Hey Dr Zoomie – seems I hear about spectroscopy a lot – alpha, gamma, neutron, and mass (or is this a Catholic thing?). I know it’s got something to do with measuring and identifying things, but that’s about it. Can you tell me what it’s about, what these are used for, and how they’re different? You never know when I might need this info to win a bet at a geek bar or something. Thanks!
Boy – talk about something I work with or use the results of almost every day! I don’t do alpha spectroscopy myself – no need, and I’m not a big fan of the chemistry that’s often required. But I’ve certainly sent environmental samples for alpha spec and I’ve used the results in any number of projects -it’s complicated to perform and it costs a lot, but when you need it, it’s absolutely essential. Gamma spec is much simpler (although it can require some chemistry as well if you’re looking for very trace amounts of a contaminant); I’ve got a few gamma spectroscopy devices I use on a regular basis, as well as giving training on how to use them and (more importantly) how to interpret the results. You can do beta spectroscopy, too, although the spectra often require some experience and skill to interpret properly (I was never very good at it), and to do neutron spectroscopy well can require a literal ton or two of equipment that fills a small cargo container.
All of these are various forms of radiation – with radiation, spectroscopy means measuring the amount of energy in each bit of radiation (photon, neutron, alpha or beta particle) you measure and putting the equivalent of a tick mark in a column for that particular energy. Cs-137, for example, emits a gamma photon with an energy of 662 thousand electron volts (662 keV), so if you collect a spectrum that has a large spike at that energy, you know you’ve found some Cs-137. These use different schemes to measure the radiation’s energy – for example, scintillation detectors will measure the amount of light produced in a scintillator by the energy the radiation deposits into the scintillator. Another way to measure energy is to see how far into a detection medium a radioactive article will penetrate; one form of neutron spectrometer does this by placing detectors inside ever-larger spheres of plastic and counting the number of neutrons with sufficient energy to penetrate through to the center.
A somewhat specialized form of photon spectroscopy involves looking at the energies of x-rays emitted by atoms – for reasons of quantum mechanics, electrons occupy very specific orbits in any of a number of shells surrounding an atoms; because of this, atoms of every element emit very specific x-rays when their electrons make transitions from one shell to another. Atoms can be encouraged to emit these x-rays by bombarding them with electrons or protons; analyzing the precise energies of the emitted x-rays can identify the elements that are present in a sample. I saw this in action in an electron microscope my second year of college – electrons were used to image a sample, but they also induced x-rays when they struck it; by analyzing the emitted x-rays we could tell what elements were present.
And then we get to mass spectroscopy which, of course, measure the weightiness of Catholic services. Lighthearted sermons register differently than hellfire-and-brimstone and by measuring a career’s worth of sermons we can tell how seriously to take the priest. And I jest of course – I went to a Jesuit school and sometimes just can’t resist. Anyhow…
Just as radiation spectrometers measure the energy of photons or particles, mass spectrometers measure the mass of the atoms and molecules being studied. And – wow – do they have a lot of uses!
One form of mass spectrometry is to strip atoms of their electrons, accelerate them using an electromagnetic field, and they pass them by a strong magnet to bend their path. Heavy atoms have more inertia and their path is bent somewhat less than light atoms – by measuring where the atoms land in a series of electrically charged collectors we can tell how much they were deflected and, thus, their mass. Doing this on an industrial scale, as was done in Oak Ridge Tennessee during WWII, can separate the lighter U-235 atoms from heavier U-238, producing enriched uranium. The same setup can be used to count carbon-14 atoms in a sample, letting scientists determine the age of a sample of organic material.
Sorting out molecules by weight is a little more complex because molecules can have a huge range of masses; the trick here is to partially ionize the molecules and then try to accelerate them, to deflect them with a magnetic field, see how far they can progress along a path, No matter the technique, the principle is the same across the board – the less path changes, the less a particle progresses along a path. This is the general principle behind gas chromatography (used to identify chemical vapors), ionization mobility spectra (used in some instruments that identify explosives and drugs), And, interestingly, this is used to help sequence DNA, although with a few wrinkles that are unique to that particular line of inquiry.
Of course, there’s also visible light spectroscopy; every element has very specific colors of light that they emit – every electron gives off (or absorbs) specific wavelengths when they transition from one energy level to another (called atomic emission spectra). Under other circumstances, elements will absorb these same specific wavelengths – this is called atomic absorption spectra. Light from distant stars and galaxies that passes through clouds of hydrogen or helium gas will show black lines where these wavelengths have been absorbed by the gas. This is how astronomers can tell what distant stars are made of – by looking at the stars’ spectra. Either way, it never ceases to impress me that, looking at galaxies billions of light years away, astronomers can figure out exactly what elements they’re made of.
That’s about it on this topic – at least, without getting really into the weeds. The bottom line is that spectroscopy helps us to better understand the composition of…well…everything – radionuclides, chemical elements, molecules, and more. And not just here on Earth or in a lab, but throughout the universe. Which I just think is very cool!