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Neutron background and possibility for shallow experiments Tadao Mitsui Research Center for Neutrino Science, Tohoku University 14-16 December, 2005 Neutrino.

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Presentation on theme: "Neutron background and possibility for shallow experiments Tadao Mitsui Research Center for Neutrino Science, Tohoku University 14-16 December, 2005 Neutrino."— Presentation transcript:

1 Neutron background and possibility for shallow experiments Tadao Mitsui Research Center for Neutrino Science, Tohoku University 14-16 December, 2005 Neutrino Sciences 2005, Neutrino Geophysics, Honolulu, Hawaii Neutron simulations for 2700 and 300 m.w.e. A “naïve” estimation for geo- at 300 m.w.e. What to do for the “next-to-next” step (Thank you very much John and Steve for squeezing time for me)

2 Cosmic ray: serious backgrounds for neutrino observations Fortunately, delayed coincidence can be utilized for electron anti neutrinos, e + p  e + + n Even though, correlated (not random coinc.) b.g. are … neutron-neutron: 2 neutrons from the same muon fast neutron: proton recoil is “prompt”, neutron capture is “delayed” Spallation products, especially, 9 Li: 9 Li (  257ms)   - n (51%) Experimental sites are: several 1000 meters water equivalent (m.w.e)

3 Cost against cosmic ray is also serious Existing mines or other tunnels … main sites so far, (physics was the main objective) For the next generation of geophysics, a lot of neutrino detectors all over the surface of the Earth, like seismographs! (A. Suzuki, July, 2005) for a full-dress measurement of the Earth’s power. Diagnoses of reactors or other applied physics Sites will not always be in the existing mines or other existing tunnels. Save the cost of civil engineering …

4 Deep-sea experiments Very few people has techniques and experiences (e.g. U. Hawaii) High pressure PMT (chain reaction) Scintillator container (balloon? or acrylic vessel?)

5 Pressure difference on the balloon etc detector filling of KamLAND

6 Pressure control: 10 -3 precision 0.2 MPa for 5 months Pressure difference: +50 <  P < +100 Pa Planned (curve) Done (points) 0.2 MPa, 5 months

7 When it dives to 300 m (3 MPa)… Slightly positive pressure between +50 and +100 Pa  10 -4 precision of total pressure change of 3 MPa Shrink: ~ GPa -1  3 MPa ~ 0.3% (3 m 3 out of 1000 m 3 ) Hanohano dives 10 times deeper, but it takes acrylic vessel with more pressure difference tolerated, not an easy technique anyway 300 m submarines

8 Cheap, fast, shallow detectors “mass production” for geophysics and applied physics

9 Cosmic ray: serious backgrounds for neutrino observations Fortunately, delayed coincidence can be utilized for electron anti neutrinos, e + p  e + + n Even though, … neutron-neutron: 2 neutrons from the same muon fast neutron: proton recoil is “prompt”, neutron capture is “delayed” Spallation products, especially, 9 Li: 9 Li (  257ms)   - n (51%) Experimental sites are: several 1000 meters water equivalent (m.w.e)

10 Hybrid Monte Carlo for neutrons T. Araki, master thesis, Tohoku U., 2005

11 Mt. Ikenoyama shape and  flux  flux + Data MC cos (zenith angle) azimuth (deg.) SWNE Only one parameter, rock density tuned to be 2.675 g/cm -3 Then good agreement in zenith and azimuth distributions T. Araki, master thesis, Tohoku U., 2005

12 Neutron attenuation length: data v.s. MC data MC data MC

13 Fast neutron event: data v.s. MC T. Araki, master thesis 2005 Fast neutron event with OD hit (by muon) and delayed coincidence (proton recoil - neutron capture coincidence) Good agreement: data and MC

14 CHOOZ (300 m.w.e.) arXiv:hep-ex031017 No reactor in simulation Muon flux only Depth and detector of CHOOZ input Muon  neutron production  neutron propagation simulation in the same way as KamLAND

15 CHOOZ (300 m.w.e.) arXiv:hep-ex031017 data 0.062  0.008 MeV -1 d -1 Factor ~2 agreement MC

16 Hybrid Monte Carlo for neutrons T. Araki, master thesis 2005 Tolerable agreement from 300 to 2700 m.w.e. without any individual tuning Muon flux range: ~ 200 Fast neutron rate: > 1000

17 KamLAND at 300 m.w.e. geo- and background A full simulation is being developed “scaling result” (using neutron rate, neutron attenuation length, …) are shown here (should not be so seriously different from the full simulation … I hope)

18 KamLAND (2700 m.w.e, 400 ton, 750 days) Geo- ( , n) reactor fast-n 9 Li ( , n) and reactor are main background. Upper peak ( 238 U contribution only) is almost unseen, because of reactor background. Background from cosmic-rays (fast- n and 9 Li) are small

19 “KamLAND” at 300 m.w.e Geo- reactor fast-n 9 Li neutron-neutron ( , n) Large background from ~10 3 times higher muon flux compared with 2700 m.w.e … neutron-neutron: 2 neutrons from the same muon fast neutron: proton recoil is “prompt”, neutron capture is “delayed” 9 Li: 9 Li (  257ms)   - n (51%)

