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Fast Atoms

Quantum Mass Spectrometer | Fast Atoms

In our last article ‘Beam me Up’ we talked about how we have successfully steered a beam of ions around our new instrument. Here we discuss our next step, how we neutralise the ion beam to produce a beam of fast atoms.

The Quantum Mass Spectrometer will offer highly selective separation of atoms with different masses. For example, we will be able to determine the amount of carbon-14 versus the amount of carbon-12 or carbon-13. The extreme selectivity of the new Quantum Mass Spectrometer is made possible by the interaction of lasers and a beam of fast atoms. At 350 000 km/hr these atoms are fast enough to travel from London to New York in less than a minute.

How is a beam of charged ions converted to a beam of uncharged atoms? The magic happens within a part of the instrument called the charge exchange cell (CEC). A beam of cations (atoms missing an electron) pass into the Charge Exchange Cell where they interact with sodium vapour. Electrons from the sodium atom are passed onto cations in the ion beam converting them back to atoms. The interaction with sodium atoms doesn’t slow the beam, the resulting beam of atoms travel at the same speed as the ion beam.

A charge exchange cell has been successfully installed and tested in our experimental set-up. The cell is loaded with sodium metal, which is vaporised when heated to 250 – 300°C. To heat the charge exchange cell, four 300 Watt cartridge heaters are secured to the body of the cell with metal straps. The heaters are connected together in a parallel circuit, through which a current is passed. This causes the temperature of the heaters to increase.

The temperature of the charge exchange cell is monitored using three thermocouples, which are read using a digital multimeter. One thermocouple is placed against the centre of the cell where the alkali metal resides. The other two thermocouples are secured against each of the CEC’s stainless steel ends. The temperatures of the ends are monitored to ensure that they remain cool enough that alkali vapour condenses at the CEC’s entrance and exit before it has the chance to escape into the apparatus, which would be difficult to clean up and significantly worsen the vacuum pressure.

After installation, the CEC was heated, and a beam of argon ions was passed through the CEC. It was observed that a fraction of the ions were neutralised, verifying that the CEC is working as planned.

During the initial stages of beamline development, each component is developed, installed and tested individually, to ensure that it behaves as required. During later stages, the beamline components can start to be used together as an instrument. Observing neutralisation of the ion beam is a significant step in the development of our apparatus, as it indicates that the ion source, ion optics, vacuum systems, charge exchange cell and detector systems are all working in concert.

The final pieces of the puzzle are now the laser beams, which will be tuned to manipulate and ionise the atoms of interest and select them from the fast atomic beam. We are currently in the process of setting up the optics necessary to transport the laser beams from their outputs through the instrument, after which spectroscopy experiments can begin.

Beam Me Up

Beam me up!

We’re delighted to announce that we’ve reached a significant milestone on our journey to developing a new trace analysis technique.

Our novel instrument exploits the interaction between a beam of ions that are accelerated through a vacuum and highly tuned lasers. Our team have successfully delivered the first ions to the end of the instrument.

A key step in commissioning a new spectrometer is the process of tuning ions from their source to the end of the instrument. It is always a reassuring moment when ions can be delivered to the final detector and a signal can be observed.

The commissioning process starts with the ion source, which is the beating heart of any mass spectrometer. The performance of the machine, its capability and capacity to deliver science will have a lot resting on having an optimized ion source.

“It has taken some time to get our ion source working optimally and to tune the beamline voltages such that the resulting ion beam will reach the detector. Understanding the relative impact of changing the voltage on each lens and steering element, with the aid of some simple simulations, has been crucial. I was very happy when we finally measured ions on the detector, as this represents an important milestone in the commissioning of this instrument, and is a good indication that all elements in the beamline are working as they should be.” – Holly Anne Perrett, Team Leader

The mass spectrometer Artemis Analytical is developing uses a laser-based method to enhance sensitivity sufficiently to allow rare isotopes such as 14C and 85Kr to be detected. These isotopes have natural abundances of one part per trillion or less, which requires a very intense initial ion beam. Even at moderate ion speeds, there will be enough to make components get very hot (and even melt) if the beam hits them. Therefore, before using intense beams the first step of the commissioning process uses an intermediate intensity ion source, which produces 1/1000th of the final beam current before scaling the system up to the maximum intensities.

We have utilized a device called an electron impact gas ion source that produces beams of argon or krypton for commissioning the instrument. A controlled leak of gas is bled into the ion source to produce almost a trillion ions per second for the commissioning process. The ions are accelerated through a voltage of approximately 1000V and then directed through the vacuum chambers. At these energies, it is possible to use static voltages applied to electrodes.

The process of manipulating the ions is similar to manipulating light with mirrors and lenses and hence the components within a spectrometer are referred to as “ion optics”. To finally deliver the beam to the end of the spectrometer requires adjusting over 25 individual power supplies connected to the ion optic elements to bend and focus the ions.

The first-time ions are accelerated and directed through the instrument is a fiendishly complex puzzle, which is made much harder because unlike manipulating light with optics, there are no visible cues to where the beam is going when it is no longer detected.

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