- Email: [email protected]
The first phase of the MARE project in Milano
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 617 (2010) 509–510
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The first phase of the MARE project in Milano A. Nucciotti a,, C. Arnaboldi a, G. Ceruti a, E. Ferri a, C. Kilbourne b, S. Kraft-Bermuth a, B. Margesin c, G. Pessina a, E. Previtali a, D. Schaeffer a, M. Sisti a a
Universita di Milano-Bicocca e INFN Sezione di Milano-Bicocca, Milano, Italy NASA Goddard Space Flight Center, Greenbelt, MD, USA c Micro-ElectroMechanical Systems and Radiation Detectors, Fondazione Bruno Kessel, Trento, Italy b
a r t i c l e in fo
abstract
Available online 17 October 2009
The international project ‘‘Microcalorimeter Arrays for a Rhenium Experiment’’ (MARE) aims at the direct and calorimetric measurement of the electron anti-neutrino mass with sub-electronvolt sensitivity. The experimental strategy consists in studying the beta spectrum of 187Re near the end-point looking for the spectral distortion expected for a finite anti-neutrino mass. The MARE project has a staged approach: in the final experimental phase (MARE-2), several large arrays with as many as 10 000 detectors each will be deployed to collect the statistics required to probe the anti-neutrino mass with a sensitivity of at least 0.2 eV, comparable to the one expected for the Katrin experiment (KATRIN LoI, 2001, [1]). In the short term, smaller scale experiments are planned to reach sensitivities of the order of 1 eV (MARE-1). This contribution reports on the Milano group activity for the MARE project. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cryogenic detectors Neutrino mass Fundamental physics
1. Introduction MARE (Microcalorimeter Arrays for a Rhenium Experiment) is an international project aiming at setting up a new large experiment to directly measure the neutrino mass from the end-point of 187Re b decay (E0 2:5 keV). Large arrays of thermal detectors containing 187Re in their absorbers will be used to perform a calorimetric measurement of the beta decay [2]. This configuration removes the most severe systematic uncertainties which have plagued the traditional and, so far, more sensitive spectrometer experiments [1,3]. Reaching a sensitivity on the neutrino mass of the order of few tenths of electronvolt calls for collecting a statistics of up to 1014 187 Re b decays and for detectors with extremely good energy and time resolutions (i.e. about 1 eV and 1 ms, respectively). These requirements have been studied in details by means both of a Monte Carlo approach and of a numerical statistical analysis using expansions of involved spectra near the end-point. This latter analysis, limited to first order expansions and in the absence of background, gives the following simple expression for the 90% CL limit on mn , Sðmn Þ90 :
Sðmn Þ90 ¼ 0:74
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2E30 DE 9tE50 4 þ tM Ab Ndet 5tM Ndet DE
Corresponding author.
E-mail address: [email protected] (A. Nucciotti). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.10.079
ð1Þ
where DE, t, tM , Ab , and Ndet are the analysis energy interval and time resolution, the measuring time, the b activity of each detector, and the number of detectors, respectively. When the detector resolving time is high enough to keep the fraction fpp of piling-up decays (fpp ¼ tAb ) lower than about 105 , the expected sensitivity Sðmn Þ90 is given by the first term in the above expression and it can be shown that DE is about the detector energy resolution. Underpthese ffiffiffiffiffiffiffiffiffiffiffiffifficonditions it is apparent how the sensitivity improves as 4 1=Nev , where the total statistics Nev is given by Nev ¼ tM Ab Ndet , and the above requirements for a subelectronvolt sensitivity are confirmed. For what concerns the time resolution, a trade off must be found between t and Ab in order to keep the detector size, the measuring time tM and the number of detectors reasonable. For higher values of fpp the sensitivity becomes almost insensitive to the detector energy resolution but the dependence on the statistics remains. All these behaviors can be obtained also using higher order expansions and are confirmed by the Monte Carlo simulations which give more reliable results. The MARE project is divided into two phases: last phase (MARE-2) consists of the deployment of several large arrays of thermal microcalorimeters. The project activity currently focuses on sorting out the best suitable detector technology for this last phase. At the same time smaller scale experiments (MARE-1) are being set-up using both detectors which are already available and new ones which are being developed. The purpose of these experiments is to probe the final results of the Mainz and Troitsk spectrometer experiments with a neutrino mass sensitivity of the order of 1 eV and to explore the systematic uncertainties
ARTICLE IN PRESS 510
A. Nucciotti et al. / Nuclear Instruments and Methods in Physics Research A 617 (2010) 509–510
peculiar of the calorimetric approach with a statistics of the order of 109 events.