20 “Gd ~ 0.1 %” to reject neutron-neutron b.g. Geo- ( , n) reactor fast-n 9 Li neutron -neutron Assumptions: Muon flux ~200 Hz Veto: 500  s Neutron life time: 30  s (Gd ~ 0.1%) 200  s (KamLAND) 10% tail assumed from Gd (~8 MeV) signal

21 Geo- ( , n) reactor fast-n 9 Li neutron -neutron Delayed neutron emitting spallation products

22 Particle ID (e + /e - ) using cherenkov lights geo- lower peak region e + (neutrino): 3 p.e./MeV e - ( 9 Li b.g.): 20 p.e./MeV Calculated cherenkov light contribution in KamLAND

23 Particle ID (e + /e - ) using cherenkov lights By developing a new scintillator with less light yield, extract cherenkov ring out of the uniformly emitted scintillation light. Actually, no need to extract the “ring”, deviation from uniform light emission implies cherenkov contribution Scintillation 30 p.e./MeV, cherenkov 30 p.e./MeV (asymptotic)  rejection power > 100 (e + /e - at visible energy 1.4 MeV (geo- lower peak)) A geo- candidate 1.88 MeV Charge distribution is very well fitted with an assumption of uniform light emission (KamLAND)

24 Geo- ( , n) reactor fast-n 9 Li neutron -neutron Delayed neutron emitting spallation products

25 “Cherenkov/Sintillation detector” to identify e+/e- Geo- ( , n) reactor fast-n 9 Li neutron-neutron Real KamLAND: ~400~500 p.e. / MeV (scintillation) ~30 p.e. / MeV (Cherenkov) “Che/Sci detector”: ~30 p.e. / MeV (scintillation) ~30 p.e. / MeV (Cherenkov)

26 Lower energy resolution, but no problem Geo- ( , n) reactor fast-n 9 Li neutron-neutron Real KamLAND:  E = 6.2%/  E/MeV  (~400~500 p.e. / MeV) “Che/Sci detector”:  E = 16%/  E/MeV  (~60 p.e. / MeV)

27 Geo- ( , n) reactor fast-n 9 Li neutron-neutron Fast neutrons

28 Fast neutron event rejection by outer detector (OD) By tagging the parent muon of the fast neutron, that event can be rejected (muon is in the same event as the prompt) Simple shielding of the neutron is not enough because shielding material also creates fast neutrons.

29 KamLAND detector

30 KamLAND outer detector Outer detector thinnest part ~ 50 cm

31 KamLAND with large OD 10 kton pure water 400 PMTs (c.f. present KamLAND = 225 PMTs) 600 Hz (300 m.w.e) (mean interval: 1.5 ms) ~5-MeV neutron   ~0.1  30m~1  s Dead time: ~ 0.1 % 4m

32 Geo- ( , n) reactor fast-n 9 Li neutron-neutron Fast neutrons

33 Fast neutron rejection by “large (4-m thick) OD” Geo- ( , n) reactor fast-n 9 Li neutron-neutron Large (4-m thick) OD detects the parent muon of fast neutrons Not only for a shield, but an active OD is needed to reject fast neutron background. OD inefficiency should be < 10 -3  monolithic OD

34 If reactor and ( , n) are also reduced … Geo- ( , n) reactor fast-n 9 Li neutron-neutron Site with no neaby powerful reactor (like Hanohano) 13 C( , n) …... (from 222 Rn) Lesson of KamLAND …

35 signal intensity v.s. depth Sudbury Kamioka CHOOZ full paper (arXiv:hep-ex/0301017) signal (geo-, reactor-  at ~180 km)  flux at 2700 m.w.e.

36 signal intensity v.s. depth Sudbury Kamioka CHOOZ full paper (arXiv:hep-ex/0301017) geo-  at 300 m.w.e. 300  2700 mwe

37 signal intensity v.s. depth Sudbury Kamioka CHOOZ full paper (arXiv:hep-ex/0301017) Challenging signal/noise ratio: Not easy Simple expectation in previous slides are just simple and naive  Developing “shallow detector” for the next (or next-to-next) step is needed to start now! 300  2700 mwe < 1/1000 weaker signal

38 signal intensity v.s. depth Sudbury Kamioka CHOOZ full paper (arXiv:hep-ex/0301017) The most successful shallow experiment so far is Palo Verde. It rejects cosmogenic b.g. with the segmented detector. For geo-, kton is needed so segmentation is not realistic. Here another idea: large OD and cherenkov PID 300  2700 mwe

39 signal intensity v.s. depth Sudbury Kamioka CHOOZ full paper (arXiv:hep-ex/0301017) Here another idea: large OD and cherenkov PID R&D of those techniques with a reactor site … usuful KamLAND front detector? (to confirm reactor spectrum before oscillation) 300  2700 mwe

40 Conclusions Cheap, fast, shallow detector is needed for geophysics and applied physics. Delayed coincidence may make it possible. Is geo- at 300 m.w.e possible?

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