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PLUME
Ref: PLM-PAY-4thYearRep08-029-1
Date: 03/03/2008
4th Year Project (2008)
P. Molyneux
Date
Updated Reference Number
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03/03/2008
PLM-PAY-4thYearRep08-029-1
final version 4th year report
High-Sensitivity Nanometeoroid Detector for a Student
Satellite
Philippa Molyneux 03/03/08
Supervisor: Dr. N. Bannister
Abstract
PLUME (Picosatellite for Leicester University’s Micrometeoroid Experiment) is a student satellite
being developed by undergraduates at the University of Leicester. Its primary payload is a highly
sensitive nanometeoroid detector based on the concept of thin film penetration by dust. The PLUME
detector will make use of a microchannel plate (MCP) to support the film, allowing a thinner film
layer to be used which results in increased sensitivity to low mass particles.
This report summarises current knowledge regarding dust in low Earth orbit and reasons to study it,
before reviewing common detection techniques. Two possible designs for the PLUME dust detector
are then outlined, and their feasibilities examined. The design that initially seems most promising
works by using a photomultiplier tube (PMT) to detect the flash produced when a high-velocity dust
particle hits a foil target and is vaporised. The response of the PMT to a typical light flash is modelled
and found to be poor for low-energy particles. Hence, the other design option (in which a potential
difference is run across the MCP to create a charge multiplier) may be more suitable for the CubeSat
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applications. Prototypes of the two designs will be built and tested at a dust accelerator facility in
Heidelberg, Germany.
Contents:
1. Introduction......................................................................................................1
2. Dust in Low Earth Orbit.................................................................................2
2.1 Sources of Cosmic Dust...................................................................2
2.2 Why Study Cosmic Dust?.................................................................3
2.3 Dust Flux in Low Earth Orbit.........................................................5
3. Typical Dust Detector Designs.......................................................................6
3.1 Remote Sensing...............................................................................6
3.2 Impact Sensing................................................................................6
3.2.1 Solid Target Detectors.............................................6
3.2.2 Thin Film Detectors.................................................7
3.2.3 Impact Ionisation Detectors.....................................7
3.3 Lobster ISS samples: An Accidental Dust Detector........................7
4. Design Options for a CubeSat Dust Detector...............................................8
4.1 The MCP Base.................................................................................8
4.2 Impact Light Flash Detector............................................................9
4.3 Electron Cascade Design................................................................9
4.4 Scintillator Design.........................................................................10
4.5 Chosen Design...............................................................................10
5. Feasibility Study for an Impact Light Flash Detector................................10
5.1 Light Flash Theory.........................................................................10
5.1.1 Light Flash Studies.................................................11
5.1.2 Signal Timing of Flash...........................................11
5.1.3 Temperature of Flash.............................................11
5.1.4 Impact Flash Intensity and Energy........................11
5.1.5 Intensity as a function of Target Melting Point......12
5.2 Photon Detection System................................................................12
5.3 Photomultiplier Theory..................................................................12
5.4 Modelling the expected response of the PMT................................13
5.4.1 Choice of PMT model.............................................13
5.4.2 Detector Spectral Response....................................14
5.4.3 Discussion of parameters used in model................15
6. Conclusions.....................................................................................................17
6.1 Further Work Required..................................................................18
7. References........................................................................................................19
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High-Sensitivity Nanometeoroid Detector for a Student
Satellite
1.Introduction
PLUME (Picosatellite for Leicester University’s Micrometeoroid Experiment) is a student satellite
being developed by undergraduates at the University of Leicester, conforming to the CubeSat
standard developed by California Polytechnic State University and Stanford University’s Space
Systems Development Lab. It is expected to launch in mid 2009 and its primary payload will be a
high-sensitivity nanometeoroid detector, the design of which will be discussed in this report.
The CubeSat programme was created primarily to allow undergraduate students to plan and execute
a real space mission, gaining valuable skills and experience in the space industry. All CubeSats must
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conform to a standard size and shape- a 10 cm sidelength cube of total mass not exceeding 1 kg,
with a centre of mass within 2 cm of the geometric centre of the satellite. This clearly puts severe
restrictions on the types of payload that can be carried by CubeSats: developers must be creative in
order to design scientifically interesting experiments that fit within such strict mass and volume
limits.
The dust detector being developed to fly on PLUME is based around a microchannel plate (MCP- a
thin glass plate with many parallel channels running through it). The MCP will be used to support an
extremely thin foil, and below this structure will be a system that detects the effects of dust
collisions with the foil. This payload is ideal for flight on a CubeSat, as MCPs are thin and lightweight
and the exposed surface of the detector may be scaled up or down to fit as much area as is available
on the satellite
surface after solar panels have been attached. Foils previously flown with MCP bases have been
found to contain impact holes apparently caused by dust particles smaller than any previously
detected [1], but none have been flown specifically in order to detect dust. This means that the
PLUME detector should be able to make a significant contribution to models of dust populations in
the near Earth environment. More importantly, the PLUME mission will act as a proof of concept for
the detector design: if successful, similar dust detectors may well be flown in the future on missions
to low Earth orbit (LEO) and beyond.
