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Ask an Astrophysicist: X-ray/Gamma-ray Astronomy

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X-ray/Gamma-ray Astronomy

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Library of Past Questions and Answers

    Medical Use and Medical Effects of Radiation

This subject is outside our areas of expertise. You might want to try the Health Physics Society site.
    Electromagnetic Radiation towards the Higher Energies

Where can I get a poster of the Electromagnetic Spectrum? (Submitted February 06, 1997)

The Question

Where can I get a poster of the electromagnetic spectrum? I have seen them at some places. If you know it would be greatly appreciated.

The Answer

Unfortunately I do not know where to get an electromagnetic spectrum poster.

Two nice catalogs for stuff like this are:

1. Sky publishing: http://skyandtelescope.com

2. The Astronomical Society of the Pacific http://www.astrosociety.org

However, neither one has an electromagnetic spectrum poster in its current catalog.

Good Luck,

Jonathan Keohane
(for Imagine the Universe!)

PS. A reader recommended the on-line catalogue from Exploratorium in San Francisco as another possible resource. See http://www.exploratoriumstore.com/.

Question ID: 970206b

Where could I find some pictures of UV radiation? (Submitted December 02, 1996)

The Question

Where could I find some pictures of UV radiation (if it's possible). Please send me the URL address.

The Answer

High-energy at the HEASARC covers the energy range of 100 eV on up, X-rays and gamma-rays, which is beyond what most astronomers consider to be the UV. However, if you are open minded about it, we have some great images from the soft X-ray (100 - 2000 eV) band. Many of these can be found in the ROSAT (Roentgen Satellite, a German-UK-US collaboration) Guest Observer Facility pages: http://heasarc.gsfc.nasa.gov/docs/rosat/rosgof.html in the Public Gallery area: http://heasarc.gsfc.nasa.gov/Images/pretty_pictures_rosat.html. HEASARC has images from a vast array of experiments (66), although not all have images of astrophysical objects. These can be found at: http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/images/pretty_pictures.html.

The Hubble Space Telescope ventures a bit into the near UV. Their pages can be found at: http://www.stsci.edu/pubinfo/edugroup/educational-activities.html.

In addition, the Resources button on the Imagine the Universe! pages can lead you to pages of other groups with outreach activities.

Steve Snowden for Imagine the Universe!.

Question ID: 961202a

What are the wavelengths and frequency ranges for the subdivisions of the EM Spectrum? (Submitted March 07, 1997)

The Question

What are the respective wavelength and frequency ranges for the main six subdivisions of the electromagnetic spectrum (i.e gamma, x-rays, ultraviolet, visible, infrared, radio) and what is the name of the quanta of a gamma-ray?

The Answer

There are no "hard" numbers for the wavelengths/frequencies of the various parts of the electromagnetic spectrum. For example, what is considered a high-energy X-ray and what is considered a low-energy gamma-ray is very blurry. But here is a "ball park" guide:

Frequency Range (Hz) Wavelength Range Type of Radiation
10E20-10E24 10E-12 - 10E-16 m gamma-rays
10E17 - 10E20 1 nm - 1 pm x-rays
10E15 - 10E17 400 - 1 nm ultraviolet light
4.3 - 7.5x10E14 700-400 nm visible light
10E12 - 10E14 2.5 um - 700 nm infrared light
10E8 - 10E12 1 mm - 2.5 um microwaves
10E0 - 10E8 10e8 - 1 m radio waves

All quanta in the electromagnetic spectrum, regardless of its wavelength, is called a photon.

You can read more about the electromagnetic spectrum at:

http://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html

Regards,
Laura Whitlock
for the Ask an Astrophysicist Team

Question ID: 970307b

What are X-rays and gamma-rays? (Submitted October 28, 1996)

The Question

What is a gamma-ray? And how does an X-ray in space work?

The Answer

X-rays and gamma-rays are like the light we can see with our eyes and the radio waves we can detect with radio and TV sets. The only difference is how fast they vibrate. Radio waves vibrate the most slowly, then microwaves and infra-red (heat) waves, then the colors red to violet, then ultra-violet radiation (which causes sun-burn among other things) then X-rays, and finally (vibrating the most quickly) gamma-rays.

