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PH507
Astrophysics
Dr Mark Price
1
Lecture 3: Infra-red and sub-millimetre detection techniques.
The Golay-Cell
Invented in 1947 by M. J. E. Golay, the Golay cell consists of an absorbing film
placed inside a pressure cell. E-M radiation falling onto the film will be
absorbed, and the film will heat up and heat the gas inside the cell. This gas will
then expand, and the pressure in the cell will increase. At the back of the cell is
a very thin, half silvered, membrane. As the pressure increases the membrane
will flex outwards, this tiny movement is detected by shining a laser onto the
film and measuring its deflection.
Below is a very simplified schematic of a cell.
Its major advantages are:
1) That it is responsive to all radiation falling onto the absorbing film.
Spectral sensitivity is tuned by selecting the front window material
appropriately.
2) Doesn’t require cooling. Operates at room temperature. Thus cheap and
easy to use.
However it does suffer from several disadvantages
1) The half-silvered membrane is extremely fragile, and the cell must be
handled with care.
2) It has a slow response, due to the thermal response in warming and
cooling the gas within the cell. This restricts their maximum speed of
useful operation to ~20 Hz.
PH507
Astrophysics
Dr Mark Price
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A commercially available Golay-Cell.




Operating temperature = 300K
Wavelength range (polyethylene window) = 20 to 1000 μm
Optical responsivity at 15Hz ≥ 100 kV/W
Optical N.E.P at 15Hz : < 100 x 10-12 Watt.Hz-1/2
Undergoing somewhat of a revival due to it’s ability to operate at room
temperatures. NASA are developing a silicon micromachined tunnelling
infrared detector based on the principle of the original Golay cell, but which
uses changes in quantum tunnelling, instead of the deflection of a light beam, to
measure pressure changes within the cell.
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Astrophysics
Dr Mark Price
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The bolometer
Basic description.
The bolometer, in principle, is a very simple detector. It effectively consists of a
small (<1mm3) crystal of very pure semiconductor (normally germanium)
whose heat capacity is very well known, and which has a very well
characterised current-voltage characteristic.
The bolometer is attached to a heat sink via a weak thermal link. When incident
radiation of any frequency is incident upon the crystal it absorbs the radiation
and warms up. As the crystal changes temperature, it’s resistance changes
(where R is the slope of the I-V characteristic). Therefore if we supply a constant
current, I, of, say, 10 nA (typical) and measure the voltage, V, across the device.
Any change in V corresponds to a change in the resistance of the device (via
Ohms law).
Therefore if we measure the change in voltage, we can calculate the change in
temperature experience by the device, and thus the amount of energy absorbed,
Q, (and hence the incident photon energy) as we know the heat capacity, C, of
the device.
A schematic diagram of a typical bolometer set-up is show below
A bolometer has several advantages and disadvantages.
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Astrophysics
Dr Mark Price
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Advantages:
1) Ease of operation. It is a much simpler device to operate than a
heterodyne receiver. Only requiring a fixed current source and careful
calibration. This is illustrated below with typical bolometer read-out
circuit.
2) It is sensitive to all incident radiation, and thus can be used where no
other device could be. This is particularly true in the sub-millimetre.
3) Growing ultra-pure semiconductors is a mature technology perfected
from the semi-conductor industry and therefore pure germanium
crystals are reasonably easy to source.
Disadvantages:
1) It is sensitive to all incident radiation. This may sound contrary to what
was stated above, but for spectral line work, or narrow band photometry
we want a detector that is only sensitive to the frequencies we are
interested in.
2) Slow frequency of operation (typically 10 Hz). As the bolometer is only
weakly connected to it’s heat sink, it takes a certain length of time for the
bolometer to dissipate any absorbed energy. This greatly restricts the
speed of the devices. A stronger coupling to the heat-sink increases the
speed, but reduces the sensitivity of the device.
3) Cooling requirement. The most sensitive bolometers operate at
temperatures of milli-kelvin (SCUBA=100 mK, SCUBA-2=50 mK). This
greatly increases the cost of operation and the complexity of the overall
instrument.
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Astrophysics
Dr Mark Price
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Q: Why do we cool bolometers to such low temperatures?
A: To reduce noise, and increase sensitivity.
There are several, contaminating noise sources present in a bolometer.
1) Thermal noise. Caused by the random, thermal motions of electrons in
the lattice.