The novel design of the detector has attracted interest from other CubeSat teams looking for
payload ideas. The PLUME team has already been in contact with groups from the University of
Florida in the USA, the University of Rome in Italy and the University of Patras in Greece, all of whom
wish to fly copies of the detector on their satellites. This is clearly beneficial in terms of outreach
between Leicester and the collaborating universities, and the more data is collected the more
reliable any conclusions drawn from the experiment will be. It will also be interesting to compare the
data obtained by detectors flying on satellites in different orbits.
2.Dust in Low Earth Orbit
2.1 Sources of cosmic dust
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The phrase ‘cosmic dust’ is used to describe dust particles in interstellar or interplanetary space, but
does not distinguish between particles of different origins. The dust in near-Earth space, for
example, may come from one of a number of sources. Some of the dust will have been ejected from
celestial bodies elsewhere in the solar system or galaxy and travelled towards the Earth, while other
particles may be due to electrostatic blow-off from the moon (see figure 1). Some particles near the
Earth will be artificial: Al2O3 grains from solid rocket motors, for example, or fragments ejected when
artificial satellites experience explosions or collisions.
Some of the particles in low Earth orbit may be associated with dust grains ejected when
interplanetary material collides with the moon. The percentage of ejected matter that ends up in the
Earth’s magnetosphere depends on the phase angle of the moon, but at certain times in the lunar
cycle may be up to 80% [2]. The dust particle experiment on the 1959 Eta (Vanguard III) satellite
detected a particularly large number of events over a period which coincided with the Leonid
meteor shower [3]. It is believed that this was due to dust being ejected from the lunar surface into
the near-Earth environment after impacts by Leonid meteors. It has been suggested that these
secondary ejecta make the moon the primary source for all particulate matter in the magnetosphere
with radii less than 0.5μm [3].
The authors of [2] have a different explanation for the increased flux of dust during the Leonid
shower. They investigated the possible effects of the Earth’s magnetosphere on the motion of
submicron-sized dust grains associated with the Leonid meteor stream. The parent comet of the
Leonids is 55P/Tempel-Tuttle, which has an eccentric retrograde orbit. This causes the Leonid dust
grains moving towards Earth to have large approach velocities (~70km s-1). Generally, since dust
particles carry a charge, the effects of the Earth’s magnetic field and currents in the magnetosphere
cause the dust grains to be ejected from the magnetosphere or to crash into the Earth a short time
after entering the region. (For a detailed description of the effects of the magnetosphere on dust,
see [4].) However, it has been found that magnetospheric perturbations are not strong enough to
change the orbits of dust with velocities as high as those reached by the Leonid dust [2]. This is true
for dust particles produced by the parent comets of most meteor showers.
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1) Interplanetary Dust
Primary
Internal
1) Electrostatic blow-off from the
surface of the moon.
2) Al2O3 grains etc. from solid rocket
motors
Sources
Secondary
1) Impact of interplanetary dust with moon.
2) Disruption of particles by electrostatic force/rotation
of interplanetary dust.
3) Fragments of artificial satellites from
explosions/collisions etc.
Sinks
1) Impact with Earth/satellite.
2) Gradual erosion due to sputtering (bombardment causing atoms to
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be ejected into gas phase) by energetic ions.
Fig.1: Sources and Sinks of dust in the Earth’s magnetosphere, adapted from fig. 1 in [5]
2.2 Why study cosmic dust?
Once a dust particle has been ejected from an object in space, it moves through interplanetary or
interstellar space, carrying information about the composition and structure of its parent body. The
dust particle is then affected by the interplanetary material in a number of ways. Its composition will
be changed by photolysis and erosion (sputtering) by the solar wind, volatile molecular species will
be evaporated from it, and it will also be affected by temperature changes.
Dust particles (whether interstellar, interplanetary, cometary or meteoritic) are known to contain a
type of molecule called polycyclic aromatic hydrocarbons (PAHs) [6]. The authors of [7] recreated
the effects that PAHs experience as they travel through space by bombarding frozen anthracene (a
PAH) containing water with low energy hydrogen ions. By doing so, they were able to synthesize
quinones, a class of molecule that is present in most living organisms. Hence it is hypothesised that
the action of the solar wind on dust particles could lead to the production of biologically active
molecules like quinones, which may later undergo physical or chemical changes that could lead to
the evolution of cells and eventually more complex organisms. Since dust grains are so small, they
are decelerated gently enough while entering the Earth’s atmosphere for some of the material
carried by them to survive and fall to Earth. Because of this, research into dust particles and their
constituent molecules is central to the study of theories of the origin of life on Earth, particularly
panspermia.
The study of dust also has a practical application to the field of X-ray astronomy. X-rays cannot be
focussed in the same way as visible light. While the index of refraction for visible light varies
depending on what medium is considered, the index of refraction for X-rays is approximately one for
all materials. This means that the path of the X-rays is not changed significantly when they move
through one material into another, and so the focal length for an X-ray lens would be impractically
large. Parabolic mirrors (which are often used in optical telescopes) cannot be used either, since
when X-rays hit a material at near-normal incidence they are generally absorbed rather than
reflected. However, when X-rays hit a mirror at grazing incidence, they do get reflected, and so X-ray
telescopes use mirrors that lie nearly parallel to the telescope's line-of-sight (Wolter type 1 mirrors
[8]), with a Charge Coupled Device (CCD) detector at the focal point of the system. This configuration
leaves the CCD more exposed to the space environment than the detectors in conventional
telescopes (see figure 2), and so it is vulnerable to dust impacts. A CCD detector on the European
Space Agency’s (ESA) cornerstone x-ray observatory XMM Newton has already been damaged by a
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micrometeoroid impact (figure 3) and a knowledge of the dust population in space is crucial for
spacecraft risk assessments in future X-ray astronomy missions.