Imagine the Universe! provides a great deal of information about X-rays and gamma-rays at the high school level. If you are younger than that you might also want to take a look at the StarChild site.

Question ID: 961028a

Is there an upper limit to the Electromagnetic Spectrum (Submitted April 12, 1997)

The Question

At the upper end of the electromagnetic spectrum are Gamma-rays. These "Gamma-rays" have the highest energy content in the electromagnetic spectrum. What is never discussed by is the following: Is there a an upper limit (frequency) to the electromagnetic spectrum? To wit: What is the "highest frequency" Gamma-ray ever detected and is there reason to believe that there are Gamma-rays with even higher levels of energy and if so....does the electromagnetic frequency spectrum have an upper limit....or does it go out to infinity?

The Answer

Thank you for your very good question about the highest energy gamma-rays. Historically, all particles with frequencies greater than about 1019 Hertz (or about 50,000 electron Volts (5x104 eV) where a typical optical photon carries 2-3 eV) are called gamma-rays. Theoretically, there is no hard limit to the energy that a gamma-ray can have. However, there are a number of practical considerations that one needs to take into account involving both astrophysical sources and basic physics.

Before we address this, however, let's tackle the question about the highest energy gamma-rays yet detected. The highest energy measurements of gamma-rays are accomplished using ground-based instrumentation which also measure cosmic rays. Reliable detections of very high energy gamma-ray radiation from individual astrophysical sources, specifically from a couple of active galaxies and from the Crab Nebula, have extended up to about 1027 Hz (5 x 1012 eV). Aside from these individual sources, there is also expected to be a diffuse emission of gamma-rays which accompany the isotropic flux of cosmic rays. This diffuse gamma-ray emission is well measured below around 1024 Hz (109 eV) or so, and is expected to extend up to at least 1030 Hz (1015 eV). There have been reports of measurements of diffuse gamma-ray emission above 1029 Hz, but many other groups have only reported upper limits to emission at these energies. The measurement is exceedingly difficult since cosmic rays can outnumber gamma-rays at these energies by a factor of 10,000 to 1 or more! So you have to sift through a lot of cosmic rays to try to find the gamma-ray signal - a very difficult task.

The truth is we may never actually know to how high an energy nature sees fit to produce gamma-rays. As the gamma-ray is making its way to our telescopes, it has to traverse through space, where there are photons and particles all around us, for example the microwave background. At the highest energies, these photons will scatter down to lower energies before they arrive at Earth. In addition, many sources could produce very high energy gamma-rays which are absorbed and re-processed within the source. As a result, at the most extreme energies, we should see only those gamma-rays produced by relatively nearby sources. In addition, while we expect diffuse gamma-rays up to 1030 Hz, at energies beyond this the basic physics of particle interactions and gamma-ray production is less clear. There could be many surprises.

Nevertheless, from the distribution of gamma-ray energies observed we know we should be able to detect gamma-rays with energies higher than those stated above. There are currently a number of projects being developed that will collect ultra-high energy gamma-rays from cosmic sources, such as OWL and MILAGRO. See

http://owl.gsfc.nasa.gov/

for details on these exciting gamma-ray astronomy projects.

Thanks for you interest,

Padi Boyd and Daryl Macomb,
for the Ask an Astrophysicist Team

Question ID: 970412e

    Space Astronomy

What is Space Astronomy? (Submitted December 18, 1996)

The Question

Hi, I was wondering what space astronomy was?

The Answer

Space Astronomy refers to the study of astronomical objects using instruments flown on satellites. These instruments may detect gamma-rays, x-rays, ultraviolet light, optical light or infrared. The Hubble Space Telescope, the Rossi X-ray Timing Explorer, and the Extreme Ultraviolet Explorer are examples of satellites used in the study of space astronomy. Space Astronomers study a wide range of objects, from normal stars to black holes to active galaxies.

As I'm sure you've noticed, our web site concentrates on x-rays and gamma-rays. It has more information about the objects we study and the satellites we use.

