ASTR 3130, Majewski [SPRING 2015]. Lecture Notes

## DETECTORS

Why are detectors used in astronomy?

• Increase sensitivity
• More wavelengths accessible than eye
• Can detect individual photons
• Can integrate with time to increase signal
• Respond linearly, or semi-linearly, to input photon signal (eye logarithmically)
• Can store permanent record
• Manipulate later

## PHOTOELECTRIC EFFECT

Most detectors in astronomy work on the principle of the Photoelectric Effect (Einstein 1905) or related photoelectrical phenomena:

• Photons of sufficient energy hitting surface of metal releases electrons.

Example: The electroscope:

A neutral electroscope with metal foil leaves is connected to a metal plate. When light shines on the plate, photoelectrons are ejected and the foil leaves then become positive and repel one another.

• Note that this effect was used to prove that light sometimes acts like a particle, rather than a wave:

The energy of the released electrons depends NOT on intensity of light (which one might expect if light were acting like a wave in this context), but rather on the frequency of the light. Moreover, the kinetic energy of the electrons released by the photons (i.e., the "photoelectrons") is linearly related to the frequency of the incoming light (not its intensity), with a proportionality given by the Planck constant:

A plot of data from a photoelectric effect experiment. Light below a certain frequency (energy) cannot release the electrons from the plate. However, once light has a frequency beyond the certain limiting frequency (energy), the electrons are emitted, and with a kinetic energy that is proportional to the difference between the incoming and limiting frequencies.

• An important aspect of the photoelectric effect is that there is a minimum threshold frequency that must be achieved before any photoelectrons can be emitted. This threshold frequency, fmin, corresponds to a minimum energy, W, called the work function that is particular to each metal.
• Thus:
Ee- = Ep - W = hf - W = h(f-fmin)

 where: Ee- = KE of released electron Ep = Energy of photon W = Work function of metal h = Planck's constant f = Photon frequency fmin = Minimum photon frequency particular to the metal

## PHOTOCONDUCTION

Photoconduction occurs when liberated photoelectrons are collected in some way to provide an electrical current.

• A rather practical application of this concept is the photocell or solar cell, which is used to create electrical power from light energy:

A basic solar cell using semiconductor (rather than metallic) materials in a so-called "p/n junction", which we will discuss later. For now, you can imagine the cell being of a metallic-like substance. The essential concept is that the photons free photoelectrons that are free to flow to the contact of the cell and complete a circuit that drives a load.

## PHOTO-EMISSIVE DETECTORS -- THE PHOTOMULTIPLIER

REFERENCE: Photomultipliers are discussed in Section 18.11 of Roy & Clarke and in Chapter 8 of Birney.

Of course, in astronomy, we are interested in the process of photon detection -- that is, we want to measure how many photons have been detected.

• Obviously, we could use the concept of the photocell to do this task. For example, if the photocell were connected to an ampmeter, we could measure the current generated by the photocell and use this to measure the photon flux on the cell.

• One type of such a detector, where the photoelectrons leave the metal surface to be measured elsewhere, is called a photo-emissive detector.

• Photons are not the only type of particle that can liberate electrons from a metal. Electrons themselves, when moving with sufficient kinetic energy, can release other electrons from metals efficiently, and, moreover, with amplification, meaning that more electrons come out of the metal than went in.

This phenomenon, in conjunction with the photoelectric effect, can be used to amplify weak light signals, as is done in a photomultuplier.

A photomultiplier tube, or "PMT", useful for light detection of very weak signals, is a photo-emissive device in which the absorption of a single photon results in the emission of an initial single photoelectron. By attracting that single photoelectron to another metal surface, called a dynode, more electrons are released which, in turn, are collected to other dynodes, resulting in a growing cascade of many electrons at the end of the dynode chain. Thus, these detectors significantly amplify the single electrons generated by the photocathode exposed to a weak photon flux.

Photomultipliers acquire light through a glass or quartz window that covers a photosensitive surface, called a photocathode, which then releases electrons that are multiplied by electrodes known as metal channel dynodes. At the end of the dynode chain is an anode or collection electrode. Over a very large range, the current flowing from the anode to ground is directly proportional to the photoelectron flux generated by the photocathode. In order to ensure that the electrons from one dynode are attracted to the next, the dynodes are pre-charged in a progression of increasing positive voltage.

• A device like a photomultiplier, in which individual photons can be detected and measured is called a photon counter, and as you might guess, play an important role in many astronomical applications.

• Photomultipliers (on a very small scale) are also used in night vision goggles.

(left) Night vision image from military night vision goggles. Image from the blog of Dr. Sudip Bose in Iraq, http://www.sudipbose.com/archives/2004/09/. (right) Night vision image of Keck Observatory, from http://www.astr.ua.edu/keel/stargaze/nitevis.html.

• In astronomy, photomultiplier detectors have traditionally been in a "tube" configuration, like that shown here:

Photomultiplier tube with its protective covering removed. The sensitivity of the detector depends on the photocathode substance used.

