1. AMBER - A novel, non-invasive approach to direct neutrino mass measurement J.A.Thornby, M.J.Hadley, A.Lovejoy, Y.A.Ramachers Department of Physics, University of Warwick, Coventry, CV4 7AL AMBER (Anti-neutrino Mass Balance using Electrostatics and Radioactivity) is a novel technique for direct neutrino mass measurement. It is a beta decay endpoint experiment but differs in two important regards from conventional experiments, such as KATRIN [1]. AMBER will use a Ni-63 source, as opposed to Tritium, and substitutes traditional energy measurements for high precision Voltage measurements. SOURCE BIAS & HIGH VOLTAGE SUPPLY Collecting enough electrons to yield the Q-value potential of 66.9 KeV would be very slow. The AMBER source is biased (and electronics are floated) at a high voltage near the Q-value. → Only have to complete the last part of the decay spectrum. Therefore AMBER requires a stable High Voltage controller – a limiting factor of the experiment. KATRIN at the limit of technology here [1], but Warwick are developing a new such device in-house. CURRENT RESULTS & THE FUTURE OF AMBER AMBER still in its infancy, only a few months old! But numerous design iterations to date. Still much to be done: → DAQ/F-E design; Calibration using Cd-109; HT stability; Measure “zero” for absolute reference. However a very versatile technique, with other secondary applications: → Search for tensor currents; Creation of High Voltage calibration standard based on nuclear physics; Possible sensitivity to moon phases; Gas conductivity measurements. Some time has been spent testing AMBER in air, where there is leakage to ground. Unusual voltage decay characteristics have prompted further study. Added ability to set ball voltage before levitating – in order to observe decay curves. → Measure conductivity of air – not very well understood in atmospheric physics! Shown below are a series of decay curves – note the unusual dips for decays from positive voltages. → More work needed to understand the technique – systematic effect or new physics? MEASURING VOLTAGES – The Kelvin Technique AMBER’s capacitor “plate” is a levitating ball – suspended by two permanent magnets and an electromagnet. Levitation serves multiple purposes and is controlled by unique electronics. Levitation coils (see circuits) use only μW of power to maintain the ball’s equilibrium. Combined with vacuum conditions ~ 10 -5 mbar means: → Ball has no mechanical couplings to anything and has no leakage path to ground. Ball’s potential measured non-invasively relative to pickup plate (see circuits) serving as an inverse Kelvin probe. Alternating current in the levitation coil causes the ball to oscillate vertically at ~ 11 Hz. Oscillation causes change of capacitance between ball and pickup plate. Electronics measures ball’s potential (wrt pickup plate) in response to capacitance change due to oscillation. BASIC PRINCIPLE Ni-63 beta decay source used as an electron current source to charge a capacitor. Voltage measurements are converted to collected charge as a function of time. → Yields an integrated beta decay spectrum, used to reconstruct beta spectrum. Process is self quenching - only the most energetic electrons are able to overcome the electrostatic repulsion of the amassed charges. Eventually charge on the plate stabilises when no more electrons can be gathered. Potential created by stored charge corresponds to the beta decay endpoint energy. ELIMINATING EXTERNALS Ball has a potential with respect to all of its surroundings: → Coat the ball and its container in Nickel to eliminate contact potentials, using surface evaporation techniques. → AMBER’s electronics drive the container at a voltage that follows the ball’s potential. → Net result: Ball sees no external potentials and has no capacitance to its surroundings – except to Ni-63 source. PRECISION High precision essential for accurate endpoint and spectrum determination. AMBER prototype currently has 10mV precision, 1mV precision is tenable. KATRIN overall sensitivity to neutrino mass of 0.2 eV. Beta decay of Ni-63 has a Q-value of 66.945 keV. Kurie plot below shows endpoint energy resolution effects. Using new “magic wand” technology (pictured right, patent pending) and a modified “Cockcroft Walton ladder”. Above: The AMBER prototype vacuum chamber. Left: The AMBER prototype module, demonstrating: 1) Levitating ball 2) Pickup plate 3) Permanent magnets 4) EM levitation coil 5) AMBER electronics. REFERENCES [1] KATRIN Design Report, KATRIN Collaboration, 2004. ΔE = 0.01 eV ΔE = 0.93 eV 1 2 3 3 4 5