2) Shot noise. Caused by the quantum nature (ie. single electrons) travelling
in the lattice/connecting wires.
3) Photon noise. Caused by the emission, and absorption of photons by the
crystal from/into it’s surroundings.
4) 1/f noise. Caused by slow changes in the system. For example, changes
in the amplifier gain, slow drifts in the heat sink temperature etc.
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Astrophysics
Dr Mark Price
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It can be shown, although it’s non-trivial, that the sensitivity of the bolometer,
called it’s Noise Equivalent Power (NEP), is given by:
NEP=4 ( A S kT 5 B)
Watts/Hz0.5
Where A is the detector’s absorbing area (m2),  S is Stefan’s constant
(5.67 x 10-8 W m-2 K-4), k is Boltzmann’s constant (1.3805 x 10-23 J K-1), T is the
physical temperature of the bolometer (K) and B is the detection bandwidth of
the system (Hz).
NEP can be thought of as the power that would give rise to a signal-to-noise
ratio=1.
[Q: Does everyone know what a signal-to-noise ratio is?]
For example, a detector with A=1 mm2 , B=1 Hz and T=300 K has an NEP of:
NEP300K = 5.5 x 10-12 Watts/Hz0.5
Meaning that a power of 5.5 x 10-12 Watts would be detected with a signal-tonoise ratio of 1, in 1 second with a bandwidth of 1 hertz. However, the same
system cooled down from 300K to 3K would have an NEP of:
NEP3K = 5.5 x 10-17 Watts/Hz0.5
A factor of 100,000 lower! Thus is can be clearly seen that the sensitivity of the
detector is a very strong function of temperature, in fact:
Detector sensitivity  T
5
2
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Astrophysics
Dr Mark Price
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The picture below is of a typical, commercially available liquid helium
dewar.
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Astrophysics
Dr Mark Price
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The photoconductor
Review of band-gap theory.
As the next two devices we shall look at, the photoconductor and the CCD, require a
simple knowledge of semiconductors, we need to review band-gap theory.
[Q: How much band-gap theory do you know?]
All materials can be broken down into three main electrical conduction classes.
Insulators (nylon, diamond, paper etc.), metals (copper, silver, gold etc.) and
semiconductors (germanium, silicon, indium antimonide etc.).
The difference between them is caused by the size of the energy gap between the
valence band, and the conduction band.
The valence band can be thought of as the region where electrons are bound ‘in orbit’
around the nucleus of an atom. The conduction band is where the electrons are “free”
and can move within the lattice of the material.
In a conductor the conduction band and valence band overlap, and electrons can freely
move within the lattice. In an insulator, the energy gap is very large (5.4 eV for
diamond) and electrons (under normal conditions) cannot ‘escape’ from the valence
band into the conduction band.
However, in a semiconductor, the energy gap is sufficiently small that a small number
of electrons can move into the conduction band.
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Astrophysics
Dr Mark Price
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The figure above shows the number of conduction electrons per cubic centimetre for
various semiconductors, semimetals and metals.
It is the energy gap in semiconductors that is of interest to us, and provides the physical
mechanism by which we can use these materials as detectors.
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Astrophysics
Dr Mark Price
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The table below shows the energy gap of various materials (in eV) and the
corresponding photon frequency and wavelength.
Material
Diamond
Silicon (pure)
Germanium (pure)
InSb
Ge:Be
Ge:Ga
Energy gap (eV)
Photon wavelength
(microns)
Photon frequency
(Hz)
5.4
1.17
0.79
0.23
0.0238
0.0108
0.23
1.06
1.57
5.2 (near infra-red)
52 (mid infra-red)
114 (far–infrared
/submm)
1.303 x 1015
2.828 x 1014
1.909 x 1014
5.765 x 1013
5.765 x 1012
2.630 x 1012
The most important thing to notice is that certain semi-conductors have energy gaps
which correspond to the energy of infra-red photons. This means they can be used as
detectors of radiation via the mechanism of photoconductivity.
Q: What is photoconductivity?
Photoconductivity is the process where an electron is ‘assisted’ to escape into the
conduction band by absorbing the energy of an incident photon.
As illustrated in the diagram below.
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Astrophysics
Dr Mark Price
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Therefore if a photon arrives with an energy = hf which is greater, or equal, to the
band-gap energy of the material, an electron can be escape from the valence band, into
the conduction band and be detected as an increase in electrical current in the device.
