CHOOZ Double Chooz réalité mythe ?

Yves Déclais , IPNL (CNRS-IN2P3/UCBL)

Questions (sur le bruit) de fond

All numbers used in these slides can be found in :

• CHOOZ proposal : http://duphy4.drexel.edu/chooz_pub/• CHOOZ publi : hep-ex/0301017• Palo-Verde : hep-ex/0107009• Double Chooz LOI : hep-ex/0405032• Double Chooz US proposal : hep-ex/0410081

In these slides the naming convention recommended in the Neutrino News is followed:

The CHOOZ reactor neutrino experiment was usually capitalized, not because it was an acronym, but because people thought it was one. The Double Chooz experiment, which met in the U.S. for the first time in February, has chosen a convention for its name, which is Double Chooz, without a dash and without all caps.

I will limit the discussion on only one point « the accidental background »many others could be discussed

CHOOZ Double Chooz

Inner : 192 PMTs , 15% coverageOuter : 24 PMTs

Inactive volume

Inner : 507 PMTs , 13% coverageOuter : 256 PMTs

2 tons StainlessSteel vessel

Active volume 26 m3 40 m3

Radiopurity

Th U K

EMI 9351 CHOOZ proposal 100 ppb 70 ppb 150 ppm

EMI 9351 CHOOZ used (glass :B53 ) 20 ppb 10 ppb 60 ppm

Double Chooz 30 ppb 30 ppb 60 ppm

Used :192 + 24

Individual contributions to single rates

Measured @ CHOOZ ( Ee > 1.3 MeV, dgéode > 30cm )

OK within a factor 265/sec

18.3

65/sec

B53 glass

Double Chooz : NPMTsx 2.6Joker : gravel Iron shot …. but threshold .5 MeV … but …

Accidental Background Raccidental = Rpositron x Rneutron x Δt (100 μsec) x dcut

proposal measured

Rpositron 100 /sec 65 /sec

Rneutron 1 /hour 45 /hour

dcut 1. 0.12

Raccidental 0.24 /day 0.42 /day

Double Chooz will not use it ?

Where is the mistake ?

Rneutron :• neutron capture on Gd from spontaneous fissions : < 6 10-3 /day

the dominant source of the U is located inside the PMTs• neutron capture on Gd from spallation neutrons outside the detector : << 1 /hour• high energy γ from neutron capture (αn reactions) in the surrounding rock : << 1 /hour in Region 1+2

beware to neutron capture on heavy nuclei inside the detector …

The remaining hypothesis is bremstrahlung γ associated to cosmic muonsIt would be sensitive to the overburden as well as the correlated background

Conclusion (1)• the price to pay for increasing the target mass of the Double Chooz experiment within boundaries conditions (using the existing setup) is to

worsen the background

So the S/B improvement as claimed by the Double Chooz authors is not realistic

• the accidental background will be strongly correlated with the overburden and so the near to far detector comparison will be affected by systematical uncertainties which have not been taken into account

• in the Double Chooz proposal there are no convincing arguments to prove that a CHOOZ type reactor neutrino detector can be efficiently operated at small overburden

Palo Verde data could be used to understand where are the

difficulties (bkgd : 300/day, efficiency corrected, for ~1000 ν )

a (cheap) demonstrator is mandatory before spending 10th M€

Conclusion (2)

1.00

Sin2(2θ13)=0.05

At such a small L/E :Accidental signal substraction

can mimic or cancel an oscillation signal« remember Bugey I »

For a safe oscillation search the oscillation signal should be localised where there is no background structure which could mimic it …

Conclusion (3)

Θ13 search @ reactors disappearance experiment hard with Gd Loaded scintillator

Low level of systematics : long and exhacting task

incompatible with « expeditiously deployed multi detector reactor experiment »

internal cross check for the validity of the result :

L/E should be large enough ( d~2km ) for an independant analysis of

+ the energy spectrum distortion

+ the disappearance effect on the rate