phys 2022: observational astronomy astronomical detectors

PHYS 2022: Observational AstronomyAstronomical 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 levelsStarlight Xpress SXV-H9 CCD camera

Range o

ver 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 Pictures taken with different smartphone cameras. Which picture has the

lowest/highest contrast?

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 In astronomy, do we need time responses as short as for human eye?

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)

- photographic film/plate and CCDs can be exposed for hours

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




Human Eye A thin biconvex lens.

Focal length ~14-17 mm.

Aperture diameter ~2-7 mm depending on scene brightness.

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? SMost likely a biological adaption to take advantage of the fact that the solar spectrum peaks at ~550 nm.olar spectrum peaks at ~5

QE of rods is ~10%, QE of cones is ~3%.

Human Eye Note that eye does not respond over

full adaptation range at a given time, but over a limited range as indicated by dashed line

Dynamic range: ~1,000 to ~10,000 depending on scene brightness and complexity

Do you now understand why a lighted room looks dark from the outside on a sunny day, but looks bright from the outside at night?



Human eye




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 eBrAgradiationhcrystalBrAg )()(

)(atomAgeAg

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 photoelectron. 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.



Human eye




Photoelectric Effect Light incident on metallic surface release electrons provided light frequency

exceed threshold frequency. Ejected electrons are called photoelectrons. (More generally, free electrons generated by the absorption of photons are called photoelectrons.)

Increasing the light intensity increases the number of photoelectrons and hence measured current. (The tube is evacuated to minimize collisions between photoelectrons and air molecules.)

metal plate

metal plate

evacuated tube

Photoelectric Effect By reversing the battery, measure the voltage required to prevent a current from

flowing. This voltage is a measure of the maximum kinetic energy of the photoelectrons.

In contradiction to expectations if light have wave-like properties:- increasing the light intensity (amplitude of the electromagnetic wave) does

not increase the maximum kinetic energy of the photoelectrons- increasing the light frequency increases the maximum kinetic energy of the

photoelectrons

Photoelectric Effect To explain the photoelectric effect, Einstein postulated that light interacts with the

metal (matter) not as a wave but as a particle (photon) with an energy given by

A photon must have an energy exceeding the minimum energy necessary to eject an electron from the metal surface before a photoelectron can be produced. This minimum energy is known as the work function, W, of the metal.

Kinetic energy of photoelectron:KEe =

Ephoton – W = hυ – W = h(υ-υmin)

Planck’s constant

wavelengthfrequency

threshold frequency

(symbol for a single photon)

Photoelectric Effect To conserve linear momentum, should photoelectrons not be ejected on the other

side of the metal plate than drawn below? For this drawing to be correct, what must have happened to photoelectrons released in the metal plate? Collided with other atoms/electrons in the metal plate and “bounced backwards.”

Why does the current increase slowly with increasing light frequency?

metal plate

metal plate

evacuated tube

Photoelectric Effect Why does the current initially increase steeply with increasing light frequency?

metal plate

metal plate

evacuated tube

Photoelectric Effect Why does the current then increase more slowly (up to a limit) with increasing

light frequency?

metal plate

metal plate

evacuated tube

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

metal plate

metal plate

evacuated tube

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.

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).

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Multiplicative factor can be designed to be very high, thus producing strong current even for weak incident light.

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

A supernova explosion of a massive star can leave behind a neutron star (i.e., the remnant core of the star, having a mass of ~1 M and radius ~10 km). A rapidly-rotating neutron star can produce beams of radiation along its magnetic poles (a good analogy is a lighthouse). If the beams of radiation sweep past the Earth, the neutron star is seen as a pulsar.

A neutron star can rotate as far as once every millisecond.

Photomultiplier Tube Optical light pulses from the Crab pulsar (rotation period of 33 ms) sampled with

microsecond (10-6 s) temporal resolution. The pulse profile changed over the period of the observations.

The Crab pulsar resulted from a supernova explosion in 1054 that was recorded by Chinese astronomers.

Photomultiplier Tube Disadvantages of PMT over other astronomical detectors:

- does not immediately produce an image- moderate quantum efficiency (up to ~30%); 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 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?


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


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)




- cosmic rays and energetic particles produced by radioactive decay thatliberate 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?




- cosmic rays and energetic particles produced by radioactive decay thatliberate 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.



