AMBER - A novel, non-invasive approach to direct

neutrino mass measurementJ.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.

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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.

Mini Voltage Decays

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Time (10 s)

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REFERENCES

[1] KATRIN Design Report, KATRIN Collaboration, 2004.

ΔE = 0.01 eV ΔE = 0.93 eV

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