investigation of neutron background in lsm
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1 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Investigation of ambient and µ-induced neutron background in LSM
measurements offast neutronsthermal (epithermal) neutronsµ-induced events (neutrons)
within the framework of EDELWEISS at LSM
V. Sanglard(DM session Friday)
n´s @ LSM I. SavvidisDM session tomorrow
2 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
neutrons as background fordirect Dark Matter search
χ χ n n
elastic scatteringoff nuclei (e.g. 72Ge)
v(χ) ~ 300km/sm(χ) ~ 70GeV
Erec, max ~ 35 keVErec, max ~ 35 keV
E(n) ~ 0.6MeV
3 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
sim. neutron energy spectra fromrock activity at LSM rock boundary
simulated neutron spectrum from U and Th traces in the Modane rock
R Lemrani et al., Nucl.Instrum.Meth. A560 (2006) 454-459
neutron spectra as simulated for LSM
4 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
4 3He counters for detection of thermal neutrons inside a polyethylene moderator for measurement of fast neutrons:
n +3He -> p +T (σcapture= 5333b) with Q=764 keV12cm
The main background is due to natural activity (α) in counter walls
stainless tube with inside coverage of 1. 50-60 μm Teflon and 2. 1μm electrolytic copper
1m
20cm Helium-3
neutron detection at LSM
5 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Average signal: 160 cpd (70% fast n´s,
30% thermal n´s)
internal background: n´s in ROI (exp.): 2.0(7) cpdα´s (extrap.): 0.6(1) cpd
Sensitivity: o(10-8) neutrons cm-2 sec-1
expected flux: o(10-6) n´s cm-2 sec-1
V. Chazal et al., Astrop.Phys.9(1998)163:Φ=(4.0 ± 1.0)x10-6 n/s/cm2
max. detect. eff.(MC): ~10% at 1 MeV preliminary!
neutron detection at LSMcalibration with PuBe
n +3He -> p +T
LSM layoutwith expt. locations
6 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
simulation of 3He(n,p)3H with GEANT4
0 1 2 3 4 5 6 7 80
0.1
0.2
0.3
0.4
0.5
0.6
Neutron energy spectrum
E(n) [MeV]
GEANT4 QGSP_BIC_HP
Lemrani et al,NIMA560(2006)454
neutron flux (input)
18 m
m
20 m
m
25 mm
118 mm
20 m
m
20
mm
2
0 m
m
20 mm
20 mmAl tubeDout=35 mmDin=33.5 mm
AirCounter
PE
PE
68 mm
NIE_tot_293Entries 18418
Mean 0.0746
RMS 0.1156
eV-410 -310 -210 -110 1
Co
un
ts
0
50
100
150
200
250
NIE_tot_293Entries 18418
Mean 0.0746
RMS 0.1156
Neutron energy (4 tubes, T_m=293 K)
NIE_tot_293Entries 18418
Mean 0.0746
RMS 0.1156
/ ndf 2χ 1370 / 699
p0 0.28± 40.14
p1 0.1± 16
eV-410 -310 -210 -110 1
Co
un
ts
0
50
100
150
200
250
NIE_tot_293Entries 18418
Mean 0.0746
RMS 0.1156
/ ndf 2χ 1370 / 699
p0 0.28± 40.14
p1 0.1± 16
Neutron energy (4 tubes, T_m=293 K)
NIE_tot_493Entries 18037Mean 0.09973
RMS 0.1193
/ ndf 2χ 1130 / 744
p0 0.17± 25.45
p1 0.13± 16.24
eV-410 -310 -210 -110 1
Co
un
ts
0
20
40
60
80
100
120
140
160
180 NIE_tot_493Entries 18037Mean 0.09973
RMS 0.1193
/ ndf 2χ 1130 / 744
p0 0.17± 25.45
p1 0.13± 16.24
Neutron energy (4 tubes, T_m=493 K)
tube0Entries 4044
Mean 728.5
RMS 125.4
keV0 100 200 300 400 500 600 700 800 900
Co
un
ts
1
10
210
310 tube0Entries 4044
Mean 728.5
RMS 125.4
Counter #0 energy deposit
tube1Entries 5056Mean 726RMS 126.8
keV0 100 200 300 400 500 600 700 800 900
Co
un
ts
1
10
210
310 tube1Entries 5056Mean 726RMS 126.8
Counter #1 energy deposit
tube2Entries 5049
Mean 730.7
RMS 118.7
keV0 100 200 300 400 500 600 700 800 900
Co
un
ts
1
10
210
310 tube2Entries 5049
Mean 730.7
RMS 118.7
Counter #2 energy deposit
tube3Entries 4425
Mean 727.4RMS 125.4
keV0 100 200 300 400 500 600 700 800 900
Co
un
ts
1
10
210
310 tube3Entries 4425
Mean 727.4RMS 125.4
Counter #3 energy deposit
E(nthermal)before capture
energy depositin 3He counter
preliminary
7 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
5-12
/01/
2007
7-8/
03/2
007
15/0
6/20
0716
/07/
2007
operation with weak neutron calibration source (20 n/sec) at EDELWEISS area
neutron measurement at LSM (pos.#4)2006/2007 campaign
8 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Laser-controlledposition of PE shielding sinceJanuary 2008
no fluctuation of the flux observedwith fixed geometry
from winter to end of summer 2008:δΦ/Φ(n) < 4% with 90% CL
neutron flux in long term measurement
9 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
thermal neutron measurementsince October 2008
measurements at LSM(35 days in position 1)
MCEfficiency, whole counter, ROI 660-830 keV
51% 54%
Sensitivity, active volume, ROI 660-830 keV
248.5 cps for 1 n/cm2/sec 266.5 cps for 1 n/cm2/sec
10 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
thermal neutron measurementsince October 2008
preliminary
!