Human eye




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.

nucleus

electron

Atoms Energy level diagram of hydrogen, showing atomic orbitals having different

energies.

(first shell)

(second shell)

(third 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 an atom can have many different energy levels, a solid can have many

different energy bands. Adjacent energy bands are separated by a band gap. Electrons in a molecule can be excited/dexcited from one energy band to another.

(Ground state)

(1st excited state)

(2nd excited 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

Band Theory of Solids If we can store and measure the number of photoelectrons in the conduction band

(electrons excited from the valence to conduction band by the absorption of photons) of a semiconductor, we can use a semiconductor to detect and measure the intensity of incident light. This is the underlying concept of a CCD.

Semiconductors Elemental semiconductors: column IVa , most popular Si, Ge.

Compound semiconductors: elements in columns Ib, IIb, IIIa, Va, VIa, VIIa symmetrically spanning column IVa to form diatomic or triatomic molecules such as GaAs, InSb, HgCdTe.

Semiconductors Key to usefulness of semiconductors as detectors at optical and infrared

wavelengths is that their bandgap energies match those of visible/IR photons.

For each semiconductor, long wavelength cutoff λlong,cut = hc/Eg.

Semiconductor Egap(eV) Detection

InSb 0.18 IR-Optical

Ge 0.67 IR-Optical

Si 1.11 NIR-Optical

GaAs 1.43 Optical

AgBr 2.81 Optical

SiC 2.86 Optical

1.11 μm

0.87 μm

6.89 μm

1.85 μm

0.44 μm

0.43 μm





InSb 0.18 IR-Optical

Ge 0.67 IR-Optical

Si 1.11 NIR-Optical

GaAs 1.43 Opt

AgBr 2.81 Opt

SiC 2.86 Opt

1.11 μm

0.87 μm

6.89 μm

1.85 μm

0.44 μm

0.43 μm





InSb 0.18 IR

Ge 0.67 IR

Si 1.11 NIR-Optical

GaAs 1.43 Optical

AgBr 2.81 Optical

SiC 2.86 Optical

1.11 μm

0.87 μm

6.89 μm

1.85 μm

0.44 μm

0.43 μm

Semiconductors Consider how an electric current can be conducted in a semiconductor.

Electrons excited to the conduction band leave behind electron “holes”, i.e. unoccupied states (i.e., unoccupied molecular orbitals) in the valence band.

In this situation, what charge carriers can conduct an electrical current?

Valence band

Conductionband

Semiconductors When a neighboring electron moves to fill an unoccupied molecular orbital

(hole), the electron leaves an unoccupied molecular orbital (hole) where it came from, which is filled by a neighboring electron, and so on. A hole therefore appears to move in the opposite direction to the motion of a series of electrons, and behave as if it was an actual positively charged particle.

Semiconductors It is easier to imagine the conduction of electricity by electrons in the valence

band by considering the motion of holes rather than valence electrons.

Although holes are discussed as if they are real positively-charged particles, remember that they are not real particles and all the effects we see are actually caused by the movement of valence electrons.

Semiconductors Electrical conductivity of a semiconductor can be radically altered by adding an

impurity (different element), a process called doping.

Consider adding an element with 5 valence electrons (e.g., P or As) to Si to produce a compound semiconductor. 4 of these 5 electrons bond with adjacent Si atoms as before, but the 5th electron cannot form a bond. This electron can easily be excited into conduction band.

Resulting crystal (which is neutral) has an excess of current-carrying electrons and is designated "N-type" (negative type).

Note: N-type semiconductor is electrically neutral.

Semiconductors Alternatively, consider adding an element with 3 valence electrons (e.g., B or Al)

to Si to produce a compound semiconductor. These 3 electrons bond with adjacent Si atoms as before, but the expected fourth bond cannot form thus leaving a hole (unoccupied molecular orbital).

Electrons from the valence band can be easily excited into the hole, leaving behind a hole in the valence band.

Resulting crystal (which is neutral) has an excess of current-carrying holes and is designated “P-type" (positive type).

Note: P-type semiconductor is electrically neutral.

Semiconductors Of course, what is really happening is that valence electrons can easily move to

fill holes, leaving behind holes that can be easily filled by other valence electrons.

phys 2022: observational astronomy astronomical detectors

Documents

nonlinear detectors

electrical charge

linear dynamic range

incident optical photons

human eyes

coupled device

response detector output

bandwidthwavelength