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Transcript
PH507 Astrophysics Dr Mark Price Lecture 2: The Electromagnetic Spectum and its detection. Diagram showing the E-M spectrum. 1 PH507 Astrophysics Dr Mark Price 2 The E-M spectrum can be grouped into several, broad categories: 1. The radio region. For astronomers this is generally in the frequency range 20 MHz – 900 GHz. 2. Microwaves. Radio frequencies within the region of 5 GHz – 1 THz (wavelength= 6 cm – 0.03 cm) 3. Infra-red. Here things get confusing and people generally start talking in terms of wavelengths, not frequency. General wavelength range of 1 micron (300 THz = 1 x10-6 metres) – 300 microns (1 THz). 4. Visible. Again a change of units from microns to nanometers. A very small part of the spectrum relatively speaking extending only from 700 nm (red) – 350 nm (violet). Equivalent to 428 THz – 856 THz. Note the edges of the visible range vary from person to person. People with artificial lenses can see further into the violet than a person with a normal lens. 5. Ultra-violet. The region from 350 nm – 10 nm. Another unit is sometimes used called the Angstrom. 1 Angstrom = 1 x 10-10 metres, it’s symbol is a capital “A” with a circle above, Å. 6. X-ray. Again, a further change of units from nanometers to electron volts (eV). Generally from 50 eV – 5000 eV for “soft” X-rays, and >5000 eV for “hard” X-rays. Thus to convert to frequency/wavelength you have to use the equation below, where E is the photon energy in electron volts and e is the charge on an electron (in coulombs). Ee hf hc 7. Gamma-rays. Again, any photon with an energy > 5000 eV can be called a gamma-ray. Note, the only distinction between “hard” X-rays and gamma-rays is their method of production! A photon is referred to as a gamma ray if it has been produced because of a nuclear reaction (ie. radioactive decay) and as an X-ray if it was produced due to the acceleration (or energy transfer) to an electron. Note in the figure above, there is a small amount of overlap between the various categories. This is also seen in the way that radiation between, for example, the radio and the infra-red is detected. PH507 Astrophysics Dr Mark Price 3 The above shows varying black-body curves for differing temperatures and at what part of the E-M spectrum the curve is a maximum. [Q: Does everybody know about blackbody radiation?] Astronomers use different parts of the E-M spectrum to investigate different astrophysical processes. PH507 Astrophysics Dr Mark Price 4 Radio: Used to detect individual atomic and molecular species. The table below is a few of the many hundred different species and their emission (absorption) frequency. Molecule H2 C 12CO 13CO “ “ “ 18 C O CS “ HCO+ Freq. (GHz.) 1.4 984.0 230.54 110.19 220.39 330.58 661.19 219.56 97.97 244.94 89.19 Transition 21 cm line 3P0 -> 3P1 2 -> 1 1 -> 0 2 -> 1 3 -> 2 6 -> 5 2 -> 1 2 -> 1 5 -> 4 1 -> 0 As radio telescopes can be constructed to a very large size (greater than a factor of ~10 large than optical telescopes), they can have a very much higher resolution (sub-arcsec) than their optical counter-parts. This allows astronomers to probe the very centre of star forming regions, galaxies etc. The diagram below is an attempt to illustrate this with a comparison of some optical images of a several starburst galaxies (taken with the Hubble Space Telescope) and some radio mapping images of the same object. Note the much higher resolution in the radio images. PH507 Astrophysics Dr Mark Price 5 Microwave: The technology used to detect microwaves is very similar to that for shorter and longer wavelengths. Potential sources of interest: 1. Cosmic microwave background. 2. Solar flares. 3. Pulsars. 4. Active Galactic Nuclei (AGNs). 5. Masers. The Swedish Odin satellite is a microwave satellite as was COBE (Cosmic micrOwave Background Explorer), which was the first instrument to measure the theoretically predicted differences in the cosmic microwave background. PH507 Astrophysics Dr Mark Price 6 Infra-red. Arguably one of the most important regions of the E-M spectrum for astronomers as it is within the infra-red ragion that the black-body spectrum of objects such as star formation regions, interstellar dust etc. peaks. The infra-red region is also further subdivided into the far-infrared (20 um -> 100 um) and the sub-millimetre (100 um -> 300 um). The sub-millimetre is particularly important for investigating star formation regions, and will be discuss further later. Space missions. IRAS (InfraRed Astronomical Satellite) ISO (Infrared Space Observatory) SIRTF (Space InfraRed Telescope Facility) PLANCK SPIRE PH507 Astrophysics Dr Mark Price 7 ASTRO-F (launched at the end of Feburary this year by the Japanese space agency, JAXA, the University of Kent had a role in its development). Visible By