Jim Lochner
for Imagine the Universe!

Question ID: 961218

What are the limitations of ground-based astronomy? (Submitted January 17, 1999)

The Question

I'm in 12th grade and plan to major in astrophysics in college. But I was wondering, what are problems with ground-based telescopes? Why is the HST more effective than telescopes on the ground?

The Answer

The short answer is that ground-based telescopes have to look through the Earth's atmosphere. The stars appear to twinkle when we look up at the night sky because their light has to pass through air which is moving and is at different temperatures, and thus has different refractive indices. To get a clearer view, the Hubble Space Telescope was placed in orbit, above the Earth's atmosphere.

You can find out more about the Hubble Space Telescope at: http://www.stsci.edu/.

Damian Audley and Sean Scully
for Ask an Astrophysicist.

Question ID: 990117a

What is astrophysics? What is high energy astrophysics? (Submitted March 27, 1997)

The Question

I was just wondering exactly what astrophysics is? Also what is the difference between astrophysics and high-energy astrophysics?

The Answer

Astrophysics is the part of astronomy that deals with the physics of stars, stellar systems, and interstellar material. It applies the laws of physics to astronomical bodies in order to help us understand how these bodies formed, how they interact with other bodies, and how they cease to be.

High-energy astrophysics is a sub-branch of astrophysics which utilizes information obtained from astronomical objects in X-ray and gamma-ray wavelengths of the electromagnetic spectrum. X-rays and gamma-rays have higher energies than visible and ultraviolet light.

Jim Lochner
for Ask an Astrophysicist

Question ID: 970327b

    X-rays

How do X-rays work? (from a 5-year old) (Submitted December 15, 1996)

The Question

I am 5 years old. How do X-rays work?

The Answer

Here at NASA we catch X-rays from far away things. From these X-rays we can learn about those far away things -- like stars and galaxies.

When you go to the doctor an get an X-ray, it is similar. The doctor shoots X-rays in one side of you, and then film catches the X-rays on the other. This makes a picture of your inside, showing your bones really well.

Thank you for asking.

Jonathan Keohane
for Imagine the Universe!

Question ID: 961215a

Who were some of the pioneers of X-ray astronomy? (Submitted April 17, 1998)

The Question

Can you tell me who some of the pioneers of X-ray astronomy were? I've looked at the resources that give a bit of history of rocket-born and satellite work, but I was wondering about the astronomers/astrophysicists who started the field of study.

The Answer

X-ray astronomy began after WWII when a large number of captured V2 rockets were made available to scientists for small experiments in sub-orbital flight. The led to detection of X-rays from the Sun by Herbert Friedman (Naval Research Laboratory) and collaborators in the 1950s. The bigger event was the detection of X-rays from Sco X-1 by a rocket flight (not a V2) in 1962 by Bruno Rossi, Riccardo Giacconi, and Frank Paolini (MIT). They were supposedly looking for X-ray fluorescence off of the moon, an effect which wasn't actually observed until 30 years later with the ROSAT satellite. Giacconi went on to promote X-ray astronomy at American Science and Engineering and the Harvard Smithsonian Astrophysical Observatory, which led to the UHURU and HEAO 1 and 2 satellites.

I assume you have looked at our lab history page:

http://astrophysics.gsfc.nasa.gov/xrays/history.html

Tim Kallman
for 'Ask an Astrophysicist'

Question ID: 980417b

How does an X-ray Telescope work? (Submitted February 23, 1998)

The Question

I'm a student from Belgium. I'm writing a paper on applications of foil. Can you tell me why precisely you use foil for the making of the X-ray telescope. Thank you.

The Answer

Thank you for your question. The basic reason why we use foil for X-ray telescope mirrors is because X-rays only bounce at shallow angles. So the mirrors must deflect the X-rays just only a little from their path.

As you can imagine, this means that the mirrors must be shaped like a cylindrical tube.

The problem, however, with this comes in collecting area. If you have a tube shaped mirror, it will not collect very many X-rays.

