To control for unwanted particle signals,
the TPC is surrounded by a tank of gado-linium-doped scintillator fluid, and housed
in another 8 x 6 meter-high water shield
containing 70,000 gallons of purified water. This will further reduce external background signals to the detector, particularly
neutrons and gammas.
A three-year run of the experiment will
achieve a sensitivity close to the fundamental limits from the cosmic neutrino
“When completed, the LZ experiment
will be the world’s most sensitive experiment for WIMPs over a large range of
WIMP masses,” said Harry Nelson, physicist at the University of California, Santa
To get the LZ assembled, moved underground and working, LEO A DALY is
designing two highly specialized controlled
environments—one above ground for the
unit’s assembly, and one below ground in
the Davis Campus.
Cleanrooms are necessary in astrophysical research because the detectors are so
sensitive. Even a tiny amount of dust, airborne microbes, aerosol particles or chemical vapors may alter the results or damage
the equipment. A typical cleanroom uses
special air handlers equipped with HEPA
filters of a certain tolerance, and change
the air a certain number of times per hour,
depending on the needs of the environment.
The cleanrooms used for the LZ experiment
are rated as class 1,000 cleanrooms, meaning there should not be more than 1,000
particles greater than . 5 microns in any
given cubic meter of air. The air handler is
calibrated to change the room’s air between
150 and 480 times per hour. For comparison, a house has an air exchange rate of . 5
to 2 air exchanges per hour.
What is unusual about the LZ’s cleanroom is that its design also keeps out
radon, a radioactive noble gas that naturally leeches out of the ground. The sensitive
nature of the LZ experiment requires an
environment free of radon, and required
the design team to create an extremely rare
radon-elimination system. Working with
a specialized cleanroom designer, the team
devised a two-step approach to keeping
the radon out: 1) all air that is pumped
into the cleanroom goes through a radon
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filtration system before entering the room,
and 2) the cleanroom’s walls are lined with
metal sheeting to reduce radon coming
from the walls.
Once the cryostat has been assembled
above-ground, the challenge is to coordi-
nate a perfectly timed method of delivering
construction materials, and the detector,
piece by piece, so they can be assembled in
the limited space available.
The Davis Campus is accessed via the
4,850 level. The 14,000-pound “cage”—a