1. Third wall/skin concept
GERDA Collaboration Meeting
Tubingen, November
A. Bettini. Padova
18 January, 2019
A. Bettini. INFN

2. Reason for the 3rd wall/skin
A cryostat in contact with a large heat reservoir is considered to be dangerous in principle by LNGS
Dangers to be avoided
LN2 & H2O (explosive) mixing in case of major cryostat failure
leak of the cryostat inner skin  LN2 gets heat from water heat source
leak from cryostat outer skin  puts heat source in thermal contact with LN2
The 3rd wall/skin must avoid direct thermal contact of water with cryostat
It can be mechanically coupled or uncoupled from the cryostat
The design of the third wall/skin must
not introduce additional background (more generally not compromise the physics)
assume the existing design of both the cryostat and the WT as given
be as simple as possible (money, time,…)
18 January, 2019
A. Bettini. INFN

3. Twice 2 possibilities Mechanically
independent from the cryostat both for static forces and dynamic
independent from the cryostat for static forces only
Material
Copper
Lexan
A polycarbonate with good cryogenic properties (H. Leutz built the LeBC H2 bubble chamber at CERN out of it), but difficult to find engineering and industrial experience and possible problems with soldering at low temperatures
18 January, 2019
A. Bettini. INFN

4. Before the 3rd wall/skin
The cryostat is designed to resist the inertial forces due to the water mass acceleration due to a quake
These forces are uniformly distributed on the surface
18 January, 2019
A. Bettini. INFN

5. 3rd wall/skin - Full mechanical independence
Third wall must resist the inertial forces due to the water mass acceleration due to a quake
Wall must be thick enough
We lose part of the water screening
“No” interference with cryostat
18 January, 2019
A. Bettini. INFN

6. 3rd wall/skin - Static independent
The skin is as thin and as close to the cryo as possible; transmit the inertial forces to cryostat with as many rings as needed to make them smooth enough
The structures are still independent (statically)
Thinner wall, but interference with cryostat must be shown to be tolerable
Minimum water screening lost
Must be careful on the soldering
Cheaper?
18 January, 2019
A. Bettini. INFN