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Rejection of surface radioactivity in TeO 2 bolometers

Rejection of surface radioactivity in TeO 2 bolometers. INFN – Milano Bicocca, Italy. University of Insubria – Como, Italy. Chiara Salvioni – Physics of Massive Neutrinos - Blaubeuren, Germany, 3 July 2007. The problem with CUORE background 1/19. Q  ~ 2.5 MeV. Goal: () of 130 Te.

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Rejection of surface radioactivity in TeO 2 bolometers

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  1. Rejection of surface radioactivity in TeO2 bolometers INFN – Milano Bicocca, Italy University of Insubria – Como, Italy Chiara Salvioni – Physics of Massive Neutrinos - Blaubeuren, Germany, 3 July 2007

  2. The problem with CUORE background 1/19 Q ~2.5 MeV Goal:() of 130Te Predictions on the future background expected for CUORE from Cuoricino background analysis and Monte Carlo simulations... BKG = 0.18 ± 0.01 c/(keV kg y) ~ 1000 TeO2 bolometers bb (0n)130Te Cuoricino Experimental data and simulations suggest one major contribute for CUORE background in the DBD region: a and b degraded particles emitted by 238U and 232Th surface contaminations on the Cu frame and on the crystal surface.

  3. An active approach to bkg reduction 2/19 Copper Surface Sensitive Bolometer Not rejected Not rejected bb(0n)-decay TeO2 TeO2 Rejected by the granularity of the detector What happens when we read coincident signals on the two thermistors… Main thermistor Auxiliary thermistor bb(0n)-decay Mimicking event V V t t “Bulk event” “Surface event”

  4. Bulk event Simulated decay time structure Surface event Shield dep. Bulk energy dep. The simulated behavior of SSBs 3/19 Simulated rise time structure The supposed behavior was first confirmed by simulations Simulated coincident pulses Simulated scatter plot

  5. Como tests 4/19 Goals • selection of materials • study of thermal couplings • tests of mechanical feasibility • analysis and selection of discrimination parameters Notes • energy resolution not optimized

  6. Experimental discrimination of events 5/19 -Decay time structure (main thermistor) -Parallel readout -Rise time structure -Pulse amplitude

  7. Surface events Mixed events Bulk events LNGS – SSB test 1 6/19 Goal • test of technical feasibility Notes • no attention on cleaning procedure • no single slabs readout (only parallel) Pulse decay time on main NTD [ms] Bkg compatible with Cuoricino… Pulse amplitude on main NTD [mV]

  8. Active slabs Passive slabs LNGS – SSB test 2 8/19 Goal • test of background reduction • evaluation of the most suitable discrimination parameters Notes • single slabs readout • slabs: both active & passive

  9. Results from the last LNGS test 9/19

  10. Full coverage of the main crystals Slab cleaning New ideas to proceed 10/19 Why didn’t we get a deeper background cleaning from surface events in the last LNGS test? Possible answers: • no clean slabs • lost channels • no total main detector coverage We proposed a new run: it will not be focused on bkg reduction but on understanding the bkg  diagnostic test. Fundamental requests: active slabs (all the elements in the new SSBs will be read) Most important…

  11. 8 mm 8 mm 50.5 mm 50.5 mm Future slabs -1 11/19 TeO2 is still our choice because of its thermal behavior, which is suitable for our detectors. Moreover, we can have a good control on the production of the slabs: they are manufactured by the same factory which gave us the main crystals for Cuoricino and the current R&D. The cut was eased by using a radioactively contaminated oil (not done with the main crystals!) Problem with the last run: the slabs were contaminated during the production process • Square shape • Top slabs with a diagonal cut to leave room for a NTD and a heater • Size 0.9 x 50.5 x 50.5 mm3 (standard for main crystals: 50.7 mm) Requests on geometry

  12. Future slabs -2 12/19 The main crystals in the new SSBs will not see the holder’s materials; they will see just the NTD, the heater and their own gold wires (not really a problem) Special requests on manufacturing Request for the same treatment as TeO2 5x5x5 cm3 crystals • Identical cutting tools • Same cutting and treatment steps (no lapping!) • No oil (higher cost because of the higher number of broken slabs) The delivery started last week…

  13. Slabs cleaning 13/19 In the meantime, we performed a test in Como in order to have some indications about the level of radioactivity of TeO2 slabs… Slabs analyzed: those used in the last LNGS test Special cleaning procedure: the one developed for main crystals at LNGS • Acidization in HNO3 • Rinsing with ultrapure water • Drying with gaseous nitrogen • Storage in clean, N2-filled bags

  14. Ntd Glue spots The “slab skyscraper” 14/19

  15. Preliminary conclusions… 15/19 We have just finished to acquire data: the analysis is currently in progress. A sample of the same slabs has been measured with a surface barrier detector by E. Previtali Preliminary results suggest that there is no evidence for contamination

  16. Assembly of a fully covered detector 16/19 With this better cleaning, the slabs could show an improved behavior in the discrimination of surface events comparing to the last test. There is still one question… The assembly of a fully covered detector • Three problems: • Gluing slabs to the main crystal • Holding the crystal • Different thermal network to represent the SSB a) Possible procedure (tested) b) Different solutions…

  17. Assembly possible solutions 17/19 The SSB can be secured in the copper frame by the usual PTFE holders… …or by new copper clamps. Another solution: the SSB “sitting” on the lower side of the frame… Both PTFE and copper clamps hold the SSB by pressing the slabs. Major issue: the holders could break the slabs while pressing (thin TeO2 foils are extremely fragile)

  18. Assembly -3 18/19 While the last solution should not cause any mechanical inconvenient during the assembly, there is still a problem: c) the thermal network is different To be modified!

  19. Conclusions 19/19 • The preliminary results about the effectiveness of the cleaning procedure for TeO2 slabs must first be verified by analyzing data from the two measurements. This is a necessary condition to the effectiveness of the run with fully covered SSBs. • We will perform a set of simulations in order to achieve some information about possible changes in the behavior of the detector due to the different thermal network. • Before testing real CUORE-size crystals at LNGS, in Como we will measure small detectors matching the same conditions as the final run. • Assembly tests will be performed to verify the concrete possibility to realize four such detectors without any unexpected mechanical problems. • If all these conditions are satisfied, the LNGS run will take place, based on four fully covered detectors with slabs both cleaned and provided with electronic readout.

  20. Background consideration RAD SSB plane (in CAW2 run) (as passive) CAW1 (same crystals without slabs) This region is dominated by slab contamination alpha particles emitted by the slab that produce a pulse not sufficiently “deformated” for our present cuts

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