Deep Underground Cryostat for a LArTPC

Carl Bromberg

Michigan State University

September 12, 2006

I. Deep underground physics experiments

A number Particle Physics and Astrophysics experiments can take
advantage of a facility like the Deep Underground Science and
Engineering Laboratory (DUSEL) currently in the site selection stage^1^.
In these experiments only neutrinos and very high energy muons can
generate background events that lessen their sensitivity. One class of
experiment searches for proton decay or cosmic neutrinos sources and,
with an appropriately designed and aimed neutrino beam, investigates
CP-violation in neutrino oscillations.

II\. Liquid Argon Time Projection Chamber

A Liquid Argon Time Projection Chamber (LArTPC) is capable of
extraordinary precision in neutrino and proton decay studies. The ICARUS
collaboration has pioneered this technology by building a series of
detectors increasing in size and performance culminating in the T600
detector (600 tons of liquid Argon at 87 Kelvin) being installed in
Hall-B of the Gran Sasso Laboratory^2^. The halls at the Gran Sasso have
the approximate shape of a horizontal cylinder 10 m in radius and 100 m
long. The T600, two 4 m x 4 m, and 20 m long cryostats constructed from
LN2 cooled aluminum panels and the associated cyrogenic systems easily
fit into this hall. However, to address CP-violation in neutrino
oscillations single detectors employing a mass of least 30 kilotons will
be required. If the mixing angle conspires against easy observation of
electron neutrino appearance, detectors many times this size may be
required. Scaling the construction methods of the T600 detector to a
such a massive detector unfortunately also leads to massive costs.
![](media/image1.pct){width="2.6590277777777778in"
height="2.8270833333333334in"}

To address this difficulty it has been proposed that the cryostat of a
LArTPC be built in a upright cylindrical tank similar to those used to
store Liquid Natural Gas (LNG) at a temperature of 110 Kelvin. These
tanks are constructed with an inner vessel of 9% nickel (or stainless)
steel with an open top that is loosely closed with a thermally
insulating plug. An outer tank of carbon steel protects the inner tank
from the elements, and creates a gas tight volume. A layer of insulation
usually Perlite (expanded volcanic glass) fills most of the space
between the walls of the two tanks. Key to the design is the self
supporting nature of the thick walls of the inner tank. The insulation
is not attached to the walls to allow the inner tank to shrink as it
cools. Therefore, the insulation cannot not provide any mechanical
strength for the inner tank. The largest LNG tanks are cylinders 40
meters high with an 80 meter diameter. If an appropriate tank this large
were filled with Argon it would hold over 200 kTon of liquid.

A recent R&D proposal^3^ lays out the plans to build a series of these
tanks and detectors culminating in the 15 kTon detector, as shown in
Figure 1, with an inner tank 19 m high, 26 m in diameter. The tank
design for an LArTPC modifies the standard LNG tank to maintain
ultra-pure Argon in a completely sealed inner tank with wire planes
strung between its roof and floor. Signal towers that start just above
the liquid level allow cables to pass through the insulation to a gas
tight feed-through.

The height of a LArTPC tank may be limited because capacitance (and
therefore the electronic noise) of the readout increases linearly with
the wire length. Thus a short and squat cylinder is preferable to one
that is tall and narrow. Underground, this implies excavation of a
cavern with large spans, which is the type of cavern mine engineers try
to avoid. As discussed in the Homestake reference design study^4^ mine
engineers prefer a horizontal cylinder, perhaps modified into the "rural
mailbox" shape, or a tall and narrow vertical cylinder. Fortunately, a
new class cryogenic tank has been developed in the last few years which
has the shape flexibility to conform to cavern shapes preferred by mine
engineers.

III\. LNG tanker design

Early generation of ships carried compressed natural gas (CNG) in a
large number of high pressure gas (250 bar) cylinders. The gas was
subsequently liquefied for storage on land. Later, ships were developed
to carry LNG in low pressure spherical tanks of nickel-steel or
aluminum. A typical LNG ship of this type carries 4 spheres each with a
volume of \~35,000 m^3^ . Only recently have LNG tankers been built to
carry the liquid in tanks that conform to the shape of the hull.

