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 2007 - Paris

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

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)

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 

! 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 !

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 .

EDELWEISS-I and Cuoricino Direct search for Dark Matter Underground Laboratory of Modane located in the Fréjus tunnel 4800 m.w.e. 4μ /m²/d FRANCE Neutrinoless DBD Cuoricino (hall A) Underground National Laboratory of Gran Sasso located in the highway tunnel 3500 m.w.e. 24μ /m²/d ITALY

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

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

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

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

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

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

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

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.

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

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.

NbSi film equipped Ge bolometers: main results 109Cd source facing the top (A) NbSi sensor 109Cd energies and absorption lengths 18 keV electrons 4 µm charge deficit 22 keV X-ray 58 µm small deficit 25 keV X-rays 82 µm small deficit 62.5 keV electrons 34 µm charge deficit 84 keV electrons 55 µm charge deficit 88 keV gammas 2.5 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

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

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%) - 5443 (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

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

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).

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

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

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.