By Bob Silberg, Science Writer
The instruments employed at UCLA, UCSD, and Open University differ in many respects, but each has a method to free the solar oxygen atoms from the collection material, and each uses a mass spectrometer to separate and count the isotopes.
Each mass spectrometer is curved like a racetrack and rigged with a series of magnetic and electric fields that allow only the desired atoms (the oxygen isotopes from the Genesis sample) to travel around all the curves to the finish line, where the detectors count them. Everything else crashes into the walls of the instrument.
Here's how UCLA's "MegaSIMS" instrument works in a little more detail. The instruments at UCSD and Open University accomplish similar tasks by somewhat different means. For example, while the UCLA team uses a beam of cesium ions as a microscopic sandblaster to free the solar oxygen atoms embedded in the collection material, the teams at UCSD and Open University use powerful lasers.
It takes about a millionth of a second for an atom to go through all these steps, from its starting position on the sample-collection wafer to the detector at the other end of the machine.
- Solar wind particles are blasted free from the collection material. This creates a cloud of atoms, including atoms from the solar wind (a small portion of which is oxygen) and atoms from the silicon carbide in which they were embedded. Some atoms in this cloud combine into molecules, and a small fraction of the molecules and free atoms gain electrons and take on a negative charge. Those negatively charged particles are repelled by the sample (which has a negative potential) and pushed into the next part of the instrument.
- A magnet filters out particles with more or less mass than the oxygen atoms. The particles pass through a magnetic field that diverts the path of particles that have a mass of 16, 17, or 18 just the right amount to enable them to continue flying through the first curved part of the instrument. This includes the three oxygen isotopes and also any molecules that happen to have one of those masses. Particles with less mass are diverted too much and particles with more mass are diverted too little. Both hit the walls of the instrument and don't make it through to the detectors at the end.
- A "stripper tube" breaks up molecules that have the same mass as the oxygen atoms. The particles that are able to continue flying through the instrument include the targeted oxygen atoms and any molecules that happen to have similar mass. The molecule that poses the biggest problem is OH, which consists of an atom of either oxygen-16 or oxygen-17 attached to an atom of hydrogen, which has a mass of one.
To get rid of these molecules, the surviving particles are all driven through a tube of argon gas at very high speed. As they enter the tube, each particle of a given charge has the same amount of energy, whether it's a molecule or free atom. Collisions with the argon molecules break the traveling molecules apart into their constituent atoms, each of which has only a fraction of the energy of the original molecule. So as they leave the tube, none of the constituents of the broken-up molecules has the same energy as the solar oxygen atoms.
- An electric field sorts by energy, filtering out everything other than solar oxygen atoms. The particles fly through an electrostatic analyzer (ESA) which uses an electric field to perform a function similar to that of the previous magnetic field. It curves the path of particles that have a particular energy just the right amount to permit them to continue flying through this curved section of the instrument. Particles with higher or lower energy crash into the walls and end their journey. The only particles with the right energy to make it through this section are the oxygen atoms that were not parts of molecules.
- A final magnet separates the 3 oxygen isotopes, sending each to a separate detector. Now that only oxygen atoms are continuing through the instrument, all that remains is to separate and count the 3 oxygen isotopes. Thanks to the ESA, all of the atoms of each type of isotope have the same amount of energy. That enables this final magnetic field to separate the atoms by mass. It does this by diverting the paths of the oxygen atoms by amounts proportional to their masses. Oxygen-18, which is the most massive of the three, has its path least affected by the magnet. Oxygen-17 is affected a little more. Oxygen-16 has its path affected the most. Each of their paths takes them to a separate detector, which counts the number of atoms that hit it. By comparing these numbers, the researchers can calculate the relative abundance of each isotope in the solar wind sample.
More on UCLA's MegaSIMS instrument
Last Updated: 4 December 2012