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MIBETA 2 Semiconductor microbolometers for a direct neutrino mass search

MIBETA 2 Semiconductor microbolometers for a direct neutrino mass search. Alessandro MONFARDINI – ITC-IRST and INFN Trento On behalf of the MIBETA group: INFN sez. Milano and Università di Milano-Bicocca, Dipartimento di Fisica, ITALY

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MIBETA 2 Semiconductor microbolometers for a direct neutrino mass search

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  1. MIBETA 2Semiconductor microbolometers for a direct neutrino mass search Alessandro MONFARDINI – ITC-IRST and INFN Trento On behalf of the MIBETA group: INFN sez. Milano and Università di Milano-Bicocca, Dipartimento di Fisica, ITALY Università dell’Insubria, Dipartimento di Fisica e Matematica, ITALY ITC-IRST and INFN, Trento, ITALY Goddard Space Flight Center, NASA, Greenbelt, Maryland, USA University of Wisconsin, Madison, Wisconsin, USA Paris - 14/11/2005

  2. Science case: (m  0)  (m = ?) Neutrino oscillations (Δm2 only): atmosphericΔm232  210-3 eV2(SK evidence + CHOOZ constrains) solarΔm122  710-5 eV2(SNO + KAMland) Neutrinoless double beta decay - 0 (model-dependent): ..but direct insights on the neutrino nature (Majorana ?) and access to Majorana phases Effective Majorana mass:mee < 0.35 eV (Heidelberg-Moscow 76Ge) mee < 0.2 ÷ 1.1 eV(CUORICINO 130Te) mee = 0.1 ÷ 0.9 eV (Klapdor: 76Ge reanalysis) Cosmology (indirect): U. Seljak, Physics Review D 71 (2005) 103515mi < 0.42 eV (CMB+SDSS+SN) Direct ( decay) SAFE determination NEEDED !! … deserves more than just KATRIN (to start data taking in 2010) Paris - 14/11/2005

  3. Calorimetric technique: status Lowest-Q (2.5keV)beta decay (most sensitive to small m): 187Re GOALS:- eliminate as much systematics as possible (sub-eV!!) - scaling (in principle) possible up to the nth generation Source  Detector(neutrino is the only allowed to escape from the bulk) Published results:< 15 eV (90% C.L.) Milano MIBETA (AgReO4) < 26 eV (95% C.L.) Genova MANU (metallic Re) STATUS:at present about one order of magnitude worse than spectrometers (best published result: 2.2eV @ 95% C.L.). Some pros in principle: micro vs. macro approach (scalability) and systematic effects (but to be studied carefully..). Paris - 14/11/2005

  4. MIBETA2 and MARE An ambitious experiment has been proposed by an international collaboration to directly compete with KATRIN using Cryodetectors (Ref. D. Pergolesi presentation). MARE (Microcalorimeters Arrays for a Rhenium Experiment) will be a two phases effort. MIBETA2 is part of MARE phase I MARE Phase I:- Present technology detectors (2006-2009)- Scaling up to hundreds of devices  MIBETA2 Semiconductor bolometers  MANU2 TES Scientific Goal: - m < 2eV before KATRIN - phase II preliminary (systematics, technology..) Paris - 14/11/2005

  5. MIBETA 2 brief description STARTING POINT: MIBETA GOAL: significantly increase the statistics SOLUTION: scaling up to 200 detectors the MIBETA concept ABSORBER: AgReO4 THERMISTOR: semiconductor (Si or Ge) Single absorber mass  500g • ~ 1010 beta decays required • E and fpup achievable fpup ~ AR Paris - 14/11/2005

  6. MARE phase I: MIBETA2 options NASA66 silicon array (XRS2). STATUS:encouraging first results with 450g AgReO4. Coupling and electronics to be optimized. NTD Ge array (LBL+Bonn). STATUS:preferred for the larger e-ph thermal coupling. Reproducibility to be demonstrated. ITC-IRSTTMAH micromachined arrays with SU8 supports for the absorbers. Implanted silicon with the technology developed for the MIBETA single devices. STATUS:10 devices arrays fabrication ongoing. IRST process BL12 Paris - 14/11/2005