Fig. 2: Method of X-ray focussing used for X-ray telescopes. Dust particles may also move down the
mirrors and cause damage to the CCD camera. [9]
Fig. 3: Electron microscope image of damage to CCD on XMM Newton caused by impacting iron
particle (left); change of the dark CCD image after impact (centre); image produced at the scatter
particle impact time (right). [10]
2.3 Dust Flux in Low Earth Orbit
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The Grün interplanetary flux model gives the average flux of micrometeoroids above a certain mass,
m, per m2 per year incident on a randomly tumbling plate at 1 AU from the Sun, ignoring the
presence of the Earth. Mathematically, it is described by the following equations [11]:
F(m) = 3.15576x107 [F1(m) + F2(m) + F3(m)] ,
(1)
where
F1(m) = (2.2x103 m0.306 + 15.0)-4.38
F2(m) = 1.3x10-9 (m + 1011 m2 + 1027 m4)-0.36
F3(m) = 1.3x10-16 (m + 106 m2)-0.85
(1a)
(1b)
(1c)
The shape of the flux model is shown in figure 4.
Fig.4: The Grün Interplanetary Flux Model [11]
While this model has been found to fit closely with the fluxes measured by previously flown dust
detectors, the most sensitive detector ever flown- the dust detector on HEOS 2- was sensitive only
to particles of mass greater than 2x10-16g [1], so the model has not been tested at the less massive
end of the micrometeoroid spectrum. The other factor that affects the reliability of the model is the
presence of the Earth: Earth shielding will reduce the flux seen by a spacecraft (this effect decreases
as the altitude of the spacecraft increases), while at the same time the Earth’s gravitational field will
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attract dust particles, increasing the flux. For a discussion of how these effects may be compensated
for, see [11].
3. Typical Dust Detector Designs
In-situ methods of detecting cosmic dust can be broadly separated into two categories: remote- or
pre-impact- sensing where the dust is detected before it hits the spacecraft, and impact-sensing, in
which the physical effects of the dust impacting the detector are studied.
3.1 Remote sensing
It is possible to observe dust particles using an imaging telescope in space, because individual sunilluminated particles passing the spacecraft will appear in images as streaks of light. In some cases, it
might be preferable to provide an artificial light-curtain to illuminate the particles rather than relying
on the sun [12]. Another remote detection technique involves the use of a thin wire or other
conductor. When a dust particle comes close to the conductor, it induces a charge on it, since the
particle itself is naturally charged. It may be possible to deduce from the induced charge the velocity,
trajectory and orbit of the particle [12].
3.2 Impact sensing
3.2.1 Solid target detectors
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 [13]). When such collisions
take place between a dust particle and a solid target, shock waves propagate outwards from the
collision point through each of the colliding bodies, compressing and heating them. If the shock
waves reach the back edge of the dust particle (the edge that did not collide with the target),
rarefaction waves start to propagate back through the material, and this compression and
rarefaction causes the particle to break up, melt, and evaporate. The vapour produced emits light in
the form of a blackbody spectrum. Other products of the collision include an impact crater in the
target, and ejecta which may include solid and liquid particles, neutral and ionised molecules and
electrons [12].
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The impacting particle and ejecta all transfer momentum to the target and generate a pulse of high
pressure (up to 5 TPa) and temperature (up to 0.5 MK) [12]. The ejecta expand outwards from the
collision point and strike adjacent surfaces where they may produce more craters, light flashes and
ejecta. Any of these effects may be measured in a dust detection system based around particle
impact onto a solid target.
3.2.2 Thin film detectors
When a fast-moving dust particle hits a thin foil, shock waves propagate through both particle and
foil in the same way as in collisions between dust and solid targets. In this case, however, shock
waves may reach the back surface of the foil as well as the edge of the particle, causing the back of
the foil to buckle. If the foil is thin compared to the diameter of the particle (particle diameter / foil
thickness ~ 0.33 [12]) this buckling, coupled with the crater caused by the impact, will result in an
impact hole through the foil.
The physical effects experienced by the particle and target in a solid target detector are all present in
thin film detection systems. A dust particle impact with a foil will result in similar ejecta, a crater or
hole, a light flash and momentum transfer. Secondary ejecta and craters may also be produced.
Ejected material may emerge from both front and back surfaces of the foil.
An example of a dust detector design based on thin foil perforation is the pressurised cell detector,
in which a drop in the cell pressure- caused by a dust particle perforating the cell wall and allowing
gas to escape- triggers the system to register a dust impact. This design was first flown in 1962 on
the Explorer 16 satellite [12]. Similar detectors were later flown on the Pioneer 10 and 11 spacecraft
[12].
3.2.3 Impact Ionisation detectors
High-speed collisions between dust grains and both solid and thin film targets result in the
production of ejecta including ions and electrons. In impact ionisation detectors, these are separated
by an electric field into positive ions and negative ions and electrons, which can be detected, and in
the most sensitive systems even counted individually [12]. The addition of a mass spectrometer
allows different ion species to be separated, and their relative concentrations measured. The total
number of electrons detected is related to the mass and velocity of the impacting dust particle. An
advantage of impact ionisation detectors is that it is easy to distinguish between the signal created
by a particle impact and noise: if the signal is due to a particle, electron and ion signals will be seen
simultaneously.
