PHYS 2022: Observational Astronomy

PHYS 2022: Observational Astronomy Astronomical Detectors

Learning Objectives • Astronomical detectors: - major functions - main characteristics • Human eye • Photographic film/plate • Photomultiplier tube - photoelectric effect - photomultiplier • Operation of Charge-Coupled Devices: - band theory of solids - semiconductor - metal oxide semiconductor (MOS) capacitor - charge coupling

Learning Objectives • Properties of Charge-Coupled Devices: - plate scale - quantum efficiency - charge transfer efficiency - output - binning • Noise considerations for Charge-Coupled Devices: - photon noise - readout noise - dark noise • Reducing CCD data: - bias frame - dark frame - flatfield frame

Functions of Astronomical Detectors • Astronomical detectors either initiate a chemical change in a compound (e.g., eye retina, photographic film or plate) or transform energy from electromagnetic radiation to electrical charge (e.g., photomultiplier tube, charged coupled device). • What are advantages of man-made astronomical detectors over human eyes?

Functions of Astronomical Detectors • Astronomical detectors either initiate a chemical change in a compound (e.g., eye retina, photographic film or plate) or transform energy from electromagnetic radiation to electrical charge (e.g., photomultiplier tube, charged coupled device). • What are advantages of man-made astronomical detectors over human eyes? - higher intrinsic sensitivity. Human eye detects about 3 (daytime) to 10 (nighttime) out of every 100 incident optical photons. Similarly, photographic film/plate also detect ~1-10 out of every 100 incident optical photons. By contrast, CCDs can detect up to about 90 out of every 100 incident optical photons.

Characteristics of Astronomical Detectors • Quantum efficiency (QE): - QE = number of detected photons/number of incident photons - detections can be chemical changes (eye, photographic emulsion), photoelectrons released (PMT), and charge-pairs created (CCD) - generally a function of wavelength • Spectral bandwidth (or simply bandwidth) - wavelength range over which photons can be detected

Characteristics of Astronomical Detectors • Linearity: - response (detector output) linearly proportional to incident number of photons (energy delivered to detector per unit time × QE × integration time) - examples of non-linear detectors are human eyes and photographic emulsions - examples of linear detectors are PMTs and CCDs, although in practice over limited range of input levels Starlight Xpress SXV-H9 CCD camera Range over which response linear

Characteristics of Astronomical Detectors • Dynamic range (or contrast, in popular usage): - ratio between largest and smallest values of detector output - overall dynamic range may differ from dynamic range over which the response is linear dynamic range = 2048 dynamic range = 128 (depends on scene brightness and complexity)

Characteristics of Astronomical Detectors • Time response: - minimum time interval over which changes in photon rate are detectable - minimum time interval over which human eye can respond is 10-15 ms for cones (daylight) and 0.1-0.2 s for rods (nighttime)

Characteristics of Astronomical Detectors • Integration time: - human eye integrates over 10-15 ms for cones (used in daylight, perceives colors) and 0.1-0.2 s for rods (used in nighttime, cannot perceive color) • Size: - size of photographic film/plate or CCD, which limits field of view • Noise: - measurement uncertainties imposed by properties of emitted light (photon noise) and nature of detector used - e.g., sources of CCD noise are dark current, readout noise, etc.

Human Eye • A thin biconvex lens. • Focal length ~14-17 mm. • Aperture diameter ~2-7 mm depending on scene brightness. • Dynamic range: ~1,000 to ~10,000 depending on scene brightness and complexity.

Human Eye • Photon sensor on the retina: - rods (scotopic vision) at night, - cones (photopic vision) in daylight • Rods comprise ~100 million cells, cones comprise ~5 million cells. • Scotopic spectral response peaks at ~500 nm. • Photopic spectral response peaks at ~550 nm. Why? Solar spectrum peaks at ~550 nm. • QE of cone is ~3%, QE of rod is ~10%.

Photographic Film/Plates • Photography invented in 1800s; use in astronomy became popular in 1900s. • Photographic film/plate comprises a thin coating of silver halide (e.g. AgBr, micron-size crystals) suspended in a gelatin emulsion on a plastic film/glass plate. • When a photon strikes, The silver ion can then combine with the freed electron Group 17 of periodic table (halogens)

Photographic Film/Plates • Chemicals used to remove the silver halide (“fixing”), leaving behind the metallic silver. Metallic silver, which is opaque to optical light, makes up the latent image (the negative). • QE of ~1-2%, reaching up to ~10% with special sensitizing techniques. • Can integrate longer than human eye to detect fainter objects.

Photographic Film/Plates • Response of photographic film/plate is non-linear. At low light levels, response is determined by physics of silver activation; as the film becomes progressively more exposed, each incident photon is less likely to impact a still-unexposed grain.

