A Novel Atto-Coulomb Charge Spectrometer and Applications

J.A. Thornby,aM.J. Hadley,a A. Lovejoy,a B. Morgan,aS. Navin,aD.Y. Stewart,a Y. Ramachersa,[*]

aDept. of Physics, University of Warwick, CoventryCV4 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

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 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 prototype and give a first performance characterization from initial data collected. A preliminary list of possible applications is also discussed.

Instrument description

A working prototype of the instrument has been built to demonstrate its feasibility and to study 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.

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 (sphericalcollector) 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, animportant 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.

The voltage measurement

The Kelvin technique represents a standard non-invasive 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, 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.

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.

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].

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 changescould 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 to be published in Rev. Mod. Phys.

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