're more expensive than tungsten, but for a space instrument the cost of materials is nothing compared to the cost of launch.

That is incorrect. Yes, the launch is expensive, but the instrument is not cheap either. Its a very complex detector and the components are not inexpensive. I do not have any exact figures, but we are talking a multi-million dollar detector here. --- Btw, I should add that I used to be a member of the GLAST collaboration.

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[mako1138]

Some relevant documents:
http://heseweb.nrl.navy.mil/glast/CALPDR/PDR_Summary_Report_16July.pdf [navy.mil]
http://www-glast.slac.stanford.edu/software/AnaGroup/Atwood-GLASTEnergy-9dec02.ppt [stanford.edu]

According to the preliminary design report, the calorimeter is 8.5 radiation lengths deep, with 1.5 in the tracker. I forget my shower mechanics but 10 rad lengths seems like enough. The design goal is 20% accuracy for a high-energy range, and 10% and 6% at progressively lower energies.

This stuff makes me feel lucky that I work with l

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[niklask]

Some relevant documents: http://heseweb.nrl.navy.mil/glast/CALPDR/PDR_Summary_Report_16July.pdf [navy.mil] http://www-glast.slac.stanford.edu/software/AnaGroup/Atwood-GLASTEnergy-9dec02.ppt [stanford.edu] According to the preliminary design report, the calorimeter is 8.5 radiation lengths deep, with 1.5 in the tracker. I forget my shower mechanics but 10 rad lengths seems like enough. The design goal is 20% accuracy for a high-energy range, and 10% and 6% at progressively lower energies. This stuff makes me feel lucky that I work with lots of lead glass and PMTs.

What is enough in terms of shower containment depend on what you want to do. To detect GeV photons, 10 radiation lengths is plenty enough. For a 100 GeV photon, there will be shower leakage, especially if the photon has a large incident angle. As one can expect, the LAT was optimized to allow detection up to a few hundred GeV.

Google has a Large Area Space Telescope?

[Alascom]

I see GLAST and assume is was a new Google product...

Anything beginning with a "G" in front says Google to me these days... :)

nit-pick

[Guppy06]

"launched Wednesday at 1605 GMT."

So we're talking 0405 UTC?

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[mefein]

16:05 GMT = 16:05 UTC (12:05 pm EDT)

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[Guppy06]

In GMT, days start at noon, not midnight.

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[mefein]

Hmmm, Julian day starts at noon. Greenwich mean time starts at midnight, just like any other timezone. http://en.wikipedia.org/wiki/Western_European_Time [wikipedia.org]

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[Guppy06]

"Hmmm, Julian day starts at noon."

And you don't think the two are related? http://en.wikipedia.org/wiki/Gmt [wikipedia.org]

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[mefein]

There is probably something I am fundamentally misunderstanding here. I live in the UK. In winter I am familiar with the timezone being called GMT. 6:00am is definitely in the morning (and I am usually asleep).