The solution is to make many thin tube shaped mirrors and nest them. Put one inside the other. If they were not thin, it would be hard to put them inside each other --- hence the need for foil.

Pictures and other descriptions are at:

http://imagine.gsfc.nasa.gov/science/toolbox/xray_telescopes2.html

http://imagine.gsfc.nasa.gov/science/toolbox/xray_detectors1.html

For more elementary information, try reading this:
http://heasarc.gsfc.nasa.gov/docs/xte/learning_center/universe/universe.html

Good luck,

Jonathan Keohane
for Ask a NASA scientist

Question ID: 980223b

What are the physical mechanisms that cause X-ray radiation? (Submitted March 13, 1997)

The Question

Please send me some information about the mechanism behind X-ray radiation in the interstellar medium. For example, the plasma mechanism.

The Answer

X-rays in space come from a variety of sources. These include objects, such as supernova remnant, active galactic nuclei (including quasars), stars, and compact objects (black holes or neutron stars) in binary orbits with more normal stars. In addition, X-rays are likely to be emitted by diffuse gas in the interstellar medium. The relative contributions or these various sources to the total X-ray flux received at earth is a subject of some debate, and it varies with the X-ray energy.

It is customary to divide the emission mechanisms for X-rays into "thermal" and "non-thermal", according to whether the velocity distribution of the emitting electrons is Maxwellian. Among thermal mechanisms, the most common is almost certainly bremsstrahlung, in which radiation occurs as the result of coulomb collisions between electrons and nuclei in an ionized gas. This mechanism is likely to be operating in virtually all X-ray sources, and dominates the emission from many of them. One of the most common non-thermal mechanisms is synchrotron emission, in which electrons radiate as the result of their gyroscopic motion in a magnetic field. This mechanism, and variation, called synchrotron-self Compton, is likely to dominate in some supernova remnants and in some quasars. Both of these mechanisms are described in electricity and magnetism texts, such as Jackson's "Classical Electrodynamics". More details can be found in a "Radiative Processes in Astrophysics" by Rybicki and Lightman.

I hope this helps!

Tim Kallman
for the Ask an Astrophysicist team

Question ID: 970313a

Did anyone analyze X-ray emission from Comet Hale-Bopp? (Submitted July 16, 1997)

The Question

I have read the article on the X-ray emissions from comet Hyakutake. Your hypothesis on the water cloud around the nucleus is interesting but did you analyze the same activities on comet Hale-Bopp? If so what are other hypothesis or conclusions?

The Answer

Last May in Baltimore, Maryland, USA we had a meeting of all the people interested in the cometary X-ray emission problem. the first time all of us had gotten together since the discovery in 1996. We now have detections of X-rays from some 8 comets, and all bright, nearby comets seem to emit X-rays! (Including Hale-Bopp, although it was much fainter than we expected in the X-ray for such an optically bright and productive comet. It has been proposed that the extremely large amount of dust emitted by the comet, as compared to other comets, may be somehow damping the X-ray emission.)

At the meeting, it became apparent that 3 mechanisms are possible causes of the emission, in oder of likeliness: charge exchange between solar wind heavy ions and cometary neutrals, bremsstrahlung emission, and magnetic field recombination. All of these mechanisms involve interactions between the solar wind and the comet's extended atmosphere and ionosphere.

It is clear that we need more observations to figure out exactly what is going on, though! But it does seem that we will be able to use the X-rays to probe the nature of the solar wind and magnetic field throughout the solar system.

Hope this helps!

Casey Lisse
for Ask an Astrophysicist

Question ID: 970716a

How are X-rays and gamma-rays produced in space? (Submitted May 26, 1997)

The Question

As a Radiographer for 14 years, I am familiar with how diagnostic X-rays are produced by man. Other, than the obvious difference of mechanical means of producing the X-rays that Dr. Roentgen discovered; or even the radiation such as that noted at Chernobyl (please bear my ignorance) are we talking a natural phenomena when you say High-Energy Astrophysics--X-rays & gamma-rays or something other than the mechanized means of production? By now, you see how little knowledge I have of what is titled "High- Energy Astrophysics"...

