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Future Dark Matter Detectors at LNGS
Water and Liquid Scintillator Shielding Concepts to Suppress Neutron Backgrounds

Frank Calaprice WONDER Meeting LNGS, March 22, 2010


Future of DM Searches
Current sensitivity (CDMS, Xenon): ~10 10 10
-47 -48 -44

cm2

Desired sensitivity & exposure for xenon/argon detectors: cm2: cm2: 1 ton-yr Xe, 10 ton-yr Ar 10 ton-yr Xe, 100 ton-yr Ar

Fundamental backgrounds limit ultimate sensitivity to 10-48 - 10-49 cm2 Solar pp neutrino-electron scattering for Xe Atmospheric neutrino-nuclear coherent scattering Xe/Ar Need multi-ton detectors, negligible background, low threshold energy.


Future DM Detectors at LNGS
DM detectors with exposures < 100 kg-yr are relatively safe from cosmogenic neutrons at LNGS, even with modest shielding. Multi-ton-yr exposures at LNGS face serious backgrounds from cosmogenic neutrons produced in the rock and passive shields, due to the shallow depth. Merging the successful Borexino technology of water and liquid scintillator shields with DM detectors can render cosmogenic neutrons harmless at LNGS. This talk evaluates the water-scintillator shields for dark matter detectors and suggests a straightforward program to develop them at LNGS. In addition to mitigation of cosmogenic neutrons, the choice of an active liquid scintillator veto can provide extremely powerful rejection of radiogenic neutrons.
Key is the adoption on novel scheme in neutron detection:
10



B(n,)7Li


Borexino Concepts Applied to Dark Matter
Borexino is a 300-ton liquid scintillator designed to detect solar neutrinos. Borexino achieved unique, unprecedented low backgrounds with active water and liquid scintillator shields. Successful measurements include:
7Be and 8B solar neutrinos Geo-neutrinos Nuclear reactor anti-neutrinos from sites > 1000 km away. More on the way...


The Borexino Detector
Outer Water Detector Inner Detector Buffer Scintillator Fiducial Volume (100 tons)
Steel plates in concrete for extra shielding10m x 10m x 10cm 4m x 4m x 4cm Rope tendons 2200 Thorn EMI 8" PMTs (1800 with light collectors 400 without light collectors) External water tank 18m Stainless steel sphere 13.7m Nylon outer vessel 11.0 m Nylon inner vessel 8.5m Fiducial volume 6.0m

Water Buffer Scintillator


Borexino Filled 2007


Borexino Results

7

Be Solar Neutrinos (2008)

Geo-neutrinos (2010)

~4 geo-neutrinos/(100 ton-yr) with negligible neutron background Defeated neutron-related background (<<1 background event/(250 ton-yr))


Borated Liquid Scintillator Best Active Veto for Radiogenic Neutrons
Radiogenic neutrons major background issue for all dark matter experiments:
Radiogenic neutrons emitted from detector parts.
(,n) reactions and fission from U, Th in PMT's, cryostat,... surfaces contaminated with 210Po External neutrons

(n,n') mimics WIMP events (W,W')

Borated scintillator was studied for BOREX to measure charged and neutral current solar neutrino rates (Raghavan). Suppress radiogenic neutrons with active veto made of boron loaded scintillator:
Detect charged particles:
10

B(n,)7Li:

thermal

= 3800 b


Neutron Capture on Boron
B + n -> 7Li*+ , 7Li* -> 7Li + (480 keV) -> 7Li (g.s.)+ Q-value: + 2.79 MeV
10

94% 6.7%

Excited state: E() = 1471 keV; Ground state: E() = 1775 keV;

E(7Li) = 839 keV E(7Li) = 1014 keV

Quenching of reaction products: ~ 60 keVee. (~1/40) Neutrons travel less than 20 cm before capture. Unlike (n,) capture, detection of charged particle products is very efficient:
Neutron veto efficiency: ~99.8% for 1-m thick scintillator. Limited by invisible capture on inert detector materials: many 9's possible for neutrons not captured within detector.


