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Status of Double Chooz
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Nuclear Physics B (Proc. Suppl.) 221 (2011) 236–240 www.elsevier.com/locate/npbps
Status of Double Chooz D. Reyna for the Double Chooz Collaboration Argonne National Laboratory, HEP Division, 9700 S. Cass Ave., Argonne, IL 60439 E-mail: [email protected] Abstract. The Double Chooz experiment will be the next reactor based neutrino oscillation measurement. The collaboration has made significant progress toward the initiation of the experimental construction. Here, we present details of the design, testing and development that have been ongoing, as well as the expected schedule for construction and installation of the experiment. We are currently on our target path to begin data taking in 2008.
1. Introduction The Double Chooz experiment will be the next reactor based neutrino oscillation measurement. The multi-national collaboration includes 26 institutions from France, Germany, Italy, Spain, Russia, the U.K. and the United States. This collaboration will attempt to make almost an order of magnitude improvement on our knowledge of the last unmeasured neutrino mixing parameter θ13 [1] by looking for the disappearance of electron antineutrinos emitted by the cores of nuclear power reactors. To minimize the time and expense required to construct this experiment, the collaboration proposes to perform this measurement at the site of the successfully concluded CHOOZ experiment[2] in the Ardennes region of France. In addition, by using the baseline designs of the CHOOZ experiment, which provides the current best limit on the value for θ13 , and making improvements to those aspects which dominated the systematic errors, we are confident that the desired goals can be achieved. A much more complete description of the experiment can be found in [3]. 2. Experimental Design The experiment will be located on the site of the Chooz nuclear power plant, which is operated by the French company Electricit´e de France (EDF). The source of antineutrinos will be the two N4 class PWR reactors of 4.27 GWth each. The detection technique will be similar to the previous CHOOZ experiment by identifying the inverse beta-decay event signature in monolithic liquid scintillator detectors viewed by 8” photomultiplier tubes. Since the previous limit on θ13 by the CHOOZ experiment was equally limited by statistical and systematic errors, the design of the Double Chooz experiment has evolved in ways that will improve or eliminate the largest sources of the systematic errors, allowing the useful application of increased statistics. The dominant error in CHOOZ was the knowledge of the antineutrino flux and spectrum from the reactor cores. To eliminate this effect, a second detector—identical in construction to the first—will be constructed and installed at a laboratory 280m from the reactor cores. This second (Near) detector will provide an unoscillated reference measurement which can be compared with a similarly constructed (Far) detector to be installed at the original CHOOZ 0920-5632/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2011.09.008
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laboratory, located an average of 1051m from the reactor cores—very near to the oscillation maximum. The near laboratory location has been chosen by working closely with the engineers of EDF to minimize the average distance from the cores, maintain the same ratio of fluxes between the two cores as seen by the far detector, and satisfy the needs of the reactor complex for safety and security. The laboratory will be constructed at the bottom of a 40m shaft and is intended to maintain a minimum overburden of 30m of rock (approximately 80 meters of water equivalent) in all directions. The detector design has also evolved in order to minimize the effects of random singles backgrounds which adversely impacted the CHOOZ experiment. The largest source of these backgrounds in CHOOZ was the radioactivity of the PMT glass itself, emitting gammas directly into the active scintillator. To reduce this, the PMTs for Double Chooz will be installed in a 1.05m thick non-scintillating mineral oil buffer which will completely surround the active scintillating region. In addition, the natural radioactivity of the surrounding rock will be shielded in Double Chooz by the use of 17cm of steel surrounding the entire active detector system. The use of steel, instead of the 1 meter thick low-radioactivity sand shielding used in CHOOZ will reduce the external gamma background by almost two orders of magnitude while simultaneously allowing the central fiducial volume to be increased by a factor of two. The combination of the steel shielding and the mineral oil buffer will dramatically reduce the random singles rate. It is expected that this will allow the elimination of several analysis criteria used for event selection in the CHOOZ analysis which relied on reconstructed vertex positions and had systematic errors on the order of 1–1.5%. Including all modifications, the final detector design to be used for both detectors in Double Chooz (shown in Fig. 1) can be described, from the center outward, as follows:
Figure 1. Design graphic for the complete Double Chooz Detector. This design has a total diameter of ∼7m and a total height of just over 7m.