far the most common form of astronomy, it’s main scientific uses are in solar system/planetary studies, studying the dynamics of stars and solar physics. The visible “colours” of stars are also used to classify their spectral type (O, B, G etc.). Space Missions. HST (Hubble Space Telescope). NGST (Next Generation Space Telescope – shelved ?). Ultra-violet There are two primary sources of UV continuum radiation. Active Galactic Nuclei (AGNs) emit nonthermal UV. This light is produced as part of the radiation from the material in the surroundings of a massive black hole. The second class of UV emitters, massive stars, emit thermal UV light. In their case the energy of the radiation is a measure of their surface temperature. Ultra-violet radiation is emitted by plasmas with temperatures >100,000K . Space missions: UVE (UltraViolet Explorer) EUVE (Extended UltraViolet Explorer) PH507 Astrophysics Dr Mark Price 8 X-ray / gamma-ray X-rays/gamma-rays are emitted by extremely hot gas with a temperature in excess of 1 x 106 K, and thus are only found in regions of extremely high gravitational fields such as those around neutron stars and black holes. The above is an X-ray image of the moon. The “glow” is from X-ray fluorescence of the moon’s surface. The interesting part of the image is the dark side of the moon showing less dots. Thus the dots are not just noise, but are actually X-ray sources. Gamma rays are generally confined to emission by so-called “gamma ray bursters” and the reason for these bursts remains unknown, but it is speculated that it is due to supernovae exploding in distant galaxies, or material infall into a black-hole. Space missions: ROSAT CHANDRA INTEGRAL PH507 Astrophysics Dr Mark Price 9 E-M RADIATION DETECTION METHODS. Radio detection techniques. Heterodyning. This is probably one of the most sensitive methods of E-M detection, and allows very high resolution spectrography to be used (Q=Δf/f ~ 1/10000). It uses the non-linear properties of a mixing element to produce harmonics of a known frequency (referred to the a local oscillator), with an unknown frequency (emitted by the object of interest). A schematic of a typical heterodyne system is shown below. How does it work ? PH507 Astrophysics Dr Mark Price 10 It relies on the non-Ohmic behaviour of the mixing element. A resistor obeys Ohm’s law: V IR However, a non-linear device (such as a tungsten light bulb) does not, and may have an I-V characteristic such as the one shown below. Such an I-V characteristic is typical of a diode. Where: V 0 : I 0 (reverse direction) V 0: I V2 (forward direction) Now imagine we have two input signals: one at a known frequency from the local oscillator, V LO , and one from the source, VS . Thus: MAX V LO V LO sin (2f LO t ) and similarly: PH507 Astrophysics Dr Mark Price 11 VS VSMAX sin (2f S t ) Therefore the total voltage seen at the diode, VTOT , is simply: VTOT = VLO VS and therefore the diode current, I, is given by I [ VTOT ] 2 I [ VLO VS ] 2 MAX sin (2f LO t ) + VSMAX sin (2f S t ) ] 2 [ VLO Using the trignometric identities: sin 2 ( x ) 1 [1 – cos ( 2x )] 2 sin(a) sin(b) = 1 [cos(a-b) – cos(a+b)] 2 we find that the diode current I cos ( 2 f LO ) + cos ( 2 f S ) + cos f LO f S + f LO f S And thus it can be seen that we have additional components at frequencies of: 2 f LO ,2 f S , f LO f S , f LO f S For the vast majority of purposes we are only interested in the difference frequency or (more commonly) the Intermediate Frequency (IF) given by: IF = f LO f S All the unwanted frequencies are filtered out and the IF frequency is passed on to the pre-amplifier and finally to the spectrometer. The usefulness of this can be illustrated if we wanted to look at the neutral carbon transistion line at 820 GHz. This frequency is far too high to measure using current electronics. Therefore, we setup a local oscillator to output a known signal at, say, 821 GHz. This produces an IF frequency of 1 GHz which can be easily amplified and measured. PH507 Astrophysics Dr Mark Price 12 Upper and lower sidebands. One of the side effects of using this technique is the existence of sidebands. In the example above I used a local oscillator frequency of 821 GHz. If I had a contaminating source in my telescope beam which happened to be emitting at 822 GHz, I would also see a contribution from that source in my IF passband. This can be illustrated with a diagram. Real devices: SIS junctions. A Superconductor-Insulator-Superconductor (or SIS) tunnel junction is an electronic device consisting of two superconducting electrodes separated by a very thin insulating barrier. Electron tunnelling across this barrier gives rise to a very non-linear current-voltage characteristic. The diagram below shows I-V characteristics for three SIS junctions comprised of different PH507 Astrophysics Dr