CRACOW EPIPHANY CONFERENCE ON NEUTRINOS AND DARK MATTER 5 - 8 January 2006, Cracow, Poland

The search of dark matter with ArDM detector

Piotr Mijakowski

The Andrzej Sołtan Institute For Nuclear Studies (IPJ), Warsaw, Świerk

OUTLINE

I

ArDM (Argon Dark Matter)

II

Neutron background in dark matter underground searches III Geant4 neutron background studies

P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 2

part I – ArDM

http://neutrino.ethz.ch/ArDM

A.Badertscher, R.Chandrasekharan, L.Kaufmann, A.Knecht, L.Knecht, M.Laffranchi, M.Messina, G.Natterer, P.Otiougova, A.Rubbia, J.Ulbricht

ETH Zurich, Switzerland C.Amsler, C.Regenfus, A.Buechler-Germann Zurich University, Switzerland A.Bueno, M.C.Carmona-Benitez, J.Lozano-Bahilo, S.Navas-Concha University of Granada, Spain I.Gil-Botella, P.Ladron de Guevara, L.Romero

CIEMAT, Spain T.Kozłowski, P.Mijakowski, E.Rondio

Soltan Institute (IPJ) Warsaw Świerk, Poland H.Chagani, E.Daw, P.Majewski, V.Kudryavtsev, N.Spooner

University of Sheffield, England ArDM : a proposed ton-scale liquid Argon experiment for direct detection of Dark Matter as WIMPs (Weakly Interacting Massive Particle,

c

) P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 3

ArDM (cont.)

Elastic scattering reaction:

c

+ Ar at rest

 c

+ Ar recoil

• Measurement of the recoils of target nuclei [10-100 keV].

• Recoil energy  scintillation & ionization of Argon

GOAL

: independently detect the light (PMTs) and the charge (Large Electron Multiplier) • light/charge ratio allows to discriminate background events ( e/ g vs. nuclear recoils )

ArDM@CERN 170 cm P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow detector design LEM Ar (10 cm) drift field ≈ 4 kV/cm LAr (120 cm) PMTs 4

ArDM status

R&D @ CERN (1 ton prototype):

• High voltage system • LEM based charge readout • Light detection system (PMTs + VUV reflecting mirrors) • CAD design and assemby of the detector

KEY POINT: UNDERSTAND THE DETECTOR PERFORMANCE

FIRST GOAL

: proof of principle: 39 Ar rejection (intrinsic background, beta emitter with decay rate of ≈ 1 kHz in a 1 ton detector)

Simulations: full detector geom., experimental background P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 5

ArDM prospects

TIME SCALE

: 

2006

: assembly of detector at on surface

CERN

; test 

2007

: transport to the Underground Laboratory (

Canfranc

, Spain); installation in experimental hall and mounting of infrastructure + neutron shield 

2007

: first data taking

CANFRANC LAB (2450 m w.e.) P.Mijakowski

pictures: E.Coccia@TAUP05 Young Researchers Session, 8 January 2006, Cracow 6

Estimated event rates

P.Mijakowski

≈ 100 event/ton/day

Assuming 30 keV recoil energy threshold, M

c

= 100 GeV/c 2

≈ 1 event/ton/day for

s

= 10 -46 : ≈ 1 event/ton/100 day Young Researchers Session, 8 January 2006, Cracow 7

part II – Neutron background

Background events: • the same signal from WIMP & neutron interactions !!! • only possible way to distinguish: neutron multiple scattering

nuclear events e-like events possible to discriminate MOTIVATION: neutron background limits detector sensitivity to WIMPs NEUTRON BACKGROUND SOURCES:



local radioactivity (surrounding rock, detector components)

- spontaneous fission 238 U - ( a ,

n

) reactions; a ’s from radioactive chains of U/Th 

muon-induced neutrons P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 8

Neutron background sources

1 neutrons from surrounding rock 238 U and (

a

,n) reactions flux:

