Surface sensitive bolometers (SSB) for the rejection of continuous alpha background in CUORE

1. Surface sensitive bolometers (SSB) for the rejection of continuous alpha background in CUORE MARISA PEDRETTI INFN - Milano

2. Outline • The radioactivity background problem • Basic idea of Surface Sensitive Bolometers • Test Runs @ Insubria University (2004-2005) • Run I @ LNGS (July – August 2005) • New Small SSB Test @ LNGS (Sept – Oct 2005) • The Future Run II @ LNGS

3. bb (0n) 130Te CUORE and Cuoricino CUORE will use bolometer technique for the searching for 0nDBD of 130Te CUORE sensitivity will depend on the radioactivity background in the 0nDBD region Cuoricino is not only a self consistent already running experiment but it is also a test for CUORE experiment. The Cuoricino background in 0nDBD region is 0.18±0.01 c/(keV kg y)

4. TeO2 TeO2 208Tl 214Bi Cu 60Co p.u. CUORE and Cuoricino MC: the background in CUORICINO is due to degraded alpha’s which release only a part of their energy in the detector (surface contamination) Cuoricino background

5. Heat sink 5 cm Cu holder Thermal coupling Thermometer NTD sensor particle DT= E/C Signal: Crystal absorber TeO2 cristal Basic principle of bolometric technique

6. Classical pulse Classical pulse If we make a scatter plot of coincident pulse amplitude (pulse amplitude obtained with thermistor on secondary crystal versus those obtained with thermistor on TeO2 crystal) we obtain two well separated behaviours DV Classical pulse Fast high and saturated pulse TeO2 thermistor The Surface Sensitive Bolometers background discrimination The idea is to use auxiliary thin bolometers to control surface contamination An other possibility is to discriminate by using the shape of coincident pulses

7. Pros • Low cost • Pros • Thermal contractions with main absorber • Known material • Cons • Low purity • Cons • Fragilty • Thermal coupling with thermistors SSB @ Insubria University Various materials for active shields were tested on small size detectors (main absorbers 2×2×2 - 2×2×0.5 cm3) Ge shields Si shields TeO2 shields • Pros • Excellent results • High purity • Cons • High cost

8. Germanium alfa peaks Events in the main bulk Example of experimental Scatter Plot 212Po 216Po 220Ra 224Ra

9. SSB: discrimination by using pulse time •  ~ 6.4 ms: events on Ge bolometer •  ~ 9.6 ms: events on TeO2 bolometer There are 2 well separated families: fast pulses corrispond to surface events whereas slow pulses correspond to bulk events Counts A selection of the events by their rise time clearly identifies the two event bands Rise time on Ge_slab [ms]

10. SSB @ LNGS: description Above ground tests are difficult due to cosmic rays In summer 2005 we test large SSB bolometers at LNGS Absorber TeO2 5x5x5 cm3 Si shields Parallel read-out of signals in order to reduce the read-out channels

11. SSB @ LNGS: pictures Test in NOT clean condition! Many wiring and construction problems (very complicated assembly)

12. SSB @ LNGS: results The scatter plot is coherent with the expected behaviour. However, there are some problems. For example… • There is a considerable contamination (a) in the slabs or close to them. Surface events Mixed events • While surface events may be identified, the parallel configuration seems to confuse the physical understanding of each element in the plot. Amplitude on slabs (a.u.) Bulk events Amplitude on TeO2 (a.u.)

13. Surface Sensitive Bolometer Scatter plot τrvs amplitude It is possible to identify all the families in the scatter plot by using rise time on slabs. Rise time (ms) Amplitude on slabs (mV)

14. Cuoricino- like detector without SSB detector with slabs (SSB) anomalous events SSB: decay time on main bolometer Decay time vs Energy for pulses read by the main TeO2 absorber Decay time on main bolometer Amplitude on main bolometer Decay time on main bolometer Amplitude on main bolometer

15. E [MeV] 2.9-3.2 3.2-3.4 3.4-3.9 bkg (no shields) [c/kg keV y] 0.44 ± 0.06 0.58 ± 0.08 0.51 ± 0.04 bkg (shields) [c/kg keV y] 0.18 ± 0.08 0.51 ± 0.16 0.29 ± 0.08 SSB @ LNGS: results Cutting on the DT distribution allows to isolate a great deal of unwanted events. Good efficiency in the active backgroud discrimination by the decay time read on the main absorber Important result: the bkg obtained with high contamination is similar to the one obtained after a careful cleaning Good active efficiency

16. Surface Sensitive Bolometer SLAB 1 MAIN SLAB 2 D3 URANIUM SOURCE 2 cm VACUUM GREASE GLUE 0.5 cm 0.5 mm 0.5 mm TEST RUN: we studied in which way contamination in different point of the detector contribute in a scatter plots

17. 238U alphas ( ≈ 4.2 MeV) 234U alphas ( ≈ 4.7 MeV) surface events mixed events bulk events Surface Sensitive Bolometer SLAB 1 (implanted)

18. MC simulations: contamination in slab MC - Simulation

19. surface events bulk events Surface Sensitive Bolometer SLAB 2 (see the implanted side of the crystal ) mixed events

20. SSB @ LNGS : future test Goal: see if it is possible to reduce the background level clean condition. • 4 crystals with SSB • 0.9 mm thickness TeO2 SSB • connection between slabs and main crystal made by 12 glue spots • 2 crystals with ntd on all the slabs read independently • 2 crystals with 5 slabs without ntd and with only a read out slab. • Coupling between ntd’s and slabs: epoxy glue

21. SSB @ LNGS : future test