Investigation of ambient and
µ-induced neutron background in LSM
measurements of
¾
fast neutrons
¾
thermal (epithermal) neutrons
¾
µ-induced events (neutrons)
within the framework
of EDELWEISS at LSM
Æ V. Sanglard
(DM session Friday)
n´s @ LSM Æ I. Savvidis
DM session tomorrow
1
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
neutrons as background for
direct Dark Matter search
χ
χ
n
elastic scattering
off nuclei (e.g. 72Ge)
v(χ) ~ 300km/s
m(χ) ~ 70GeV
Æ Erec, max ~ 35 keV
2
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Erec, max ~ 35 keV
Æ E(n) ~ 0.6MeV
n
neutron spectra as simulated for LSM
R Lemrani et al., Nucl.Instrum.Meth. A560 (2006) 454-459
simulated neutron spectrum from
U and Th traces in the Modane rock
3
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
sim. neutron energy spectra from
rock activity at LSM rock boundary
neutron detection at LSM
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 keV
Helium-3
20cm
1m
The main background is due to
natural activity (α) in counter walls
Î stainless tube with inside coverage of
1. 50-60 μm Teflon and
2. 1μm electrolytic copper
4
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
12cm
neutron detection at LSM
calibration with PuBe
Average signal:
160 cpd (70% fast n´s,
30% thermal n´s)
n +3He -> p +T
internal background:
n´s in ROI (exp.): 2.0(7) cpd
α´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!
5
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
LSM layout
with expt. locations
simulation of 3He(n,p)3H with GEANT4
0.0746
RMS
0.1156
250
Counts
0.5
GEANT4
200
QGSP_BIC_HP
10
200
102
Lemrani et al,
NIMA560(2006)454
100
0.3
Entries
18418
Mean
0.0746
RMS
0.1156
χ2
/ ndf
1370 / 699
10
50
p1
Mean
150
103
40.14 ± 0.28
p0
150
0.4
250
3
NIE_tot_293
E(n
thermal)
tube0
before
Entries 4044
capture
Counter #1 energy deposit
Counts
18418
Mean
Counts
Entries
Counts
neutron flux (input)
0.6
Counter
#0energy
energy
Neutron
(4deposit
tubes, T_m=293 K)
NIE_tot_293
Neutron energy (4 tubes, T_m=293 K)
Neutron energy spectrum
728.5
16 ± 0.1
RMS
102
125.4
100
-2
10
-1
10
10
50
0
1
eV
Neutron energy (4 tubes, T_m=493 K)
68 mm
PE
118 mm
Entries
Mean
140
3
RMS
120
p0
25.45 ± 0.17
p1
16.24 ± 0.13
4
χ 2 / ndf
100
18037
0.09973
5
6
0.1193
8
102
PE
0
10-4
-3
10
20 mm
Al tube
Dout=35 mm
Din=33.5 mm
6
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
20 mm
20 mm
20
-2
10
20 mm
40
Counter
0
100 200 300 400 500 600 700 800 900
keV
Counter #3 energy deposit
tube2
Entries
103
5049
Mean
730.7
RMS
118.7
102
tube3
Entries
4425
Mean
727.4
125.4
RMS
10
10
1
1
20 mm
60
Air
103
E(n) [MeV]
1130 / 744
80
7
20 mm
25 mm
2
NIE_tot_493
160
18 mm
1
Counts
0
0
Counts
180
1
0
-3
100
800 900 1
10-4 200 300
10 400 500
10-2600 70010-1
keV eV
Counter #2 energy deposit
0.1
5056
726
126.8
Counts
0.2
-3
Entries
Mean
RMS
10
1
0
10-4
energy deposit
in 3He tube1
counter
-1
10
1
eV
0
100 200 300 400 500 600 700 800 900
keV
ry
prelimina
0
100 200 300 400 500 600 700 800 900
keV
7
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
/20
07
16
/07
/20
07
operation with weak
neutron calibration
source (20 n/sec) at
EDELWEISS area
15
/06
00
7
03
/2
7-8
/
5-1
2/0
1
/20
07
neutron measurement at LSM (pos.#4)
2006/2007 campaign
neutron flux in long term measurement
Laser-controlled
position of PE
shielding since
January 2008
Æ no fluctuation of
the flux observed
with fixed geometry
from winter to end of summer 2008:
δΦ/Φ(n) < 4% with 90% CL
8
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
thermal neutron measurement
since October 2008
MC
9
Efficiency, 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
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
measurements at LSM
(35 days in position 1)
thermal neutron measurement
since October 2008
!
