Ion beam cooler for selective optical filtering of anions

Accelerator mass spectrometry (AMS) has proven to be a powerful tool for the detection of long-lived radioisotopes at typical abundances in the range of 10-12 to 10-16, not only for radiocarbon dating in archeology, but also for research in many other fields such as earth sciences, environmental sciences and biomedicine. However, being a mass spectrometric method, AMS is limited by the ability of separating neighboring isobars. Various methods exist to overcome this issue like the use of negatively charged ions, the use of special molecules or separating the isobars via their different energy loss in matter. The latter depends strongly on the available particle energy and therefore sets a lower limit for the size of the accelerator. In the framework of this project we want to propose a novel approach in separating isobars using a method called "selective optical filtering". Negatively charged ions are neutralized by interaction with a laser beam in a process called "laser photodetachment". Photodetachment can occur if the photon energy is higher than the electron affinity of the anion. As the electron affinity is dependant only on the electronic structure of the atom this process is element sensitive. With a suitable choice of laser wavelength it is possible to selectively deplete interfering isobars while leaving the isotope of interest unaffected. However, sufficient knowledge of allowed transitions in the negative ions of interest is indispensable. Especially for the case of molecular negative ions very little is known about allowed transitions. Due to the small cross-sections of the photodetachment process a long interaction time together with a high photon density is essential to achieve sufficient suppression of the interfering isobar. This requires a decent overlap of the laser beam with the negative ions, which can be achieved by slowing down the ions in a gas-filled radio frequency quadrupole cooler. Another advantage of such an ion beam cooler is its ability to deplete excited states in order to produce pure ground state negative ion beams. This is essential for studies of fundamental properties of negative molecular ions as molecules produced in the sputtering process tend to occupy highly excited molecular states and only a small fraction of the beam consists of ground state molecules. In order to study the fundamental properties of negative molecular ions we propose to develop a gas-filled radio frequency quadrupol ion beam cooler specially suited for capturing and confining negatively charged molecular ions. For testing the device a small negative ion beam spectrometer consisting of a cesium sputter ion source and a combined electrostatic and magnetic mass filter providing a mass separated negative ion beam will be installed. The setup will be designed to provide negative ions with masses up to ~300 amu. To assess the performance and the ion optical properties as well as to study fundamental properties of selected atomic and molecular negative ions standard laser equipment is part of this proposal. For further fundamental studies more sophisticated tunable laser systems are necessary. To perform such studies the ion cooler will be temporarily installed at the negative ion beam facility GUNILLA (Göteborg University Negative Ion (co)Linear Laser Apparatus) at the University of Gothenburg. The required provisions for combining the cooler device with GUNILLA will already be taken into account in the design phase of the device.

For the analysis of samples with accelerator mass spectrometry (AMS) it is of key importance to make sure that the results are not disturbed by other particles having a similar mass. Within the scope of this research project a novel approach to prevent this has been pursued. The in the beginning negatively charged unwanted particles will interact with laser light which will take away the excess electron. This allows the measurement of the sought-after particles. This so-called laser photodetachment is a process solely dependent on the electronic configuration of the ion and thereafter dependent on the nuclear charge and not on its mass. By careful selection of laser light of suitable wavelength one can make sure, that only the unwanted particles are neutralised by interaction with the photons and the ions of interest remain intact. A major drawback of this method is the low cross-section at the currently available laser powers. To increase the efficiency of this process the particles will be stopped in a gas-filled radio-frequency quadrupole cooler which ultimately increases the interaction time between the laser photons and the ions. Such a device was constructed and tested at a specially constructed test facility. The results are promising and show transport efficiency through the cooler of up to 80 %. At ion currents of 80 nA and a continuous wave laser having 10 W output power a suppression of the unwanted particles by a factor 105 could be achieved. This gives us confidence, that the developed ion cooler fulfils its demands. The next step is to connect the device with an accelerator mass spectrometer to be able to demonstrate the applicability of our methods under real-world conditions. This should allow for the detection of rare particles, which could up to now only be measured at large facilities or not at all. One example is the isotope 60Fe, which is produced in supernova explosions. The detection of this particle on earth proofs the existence of an earlier supernova near the earth. Iron-60 could up to now only be detected at large facilities. Another example is the isotope 182Hf, which is also produced in supernova explosions. This particle could up to now not even with the largest facilities be detected in sediment layers on earth free of doubt.