Cross section of a photomultiplier tube showing how the dynode chain can be compacted into a circular space.

## NON-PHOTO-EMISSIVE PHOTON DETECTORS

In non-photo-emissive photon detectors, photoelectrons are released in the metallic (or semiconductor) material, but the "freed" photoelectrons are retained in the detector. One then can either:

• Monitor a continuous electric current, e.g., as you might do with the photocell example above.

• Build-up and store charge to be measured as an accumulated electron number later on, all at one time.

• The time during which the photoelectrons are allowed to build up before measuring their total number is called an integration.

• The process of measuring the accumulated electron number at the end of the integration is called a readout.

There are various kinds of non-photo-emissive detectors, based on the nature of the storage and "readout" process. In all cases photons enter a substrate and release bound electrons from parent atoms. Thus, the net effect of the freed electrons might induce:

• A simple build up of electrical charge that is stored in a potential well and then read out at the end as an accumulated voltage.

• This is the basis of the charged couple device (CCD) and many other detectors.

• A chemical change in the substrate of the detecting medium.

• This is the basis of the photographic detector.

• In photography, grains of certain types of molecules in the emulsion absorb photons, which excites the electrons in the grain and results in a chemical change in the grain.

These excited grains create a so-called latent image that is amplified later in the darkroom.

• In the "readout" process of developing, those grains that have been excited in this way are catalyzed to undergo further chemical change that darkens the grains.

The number of grains in an area undergoing this process (giving an overall darkness) indicates the number of photons that have entered that region.

An electron micrograph image of AgBr and AgCl grains in a photographic emulsion. Kodak image.

## BANDGAP ENERGY

An important aspect of the detection process relates to the "work function" of the detecting medium.

• To generalize the "work function" concept to include both photo-emissive and non-photo-emissive detectors, we will refer instead (and technically more correctly) to the bandgap energy of the detector. The meaning of the name will become clear later on.

• The bandgap energy, Egap, is that minimum energy required to free "bound" electrons in the atoms of the detecting medium and make them "storable" as free charges, whether or not they are emitted from the detector.

As in the photoelectric effect, we have that the energy in the electron is given by

Ee- = Ep - Egap

where: Egap = Bandgap energy
• As in the photoelectric effect, the ability of a detector to create photoelectrons depends on the bandgap energy of that detector and the energy of the light coming in.

• We must resort to different substances with different bandgap energies (or play tricks to alter the bandgap energies) to detect photons of different energies/wavelength.

E.g., X-ray, UV, visible, IR -- require different detector substrates, even if the same basic principles of detection are used.

## PARAMETERS RELATING TO DETECTORS

At this point it is helpful to introduce some concepts related to the detection process as described so far.

• Quantum Efficiency (QE): The fraction of incoming photons converted into signal.
• CCDs tend to be rather efficient, with QE's reaching 80% or more.

• Photographic film has a QE of only a few percent.

• The QE of a detector is generally a function of wavelength (more on this later).

Examples of CCD quantum efficiency curves under different illumination configurations. Courtesy Apogee Instruments, Inc.

• Spectral response: Wavelength region / frequency range over which photons can be detected reasonably. See, for example, the image above showing the spectral response of the CCD.
• Noise: Uncertainty in output signal. Ideally, only statistical fluctuations in the input signal. But not usually.
• Image from a CCD detector uniformly illuminated. That the image is not uniform shows the existence of various types of noise that entered into the detection process.

• Linearity: Degree to which output signal is proportional to incoming photon numbers.

• As an example, photographic plates are not linear at all signal levels.

They follow a "characteristic curve" (also called a Hurter-Driffield curve or "H-D curve"), which relates the number of incoming photons (the "exposure") to the density of blackened grains.

The characteristic curve shows a (1) "toe" that corresponds to a minimum necessary number of received photons required to activate a grain, a linear part (2-3), and a "shoulder" corresponding to where the grains are "filling up" (3-4) and a point (4) where the grains are "saturated" and can no longer absorb any more photons. AGFA image.

• Dynamic range: Maximum variation in signal over which detector output can represent photon flux without losing signal.

• Pixels: Individual, independently detecting elements in multiple detector system; "Picture Elements".

• Magnified image (left) of CCD camera (right) and showing the pixel structure.

• Time response: Minimum time interval over which changes in photon rate are detectable. E.g. photomultiplier:

Plot of the photoelectric effect graph taken from hyperphysics.phy-astr.gsu/edu/hbase/imgmod. Image and text relating to the linear photomultiplier taken from http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/images/photomultiplier.jpg. The cross-section of the PMT taken from http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/uvvisab3.htm and the photo of the cosmic ray PMT from Image from http://www2.slac.stanford.edu/vvc/cosmicrays/ccrtparts.html. Solar cell image from www.bpsolar.com/images/ContentImages/ pn_junction.jpg. CCD pixel image from Matthew Bershady, U. Wisconsin. All other material copyright © 2002,2006,2008,2012,2015 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 313 and Astronomy 3130 at the University of Virginia.