Photographic Film/Plates • Silver Halide grains are manufactured by combining Silver Nitrate and Halide Salts (Chloride, Bromide, and Iodide) in complex ways that result in a range of crystal sizes, shapes, and compositions. • These primitive grains are then chemically modified on their surface to increase their light sensitivity. The unmodified grains are only sensitive to the blue portion of the spectrum, and they are not very useful in camera film. Organic molecules known as spectral sensitizers are added to the surface of the grains to make them more sensitive to blue, green and red light. These molecules must adsorb (attach) to the grain surface and transfer the energy from a red, green, or blue photon to the silver halide crystal as a photo electron. Other chemicals are added internally to the grain during its growth process, or on the surface of the grain. These chemicals affect the light sensitivity of the grain, also known as its photographic speed (ISO, or ASA rating).

Photographic Film/Plates • When a photon of light is absorbed by the spectral sensitizer sitting on the surface of a silver halide grain, the energy of an electron is raised into the conduction band from the valence band, where it can be transferred to the conduction band of the silver halide grain electronic structure. A conduction band electron can then go on to combine with a positive hole in the silver halide lattice and form a single atom of silver. This single atom of silver is unstable. However if enough photoelectrons are present at the same time in the crystal lattice, they may combine with enough positive holes to form a stable latent image site. • It is generally felt that a stable latent image site is at least 2 to 4 silver atoms per grain. A silver halide grain contains billions of silver halide molecules, and it only takes 2 to 4 atoms of uncombined silver to form the latent image site. Modern color films generally take 20 to 60 photons per grain to produce a developable latent image.

Photoelectric Effect • Light therefore has particle-like in addition to wave-like properties. • Light, in the form of electromagnetic waves, shows its wave-like properties as it propagates through space; e.g., refraction, diffraction, interference. • Light, in the form of photons, shows its particle-like properties as it interacts with matter; e.g., photoelectric effect, Compton effect, excitation of atoms and molecules.

Photoelectric Effect • The setup below can therefore be used to count photons; i.e. the measured current (i.e., the number of photoelectrons) is proportional to the incident number of photons (which exceed the threshold frequency). • What if we wanted measure an individual or a small number of incident photons? The current produced may be too small to measure precisely; e.g., the number of photoelectrons passing through the current meter may only be comparable to the net number of electrons that pass through the current meter due to random thermal motion. metal plate metal plate evacuated tube

Photomultiplier • Incident photons on photocathode (metal plate) releases photoelectrons. Photoelectrons focused by electrode, then accelerated by first dynode (metal plate biased to a voltage that is positive with respect to photocathode and electrode). Impact of photoelectrons with first dynode releases more electrons, which are then accelerated by second dynode (metal plate biased to a higher positive voltage with respect to first dynode) in a process that multiplies the number of electrons released before finally being collected at the anode (metal plate with highest positive voltage).

Photomultiplier • Multiplicative factor can be designed to be very high, thus producing strong current even for weak incident light. (Total electrons per photoelectron)

Photomultiplier Tube • Photomultiplier therefore converts weak incident light into strong electrical signal that can be accurately measured. • Advantages of photomultiplier tube (PMT) over other astronomical detectors: - linear over wide range of incident light intensity - ultrafast time response (~0.0001 ms) - low noise • Optical light pulses from the Crab pulsar sampled with microsecond (10-6s) temporal resolution. The pulse profile changed over the period of the observations.

Photomultiplier Tube • Disadvantages of PMT over other astronomical detectors: - does not immediately produce an image - moderate quantum efficiency (1%-10%); why so low? - lifetime limited by buildup of charge at anode Why does QE drop away at long λ? Why does QE drop away at short λ?

Photomultiplier Tube • Disadvantages of PMT over other astronomical detectors: - does not immediately produce an image - moderate quantum efficiency (1%-10%); why so low? Will return to this later. - lifetime limited by buildup of charge at anode Why does QE drop away at long λ? Threshold frequency. Why does QE drop away at short λ? Will return to this later.

Photomultiplier Tube • Even when the PMT is shielded from light, it generates a weak current known as dark current. That is, electrons continue to be released from the photocathode. What processes are responsible for the dark current?

Photomultiplier Tube • Even when the PMT is shielded from light, it generates a weak current known as dark current. Processed responsible for the dark current are: - electrons liberated from the photocathode and dynodes because they have kinetic energies due to thermal motion sufficiently large to overcome the work function of the respective metals

Photomultiplier Tube • Even when the PMT is shielded from light, it generates a weak current known as dark current. Processed responsible for the dark current are: -cosmic rays and energetic particles produced by radioactive decay that liberate electrons through ionization or transfer of kinetic energy (typically relativistic proton)

Photomultiplier Tube • Even when the PMT is shielded from light, it generates a weak current known as dark current. Processed responsible for the dark current are: - electrons liberated from the photocathode and dynodes because they have kinetic energies due to thermal motion sufficiently large to overcome the work function of the respective metals -cosmic rays and energetic particles produced by radioactive decay that liberate electrons through ionization or transfer of kinetic energy • These are the main sources of noise in a photomultiplier. • How can dark current produced by thermal motions be minimized?