This is referred to as intrinsic photoconductivity. InSb is an example of an intrinsic
photoconductivity which is commercially available as an infra-red detector.
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Astrophysics
Dr Mark Price
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Extrinsic photoconductivity.
In order to control the size of the bandgap, and effectively ‘tune’ the photoconductor,
different dopants are deliberately added to the host crystal. The amount of dopant is
very small, or the order of 1 in 1010 atoms!
Dopants generally tend to be in Group-III, or Group-V of the periodic tables, and are
thus tri-valent or penta-valent respectively. Germanium and silicon are in Group-IV and
are quad-valent.
By addition of a Group-III dopant to germanium we have (effectively) reduced the
number of electrons/unit volume in the valence band, and the material is referred to as
“p-type” (‘p’ = ‘p’ositive). Conversely, when we add a Group-V element, we have
increased the number of electrons/unit-volume in the valence band, and the material is
referred to as “n-type” (‘n’=’n’egative).
From the table above, it can be seen that by deliberately adding gallium to germanium
you can obtain a semiconductor with a bandgap of 0.0108 eV. But this is very different
from pure germanium, so how does this work?
Conduction (and subsequent current flow) is now obtained by exciting electrons from
the dopant, gallium, into the conduction band.
Thus by changing the dopant, and the concentration of the dopant, we can ‘tune’ the
detector to be sensitive to a wavelength we are interested in.
PH507
Astrophysics
Dr Mark Price
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The table below shows how the energy gap in germanium, silicon and gallium arsenide
is affected by adding differing dopants.
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Astrophysics
Dr Mark Price
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Basic mode of operation.
So, now we know the physics behind their operation, how would we use one in reality?
Simply, we treat the photoconductor as a resistor in series with a capacitor and a ‘reset’
switch.
As photons are absorbed by the photoconductor, electrons are excited into the
conduction band and are swept out of the crystal by a bias voltage applied across the
crystal. These electrons then go to charge up a capacitor, C.
Thus the voltage across the capacitor tells us how charge has been collected, and
therefore the intensity of the illuminating radiation.
From simple circuit theory, the voltage across a capacitor in response to a step input (ie.
suddenly shining photons onto our detector) is given by:
V (t )  V0 [1  exp(
t
)]
RC
For t << RC, and by expanding exp(-t/ RC) to first order, the above equation becomes:
V (t )  V0 [1  (1 
V t It
t
)]  0 
RC
RC C
and on differentiating simply becomes:
dV
I

dT C
Thus if the rate of change of voltage can be measured, then the current, I, can be
calculated.
This is exactly how photoconductors are operated in practise and signal currents of
electron per second can be measured, making them very sensitive devices.
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Astrophysics
Dr Mark Price
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Astrophysics
Dr Mark Price
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The diagram below shows some representative signal ramps (effectively the ‘tail’ of the
RC charge curve) taken from a detector flown on the ISO space mission.
Of course, one of the problems with a space mission is that the spacecraft is constantly
bombarded by the solar wind, and cosmic rays.
These highly energetic particles with energies > 1 MeV can seriously compromise the
sensitivity of photoconductors.
How badly was seen very soon after the ISO space-craft was launched in 1995.
From previous we saw that extrinsic photoconductivity is produced by exciting
electrons into the conduction band from dopants within the main host lattice (normally
germanium). However, a photon with a high enough energy (such as a gamma-ray) does
not just excite electrons from the dopants, but from the host lattice itself!
As shown in the next diagram.
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Astrophysics
Dr Mark Price
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This causes a lauch number of electrons to be liberated at once, and to be swept out of
the crystal. This results in a sudden increase in voltage at the output.
Below is an example of a real gamma-ray event as detected in the lab.
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Astrophysics
Dr Mark Price
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If we were simply to measure the slope of this ramp to measure dV/dt we would
calculate an erroneous value for I due to the radiation induced discontinuity present in
the ramp.
The photoconductor described here is only one type of a range of infrared and
submillimetre detectors.
Others include BIBs (Blocked-Impurity-Band), NTDs (Neutron Transmutation Doped
detectors), TEBs (Transition Edged Bolometers) etc. etc.
In next week’s thrilling installment!
CCDs, X-ray & exotic detectors, ground and space based observatories.
Note: assignment #7 due back Wednesday 15th.