Central Detector The fiducial “target” volume is contained within an 8mm thick transparent acrylic vessel. The target contains 10.3 m3 of Gd-loaded dodecane+PXE scintillator. The target is surrounded by a layer of unloaded dodecane+PXE scintillator, used to detect gammas from the n-Gd capture events which exit the target region—reducing the loss in detection efficiency near the edge of the target volume. This so-called “gamma catcher” is contained within a second transparent acrylic vessel of thickness 12mm and volume 22.6 m3 . The acrylic vessel is surrounded by the non-scintillating dodecane buffer, mentioned above, which is contained within a stainless steel tank. Installed on the inner wall of the stainless
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steel tank, 534 8” PMTs—providing an active coverage of ∼13%—collect the light from the central scintillating volumes. Inner Veto A 50cm thick cylindrical “veto” region, filled with liquid scintillator, will surround the central detector at the far site. A slightly thicker inner veto—as much as 100cm—will be used at the near site to reduce the effect of the increased muon rate at the shallower depth. This system provides the dual purpose of identifying muons which pass near the central detector and can create spallation neutrons—a correlated background—as well as attenuate and identify any background coming from outside the detector area. Information from the inner veto will be used, offline, to further reduce the effects of correlated and uncorrelated backgrounds that may lie within the event sample. The inner veto is completely surrounded by the steel shielding mentioned above. Outer Veto Placed above the previously described systems, an active external tracking system will be used to further identify “near-miss” muons. By covering an extended region from 2–4m beyond the edge of the inner veto, the rate of unidentified spallation neutrons entering the central detector can be reduced by an additional factor of 5–10. In addition, providing an entry point and/or track direction for muons which cross the central detector is expected to provide useful information for the rejection of correlated events arising from cosmogenic radioactive isotopes such as 9 Li. The projected systematic errors for Double Chooz, compared with those from the original CHOOZ experiment, are shown in Table 1. Table 1. Total systematic error on the normalization between the detectors. CHOOZ Double Chooz ν flux and σ 1.9% 0–0.1% Reactor Induced Reactor Power 0.7% 0–0.1% Two identical detectors Energy per fission 0.6% 0–0.1% Solid Angle 0.3% 0–0.1% Relative Measurement Volume 0.3% 0.2% Identical measurement device Density 0.3% 0–0.1% Accurate temperature control Detector Induced H/C/Gd Ratios 1.2% 0–0.1% Single scintillator batch Spatial Effects 1.0% 0–0.1% Identical target geometry Live Time few% 0.25% Redundant measurements Analysis Event Selections 1.5% 0.2–0.3% No vertex requirements Total 2.7% 0.6%
3. Prototype and Testing In preparation for the construction of the experiment, almost every system has gone through extensive design and development work. Prototype electronics boards have been constructed, PMTs from multiple manufacturers have been compared, and procedures for everything from measuring the mass of the scintillator to demagnetizing the steel shielding have been tested— just to name a few. Two systems, however, deserve special comment: the liquid scintillator development and the design of the double-walled acrylic vessel. Liquid Scintillator Development The performance of the liquid scintillator, particularly the Gd-doped target scintillator, is of critical importance to the success of the experiment. Stable, high quality liquid scintillator has become rather commonplace, however doping these scintillators with certain metals have often been more troublesome. It was the
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optical degradation of the Gd-doped scintillator in the original CHOOZ experiment which ultimately terminated data taking after 1 year. To achieve the Double Chooz goals, a stable scintillator will be required which can provide high performance for at least 3–5 years. With that understanding, groups from MPIK–Heidelberg and LNGS/INR have been working for more than 4 years to understand the previous degradations and to develop new chemical compositions which will satisfy the Double Chooz requirements. A base scintillator composed of PXE and dodecane has been chosen due to it’s high flashpoint and relative compatibility with the acrylic vessels. Two Gd formulations have been developed which show good performance—one based on carboxylic acids and the other on Gd-β-diketonates. Both of these candidates have been extensively tested under multiple environmental conditions and have already demonstrated stability for periods of greater than 400 days. The preferred candidate is currently in transition to industrial production. Samples of 50g and 400g have been produced by an industrial firm and synthesized into ∼80 liters of scintillator. A final complete test batch of 700g (∼150 liters of