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Ref: PLM-PAY-DetectorOpts-005-1
Date: 07/08/2007
Detector Design Options
P. Molyneux
Date
Updated Reference Number
change
07/08/2007
PLM-PAY-DetectorOpts-005-1
first version issued
1. Thin foil with Microchannel Plate (MCP)
The Microchannel Plate consists of a large number of channels with diameters of 12.5µm.
The distance between the centre of one channel and the centres of the channels
surrounding it is 15µm. This gives an open area fraction for the MCP of 63%.
The detector may be a CCD or CMOS detector or a metal plate, depending on the detection
technique used.
1.1 Impact Light Flash Detector
Collisions in interplanetary space tend to occur at hypervelocity speeds: speeds which are
greater than the speed of sound in the colliding objects (around 5 km s-1 or faster [1]). When
such collisions take place, shock waves propagate outwards from the collision point through
each of the colliding bodies, compressing and heating them. If the shock waves reach a free
surface, rarefaction waves will start to propagate back through the material, and this
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Ref: PLM-PAY-DetectorOpts-005-1
Date: 07/08/2007
compression and rarefaction will cause the material to break up, melt, and evaporate. The
vapour produced emits light.
A CCD or CMOS detector could use the impact light flash to detect particles impacting on
thin foil. Experiments have been performed [2] into the intensity of light flashes, the results of
which suggest that the intensity can be related to the mass and velocity of the impacting
particle by the equation I = C1mαvβ, where C1, α and β are constants. The constants α and β
were measured by the author of [2] for different combinations of projectile and target
materials. It was found that α was always approximately equal to 1. β had a value in the
range of 3.8 to 4.6, with an average value of 4.1.
The author of [2] also measured the spectral distribution of light flashes in order to determine
the temperature of the plasma produced. This was estimated to be between 2500K and
5000K, depending on the impact velocity.
Design Issues
It is important to know what direction the plasma produced by impact will move in, so that we
know for sure that there will be enough moving down the microchannel to the detector to
allow distinction between particles of different masses. All previous research into this area
seems to be based upon impacts with a semi-infinite target rather than a thin foil, so this is
something that needs to be considered if the light flash design is chosen.
CCD/CMOS detection of light flash
The CCD or CMOS detector chosen should be sensitive enough to detect light flashes from
impact by dust particles smaller than particles that have been detected using other methods.
The amount of light energy emitted in hypervelocity impact was investigated in [2] for various
combinations of projectile and target materials. It was found to vary from 2 x 10-6 times the
projectile energy to 10-2 times the projectile energy. The kinetic energy of a 10-19 kg particle
travelling at 5km s-1 is 1.25 x 10-12 J (= 7.8MeV). The light energy emitted when this particle
is involved in a collision is therefore in the range of 2.5 x 10-18 J – 1.25 x 10-14 J (= 15.6eV –
78keV).
Since the temperature of the light flash has been found to lie in the range from 2500K to
5000K, the peak wavelength of emitted radiation can be found using Wien’s Law:
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Ref: PLM-PAY-DetectorOpts-005-1
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 peakT  2.898 10 3 m  K . This gives a range of peak wavelengths from 579nm to 1160nm.
The CCD data available from Kodak [3] gives quantum efficiencies for light with a
wavelength of 650nm, so in order to gain an estimate of the detectability of the light flash it
will be assumed that the majority of the light energy is emitted at this wavelength. From
Wien’s Law it can be shown that a peak wavelength of 650nm corresponds to a temperature
of ~4500K. The photon energy at this wavelength (calculated using E = hc/λ) is ~3.06 x 10-19
J (~ 1.91eV). If this is compared to the calculation of emitted light energy above, the number
of photons emitted in the flash can be estimated as between 8 and ~40840.
If too few photons fall on the CCD from the flash, they will be indistinguishable from photons
generated as noise. In order to estimate the minimum number that will give a clear signal,
the signal to noise ratio of the CCD must be considered. This can be calculated using
QE  N photons
S

2
2
2
N
 dark
  signal
  readout


1/ 2
, where QE is the quantum efficiency of the detector, Nphotons
is the number of photons incident on the detector, δdark is the dark noise of the detector (the
amount of charge generated thermally rather than by photons), δsignal is the signal noise
(associated with the random arrival of photons at the detector, and given by δsignal = (QE x
Nphotons)1/2), and δreadout is the readout noise of the detector.
For an ideal detector
S
 QE  N photons . In [4] it is shown that to achieve a good signal to
N
noise ratio, the number of photons reaching the detector should be greater than or equal to
2
 readout
QE
.
Data sheets from the Kodak website [3] give separate quantum efficiencies for CCDs with
and without microlens arrays. Microlenses are tiny lens systems that focus photons onto the
photosensitive areas of the CCD (see [5]). Most of the CCDS currently available from Kodak
have quantum efficiencies at 650nm of 77% with microlenses and 65% without. A value of
15 electrons is given as the readout noise value of most. The number of photons required to
produce a good signal is therefore 152/0.77 = 293 for a CCD with microlenses or 152/0.65 =
347 without. This does lie within the range of photons emitted calculated above, but
unfortunately [2] does not contain information about any projectile-target combinations other
than the two extreme cases, so it is not possible to say which end of the range the energy
from a dust particle impacting aluminium foil is likely to fall in.
It may be that a CMOS detector would be more suitable for the detection system than a
CCD. CMOS chips tend to be cheaper than CCDs and are more resistant to radiation
damage. However, they typically have higher noise levels and lower quantum efficiencies.
