Systematic errors of reactor neutrino experiments and ideas about new detectors
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Systematic errors of reactor neutrino experiments and ideas about new detectors. Yifang Wang Institute of High Energy Physics, Beijing Nov.28, 2003. How big sin 2 2 q 13 is ?. J. Bahcall et al., hep-ph/0305159. M. Maltoni et al., hep-ph/0309130.

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Systematic errors of reactor neutrino experiments and ideas about new detectors

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Systematic errors of reactor neutrino experiments and ideas about new detectors

Systematic errors of reactor neutrino experiments and ideas about new detectors

Yifang Wang

Institute of High Energy Physics, Beijing

Nov.28, 2003


How big sin 2 2 q 13 is

How big sin22q13 is ?

J. Bahcall et al., hep-ph/0305159

  • M. Maltoni et al., hep-ph/0309130

Sin22q13 ~ 3%, Chooz limit: Sin22q13 < 10%


Summary about systematic errors

Summary about systematic errors

  • There are three main types of errors: reactor related(3%), background related(0.5-4%) and detector related(3%)

  • Use two detectors, far/near to cancel reactor related errors completely and some of detector/background related errors

  • Use movable detectors, near-far, to cancel all backgrounds and some of detector related errors

  • Sufficient shielding to reduce backgrounds

Can we do better than 1% ???


Possible arrangement

Possible arrangement

500m

500m

1500m

1500m

Nf=P1500(A+B)/15002

Nn=P500(A+B)/5002

Reactor errors

canceled out exactly


Another possibility

Another Possibility

1200m

300m

300m

1500m

1500m

Nf=P1500(A+B)/15002

NnA=P300(A)/3002 +P1237(B)/12372

NnB=P300(B)/3002 +P1237(A)/12372

Reactor errors:

~0.1% @ DA ~ 1.5%


Systematic errors of reactor neutrino experiments and ideas about new detectors

100m is better than 500 m

100/1500

500/1500


Systematic errors of past reactor experiments

Systematic errors of past reactor experiments


Kamland

KamLAND

1000t scintillators

Shielding:

3000 MWE/3m Water

180 km baseline

Signal: ~0.5/day

Eff. ~40%

BK:

corr.: ~0.001/day

uncorr. ~0.01/day


Large error on fiducial volume

Large error on fiducial volume


Systematic errors

Systematic errors


Experience gained

Experience gained

  • Very good shielding

  • Balloon not good  target mass not well defined

  • Light transport in scintillator unknown  particularly bad for Large detectors  large error on position reconstruction

  • Background from 8He/9Li

  • Not good enough veto tracking system

  • 3m water shielding gives a neutron reduction of >13*106(high energy).


Systematic error comparison

Systematic error comparison


Important point to have small systematic error

Important point to have small systematic error

  • Energy threshold less than 0.9 MeV

  • Homogeneous detector

  • Scintillator mass well determined

  • Target scintillator all from one batch, mixing procedures well controlled

  • Not too large detector

  • Comprehensive calibration program

  • Background well controlled  good shielding

  • Be able to measure everything(Veto ineff., background, energy/position bias, …)

  • A lot of unforeseen effects will occur when looking at 0.1% level


Further reduce systematic errors multiple modules

Further reduce systematic errors: multiple modules

  • Many modules, 8t each, 100-200 8”PMT/module

  • 1-2 at near, 4-8 at far, small enough for movable calibration

  • Correlated error cancelled by far/near

  • Uncorrelated error can be reduced

  • Event rate:

    near: ~500-2000/day/module

    Far: ~40/day/module

  • 100 days calibration at the near pit

     0.2-0.5% statistical error

  • Two reference modules 100 days,

    others ~ 10 days calibration

oil

Gd-scintillator


Advantages with multiple modules

Advantages with multiple modules

  • Smaller modules have less unknowns

  • Multiple handling to control systematic error

  • Easy construction

  • Easy movable detector

  • Scalable

  • Easy to correct mistakes


Systematic errors of reactor neutrino experiments and ideas about new detectors

Fe or low acticity concrete

water

RPC

module


Structure of the module

Structure of the module

III

I

II

  • Three layers module structure

    I. target: Gd-loaded scintillator

    II. g-ray catcher: normal scintillator

    III. Buffer shielding: oil

  • Advantages:

    • Well defined fiducial volume

    • No cut on position  small systematics

  • Disadvantages:

    • Complicated mechanical structure

    • Light yield matching/energy bias ?

      Need careful MC study


Preliminary results 100pmt 8t

Preliminary results: 100PMT/8t

Energy resolution


Systematic errors of reactor neutrino experiments and ideas about new detectors

position resolution

Cubic

Cylindrical

Position bias


Cylinder with reflection

Cylinder with reflection

8MeV, Light yield 7000/MeV, QE=20%, QED1=60%,

Buffer=50cm, 100 8” PMT


Background correlated

Background - correlated

  • Cosmic-muon-induced neutrons:

    • B/S < 0.005  1/day @ ~1km

    • Can be measured by veto tagging, accuracy<20%

    • Veto rate < 1KHz, 2-3 layers RPC(1600-2400 m2) ?

    • Methods:

      • Overburden > 100 MWE

      • Active Veto, ineff. < 0.5%, known <0.2%

    • Three scenarios:


Other correlated backgrounds

Other correlated backgrounds

  • b-neutron instable isotopes from cosmic m

    • 8He/9Li, Br(n) = 12%/48%, 9Li dominant

    • Production rates = fm·NA ·s ·Br

    • Depth > 300 MWE, best 1000 MWE


Background uncorrelated

Background - Uncorrelated

  • B/S < 0.05  < 8/day @ far site

  • Can be measured by swap method, precision ~ B/s=2.5%/day

  • single rate @ 0.9MeV < 50Hz

    2· Rg · Rn· t < 0.04/day/module

  • Methods:

    • Low activity glass for PMT, > 0.5m oil shielding (dominant!)

    • 3 MWE shielding, low activity sand/aggregate or Fe ?

    • Rn concentration < 20 Bq/m3, N2 flushing ?

    • (U, Th, K) in Scintillator < 10-13 g/g, clean Gd

    • All mechanical structure made of low activity materials

    • Calibration gadget made of clean materials such as Teflon, …

    • Clean everywhere, no dust, no …


Radiopurity of some materials

Radiopurity of some materials


Our estimate of systematic errors per module

Our estimate of systematic errors per module

  • Reactor < 0.1%

  • Background < 0.2%

  • Energy cut ~ 0.2%

  • Position cut ~ 0.2%

  • Time cut < 0.1%

  • Livetime ~ 0.1%

  • Other unexpected <0.2%

  • Total < 0.5%


Budget for detector 6 module

Budget for detector(6 module)


R d efforts

R&D efforts

  • Low radioactive glass PMT

  • Low radioactive Gd-loaded liquid scintillator

    • Development

    • Mixing/purification

    • Monitor/Aging study

  • Mechanics of shielding structure

  • A lab for Low radioactive materials

  • Mass production of glass RPC


Summary

Summary

  • Measuring Sin22q13 is a very important experiment, an unique opportunities for us

  • Systematic error can be controlled to ~0.5%.

  • Daya bay power plant is a good choice for the experiment


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