scintillator) is currently being synthesized completely by the industrial firm as a last test of the procedures. Once successful production at these scales has been established, the final production run of 100 kg (needed for both detectors) will begin. Acrylic Vessel Design The nested acrylic cylinders, which will contain the target and gammacatcher liquids, present challenges from the perspective of both fabrication and longevity. The material must be transparent to wavelengths above 400nm and must resist leakage for more than 10 years. By far the most significant constraint, however, is that the acrylic must demonstrate a level of chemical compatibility with all of the central detector liquids such that over the 5 year lifespan of the experiment, no degradation of either the liquids (scintillation and absorbency) or the acrylics (cracking and crazing) will occur. These requirements put severe limits on the quality of the acrylic welding processes and the levels of residual stresses which can exist within the acrylic structure itself. A significant amount of effort has gone into studying the response of the acrylic under various chemical and environmental conditions. In addition, a detailed finite element analysis has been performed to study not only the static forces on the acrylic structure, but also the stresses that will be born during transport from the factory to the experimental laboratories. As a final test of our understanding of the acrylic, the scintillator and ancillary systems, a 1:5 scaled prototype of the liquid vessels has been constructed. This model contained a nested acrylic structure which was filled with the Gd-doped carboxylate type scintillator in the central volume and the expected unloaded PXE/dodecane scintillator in the outer volume. The acrylic was placed inside a stainless steel vessel which contained non-scintillating mineral oil which was itself contained within another steel vessel filled with scintillator. This prototype assembly enabled testing of all expected cleaning and installation procedures for the components. In addition, a complete liquid handling system was required. Such a test setup—almost an experiment on its own—has been very educational for the collaboration. In addition to the opportunity to practice installation and filling procedures, some weaknesses in the liquid connections and pressure relief systems were seen. 4. Expected Schedule and Results As of this writing, funding from most of the European groups has either already been established or is in the final stages of approval. In addition, the French scientific agencies have committed to doubling their contribution, as needed, in order to insure that the intended experimental schedule is maintained. As such, work by EDF to renovate the infrastructure at the far laboratory has already commenced. It is expected that the collaboration will gain beneficial occupancy in November of 2006. Detailed engineering layouts of the completed laboratory have already been
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performed and procedures for transport and installation of all liquids and major components have been established. Complete installation of the detector will take about one year such that data taking with the far detector alone is expected to begin in 2008. The near laboratory is undergoing a final refinement of the civil design by the engineers of EDF. Construction of the laboratory will begin after completion of the final design and the competitive bidding process—expected by the end of 2007. The completed laboratory is expected to be available to the collaboration by the fall of 2008. After a similar 1 year detector construction program, the collaboration should begin full 2 detector data taking near the end of 2009. The expected sensitivity from Double Chooz is shown in Fig. 2. Notice that running the far detector alone will be sufficient to confirm and even surpass the previous CHOOZ results in only a few months. This is primarily due to the significantly increased statistics from both reactor power and fiducial volume, as well as the improvements to the detector designs.
Figure 2. Expected sensitivity of Double Chooz under the current schedule. The 90% confidence level limit on sin2 (2θ13 ) is shown for an assumed null measurement and Δm2 = 2.5 × 10−3 eV2 known to 20%.
5. Conclusion The Double Chooz collaboration has made significant progress in the design and prototyping of the experiment and it’s necessary components. Construction is already underway at the EDF facility in France. We expect to provide a substantial improvement to the previous limit on θ13 , surpassing the previous bounds within 6 months with the far detector alone. With the expected schedule, Double Chooz should provide a 90% confidence limit on sin2 (2θ13 ) of ∼0.05 in 2009 and between 0.02–0.03 in 2011. References [1] W. M. Yao et al. [Particle Data Group], Chapter 13: “Neutrino Mixing”, J. Phys. G 33, 1 (2006). [2] M. Apollonio et al. [CHOOZ Collaboration], Eur. Phys. J. C 27, 331 (2003). [3] F. Ardellier et al. [Double Chooz Collaboration], arXiv:hep-ex/0606025.
