Transcript Part C
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? Pic du Midi Observatory in the French Pyrenees 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 Problems associated with Earth based observation What did astronomers do to get around this problem? 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. 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 Problems associated with Earth based observation What did astronomers do to get around this problem? NASA 1990s X-ray measurements 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 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 earthorbiting 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 Launched in 1999 Looks for: X-ray bursters X-ray pulsars Fermi Gamma ray space telescope Launched in 2008 Looks for: Quasars AGNs Gamma ray bursters Techniques for detecting X-rays and gamma-rays Photoelectric effect E kineticenergy h f E 0 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. E3.5 Z 5 Techniques for detecting X-rays and gamma-rays Photoelectric effect E kineticenergy h f E 0 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). Momentum of wave p h Collision between X-ray and electron Momentum of electron changes Wavelength of photon changes h f i (1 cos ) me c Techniques for detecting X-rays and gamma-rays Compton effect h (1 cos ) At what angle does maximum energy loss occur ? f i me c Figure shows energies of a 500 keV photon and electron after Compton scattering. 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? p h h f i (1 cos ) me c