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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 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


Goddard Space Flight Center, NASA, Greenbelt, Maryland, USA

University of Wisconsin, Madison, Wisconsin, USA

Paris - 14/11/2005

science case m 0 m
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

calorimetric technique status
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

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


Scientific Goal:

- m < 2eV before KATRIN

- phase II preliminary (systematics, technology..)

Paris - 14/11/2005

mibeta 2 brief description
MIBETA 2 brief description


GOAL: significantly increase

the statistics

SOLUTION: scaling up to 200

detectors the MIBETA concept


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

mare phase i mibeta2 options
MARE phase I: MIBETA2 options

NASA66 silicon array (XRS2).

STATUS:encouraging first

results with 450g AgReO4.

Coupling and electronics to be


NTD Ge array (LBL+Bonn).

STATUS:preferred for the

larger e-ph thermal coupling.

Reproducibility to be


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

nasa array results
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

germanium ntd results
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

systematic effects for mibeta2
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

befs and a nuclear physics result
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

44 ti gamma deeply excited re
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


determined by

an external

44Ti source

External 55Fe


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

itc irst technology for cryodetectors
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

irst silicon arrays fabrication steps
IRST siliconarraysfabricationsteps

Present status:

in production

(multiple implantations

for main doping and

compensation ongoing)

Paris - 14/11/2005

wiring and old itc irst cryoflats
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

new irst cryoflats for mibeta
New IRST cryoflats for MIBETA


Old problems:

hopefully all understood

and solved with the new


Present Status:

Ready for fabrication


Paris - 14/11/2005


• 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