Nuclear Physics B (Proc. Suppl.) 221 (2011) 236–240 www.elsevier.com/locate/npbps
Status of Double Chooz D. Reyna for the Double Chooz Collaboration Argonne National Laboratory, HEP Division, 9700 S. Cass Ave., Argonne, IL 60439 E-mail: [email protected] Abstract. The Double Chooz experiment will be the next reactor based neutrino oscillation measurement. The collaboration has made significant progress toward the initiation of the experimental construction. Here, we present details of the design, testing and development that have been ongoing, as well as the expected schedule for construction and installation of the experiment. We are currently on our target path to begin data taking in 2008.
1. Introduction The Double Chooz experiment will be the next reactor based neutrino oscillation measurement. The multi-national collaboration includes 26 institutions from France, Germany, Italy, Spain, Russia, the U.K. and the United States. This collaboration will attempt to make almost an order of magnitude improvement on our knowledge of the last unmeasured neutrino mixing parameter θ13 [1] by looking for the disappearance of electron antineutrinos emitted by the cores of nuclear power reactors. To minimize the time and expense required to construct this experiment, the collaboration proposes to perform this measurement at the site of the successfully concluded CHOOZ experiment[2] in the Ardennes region of France. In addition, by using the baseline designs of the CHOOZ experiment, which provides the current best limit on the value for θ13 , and making improvements to those aspects which dominated the systematic errors, we are confident that the desired goals can be achieved. A much more complete description of the experiment can be found in [3]. 2. Experimental Design The experiment will be located on the site of the Chooz nuclear power plant, which is operated by the French company Electricit´e de France (EDF). The source of antineutrinos will be the two N4 class PWR reactors of 4.27 GWth each. The detection technique will be similar to the previous CHOOZ experiment by identifying the inverse beta-decay event signature in monolithic liquid scintillator detectors viewed by 8” photomultiplier tubes. Since the previous limit on θ13 by the CHOOZ experiment was equally limited by statistical and systematic errors, the design of the Double Chooz experiment has evolved in ways that will improve or eliminate the largest sources of the systematic errors, allowing the useful application of increased statistics. The dominant error in CHOOZ was the knowledge of the antineutrino flux and spectrum from the reactor cores. To eliminate this effect, a second detector—identical in construction to the first—will be constructed and installed at a laboratory 280m from the reactor cores. This second (Near) detector will provide an unoscillated reference measurement which can be compared with a similarly constructed (Far) detector to be installed at the original CHOOZ 0920-5632/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2011.09.008
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laboratory, located an average of 1051m from the reactor cores—very near to the oscillation maximum. The near laboratory location has been chosen by working closely with the engineers of EDF to minimize the average distance from the cores, maintain the same ratio of fluxes between the two cores as seen by the far detector, and satisfy the needs of the reactor complex for safety and security. The laboratory will be constructed at the bottom of a 40m shaft and is intended to maintain a minimum overburden of 30m of rock (approximately 80 meters of water equivalent) in all directions. The detector design has also evolved in order to minimize the effects of random singles backgrounds which adversely impacted the CHOOZ experiment. The largest source of these backgrounds in CHOOZ was the radioactivity of the PMT glass itself, emitting gammas directly into the active scintillator. To reduce this, the PMTs for Double Chooz will be installed in a 1.05m thick non-scintillating mineral oil buffer which will completely surround the active scintillating region. In addition, the natural radioactivity of the surrounding rock will be shielded in Double Chooz by the use of 17cm of steel surrounding the entire active detector system. The use of steel, instead of the 1 meter thick low-radioactivity sand shielding used in CHOOZ will reduce the external gamma background by almost two orders of magnitude while simultaneously allowing the central fiducial volume to be increased by a factor of two. The combination of the steel shielding and the mineral oil buffer will dramatically reduce the random singles rate. It is expected that this will allow the elimination of several analysis criteria used for event selection in the CHOOZ analysis which relied on reconstructed vertex positions and had systematic errors on the order of 1–1.5%. Including all modifications, the final detector design to be used for both detectors in Double Chooz (shown in Fig. 1) can be described, from the center outward, as follows:
Figure 1. Design graphic for the complete Double Chooz Detector. This design has a total diameter of ∼7m and a total height of just over 7m.
