john thornby 4 th april 20071 development of a novel charge spectrometer iop nuclear and particle...
TRANSCRIPT
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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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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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Acknowledgements & Disclaimer
Acknowledgements:
Dr. Yorck Ramachers, Adrian Lovejoy,
Disclaimer:
This is not strictly a Nuclear Physics talk!
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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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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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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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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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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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Levitation Electronics Ball equilibrium maintained with μW Power!
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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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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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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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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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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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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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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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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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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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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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The Kurie plot K(Ee) is a convenient linearization of the beta spectrum
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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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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High Voltage System (work in progress)