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Muon Drift Tube Gases
Chris Clark
Advisors: Rachel Avramidou, Rob Veenhof
Muon Drift Tubes
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Incoming muons
ionize gas molecules
Electrons produced
in ionizations are
accelerated toward
the anode wire by
the electric field
Electron Avalanche
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Fast electrons can
cause additional
ionizations near wire
Exponential growth
of electrons
Repulsive force of
many electrons near
the wire causes
displacement of
electrons in the wire
GARFIELD
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Simulates passage of
a muon through a
drift tube and
generates the
resulting drift-lines of
electrons and ions
Produces a
distribution of drift
times (the time it
takes for an electron
to reach the wire*)
*Actually it takes about 20 electrons due to the discriminator
Simulation Results
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We recorded the maximum time from a drift
time distribution for electrons that started just
inside the tube wall
We did this many times after tweaking the
temperature, pressure, or CO2 fraction of the
gas and found linear fits for the drift times.
Conclusions
The slopes of the linear fits were farther
from the experimental values than
previous simulations
 We ran all the simulations again with
10x higher statistics and the slopes were
only slightly affected
 Possibly this means that the experiments
are not controlled tightly enough
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Penning Ionization
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Penning Ionization
occurs when an
excited molecule
ionizes another
molecule by collision
In this section I will
explain the ideas
behind a simple
equation that I
developed to predict
the probability of
Penning ionization
Definition
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Only some excited states actually have enough
potential energy to ionize another gas species
We define the probability of Penning ionization
to be the fraction of the energy in the
molecules in these excited states that will
eventually end up causing ionization by the
Penning process
We use the terms ‘energy’ and ‘eventually’ so
that the energy can be transferred to another
molecule before Penning ionization
Sample Gases
We used the gas that will be found in
the ATLAS muon drift tubes:
93% Argon, 7% CO2
 Another gas with good information
already available is the gas in ALICE:
90% Neon, 10% CO2
 For both of these gases there is only
one type of Penning ionization:
Ar*+CO2 and Ne*+CO2
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Energy Destinations
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A good way to understand the problem
is to look at where the energy can end
up in a stable form (excited states will
deexcite)
Conceivable options are:
 Ionization of CO2
 Kinetic Energy
 Escape from the system
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Photon Deexcitation
Natural Radiative Lifetime of Argon in
the D-Level excited state is probably
around 3.7 ns
(given by inverse of Transition
Probability or Einstein A Coefficient)
 The mean free time of an Argon atom in
this gas is about 1.5 ns
 Somewhere around 1/3 of excited Argon
atoms will deexcite before undergoing
any collisions
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Photon Deexcitation
A photon sees roughly the same cross
sections as an excited Argon atom
because the cross sections are primarily
determined by the target
 The photons won’t escape the system
because of their short mean free paths
and because the walls of the tube are
Aluminum, which is very reflective
 Can pretend that photon deexcitation
never happens and it won’t affect the
Penning ionization probability
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Concerns
Kinetic-Assisted Ionization – If a lower
energy excited state had enough kinetic
energy it could still cause ionization, but
this will never happen at our
temperatures
 Associative Ionization – If two excited
molecules collide then their combined
excitation energy can cause an
ionization, but this type of collision
seems less probable
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Collisions of the 2nd Kind
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Inelastic collisions of the second kind are
collisions in which the excitation energy of one
molecule is released and ends up in the kinetic
energy of the other molecule
After such a collision ionization will probably
not happen because it is thermodynamically
unfavored
Together with inelastic collisions of the first
kind, these collisions are responsible for
maintaining the relation specified by the
Boltzmann factor
Summary
The energy from the D-Level excited
states can only end up as Penning
ionization or kinetic energy
 Penning ionization only occurs if an
excited Argon atoms collides with a CO2
 Inelastic Collisions of the second kind
are relegated to occurring upon collision
with another Argon atom
 No energy escapes and these are the
only significant processes
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Penning Ionization Probability
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Here, f is the fraction of CO2, σPenning is the cross
section for Penning ionization, and σInelastic is the cross
section for inelastic collisions of the second kind
σInelastic is approximated by the van der Waals radius of
Argon
σPenning is taken from experimental values, which are
hard to find
Empirical Comparison
*
Equation
Empirical
ATLAS Gas
0.24499**
0.23156
ALICE Gas
0.41719
0.4-0.5***
*This is so theoretical there are no significant figures!
**The Penning cross section for Ne*+CO2 was used here
***This value was produced by Rob Veenhof using a more
sophisticated method that yields a range values
Conclusion
The equation for Penning ionization
probability is not ready to be used as a
tool – it needs further testing, further
consideration of some factors, and more
precise cross section measurements
 However, with the ideas in place,
hopefully the hard part is over
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Acknowledgements
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Funding Sources:
University of
Michigan
National Science
Foundation
Ford Motor Company
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Help Sources:
Rachel Avramidou
Rob Veenhof
Peter Cwetanski
Adrian Fabich
Homer Neal
Jean Krisch
Jeremy Herr