1
A Novel Atto-Coulomb Charge Spectrometer and Applications
J.A. Thornby,a M.J. Hadley,a A. Lovejoy,a B. Morgan,a S. Navin,a D.Y. Stewart,a Y.
Ramachersa, *
a
Dept. of Physics, University of Warwick, Coventry CV4 7AL, UK
Do not use this line: Received date here; revised date here; accepted date here
Abstract
We present a new experimental concept for charged particle spectrometry. The design and selected applications will be
demonstrated. Physics applications ranging from atmospheric to particle physics have been identified so far. Our main
application targets precision measurements of electron energy spectrum shapes from beta-decay sources with applications in
particle physics. A preliminary energy resolution of the order of ten meV, corresponding to an extreme sensitivity of the
apparatus down to atto-Coulomb, appears to be achievable. The new concept is based on replacing the energy observable with
voltage measurements utilizing an inverse Kelvin-probe technique. An electrostatic charge integrator has been built;
magnetically levitated in vacuum for optimal insulation and whose charge is read out as a voltage by the non-invasively
coupled inverse Kelvin-probe. Sensitivity limits to rates of charge accumulation or static charge arise from stray capacitances
in our prototype set-up and the precision control of the high-voltage power supply for the case of beta-decay spectrometry.
Keywords: beta-decay; spectrometry; charge measurement; high-voltage
PACS: 29.30.Aj
1. Introduction
A novel instrument for charge spectroscopy has
been developed, enabling researchers in basic and
applied sciences to conduct measurements of
electrical charge, either transferred from a general
source unimpeded through vacuum or through a
medium. The original application for creating this
instrument is to measure charges delivered from
radioactive
beta-decay sources to high precision. The
———
*
Corresponding author. e-mail: y.a.ramachers@warwick.ac.uk
basic idea, as opposed to well-known detector
technology for beta-radiation, is to exploit a voltage
measurement instead of an energy measurement,
since voltages can be measured far more precisely.
The concept of this device is to integrate charges
(ideally loss-free) on a suitable collector, building up
a voltage which is then measured non-invasively.
Solutions to these challenges are obtained in the form
of insulating the collector by magnetic levitation in a
vacuum and utilizing the Kelvin-probe technique [1]
for precision non-invasive voltage measurements [2].
In the following, we describe the instrument
2
prototype
and
give
a
first
performance
characterization from initial data collected. A
preliminary list of possible applications is also
discussed.
2. Instrument description
A working prototype of the instrument has been
built to demonstrate its feasibility and to study
Fig. 1 Photo of the prototype outside the vacuum chamber.
performance characteristics for later optimization
(see Fig. 1).
The relevant structural parts visible in Fig. 1 are:
 Mounting as a rigid vacuum insert on flange.
 Two cylindrical permanent magnets for
levitation of collector.
 Small coil for levitation balancing and
modulation control.
 Levitated spherical collector magnet.
 Electrostatic shielding box surrounding
collector.
 Kelvin-probe pick-up plate beneath collector.
3. The collector insulation mechanism
In the prototype a spherical dipole magnet, 9 mm
diameter, is used as a collector. This magnet is
magnetically levitated in order to achieve best
possible insulation when operating in vacuum. Fig.1
depicts the levitation mechanism which consists of
two cylindrical magnets (23 mm diameter, 20 mm
height) in a push/pull configuration, mounted above
and below the specimen to be levitated, and a small
electromagnetic coil mounted in between but below
the specimen. All permanent magnets consist of
NdFeB compound, grades N48 (cylindrical) and N42
(spherical collector) corresponding to a magnetic
remanence of 1.38 Tesla and 1.28 Tesla respectively.
The positions of the levitating magnets are chosen
such that the small coil only serves for minute
corrections and hence requires only μW during
operation. As the sole active device, the coil is
regulated by simple control electronics, employing a
Hall probe. An additional feature of this levitation
mechanism is the ability to move the sphere vertically
in a stable range of about 15mm, an important degree
of freedom for the Kelvin technique. Once a suitable
point of equilibrium is determined, the control circuit
can be switched into an integrating mode such that
the sphere position is fixed and re-established after
any disturbance, creating a stable and reproducible
configuration. For any superposed periodic motion,
see below, this stable equilibrium position remains
fixed.
4. The voltage measurement
The Kelvin technique represents a standard noninvasive voltage measurement procedure in surface
physics. A specimen can be investigated for surface
charges by a non-contacting sensor close to the
surface. A voltage difference between the probe and
sensor can be measured precisely. If the sensor
oscillates it produces an AC current at its output due
to the change in capacitance. By tuning the bias on
the sensor this current can be made to disappear when
the probe and sensor show equal bias voltage, i.e.
when the capacitance between them disappears. This
null-measurement technique is only limited in
sensitivity by the noise on the current measurement,
3
hence can be arranged as a high precision voltage
measurement. Furthermore, the absolute voltage
cancels and identical precision measurements can be
obtained on a 1 V level as on a 90 kV level. Merely
practical considerations of working at 90 kV warrant
special attention.
For our spectrometer we prefer to oscillate the
probe as opposed to the sensor, which is achieved by
delivering an AC modulation via the EM coil. Our
sensor is a simple pick-up plate, mounted rigidly
below the oscillating sphere. A non-negligible
advantage of this inverse mounting which we refer to
as inverse Kelvin-probe, is the possibility to mount
the pre-amplification electronics directly on the
sensor. After pre-amplification, the signal is fed into
a Lock-in amplifier to retrieve the reference
frequency, now in the form of an AC voltage input
signal.