The breakthrough in design^5^ is made possible by the use of Invar, a
high (36%) nickel content steel that remains malleable at cryogenic
temperatures and has a nearly zero coefficient of expansion. The
insulation can now be glued between the inner Invar tank(s) and the
outer mild steel hull structure, and can provide the mechanical strength
to support the weight and side pressure of the liquid. Invar, however,
is very expensive and therefore must be as thin as possible. The basic
structure, as shown in Figure 2, uses two "membranes" of 0.7 mm Invar
separated from each other and from the vessel's hull by plywood boxes
filled with perlite insulation,. The two layers of Invar and two layers
of insulation have a total thickness of only 0.53 m, that limits the
"boil-off" to 0.15% per day, sufficient for the 10-15 day sail from the
gas well to a port. For long term storage, thicker insulation would
reduce the refrigeration needs to achieve a much lower boil-off
rate.![](media/image2.pct){width="3.654861111111111in"
height="2.147222222222222in"}

A number of ships have been built with this initial design^5,6^. The
last stage in the construction is welding of the inner Invar membrane as
shown in Figure 3. Once each sealed compartment is completed, the welds
are inspected and leak tested by pressurizing the insulation layer with
20% helium and scanning a helium leak detector over all the welds.
![](media/image3.pct){width="3.7666666666666666in"
height="2.5131944444444443in"}![](media/image4.jpeg){width="1.9020833333333333in"
height="2.567361111111111in"}

Two companies hold patents on similar membrane designs, Gaz Transport
and Technigaz. With their merger into a single company, GTT, a new
design has evolved, patent name CS1 as shown in Figure 4, that combines
the best properties of each company's approach. The inner membrane of
Invar is retained (but now corrugated in the corners), while Triplex, a
material composed of fiberglass and aluminum that is more flexible than
Invar is used for the second membrane, while reinforced polyurethane
insulation replaces the Perlite. The original and the CS1 designs have
undergone extensive finite element analysis and these have been compared
to experience with existing ships. It is expected that this new design
can be certified against material fatigue failure for 40 years, of
operation in the most adverse working conditions, such as those to be
found in the North Atlantic.
![](media/image5.pct){width="3.1381944444444443in" height="2.81875in"}

IV\. LArTPC Membrane Style Cryostat

The ES&H requirements for a land based LNG tank, or a LNG tanker are
harder to satisfy than for the same vessels carrying liquid Argon. A
ship carrying LNG can safely sail rough seas, and then unload its cargo
in a port near populated areas. I will assume, therefore, that a LArTPC
tank built with the same principles to carry a similar amount of liquid
(now Argon) can be engineered for safe operation in a deep underground
location.

The design of a ship incorporates aspects that would be unnecessary for
a land based system. The double hull is designed to fill with ballast
water as the cargo is unloaded, and to protect the inner hull in the
event of a grounding or a minor collision. Also, the cargo is broken up
into 4 or 5 separate compartments because the ship must be balanced at
all times, e.g., during loading and unloading of cargo. There doesn't
seem to be any safety reasons for the compartments, except that during a
return voyage the ship could remain afloat if one of the empty
compartments filled with water. One clear difference is that Argon is
nearly three times as heavy as natural gas so that stronger versions of
the insulation packaging will have to be developed. An engineering study
of the membrane approach must be done to determine how it can be safely
simplified and strengthened to carry liquid Argon.

Another important engineering issue is the role of the cavern walls in
supporting the cryostat. Just placing a full ship design into a cavern
results in a cavern with a minimum diameter of 42 m, and an Argon cross
section of about 800 m^2^, as shown on the left in Figure 5. The liquid
level (dashed line) could not be this high unless the preamplifiers can
be located in the liquid, an option that is under investigation. A
conservative design would require that the electronics be located
outside of the cryostat and the liquid level end at the top of the
vertical walls. This lowers the cross sectional area to about 500 m^2^.
Shown on the right in Figure 5 is a detector using the cavern walls for
support that results in diameter of 36 m, and with it a significant
savings in costs for steel and excavation. The figure shows the top of
the cryostat supported from the cavern roof, however, with cold preamps
the entire octagonal tank could fit into this cavern. Thicker insulation
might be needed to keep the rock warm, or a naturally available water
flow could warm the outside resulting in an increased refrigeration
load. ![](media/image6.pct){width="5.759027777777778in"
height="2.5083333333333333in"}

