Transcript ppt

Lessons from Braidwood with
Relevance to Daya Bay
Jonathan Link
Virginia Polytechnic Institute
Workshop on Future PRC-U.S. Cooperation in
High Energy Physics
June 12, 2006
Focus on the Veto System
I’m not suggesting this as a design
for Daya Bay (yet), but it seems
there are relevant questions that can
be answered from the work that was
done.
Concrete Shielding
10 m
7m
1m
The Braidwood muon tagging
system was a compact design,
utilizing heavy concrete (3.5
g/cm3) and multi-layer surface
detectors.
Each detector had its own
tagging and shield, and the
shield was assumed to be a
cube of 100 m2 on a side.
1 meter of heavy concrete has
75% more efficient at slowing
GeV neutrons than 2 meter of
water. MeV neutrons are
mitigated by the hydrogen rich
buffer.
Surface Tracker System Efficiency
The module efficiency is given by:
where is the per plane efficiency
and hits in m of n planes are required for a hit.
Random Firing Rate and Dead Time
The system random firing rate is given by:
where R is the per plane rate (0.05 Hz/cm2 for Belle RPCs)
A is the coincidence area (2.4 m2 for 1.55 m×1.55 m module)
and Δt is the coincidence time window (100 ns)
Rsys=0.87 Hz/module (2 of 3)
The dead time due to tagging
random firing is:
Which results in tolerable dead
times for all configurations, with
reasonable assumptions.
Operating Mode
Dead Time (%)
2 of 3
1.1
2 of 4
2.1
3 of 4
1.7×10-4
3 of 5
4.2×10-4
3 of 6
8.5×10-4
Conclusions on Surface Systems
For a per plane efficiency of > 95%, an operating more of 2
out of 3 results in >99% system efficiency. Belle RPC
efficiency is reported as 95% and the inefficiencies are
largely associated with edges and spacers.
Requiring 2 hits out of 4 layer works down to 90% per plane
efficiency.
Both configurations have tolerable dead times from random
firing assuming the Belle RPC firing rate (which has a
significant contribution from cosmics).
In a multi-layer surface system the per plane efficiencies can
easily be measured allowing you to determine a precise,
position dependant system efficiency.
RPC Module Concept
This is actually a 6 layer
module concept (don’t
ask), but the important
part is the overlap design.
Module Layout
Modules are arrayed
such that they only
abut other modules at
their corners.
Mounting hardware,
gas, high voltage and
signal connectors are
on the open edges.
The full 7×7 array is
completed with a
complimentary second
layer which overlaps
the modules of the
first layer.
Module Layout
Modules are arrayed
such that they only
abut other modules at
their corners.
Mounting hardware,
gas, high voltage and
signal connectors are
on the open edges.
The full 7×7 array is
completed with a
complimentary second
layer which overlaps
the modules of the
first layer.
Surface Module Layout Comments
With an overlapping layout, geometrical inefficiencies can
be reduced to nearly zero.
The Braidwood concept has 36×25 cm2 dead spots in a 100
m2 surface for a geometric inefficiency of <10-5.
If β-decay firing rates are high, layers can be separated by
thin steal plates which could also act as a first line of
defense against neutrons.
The Belle firing rate is beam off, in-situ, so it includes
cosmic and environmental hits.
Possible Design for Daya Bay
If radioactive background from external materials are a major
problem in the central detector then consider this possibility…
Concrete Shielding
1. The outer shield is constructed
of low background structural
concrete (it must be possible to
get concrete that is significantly
cleaner than Daya Bay granite.
2. Inner layer of a dense, very low
background, and preferably
solid (self supporting?)
material.
Dense plastic or some other
low background material
3. Tank steal and buffer volume
also provide gamma
attenuation.
Conclusions
• The Braidwood tagging and shield system design is compact and highly
efficient.
• The formulas for calculating efficiency and dead time depend only on the
hit definition, per plane efficiency, and plane firing rate. These calculation
can be applied directly to the Daya Bay muon tracker.
• Actual efficiencies and dead times have be calculated for a range of
operating modes, which indicate that as few as three layers could have
satisfactory efficiencies and dead time rates.
• A concept for a geometrically efficient array of RPCs was shown. This
concept should be applicable to other detector technologies such as plastic
scintillator.
• A concept for a completely dry Daya Bay veto was shown. This system
likely has advantages in cost, engineering and complexity, but work is
needed to understand radioactive backgrounds.