From: Zelimir Djurcic (zdjurcic@nevis.columbia.edu)
Date: Fri Jan 20 2006 - 14:34:59 CST
Dear Braidwood,
I wanted to remind you about an important issue. This is probably not
crucial for the current EOI, but is something that we need to think about
when building the experiment.
Material selection is an important aspect of a high-sensitivity experiment
such as the Braidwood.
The contribution of radioctivity to the detector background should be kept
to a minimum in order to limit the uncorrelated backgrounds.
This is important to achieve a high sensitivity of theta 13 measurement
and essential for other physics goals (i.e. weak mixing angle etc.).
The design and construction of the experiment does not permit a phased
approach to obtaining these goals. There do not exist relistic
decontamination methods to reduce radioactive backgrounds from the
detector components once the experiment is constructed.
The most-sensitive materials include the acrylic vessel, the vessel supporting
structure, access chimney, calibration devices, scintillator piping and
sampling devices, and liquid scintillator.
The materials used in the buffer region have the next stictest requirements
including the buffer oil, PMTs, PMT cables, and stainless steel vessel.
Vero system would have the loosest radio-purity requirements (if any).
Additional concern is the emanation of Rn (noble gas with a 3.8 days half-life
in 238U chain). The release of this gas by detector components
in different regions can result in higher backgrounds throughout the
detector as the radon mixes in the oil and LS.
Therefore, in addition to our concern for the intrinsic radioactive
contamination of the detector components, the requirements for
surface contamination are equally rigorous.
Aside is an environmental Rn concern: We would like to prevent
Rn collection in a clean detector.
(KamLAND, for example, had very rigorous material selection, but the Rn was out
of control when the detector was filled).
There is a lot of experience in the material selection from KamLAND (actually,
I write this text using a part of KamLAND proposal and experience), SNO, EXO
and other experiments.
In order to find out allowed concentrations of U,Th,K (some ppt levels, I
guess) in the detector components (for the different physics goals) in the
Braidwood, we can use RAT simulation (probably summer student job).
We will need an access of various facilities to select the detector materials
for desired purity (the Braidwood Collaboration may already have some, or
we may need to build them).
The liquid scintillator and all materials in (close by) the inner detector
might need to be surveyed to extremly tight activity limits.
Neutron activation analysis and mass specroscopy were used in KamLAND.
Detection limits of <2.4x10^-15 g(40K)/g LS, 5.5x10^-15 g Th/g LS, and
<8x10^-15 g U/g LS have been achieved with NAA.
Chemistry lab and a clean root were used for a sample preparation.
High-purity germanium detectors were available to perform the analysis of the
irradiated samples.
Other construction materials and detector components can be monitored
using low-background counting facilities: well-shielded high-purity Ge
detectors.
Material selection and detector monitoring is an important part of R&D program
to consider and work on.
We will need to discuss this issuee at some point.
Cheers,
Zelimir
Zelimir Djurcic
Columbia Group
Fermi National Accelerator Laborartory P.O. Box 500, Mail Stop 309
Batavia, IL 60510
Phone: 630-840-2092
Fax: 630-840-3867
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