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3.3 Lobster ISS samples: an accidental dust detector
Between 2002 and 2004, microchannel plates coated with a thin (60 nm) film of aluminium were
exposed to the space environment outside the International Space Station [1]. Similar MCPs were
intended to form the optics of the Lobster X-ray telescope, and the purpose of the ISS exposure was
to test their suitability for this application. The walls of the channels in MCPs focus X-rays in a similar
way to the mirrors shown in figure 2: X-rays entering the telescopes at angles almost perpendicular
to the plates are reflected down the channel walls towards a detector.
The aluminium film on the Lobster samples was intended to act as a thermal control layer for the
system. When the MCPs were returned to Earth, they were examined at the University of Leicester
to determine the effects of the exposure on the absorptivity and emissivity of the film. The
aluminium layers were found to have been perforated by a large number of tiny holes from
impacting particles. The foils were studied using a Field Electron Gun Scanning Electron Microscope
(FEG-SEM), and impact holes as small as 100 nm in diameter were found [14] (see figure 5). The
Carey-McDonnell-Dixon (CMD) formula [15], which relates the diameter of a particle to the diameter
of hole resulting from its impact with a thin foil, predicts that a 100 nm hole through a 60 nm foil
may be created by a 33 nm particle impacting at 5 km s-1, or by a 13 nm particle at 20 km s-1 [14].
Dust detectors measuring the flux of particles penetrating a thin foil have been flown before, but
since none had a supporting base like an MCP, their ability to detect the smallest, lightest particles
was limited by the fact that they had to be thick enough to support themselves. A detector making
use of a supporting MCP base may therefore increase our knowledge of the distribution of the
smallest dust particles in space, allowing models such as the Grün flux model to be tested in the low
mass particle regime.
The impact features found on the Lobster sample plates are believed to have been produced by
nanometeoroids with masses as small as ~10-18 g [1]. This makes the filmed MCPs sensitive to
particles approximately 200 times smaller than those detected by the dust detector HEOS 2, the
most sensitive detector flown to date.
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Fig. 5: one of the smallest holes observed in the Al film [14]
4. Design options for a CubeSat dust detector
The PLUME nanometeoroid detector will be an active version of the passive dust detection system
that was flown on the ISS in the form of the Lobster samples, allowing information about dust
impacts to be transmitted to Earth without the need for retrieval of the detector. The development
of an active detector will also allow future use of the system on unmanned missions anywhere in the
solar system.
4.1 The MCP Base
The two main designs considered for the PLUME dust detector are built around a microchannel plate
structure. The MCP consists of a thin (~1 mm) glass plate, through which run a large number of
parallel channels with diameters of 12.5µm. The channels are arranged hexagonally, as shown in
figure 6, with a distance between the centre of one channel and the centres of the channels
surrounding it of 15µm. This gives an open area fraction for the MCP of 63%.
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Fig. 6: channel spacing in an MCP
4.2 Impact Light Flash Detector
Fig. 7: Design concept for a light flash dust detector
The first design considered works by detecting the light flash produced when a hypervelocity dust
grain hits a thin foil and creates a clear hole by vaporising part of the target foil. At least a fraction of
the photons emitted in the flash moves through the perforated foil and down the MCP channel
towards a detector. This detector is either a CCD (charge coupled device) or CMOS (complementary
metal-oxide semiconductor) imaging chip or a photomultiplier tube (PMT).
4.3 Electron Cascade Design
Fig. 8: Design concept for an electron cascade dust detector
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As an alternative to the light flash detector, a detector that measures the charge released during
particle impact may be flown. If a voltage is applied across an MCP, the plate can be used as a charge
multiplier. Incoming radiation or particles hit the wall of a channel and excite electrons out of the
glass. These electrons are accelerated down the channel by the potential difference. They hit
another wall, exciting more electrons which in turn are accelerated down the channel, exciting yet
more electrons. This electron cascade results in a large charge which is detected at the base of the
MCP. 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 always large enough to cancel out the
electric field gradient down the channel. This means that any event causes the same size signal at
the detector. If used in low gain (lower 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.
In the electron cascade dust detector, a voltage is applied across the MCP and it is the charged
particles in the plasma produced during impact that cause the electron cascade. The charge is
collected on a metal plate at the base of the MCP. The voltage across the MCP should be chosen
such that dust impacts will cause saturation but impacts by cosmic rays or photons will not, allowing
genuine signals to be distinguished from noise.
4.4 Scintillator Design
It has been suggested that instead of an MCP-based dust detector, PLUME could fly a detection
system consisting of a scintillator to convert the energy of the particle into visible light, and an
imaging chip to detect this light. Scintillators are normally used to convert high energy radiation such
as gamma rays into lower energy radiation like visible light. It is not well known how scintillators
respond to particle impact, or how sensitive such a detector would be.
