- Email: [email protected]
The microcalorimeter arrays for a Rhenium experiment (MARE): A next-generation calorimetric neutrino mass experiment based on the study of 187Re β spectrum
Progress in Particle and Nuclear Physics 57 (2006) 68–70 www.elsevier.com/locate/ppnp
Review
The microcalorimeter arrays for a Rhenium experiment (MARE): A next-generation calorimetric neutrino mass experiment based on the study of 187 Re β spectrum S. Sangiorgio ∗ on behalf of the MARE collaboration Dip. di Fisica dell’Universit`a dell’Insubria, Como and INFN, Sez. di Milano, Italy Accepted 17 November 2005
Abstract Even if neutrino oscillation experiments have proved undoubtedly that neutrinos do have mass, the puzzle is still far from being solved. An important contribution to this quest for neutrino mass can come from the project microcalorimeter arrays for a Rhenium experiment (MARE) which has been proposed to measure the neutrino mass directly through observation of the 187 Re β spectrum. Two experiments, MIBETA and MANU, are pooling their experience in this field with other groups to form an international collaboration to address the challenging goal of an experiment with 0.2 eV sensitivity. The present situation will be discussed here along with MARE’s evolution and prospects. c 2005 Elsevier B.V. All rights reserved.
1. Introduction and science motivation The existence of a finite neutrino mass has been stated with a high degree of confidence by several experiments on neutrino oscillations. However, the absolute values of neutrino masses are still unknown. Among the possible ways to investigate this issue, direct observation of β decay has the advantages of being model-independent and safe, but is unfortunately limited by the difficulty of reaching very low sensitivity. For this reason, these kinds of experiments are only able to investigate the quasi-degenerate region of neutrino masses. ∗ Tel.: +39 031 238 6244; fax: +39 031 238 6209.
E-mail address: [email protected]. c 2005 Elsevier B.V. All rights reserved. 0146-6410/$ - see front matter doi:10.1016/j.ppnp.2005.11.004
S. Sangiorgio / Progress in Particle and Nuclear Physics 57 (2006) 68–70
69
The best limit to date for the νe mass comes from electrostatic β spectrometers measuring decay end-points, with limits of 2.2 eV [1]. KATRIN [2], the next-generation spectrometer, could reduce this limit to about 0.2 eV if successful. It will, however, approach the ultimate limit for these devices. An alternative and complementary technique able to eliminate the systematic effects related to the separation between the source and the detector is the calorimetric technique, by means of very sensitive cryogenic thermal bolometers that couples well to the study of the 187 Re β spectrum thanks to the smallest end-point available, good β activity (1 Hz/mg) and thermo-mechanical properties of both metallic Re and its compounds. The best published calorimetric limit of m ν < 15.6 eV (90% c.l.) was obtained by the MIBETA [3] experiment, an array of microbolometers based on semiconductor thermistors and dielectric (AgReO4 ) absorbers. Similar results have been achieved by the MANU group (m ν < 26 eV—95% c.l.) [4] with a single metallic rhenium crystal. To reach sub-eV sensitivity on neutrino mass, the present experimental sensitivity must be increased by two orders of magnitude. To accomplish this goal, a collaboration has been started for microcalorimeter arrays for a Rhenium experiment (MARE) [5]. This will be a two stage effort in which the intermediate step will aim to achieve m ν ∼ 2 eV and will be accompanied by parallel R&D activity towards the final setup. 3H