The Answer

Yes, the X-rays we observe are natural -- not man made. They are produced very far away in the myriad of phenomena described in Imagine the Universe! (http://imagine.gsfc.nasa.gov/). The X-rays then travel for hundreds to billions of years before they happen to hit the detectors on our X-ray telescopes.

In fact there are two basic natural methods of producing extraterrestrial X-ray -- thermal and non-thermal. Two members of our ask_astro team replied to your message -- one describing the thermal processes and the other the non-thermal processes. I have attached a brief description of each with this E-mail (see below), and as I mentioned above there is much information on this at our Web site.

Sincerely,

Jonathan Keohane and much of the Ask an Astrophysicist Team


Appendix A: Thermal X-rays (by Mike Arida)

At 37 C the human body emits infrared radiation. This is called blackbody radiation.

The hotter the object, the higher the energy of the photon's emitted.

When you heat a piece of metal in a fire till it glows read you have energized some of those photons to the red part of the visible spectrum.

The Sun, at 5,000 C emits most of its energy in the yellow/green part of the visible spectrum.

A tungsten light bulb get to be about 10,000 C and emits in the bluish/white end of the spectrum.

X-rays, being much more energetic than visible light, require a hotter source, in the 1 - 10's of millions of degree range. So one method of X-ray production is in the very hot gas expanding outward after a supernova explosion, or the gas heated as it spirals (and accelerates) into a black hole.


Appendix B: Non-Thermal X-ray (by David Palmer)

Many X-rays studied by high-energy astronomers are produced by high-energy electrons being accelerated or decelerated, either by being deflected by a magnetic field, or by hitting other particles. X-ray tubes used in radiography work the same way: a beam of electrons is fired into a metal target, and as they stop the electrons produce X-rays.

There are also gamma-rays produced by the decay of radioactive isotopes. These are produced on Earth by reactors such as Chernobyl, and in the sky by reactors such as novae and supernovae.

Question ID: 970526a

What are the principle sources of X-rays and gamma-rays? (Submitted November 10, 1996)

The Question

I am an Italian girl and I'm eleven years old. I should like to know the principle sources known of the X-rays and Gamma-rays in the Universe. Thank you very much .

The Answer

Thank you for you interest. It is impressive that someone so young is interested in these topics. There are many types of sources that produce X-rays, gamma-rays or both. Two types of objects, "active galactic nuclei" (which include quasars), and "X-ray binaries" produce X-rays and to a lesser extent, gamma-rays. Active galactic nuclei are most likely powered by supermassive black holes (as massive as millions to billions of suns). Some X-ray binaries may also contain black holes. Supernova explosions also produce a lot of X- and gamma radiation. There is hot gas in some galaxies, including our Milky Way, that produces X-rays. Stars, including our Sun, produce X-rays, particularly in their coronae. There is also a mysterious phenomenon called "gamma-ray bursts" which are now being detected daily by the Compton Gamma-Ray Observatory. Scientists are not sure yet what these are, but they are very energetic. For more information on these topics, try looking at the Learning Center Web pages.

Have fun,
Andy Ptak and the Ask an Astrophysicist team

Question ID: 961110

    Gamma-rays

Who discovered Gamma-rays? (Submitted February 09, 1998)

The Question

I know that gamma rays were discovered shortly after alpha and beta particles, but who is given credit for the discovery?

The Answer

Paul Villard, a French physicist, is credited with discovering gamma rays. Most sources put this in 1900, although I've seen a few sources use 1898. Villard recognized them as different from X-rays (discovered in 1896 by Roentgen) because the gamma rays had a much greater penetrating depth. It wasn't until 1914 that Rutherford showed that they were a form of light with a much shorter wavelength than X-rays.

A good web site on the history of the discovery of radiation, written by Michael Fowler at University of Virginia, is:

http://www.phys.virginia.edu/classes/252/rays_and_particles.html

Jim Lochner
for Ask an Astrophysicist

Question ID: 980209c

What do Gamma-ray Astronomers do? (Submitted September 14, 1997)

The Question

What does a Gamma-ray Astronomer do and what deals with Gamma-rays today?