Kodak make colour CMOS chips which have quantum efficiencies of 40% at 450, 550 and
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650nm, and monochrome chips with a quantum efficiency of 60% at 550nm. A typical
readout noise is 30 electrons. Assuming that the signal to noise relation is the same as for
CCDs, the number of photons required to produce a good signal is 302/0.6 = 1500 for a
monochrome chip and 302/0.4 = 2250 for a colour one.
1.2 Secondary Electron Cascade Detector
In this design the CCD/CMOS detector is replaced by a metal plate. Incoming dust
particles hit the foil and charged particles (e.g. e-, p+ or ions) from the plasma produced hits
the wall of a microchannel, releasing electrons. A voltage across the microchannel plate
accelerates these electrons, causing them to crash into another wall and release more
electrons, and so on down the channel. Generally, MCPs are run in saturation (high voltage)
mode, which means that the overall charge in the cloud of electrons that reaches the
detector is large enough to cancel out the electric field gradient down the channel. If used in
low gain (low voltage) mode, information about the charge that initially caused the electron
cascade may be deduced from the magnitude of the charge that hits the detector.
Design Issues
Since dust particles will produce a large number of charged particles when they vaporise,
dust of any size may well cause saturation of a microchannel, in which case it won’t be
possible to distinguish between small and large dust particles. However, it should be
possible to determine whether an electron cascade is caused by dust or by charged particles
that are not associated with dust, since dust impacts will produce a much larger charge than
individual electrons or protons etc. By measuring the flux of dust impacts, and comparing
this to the expected flux for certain minimum particle sizes, it should be possible to estimate
the minimum mass of particle the detector responds to, and therefore say whether the thin
foil is more sensitive than previous detectors.
1.3 Gas-Filled Microchannel Plate
This design is similar to the secondary electron cascade design. The difference is that
instead of exciting electrons from the walls of a channel, incoming charges excite electrons
in a gas inside the channel. These are accelerated as in the previous design, but the gain is
much lower than it would be without the gas. The charges imparted on the metal plate
should therefore be more dependent on the mass of the impacting dust particle, and the
signal recorded by the detector should provide more information about the distribution of
different sized dust grains in Low Earth Orbit.
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Ref: PLM-PAY-DetectorOpts-005-1
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Design Issues
Since space is essentially a vacuum, the gas in the microchannel plate will put a lot of
pressure on the layer of foil, and probably burst out through it, at least in places. Once the
gas has escaped the detector will not function as designed. Another possible issue is the
fact that each channel can only fire once, limiting the number of particles that is detectable.
However, calculations of the expected particle flux show that the number of particles
expected to impact the foil is below this limit (see payload science requirements document).
2. Scintillator Dust Detector
A scintillator is a substance that absorbs radiation of certain energies and then emits
radiation of lower energy. This detector design would use a scintillator that emits visible light
after particle impact, and this light would be detected by a CCD/CMOS detector. The three
types of scintillator that may be used in this design are organic crystals, organic plastics, and
inorganic crystals.
Organic Crystals
The most commonly used organic crystal scintillator (and the best in terms of light output) is
crystalline anthracene. The emission spectrum of anthracene has a peak around 400-440
nm, in the violet part of the visible spectrum. An interesting feature of anthracene scintillators
is that their light response to heavy particles is dependent on the direction from which the
particles impact. This may lead to confusion as to how energetic a particle is.
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Plastics
A common plastic scintillator used for particle and gamma ray detection is polyvinyl toluene
(PVT). This emits light with a peak wavelength of 423nm. The amount of visible radiation
emitted is proportional to the amount of radiation absorbed. If PVT is used as a particle
detector, the emitted radiation is proportional to the stopping power of the particles- i.e. their
rate of energy loss within the scintillator. If plastic scintillators are exposed to radiation with
high stopping powers, bonds within them may be broken, significantly reducing the
proportion of absorbed energy that they emit as light. The surface of organic plastic
scintillators can be damaged by the formation of microcracks, which cause light to be lost by
reflection.
Inorganic Crystals
Inorganic crystals are not easily damaged by high-energy radiation. The better inorganic
scintillators have higher light outputs than anthracene. There is generally a linear
relationship between the energy of a particle and the size of the light response it produces in
the scintillator. Particles produce a smaller response than gamma rays. One of the best
inorganic crystal scintillators is barium fluoride (BaF2), but it emits light in the UV region and
so is not suitable for use with a CCD/CMOS detector.
References
[1] H. Fechtig, E Grün and J. Kissel: Laboratory Simulation (3. High Velocity Impact
Processes), Cosmic Dust, edited by J.A.M McDonnell, pp. 622-637, Chichester, Sussex,
England and New York, Wiley-Interscience, 1978.
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[2] G. Eichhorn: Impact Light Flash Studies- Temperature, Ejecta, Vaporisation.
Interplanetary dust and zodiacal light; Proceedings of the Colloquium, 31st, Heidelberg,
West Germany, June 10-13, 1975. Berlin and New York, Springer-Verlag, 1976, p. 243-247
[3] Kodak Image Sensor Solutions:
http://www.kodak.com/US/en/dpq/site/SENSORS/name/ISSHome
[4] Andor Technology: Signal to Noise Ratio
http://www.andor.com/library/digital_cameras/?app=318
[5] Olympus: Microscopy Resource Centre
http://www.olympusmicro.com/primer/java/photomicrography/ccd/microlens/index.html
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