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].
MEASURING VOLTAGES – The Kelvin Technique
SOURCE BIAS & HIGH VOLTAGE SUPPLY
• AMBER’s capacitor “plate” is a levitating ball – suspended by two permanent magnets and an electromagnet.
• Collecting enough electrons to yield the Q-value potential of 66.9 KeV would be very slow.
• Levitation serves multiple purposes and is controlled by unique electronics.
• The AMBER source is biased (and electronics are floated) at a high voltage near the Q-value.
AMBER will use a Ni-63 source, as opposed to Tritium, and substitutes traditional
energy measurements for high precision Voltage measurements.
→ 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.
Using new “magic wand” technology
(pictured right, patent pending) and a
modified “Cockcroft Walton ladder”.
• Voltage measurements are converted to collected charge as a function of time.
→ Yields an integrated beta decay spectrum, used to reconstruct beta spectrum.
• Levitation coils (see circuits) use only μW of power to maintain the ball’s equilibrium.
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• AMBER still in its infancy, only a few months old! But numerous design iterations to date.
→ Ball has no mechanical couplings to anything and has no leakage path to ground.
• Still much to be done:
→ DAQ/F-E design; Calibration using Cd-109; HT stability; Measure “zero” for absolute reference.
t
I
• Combined with vacuum conditions ~ 10-5 mbar means:
source
(U C (t )) dt
• 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.
• Process is self quenching - only the most energetic electrons are able to overcome
the electrostatic repulsion of the amassed charges.
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• Eventually charge on the plate stabilises when no more electrons can be gathered.
• Some time has been spent testing AMBER in air, where there is leakage to ground.
• Potential created by stored charge corresponds to the beta decay endpoint energy.
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PRECISION
• Unusual voltage decay characteristics have prompted further study.
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• 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.
• 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.
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→ More work needed to understand the technique – systematic effect or new physics?
Above: The AMBER prototype vacuum chamber.
Mini Voltage Decays
Left: The AMBER prototype module, demonstrating: 1)
Levitating ball 2) Pickup plate 3) Permanent magnets 4)
EM levitation coil 5) AMBER electronics.
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• 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.
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Voltage (100 V)
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UC 
C
CURRENT RESULTS & THE FUTURE OF AMBER
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ELIMINATING EXTERNALS
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Time (10 s)
• 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.
ΔE = 0.01 eV
ΔE = 0.93 eV
→ 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.
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