AMBER - A novel, non-invasive approach to direct neutrino mass measurement

1. ΔE = 0.01 eV ΔE = 0.93 eV 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. • 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 inverseKelvin 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. • 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. • 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. Using new “magic wand” technology (pictured right, patent pending) and a modified “Cockcroft Walton ladder”. • 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? 3 1 5 2 • 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 66.945keV. • Kurie plot below shows endpoint energy resolution effects. 4 3 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. • 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. REFERENCES [1] KATRIN Design Report, KATRIN Collaboration, 2004.