4.5 Chosen Design
The design chosen by members of the PLUME payload team for further development was the light
flash design. It was felt that this was the most elegant design, and the fact that the imaging system
could be bought off the shelf, making development faster and simpler, was seen as a major
advantage. Another advantage it has over the electron cascade design is that the cascade system
relies on a high voltage which requires a lot of power. Power available on a CubeSat is limited to the
amount of power that the solar panels can generate to charge batteries, and this must be shared
between the communications, on-board data handling and attitude determination subsystems, as
well as the dust detector and camera.
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5. Feasibility study for an impact light flash detector
The following section is a feasibility study for a dust detection system in which the intensity of the
light flash produced by the vaporisation of a dust particle impacting foil is measured by a PMT.
5.1 Light flash theory
As stated in the section dealing with typical dust detector designs, hypervelocity collisions between
dust particles and targets cause shock waves to travel through both objects, compressing and
decompressing the materials in turn and resulting in the vaporisation of the particle. The flash seen
when this happens has been studied using dust accelerators. It should be noted that all of the
research summarised in the rest of this section was gathered from experiments involving dust
impacting solid targets: it has not yet been experimentally determined whether or not the flash
looks the same for thin foil targets.
5.1.1 Light Flash studies
Studies of the light flashes emitted during the collision of high velocity dust particles with solid
targets of various different materials have been carried out using dust accelerators [16]. In these
accelerators, dust particles are first electrically charged and then accelerated by a 2MV Van de
Graaff generator towards a target in a vacuum chamber. Using this set-up, the dust particles are
accelerated to a velocity given by
v  (2qV / m)1 / 2 ,
(2)
where V is the accelerating voltage, m is the mass of the particle and q is the charge on it. The light
emitted in the flash is usually detected using photomultiplier tubes.
5.1.2 Signal timing of flash
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The light flash comprises of two parts: the main impact flash and a second signal due to secondary
impacts of ejecta from the projectile and target [17]. The first part of the flash has a rise time shorter
than 100 ns and a length of 10-20 µs. The length of the second part is velocity-dependent, and can
be 200 µs at low velocities. The relationship between the amplitudes of the two peaks is
independent of particle mass but increases with impact velocity, until at ~8 or 9 km s-1 the second
peak is covered by the first peak as it falls [18].
5.1.3 Temperature of flash
In some studies, a number of photomultiplier tubes are used to detect the light flash, each with a
different wavelength filter. The light energy signals from the tubes are plotted as a function of
wavelength. This distribution is integrated over all wavelengths to give the total energy in the flash.
If a blackbody curve is fitted to the plot, the temperature of the light flash can be estimated using
Wien’s law:
T = (2.897 768 × 10–3 m.K) / λpeak.
(3a)
This method gives a plasma temperature estimate in the range of 2500 K to 5000 K for typical light
flashes [19].
5.1.4 Impact flash intensity and energy
The maximum intensity of the flash, I, and the total energy in the flash, E, depend on the velocity, v,
and mass , m, of the impacting particle as I = c1mα1vβ1 and E = c2mα2vβ2. The parameters c1, β1, c2, and
β2 all depend on the projectile-target combination used. β1 has a value between 3.8 and 4.6, with
an average of 4.1, while β2 varies from 3.0 to 3.6 with an average of 3.2. α1 and α2 are both equal to
1[19]. Hence:
I = c1mv4.1,
(4)
E = c2mv3.2.
(5)
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Depending on the projectile and target materials, the light energy in the flash varies from 2 x 10-6 to
10-2 in units of the projectile energy [19].
5.1.5 Intensity as a function of target melting point
It has been shown that the intensity of the first peak in the light flash is higher for targets made from
metals with high melting points (e.g. molybdenum, rhenium, tantalum and tungsten) than for other
targets [19]. These metals also only show one peak, instead of the normal two. A suggested
explanation for this is that the temperature generated in the flash (assumed to be 2500 K) is not hot
enough to liquefy these metals significantly and produce enough ejecta to have a second peak [19].
The energy that would be used for melting other targets is instead used for heating and evaporating
the projectile, generating more hot gas and plasma than usual.
5.2 Photon detection system
When the impact flash detector was initially proposed, it was intended to use a CCD or CMOS
imaging chip under the filmed MCP to detect each flash. CCDs are often used in digital cameras, but
they are quite difficult to use in space as they must be cooled and are highly susceptible to radiation
damage. CMOS chips are much better at coping with exposure to radiation, and if radiation damage
to a pixel does occur it is easy to isolate and ignore the pixel. This is not possible for CCDs. However,
CMOS images are typically much noisier than CCD images, so the ability of the system to detect the
lowest energy particles would be compromised if a CMOS chip rather than a CCD was used.
Since neither CCD nor CMOS chips are particularly well-suited to application as the imaging system
for an impact flash detector, it was suggested that photomultiplier tubes (PMTs) should be
considered instead. Photomultiplier tubes have already been successfully used to study dust impact
flashes with solid targets on Earth. They require high-voltage power supplies, which is not ideal for
CubeSat applications, but are available with built-in power supply units that require a 5V supply.
These PMT units would be well-suited to use in the PLUME detector.
5.3 Photomultiplier Theory
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Fig. 9: Schematic diagram of a photomultiplier tube [20]
A Photomultiplier tube converts light into electrical signals via the photoelectric effect (see figure 9
for an illustration of PMT operation). Light enters the tube through a glass window and hits the
photocathode, causing photoelectrons to be emitted into the vacuum. The photoelectrons are
accelerated and focussed onto the first dynode, depositing energy which causes secondary electrons
to be released from the dynode surface. These electrons go on to excite more electrons from the
next dynode, and so on down the tube. The electrons emitted from the last dynode are collected at
the anode.