The Answer

Gamma-ray astronomers study the universe as revealed by the most energetic portion of the electromagnetic spectrum. They study a variety of objects, including solar flares, neutron stars, black holes, active galaxies and gamma-ray bursts. You can learn lots about these objects via our Basic and Advanced areas on this web site.

Jim Lochner
for Ask an Astrophysicist

Question ID: 970914b

Do gamma-rays penetrate our atmosphere? (Submitted July 18, 1997)

The Question

I want to know more about the gamma-rays that do make it through the atmosphere. Are these the same as the 'cosmic rays' that are constantly bombarding earth and have a bearing on mutation? Do any gamma-rays get through, what happens to them, are they measured down here? Would the quality of our atmosphere have any bearing on how many rays get through? I know that rays do not cause the ozone hole, but does the hole in the ozone let more rays in? Do our increased levels of CO2 in the atmosphere cause changes in the 'cosmic ray' barrier?

The Answer

Very few gamma-rays make it through the atmosphere. The atmosphere is as thick to gamma-rays as a twelve-foot thick plate of aluminum. Gamma-rays are very very unlikely to go through that much material. However, they can strike the material and produce 'secondary' particles which are more penetrating, and can go through the material.

Most of the cosmic rays which reach the Earth's surface are 'secondary cosmic rays', produced by gamma-rays or (much more commonly) 'primary cosmic rays' hitting the top of Earth's atmosphere. These primary cosmic rays are high energy particles (such are protons and the nuclei from iron atoms) moving at very close to the speed of light. These primary cosmic rays have a hard time even getting to the top of our atmosphere--the Earth's magnetic field deflects most of them away. If Earth didn't have a magnetic field, there would be many more primary cosmic rays hitting the atmosphere, and many more secondary cosmic rays hitting us.

There is a page in Imagine the Universe! about observations of the light produced when cosmic rays and gamma-rays hit the top of the atmosphere. It is at:

http://imagine.gsfc.nasa.gov/science/toolbox/gamma_detectors2.html

The cosmic rays are not very sensitive to the quality of the air (the chemical composition--how the nitrogen, oxygen, carbon and other elements in the air are joined together to make ozone, smog and other chemicals). They are more affected by the quantity of the air, because most interactions depend only on the nuclei of the atoms, and not on entire molecules. Three O2 molecules and two O3 (ozone) molecules look exactly the same to a cosmic ray. A carbon atom looks only slightly different from an oxygen or nitrogen atom, so the increased CO2 level has almost no effect. Nothing we do is likely to significantly change the number of cosmic rays hitting Earth.

David Palmer
for Ask an Astrophysicist

Question ID: 970718

I have questions about gamma-ray showers and Cerenkov detectors. (Submitted November 14, 1996)

The Question

  1. Why is the shape of shower different for a gamma-ray compared to a cosmic ray? Is it to do with the initial interaction of the gamma-ray produces a positron/electron pair that go off at some angle.

  2. At the CGRO learning center I was expecting to see plots of the pool of Cerenkov light to be elliptical for a gamma-ray and roughly circular for a cosmic ray. I don't see this in the plots shown, especially the cosmic ray plot, which shows a scattered distribution that I don't understand.

  3. On the actual imaging telescope why are there multiple mirrors instead of just one dish with the PMTs behind focal plane. Also why are the individual mirrors hexagonal and not say squares or pentagons?

  4. Since the pool of light is much bigger than the area of telescope how can the shape of the pool be determined? Does the intensity of light fall off or increase in concentric contours from the edge of the pool to the center?

The Answer

> 1. why is the shape of shower different for a gamma-ray
> compared to a cosmic ray? Is it to do with the initial
> interaction of the gamma-ray produces a positron/electron
> pair that go off at some angle.
>

Yes, that is the key. The initial interaction of the gamma-ray is a pair production interaction which has a relatively small opening angle for the created pair. On the other hand, the first cosmic ray interaction is some exotic nuclear interaction where many different particles can result, each splintering off in different directions, some at large angles. Further interactions tend to accentuate this. Even though the gamma-ray showers broaden lower down in the atmosphere as the particles lose energy, the cosmic-ray showers are still much more extended.

