Transcript ns_SAW_UTC_2004

J.A.Thornby – Department of Physics, University of Warwick, CV4 7AL
β-Decay and Neutrino Mass
Measuring the β Spectrum with AMBER
The neutrino – a tiny, electrically neutral, almost undetectable particle
– presents one of the greatest mysteries in modern physics. For
many years it was thought that the neutrino had no mass whatsoever,
and although recent evidence shows this not to be the case it has
thus far eluded measurement. Although small, the absolute mass of
the neutrino has far-reaching implications for physics and cosmology
and nuclear β-decay provides an excellent tool for probing it.
To accurately measure the end-point energy of β-decay a high
precision charge spectrometer with excellent resolution near to Q is
required, to avoid the smearing effects shown in Fig. 3. Enter AMBER.
N
P
d
u
d
d
u
u
W-
N → P + e- + υe
Vacuum Flange Mounting
1.38T Support Magnet
Levitating Ball
Aims and Proof of Principle
In order to accurately determine
the β end-point energy, AMBER
is required to measure the ball’s
potential, V, very precisely. Fig. 6
shows data taken over 24 hours
and demonstrates:
• Perfect ball insulation & stability
• Voltage resolution of ±1mV
Figure 6: Ball voltage
stability and resolution
eυe
Figure 1: β-Decay – Cartoon, Feynman Diagram & Nuclear Equation
dV/dt small
β-Decay is a process, occurring naturally within the nucleus of some
unstable atomic species, in which a Neutron decays to a Proton
accompanied by the emission of an electron and a neutrino (Fig. 1).
Kelvin Pickup Plate
Relative Decay Probability
Electromagnetic Coil
1
0.8
End-Point
Energy
0.6
0.4
0.2
0
Electron Energy
Emax
The electron has a spectrum of
energies (Fig. 2), up to a maximum,
fixed by the Q-value of the decay.
When Ee = Emax (“end-point energy”)
Eυ = 0, as total energy is fixed by Q.
If Emax ≠ Q then the discrepancy
corresponds to the energy required to
produce the neutrino. This is seen
more clearly in a Kurie Plot (Fig. 3)
dV/dt large
1.38T Support Magnet
AMBER employs a levitating ball bearing in a 1 × 10-4 mbar vacuum.
The ball is perfectly electrically insulated, meaning accumulated
charges cannot escape. The principle behind AMBER is to charge the
ball up using β electrons, while simultaneously and continuously
measuring its electrical potential.
Retarding force
K(E)
Zero neutrino mass
e-
Q-mνc2
Q
negative
Finite neutrino mass
Incoming β-electron
Effect of:
 Background
 Energy resolution
 Excited final states
Figure 5: The charge collection/repulsion process
Q
(dN/dE) dE  2(dE/Q)3
Q-dE
Figure 3: The Kurie Plot, a convenient linearisation of the β-spectrum
As electrons are collected the ball will become negatively charged,
thereby repelling electrons. Only electrons energetic enough to
overcome the repulsion can be collected. As the ball’s potential
increases the retarding force increases, making it progressively harder
for subsequent β-electrons to be gathered. A point will be reached at
which even the most energetic β-particles are insufficient to overcome
the repulsion of the ball. At this point the ball’s potential is equivalent
to the end-point energy.
AMBER aims to measure the
collected charge, inferred from
the change in the ball’s potential.
From this an integrated βspectrum (Fig. 7) can be
plotted. The true β-spectrum and
end-point energy can later be
recovered from this integrated
spectrum.
Figure 7: Integrated
β-spectrum of 63Ni
Figure 4: The AMBER apparatus – schematic drawing and prototype
Figure 2: The β–spectrum

2 mV
ΔV0
Increasing electron
source voltage
ΔV1
ΔV2
ΔV3
It is also necessary for AMBER
to demonstrate that the charge
collection process will work on
the ball a in a vacuum.
Fig.
8
demonstrates
this
process, using a variable high
voltage electron source.
ΔV0 =ΔV1+ΔV2+ΔV3
Figure 8: Demonstrating charge
collection in the vacuum
The AMBER prototype has demonstrated, in principle, the ability to
measure the neutrino mass but there is much work still to be done.
Acknowledgements
Dr. Yorck Ramachers, Mr. Adrian Lovejoy &
AMBER logo courtesy of Mr. Chris Allen.