investigation of neutron background in lsm

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


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


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 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


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


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


Mean 0.0746

RMS 0.1156

Neutron energy (4 tubes, T_m=293 K)


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


Mean 0.0746

RMS 0.1156

/ ndf 2χ 1370 / 699

p0 0.28± 40.14

p1 0.1± 16


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


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


tube2Entries 5049

Mean 730.7

RMS 118.7

keV0 100 200 300 400 500 600 700 800 900

Co

un

ts

1

10

210


Mean 730.7

RMS 118.7


tube3Entries 4425

Mean 727.4RMS 125.4

keV0 100 200 300 400 500 600 700 800 900

Co

un

ts

1

10

210


Mean 727.4RMS 125.4


E(nthermal)before capture

energy depositin 3He counter

preliminary


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


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


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


thermal neutron measurementsince October 2008

preliminary

!


μμ

nn

nn

Edelweiss experimental setup

µ veto systemA=100m2 in42 modules


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


• 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


coincidences in µ-veto and Ge bolometers

interactivevisualisationof coincidences

µ track reconstruction

time [ns]


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


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!


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


picture gallery of installation (Sept/Nov 2008)


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:


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


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]


„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


preliminary


„Background“ run, Δtsec (Δtsec>1000 ns)

~12.6 days

<Δtsec(AmBe)> = 18.3µs

preliminary


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


coincidences with µ veto system (any hit)

(Δtsec>1000 ns)

preliminary


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


acknowledgements

Evgeny Yakushev and Dubna groupEDW collaborationILIAS-TARI P2006-05-LSM & P2008-01-LSMJOULE LSM-Dubna-PragueDFG via SFB-TR27

Edelweiss collaboration

transregional collaborativeresearch centre“neutrinos & beyond”HD-KA-TÜ-M

investigation of neutron background in lsm

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