1. Status of Surface Sensitive Bolometers University of Insubria – Como, Italy INFN – Milano, Italy Prague, 20.04.2006 Chiara Salvioni

2. Outline of the presentation (1) Outline of the presentation (1) Topics to be discussed Brief summary of Surface Sensitive Bolometers activity Latest experimental tests Detector simulations Next SSB test @ LNGS

3. Surface background in CUORE Surface background in CUORE Goal:  (  ) of 130 Te ~ 1000 TeO 2 bolometers Q  = 2530.3 keV Predictions on the future background expected for CUORE from Cuoricino background analysis and Monte Carlo simulations... Experimental data and simulations suggest one major contribute for CUORE background in the DBD region:  and  degraded particles emitted by 238 U and 232 Th surface contaminations on the Cu frame and on the crystal surface. BKG = 0.18 ± 0.01 c/(keV kg y) T 1/2  > 2×10 24 y @ 90% C.L. Cuoricino   130 Te TeO 2 Cu TeO 2

4. Surface Sensitive Bolometers Surface Sensitive Bolometers Background reduction may be achieved through both passive and active methods Creation of a new kind of detectors able to recognize surface events Identification of background events S urface S ensitive B olometers Auxiliary bolometer Main bolometer SSB Classic pulse High and fast pulse Dynamic behavior: Event originating inside the main bolometer (DBD event) Event originating outside the main bolometer (degraded  ) The difference between heat capacities generates a difference in pulse height and shape... Idea: cover each face of a classic bolometer by gluing an active layer, in order to provide a 4  shielding

5. First SSB experimental results (Como) First SSB experimental results (Como) Amplitude comparison According to the described dynamic behavior, various pulse parameters proved to be effective in discriminating surface events. (Scatter plot) -Individual thermistor read-out -Parallel thermistors read-out  r on auxiliary thermistor Bulk events Surface events Pulse amplitude on auxiliary NTD [mV] Pulse amplitude on main NTD [mV]  d on main thermistor Surface events Bulk events Pulse amplitude on main NTD [mV] Pulse amplitude on auxiliary NTD [mV] “Slow” bulk events “Fast” surface events (To be investigated) Pulse amplitude on main NTD [mV] Pulse decay time on main NTD [ms]

6. Outline of the presentation (2) Outline of the presentation (2) Brief resume of Surface Sensitive Bolometers activity Latest experimental tests Detector simulations Next SSB test @ LNGS

7. Recent LNGS tests – Run 1 Recent LNGS tests – Run 1 SSB Run 1 Tests performed in Como had all small main TeO 2 absorbers (2 cm 3 ); moreover, various shield materials and different techniques to couple layers and crystals had already been tried. Features: Main absorber 5×5×5 cm 3 Full coverage of 4 TeO 2 crystals Si active shields Parallel thermistors read-out

8. Run 1: scatter plot Run 1: scatter plot Surface events Bulk events Mixed events Recap of the results: Parallel read-out

9. Run 1: decay time on main thermistor Run 1: decay time on main thermistor We also found a structure in the decay time distribution vs amplitude for pulses read by the thermistor on the main TeO 2 crystal: Pulse decay time on main NTD [ms] Pulse amplitude on main NTD [mV] SSB Pulse decay time on main NTD [ms] Pulse amplitude on main NTD [mV] Not shielded 3D plot Note

10. Recent LNGS tests – Run 2 Recent LNGS tests – Run 2 SSB Run 2 Aims Features: Main absorber (small) 2×2×0.5 cm 3 One TeO 2 crystal with two shields (no full coverage) TeO 2 active shields Independent thermistors read-out Alpha source implanted in two different points of the detector TeO 2 shield 1 TeO 2 shield 2 Vacuum grease Glue Alpha source -verify how contaminations in different points of the detector contribute to scatter plots -read shield thermistors independently Very important to understand if we can identify contaminations that are not just external, but also internal to the detector itself

11. Run 2: scatter plots (1) Run 2: scatter plots (1) Shield 1 (implanted) Surface events Bulk events Mixed events 3D plot 238 U  ’s (~4.2 MeV) 234 U  ’s (~4.7 MeV) Surface events Bulk events Mixed events Shield 2 (facing the implanted side of the main crystal)

12. Run 2: scatter plots (2) Run 2: scatter plots (2) Shield 1 (implanted) Surface contaminations of the thin TeO 2 layer MC simulation Origin: due to nuclide recoil, there is a fixed maximum energy that can be released in the main absorber (dependence on contamination depth)