F

rock ~ 3.8 × 10 -6 n/(s ·cm 2 ) @ CANFRANC ArDM input (preliminary): ~ 13200 n/day

supression: neutron shielding 2 neutrons from det. components 238 U and (

a

,n) reactions flux: „detector dependent”

ArDM input (preliminary): ~ 74 n/day

supression: high-purity materials 3 muon-induced neutrons – production in hadronic & e-m cascades init. by

m

’s flux:

F m

-ind ~ 1.7 × 10 -9 n/(s ·cm 2 ) @ CANFRANC ArDM input (preliminary): ~ 6 n/day

supression: active veto P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 9

Neutron background energy spectra

[1] rock [3]

m

-ind [2] detector components only fission spectrum plots 1 & 3 : M.J. Carson, J.C. Davies et al., Astroparticle Physics 21(2004) 667-687 P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 10

part III –

Geant4

neutron background studies

How many neutron recoils we will have in our experiment?

(how many neutrons will enter the detector? what energy spectrum? how many would interact and produce visible recoils? how many would undergo multiple scattering?) ArDM simulation tasks:

 

detailed detector geom. (Geant4) verifiaction of sim. processes (elastic scattering, neutron capture)

  

rock neutrons neutrons from det. components muon-induced neutrons



other background sources:

g

’s, 39 Ar full detector sim. (GRANADA) P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 11

Geant4

neutron background studies – elastic scattering in liquid Argon

Argon recoil spectra from G4 simulation T

n = 2 MeV

T

n = 5 keV

T

n = 15 MeV T

n <<

M

n n on relativistic

dN Ar T Ar

~

d



cos  

T n

 1  

m n



m

2

n M Ar

 2   cos  

d M m n

2 2

Ar d

s  cos  sin 2    2  

dT Ar

 (

M Ar



d d

s



cos  

m n

) 

T n



 2 

m n M



Ar M



m n

 2

Ar

 ( 1  cos  )

P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 12

Geant4

neutron background studies – rock neutrons analysis example

SIMULATION: - neutron source placed randomly on the walls of fiducial volume - neutrons going out of the detector are neglected

F n = 3.8•10 -6 n/(s ·cm 2 )

assumed total neutron flux at the walls of fiducial volume IN OUR GEOMETRY: 1 neutron per ~ 6.5 sec.

13200 neutrons per day OUTPUT INPUT neutrons from rock input energy spectrum P.Mijakowski

10 keV threshold Young Researchers Session, 8 January 2006, Cracow 13

Geant4

neutron background studies – rock neutrons analysis example no of incoming neutrons - correction for interacting neutrons - correction for multiplicity + correction for spatial resolution (2 cm) - shielding (CH 2 ) reduction factor 10 4 -10 6 P 13200 n/day int ~ 57% 7500 n/day P multi ~ 53% 7500 – 4000 = 3500 n/day 3700 n/day 1-135 events per year

P.Mijakowski

P P inter multi

Young Researchers Session, 8 January 2006, Cracow

~57 % ~53 %

assumed 2cm spatial resolution visible

95%

14

Summary & Outlook

• • •

ArDM: a new project aiming at developing and operating a 1 ton-scale liquid Argon detector for direct detection of WIMPs With a 1 ton prototype we want to show the validity of this design (in particular 39 Ar rejection) Neutron shieldings will be addressed in a second phase

•

Investigations on neutron background sources and their interactions inside the detector are also performed (simulations in Geant4):

–

evaluation of expected number of neutron events (data analysis)

–

simulations will help to specify requirements for detector veto and shielding SIMULATIONS OUTLOOK:

•

Energy spectra and flux of incoming neutrons

•

Detailed detector geometry in Geant4 P.Mijakowski

Young Researchers Session, 8 January 2006, Cracow 15

BACKUP

Geant4

neutron background studies – neutron capture in liquid Argon

capture on natural Argon ( 40 Ar - 99,6%, 36 Ar - 0.337%, 38 Ar - 0.063%) 6.099 MeV Initial neutron energy = 10 eV 6.598 MeV 8.788 MeV P.Mijakowski

Average number of

g

’s produced = 3.5

Young Researchers Session, 8 January 2006, Cracow 17