only two arrays a sensitivity of about 4.5 eV at 90% CL is expected in three years running time.
2. MARE-1
3. MARE-2
One of the MARE-1 experiments is carried out in Milano as a joint effort of the groups at Milano-Bicocca University, Insubria University, NASA/GSFC at Greenbelt, Wisconsin University at Madison and Memsrad/FBK at Povo. Here we report the status of this experiment being set-up in one of the dilution refrigerators installed at the Milano-Bicocca University. It uses the 6 6 arrays of silicon implanted thermistors developed by the NASA/GSFC group as focal detectors for the XRS2 experiment on the ASTROE-2 mission. Although these arrays have been optimized for X-ray spectroscopy, it turned out they could be successfully adapted to the measurement of the 187Re b spectrum by applying AgReO4 absorbers. An R&D effort has been devoted to devise the best AgReO4 crystal geometry and the optimal crystal–sensor coupling. It has been shown that with 600 600 200 mm3 AgReO4 crystals (about 0.5 mg, giving 0.27 decay/s) it is possible to achieve an energy resolution of about 30 eV FWHM at the 187Re end-point energy and a time resolution of about 250 ms. Keeping these average performances, eight XRS2 arrays with a total of 288 pixel instrumented with AgReO4 crystals would yield a neutrino mass sensitivity of about 4.6 (3.5) eV in one (three) years measuring time at 90% CL with a total statistics of about 2:4 109 (7:3 109 ) beta decays. The low temperature experimental set-up installed in the dilution refrigerator can host up to eight arrays although only two have been funded so far. Therefore all the necessary equipments for 80 channels are being deployed. This includes one 4 K preamplifier box which contains 80 JFETs on a thermally decoupled board. When biased the JFETs warm up to their optimal working temperature of about 135 K. The box is placed few centimeters below the detectors to minimize the microphonic noise and the parasitic electrical capacitance. For the expected operating conditions the JFETs have a noise of about 3 nV=OHz. These JFETs performs 30% better than the ones used for the detector R&D therefore we expect a similar improvement for the final detectors. The read-out cryogenic wiring, the room temperature detector and JFET bias network, the preamplifiers, the antialiasing filters, the triggers and the DAQ system have been installed for 80 channel as well. A fluorescence calibration source which can be opened and closed from room temperature completes the set-up. A first test run with two arrays but only 11 AgReO4 crystals attached is planned to start shortly. The schedule has been delayed by about two months for an unexpected failure of the micro-bridges inside the 4 K preamplifier box. The micro-bridges are micromachined devices designed to provide electrical connection while keeping the thermal isolation: they are used to connect the JFET terminals at 135 K to the surroundings at 4 K and are made of thin free-standing meanders of aluminum on polyimide. After the test run aiming at checking the functionality of all channels and at their characterization, the remaining AgReO4 crystals will be attached to the arrays and the measurement will start. This is expected to happen in September 2009. Depending on the results shown by the first two arrays, a decision will be made about the deployment of the remaining six arrays. With
The ambitious final goal of the MARE project calls for a novel detector technology. Kinetic Inductance Detectors (KIDs) are one very promising option which is being explored in Milano. KIDs are devices in the superconducting state presenting a sharp frequency resonance in the microwave range (1–10 GHz): energy absorption in the device increases the quasi-particle (qp) density, shifting both the central frequency and the width of the resonance. KIDs are fabricated by means of microelectronic techniques by patterning superconducting material thin films (e.g. aluminum, titanium or niobium) to the proper resonator shape (e.g. quarter wave Coplanar Waveguide or CPW) [4]. KID fabrication involves relatively simple standard microelectronics processes, while inexpensive and powerful room-temperature read-out electronics can exploit the microwave integrated circuits developed for wireless communication. So far progresses on this technology have been fueled by the application to astronomy and astrophysics: the application to MARE requires a specific R&D program centered on coupling KIDs to rhenium absorbers. In principle qp detectors like KIDs are the best solution for