ry
prelimina
10
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Edelweiss experimental setup
μ
n
n
µ veto system
A=100m2 in
42 modules
11
| 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 ns
diagonal tracks: Δl(max)=5.8m Æ Δt ≈ 19.5 ns
Γdownward going muons” only
12
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
simulation of µ-induced events
• 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
n→N
π− abs
p→N
μ→N
others
13
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
µ
coincidences in µ-veto and Ge bolometers
time [ns]
interactive
visualisation
of coincidences
µ track reconstruction
14
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
coincidences in µ-veto and Ge bolometers
Candidates for
coincident
veto-bolometer
events
1keV<Edep<250keV
accidental
coincidences
Selection of coincident candidates:
Geant 4 simulation of µ-induced Ge-hits
¾ > 1 veto module hit
¾ 15 ms < Δt < 35 ms
Erecoil < 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
15
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Expected from Geant4 simulation:
~0.03 events/kg.d
Measured:
0.04 events/kg.d
ry
prelimina
coincidences in µ-veto
and Ge bolometers:
µ-induced n´s?
GEANT4
nuclear recoils
most likely!
bolo events outside
coincidence window:
11(Q<0.6)/84(Q>0.6)
µ-veto coincidences
16
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
a dedicated neutron counter (NC)
for µ-induced neutrons
µ
Gd-loaded scintillator
paraffin
(PC based, 50x100x200cm3)
8 LED‘s,
λ=420 nm
157Gd(n,
γ)158Gd Eγ=8 MeV
Signature:
17
Pb bricks
μ-n coincidence or multiple n‘s
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
n´s
8x2 8“ PMTs (neutrons)
+
2x2 2“ PMTs (µ)
Neutron count rate:
~1 /day (μ),
~8000 /day (radioactivity)
picture gallery of installation (Sept/Nov 2008)
18
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
calibration with AmBe source: n + γ
DAQ time window:
μ, e/γ
n
n
n
„sequential n´s“
56 μs
0
γ
n
n
AmBe
241Am
α + 9Be → 13C
12C
+n
12*C + n (~60% ?)
12C + γ (4.4 MeV)
19
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
AmBe neutron measurement
Δ=36418.4
~17 hours
Neutron source ~20 n/s,
TDAQ=53.4 μs
Δ=45.7
(η = 1.07e-3)
Δ=1
# of secondary evts. within 53.4µs
20
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
(Δt>1000 ns to avoid
PMT after-pulses)
AmBe neutron measurement
GEANT4 MC simulations
AmBe source: ~20 n/s
This example:
En = 3 MeV
Gd content = 0.5%
En up to ~11 MeV, max. 3 MeV
Number of events
measurement
fit: exp(-Δt/τ)
τ = 17.4 ± 0.2 μs
17.4
Δt [µs]
21
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
„Background“ run (Δtsec>1000 ns)
No specific selection
~12.6 days
μ <300GeV>
Veto
Δ=17.2
Polyethylene
n
rock
ry
prelimina
(Cryostat)
μ´
p,π,α
# of secondary evts. within 53.4µs
22
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Cu
Ge
n
μ
Veto
Pb
Gd-loaded
scintillator
Pb
„Background“ run, Δtsec (Δtsec>1000 ns)
<Δtsec(AmBe)> = 18.3µs
~12.6 days
ry
prelimina
23
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
coincidences with µ veto system (any hit)
(Δtsec>1000 ns)
~51 days
μ <300GeV>
Δ=218
Veto
ry
a
n
i
m
i
l
e
r
p
Δ=1
n
rock
Δ=2
Polyethylene
(Cryostat)
μ´
p,π,α
# of secondary evts. within 53.4µs
24
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Cu
Ge
n
μ
Veto
Pb
Gd-loaded
scintillator
Pb
summary & outlook
¾ fast neutrons: measurement campaign 2006-2008
Æ transferred to LNGS
¾ thermal neutrons: new setup since 10/08
Æ publication under preparation
¾ µ-induced neutrons: new setup since 11/08 operational
Æ long-term measurement
¾ comprehensive programme
of background investigation
Æ next generation experiments
25
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
acknowledgements
¾ Evgeny Yakushev and Dubna group
¾ EDW collaboration
¾ ILIAS-TARI P2006-05-LSM & P2008-01-LSM
¾ JOULE LSM-Dubna-Prague
¾ DFG via SFB-TR27
transregional collaborative
research centre
“neutrinos & beyond”
HD-KA-TÜ-M
26
| K. Eitel | TAUP2009 | Rome/LNGS, July 1-5, 2009
Edelweiss collaboration