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Yifang Wang: Daya Bay II

March 13, 2013

The idea of the daya Bay-2 experiment is to deploy a large acrylic tank of 34.5 meter diameter. A liquid scintillator detector, increasing the volume 20-fold, and increase the light yield by a factor of five for increased resolution

The sensitivity to the mass hierarchy of such a large detector, thanks to a large value of theta_13, can be a 3-sigma measurement in six years, with large probability. This is based on collecting several tens of thousand events.

The value of dm^2_23 value measured by T2K and NOVA to 1.5% precision can be used in the future to reach a significance of 4.5 standard deviations. With 1% precision the significance can exceed 5 standard deviations.

The optimization is studied, the baseline is such that there is a maximum of the oscillation in theta_12. Multiple reactors might cancel the oscillation structure, so the baseline difference should be controlled to be less than 500 meters. One can thus probe the unitarity of the mixing matrix parameters to 1% level. These determinations have the potential to exceed those of the CKM matrix elements.

For supernova neutrinos a the detector can detect a significant signal. One can distinguish the kinds of events, understanding more of the SN flux. For geoneutrinos, the systematics complicate the matter because of the huge reactor spectrum.

This is a challenging experiment, because 20ktons of liquid scintillator implies large dimensions, and a required energy resolution of 3%/sqrt(E) implies to get 1200 photoelectrons per MeV.  This may be achieved by highly transparent liquid, with high light yield, and high photocathode coverage, and high quantum-efficiency phototubes. 3% is achievable.

Prototypes of the new kind of phototube that is envisioned are being constructed in Beijing. By the end of the year they hope to get PMTs which fulfil the demanding specifications. For the light yield in the scintillator, one can think of a lower temperature, a fluor concentration optimization, and the use of a wavelength shifter. To increase the attenuation length they must improve the raw materials, and improve the production process and the purification cycles.

The question being often asked is whether after reconstruction the 3% value of resolution can be really achieved. This appears to be achievable if we believe simulations.

The signal being looked for is that of inverse beta decay. The event rate expected is of 40 events per day. They decided to use no Gadolinium dopant to not affect the attenuation length, and to make sure that the irreducible background from the scintillator is lowered. Of course the price to pay is the capture time, which is longer, and the lower energy of the capture.

Calculation of backgrounds assumes an overburden of 700 m, and single rates from LS and PMT are of 5 Hz.

Building such a large tank either with steel or acrylic is very hard. One may go with just an acrylic tank, or a steel tank with an acrylic box or wall. They are looking at the various options for the tank construction. Without a steel tank one can build a honeycomb support structure. However the different density of the water and mineral scintillator creates a higher pressure to the tank. This boils down to the pressure of a normal aquarium, which is manageable.

A 12-cm thick acrylic tank but in reality one must look at other examples. SNO, for a 1-kton tank, took two years to a 10-person team, so it is a tough call to know how feasible this is. A variation is a acrylic wall. A combed structure with tiles of acrylic can be envisioned. This is easier to assemble.

Another option is a box of acrylic, but this also involves technical problems connected with the pressure differences.

A balloon is cheaper, and quicker to install. Experience here comes from Borexino (0.5 kton) and KamLAND (1 kton).

Finally, one can think of building just a steel tank. The PMT protection is a concern though. Also, resolution may be affected by pile-up of background events. If the PMT glassi s the same as in Daya Bay, the radioactivity will cause MHz of background. Online background suppression can be provided by divide PMTs in 1476 regions, and look at the charge ration, can bring event rates to 0.6 kHz. Resolution overall is affected, although not dramatically.

A Veto can be constructed with the signal seen in water is sufficient. On the top surface one can use resistive plate chambers or other technologies. The speaker said “it is just a matter of money”!

They are looking at a new site for the experiment. Both considered sites have reactors with large outputs, 17 and 18 GW power. THe geological surveys evidence no show-stoppers. The total time for the civil construction of the underground hall is of three years. THe schedule sees the preparation and hall construction until 2017, detector R&D in 2013-2016, and PMT production starting in 2016. Filling and data taking should finally happen in 2020. After a number of reviews, the project was approved by the CAS.

In summary, DAYA Bay II is a very rich project, with a beautiful physics program. It is challenging, but not impossible. They welcome collaborators!


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