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Space Science

New Map From Fermi Gamma-Ray Space Telescope 34

I Don't Believe in Imaginary Property writes "NASA has received interesting results from the Fermi Gamma-ray Space Telescope, originally known as GLAST, which has allowed them to create new map of the gamma-ray sky. The secret to its ability to resolve gamma-rays is that they use layers of tungsten interleaved with silicon detectors. When a gamma-ray strikes tungsten, it produces an electron/positron pair due to the photoelectric effect, which cascades as it goes through further layers of tungsten. Meanwhile, they record which silicon detectors had electrons or positrons pass through them to determine the direction of the source and they also record the total energy of the electron/positron pairs to calculate the wavelength of the gamma-ray using Planck's Law. The data gathered in just its first few hours of operation is reportedly comparable to the data from the Energetic Gamma-Ray Experiment Telescope, which gathered data for nine years back in the 1990's and there are hopes that it could detect dark matter in the form of weakly interacting massive particles (WIMPs)."
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New Map From Fermi Gamma-Ray Space Telescope

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  • fizick (Score:5, Informative)

    by fizick ( 1352613 ) on Wednesday August 27, 2008 @10:28PM (#24774255)
    The physics of the detectors described by samzenpus is a bit wonky. Electron/positron pair production has nothing to do with the photoelectric effect. The first is a result of a high energy gamma ray photon interacting with an atomic nucleus, the second is a photon interacting with an atom's electrons.
    • by Anonymous Coward on Wednesday August 27, 2008 @10:51PM (#24774417)

      Actually, that's my fault. I submitted the story, samzenpus just approved it. I did my best to figure out why a gamma ray produces an electron/positron pair and the photoelectric effect seemed right. Well, except for the positron part, I guess. I should've noticed that, because you're right--it just ejects electrons, it doesn't create antiparticles. It's been too long since my last college physics class. None of the articles I linked to bothered to explain that point, so I did my best to look up the reason.

      I also figured that if I was wrong, it would spark some discussion and I'd learn the actual reason. Care to explain more? I would like to know.

      - I Don't Believe in Imaginary Property []

      • by srjh ( 1316705 ) on Wednesday August 27, 2008 @11:28PM (#24774697)

        Well, except for the positron part, I guess.

        And for the electron part as well.

        Pair production is where a high energy photon (i.e. higher than the rest energy of a positron and an electron) contributes its energy to the creation of an electron/positron pair. The electron doesn't exist to begin with either.

        With the photoelectric effect, the energy of the photon contributes to the ejection of an existing electron from the surface of a material. This happens at a much lower energy.

    • Not only is the poster wrong about how the silicon microstrip tracker works, he is also wrong about how the total energy of the incident gamma ray is determined. The energy is measured by the calorimeter and that has nothing to do with Planck's law.
  • Detecting WIMPs? (Score:5, Informative)

    by CyrusOmega ( 1261328 ) on Wednesday August 27, 2008 @10:47PM (#24774387)
    According to []

    "Because they [WIMPs] do not interact with electromagnetism they cannot be seen directly"


    "As more and more WIMPs thermalize inside the Sun, they begin to annihilate with each other, forming a variety of particles including high-energy neutrinos.[1] These neutrinos may then travel to the Earth to be detected in one of the many neutrino telescopes, such as the Super-Kamiokande detector in Japan"

    It would seem impossible to actually "detect" WIMPs, but rather their possible effects which is also dependent on the mass of the Higgs boson particle. Maybe I am missing something?
    • by RuBLed ( 995686 )
      The explanation is wishy-washy at best.
    • Re:Detecting WIMPs? (Score:5, Informative)

      by habig ( 12787 ) on Wednesday August 27, 2008 @10:56PM (#24774461) Homepage

      Correct - GLAST would see WIMPS only indirectly. If WIMP/antiWIMP pairs are out there in the quantities needed to make up all that missing mass, quite a lot of them will be colliding and annihilating anywhere you look at any given time. Some of those annhilation products will be gamma rays. Since WIMPS get pulled around by gravity, you might get a lot of annhilation in the bottom of a potential well, but then the only resulting products which we could see would be whatever eventually decays to neutrinos, which we might see in Super-K.

      You're right that exactly what the mixture of decay products and their energies might be are highly model dependent (not the least of which is that we don't know what the higgs weighs), but seeing something is a good first start, it's easier to put the puzzle together when you have some non-invisible pieces to play with.

    • Re: (Score:2, Funny)

      by Kratisto ( 1080113 )
      This is incorrect. Detecting WIMPs is a simple matter of computing CM^2/V, where C is equal to courage in the presence of the other gender, M is equal to total muscle mass, and V is equal to vocal frequency.
  • by the_denman ( 800425 ) <> on Thursday August 28, 2008 @01:47AM (#24775533) Homepage
    I used to do work study for some of the folks working with the GLAST project at Iowa State University their website is here [] and has some more information about Gamma Ray Astrophysics.
  • There was one thing TFA didn't make clear to me: what does this tell us about man-in-the-moon marigolds?
  • I read that as "Ramma-Gay" instead of "Gamma-Ray".
    Freudian slip? I hope not.

"The following is not for the weak of heart or Fundamentalists." -- Dave Barry