> 2. At the CGRO learning center I was expecting to see
> plots of the pool of Cerenkov light to be elliptical
> for a gamma-ray and roughly circular for a cosmic ray.
> I don't see this in the plots shown, especially the
> cosmic ray plot, which shows a scattered distribution
> that I don't understand.
>

The plots at the learning center are actually hit positions of photons from simulated air showers. If many more events were plotted, and some smoothing were done, the shapes you describe would start to be apparent.

> 3. On the actual imaging telescope why are there multiple
> mirrors instead of just one dish with the PMTs behind
> focal plane. Also why are the individual
> mirrors hexagonal and not say squares or pentagons?
>

Some observatories use mirrors originally intended for other work - such as solar power studies. For this type of work, and air Cerenkov work, it is the size of the mirrored surface, not the quality of the mirror which is important. Using the Whipple Observatory example, it would be very expensive to manufacture a smooth mirror with a 10 meter diameter. On the other hand, a 10 meter diameter mirror which is a mosaic of smaller mirrors is cheap, has a large collecting area, and provides adequate optics for the relatively broad structures being measured (the size of the image is typically a few tenths of a degree). The shape of the individual facets is not really important.

> 4. Since the pool of light is much bigger than the area of
> telescope how can the shape of the pool be determined?
> Does the intensity of light fall off or increase in concentric
> contours from the edge of the pool to the center?
>

Detectors are not actually measuring the shape of the entire pool (which is roughly a large pancake), but the "shape" arising from the angular distribution of light in the local part of the pool that the mirror reflects. This shape changes depending upon where you are in the "pool". Even gamma-ray showers look circular at the center of the pool. The light intensity in the pool does have an interesting radial dependence. For gamma-ray initiated showers, if you measured the intensity from the center of the shower, you would find that it was approximately constant out to a distance of 100 meters or so, falling off rapidly after that.

Daryl Macomb
Compton Gamma Ray Observatory team

Question ID: 961114

Can gamma-rays react with CO2 in our atmosphere? If so, how? (Submitted November 29, 1996)

The Question

Can gamma-rays react with CO2 in our atmosphere? If so how?

The Answer

Gamma-rays are usually absorbed in the upper atmosphere (most would not even reach the top of the Everest). Higher in the atmosphere, it can interact with CO2 molecules, but they do not react with CO2 more than with other kinds of molecules, if that was your question.

Koji Mukai
for Imagine the Universe!

Question ID: 961129b

What happens to people who have been exposed to a lot of gamma-rays? (Submitted October 13, 1997)

The Question

What happens to people who have been exposed to a lot of gamma-rays?

The Answer

There is some general information concerning radiation exposure, including gamma-rays, at

http://www.hps.org/publicinformation/ate/cat25.html

Andy Ptak
for the Ask an Astrophysicist

Question ID: 971013b

Why are gamma-rays more dangerous than radio waves? (Submitted January 09, 1998)

The Question

Hi, I'm in 8th grade and my science class is learning about the the electromagnetic spectrum. My question is why are gamma-rays so much more harmful than radio waves? I already understand that gamma-rays have a higher energy level in them than radio waves, but what makes this energy so harmful?

The Answer

The reason gamma-rays are more harmful then radio waves is because light can be thought of as particles (photons) as well as electromagnetic waves. A radio photon doesn't have much energy and doesn't travel through matter well (that's why you don't pick up radio well in a tunnel). A gamma-ray photon has enough energy to damage atoms in your body and make them radioactive, and gamma-rays can easily penetrate into your body. It's like the difference between getting hit by sand or a bullet. It takes a lot of sand to do any damage, but only one bullet.

Thanks for your question.

Eric Christian
for Ask an Astrophysicist

Question ID: 980109b

Is it possible to have high-energy gamma-ray emission from de-excitation of nuclei? (Submitted August 06, 1996)

The Question

I am a PhD student at the Observatory in Torino (Italy).

