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submitted to Journal of Vacuum Science and Technology A
Geant4 Applications for Modeling Molecular Transport
in Complex Vacuum Geometries
Jack Singal,* J. Brian Langton, and Rafe Schindler
Kavli Institute for Particle Astrophysics and Cosmology
SLAC National Accelerator Laboratory and Stanford University
Menlo Park, CA 94025
(Dated: February 13, 2013)
This letter discusses a novel use of the Geant4 simulation toolkit to model molecular transport in
a vacuum environment, in the molecular flow regime. The Geant4 toolkit was originally developed
by the high energy physics community to simulate the interactions of elementary particles within
complex detector systems. Here its capabilities are utilized to model molecular vacuum transport in
geometries where other techniques are impractical. The techniques are verified with an application
representing a simple vacuum geometry that has been studied previously both analytically and
by basic Monte Carlo simulation. We discuss the use of an application with a very complicated
geometry, that of the Large Synoptic Survey Telescope camera cryostat, to determine probabilities
of transport of contaminant molecules to optical surfaces where control of contamination is crucial.
PACS numbers: 07.30.Kf, 02.70.Uu
Keywords: gas flow dynamics; pumping systems; contamination sources
The cryostat of the Large Synoptic Survey Telescope
camera (LSSTCam)  represents a significant challenge
for vacuum design. With a volume of 2.9 m3, it will
contain nearly 1000 kg of material, and have more than
40 m2 of exposed surface area within. The geometry
of the LSSTCam cryostat presents an environment too
complicated for vacuum transport to be modeled by ana-
lytical calculations, or by techniques such as thermal con-
ductance calculations in conjunction with finite element
analysis. Instead, a fully three dimensional model of
the geometry is required, with a full Monte Carlo treat-
ment of the initial conditions and subsequent propagation
of particles through their interactions with surfaces. Such
calculations have been performed previously for relatively
simpler geometries. The complexity of the LSSTCam
cryostat would render the specification of the geometry
by similar means impossible. Therefore, the Geant4 sim-
ulation toolkit [5, 6] presents itself as an ideal platform
for developing an application to model in fine detail vac-
uum transport in the LSSTCam cryostat and similarly
complex vacuum structures.
Geant4 is a Monte Carlo simulation toolkit used ex-
tensively in particle physics, nuclear physics, and medical
physics applications. It is a flexible, self-contained soft-
ware package that allows the definition of a volume con-
taining real materials arranged into complex geometries
and then the transport of elementary or composite parti-
cles within the volume. Geant4 follows any fundamental
interactions of the particles with the materials, as well as
any products of those interactions. The toolkit consists
of a large number of C++ class bases which the applica-
*Electronic address: firstname.lastname@example.org
t V I!r
FIG. 1: CAD rendering of an LSST camera cryostat design in
cross section (this particular scenario is known as "option 3").
A brief summary of the cryostat is given in 1 and its geometry
is discussed in IV. Shown here is one science raft tower mod-
ule containing CCD detectors, electronics boards and other
structures, the grid (rendered in blue), the cryoplate (pink
and dark orange), the cold shroud (red), the lower shroud (or-
ange), some of the pumping plenum and chimneys (dark green
and white), the feed-thru flange (transparent light green), and
the pump plate (blue). The cryostat body and the L3 lens
are rendered in grey. Some structures are not shown.
tion developer uses in order to create a specific applica-
tion that models a physical system. To develop a Geant4
application, particles, their interactions, a detector ge-
ometry, and relevant cataloging of outcomes (known as
"scoring") must be implemented.
In this letter we describe Geant4 applications that were
developed to model vacuum transport in the molecular
flow regime. The general strategy is summarized in II.
In III an application with a simple geometry to verify
Work supported in part by US Department of Energy contract DE-ACO2-76SF00515.
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Singal, J.; Langton, J. & Schindler, R. Geant4 Applications for Modeling Molecular Transport in Complex Vacuum Geometries, article, February 15, 2013; United States. (digital.library.unt.edu/ark:/67531/metadc842155/m1/1/: accessed January 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.