2. MARE phase I: Present status and simulations The main goal of MARE phase I is to reach a sensitivity comparable with that of current spectrometers, and this will be achieved by improving the present detector technology used in MIBETA and MANU to MIBETA2 and MANU2. This phase has already started, with encouraging preliminary results. MIBETA2 is testing three different kinds of thermistor technologies: (1) arrays of implanted micromachined silicon thermistors provided by ITC-IRST; (2) NASA silicon micromachined arrays; (3) neutron transmutation doped germanium arrays. Concurrently, MANU2 is already using the Ir–Au TES (transition edge sensor), realized by laser ablation techniques on a silicon substrate to get faster rise times and better signal-to-noise ratio (S/N). Along with detector optimization, it will also be necessary to scale up to hundreds of devices. A second but no less important goal for this first phase will be an improvement in the understanding of experiment systematics which could affect neutrino mass calculation. Systematics includes the theoretical spectral shape of the decay, solid state Beta Environmental Fine Structure effect, detector response function, unidentified pile-up, and data reduction. A Monte Carlo (MC) code has been developed to explore different configurations for MARE phases I and II. The main parameters are: Nev , the total number of beta decays; E, the detector energy resolution; and f pup , the pile-up fraction, which is related to the beta rate per detector Aβ and the pulse rise time t R . From MC, assuming that E = 10 eV (the present resolution is ∼80 eV) and f pup = 5 × 10−5 , we could estimate that Nev ∼ 5 × 109 β decays are needed to reach ∼2 eV sensitivity. 3. MARE phase II: Perspective and conclusions MARE phase I will be able to scrutinize the Mainz/Troitzk results with a completely independent approach. However, MARE would be less interesting if phase I were just a dead end. Moreover, the number of β events has to improve dramatically to Nev ∼ 5 × 1014 . This cannot be reached by a brute-force expansion of the MARE phase I experiment. Instead, substantial
70
S. Sangiorgio / Progress in Particle and Nuclear Physics 57 (2006) 68–70
improvements are needed. The performances required by the detectors are in fact beyond the capabilities of the existing devices. Luckily enough, devices that could suite these needs are already under study. Among these, TES or MMC (metallic magnetic calorimeters) equipped with multiplexed superconducting quantum interference detector (SQUID) read-out seem the most promising detectors to fill the gap. Another important feature will be the scalability of the detector obtained by taking advantages of the intrinsic modularity of the technique. The intention of the MARE proponents is to start phase II data gathering in 2010, in order to be competitive with KATRIN. References [1] [2] [3] [4] [5]
J. Bonn et al., Nuclear Phys. B Proc. Suppl. 91 (2001) 273. The KATRIN collaboration, hep-ex/0109033. M. Sisti et al., Nucl. Instrum. Methods A 520 (2004) 124. M. Galeazzi et al., Phys. Rev. C 63-1 (2001) 0114302. MARE proposal, 2005, Downloadable from: http://crio.mib.infn.it/wig/silicini/publications.html.
Review
The microcalorimeter arrays for a Rhenium experiment (MARE): A next-generation calorimetric neutrino mass experiment based on the study of 187 Re β spectrum S. Sangiorgio ∗ on behalf of the MARE collaboration Dip. di Fisica dell’Universit`a dell’Insubria, Como and INFN, Sez. di Milano, Italy Accepted 17 November 2005
Abstract Even if neutrino oscillation experiments have proved undoubtedly that neutrinos do have mass, the puzzle is still far from being solved. An important contribution to this quest for neutrino mass can come from the project microcalorimeter arrays for a Rhenium experiment (MARE) which has been proposed to measure the neutrino mass directly through observation of the 187 Re β spectrum. Two experiments, MIBETA and MANU, are pooling their experience in this field with other groups to form an international collaboration to address the challenging goal of an experiment with 0.2 eV sensitivity. The present situation will be discussed here along with MARE’s evolution and prospects. c 2005 Elsevier B.V. All rights reserved.
1. Introduction and science motivation The existence of a finite neutrino mass has been stated with a high degree of confidence by several experiments on neutrino oscillations. However, the absolute values of neutrino masses are still unknown. Among the possible ways to investigate this issue, direct observation of β decay has the advantages of being model-independent and safe, but is unfortunately limited by the difficulty of reaching very low sensitivity. For this reason, these kinds of experiments are only able to investigate the quasi-degenerate region of neutrino masses. ∗ Tel.: +39 031 238 6244; fax: +39 031 238 6209.