Liquid Scintillator
Veto for external cosmogenic neutron
Most serious background comes from cosmogenic neutrons generated in the rock by muons. Above a few hundred Mev, the neutrons flux decreases and contributes little to background. At lower energies the neutrons are more easily absorbed by shielding.
10 10 10 10 10 10
11 12 13 14 15 16

Neutron flux cm2 s

D. Mei, A. Hime PRD 73 053004 (2006)

1

0

500 1000 1500 2000 2500 3000 3500 En MeV

E n 90 MeV mean energy of neutrons




Liquid Scintillator
External Cosmogenic Neutron Veto
Neutron Energy 10 MeV 50 MeV 100 MeV >200 MeV Mean Free Path in Liquid Scintillator ~ 30 cm ~50 cm ~55 cm ~70 cm Attenuation Length in Water ~25 cm ~50 cm ~80 cm ~120 cm

Fast neutrons with E ~ 100 MeV are difficult to stop in passive shield of water. Liquid scintillator of same thickness is more effective than water because it offers a veto signal based on mean free path, in addition to absorbing the neutrons. Scintillator thickness effectively doubled by requirement that neutron must pass through the scintillator twice (in and out) to mimic WIMP signal.


Neutron Attenuation


Muon Flux versus Depth
SNO-Lab at Sudbury is at depth of ~6000 mwe. Muon flux at SNO-Lab:
~70 times lower than LNGS.

Shallower depth of LNGS can be overcome with water or scintillator shielding equivalent to 4-5 attenuation lengths. For 100 MeV neutrons this is ~ 4-5 meters of water or scintillator. Detailed simulations are needed for better estimates.


A Case Study: Dark Matter with DAr
A DAr detector with depleted argon has been proposed as the prototype of a series of a large-scale dark matter detectors (See talk of C. Galbiati tomorrow.) Novel low background features make this a powerful instrument to search for dark matter, as well as a prototype for new technology.
Underground argon depleted in 39Ar Low background high quantum efficiency photodetectors Boron-loaded scintillator for efficient rejection of neutron backgrounds. Two-phase TPC detector with double discrimination against / events, based on pulse shape and


DAr Detector in Water and Scintillator
11 m CTF Water Tank (11 m x 10 mH) Borated Liq. Scint. 3 m x 4 mH (25 tons) with 50 PMT's 10 m Xe/Ar detector PMTs for Cerenkov Detection of Muons Depleted Ar Detector in cryostat


Cosmogenic Neutrons
With passive shielding:
40 cm polyethylene. + 20 cm Pb + 15 cm Steel: ~3,000 background events/(ton-yr)

With active (muon) shielding accomplished with 1 m of liquid scintillator or 5 m of water:
~2 events/(ton-yr) (10
-46

cm2 in 1 ton-year of DAr)

With active shielding, 4 m water + 1 m borated scintillator:
<0.1 events/(ton-yr) (10
-47

cm2 in 10 ton-years of DAr)

With 5 m scintillator:
Background is tiny!


Background and possible reach with Borexino size water/scintillator
MEASURED: Dark matter background can also be measured by Borexino
Muon crossing BX, WT, fast cosmogenic neutron enters BX IV, induces proton recoil, then captures in BX IV Distinctive signal of proton recoils in BX scintillator: excellent pulse shape discrimination First experimental indication from geo-anti-nue analysis: <<1 event/ (10 ton-yr) when extrapolated to a Xe/Ar recoil [nominally equivalent 10-47 cm2 for Ar, 10-48 for Xe]

ESTIMATED: Cosmogenic neutron-induced background calculated in Borexino (geo-anti-nue paper):
Fast cosmogenic neutron enters BX IV, induces proton recoil, then captures on 1H Background estimate in geo-neutrino paper is <<1 antinuebackground event/(250 ton-yr) [nominally equivalent 5x10-49 cm2 for Ar, 5x10-50 cm2 for Xe]


Conclusions
Large liquid scintillators are very effective in vetoing both radiogenic and cosmogenic neutrons. Water/scintillator shields can reduce cosmogenics to the levels of the deepest underground laboratories and enable ultimate dark matter experiments at LNGS. Possibility of achieving a background of ~10-48-10 cm2 at shallow LNGS depth can be evaluated experimentally using current Borexino data.
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