Geant4 Applications for Modeling Molecular Transport in Complex Vacuum Geometries Page: 3 of 5
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FIG. 2: View of an OpenGL rendering of the "option 3" ge-
ometry used in the LSST camera cryostat Geant4 application.
For clarity, certain structures are rendered in wireframe and
others are rendered invisible.
(L) to the radius (R) of the cylinder, and have been
calculated analytically by Clausing in 1930 and sub-
sequently by Davis using simple Monte Carlo meth-
ods. A Geant4 application for this geometry is imple-
mented and the results for various L/R values compared.
The application has regions specified at the two ends of
the cylinder that terminate a given molecule's trajectory
when they are entered, and the terminating region is
recorded in the output file as described in gII. Table
1 shows the probabilities for a molecule to reach the op-
posite opening for different values of L/R, as calculated
analytically by Clausing, via Monte Carlo methods by
Davis, and by the Geant4 application. The statistical
uncertainty on the probabilities we have obtained can
be estimated from standard theory as a [\n/M]/M,
for a situation where there are n randomized trials with
M possible discrete outcomes. In this case, c=0.005.
The results from the Geant4 application show very good
agreement with the probabilities obtained by the other
IV. LSSTCAM CRYOSTAT APPLICATION
Figure 1 shows a CAD rendering of an LSSTCam cryo-
stat layout. The strict requirements on LSST's photon
throughput necessitate a detailed knowledge of the effect
of potential molecular contaminants that may deposit on
the cold CCD focal plane surface within the cryostat.
One crucial input to any model that addresses this ques-
tion is the transport of molecules within the cryostat. In
particular, how likely is a contaminant molecule from the
bulk of the vacuum space where most of the electronics
and other structures are located to reach the focal plane
region rather than being removed from the system else-
TABLE II: Likelihood of three different outcomes (exiting via
a pump port, encountering a getter pump, or encountering the
front surface of a CCD) occurring first for molecules in the
LSSTCam cryostat application, for several different geome-
tries. The geometries are discussed in IV. Results are shown
both for molecules originating with random positions and mo-
mentum directions from the entire region within the cryostat
behind the focal plane ("All loc."), and for molecules origi-
nating with random positions and momentum directions only
within the envelope containing all of the electronics boards
("Elec. reg."). Values may not total 100% due to rounding.
Geometry Pump Getter CCD Front Surf.
Baseline 12% 32% 56%
Option 2 15% 32% 54%
Option 3 8% 36% 56%
Option 3 + 2xI 23% 29% 47%
Option 3 + 4xI 50% 20% 30%
Geometry Pump Getter CCD Front Surf.
Baseline 13% 31% 57%
Option 2 13% 35% 52%
Option 3 8% 37% 55%
Option 3 + 2xI 24% 24% 52%
Option 3 + 4xI 50% 17% 33%
A brief description of the LSSTCam cryostat geometry
follows: The cryostat body, which forms the vacuum en-
closure, is a truncated conical section with small interior
dimension (ID) 0.95 m and large ID 1.06 m. It is sealed
at the smaller "front" end by a glass lens designated
"L3". Directly behind L3 is the focal plane consisting
of the 3.2 billion 10 pm CCD pixels arranged in 21 raft
tower modules (RTMs). Each RTM contains nine 4kx4k
pixel CCD detectors, and is itself a self-contained cam-
era with the CCD packages, a Silicon Carbide "raft" on
which they are mounted and aligned, conductance barri-
ers, electronics boards, thermal and wall structures, and
connectors and cabling. Four triangular shaped RTMs
carrying guide and wavefront sensors are located at the
corners of the focal plane. The RTMs pass through a Sil-
icon Carbide ceramic honeycomb structure known as the
grid, which kinematically supports the raft. Behind the
grid, the cryoplate is a mechanical structure that carries
the balance of the load of the RTM. The cryoplate con-
tains cryogenic refrigerant channels that provide cooling
to the RTM electronics and other structures. Behind the
cryoplate, a separate cold circuit removes heat from the
rearmost electronics at a higher temperature. There are
also various shrouds, chimneys, and plenums that provide
thermal radiation shields and direct molecular transport.
The rear of the cryostat is an annular feed-through flange
containing hermetic signal and cryogenic feed-throughs,
and an octagonal plate containing ports for turbomolecu-
lar and ion pumps and gauges. In combination, they seal
the rear of the vacuum space. Two large molecular sieve
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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/m1/3/: accessed August 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.