Poster Excerpt 14: HOLMES, an experiment for a direct measurement of neutrino mass
(The following text was submitted by Valentina Ceriale, Giulio Pizzigoni, and Andrei Puiu for the Holmes collaboration)
Measuring the neutrino mass is one of the most compelling challenges of modern physics. Experiments studying oscillation phenomena have clearly shown that neutrinos have non vanishing mass and at least three neutrinos with different masses exist; yet the absolute values of these masses remain unknown. There are several methods from which neutrino mass can be assessed: Cosmological observation, neutrinoless double beta decay and beta or electron capture spectrum end-point study. The latter is currently the one and only which can provide a model independent measurement of the absolute scale of neutrino mass.
Within this framework the European Research Council has recently funded HOLMES, a new experiment to directly measure the neutrino mass. HOLMES will perform a calorimetric measurement of the energy released in the electron capture decay of 163Ho.
This measurement was originally proposed in 1982 by A. De Rujula and M. Lusignoli, but only in the last decade the technological progress in detectors development allowed to design a sensitive experiment.
In a calorimetric measurement the energy released in the decay process is entirely contained into the detector, except for the fraction taken away by the neutrino. The typical spectrum of holmium electron capture is shown in the figure below; the region of interest for the neutrino mass measurement is the end point of the spectrum.
The most suitable detectors for this type of measurement are low temperature thermal detectors, where all the energy released into an absorber is converted into a temperature increase that can be measured by a sensitive thermometer directly coupled with the absorber.
HOLMES will be an important step forward in the direct neutrino mass measurement with a calorimetric approach as an alternative to spectrometry. It will deploy a large array of low temperature microcalorimeters with implanted 163Ho nuclei. The resulting mass sensitivity will be as low as 0.4 eV and it will also establish the potential of this approach to extend the sensitivity down to 0.1 eV and lower.
In order to reach the sensitivity of 0.4 eV it is necessary to collect statistics higher than 1013 decays.
To fulfill this task the best experimental configuration has been defined after Monte Carlo simulations: in its optimal configuration HOLMES will collect about 3×1013 decays with an instrumental energy resolution of about 1 eV FWHM and a time resolution of about 1 μs. For a total measuring time of 3 years, this requires a total 163Ho activity of about 3×105 Bq. These Ho nuclei will be implanted in 1000 detectors.
The detectors for HOLMES are absorbers of Gold-Bismuth, in which the radioactive source is embedded, coupled to a Transition Edge Sensor read out using a RF-SQUID, for microwave multiplexing purposes. The detectors will be cooled down to temperatures as low as 10 mK with a dilution refrigerator. It is very important to be able to read out simultaneously as many detectors as possible with a single transmission line in order to reduce the heat load of the coldest point of the refrigerator. For this purpose a special RF multiplexed system is being developed.
Specifically the temperature sensor, a Transition Edge Sensor (TES) will be a detector made of superconducting material used its resistive transition. Our experiment will use a Mo/Cu TES on SiNx membrane.
The TES microcalorimeters will be fabricated in a two step process. The first steps will be carried out at the National Institute for Standard and Technology (NIST, Boulder, Co, USA). The devices will be further processed in the Genova INFN laboratory. Here, the first step will be the deposition by means the ion implanter of a thin (few 100˚ A) layer of Au: 163Ho, then the bismuth absorber will be completed with a deposition of a second 1.5 µm bismuth layer to fully encapsulate the163Ho source. The second step will be a Deep Reactive Ion Etching (DRIE) of the back of the silicon wafer to release the membranes with the TES microcalorimeters.
To have more performing devices we need to work at low temperatures, in order to have low heat capacity. However the relatively high concentration of holmium (J = 7/2) could indeed cause an excess heat capacity due to hyperfine level splitting in the metallic absorber. Low temperature measurements have been already carried out in the framework of the MARE project to assess the gold absorber heat capacity (< 150 mK), both with holmium and erbium implanted ions. Those tests did not show any excess heat capacity, but further more sensitive investigations will be carried out.
To minimize the stray electrical inductance L which limits the pulse rise time, the TES will be arranged in 2 × 32 sub-arrays. This arrangement allows also to maximize the geometrical filling factor and therefore the 163Ho implantation efficiency.
As stated above, in order to perform a calorimetric measurement of the EC spectrum, about 6.5 × 1013 163Ho nuclei must be embedded in the absorber of each microcalorimeter. To avoid chemical shifts of the end-point, only holmium in the metallic chemical form must be introduced.
The embedding system is being designed and set-up at the Genova INFN laboratory: it consists of a holmium evaporation chamber to produce the metallic target and an ion implanter for embedded the holmium in the absorber.
163Ho is produced by the nuclear reaction 162Er(n,γ)163Ho. It is possible to have an erbium target enriched in 162Er at 30% in its oxided state, thus it is necessary to reduce and distill the 163Ho before implantation. This process consist in heating a mixture of metallic yttrium and oxide holmium above the Y melting point (1520°C). Due to the different standard enthalpy of formation, oxygen is captured by yttrium leaving pure metallic holmium by means of the reaction:
In order to separate the purified holmium from the other species one can rely on the higher vapour pressure of the metallic holmium at a fixed temperature (106 higher than oxide holmium).
The distilled material, collected by a cooled glass, was analyzed at XPS.In the Genoa Laboratory a set-up was made. It consists in a Knudsen cell mounted in a vacuum chamber coupled with a type C thermocouple, in order to measure the working temperature. Oxide holmium and metallic yttrium were placed in the cell and it is heated up to 1580°C.
The XPS analysis showed the only presence of metallic holmium.