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Detectors for X-Ray Astronomy 48 Proportional Counters

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Detectors for X-Ray Astronomy 48 Proportional Counters Detectors for X-ray astronomy 48 Proportional Counters Workhorses of X-ray astronomy for >10 years 1962-1970: Rockets and Balloons 1962 Sco X-1 and diffuse X-ray sky background discovered by Giacconi sounding rocket Limited by atmosphere (balloons) and duration (rockets) • 1970 → Satellite era Uhuru: First dedicated X-ray Satellite e.g. Ariel V, EXOSAT e.g. Ginga e.g. XTE e.g. ROSAT 49 Adapted from Hill, Urbino 08 Proportional counters Simplest proportional counter is made of a gas chamber with an anode to collect charges "Proportional counter avalanches" by Dougsim - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons Gas Detectors (Ar, Xe) Incident X-ray interacts with a gas atom and a photoelectron is ejected Photoelectron travels through the gas making an ionisation trail Trail drifts in low electric field to high E-field In high E-field multiplication occurs (Townsend avalanche) Charge detected on an anode 50 Adapted from Hill, Urbino 08 One can add 2nd chamber to discriminate between X-ray photons and cosmic rays 51 Gain of 105 or more Duration about 1 ns → short pulse! 52 Quenching molecules The positive atom moves slowly to the cathode wall of the chamber. They are neutralized by gaining an electron This causes an energy release which can further ionize the gas, creating a charge pulse! To eliminate (or quench) this signal, one uses a 10-15% proportion of organic gas (e.g., methane), which has low electron affinity Thus, the positive ions will take electrons from the quenching gas molecules, and only the ionized molecules will reach the cathode The recombination of the ionized molecules does liberate some energy, but insufficient to ionize the gas, but it breaks the molecules Such proportional counters have, thus, a limited time life, unless they are constantly replenished by quenching molecules (gas flow counters) 53 Escape peak If the incident X-ray photon has higher energy, E0, than the K-shell fluorescence energy of gas, Ef, an escape peak can appear at E’=E0-Ef Fluorescence X-ray photon can escape the counter, leaving a smaller energy to be measured Ar-filled PC NB: Ef(Ne)=0.85 keV No No escape peak as escape the fluorescence peak, E (Ar)=3 keV energy is almost E <E f 0 f completely absorbed in the counter 54 Typical Characteristics 0.4 Townsend Avalanche Energy Resolution is limited by: The statistical generation of the charge by the photoelectron By the multiplication process Quantum Efficiency: Low E defined by window type and thickness High E defined by gas type and pressure 55 Adapted from Hill, Urbino 08 Typical Characteristics Position sensitivity Non-imaging case: Sensitivity Area Limited by source confusion to 1/1000 Crab Imaging case: track length, diffusion, detector depth, readout elements Timing Resolution Limited by the anode-cathode spacing and the ion mobility: ~ µsec Timing variations: Sensitivity Area High Gain: 103-105 56 Adapted from Hill, Urbino 08 Background origin Charged particles Cosmic rays, sub-relativistic electrons, particles from interactions with detector/telescope Photons Forward Compton scattering of gamma rays in the gas can deposit 0.1-10 keV X-rays and gamma-rays created by cosmic rays Fluorescent X-rays created in structure Background is generally flat 57 Background rejection techniques Energy Selection Reject events with E outside of band pass Rise-time discrimination Rise time of an X-ray event can be characterised. The rise-time of a charged particle interactions have a different characteristic. • Anti-coincidence Use a sub-divided gas cell with a shield of plastic scintillator Co-incident pulses indicate extended source of ionisation 58 Adapted from Hill, Urbino 08 Capella ROSAT PSPC ( ~ 1-2) 59 Micro-channel plates 60 Gain is very high 106-108 CGCD=crossed grid charge detector Chandra HRC Micro-channel plate 61 Fine position determination 62 Low63 QE and no energy response X-ray CCDs 1977 – ASCA XMM Swift XRT CCD Chandra Swift Suzaku 64 Adapted from Hill, Urbino 08 CCDs Charge Coupled Devices invented in the 1970s Sensitive to light from optical to X-rays In practice, best use in optical and X-rays CCDs make use of silicon chips The CCD consists of (1) a p-type doped silicon substrate, (2) the charge storage (depletion) layer, which is covered by (3) a SiO2 insulating layer; upon this is (4) an array of closely spaced electrodes, which can be set to pre-defined voltage value 65 Si array Si array n-type Also Sb,P Si array p-type Also Al,Ga 66 Reminder of solid state physics Electrons in a lattice do not have discrete energies. They form energy bands: Valence band