  7. NASA array results • 66 array (XRS2) • T0 7 K • AgReO4 (450g) either mounted on • Si platforms glued to the four SU8 • spacers or directly to the thermistor • (with the help of a silicon spacer) • Best results obtained with the spacers: • EFWHM (@ 1.5keV) = 35eV • RISE = 220s • Problems: - MIBETA electronics not • well matched yet • - coupling of AgReO4 to be optimised. • But…… good baseline choice in any case. Paris - 14/11/2005

  8. Germanium NTD results • 37 devices array originally • built for astronomical purposes • - Ge NTD bump bonded on SiN • thin (0.8m) membranes • - Nb wiring to the pads • - AgReO4 (450g) glued on the • NTD (ST2850FT) • Among 10 actually bonded NTDs, the best result is summarized here: • EFWHM (@ 1.5keV) = 50eV • RISE = 270s • Problems: - NTD mechanical stress Paris - 14/11/2005

  9. Systematic effects for MIBETA2 • Under investigation using “old” MIBETA data: • - theoreticalspectral shapeof the 1st forbidden 187Re decay; • - solid stateBEFSeffect; • internaldetector responsefunction calibration; • - unidentifiedpile-upspectrum; • - external radioactivebackground; • - energy scalecalibration; • - surfaceelectron escape; • - data reduction. Briefly discussed here Briefly discussed here Paris - 14/11/2005

  10. BEFS and a Nuclear Physics result A recent re-analysis of the MIBETA fit residuals with the state-of-the-art EXAFS software (GNXAS) led to a substantial improvement of our understanding of the effect. The important result here is: F(le=1) = 0.84  0.30 Fraction of electrons emitted with l=1 (p-wave electrons). To satisfy the overall angular momentum conservation the antineutrino is mainly emitted with null orbital momentum. Re-submitted to PRL Now extrapolation to the end-point is much safer, and we know that the effect is negligible for MARE phase I. Crucial for MARE Phase II, on the other hand. Paris - 14/11/2005

  11. 44Ti gamma: “deeply excited” Re A 6keV x-ray photon can only penetrate for about 4m in an AgReO4 target, path to be compared with 300m, linear dimensions of a typical MIBETA crystal. QUESTION: are the asymmetric calibration line profiles due to escape effects ? DEDICATED RUN: a 44Ti gamma source (E=78.4keV) has been used to uniformly excite Re K,L escape peaks throughout the absorbers. Internal Re Escape peak (K2) determined by an external 44Ti source External 55Fe photons impinging onto the crystals Preliminary result: the symmetry seems recovered…but the large intrinsic width of the escape peaks (42eV) is annoying. Paper in preparation Paris - 14/11/2005

  12. ITC-IRST technology for Cryodetectors • Old single • devices are • shown here. • New run: • 10 detectors • modular arrays; • absorbers • SU8 supports; • heaters for • calibration. • ITC-IRST capabilities applied to Cryogenics Detectors: • “usual” silicon technologies (lithography down to 2m) for the thermistor implant and heater • surface micromachining for thermistor-to-absorber coupling • Bulk micromachining (chemical) for the realization of the suspended structure • Experience with thin membranes and films for other detectors concepts (e.g. MKID, IR and THz detectors) Paris - 14/11/2005

  13. IRST siliconarraysfabricationsteps Present status: in production (multiple implantations for main doping and compensation ongoing) Paris - 14/11/2005

  14. Wiring and “old” ITC-IRST Cryoflats • General problem:COLD END WIRING • detector  “cold” electronics plate 20mK  4K • plate  (e.g. JFET input) 4K  120K • In order to keep the power transmitted per wire below • 10nW for usable wire lengths and sections the first • connection has to be realised in Titanium. First RUN problems: - single front mask - aluminium - cleaving - bonding - weak suspension points Achievement: - Micromachining process tuning Paris - 14/11/2005

  15. New IRST cryoflats for MIBETA PROCESS SCHEME Old problems: hopefully all understood and solved with the new layout/process Present Status: Ready for fabrication SINGLE MEANDERS LAYOUT Paris - 14/11/2005

  16. Conclusions • MIBETA2 is now part of the two steps experiment MARE • MIBETA2 is intended to reach a 2  4 eV limit, depending on the actual performances of the detectors, the number of available channels and so on • A number of systematic effects are under investigation using MIBETA data (e.g. BEFS, line profile) • We do have a baseline design with acceptable performances • We are waiting for the first results from the new ITC-IRST run to improve the baseline expected results. Paris - 14/11/2005

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