1. Surface event tagging with NbSi films
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse - Orsay
Surface event tagging
with NbSi films
Claudia Nones
IDEA Meeting – 19/20 November Paris

2. NbSi film equipped bolometers Surface sensitive composite bolometers
Outline
The bolometric technique in the search for rare events
The problem of surface radioactivity
Two examples: EDELWEISS-I and Cuoricino
NbSi film equipped bolometers
How to fight against surface radioactivity
Surface sensitive composite bolometers
Conclusions and prospects

3. The bolometric technique
This technique measures all the energy deposited by particle in form of increase of temperature in the absorber
The original idea is very simple:
heat sink
thermal coupling
thermometer
incident
particle
crystal absorber
Absorber
DBD source
suitable WIMP target
From a very simple thermal model:
Signal:
ΔT = E/C
Time constant = C/G
Different thermometers
thermal phonons (NTD)
-> to develop high pulses the detector has to work at low temperatures (10 – 50 mK).
athermal phonons (NbSi film)

4. Thermal and athermal phonons
Energy
Phonon
number
Thermal phonons T
Before quantum interaction 
Energy
Phonon
number
Athermal phonons
Immediately after quantum interaction 
Detector operating here:
Energy
Type of quantum
Position
Initial Momentum
Detector operating here:
Perfect calorimeter
Energy
Energy
Phonon
number
Athermal phonons
Phonon energy degradation 
Energy
Phonon
number
Thermal phonons
T + T
New thermal distribution 

5. ! Background sources SOURCES
In rare event searches there are several critical background sources:
Neutrons from natural radioactivity and induced by muons
Internal contamination from natural radioactive chains
External 
SOURCES
Surface contamination of the detectors or nearby materials
Cosmogenic isotopes, produced by activation due to cosmic rays during production and transport
LIMITING
FACTOR !

6. WHY? Surface contamination in rare event search 0νββ experiment:
Surface contamination of the detectors or nearby materials
Near surface events
Main problem for different rare event experiments
(CUORE --> 0vββ, EDELWEISS --> DM)
WHY?
0νββ experiment:
Dark Matter experiment:
they may cause counts in the energy spectrum close to the Q-value, where the signal is expected.
they originate an incomplete charge collection simulating a nuclear recoil, when an ionization signal is used for particle identification .

7. EDELWEISS-I and Cuoricino
Direct search for Dark Matter
Underground Laboratory of Modane located in the Fréjus tunnel m.w.e μ /m²/d
FRANCE
Neutrinoless DBD
Cuoricino
(hall A)
Underground National Laboratory of Gran Sasso located in the highway tunnel m.w.e μ /m²/d
ITALY

8. Surface background in Cuoricino
TOTAL Cuoricino BACKGROUND SPECTRUM
214Bi
60Co
208Tl
~ 0.2 c / keV kg y
Gamma region
Alpha region, dominated by peaks and by their tails due to shallow contamination

9. Surface background in EDELWEISS
 surface contamination as in Cuoricino
’s are not the direct problem, but they are associated to β surface emission.
Surface β’s + incomplete charge collection
mimicking nuclear recoils

10. How to fight against this problem? (1)
Three different approaches are possible:
Increase the quality of the surface treatment both of crystals and of all the other materials (such as copper) facing them that can be translated into better cleaning procedures. 1
Review the design geometry of the detector mounting structure, in order to minimize the copper surface and to gain efficiency from anticoincidence between closer detectors. 2
Realize clever bolometers able to discriminate the origin of the events providing information on the particle impact point exploiting different mechanisms of solid state physics. 3

11. How to fight against this problem? (2)
Main goal: understand the origin of the events
Using the dynamics of the heat flow in the detector:
Using the athermal component of the signal:
Information on the particle impact point
ALREADY DISCUSSED IN PREVIOUS IDEA MEETINGS
able to identify events due to energy deposited at the detector surface. This capability is obtained by thermally coupling thin active layers to the main energy absorber of the bolometer, and is demonstrated by irradiating the detectors with α particles.
New techniques for an active background discrimination
Surface Sensitive composite Bolometers
NbSi film equipped bolometers

12. NbSi film equipped bolometers NbSi film equipped bolometer
crystal absorber
with interdigitized electrodes
NbSi film equipped bolometer

13. NbSi film equipped bolometers: pulse features
Typical pulse shape from a NbSi sensor
Direct absorption of athermal phonons by the NbSi
Double exponential decay time
Thermal relaxation of the whole detector to the heat sink
Athermal component
Thermal component