I would like to ask you if is it possible to have high-energy gamma emissions (that's to say up to 30 MeV) from some kind of dissections of nuclei? I know about Carbon and Oxygen at about 5-6 MeV and I know about the decay energy emission (e.g. aluminum and iron and others) not up to 10 MeV. Can one use EGRET or other similar high-energy satellites (e.g. in the future AMS) to study stellar production of some kind of nuclei (excited for example by the passage of a shock wave)?

The Answer

Thanks for your question about gamma-ray lines and satellite capabilities to study production of these nuclei. We've consulted with Dr. David Thompson (of the EGRET team here at NASA/Goddard) about your questions.

The highest energy gamma-ray line of which we are aware is one at about 20 MeV resulting from an exited state of tritium. Tritium is so unstable, however, that it is unlikely to be significant in any astrophysical situation. There are a few other lines between 10 and 20 MeV, but the vast majority of deexcitation lines, and all those which have been considered likely astrophysical candidates, have energies below 10 MeV.

EGRET does have some energy measurement capability at 20 MeV, but not with adequate resolution to see a narrow line. The same is likely to be true of AMS or GLAST. A far more promising avenue for the study of astrophysical lines is INTEGRAL, which has the high resolution and high sensitivity in the energy range of greatest interest.

Question ID: 960806

Does electron-positron annihilation always yields energy in the gamma-ray band? (Submitted July 19, 1997)

The Question

I've been reading up on the recent positron findings at our galaxy center and wondered if we know that electron-positron annihilation always yields energy in the gamma-ray band?

My background in this area is limited to college physics, and book (net) research in plasma and astrophysics.

The Answer

Thank you for your question. The answer is yes, we know the physics of electron-positron interactions quite well, because it has been measured in particle physics labs. As it turns out the mass of an electron (9.1E-28 grams) times the speed of light squared (E = m c2) is 8.12E-07 ergs of energy. In more common units this is 511 keV (kilo electron volts).

When an electron and positron annihilate they produce 2 photons, each with 511 keV of energy (so no net energy is gained or lost). When we observe a spectral emission line at 511 keV, we can be pretty sure it is caused by this positron/electron interactions.

Jonathan Keohane
for Ask an Astrophysicist

Question ID: 970719b

What is anti-matter? (Submitted May 05, 1997)

The Question

Recently I've heard about antimatter. Could you describe to me what it is? Could you tell me why is it so important for NASA to join matter and antimatter?

The Answer

The existence of antimatter was predicted by the theory of quantum mechanics of the electron by Dirac in the 1920's. The first experimental verification came with the discovery of positrons in cosmic rays by Anderson in 1932 or thereabouts. However, positrons (anti-electrons) are found in some kinds of naturally radioactive substances also. Later anti-protons and other anti particles were produced in accelerators in the laboratory.

An anti-particle is a particle whose properties are exactly opposite to its corresponding particle. Thus a positron (the particle thought to be responsible for the gamma-rays which were in the news last week) has a charge opposite (positive) to that of the electron, and 'lepton number' -1. When positrons and electrons collide they annihilate each other, and their energies are converted into gamma-rays. If the positron and electron are at rest (which is unlikely) and their spins are oriented opposite to each other they produce 2 gamma-rays, each with energy 511,002.7 electron volts. This is the radiation which was observed by the Compton Gamma Ray Observatory, and is considered to be a unique signature of electron-positron annihilation. This signature is expected to arise from annihilation even if the positrons and electrons are not at rest, but have moderate kinetic energies.

Antimatter is interesting partly because of the spectacular and violent way in which it interacts with normal matter. It is also an open question why the Universe appears to be relatively empty of antimatter; theories for the big bang predict approximately equal amounts of matter and antimatter should have been produced. The positrons which produced the gamma-rays seen by the Compton Observatory were probably produced by collisions of high energy particles (predominantly ordinary protons and electrons) accelerated near a black hole.

I hope that this helps to answer your question.

Tim Kallman
for the Ask an Astrophysicist team.

Question ID: 970505d


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