13. Run 2: rise time on shield thermistors Run 2: rise time on shield thermistors Shield 1 (implanted) Shield 2 (facing the implanted side of the main crystal) Rise time on shield 2 Amp. on main Amp. on shield 2 Surface events – fast signals:  r ~ 2 ms

14. Run 2: decay time on main thermistor Run 2: decay time on main thermistor Bulk events Surface events –shield 1 Surface events –shield 2 3D plot

15. Outline of the presentation (3) Outline of the presentation (3) Brief resume of Surface Sensitive Bolometers activity Latest experimental tests Detector simulations Next SSB test @ LNGS

16. Thermal model of SSBs Thermal model of SSBs C1C1 9.9x10 -9 ·T C6C6 4.3x10 -8 ·T 3 g 50 1.8x10 -5 ·T 2.4 C2C2 2.7x10 -8 ·T 3 g 20 9.6x10 -5 ·T 2.4 g 54 1.3x10 -1 ·T 4.37 C3C3 2.3x10 -3 ·T 3 g 21 7.0x10 -1 ·T 4.37 g 63 5.2x10 -4 ·T 3 C4C4 1.9x10 -9 ·T g 30 4.0x10 -5 ·T 2 g 65 2.6x10 -4 ·T 3 C5C5 5.1x10 -9 ·T 3 g 32 2.3x10 -3 ·T 3 T0T0 9 mK 6-node model Shield/crystal, NTD/shield and NTD/crystal thermal couplings realized with glue “Big” TeO 2 crystal (CUORE size) with one Si shield Thermistor/heat bath: gold wires Main crystal/heat bath: PTFE Parameters for static & dynamic simulations Initial values

17. Test 0: decay time on main thermistor Test 0: decay time on main thermistor Focus on: pulse decay time in the main thermistor read-out Test #0 Tests performed varying one parameter at a time (or a group of connected parameters) Energy released In the shield Energy released In the main absorber The decay time growth vs amplitude reflects the behavior observed in experimental tests

18. Tests 1 & 2 [links to the heat bath] Tests 1 & 2 [links to the heat bath] Test #1 Test #2 g 50 x 6 (shield thermistor gold wires) g 30 x 10 (main/bath PTFE) Shield energy rel. Bulk energy rel. Shield energy dep. Bulk energy dep. Shield energy dep. Bulk energy dep. @ 3.1 MeV

19. Tests 3 & 4 [passive auxiliary NTD] Tests 3 & 4 [passive auxiliary NTD] Test #3 Test #4 g 50 = 0 (shield thermistor not polarized –passive node) g 54 C 4 C 5 x 6 (shield thermistor as a passive node and with larger volume) Shield dep. Bulk energy dep. Shield dep. Bulk energy dep. Shield dep. Bulk dep. @ 3.1 MeV This decay time vs amplitude trend corresponds to experimental results g 50 = 0

20. Tests 5 [shield heat capacity] Tests 5 [shield heat capacity] Test #5 C 6 x 100 g 50 = 0 (larger shield heat capacity and passive aux. thermistor) Shield energy dep. Bulk energy dep. Shield energy dep. Bulk energy dep. @ 3.1 MeV A thermal add-on to the slab?

21. Outline of the presentation (4) Outline of the presentation (4) Brief resume of Surface Sensitive Bolometers activity Latest experimental tests Detector simulations Next SSB test @ LNGS

22. Next LNGS test: Run 3 Next LNGS test: Run 3 SSB Run 3 Features: “Big” main absorbers: 5×5×5 cm 3 4 SSBs featuring total coverage (6 shields each) TeO 2 active shields (reasons: -thermal contractions with main absorber -known material) 2 SSBs have thermistors on each layer and independent read-outs 2 SSBs have 5 “passive” slabs (no thermistor) and one readable slab Thicker TeO 2 slabs (0.9 mm) to avoid known mechanical problems Great attention for clean working conditions in order to avoid possible contamination sources

23. Run 3: single SSB Run 3: single SSB Single detector assembly

24. Run 3: assembly Run 3: assembly 4-SSB module

25. Run 3: remarks Run 3: remarks The 4-SSB module will be cooled down with other 8 unshielded crystals (possibility to compare background results) Presence of 5 “passive” slabs on 2 SSBs to understand if they work as pulse shape modifiers; the remaining shield thermistor will help evaluate if event discrimination by main channel read-out is possible Test of background reduction