The last remaining challenge then is to increase
the charge sensitivity of the device. From the basic
relation Q=CV (Q representing charge, C the
capacitance and V for voltage) it is found that
ΔV/ΔQ=1/C. Thus reducing the capacitance yields a
greater voltage response per unit charge. Therefore a
box has been built surrounding the sphere. The box
potential can be bootstrapped together with that of the
pick-up plate to follow the probe potential such that
at equilibrium, the capacitance of the sphere to the
environment (any other conductor like the vacuum
chamber walls) is minimised. Leaving one side of the
box open, a path for incoming charges is left and it is
this capacitance that dominates the spectrometer
sensitivity. Preliminary estimates yield capacitances
of the order of ten Femto Farad which would enable
the instrument to achieve Atto-Coulomb sensitivity.
Additionally, the option to set the box potential at
any fixed value is also included in the control
electronics.
5. First Data
Our first data sample results from an important
performance test for the device, i.e. the long-term
stability test under various external conditions. So
far, we can release data from operating the
spectrometer in Air but nevertheless closed in the
vacuum chamber. A minimal offset bias remains after
lifting the sphere, and we monitor this offset for a
total of 13 days, see Fig.2. Our first result can be
summarized as follows: long-term stability of the
device in Air at atmospheric pressure amounts to 0.6
mV (std. dev.) here on a mean value of 4.1 mV over
the whole period. Fig.3 displays the power spectral
density of the data. A 1/f noise contribution can be
seen as well as a constant noise-floor. The maximum
power in that constant noise corresponds to 2.3 mV
noise and should be present at all frequencies. The
smooth roll-off towards higher frequencies is a
consequence of the Lock-in amplifier filter setting.
An artifact frequency at 1/12 Hz is visible which
originates from the Lock-in Amplifier. A series of
test measurements has been started to further
characterize the device, particularly in vacuum. First
tests using a radioactive beta-decay source to charge
the collector are planned in the near future.
6. Applications
Several modes of operation for this device can be
classified: low- and high-voltage applications and
closed configuration applications. A straightforward
low-voltage application would be the measurement of
gas conductivity as function of bias. This device
would act as a new technique for such studies,
alternative to the Gerdien technique which is
routinely used in atmospheric physics for
conductivity measurements [3]. The advantage of our
spectrometer would be that no other dielectrics than
the gas are involved, hence removing any systematics
from leakage currents along similar level insulators.
High-voltage applications require a high-voltage
power supply that would have to be stabilized on a
similar level to the voltage measurement precision in
order not to dominate the uncertainties. A power
supply with 10 mV precision on 100 kV bias
(controller patent pending) has recently been
constructed and tested in our electronics workshop at
Warwick. Two main applications are planned: First
development of a high-voltage standard from first
principles, as opposed to existing ones derived from
low-voltage standards. Secondly, the precision
measurement of beta-decay spectral shapes for
studies of the weak interaction structure and neutrino
physics [4].
4
Fig. 3 Power specctrum of the 13 days data sample. Noise
contributions from 1/f, constant noise-floor and Lock-in filter
roll-off can be seen as well as a spike at 1/12 Hz.
Fig. 2 Bias measurement stability test with total duration of 13
days; for details, see text.
More speculative applications are connected with
operating the device in a closed configuration.
Assuming constancy of charge once the device is
closed, i.e. preventing further charges from being
collected, one might be able to perform weak force
measurements. Tiny mechanical changes could be
induced by weak forces, i.e. capacitance changes on
the equilibrium position of the collector with respect
to the pick-up plate. Another possibility is to
transform the closed configuration into an active
detection volume for a gas-filled counter. An
extremely constant high-bias between collector
sphere and an outer sphere could be established by
coating the collector with a radioactive source and
leaving it to charge up the floating sphere to the
maximum voltage corresponding to the maximum
decay energy. Transients from ionization in the gas
volume would then be detected by small discharge
voltage changes on the collector like any proportional
counter. However, this configuration would operate
self-powered and virtually maintenance free, offering
long-term operation, large volume detectors which
would be cost-effective and simple to produce.
We are confident this list of potential applications
is not exhaustive. Work in progress comprises the
transformation of the prototype into a more optimized
configuration for the high-voltage applications listed
above
and
continuous
performance
and
characterization tests.
Acknowledgments
Y.R. acknowledges a start-up RDF-grant from the
University of Warwick and S.N. a Nuffield
Foundation
undergraduate
research
bursary
URB/33134.
References
[1] W.A. Zisman, Rev Sci. Instrum. 3 (1932) 367.
[2] G.N. Luo et al., Rev. Sci. Intrsum. 72 (2001) 2350 and
references therein; L. Kronik and Y. Shapira, Surf. Sci. Rep.
37 (1999) 1 (ch.3.1); J.D. Baikie et al., Rev Sci. Instrum. 62
(1991) 1326
[3] K.L. Aplin, Rev. Sci. Instrum. 76 (2005) 104501 and
references therein
[4] N. Severijns, M. Beck and O. Naviliat-Cuncic, Preprint
http://arXiv.org/abs/nucl-ex/0605029 to be published in Rev.
Mod. Phys.