W4 Part 3b - Infrared Detectors

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Do curso por The University of Edinburgh

AstroTech: The Science and Technology behind Astronomical Discovery

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The University of Edinburgh

AstroTech: The Science and Technology behind Astronomical Discovery

223 ratings

"How do they know that?" Modern astronomy has made some astonishing discoveries - how stars burn and how black holes form; galaxies from the edge of the universe and killer rocks right next door; where the elements come from and how the expanding universe is accelerating. But how do we know all that? The truth is that astronomy would be impossible without technology, and every advance in astronomy is really an advance in technology. But the technology by itself is not enough. We have to apply it critically with a knowledge of physics to unlock the secrets of the Universe. Each week we will cover a different aspect of Astronomical technology, matching each piece of technology to a highlight science result. We will explain how the technology works, how it has allowed us to collect astronomical data, and, with some basic physics, how we interpret the data to make scientific discoveries. The class will consist of video lectures, weekly quizzes, and discussion forums. Each week there will be five videos, totalling approximately 40 minutes. They will be in a regular pattern - a short introduction, an example science story, an explanation of the key technology area, a look at how the technology is used in practice, and finally a look at what the future may hold.

Na lição

Detectors and the First Galaxies

Detectors, how they work, and how we use them to catch the light from the very first galaxies.

W4 Part 1 - Catching the Light from the Stars5:01

W4 Part 2 - Galaxies since the Dawn of Time4:24

W4 Part 3a - Light, Matter, and CCDs15:01

W4 Part 3b - Infrared Detectors5:36

W4 Part 4 - Galaxies since the Dawn of Time Part II7:03

W4 Part 5 - Another Revolution in Detector Technology? 1:57

Conheça os instrutores

Andy Lawrence

Physics and Astronomy
Catherine Heymans

Physics and Astronomy

[BLANK_AUDIO]

So, that's the detection of regular, visible wavelength light.

Now lets talk about the infrared.

So, first thing, why we want to measure infrared light.

Lots of very interesting things in the universe

emit more infrared light than visible wavelength light.

So, cold things, things shrouded behind dust, and in particular,

high redshift objects at the edges of the universe emit lots of infrared.

So we really do want to detect infrared light.

But here's the problem. So visible wavelength light, so, the wavelength of

light you can see with your eyes, is around 500 nanometers, or 0.5 microns.

If we take near infrared light, the sort of thing we want to measure,

at say a wave length of two microns -

if we ask, well okay, how much energy

does each photon have, for that near infrared light?

The answer is about 0.6 electron volts.

Now you may have remembered, that for silicon, that band gap between the the

bound states and the conducting states, that

gap, is 1.2 electron volts.

Now, that means for regular visible light, each photon could

easily bump you up into the conduction band of silicon.

But the infrared light can't do that trick.

The photons don't have enough energy.

So w need to do something else.

So basically, the answer is to turn to different substances.

There are other semi-conductors. So silicon is

the most popular in the electronics industry.

So, it makes very cheap devices because everybody uses it,

and it's easy to make printed circuits and so on.

But there are other semi-conductors we can use as detectors.

So for instance, the popular choice in the near infrared, a few microns,

Is an alloy of Mercury, Cadmium and

Tellurium, or usually known as Mer-cad-tel.

And in fact, if you adjust the Cadmium content, you can

change the the size of the band gap and tune it.

So that's pretty cool.

But it's got a, a smaller band gap, so the infrared light can do it.

When we move into mid infrared, 10, 20 microns,

that's the sort of wavelength warm bodies, and

most of the things in this room are

radiating at, that has even lower energy per photon.

So to detect that, well there are various choices,

but the popular one for example, is silicon arsenic.

Then we move into the far infrared at

100 microns there we use germanium and gallium.

So there are choices, there are ways we can do this, but

I won't go into details, but in order to actually make working devices,

with these other materials, it's much harder work, the cost has not

been driven down by the big electronics industry the way it has for silicon.

So those devices are more complicated and a lot more expensive.

So this is an infrared chip right here made

of Mercadtel, and this is a working model, a tester

for an infrared camera that's on a telescope

called VISTA that's in Chile, run by the European Southern Observatory.

But the camera was designed and built here in the UK in Edinburgh and Oxfordshire.

And, that is, it looks pretty similar to a CCD,

I know, it just looks like a flat, smooth, surface.

It's made up of lots and lots of pixels.

But it's not made of silicon.

It's made of Mercadtel.

So, it works very well, but, it's very expensive.

So, in infrared astronomy, it's very important to be cool.

That is, to keep everything cold.

There are two reasons for this.

The first one, is that everything around us

is radiating in the infrared, at the sort of

temperature that we are here on Earth -

your body, the telescope - everything is radiating in the infrared.

That's just the peak wavelength of radiation for warm bodies.

So we want to try minimize that to keep down the glare.

That's a background for detecting faint things.

The second reason is maybe not quite so obvious.

Let's think again about how the detector works.

Remember that we've got a set of bound states and

then a small gap, and then the conduction band, and what

we want is for light to strike the surface, knock electrons

up into the conduction band, and then we trap them.

However, if the material itself is warm the atoms will be jiggling about.

And then some atoms will bump into other atoms and those

collisions between atoms, can also bump up electrons into the conduction band.

And we get a signal that's not due to light,

but it's just due to this so called thermal noise.

So we want to cool the material down, to minimize that problem.

And this is especially bad for infrared detectors,

Which have a very small band gap.

So, what we do is to put our detectors such

as this infrared array here, I need to put it inside

a kind of super fridge basically, in a bath of liquid nitrogen,

or even liquid helium, and keep it as cold as possible.

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