From: Joseph Formaggio (josephf@MIT.EDU)
Date: Fri Jan 20 2006 - 22:17:48 CST
A preliminary study of the minimum acceptable contamination levels
(at least, within an order of magnitude) has been looked at in a bit
of detail. See the enclosed report for more information.
The study concentrated mainly on 13C activation background, which
could be serious for Braidwood if not kept in check. The study did
not look into other background issues, such as accidentals or
backgrounds for the weak mixing angle.
These levels are not achieved for free and so some care is necessary.
On Jan 20, 2006, at 3:34 PM, Zelimir Djurcic wrote:
> 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
> 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
> 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
> 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
> 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.
> 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
Massachusetts Institute of Technology
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