heavy radiation backgrounds in the lsbb · heavy radiation backgrounds in the lsbb tom girard, ana...

Heavy Radiation Backgroundsin theLSBB

Tom Girard, Ana Fernandes, Miguel Felizardo

Advanced Detectors GroupCentro de Física NuclearUniversidade de Lisboa

SIMPLE

gas

25 µm 1 mm

WIMP

superheated freon droplet

(i) E > Ec = 4πrc2γ/3ε

(ii) ∆E/∆x > Ec/arc

( Seitz : Phys. Fluids 1 (1979) 1 )

∆ E/ ∆ x ≥ 150 keV/µm

intrinsic BG rejection < 10-10

v-a-v

bolometers: 10-4,-5

xenon: 10-2

argon: 10-7,-8

• key feature: selective sensitivitytunable (P,T) double threshold device

The problem with rare event searches : S / B << 1

cosmogenics, radio-contaminants, ...

underground sitingφµ (surface) ~ 4 x 105 µ/hr

φµ (capsule) ~ 36 µ/hr

φµ (SG) ~ 105 µ/hr

but ....... you begin to see your mountain !

1260 n/hr

(0.4-4)x10-3 n/m2s-> 14 n/hr

( U/Th )

231Th227Ac

231Pr

223Fr

211Pb

219Rn

227Th

223Ra

215Po

207Tl

207Pb

232Th

228Ra

228Th

228Ac224Ra

208Pb

220Rn

216Po

212Pb

212Bi

212Po

208Tl

211Bi

235U14x109 y

α

α

α

α

α

α

α

α

7x108 y

α α

α

α

α

α

α

U / Th decay chainsα (α,n) spontaneous fission

Neutron shielding : low Z material (H, Li, B....

N(x) ~ N(0) e –x / λ

Common shielding materials :

� Concrete : ~ water

� Water : factor 5-10 reduction per 10 cm

� PE : ~ water

� Wood : factor 5-10 reduction per 20 cm

SIMPLE in GESA

Marine blue: 30-100 cm thick concrete

Cyan: 50-75 cm thick water(shield, pool)

Brown: 20 cm thick wood

Yellow: 1 cm thick steel

Grey: 50 thick concrete (floor)

alpha’s < 0.5 evt/kgd� ~4 x 10-3 evts/hr

232Th (Bq/kg) 238U (Bq/kg) 40K (Bq/kg)

LSBB 7.7 11 57

Canfranc-14m 60 30 880Canfranc-31m 8.5 4.5 37Canfranc-43m 23 9.8 43Canfranc-51m 76 31 680

GSasso ~8 ~83 --GSassso ~ 0 ~ 0 --

Modane ~ 10 ~ 10 --

shielding radio-assays

1 cm thick steel

30-100 cm thick concrete (± 4π)

232Th (Bq/kg) 238U (Bq/kg) 40K (Bq/kg)

LSBB 0.02 1.8 0.02

GESA MCNP simulations(schematic)

Figure 1. Room model with water shielding.Color code: yellow - water; blue - air;

magenta - concrete; green – SDDs.

(b) Top view

(a) Front view

Table 2. Simulation results.

File Sourcelocation

Sourcevolume

(103 cm3)

Shield’g F4 < 5 keV(x10-8)

F4 > 5 keV(x10-8)

bair Tank base 2961 No 81.9 ± 0.4% 10.4 ± 0.7%

bwat Yes 42.1 ± 0.2% 4.53 ± 0.5%

cair Ceiling andfloor

11216 No 65.8 ± 0.4% 8.49 ± 0.6%

cwat Yes 1.50E-1 ± 4% 6.00E-3 ±11%

fair End walls 320 No 12.3± 0.9% 1.41E-3 ±1.7%

fwat Yes 1.03E-3 ±22%

0

lair Side walls 10362 No 72.7± 0.4% 10.6 ± 0.6%

lwat Yes 7.60E-1 ± 2% 1.97E-2 ± 7%

concrete only: ~ 3000 evt /kgd

226Ra

222Rn

214Pb

218Po214Bi

214Po

210Pb

210Bi

210Po

206Pb

α

α

α α

1600 y

radon decayα, (α,n)

radon [ α , (α,n) ] : Cradon = 20 � 1500 α /m3s (seasonal)

Nα(x) ~ Nα(0) e – x / D

> Shielding : common materials

D (cm)

> concrete 6 – 80

> water 2

> plastics < 0.07

9cm H2O 4mm glass( 1000 α /m3s 11 α /m3s 0.04 α /m3s )

> Ventilation: air or N2 flow at 1 liter / hr

x ≥ 3D � “radon tight”

+ monitor’g

ventilation

shielding....

Instrumentation

6.3 6.4 6.5 6.6 6.7 6.8 6.9−3

−2

−1

0

1

2

3

Time [s]

Am

plitu

de [V

]

0 2000 4000 6000 8000 10000 12000 14000 16000−90

−80

−70

−60

−50

−40

−30

−20

−10

639.51 Hz

Frequency [Hz]

Pow

er [d

B]

0 2000 4000 6000 8000 10000 12000 14000 16000−90

−80

−70

−60

−50

−40

−30

−20

−10

3.44 kHz

Frequency [Hz]

Pow

er [d

B]

microleak FFT

0 500 1000 1500 2000−90

−80

−70

−60

−50

−40

−30

−20

−10

34.37 Hz

Frequency [Hz]

Pow

er [d

B]

fracture FFT

0 20 40 60 80 100−100

−95

−90

−85

−80

−75

−70

−65

−60

−55

−50

40.33 Hz

Frequency [Hz]

Pow

er [d

B]

trapped N2 gas FFT

MCE-200 microphone+

PGA2500 adaptive amplifiernucleation

event

SIMPLE measures ( 27/10/2009 – 05/02/2010 )

3.43 ± 0.51 ± 0.12 evt/kgd

consistent with diffusionof 15 Bq/m3 radon level

1.11 ± 0.28 ± 0.22 evt/kgd

consistent with MCNP1.09 ± 0.02 ± 0.07 evt/kgd

based on materials’radio-assays

450 500 550 600 650 700 750100

1000

10000

100000

Pow

er [u

W]

Frequency [Hz]

true event

103 µW

α

n

CONCLUSIONS

1. Neutron flux into GESA ~ 3000 evt / kgd

> mainly from concrete (rock shielding efficient)

> below health & safety standards (LSBB NOT a reactor ! )

> of possible concern to some experiments

> reduced to 1 evt / kgd in SIMPLE

2. Radon flux into GESA ~ 0.26 - 3.9 x 108 evts / kgd

> consistent with 15 Bq /m3 ( x10 < 102 measured – water flow?)

> sometimes above health & safety standards ( ~ 1/2 year)

> of possible concern to some experiments

> of concern to human activity...

> reduced to ~ 2.8 evt/ kgd