11 c background suppression in the cno-pep solar neutrino energy region with borexino

11C background suppression in the CNO-pep solar neutrino energy region with Borexino

Davide Franco for the Borexino collaborationMilano University & INFN

XCIV Congresso Nazionale della Società Italiana di Fisica22-27 Settembre 2008, Genova

Davide Franco – Università di Milano & INFNSIF 2008

pep and CNO neutrinos

pep neutrinos: Directly related with the pp

fusion reaction in the Sun Improves our knowledge of

the solar neutrino luminosity

Test of the MSW-LMA

CNO neutrinos: First experimental

observation of the CNO cycle presence in the Sun

Helpful in the age estimation of the Globular Clusters

Non-standard interactions, mass-varying neutrinos, CPT…


Detection signature in Borexino

Neutrino elastic scattering off electrons

Energy range of observation: 0.8 – 1.4 MeV

Expected flux in BOREXino-like scintillator (BS07++LMA): pep-: 9x10-3 d-1 ton-1

CNO-: 6x10-3 d-1 ton-1

Eve

nts

/3ye

ars/

100

ton

s/0.

05 M

eV

3 years statistics in 100 tons of scintillator

Overall signal rate: 1.5 c/d in 100 tons


Detector layout and main features

Water Tank: and n shield water Č detector208 PMTs in water2100 m3

20 legsCarbon steel plates

Scintillator:270 t PC+PPO in a 150 m thick nylon vessel

Stainless Steel Sphere:2212 PMTs 1350 m3

Nylon vessels:Inner: 4.25 mOuter: 5.50 m


The signal and the background

Energy range: 0.8 – 1.4 MeV

11C contamination


11C production and the three-fold coincidence

(+ secondaries) + 12C → (+ secondaries) + 11C + n

n + p → d +

11C → 11B + e+ + e

Triple coincidence among:

• cosmic muon:

• rate at LNGS (3700 m.w.e.): 1.16 hr-1 m-2

• average energy: 320 GeV

• gamma from neutron capture:

• energy: 2.2 MeV

• capture time: 250 s

• positron from 11C decay:

• deposited energy between 1.022 and 1.982 MeV (but quenched!)

• mean life: 30 min!!!

11C

n


Vetoing the detector

PC+PPO11C

n

Depends on detector efficiencies in tagging neutrons and in tagging and tracking muons


11C expected rate from NA54 Cern experiment

S/B ~1/6


Detecting (and rejecting) cosmic muons

are identified by ID and OD OD eff: ~ 99% ID based on pulse shape analysis

Rejection factor > 103 (conservative) crossing the buffer only

crossing the scintillator

pulse

scintillation pulse

R = (1.21±0.05) h-1m-2


-track (in progress)


Cross sections for 11C production from 12C as a function of energy

Cumulative range of -induced secondaries

SNOLab

C. Galbiati et al., Phys. Rev. C 71, 055805 (2005)

Kamioka LNGSNA54

Neutrons are produced in association with 95.5% of the muon-induced 11C

SNOLab

Intrinsic inefficiency


Muon induced neutrons

• Since January 2008 we were unable to identify high-multiplicity events• Electronics has been properly modified: a very long gate (1.6 ms) is opened after each muon• New clustering algorithm has been developed• A parallel FADC multichannel system is under development to estimate the efficiency• From preliminary results > 90% neutrons detected• Still problems in very high multiplicity events


Test of the coincidence technique with the Counting Test Facility

4 tons of scintillator 1 m radius vessel housing the

scintillator 2 m radius “shroud” 3.6 p.e./PMT for 1 MeV electron Muon veto 100 PMT (OC: 21%) Buffer of water Energy saturation: 6 MeV


Neutron selection• For each detected , the following event in the time window Tn = [20, 2000] μs is selected as a candidate event for a neutron capture γ • E < 2.6 MeV

Muon selection• cut on the number of photoelectrons detected by the muon-veto

11C selection• After each coincidence, 11C candidates are selected in a subsequent time window Tw = 300 min, 10 times the 11C mean life.• Optimal energy range: 1.15 < E < 2.25 MeV • Distance between 11C event and gamma < 35 cm

Data selection in CTF


Measured 11C production rate in CTF

R(11C) = [13.0 ± 2.6(stat) ± 1.4(syst)]×10-2 d-1 ton-1

= 0.89 ± 0.20Expected Rate(NA54)

Measured Rate

Goal reached?

Phys.Rev.C74:045805,2006.


Three Fold Coincidence in Borexino

S = 500 ± 12 pe/MeV

Q/E ~ 80 %

The total rate is not known yet:- Unknown efficiency in tagging neutrons- Too low statistics

sample of 11C with strong cuts


…larger rate in Borexino

from spectral analysis

= 1.7 ± 0.1Expected Rate(NA54)

Measured Rate Inconsistency between NA54/CTF and Borexino?

• different size of the detector?• other background sources?


Conclusion

Electronics and DAQ have been properly modified and strongly improved

New FADC system for increasing the neutron detection efficiency is almost ready

The CTF measurement has demonstrated that the three-fold coincidence technique is powerful in localizing in space and time 11C decays

11C can be removed by blinding detector volumes around it

Still problems: inconsistency between the rate measured by CTF and NA54 and Borexino

Waiting for more statistics

pep and CNO neutrino spectroscopy is coming soon!!

11 c background suppression in the cno-pep solar neutrino energy region with borexino

Documents

c production

c decay

c snolabsif

c candidates

c expected rate

muon induced neutrons

c background suppression

muonveto11c selection