The output of a photomultiplier tube is a current pulse, the charge in which is proportional to the
energy absorbed at the photocathode and the gain of the PMT (e.g. the number of secondary
electrons that are excited by each photoelectron). The pulse may be small, so a preamplifier is used
to convert it into a voltage pulse large enough to be analysed properly. The voltage pulse falls away
exponentially in time, but if the time constant of decay is large and the time between signals (dust
impacts) is small, the pulses may pile up, making it difficult to analyse each one individually. For this
reason a shaping amp is used to shorten the pulse. It also smoothes out the peak in order to make
detection of the maximum value possible. Without the shaping amplifier the maximum would be too
sharp to be read out properly. The shaped pulse is then converted into a digital signal.
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Fig. 10: Signal processing for a PMT system
At this point the signal is ready to be analysed to determine the energy that was absorbed at the
photocathode: the energy in the impact light flash. In the case of the PLUME dust detector, data
about the shaped pulse will be written to memory stack by direct memory access (DMA). If it is
determined that the pulse is a valid signal rather than noise (e.g. if the energy in the pulse is similar
to that expected to be contained in the light flash), an interrupt signal will be sent to the satellites
on-board data handling (OBDH) system and the data will be written to non-volatile memory.
5.4 Modelling the expected response of the PMT
5.4.1 Choice of PMT model
The material from which the photocathode is produced determines the spectral response of a PMTits sensitivity to different wavelengths of light. PMTs are available which respond best to UV, visible
or IR radiation. In order to choose a suitable PMT unit to use in the dust detector, the spectrum of
the light flash must be considered. The temperature of the flash is known to be between 2500 and
5000 K. The peak wavelength of the flash can be determined from Wien’s Law,
 peak  (2.897768  103 m  K ) / T ,
(3b)
and is found to be between 579 nm at T=5000 K (towards the yellow part of the spectrum) and 1160
nm at T=2500K (in the near infrared).
Hamamatsu produce a wide range of photomultiplier tubes with cathodes of various materials. The
main photocathode types are: Cs-Te (caesium telluride) or Cs-I (caesium iodide), which are sensitive
to UV rays only; bialkali materials, which are typically sensitive to UV and visible radiation; multialkali
cathodes, which respond well in the visible and near IR regions of the spectrum; and GaAs (gallium
(II) arsenide) cathodes, which cover the UV, visible and near IR regions. For the dust detector, a
multialkali model was chosen, as these are readily available and cover a large proportion of the
spectral range that the light flash’s peak wavelength is likely to fall into. For the purposes of
modelling the dust detector’s response to particle impacts, a specific PMT model was chosen: the
Hamamatsu R7400U-02. This unit has a multialkali cathode and responds to light with wavelengths
between 300 and 880 nm [21].
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5.4.2 Detector spectral response
The typical spectral responses of multialkali PMTs in the Hamamatsu R7400U series are shown in
figure 11.
Fig. 11: Typical multialkali detector spectral response [21]
The spectral response of the detectors is given in terms of the cathode radiant sensitivity. This can
be converted to quantum efficiency using the following equation:
Q.E. 
hc
1240
S   100% 
S  100% ,
e
 
(6)
where h is Planck’s constant, c is the speed of light in a vacuum, e is the electron charge, λ is the
wavelength of incident light in nanometres and Sλ is the radiant sensitivity at that wavelength.
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5.4.3 Discussion of parameters used in model
In order to determine whether a PMT would be a suitable photon detection system for the dust
detector, a spreadsheet model was produced in which the spectral response curve of the
Hamamatsu R7400U-02 PMT was convolved with the expected spectrum of a typical hypervelocity
light flash. This was achieved by treating the light flash as a blackbody. The temperature of a typical
light flash is between 2500 K and 5000 K, so two blackbody curves were plotted based on these two
extreme temperatures. This allowed the minimum and maximum expected PMT response to be
compared.
A blackbody can be described by Planck’s law:
I ( ) 
2h 3
c
1
e
h
kT
 ,
(7)
1
where I(ν)Δν is the energy per unit surface area, per unit time, per unit solid angle emitted in the
frequency range ν to ν + Δν, h is Planck’s constant, c is the speed of light in a vacuum, k is
Boltzmann’s constant and T is the temperature of the blackbody.
The purpose of the spreadsheet was to calculate the number of photons from the flash that would
be detected by the PMT. The calculation was split up so that the number of photons in each of a
large number of frequency bins was counted, giving an idea of the range of wavelengths over which
the strongest signals would be expected from the detector.
The first step in creating the spreadsheet was to take readings from the spectral response curve
(figure 11) at regular wavelength intervals and convert them into quantum efficiencies using
equation (6). It was only possible to take readings every 50 nm along the curve because of the small
size of the graph, so the radiant sensitivity was extrapolated between these readings to provide
estimates for the quantum efficiency every 10 nm. The wavelengths were then converted to
frequencies to allow easier use of Planck’s law.