making thermal detectors with superconducting absorbers like metallic rhenium. By implementing a proper qp trapping scheme KIDs can be coupled to relatively large rhenium crystal absorbers: performances then depend on the qp diffusion length in the rhenium and not on its thermal capacity. The expected speed and energy resolution are in the range of the microseconds and electronvolts, respectively. Furthermore the signals from thousands of KIDs can be multiplexed in the frequency domain on a single coaxial cable with a single microwave amplifier. This possibility is of enormous value for an experiment with possibly hundreds of thousands of channels like MARE. With KIDs based detectors it may be possible to use rhenium absorbers as large as 20 mg each (corresponding to an activity Ab of about 20 decay/s): with an energy resolution of about 5 eV and a resolving time of about 1 ms, a sensitivity of 0.2 eV at 90% CL would be reached with an exposure of about 105 detector year.
4. Conclusions The first of the MARE-1 experiments is getting ready to start in Milano using silicon implanted thermistor arrays. The experiment starts with about 72 channels but can be readily expanded to 288 channels for better sensitivity. This small scale experiment will help demonstrating the feasibility of the MARE project by allowing the study of 187Re b spectrum with a statistics of the order of 109 decays. At the same time a R&D program aiming at the application of KIDs in MARE is starting. References [1] [2] [3] [4]
KATRIN LoI, 2001, hep-ex/0109033. MARE project proposal: /http://crio.mib.infn.it/wig/silicini/proposal/S. Ch. Kraus, et al., Eur. Phys. J. C 40 (2005) 447. P.K. Day, et al., Nature 425 (2003) 817821.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The first phase of the MARE project in Milano A. Nucciotti a,, C. Arnaboldi a, G. Ceruti a, E. Ferri a, C. Kilbourne b, S. Kraft-Bermuth a, B. Margesin c, G. Pessina a, E. Previtali a, D. Schaeffer a, M. Sisti a a
Universita di Milano-Bicocca e INFN Sezione di Milano-Bicocca, Milano, Italy NASA Goddard Space Flight Center, Greenbelt, MD, USA c Micro-ElectroMechanical Systems and Radiation Detectors, Fondazione Bruno Kessel, Trento, Italy b
a r t i c l e in fo
abstract
Available online 17 October 2009
The international project ‘‘Microcalorimeter Arrays for a Rhenium Experiment’’ (MARE) aims at the direct and calorimetric measurement of the electron anti-neutrino mass with sub-electronvolt sensitivity. The experimental strategy consists in studying the beta spectrum of 187Re near the end-point looking for the spectral distortion expected for a finite anti-neutrino mass. The MARE project has a staged approach: in the final experimental phase (MARE-2), several large arrays with as many as 10 000 detectors each will be deployed to collect the statistics required to probe the anti-neutrino mass with a sensitivity of at least 0.2 eV, comparable to the one expected for the Katrin experiment (KATRIN LoI, 2001, [1]). In the short term, smaller scale experiments are planned to reach sensitivities of the order of 1 eV (MARE-1). This contribution reports on the Milano group activity for the MARE project. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cryogenic detectors Neutrino mass Fundamental physics
1. Introduction MARE (Microcalorimeter Arrays for a Rhenium Experiment) is an international project aiming at setting up a new large experiment to directly measure the neutrino mass from the end-point of 187Re b decay (E0 2:5 keV). Large arrays of thermal detectors containing 187Re in their absorbers will be used to perform a calorimetric measurement of the beta decay [2]. This configuration removes the most severe systematic uncertainties which have plagued the traditional and, so far, more sensitive spectrometer experiments [1,3]. Reaching a sensitivity on the neutrino mass of the order of few tenths of electronvolt calls for collecting a statistics of up to 1014 187 Re b decays and for detectors with extremely good energy and time resolutions (i.e. about 1 eV and 1 ms, respectively). These requirements have been studied in details by means both of a Monte Carlo approach and of a numerical statistical analysis using expansions of involved spectra near the end-point. This latter analysis, limited to first order expansions and in the absence of background, gives the following simple expression for the 90% CL limit on mn , Sðmn Þ90 :
Sðmn Þ90 ¼ 0:74
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2E30 DE 9tE50 4 þ tM Ab Ndet 5tM Ndet DE
Corresponding author.