Central Detector The fiducial “target” volume is contained within an 8mm thick transparent acrylic vessel. The target contains 10.3 m3 of Gd-loaded dodecane+PXE scintillator. The target is surrounded by a layer of unloaded dodecane+PXE scintillator, used to detect gammas from the n-Gd capture events which exit the target region—reducing the loss in detection efficiency near the edge of the target volume. This so-called “gamma catcher” is contained within a second transparent acrylic vessel of thickness 12mm and volume 22.6 m3 . The acrylic vessel is surrounded by the non-scintillating dodecane buffer, mentioned above, which is contained within a stainless steel tank. Installed on the inner wall of the stainless
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D. Reyna / Nuclear Physics B (Proc. Suppl.) 221 (2011) 236–240
steel tank, 534 8” PMTs—providing an active coverage of ∼13%—collect the light from the central scintillating volumes. Inner Veto A 50cm thick cylindrical “veto” region, filled with liquid scintillator, will surround the central detector at the far site. A slightly thicker inner veto—as much as 100cm—will be used at the near site to reduce the effect of the increased muon rate at the shallower depth. This system provides the dual purpose of identifying muons which pass near the central detector and can create spallation neutrons—a correlated background—as well as attenuate and identify any background coming from outside the detector area. Information from the inner veto will be used, offline, to further reduce the effects of correlated and uncorrelated backgrounds that may lie within the event sample. The inner veto is completely surrounded by the steel shielding mentioned above. Outer Veto Placed above the previously described systems, an active external tracking system will be used to further identify “near-miss” muons. By covering an extended region from 2–4m beyond the edge of the inner veto, the rate of unidentified spallation neutrons entering the central detector can be reduced by an additional factor of 5–10. In addition, providing an entry point and/or track direction for muons which cross the central detector is expected to provide useful information for the rejection of correlated events arising from cosmogenic radioactive isotopes such as 9 Li. The projected systematic errors for Double Chooz, compared with those from the original CHOOZ experiment, are shown in Table 1. Table 1. Total systematic error on the normalization between the detectors. CHOOZ Double Chooz ν flux and σ 1.9% 0–0.1% Reactor Induced Reactor Power 0.7% 0–0.1% Two identical detectors Energy per fission 0.6% 0–0.1% Solid Angle 0.3% 0–0.1% Relative Measurement Volume 0.3% 0.2% Identical measurement device Density 0.3% 0–0.1% Accurate temperature control Detector Induced H/C/Gd Ratios 1.2% 0–0.1% Single scintillator batch Spatial Effects 1.0% 0–0.1% Identical target geometry Live Time few% 0.25% Redundant measurements Analysis Event Selections 1.5% 0.2–0.3% No vertex requirements Total 2.7% 0.6%
3. Prototype and Testing In preparation for the construction of the experiment, almost every system has gone through extensive design and development work. Prototype electronics boards have been constructed, PMTs from multiple manufacturers have been compared, and procedures for everything from measuring the mass of the scintillator to demagnetizing the steel shielding have been tested— just to name a few. Two systems, however, deserve special comment: the liquid scintillator development and the design of the double-walled acrylic vessel. Liquid Scintillator Development The performance of the liquid scintillator, particularly the Gd-doped target scintillator, is of critical importance to the success of the experiment. Stable, high quality liquid scintillator has become rather commonplace, however doping these scintillators with certain metals have often been more troublesome. It was the
D. Reyna / Nuclear Physics B (Proc. Suppl.) 221 (2011) 236–240
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optical degradation of the Gd-doped scintillator in the original CHOOZ experiment which ultimately terminated data taking after 1 year. To achieve the Double Chooz goals, a stable scintillator will be required which can provide high performance for at least 3–5 years. With that understanding, groups from MPIK–Heidelberg and LNGS/INR have been working for more than 4 years to understand the previous degradations and to develop new chemical compositions which will satisfy the Double Chooz requirements. A base scintillator composed of PXE and dodecane has been chosen due to it’s high flashpoint and relative compatibility with the acrylic vessels. Two Gd formulations have been developed which show good performance—one based on carboxylic acids and the other on Gd-β-diketonates. Both of these candidates have been extensively tested under multiple environmental conditions and have already demonstrated stability for periods of greater than 400 days. The preferred candidate is currently in transition to industrial production. Samples of 50g and 400g have been produced by an industrial firm and synthesized into ∼80 liters of