The costs of this detector can be estimated. The workers will have to
trained and must work underground. However, the containers on ships are
built from the inside so that their workers also must work in a cave.
Steel and nickel prices are rising, as is the cost of energy. A ship 300
m in length that can carry 70 kton of LNG cost \< \$200 M in 2004. The
price must be closer to \$250 M today. However, we do not have to build
an entire ship, just the inner hull and the insulated Invar tanks. My
estimate for the cost of a tank for 100 kton of liquid Argon (140 m
long, 70,000 m^3^) will cost about 150 M\$, not including the excavation
costs, which are better estimated by the mining people.

V. Early prototype LArTPC for the 300 ft level.

In the Early Implementation Program (EIP) at the Homestake site^7^ there
is a proposal to develop a shallow 300 ft level (300L) and a deep 4850
ft level (4850L). The underground laboratories that have been discussed
for the EIP are 20 m wide, 15 m high, and 50 m long. To estimate the
size of a prototype membrane style cryostat that can fit in this space,
I'll assume that the walls can be used for mechanical support and that
an outer steel shell can be mounted on the wall. Taking the insulation
thickness to be 1 m (double that used on ships) on either side, the
Argon volume would be 18 m by 10 m by 50 m long (9,000 m^3^) or an Argon
mass of 12.6 kTon. In an EIP cavern, a LNG tank of the cylinder type
discussed earlier would hold a maximum of 3.5 kT.

VI\. Conclusion

It does appear that an \~ 12 kTon prototype LArTPC could be built with
Invar membrane technology in an EIP cavern. Realistic engineering will
undoubtedly reduce the capacity of this detector. On the other hand, the
cavern size of the EIP is very conservative. Studies indicate that
caverns of the size necessary to hold 100 kTon of liquid Argon in a deep
underground location are possible.

Further studies are needed understand the engineering consequences of
the membrane technology. This paper discussed only the tank design. It
did not investigate the feasibility of signal towers or how to implement
the wire chamber supports, though a number of solutions immediately come
to mind,

VII\. References

1\. "DUSEL P5 presentation v3", Bernard Sadoulet 4/19/06;

> http://www.dusel.org/DUSELatMeetings/DUSEL_P5_Apr06BS.pdf

2\. "Design, construction and tests of the ICARUS T600 detector", NIM A
527 (2004) 329-410.

3\. "A Large Liquid Argon Time Projection Chamber for Long-baseline,
Off-Axis Neutrino Oscillation Physics with the NuMI Beam", D. Finley et
al., Fermilab Note FN-0776-E.

4\. "The National Underground Science and Engineering Laboratory at
Homestake: Project Book, Reference Design Stage", Homestake
Collaboration, nucl-ex/0308015, p 47-48

5\. "Advanced Construction Processes and Quality Improvement toward the
Next Generation of L.N.G. Membrane Vessels", F. M. Garcia and V. M.
Caridad, University of Madrid, SNAME World Maritime Technology
Conference, San Francisco 2003, paper D2(D119);

> http://www.skibstekniskselskab.dk/download/WMTC/D2(D119).pdf

6\. "Building of Advanced Large Sized Membrane Type LNG Carrier", J.
Ishimaru, et al., Mitsubishi Heavy Industries, Ltd., Technical Review
Vol. 41 No. 6 (Dec. 2004); "Key Technologies of Mitsubishi LNG
Carriers - Present and Future", ibid., Vol. 38 No. 2 (Jan. 2004)

7\. Homestake Interim Laboratory & Homestake DUSEL, K. Lesko, talk at
the XXII International Conference on Neutrino Physics and Astrophysics,
Santa Fe, NM, June 13-19, 2006;

> neutrinosantafe06.com/downloads/Session8/8.4Lesko.pdf