problems associated with earth based observation optical band = stars and planets and nebulae....
TRANSCRIPT
Problems associated with Earth based observation
Optical band = stars and planets and nebulae.
Infrared band = low energy heat sources.
Radio band = dust shrouded environments.
What about the rest ????? !!!!!
Problems associated with Earth based observation
How bad is the problem for X-rays and gamma rays?
Problems associated with Earth based observation
What did astronomers do to get around this problem?
Altitude by which half of the incoming radiation has been absorbed
Problems associated with Earth based observation
What did astronomers do to get around this problem?
Experiment Total cost Duration Cost per hour
Mountain observatory £2,000,000 10 years £50 per hour
Aircraft £240,000 1 day £10,000 per hour
Balloon £300,000 1 day £12,500 per hour
Rocket £500,000 10 minutes £3,000,000 per hour
Satellite £200,000,000 5 years £10,000 per hour
Pic du Midi Observatory in the French Pyrenees
Problems associated with Earth based observation
What did astronomers do to get around this problem?
Experiment Total cost Duration Cost per hour
Mountain observatory £2,000,000 10 years £50 per hour
Aircraft £240,000 1 day £10,000 per hour
Balloon £300,000 1 day £12,500 per hour
Rocket £500,000 10 minutes £3,000,000 per hour
Satellite £200,000,000 5 years £10,000 per hour
Variation in counting rate as function of galactic longitude from rocket borne proportional counter flown in 1967. The hard line represents the expected distribution based on known sources whilst the circles represent the data obtained in that flight.
Problems associated with Earth based observation
What did astronomers do to get around this problem?
Experiment Total cost Duration Cost per hour
Mountain observatory £2,000,000 10 years £50 per hour
Aircraft £240,000 1 day £10,000 per hour
Balloon £300,000 1 day £12,500 per hour
Rocket £500,000 10 minutes £3,000,000 per hour
Satellite £200,000,000 5 years £10,000 per hour
NASA 1990s X-ray measurements
Problems associated with Earth based observation
First artificial satellite, Sputnik 1, was launched by the Soviet Union in 1957.
Problems associated with Earth based observation
Uhuru, launched in 1970 was the first earth-orbiting mission dedicated entirely to celestial X-ray astronomy and operated for 3 years.
It consisted of two proportional counters and made the first comprehensive and uniform all sky survey.
Uhuru spun making one revolution every 12 minutes whilst mapping out a scan of space either 0.5º or 5º wide between 2 - 20 keV.
Problems associated with Earth based observation
The second NASA Satellite (SAS-2) launched in 1972 was dedicated to gamma-ray astronomy in the energy range above 35 MeV using a wire spark-chamber aligned with satellite spin axis. It provided the first detailed look at the gamma-ray sky.
Problems associated with Earth based observation
COS-B, launched in1975 by the ESA, measured high energy gamma data (~30 MeV-5 GeV) using a Gamma-Ray Telescope comprising a spark chamber and a proportional counter. It’s highly elliptical orbit enabled long observation times enabling more detailed mapping.
Problems associated with Earth based observation
Vela satellites operated by the U.S. Department of Defense in the 70s were not intended primarily for astronomical studies but rather to search for clandestine nuclear bomb tests. They did however provide much useful astronomical data such as gamma-ray bursts (0.2 to 1.5 MeV) of 1 second duration. Triangulation showed these were not confined to the galactic plane and so must be extra-galactic in origin.
What is up there now? Chandra X-ray telescope satellite
Fermi Gamma ray space telescope
Launched in 1999
Looks for:
X-ray bursters
X-ray pulsars
Launched in 2008
Looks for:
Quasars
AGNs
Gamma ray bursters
Techniques for detecting X-rays and gamma-rays
Photoelectric effect
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Photon is absorbed and energy given to an electron which is emitted. This is called a photoelectron.
Likelihood or probability that interaction occurs is called the cross section (σ) and depends on energy of the photon and the Z (atomic number) of the detector atom.
5.3 E
5Z
Techniques for detecting X-rays and gamma-rays
Photoelectric effect
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Imagine a ray of green light of wavelength λ = 530 nm incident on a detector with a work function of 1.1eV. What is the kinetic energy given to a photoelectron ejected from this target?
What is the lowest wavelength of light that can release an electron from this target?
Techniques for detecting X-rays and gamma-rays
Compton effect
Einstein had proposed that despite all the evidence that light is a wave, it also has particle-like properties (wave-particle duality).
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Collision between X-ray and electron
Momentum of electron changes
Wavelength of photon changes
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Techniques for detecting X-rays and gamma-rays
Compton effect
At what angle does maximum energy loss occur ?
Figure shows energies of a 500 keV photon and electron after Compton scattering.
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Cross section for Compton scattering increases slowly with energy of the incident photon.
Techniques for detecting X-rays and gamma-rays
Compton effect
Let’s imagine that we collide a gamma ray photon (λ = 3×10-14 m) with an electron. What is the momentum of the photon before the collision? What is the energy lost by the photon if following the collision its direction changes by 60 degrees?
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