Mark Price 13 superconducting alloys manufactured at the University of Kent. The table below tabulates the superconducting energy gap for various superconducting alloys and the corresponding frequency. Alloy composition Superconducting energy gap (meV) Frequency (GHz.). Pb:Bi - Pb:Bi 3.30 798 Pb:Au:In - Pb:Bi 2.90 701 Pb:Au:In - Pb:Au 2.65 641 Nb - AlO2 - Nb 2.8 677 It is this non-linear I-V characteristic that gives rise to the strong frequency mixing essential for good heterodyne receivers. PH507 Astrophysics Dr Mark Price 14 A SIS junction is formed from evaporated metal films deposited onto a thin quartz substrate. The SIS junctions manufactured at the University of Kent are made by the so-called, 'Dolan bridge' method. A Dolan Bridge is a suspended 3D bridge made of photoresist. A scanning electron micrograph of such a structure is shown below. The metals are thermally evaporated within a vacuum chamber and the junction is formed by in the 'shadow' region under the bridge. To operate properly, the junction must be cooled to very low temperatures, typically to the temperature of liquid helium (4.2 K). PH507 Astrophysics Dr Mark Price 15 The image below is a scanning electron micrograph of a SIS junction made at the University of Kent. The actual junction area itself is approximately 1 micron2 and is outlined in yellow. Junctions have been made at the University of Kent with areas of less than 0.3 microns2. This is important as we need the area to be as small as possible to reduce the parasitic capacitance of the junctions and to optimise the value of the omega*R*C product to a value of approximately 4. Where omega=2 x pi x frequency, R is the normal state resistance of the junction, and C is the junctions’ capacitance. It has been ascertained that an omega*R*C product of 4 gives optimum receiver performance. Thus when operating at a frequecy of, say, 850 GHz it is important to reduce the capacitance of the junction as much as possible. One other important consideration is the superconducting energy gap of the alloy used. Superconducting materials have an energy gap which is analogous to the energy gap of a semi-conductor. As the photon energy of the detected radiation increases above the energy gap of the supercondutor the Cooper pairs start to break-up and the superconductor very rapidly starts to become 'lossy' - it's conductivity starts to approach that of it's room temperature conductivity - even if the material is still well below it's critical temperature. The table above gives the energy gap of some alloys used to make SIS junctions, their energy gap and the corresponding frequency at which the Cooper pairs start to break up. It should be noted that although SIS junctions can still be used above this frequency the ohmic losses of the material rapidly start to compromise the overall performance of the receiver. The Nb-AlO2-Nb values PH507 Astrophysics Dr Mark Price 16 are included to illustrate that Pb:Bi alloys have a higher energy gap and are thus better suited for very high frequency work. Infrared detection. A very important region of the E-M spectrum for astronomers, as this is where the black-body curve peaks for objects with temperatures between 10K and 100K, such as regions of star and planetary formation. Unfortunately, it is also the region where an astronomer’s view of the universe is impaired by the atmosphere. Atmospheric attenuation above Mauna Kea (Hawaii) under average conditions. PH507 Astrophysics Dr Mark Price 17 PH507 Astrophysics Dr Mark Price 18 As can be seen, the atmosphere is totally opaque within this region, and thus to investigate within this band we need to put detectors above the atmosphere. PH507 Astrophysics Dr Mark Price 19 A world class observatory: The James Clerk Maxwell telescope. With a diameter of 15m the James Clerk Maxwell Telescope (JCMT) is the largest astronomical telescope in the world designed specifically to operate in the submillimeter wavelength region of the spectrum. The JCMT is used to study our Solar System, interstellar dust and gas, and distant galaxies. It is situated close to the summit of Mauna Kea, Hawaii, at an altitude of 4092m. It is a jointly funded telescope between the UK and Canada. This is an interesting telescope as it works in a part of the E-M spectrum which crosses from the radio into the infra-red. Below are five instruments which have been used, or are currently being used on the JCMT. 1. Receiver A3 (230 GHz). Heterodyne radio receiver. 2. Receiver B3 (345 GHz). Heterodyne radio receiver. 3. Receiver W (460 and 690 GHz). Heterodyne radio receiver. 4. SCUBA (Submillimetre Common User Bolometer Array) working at 850 um and 450 um. 5. THUMPER (Two Hundred Micron PhotomeER) working at 200 microns using a gallium doped germanium photoconductor. PH507 Astrophysics Dr Mark Price 20