14. NbSi film equipped bolometers
BULK EVENT
pulse with enhanced athermal component
pulse with reduced
athermal component
pulse with reduced
athermal component
pulse with reduced
athermal component
pulse with enhanced athermal component
Dire che la capacità è trascurabile.
able to identify events due to energy deposited at the detector surface. This capability is obtained by thermally coupling thin active layers to the main energy absorber of the bolometer, and is demonstrated by irradiating the detectors with α particles.
Impulso classico nel senso di quelli soliti visti da bolometri in cuoricino
Dire a che cosa si riferiscono gli eventi:bb o alfa proveniente da contaminazione superficiale
The main goal of this method is to understand the origin of the events.we know that other groups have achieved this goal using out of equlibrium phononDIRE CHE si vuoleva cercare un metodo per capire la provenienza degli eventi che si sa che questa cosa è già fatta da altri gruppi tramite fononi fuori equilibrio (out of equilibrium) ma che per cuore nn si vuole rivoluzionare il read out
SURFACE EVENT
N.B.:1 film would be enough for discrimination using pulse shape analysis

15. NbSi composite bolometers: the scatter plot
Plotting the athermal pulse amplitude from film A vs the athermal pulse amplitude from film B:
Ath_amplitude
Film B
Film A
surface event band
bulk event band
A bulk event would give similar
fast components in the two films.
A degraded alpha pulse, with the same fast component as a bulk event, would give a very high pulse in the film where the event occurs and a normal fast component in the other one.
Pensiamo di avere un evento…in coincidenza sui 2 termistori
Graficando l’ampiezza dell’impulso sul termistore di germanio contro l’ampiezza su quello del TeO2 per un medesimo evento si ottiene uno scatter plot che mostra due curve ben diverse a seconda che questo evento si verifichi nel tellurio o nello schermo di Ge.

16. Rejection criteria 2bulk 2surface + 1  SURFACE EVENTS  k parameter
- 1
+ 1
Energy
k parameter
Bulk events  
2surface
2bulk
CUT: 2surface > 2bulk
BULK EVENTS
SURFACE EVENTS

17. NbSi film equipped Ge bolometers
Different tests have been done at CSNSM in Orsay (France) to find the best configuration for films.
200 g or 400 g Ge absorber
a-Si sub-layer
Nb or Al electrodes (guard) – Nb 500 µm interdigitized electrodes (centre)
Two a-NbxSi1-x thin film sensors (60 nm thick, x~0.085)
Pd heating, Au pad thermal link
2 x 400g and 2 x 200g NbSi bolometers actually cooling down at the LSM underground laboratory.

18. NbSi film equipped Ge bolometers: main results
109Cd source facing the top (A) NbSi sensor
109Cd energies and absorption lengths
18 keV electrons µm charge deficit
22 keV X-ray µm small deficit
25 keV X-rays µm small deficit
62.5 keV electrons µm charge deficit
84 keV electrons µm charge deficit
88 keV gammas mm no deficit
Localization factor:
rejects 97% of near surface events
rejects events less than 2 mm underneath the NbSi
no energy dependence below 100 keV
109Cd calibration -3V
non-rejected
events
After cuts, there is a reduction of the fiducial volume.
Sea level

19. B212: data analysis
Cd source
ALL DATA
CUT |k| > 0.3
AFTER CUT

20. NbSi film equipped TeO2 bolometers
Why not to apply this tecnhique to a TeO2 crystal?
It could be very useful for the 0νββ community.
The FIRST protoype
241Am external source
2 cm
Main lines:
α lines : 5486 (85%) (13%) keV γ lines: 60, 24, 18, 14 keV
NbSi: 14x14 mm2 , thickness: 650 Å
500 Å of Ge amorphous under layer
electrodes: 400 Å of Nb + 40 Å of Ir

21. NbSi film equipped bolometers on TeO2: results
Ath_B [keV]
Ath_A [keV]
Bulk event line
 lines
60 keV line
Two different slopes are clearly visible.
ath_A ~ ath_B  bulk events
ath_A > ath_B  surface events

22. NbSi film equipped bolometers on TeO2: results
Ath_B [keV]
 lines
60 keV line
Bulk event line
ath_A/ath_B ~ 0  bulk events
ath_A/ath_B ~ 0.15  alpha events
0.05 < ath_A/ath_B < 0.35  60 keV events
Alpha events: pure surface events.
60 keV events: across surface and bulk event regions (typical absorption profile).

23. NbSi film equipped bolometers on TeO2: results
2surface
2bulk

24. NbSi film equipped bolometers on TeO2: results
Ath_B [keV]
2surface > 2bulk
Cut:

25. Conclusions and prospects
Surface background is a problem for both DM and 0νββ communities.
The identification of surface events is a powerful technique for background tagging and diagnostic.
Two different techniques have been developed to recognize surface events.
One technique used in the DM context has been successfully transfered to 0νββ one.
Future tests are foreseen to improve and optimize the two techniques.