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
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