E-mail address: [email protected] (A. Nucciotti). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.10.079
ð1Þ
where DE, t, tM , Ab , and Ndet are the analysis energy interval and time resolution, the measuring time, the b activity of each detector, and the number of detectors, respectively. When the detector resolving time is high enough to keep the fraction fpp of piling-up decays (fpp ¼ tAb ) lower than about 105 , the expected sensitivity Sðmn Þ90 is given by the first term in the above expression and it can be shown that DE is about the detector energy resolution. Underpthese ffiffiffiffiffiffiffiffiffiffiffiffifficonditions it is apparent how the sensitivity improves as 4 1=Nev , where the total statistics Nev is given by Nev ¼ tM Ab Ndet , and the above requirements for a subelectronvolt sensitivity are confirmed. For what concerns the time resolution, a trade off must be found between t and Ab in order to keep the detector size, the measuring time tM and the number of detectors reasonable. For higher values of fpp the sensitivity becomes almost insensitive to the detector energy resolution but the dependence on the statistics remains. All these behaviors can be obtained also using higher order expansions and are confirmed by the Monte Carlo simulations which give more reliable results. The MARE project is divided into two phases: last phase (MARE-2) consists of the deployment of several large arrays of thermal microcalorimeters. The project activity currently focuses on sorting out the best suitable detector technology for this last phase. At the same time smaller scale experiments (MARE-1) are being set-up using both detectors which are already available and new ones which are being developed. The purpose of these experiments is to probe the final results of the Mainz and Troitsk spectrometer experiments with a neutrino mass sensitivity of the order of 1 eV and to explore the systematic uncertainties
ARTICLE IN PRESS 510
A. Nucciotti et al. / Nuclear Instruments and Methods in Physics Research A 617 (2010) 509–510
peculiar of the calorimetric approach with a statistics of the order of 109 events.
only two arrays a sensitivity of about 4.5 eV at 90% CL is expected in three years running time.
2. MARE-1
3. MARE-2
One of the MARE-1 experiments is carried out in Milano as a joint effort of the groups at Milano-Bicocca University, Insubria University, NASA/GSFC at Greenbelt, Wisconsin University at Madison and Memsrad/FBK at Povo. Here we report the status of this experiment being set-up in one of the dilution refrigerators installed at the Milano-Bicocca University. It uses the 6 6 arrays of silicon implanted thermistors developed by the NASA/GSFC group as focal detectors for the XRS2 experiment on the ASTROE-2 mission. Although these arrays have been optimized for X-ray spectroscopy, it turned out they could be successfully adapted to the measurement of the 187Re b spectrum by applying AgReO4 absorbers. An R&D effort has been devoted to devise the best AgReO4 crystal geometry and the optimal crystal–sensor coupling. It has been shown that with 600 600 200 mm3 AgReO4 crystals (about 0.5 mg, giving 0.27 decay/s) it is possible to achieve an energy resolution of about 30 eV FWHM at the 187Re end-point energy and a time resolution of about 250 ms. Keeping these average performances, eight XRS2 arrays with a total of 288 pixel instrumented with AgReO4 crystals would yield a neutrino mass sensitivity of about 4.6 (3.5) eV in one (three) years measuring time at 90% CL with a total statistics of about 2:4 109 (7:3 109 ) beta decays. The low temperature experimental set-up installed in the dilution refrigerator can host up to eight arrays although only two have been funded so far. Therefore all the necessary equipments for 80 channels are being deployed. This includes one 4 K preamplifier box which contains 80 JFETs on a thermally decoupled board. When biased the JFETs warm up to their optimal working temperature of about 135 K. The box is placed few centimeters below the detectors to minimize the microphonic noise and the parasitic electrical capacitance. For the expected operating conditions the JFETs have a noise of about 3 nV=OHz. These