Conduction band For semi-conductors, the Fermi level is just in the middle of the conduction and valence bands. At finite temperature, some electrons of p h n the valence band can jump into the conduction o o t t o o n h band (current noise) p EG(Si)=1.1 eV (IR), EG(Ge)=0.72 eV E (C)=5.5 eV (insulator) G Hole Electron 67 (From http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html) The pn junction 68 Reverse-biased pn junction Forward-biased pn junction 69 4.1 eV 2.5 eV 1.8 eV 1.2 eV Transverse cut of CCD with buried channel Blecha (cours instrumentation) 70 The electrode has positive potential to attract the generated photoelectrons in a potential well The above MOS capacitor is 1 pixel 71 Front-illuminated CCDs Back-illuminated CCDs s s n n o o t t o o h h p p g g n n i i m m o o c c n n I I Anti-reflective coating n p 15m n 625m p They have a low Quantum The QE can approach 100%. These Efficiency due to the reflection thinned CCDs become transparent to and absorption of light in the near infra-red light and the red response surface electrodes. Very poor blue is poor. Response can be boosted by response. The electrode structure the application of an anti-reflective prevents the use of an anti- coating on the thinned rear-side. These reflective coating that would coatings do not work so well for front- otherwise boost performance. illuminated CCDs due to the surface bumps created by the surface electrodes 72 Courtesy of S. Tulloch Quantum Efficiency Comparison The graph below compares the quantum of efficiency of a thick frontside illuminated CCD and a thin backside illuminated CCD. 73 Courtesy of S. Tulloch Back-illuminated CCDs BI CCD FI CCD Thinner dead layers higher low-E QE Thinner active region lower high-E QE Increased noise, charge transfer inefficiency higher FWHM From C. Grant, X-ray Astronomy School 2007 Structure of a CCD The diagram shows a small section (a few pixels) of the image area of a CCD. This pattern is repeated. Channel stops to define the columns of the image Plan View Transparent horizontal electrodes to define the pixels One pixel vertically. Also used to transfer the charge during readout Electrode Insulating oxide n-type silicon Cross section p-type silicon Every third electrode is connected together. Bus wires running down the edge of the chip make the connection. The channel stops are formed from high concentrations of Boron in the silicon. 75 Courtesy of S. Tulloch CCD Analogy VERTICAL RAIN (PHOTONS) CONVEYOR BELTS (CCD COLUMNS) BUCKETS (PIXELS) MEASURING CYLINDER HORIZONTAL (OUTPUT CONVEYOR BELT AMPLIFIER) (SERIAL REGISTER) 76 Courtesy of S. Tulloch Exposure finished, buckets now contain samples of rain. 77 Courtesy of S. Tulloch Conveyor belt starts turning and transfers buckets. Rain collected on the vertical conveyor is tipped into buckets on the horizontal conveyor. 78 Courtesy of S. Tulloch Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn into the measuring cylinder . 79 Courtesy of S. Tulloch After each bucket has been measured, the measuring cylinder is emptied , ready for the next bucket load. ` 80 Courtesy of S. Tulloch 81 Courtesy of S. Tulloch 82 Courtesy of S. Tulloch 83 Courtesy of S. Tulloch 84 Courtesy of S. Tulloch 85 Courtesy of S. Tulloch 86 Courtesy of S. Tulloch A new set of empty buckets is set up on the horizontal conveyor and the process is repeated. 87 Courtesy of S. Tulloch 88 Courtesy of S. Tulloch 89 Courtesy of S. Tulloch 90 Courtesy of S. Tulloch 91 Courtesy of S. Tulloch 92 Courtesy of S. Tulloch 93 Courtesy of S. Tulloch 94 Courtesy of S. Tulloch 95 Courtesy of S. Tulloch 96 Courtesy of S. Tulloch 97 Courtesy of S. Tulloch 98 Courtesy of S. Tulloch 99 Courtesy of S. Tulloch 100 Courtesy of S. Tulloch 101 Courtesy of S. Tulloch 102 Courtesy of S. Tulloch 103 Courtesy of S. Tulloch 104 Courtesy of S. Tulloch Eventually all the buckets have been measured, the CCD has been read out. 105 Courtesy of S. Tulloch 106 corresponds to one pixel the most positive potential in the device where they create ‘charge packets’. Each packet Photons entering the CCD create electron-hole pairs.electronsThe are then attracted towards Charge Collectionin aCCD Charge packet incoming photons pixel boundary n-type silicon p-type silicon SiO2 Insulating layer Electrode Structure pixel boundary Courtesyof S. Tulloch Charge Transfer in a CCD 1. In the following few slides, the implementation of the ‘conveyor belts’ as actual electronic structures is explained. The charge is moved along these conveyor belts by modulating the voltages on the electrodes positioned on the surface of the CCD. In the following illustrations, electrodes colour coded red are held at a positive potential, those coloured black are held at a negative potential.
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