Photomultiplier Tube • Even when the PMT is shielded from light, it generates a weak current known as dark current. Processed responsible for the dark current are: - electrons liberated from the photocathode and dynodes because they have kinetic energies due to thermal motion sufficiently large to overcome the work function of the respective metals -cosmic rays and energetic particles produced by radioactive decay that liberate electrons through ionization or transfer of kinetic energy • These are the main sources of noise in a photomultiplier. • How can dark current produced by thermal motions be minimized? By operating the PMT at low temperatures. • How can PMTs be shielded from cosmic rays to record weak incident light?

Photomultiplier Tube • In experiments to detect and measure the flux of neutrinos or anti-neutrinos, Cherenkov light produced by the passage of neutrinos or anti-neutrinos through liquid (water or other chemicals) is detected by a bank of photomultiplier tubes. Cherenkov light in the core of a nuclear reactor Super-Kamioka Neutrino Detection Experiment

Photomultiplier Tube • To shield against cosmic rays, experiments are conducted deep underground to provide a large path length for cosmic rays to interact with matter. Neutrinos and anti-neutrinos, by comparison, are much more weakly interacting. • For example, the Super-Kamioka Neutrino Detection Experiment is located 1 km underground in the Mozumi mine.

Charge-Coupled Device • Charge-Coupled Device (CCD) invented in 1960’s, first used in astronomy in 1976. Today, standard detector for digital imaging from UV to near-infrared. • This optical picture of Uranus is believed to be the first astronomical image ever made with a CCD, taken by J. Janesick and B. Smith in 1976 using a 400 x 400 pixel CCD on the 61-inch telescope on Mount Bigelow in Arizona.

Atoms • A simplified picture of an atom is a nucleus (containing positively charged protons, as well as electrically neutral neutrons) surrounded by negatively-charged electrons in orbit around the nucleus. • Each electron describes an atomic orbital.

Atoms • Electrons fill shells, starting with the innermost shell. • Electrons in different shells have different (orbital) energies; i.e., atomic orbitals corresponding to different shells have different energies. electron nucleus

Atoms • Energy level diagram of hydrogen, showing atomic orbitals having different energies. (third shell) (second shell) (first shell)

Covalent Bonds • Electrons in the outermost shell are called valence electrons, which can participate in the formation of chemical (covalent) bonds with other atoms. Atoms/molecules with unfilled outermost shell are especially reactive, whereas those with filled outmost shells are especially inert. • Number of molecular orbitals is equal to the number of atomic orbitals in the atoms being combined to form the molecule. A hydrogen molecule has a total of two molecular orbitals.

Covalent Bonds • In a multi-electron atom, the situation is more complicated. The sharing of electrons in the outer shell can have an effect on electrons in the inner shell(s). • A single water molecule has eight molecular orbitals, only five of which are occupied at a given time. Electrons can be excited from a lower to higher molecular orbital, and vice versa.

Band Theory of Solids • In a solid, atoms pack closely together by sharing their electrons to form what resembles a single large molecule. • How many molecular orbitals are there in a solid like pure silicon? Lump of pure silicon

Band Theory of Solids • In a solid, atoms pack closely together by sharing their electrons to form what resembles a single large molecule. • How many molecular orbitals are there in a solid like pure silicon?

Band Theory of Solids • A solid – an indefinitely large molecule – has a very large number of molecular orbitals. • Theory of quantum mechanics describes how many molecular orbitals can have the same energy. Result of a very large number of molecular orbitals is an energy band rather than distinct energy levels. (Number of molecular orbitals)

Band Theory of Solids • Just as one or more electrons in an atom can be excited to higher energies in different atomic orbitals, one or more electrons in a molecule can be excited to higher energies in different molecular orbitals. Result is energy bands separated by band gaps. (2nd excited state) (1st excited state) (Ground state)

Band Theory of Solids • Any solid has a large number of bands; in theory, a solid can have infinitely many bands (just as an atom has infinitely many energy levels). • Bands have different widths, based upon the properties of the atomic orbitals from which they arise. Also, allowed bands may overlap, producing (for practical purposes) a single large band. • All but a few of these bands lie at energies so high that any electron that attains those energies will escape from the solid (i.e., classic photoelectric effect). These bands are usually disregarded within a solid.

Band Theory of Solids • Below is a simplified diagram where we consider only two electronic bands in a solid (the rest are ignored) that allows the three major types of materials to be identified: metals, semiconductors, and insulators. • Valence band = “ground states” that are almost fully occupied in an insulator or semiconductor. • Conduction band = “excited states,” corresponding to states whereby electrons can move freely in a solid, and which are partially occupied in a metal, weakly occupied in a semiconductor, and virtually unoccupied in an insulator.

Band Theory of Solids • Energy bandgap (Eg) is the minimum energy required to excite electrons from the valence to the conduction band. • What are two ways in which electrons can be excited into the conduction band?

Band Theory of Solids • Energy bandgap (Eg) is the minimum energy required to excite electrons from the valence to the conduction band. • What are two ways in which electrons can be excited into the conduction band? - thermal energy (heat); e.g., electrical conductivity of semiconductor increases with increasing temperature - absorption of photons (creation of photoelectrons); sometimes also referred to as the non-photoelectric effect Silicon Silicon

PHYS 2022: Observational Astronomy