BGO 45.7 g
ROSEBUD is dedicated to develop and optimize scintillating bolometers of different materials and to use them in Nuclear and
Particle Physics experiments focusing on DM search
Integrated in Supported by
• CNRS (Centre National de la Recherche Scientifique)• PNCG (Programme National de Cosmologie et Galaxies)• CICYT (Comisión Interministerial de Ciencia y Tecn.)• DGA (Gobierno de Aragón)
PRESENT OBJECTIVESPRESENT OBJECTIVES
N. Corona, C. Cuestab, E. Garcíab, J. Gironneta, J. Leblanca, P. de Marcillaca, M. Martínezb, Y. Ortigozab, A. Ortiz de Solórzanob, C. Pobesb, J. Puimedónb, T. Redona, T. Rolónb, M.L. Sarsab,
L. Torresa, J. A. Villarb
a Institut d’Astrophysique Spatiale, CNRS & Université Paris Sud, Bât 121, 91405 Orsay, Paris, France.b Laboratorio de Física Nuclear y Astropartículas, Universidad de Zaragoza, 50009 Zaragoza, España.
BGO SCINTILLATING BGO SCINTILLATING BOLOMETERBOLOMETER
Its application in dark matter experimentsROSEBUD (Rare Objects ROSEBUD (Rare Objects SEarchSEarch
with Bolometers with Bolometers UnDergroundUnDerground))
TAU
P 20
09TA
UP
2009
OutlineOutline
The BGO scintillating bolometerROSEBUD experimental set-up in the LSCBGO as WIMP detector and as γ-rays spectrometer
(tests performed in the Canfranc Underground Laboratory)
Characterization of the BGO scintillating bolometer Energy partition (αl
/αh ratio, scintillation at low T, Ge
optical bolometer efficiency)Energy estimate of saturated β/γ
and α
events
BGO heat and light relative efficiency factors (REFs)Conclusions
The BGO scintillating bolometer
252Cf
heat
light
heat
light
ROSEBUD Experimental Set-up in Canfranc Underground Laboratory (LSC)
LiFMass 33 g
Sapphire (Al2
O3
)Mass 50 g
Low background environment at LSC (2450 m.w.e.)• Internal shield (Cu & Pb pieces)
• Externnal shield (Pb, polyethylene, μ-metal, Rn removal)
BGO (Bi4
Ge3
O12
)Mass 46 g209Bi 100%: ↑A (heavy material) with J = 9/2It’s sensitive SI and SD interactions (209Bi, 73Ge)207Bi contamination (~3 Bq/kg) β/γ
spectrometer Z↑
BGO as WIMP detector Particle discrimination threshold and nuclear recoils
252Cf calibration
Lower particle discrimination threshold and background are needed
Background
BGO as γ-
rays spectrometerSpectrum of 71.94 h
Peak energy (keV)
FWHM(keV)
∼15 3.4 ± 0.1
88.0 3.3 ± 0.0
242.0 5.0 ± 1.5
295.2 4.4 ± 0.5
351.9 4.9 ± 0.3
511.0 6.8 ± 1.9
569.7 7.5 ± 0.1
584.7 7.6 ± 0.5
609.3 7.4 ± 0.3
657.7 7.7 ± 0.4
768.4 7.3 ± 2.2
1063.6 11.0 ± 0.3
1120.3 12.3 ± 1.7
BGO and experiment background
Scintillating bolometer: Energy partitionPhoton interaction
E
αl ·
E αh ·
E α0 ·
E
Not detectableTraps, lattice defects …
Heat channel
ε
ε
· αl ·
E
Light channel
αl
+ αh + α0 = 1
BGO αl
/ αh ratio
88.0 keV
β/γ
events Mean weighted αℓ
/αh = 0.125 ±
0.021
BGO scintillation at low T
BGO light response constant below 100 K:
23700 ± 2600 photons/MeV @ 6 KJ. Gironnet et al. NIM A 594 (2008) 358-361
Bi4 Ge3 O12 scintillation study with α
irradiation (241Am) down to 6 K
IAS (fr), Paul Scherrer Institut (ch), University of Oxford (uk)
⟨Ephoton ⟩
= 2.4 eV @ 77 K
