4th April 2007 1

John Thornby

Development of a Novel Charge Spectrometer

IoP Nuclear and Particle Physics Divisional Conference

John Thornby

University of Warwick

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John Thornby

Overview

Motivation for a new technique How it works

Empirical principles Experimental details Instrument characterisation Recent results

Review: Applications, Goals and Outlook

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John Thornby

Acknowledgements & Disclaimer

Acknowledgements:

Dr. Yorck Ramachers, Adrian Lovejoy,

Disclaimer:

This is not strictly a Nuclear Physics talk!

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John Thornby

Motivation for a new Technique Once upon a time in Warwick…

This man had a crazy idea

β-endpoint experiment View to perhaps measuring absolute υ mass Borrowing concepts from Mainz & Troitsk

BUT Laboratory scale & fraction of budget!

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John Thornby

The Idea… Past experiments basically count electrons

Replace with a continuous rate of change observable?

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The Idea Continued

t

sourceC dtIC

V0

1

β-isotope used as a current source Charges a capacitor (simply a charge collector) Charges converted to Voltages Obtain an integrated β-spectrum

VC

C

Isource

e-

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John Thornby

So, how does it work? Process self-quenches:

Accrued e- provide increasing retarding potential → Cost and noise-free!

Only most energetic e- overcome repulsion Eventually no more electrons will make it… Corresponds to end-point energy. Measure it!

63Ni e-

e-

e-

e-

e-

e-

Source Collectore-e-e- e-e- e-

e-e-

e-

e-

e-

e-

e- e-

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Integrated β-Spectrum

Most electrons energies contribute → dVC/dt large

Only rare high-energy electrons contribute

→ dVC/dt small

End-point @ dVC/dt = 0

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John Thornby

Now the clever part… Capacitor actually a dipole magnet ball bearing… Magnetically levitated & held ~ 10-4 mbar vacuum

Accrued charge cannot escape!

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John Thornby

Levitating the Ball Magnetic forces balance gravity Unique in-house designed electronics

Provides stable, reproducible configuration

Levitation coilPermanent Magnets

Hall Probe

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John Thornby

Levitation Electronics Ball equilibrium maintained with μW Power!

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John Thornby

Non-Invasive Voltage Measurement – Inverse Kelvin Technique Supply 11 Hz, 1V p-p sine wave to coil AC in levitation coil → field oscillates Ball oscillates up and down above a special

pickup plate

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John Thornby

Inverse Kelvin Technique Continued Ball oscillates, capacitance wrt pickup plate

changes → induces AC voltage on pickup

Amplify the signal and analyze AC output with PSD (Lock-in amplifier)

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John Thornby

Calibration

Induced AC voltage on pickup proportional to DC voltage on ball

PSD Returns an error voltage

AC/DC Conversion - Calibration Line

y = 0.1995x + 0.1956

R2 = 0.9987

-80

-60

-40

-20

0

20

40

60

80

-400 -300 -200 -100 0 100 200 300 400

PSD Error Voltage (mV)

DC

Bal

l V

olt

age

(V)

Contact potential

Vball(V) ~ 0.2 × VPSD(mV)

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John Thornby

Collector Insulation

2mV band

Need to know how stable voltage is in order to reliably determine the quench/end point

Justified in quoting stability of ±1mVCorresponds to 1meV energy resolution!

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Charging the Ball in vacuum

Can we charge the ball in vacuum?

β/conversion electron isotopes

Stimulated emission electrons

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Plan “B” – Stimulated Emission

Sharp needle at a large –ve potential At ~ -1.5 kV electrons are emitted Detected on the ball!

~ -1.5 kV

Tungsten needle, atomically sharp

e- Ball

~ 1.5 cm

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John Thornby

Electron Collection Demonstration

ΔV0 = 0.903 V

ΔV1 = 0.030 V

ΔV2 = 0.315 V

ΔV3 = 0.561 V

ΔV0 = ΔV1 + ΔV2 + ΔV3

Offset consistent with genuinely charging the ball

-1.5 kV

-1.75 kV

-2.0 kV

0 V

-250 V increments, every 5 minutes

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John Thornby

Air Conductivity Measurement Capacitors can be discharged too…

Low voltages are well-fit by exponential Not so good for higher voltages Physics to be investigated here Need to measure Capacitance, since

exponential decay constant is f(C,R)

Ball Voltage vs. Time

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John Thornby

Outlook, Review and “to do” list…

Exciting and innovative prototype experiment Demonstrated insulation & charging of collector Next step is to use a real source in vacuum Calibrate HT controller

109Cd, mono-energetic particles as a reference Measure Capacitance of ball to surroundings

(non-trivial) → Air/Vacuum conductivity Perform tests in a variety of configurations

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John Thornby

Potential Applications

β-endpoint → υ mass Possible sensitivity to neutrino mass hierarchy Air & vacuum conductivity - C(P,T) Gas purities via conductivity Calibration of a new High Voltage standard Possible sensitivity to Lunar activity!

SUGGESTIONS WELCOME!

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The End

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Bonus Material…

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Why 63Ni? Cheaper than Tritium! Well understood Gamow-Teller decay Easy to handle, Ni plating is easy

Can coat ball, box & plate in Ni Reduce contact potentials

Q value 66.945 keV (comparitively high) We are therefore insensitive to electrons resulting

from beta decays of lower Q value sources Can therefore use Pb shielding!

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Stray Capacitance

Require ball’s capacitance to the system

Spectrum Reconstruction: TVI

CV)(

Vacuum Chamber (Earth)

Source

To amplifier…

C1

C2

C3

NB: Not to scale

To HT system…

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John Thornby

Eliminating Externals

10-4 mbar vacuum

Ball floating (no leakage to ground)

Box “boot-strapped” to same potential as ball

All surfaces coated in Nickel (no contact p.d)

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John Thornby

Double sourcecontrol of systematic

Pre-spectrometerselects electrons with E>Q-100 eV

(10-7 of the total)

Better detectors: higher energy resolution time resolution (TOF) source imaging

Main spectrometer high resolution ultra-high vacuum (p<10-11 mbar) high luminosity

Strategy better energy resolution E ~ 1 eV higher statistics stronger T2 source – longer measuring times better systematic control in particular improve background rejection

Goal: to reach sub-eV sensitivity on Mυ letter of intent - 2001

hep-ex/0109033

KATRIN design report Jan 2005

KATRIN: Next generation MAC spectrometer

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John Thornby

The Kurie plot K(Ee) is a convenient linearization of the beta spectrum

QQ

Q–Mc2 Q

K(E

)

zero neutrino mass

finite neutrino mass

effect of: background energy resolution excited final states

Q-E

Q

(dN/dE) dE 2(E/Q)3

And on the Kurie plot…

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John Thornby

Kurie plot superposition of three different sub - Kurie plots each sub - Kurie plot corresponds to one of the three different mass eigenvalues

The weight of each sub – Kurie plot will be given by |Uej|2, where

|e = Uei |Mi i=1

3

Q – M3

Q – M2

Q – M1

Q Ee

K(Ee)

K(E

e)

Ee

Mass Hierarchy

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John Thornby

High Voltage System (work in progress)