Next, the blackbody equation was used to calculate I(ν) at the two extreme temperatures for each of
the frequencies considered. The value given was then multiplied by largest expected time signal of
the flash (200 µs) and by a solid angle of 4π. The energy value also had to be multiplied by the
surface area of the flash. The heat flow of the vapour released in a hypervelocity dust impact has
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been modelled before [17]. It was found that the temperature of the vapour is around 3000 K at a
distance of 3r from the impact site, where r is the radius of the dust particle. Hence, the surface area
of the flash was approximated to a sphere with this radius. The radius of the impacting particle was
calculated from a typical dust density, ρ, and mass, m:
2
 3m  3
r  

 4 
(8)
The particle mass used was 10-16 g, since no system previously flown has detected particles this light,
and the main aim of the detector is to expand on current knowledge of the low-mass dust
population in low Earth orbit by bettering previous detector sensitivities.
To get I(ν)Δν, the value of I(ν)calculated above was multiplied by the frequency difference between
the row of the spreadsheet being considered and the next row down; so for the first row of the
spreadsheet- with a wavelength of 280 nm- I(ν) was multiplied by the difference in frequency
between light with wavelengths 280 and 290 nm.
The energy of a photon in each row was calculated using E=hν, where the symbols have their usual
meanings. I(ν)Δν was divided by this to give an estimate of the number of photons in each frequency
bin, over all solid angles. Assuming that the photons in the flash would be evenly distributed over 4π
steradians, half of them could be ignored as they would be moving away from the detector rather
than towards it. However, not all of the photons in the remaining 2π steradians would reach the
detector. Glass is not a very reflective material, so a large proportion of the photons would be
absorbed by it if they hit the MCP channel wall. For the purposes of the spreadsheet, the worst case
scenario was considered; that in which all photons incident on the channel walls would be absorbed.
In this case, only photons in a narrow solid angle would reach the detector. The magnitude of this
solid angle was calculated using
  2 (1  cos  )
(9) [22],
where θ is the angle shown in figure 12. When calculating θ, the microchannel diameter and length
were taken to be 12.5 μm and 1.0 mm respectively.
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Fig. 12: microchannel geometry
The number of photons of a particular wavelength in this solid angle was calculated and multiplied
by the quantum efficiency of the detector to give the total number of photons detected by the PMT
in the corresponding frequency bin. This was the required output of the spreadsheet.
6. Conclusions
The finished spreadsheet is shown in full in appendix 1.
Initially, an error in the spreadsheet meant that the number of photons hitting the PMT was
multiplied by the radiant sensitivity of the detector, rather than the quantum efficiency. This
resulted in the number of photons detected being underestimated by a factor of ~100 and so,
according to the model, the PMT would detect at most 1 or 2 photons in each frequency bin (see
appendix 2). It was suggested that the MCP channel walls might be coated in a reflective material to
increase the number of photons reaching the PMT. However, it is not known whether the film
depositing technology is compatible with MCPs that have had their channels coated in this way.
Because of the apparent inefficiency of the PMT, it was decided that an electron cascade detector
(figure 8) might be a more suitable design choice for the dust detection system. The electronics
chain required to process a signal from an electron cascade detector is the same as that used by an
PMT light flash detector (figure 10), although the detector itself would require a high-voltage power
supply, unlike the PMT detector which would have one built into it. A preamplifier and shaping
amplifier have been set up in the MCP lab at the University of Leicester’s Space Research Centre. A
test pulse generator has been connected to the system, as has an oscilloscope which can be used to
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view the shape of the pulse at various stages in the signal processing chain. The components will
now be tested to find the optimum settings to produce clear voltage pulses (see appendix 4). Once
this has been completed, the detector front and high-voltage power supply may be connected and
the complete setup tested at a dust accelerator.
The error in the spreadsheet has now been noted and corrected, and the results can be seen in
appendices 1 and 3. At T = 2500 K (which corresponds to low energy particles), the PMT is still not
expected to be able to detect photons in any frequency bin. However, higher energy impacts,
producing flash temperatures of 5000 K, are expected to result in tens of photons being detected in
most of the frequency bins. This is enough to result in a charge pulse, and so the light flash detector
should be suitable for the detection of high-energy dust impacts. However, since the energy in the
flash is related to the mass of the impactor, a detector that is only sensitive to high-energy collisions
may limit the success of the PLUME mission in measuring the flux of low-mass particles in the near
Earth environment.
The electron cascade detector is potentially much more sensitive to dust impacts than the PMT
design, because the voltage across the MCP can be tweaked to give the best response to dust
collisions. For this reason, the electron cascade design will be developed further, although since the
electronics required for both designs are the same it may be possible to test both without much
extra work.
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6.1 Further work required
One of the main advantages of using the light flash detector is that it would make it easy to
distinguish between valid dust impact signals and noise. If the electron cascade design is used this
process would be considerably harder. An MCP electron multiplier responds to electromagnetic
radiation such as X-rays, and also charged particles- whether due to dust impacts, solar wind or
cosmic rays. It should be possible to select the voltage across the MCP so that dust impacts produce
a saturated signal while lower energy protons and electrons do not. However, X-rays may have
similar energies to dust particles, making them hard to exclude. An important step in the
development of the electron cascade detector is therefore to research the charged particle (proton,
neutron, alpha and beta particles) and radiation (X-ray and γ-ray) flux expected in low Earth orbit,
and also to investigate further any possible overlaps between the energy of the dust particles and
the energy of background particles and radiation.