Measured intensity at 77 K obtained by Johann Gironnet at IAS (fr), Paul
Scherrer Institut (ch)
BGO light yield: αl
= 0.057 ± 0.006
Ge
optical bolometer efficiciency
Ge optical efficiency estimated with 55Fe x-rays 5.9 keV
)keV3.841(keV9.5
EE
deposited
absorbed
ll α=
α=ε
BGO Ge optical sensor efficiency
%1201.012.0 →±=ε
Energy partition in BGOPhoton interaction
E
αl
·
E αh ·
E α0 ·
E
Not detectableTraps, lattice defects …
Heat channel
ε
ε
· αl
·
ELight
channel
α0 = 0.487 ±
0.090αl
= 0.057 ±
0.006 αh = 0.456 ±
0.090BGO
ε
= 0.12 ±
0.01
Negative correlation:
αℓ
/αh = 0.125 ±
0.021
Scintillation at low temperature:
αl
= 0.057 ±
0.00655Fe x-rays calibration:
ε
= 0.12 ±
0.01
• αl
+ αh + α0
= 1:
α0
= 0.487 ±
0.091
Full characterization including energy trapping
Energy estimate of saturated β/γ
eventsLow energy β/γ
saturated events
αsa
tura
ted
even
ts
High energy β/γ
and α
saturated events
1063
.7 k
eV11
20.3
keV
609.
3 ke
V56
9.7
keV
1633.4 keV (207Bi)
1764.5 keV (214Bi)1633.4 keV (207Bi)
1764.5 keV (214Bi)
Light channel 1400Heat amplitude
Light channel 1475Heat channel 1700
Energy estimate of saturated α
events Irradiation with 241Am α
source
Low Gain
Low GainHeat spectrum
α
eventsHigh Gain
Heat spectrumα
eventsThis method allows us to identified events from the background but loosing energy
resolution!
High Gain
Light channel 1215Heat channel 1205
α events not saturating α events saturating
β/γev
ents
β/γ e
vent
s
Heat REF(237Np nuclear recoils:β/γ)BGO irradiation with 241Am α
source
Energies of 237Np recoils92.71 ±
0.17 keV
(84.5%)
92.05 ±
0.18 keV
(13.0%)
Qα
= 5637.81 ±
0.12 keVEα
= 5485.56 ±
0.12 keV
(84.5%)Eα
= 5442.80 ±
0.13
keV
(13.0%)241Am → 237Np + α
Heat REF(237N nuclear recoils:β/γ)
Heat REF(237Np recoils:β/γ)
0.937 ±
0.015stat syst302.0004.0
+−
Mean weighted kinetic energy237Np nuclei
92.40 ±
0.09 keV
Heat pulse amplitude electron recoilsKinetic energy 92.40 ±
0.009 keV
912.56 ±
0.85stat syst mV20.273.294
+−
Heat pulse amplitude237Np recoils
854.96 ±13.36stat ±
2.98syst mV
BGO irradiation with 241Am α
+ 252Cf n sources
Light REF(β/γ:nuclear recoils)
Recoils induced by n
(mainly oxygen recoils)
Recoils induced by n
(mainly oxygen recoils)
237Np recoils237Np recoils
Light REF(β/γ:nuclear recoils)
Light REF(β/γ:nuclear recoils)
Light REF(β/γ:nuclear recoils(mainly O))
10.5 ±
0.4stat ±
0.8syst
Light REF(β/γ:237Np nuclear recoils)
39.5 ±
7.2stat ±
13.7syst
Conclusions1)
The BGO scintillating bolometer has been tested in the LSC showing discrimination capabilities of 23.5 keV
(90% CL) for
WIMP detection. But, we need lower threshold and background in the nuclear recoil band for dark matter experiments.
2)
BGO shows excellent capabilities as γ-rays spectrometer allowing to analyze the background of the experiment. Identification and estimation of the energy of saturated events in low energy experiments (dark matter) is also feasible.
3)
The energy partition (light, heat and trapping) has been studied.
4)
We have estimated heat and light REF for nuclear recoils identifying different light scintillation values dependent on the recoiling nucleus mass.