JFETs performs 30% better than the ones used for the detector R&D therefore we expect a similar improvement for the final detectors. The read-out cryogenic wiring, the room temperature detector and JFET bias network, the preamplifiers, the antialiasing filters, the triggers and the DAQ system have been installed for 80 channel as well. A fluorescence calibration source which can be opened and closed from room temperature completes the set-up. A first test run with two arrays but only 11 AgReO4 crystals attached is planned to start shortly. The schedule has been delayed by about two months for an unexpected failure of the micro-bridges inside the 4 K preamplifier box. The micro-bridges are micromachined devices designed to provide electrical connection while keeping the thermal isolation: they are used to connect the JFET terminals at 135 K to the surroundings at 4 K and are made of thin free-standing meanders of aluminum on polyimide. After the test run aiming at checking the functionality of all channels and at their characterization, the remaining AgReO4 crystals will be attached to the arrays and the measurement will start. This is expected to happen in September 2009. Depending on the results shown by the first two arrays, a decision will be made about the deployment of the remaining six arrays. With
The ambitious final goal of the MARE project calls for a novel detector technology. Kinetic Inductance Detectors (KIDs) are one very promising option which is being explored in Milano. KIDs are devices in the superconducting state presenting a sharp frequency resonance in the microwave range (1–10 GHz): energy absorption in the device increases the quasi-particle (qp) density, shifting both the central frequency and the width of the resonance. KIDs are fabricated by means of microelectronic techniques by patterning superconducting material thin films (e.g. aluminum, titanium or niobium) to the proper resonator shape (e.g. quarter wave Coplanar Waveguide or CPW) [4]. KID fabrication involves relatively simple standard microelectronics processes, while inexpensive and powerful room-temperature read-out electronics can exploit the microwave integrated circuits developed for wireless communication. So far progresses on this technology have been fueled by the application to astronomy and astrophysics: the application to MARE requires a specific R&D program centered on coupling KIDs to rhenium absorbers. In principle qp detectors like KIDs are the best solution for making thermal detectors with superconducting absorbers like metallic rhenium. By implementing a proper qp trapping scheme KIDs can be coupled to relatively large rhenium crystal absorbers: performances then depend on the qp diffusion length in the rhenium and not on its thermal capacity. The expected speed and energy resolution are in the range of the microseconds and electronvolts, respectively. Furthermore the signals from thousands of KIDs can be multiplexed in the frequency domain on a single coaxial cable with a single microwave amplifier. This possibility is of enormous value for an experiment with possibly hundreds of thousands of channels like MARE. With KIDs based detectors it may be possible to use rhenium absorbers as large as 20 mg each (corresponding to an activity Ab of about 20 decay/s): with an energy resolution of about 5 eV and a resolving time of about 1 ms, a sensitivity of 0.2 eV at 90% CL would be reached with an exposure of about 105 detector year.
4. Conclusions The first of the MARE-1 experiments is getting ready to start in Milano using silicon implanted thermistor arrays. The experiment starts with about 72 channels but can be readily expanded to 288 channels for better sensitivity. This small scale experiment will help demonstrating the feasibility of the MARE project by allowing the study of 187Re b spectrum with a statistics of the order of 109 decays. At the same time a R&D program aiming at the application of KIDs in MARE is starting. References [1] [2] [3] [4]
KATRIN LoI, 2001, hep-ex/0109033. MARE project proposal: /http://crio.mib.infn.it/wig/silicini/proposal/S. Ch. Kraus, et al., Eur. Phys. J. C 40 (2005) 447. P.K. Day, et al., Nature 425 (2003) 817821.