Another area that needs further work is the development of the signal processing system for the
detector. An electronics chain for the electron cascade detector has been set up and will be finished
once the settings have been adjusted to give optimum signals. Once this has been done a filmed
MCP and high-voltage power supply can be attached. A vacuum chamber has been sourced for the
detector to sit in while being tested, and the PLUME team have been promised time at the dust
accelerator at the Max-Planck-Institut für Kernphysik in Heidelberg by summer 2008. It is hoped that
it will be possible to test both of the detector designs that are currently being considered.
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7. References
[1] J. D. Carpenter, T. S. Stevenson, G. W. Fraser, J. C. Bridges, A. T. Kearsley, R. J. Chater and S. V.
Hainsworth (2007): Nanometer Hypervelocity Dust Impacts In Low Earth Orbit, J. Geophys. Res.,
112, Issue E8, December 2004
[2] A. Juhász, M. Horányi: Magnetospheric Screening of Cosmic Dust, Journal of Geophysical
Research, Volume 104, Issue A6, June 1999, p. 12577-12584
[3] W.M. Alexander, J.D. Corbin: Interaction of Lunar Ejecta and the Magnetosphere of the Earth,
Solid particles in the solar system; Proceedings of the Symposium, Ottawa, Canada, August 27-30,
1979. Dordrecht, D. Reidel Publishing Co., 1980, p. 425-428.
[4] M. Horányi, H.L.F. Houpis, D.A. Mendis: Charged Dust in the Earth’s Magnetosphere- 1. Physical
and Dynamical Processes, Astrophysics and Space Science, vol. 144, no. 1-2, May 1988, p. 215-229
[5] D.A. Mendis: Entry of Dust Particles into Planetary Magnetospheres, Advances in Space Research,
vol. 4, no. 9, 1984, p. 111-120
[6] L. J. Allamandola, A. G. G. Tielens, and J. R. Barker: Interstellar Polycyclic Aromatic
Hydrocarbons: the Infrared emission bands, the excitation/emission mechanism, and the
astrophysical implications, Astrophysical Journal Supplement Series 71, 733 (1989).
[7] M. Tuleta, L. Gabła, and J. Madej: Bioastrophysical Aspects of Low Energy Ion Irradiation of
Frozen Anthracene Containing Water, Physical Review Letters, vol. 87, no. 7, 2001.
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[8] Goddard Space Flight Centre: X-Ray Imaging Systems,
http://imagine.gsfc.nasa.gov/docs/science/how_l2/xtelescopes_systems.html
[9] European Space Agency - XMM Science Operations Centre: XMM Users' Handbook
http://www.mssl.ucl.ac.uk/www_xmm/ukos/onlines/uhb/XMM_UHB/node14.html
[10] E. Pfeffermann, N. Meidinger , L. Strüder, H. Bräuninger and G. Hartner: Lessons learned from
the EPIC pn-CCD camera for future missions, Mem. S.A.It. Vol. 75, 555 (2004)
[11] SPENVIS: Grün Interplanetary Flux Model
http://www.spenvis.oma.be/spenvis/help/background/metdeb/metdeb.html#METFLUX
[12] E Grün, BAS Gustafson, S Dermott, H Fechtig: Interplanetary dust, Springer, 2001.
(Instrumentation chapter, p.385-444)
[13] H. Fechtig, E. Grün and J. Kissel: Laboratory Simulation (3. High Velocity Impact Processes),
Cosmic Dust, edited by J.A.M McDonnell, p. 622-637, Chichester, Sussex, England and New York,
Wiley-Interscience, 1978.
[14] J. D. Carpenter, PhD thesis, Chapter 6: Detecting Interplanetary Dust and Space Debris with
Filmed MCPs.
[15] W. C. Carey, J. A. M. McDonnell, D. G. Dixon: An empirical penetration equation for thin
metallic films used in capture cell techniques, Properties and interactions of interplanetary dust;
Proceedings of the Eighty-fifth Colloquium, Marseille, France; 9-12 July 1984. Edited by R. H. Giese
and P. Lamy, p. 131-136. 1985
[16] Max-Planck-Institut für Kernphysik: The Heidelberg Dust Accelerator Facility, http://www.mpihd.mpg.de/dustgroup/laboratory/laboratory.html
[17] M.J. Burchell, L. Kay and P.R. Ratcliff: Use of Combined Light Flash and Plasma Measurements
to study Hypervelocity Impact Processes, Adv. Space Res. Vol. 17, No.12, p 141-145, 1996.
[18] G. Eichhorn: Measurement of the Light Flash produced by High Velocity Particle Impact, Planet.
Space. Sci., Vol.23, p.1519-1525, 1975
[19] 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
[20] Hamamatsu: Photomultiplier Tubes, principles to application.
http://sales.hamamatsu.com/assets/applications/ETD/pmt_handbook_complete.pdf
[21] Hamamatsu: Specifications for PMT model R7400U-02
http://sales.hamamatsu.com/en/products/electron-tube-division/detectors/photomultipliertubes/part-r7400u-02.php
[22] Steven L. Jacques, Scott A. Prahl (Oregon Graduate Institute): Radiant Intensity,
http://omlc.ogi.edu/classroom/ece532/class1/intensity.html
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