11 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
μμ
nn
nn
Edelweiss experimental setup
µ veto systemA=100m2 in42 modules
12 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
EDELWEIS-II muon veto system
expected time difference:vertical tracks: Δt = 5.2m/3×108m/s ≈ 17 nsdiagonal tracks: Δl(max)=5.8m Δt ≈ 19.5 ns
“downward going muons” only
13 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
• DM: n mimic WIMP recoils(~ few keV);
• 0νββ:n (>MeV) => γ rays (inelast. scatt); n (therm.) => γ rays (capture);
GEANT4 (CnH2n)γ→N
π→N
π− abs
n→N
p→N
μ→N
others
simulation of µ-induced events µ
14 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
coincidences in µ-veto and Ge bolometers
interactivevisualisationof coincidences
µ track reconstruction
time [ns]
15 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
coincidences in µ-veto and Ge bolometers
Selection of coincident candidates:
> 1 veto module hit
15 ms < Δt < 35 msErecoil < 250 keV Erecoil > 250 keV
measured events 16 28
expected accidentals 3.7 ± 0.2 2.9 ± 0.2
excess coincidences 12.3 ± 4.2 25.1 ± 5.5
signal/background 3.3 ± 1.4 8.7 ± 2.4
1keV<Edep<250keV
accidentalcoincidences
Expected from Geant4 simulation:~0.03 events/kg.d
Measured:0.04 events/kg.d
Geant 4 simulation of µ-induced Ge-hits
Candidates for coincident
veto-bolometerevents
preliminary
16 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
GEANT4coincidences in µ-veto and Ge bolometers:µ-induced n´s?
nuclear recoils
µ-veto coincidences
bolo events outsidecoincidence window:11(Q<0.6)/84(Q>0.6)
most likely!
17 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Gd-loaded scintillator (PC based, 50x100x200cm3)paraffin
Pb bricks
µ
n´s
8x2 8“ PMTs (neutrons)+2x2 2“ PMTs (µ)
8 LED‘s,λ=420 nm
157Gd(n,γ)158Gd Eγ=8 MeV Neutron count rate:~1 /day (μ), ~8000 /day (radioactivity)Signature: μ-n coincidence or multiple n‘s
a dedicated neutron counter (NC) for µ-induced neutrons
18 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
picture gallery of installation (Sept/Nov 2008)
19 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
calibration with AmBe source: n + γ
241Am α + 9Be → 13C 12C + n
12*C + n (~60% ?)
12C + γ (4.4 MeV)
0 56 μsγ n n AmBe
„sequential n´s“nμ, e/γ n n
DAQ time window:
20 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
AmBe neutron measurement
Δ=36418.4
Δ=45.7
Δ=1
~17 hours
Neutron source ~20 n/s,
TDAQ=53.4 μs
(η = 1.07e-3)
(Δt>1000 ns to avoidPMT after-pulses)
# of secondary evts. within 53.4µs
21 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
AmBe neutron measurementN
umbe
rof e
vent
s
measurement
GEANT4 MC simulations
This example:En = 3 MeVGd content = 0.5%
τ = 17.4 ± 0.2 μs
17.4
fit: exp(-Δt/τ)
AmBe source: ~20 n/s
En up to ~11 MeV, max. 3 MeV
Δt [µs]
22 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
„Background“ run (Δtsec>1000 ns)
Δ=17.2
~12.6 days
Pb
Polyethylene
Veto
rock
p,π,α
μ´
n Cu(Cryostat)
Ge
μ <300GeV>
nVeto
Pb
Gd-loadedscintillator
μ
No specific selection
# of secondary evts. within 53.4µs
preliminary
23 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
„Background“ run, Δtsec (Δtsec>1000 ns)
~12.6 days
<Δtsec(AmBe)> = 18.3µs
preliminary
24 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Δ=218
Δ=1
~51 days
Δ=2
Pb
Polyethylene
Veto
rock
p,π,α
μ´
n Cu(Cryostat)
Ge
μ <300GeV>
nVeto
Pb
Gd-loadedscintillator
μ
# of secondary evts. within 53.4µs
coincidences with µ veto system (any hit)
(Δtsec>1000 ns)
preliminary
25 | K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
summary & outlook
fast neutrons: measurement campaign 2006-2008transferred to LNGS
thermal neutrons: new setup since 10/08publication under preparation
µ-induced neutrons: new setup since 11/08 operationallong-term measurement
comprehensive programmeof background investigation
next generation experiments
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