scintillator. A final complete test batch of 700g (∼150 liters of scintillator) is currently being synthesized completely by the industrial firm as a last test of the procedures. Once successful production at these scales has been established, the final production run of 100 kg (needed for both detectors) will begin. Acrylic Vessel Design The nested acrylic cylinders, which will contain the target and gammacatcher liquids, present challenges from the perspective of both fabrication and longevity. The material must be transparent to wavelengths above 400nm and must resist leakage for more than 10 years. By far the most significant constraint, however, is that the acrylic must demonstrate a level of chemical compatibility with all of the central detector liquids such that over the 5 year lifespan of the experiment, no degradation of either the liquids (scintillation and absorbency) or the acrylics (cracking and crazing) will occur. These requirements put severe limits on the quality of the acrylic welding processes and the levels of residual stresses which can exist within the acrylic structure itself. A significant amount of effort has gone into studying the response of the acrylic under various chemical and environmental conditions. In addition, a detailed finite element analysis has been performed to study not only the static forces on the acrylic structure, but also the stresses that will be born during transport from the factory to the experimental laboratories. As a final test of our understanding of the acrylic, the scintillator and ancillary systems, a 1:5 scaled prototype of the liquid vessels has been constructed. This model contained a nested acrylic structure which was filled with the Gd-doped carboxylate type scintillator in the central volume and the expected unloaded PXE/dodecane scintillator in the outer volume. The acrylic was placed inside a stainless steel vessel which contained non-scintillating mineral oil which was itself contained within another steel vessel filled with scintillator. This prototype assembly enabled testing of all expected cleaning and installation procedures for the components. In addition, a complete liquid handling system was required. Such a test setup—almost an experiment on its own—has been very educational for the collaboration. In addition to the opportunity to practice installation and filling procedures, some weaknesses in the liquid connections and pressure relief systems were seen. 4. Expected Schedule and Results As of this writing, funding from most of the European groups has either already been established or is in the final stages of approval. In addition, the French scientific agencies have committed to doubling their contribution, as needed, in order to insure that the intended experimental schedule is maintained. As such, work by EDF to renovate the infrastructure at the far laboratory has already commenced. It is expected that the collaboration will gain beneficial occupancy in November of 2006. Detailed engineering layouts of the completed laboratory have already been
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performed and procedures for transport and installation of all liquids and major components have been established. Complete installation of the detector will take about one year such that data taking with the far detector alone is expected to begin in 2008. The near laboratory is undergoing a final refinement of the civil design by the engineers of EDF. Construction of the laboratory will begin after completion of the final design and the competitive bidding process—expected by the end of 2007. The completed laboratory is expected to be available to the collaboration by the fall of 2008. After a similar 1 year detector construction program, the collaboration should begin full 2 detector data taking near the end of 2009. The expected sensitivity from Double Chooz is shown in Fig. 2. Notice that running the far detector alone will be sufficient to confirm and even surpass the previous CHOOZ results in only a few months. This is primarily due to the significantly increased statistics from both reactor power and fiducial volume, as well as the improvements to the detector designs.
Figure 2. Expected sensitivity of Double Chooz under the current schedule. The 90% confidence level limit on sin2 (2θ13 ) is shown for an assumed null measurement and Δm2 = 2.5 × 10−3 eV2 known to 20%.
5. Conclusion The Double Chooz collaboration has made significant progress in the design and prototyping of the experiment and it’s necessary components. Construction is already underway at the EDF facility in France. We expect to provide a substantial improvement to the previous limit on θ13 , surpassing the previous bounds within 6 months with the far detector alone. With the expected schedule, Double Chooz should provide a 90% confidence limit on sin2 (2θ13 ) of ∼0.05 in 2009 and between 0.02–0.03 in 2011. References [1] W. M. Yao et al. [Particle Data Group], Chapter 13: “Neutrino Mixing”, J. Phys. G 33, 1 (2006). [2] M. Apollonio et al. [CHOOZ Collaboration], Eur. Phys. J. C 27, 331 (2003). [3] F. Ardellier et al. [Double Chooz Collaboration], arXiv:hep-ex/0606025.