E-mail address: [email protected]. c 2005 Elsevier B.V. All rights reserved. 0146-6410/$ - see front matter doi:10.1016/j.ppnp.2005.11.004
S. Sangiorgio / Progress in Particle and Nuclear Physics 57 (2006) 68–70
69
The best limit to date for the νe mass comes from electrostatic β spectrometers measuring decay end-points, with limits of 2.2 eV [1]. KATRIN [2], the next-generation spectrometer, could reduce this limit to about 0.2 eV if successful. It will, however, approach the ultimate limit for these devices. An alternative and complementary technique able to eliminate the systematic effects related to the separation between the source and the detector is the calorimetric technique, by means of very sensitive cryogenic thermal bolometers that couples well to the study of the 187 Re β spectrum thanks to the smallest end-point available, good β activity (1 Hz/mg) and thermo-mechanical properties of both metallic Re and its compounds. The best published calorimetric limit of m ν < 15.6 eV (90% c.l.) was obtained by the MIBETA [3] experiment, an array of microbolometers based on semiconductor thermistors and dielectric (AgReO4 ) absorbers. Similar results have been achieved by the MANU group (m ν < 26 eV—95% c.l.) [4] with a single metallic rhenium crystal. To reach sub-eV sensitivity on neutrino mass, the present experimental sensitivity must be increased by two orders of magnitude. To accomplish this goal, a collaboration has been started for microcalorimeter arrays for a Rhenium experiment (MARE) [5]. This will be a two stage effort in which the intermediate step will aim to achieve m ν ∼ 2 eV and will be accompanied by parallel R&D activity towards the final setup. 3H
2. MARE phase I: Present status and simulations The main goal of MARE phase I is to reach a sensitivity comparable with that of current spectrometers, and this will be achieved by improving the present detector technology used in MIBETA and MANU to MIBETA2 and MANU2. This phase has already started, with encouraging preliminary results. MIBETA2 is testing three different kinds of thermistor technologies: (1) arrays of implanted micromachined silicon thermistors provided by ITC-IRST; (2) NASA silicon micromachined arrays; (3) neutron transmutation doped germanium arrays. Concurrently, MANU2 is already using the Ir–Au TES (transition edge sensor), realized by laser ablation techniques on a silicon substrate to get faster rise times and better signal-to-noise ratio (S/N). Along with detector optimization, it will also be necessary to scale up to hundreds of devices. A second but no less important goal for this first phase will be an improvement in the understanding of experiment systematics which could affect neutrino mass calculation. Systematics includes the theoretical spectral shape of the decay, solid state Beta Environmental Fine Structure effect, detector response function, unidentified pile-up, and data reduction. A Monte Carlo (MC) code has been developed to explore different configurations for MARE phases I and II. The main parameters are: Nev , the total number of beta decays; E, the detector energy resolution; and f pup , the pile-up fraction, which is related to the beta rate per detector Aβ and the pulse rise time t R . From MC, assuming that E = 10 eV (the present resolution is ∼80 eV) and f pup = 5 × 10−5 , we could estimate that Nev ∼ 5 × 109 β decays are needed to reach ∼2 eV sensitivity. 3. MARE phase II: Perspective and conclusions MARE phase I will be able to scrutinize the Mainz/Troitzk results with a completely independent approach. However, MARE would be less interesting if phase I were just a dead end. Moreover, the number of β events has to improve dramatically to Nev ∼ 5 × 1014 . This cannot be reached by a brute-force expansion of the MARE phase I experiment. Instead, substantial
70
S. Sangiorgio / Progress in Particle and Nuclear Physics 57 (2006) 68–70
improvements are needed. The performances required by the detectors are in fact beyond the capabilities of the existing devices. Luckily enough, devices that could suite these needs are already under study. Among these, TES or MMC (metallic magnetic calorimeters) equipped with multiplexed superconducting quantum interference detector (SQUID) read-out seem the most promising detectors to fill the gap. Another important feature will be the scalability of the detector obtained by taking advantages of the intrinsic modularity of the technique. The intention of the MARE proponents is to start phase II data gathering in 2010, in order to be competitive with KATRIN. References [1] [2] [3] [4] [5]
J. Bonn et al., Nuclear Phys. B Proc. Suppl. 91 (2001) 273. The KATRIN collaboration, hep-ex/0109033. M. Sisti et al., Nucl. Instrum. Methods A 520 (2004) 124. M. Galeazzi et al., Phys. Rev. C 63-1 (2001) 0114302. MARE proposal, 2005, Downloadable from: http://crio.mib.infn.it/wig/silicini/publications.html.