Status of Surface Sensitive Bolometers

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

2. 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. bb (0n)130Te BKG = 0.18 ± 0.01 c/(keV kg y) Cuoricino T1/20n> 2×1024y @ 90% C.L. a and b degraded particles emitted by 238U and 232Th surface contaminations on the Cu frame and on the crystal surface. Experimental data and simulations suggest one major contribute for CUORE background in the DBD region: Cu TeO2 TeO2 Surface background in CUORE Q = 2530.3 keV Goal:() of 130Te Predictions on the future background expected for CUORE from Cuoricino background analysis and Monte Carlo simulations... ~ 1000 TeO2 bolometers

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

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

6. 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 Tests performed in Como had all small main TeO2 absorbers (2 cm3); moreover, various shield materials and different techniques to couple layers and crystals had already been tried. SSB Run 1 Features: Main absorber 5×5×5 cm3 Full coverage of 4 TeO2 crystals Si active shields Parallel thermistors read-out

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

9. Not shielded Pulse decay time on main NTD [ms] Pulse amplitude on main NTD [mV] 3D plot 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 TeO2 crystal: Note SSB Pulse decay time on main NTD [ms] Pulse amplitude on main NTD [mV]

10. Vacuum grease TeO2 TeO2 shield 2 TeO2 shield 1 Glue Alpha source Recent LNGS tests – Run 2 -verify how contaminations in different points of the detector contribute to scatter plots Aims Very important to understand if we can identify contaminations that are not just external, but also internal to the detector itself -read shield thermistors independently SSB Run 2 Features: Main absorber (small) 2×2×0.5 cm3 One TeO2 crystal with two shields (no full coverage) TeO2 active shields Independent thermistors read-out Alpha source implanted in two different points of the detector

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

12. Run 2: scatter plots (2) Shield 1 (implanted) Origin: due to nuclide recoil, there is a fixed maximum energy that can be released in the main absorber (dependence on contamination depth) Surface contaminations of the thin TeO2 layer MC simulation

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

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

15. 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 Parameters for static & dynamic simulations 6-node model “Big” TeO2 crystal (CUORE size) with one Si shield Shield/crystal, NTD/shield and NTD/crystal thermal couplings realized with glue Main crystal/heat bath: PTFE Thermistor/heat bath: gold wires Initial values

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

18. Shield energy rel. Bulk energy dep. Shield energy dep. Shield energy dep. Bulk energy rel. Bulk energy dep. Tests 1 & 2 [links to the heat bath] Test #2 Test #1 g50 x 6 (shield thermistor gold wires) g30 x 10 (main/bath PTFE) @ 3.1 MeV

19. Bulk dep. Shield dep. Shield dep. Shield dep. Bulk energy dep. Bulk energy dep. Tests 3 & 4 [passive auxiliary NTD] Test #4 Test #3 g54 C4 C5 g50 = 0 (shield thermistor not polarized –passive node) (shield thermistor as a passive node and with larger volume) x 6 g50 = 0 This decay time vs amplitude trend corresponds to experimental results @ 3.1 MeV

20. Bulk energy dep. Shield energy dep. Shield energy dep. Bulk energy dep. Tests 5 [shield heat capacity] Test #5 A thermal add-on to the slab? C6 x 100 (larger shield heat capacity and passive aux. thermistor) g50 = 0 @ 3.1 MeV

21. 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 SSB Run 3 Features: “Big” main absorbers: 5×5×5 cm3 TeO2 active shields (reasons: -thermal contractions with main absorber -known material) Thicker TeO2 slabs (0.9 mm) to avoid known mechanical problems 4 SSBs featuring total coverage (6 shields each) 2 SSBs have thermistors on each layer and independent read-outs 2 SSBs have 5 “passive” slabs (no thermistor) and one readable slab Great attention for clean working conditions in order to avoid possible contamination sources

23. Run 3: single SSB Single detector assembly

24. Run 3: assembly 4-SSB module

25. Run 3: remarks Test of background reduction 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