{"id":1775,"date":"2018-12-31T08:38:31","date_gmt":"2018-12-31T12:38:31","guid":{"rendered":"http:\/\/ctlsites.uga.edu\/hargretthoursproject\/?page_id=1775"},"modified":"2018-12-31T08:38:31","modified_gmt":"2018-12-31T12:38:31","slug":"a-p-xrf","status":"publish","type":"page","link":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/a-p-xrf\/","title":{"rendered":"A p-XRF?"},"content":{"rendered":"\n

by Bonnie Hester. (Appeared originally as the blog post, “What the heck is ax XRF machine?<\/a>“) <\/p>\n\n\n\n

\u2026and why should we care?? That\u2019s just the question I\u2019m going to \nanswer in this post! More specifically, I am going to try to explain in \nlayman\u2019s terms how these machines work, and also why the work we\u2019re \ndoing with them in our class is so cool.<\/p>\n\n\n\n

As you may know already<\/a>,\n in Dr. Camp\u2019s manuscripts class right now we are researching the way \nthat colors were made during the medieval era and applying that research\n to some actual manuscript fragments from the Hargrett Special \nCollections Library. Dr. Alice Hunt from UGA\u2019s Center for Applied \nIsotope Studies has been leading our classes in our analysis of the \npigments on our fragments, which we are achieving mainly through the use\n of a small device that looks a little like an odd, oversized microscope\n called a \u201cportable X-ray fluorescence spectrometer,\u201d or pXRF machine. \nUnderstanding how an XRF device works requires a little background \nknowledge of some science-y things that many humanities majors are very \nhappy to put away after high school, so bear with me a second!<\/p>\n\n\n\n

\"Illustration<\/a>
Illustration of x-ray generation within the glass bulb of an x-ray tube. Credit<\/strong>: From Doing pXRF Right<\/em>\u2026 by Hunt & Speakman (see end of post).<\/figcaption><\/figure>\n\n\n\n

Inside\n an XRF machine there is a small glass tube in which a metal wire called\n a filament, often made of tungsten, is heated so that it gives off \nhigh-energy electrons. These electrons strike a target in front of them \nmade from an element with a high atomic number (often rhodium). This \ntarget, which is positioned at an angle from the filament, gives off \nX-rays which are then directed through a small window out of the tube \nand towards the material being analyzed.<\/p>\n\n\n\n

\"Bohr<\/a>
Bohr\n model of a Uranium atom. Electron shells are indicated by the capital \nletters; adjacent numbers indicate the total number of electrons in the \nshell. Credit<\/strong>: From Doing pXRF Right\u2026<\/em> by Hunt & Speakman (see end of post).<\/figcaption><\/figure>\n\n\n\n

Once\n the X-rays hit the sample, they interact with its atoms and dislodge \nsome of their electrons. As you might remember, electrons can be thought\n of as circling the nucleus of an atom in \u201crings,\u201d or shells, which get \nbigger as they get farther away from the nucleus. The innermost shell is\n often labeled K, and the next L, and so forth (remember this for \nlater!). If an electron from a lower shell (one closer to the nucleus) \nis dislodged, another electron from a higher shell will drop down to \nthat lower level in order to keep the atom stable (so an electron from \nthe L shell might drop down to the K shell). Since electrons in higher \nshells have more energy than those in lower shells, the act of dropping \ndown will cause the electrons to release that \u201cextra\u201d energy which they \nno longer require. It is this release of energy (called radiation) which\n is measured by the machine and converted into graphical form for us to \nanalyze.<\/p>\n\n\n\n

Side note: these X-rays are very dangerous! In order to \nanalyze any given element, we have to strike it with a sufficient amount\n of energy to dislodge some of its electrons. As you go up in atomic \nnumber, it requires more and more energy \u2013 a general rule of thumb is \nthat if striking an element causes an energy discharge of a certain \namount, it will have required about twice that amount to dislodge that \nelement\u2019s electrons in the first place. Moreover, even low atomic number\n elements require a great deal of energy  \u2013 for our class, Dr. Hunt has \nbeen using an energy setting of about 40 kilovolts. For context, the \ndefibrillators used to try to start patients\u2019 stopped hearts in \nhospitals use about 300-1000 volts, which is a maximum of 1\/40th of the \nenergy we are using! If these X-rays were to strike us directly they \nwould stop our hearts immediately\u2026so no messing around in this endeavor!\n We are not generally in great danger of being hit, of course, since the\n machine has a heavy-duty safety cap lined with lead to contain the \nrays, and we are very sure to stand well out of their path, even if the \ncap is on. But I digress! Back to the science:<\/p>\n\n\n\n

\"Spectrum<\/a>
Spectrum generated using a pXRF instrument. Credit<\/strong>: From Doing pXRF Right<\/em>\u2026 by Hunt & Speakman (see end of post).<\/figcaption><\/figure>\n\n\n\n

We\n are able to put these energy readings to good use because each element \nin the periodic table has a unique, characteristic level of energy it \nemits when struck by X-rays. We can thus match up the energy spike \nrecorded to what scientists of far greater caliber than us have already \ndetermined to be the levels associated with each element. This, of \ncourse, is a much more complex task than it might sound initially, for \nthree main reasons. First, our technology is not able to perfectly \ndistinguish similar spikes from one another, so there is some overlap \nthat creates uncertainty. Second, elements with atomic numbers less than\n about 16 do not give off energy discharges big enough to be measured by\n the machine, so you cannot use XRF to test whether or not such elements\n (including carbon, a very common element) are present in a sample. \nThird, if you dislodge electrons from more than one shell level of a \ngiven element, there will be multiple \u201cdrops\u201d between shells so that the\n atom remains stable. Each of these drops will then cause a separate \nspike to register on our machine, since they involve releases of \ndifferent amounts of energy.<\/p>\n\n\n\n

Of particular importance to us are \nspikes caused by drops to the K and L shell levels, spikes which we call\n K lines and L lines respectively. The fact that elements may cause both\n types of lines (and more, possibly) to appear in our readings means \nthat what we might think is an L line for a certain element is in fact a\n K line for a different one. For instance, my project partner and I \nspent time today looking at our spectra, and we had a large spike occur \nat the energy level where you would expect the element rubidium to have a\n K line. However, nothing in our research suggested we would see \nrubidium, and this is also around the area where you might find the L \nline for gold, which we know is present. Thus it is more likely that \ngold is causing this spike, rather than this strange other element. All \nthat is to say, then, is that it isn\u2019t as easy as just matching up \ndistinct pairs of data \u2013 it takes some serious critical thinking to draw\n the best conclusions about to which elements are actually present.<\/p>\n\n\n\n

But\n enough of that science mumbo-jumbo: why does this matter at all? What \nis the point of putting ourselves through all this \nphysics-and-chemistry-induced trauma? First of all, there are a plethora\n of reasons that it is useful and important to know precisely what \nelements make up our manuscripts. For instance, knowing how they were \nmade and with what can give us insights into medieval production \ntechniques as well as into trade patterns of the time. For example, if \nwe were to examine one of our fragments made in, say, northern France, \nand discovered that its blue pigment was made using lapis lazuli, then \nwe would know that most likely, this area of France was trading through \nVenice to obtain this rare mineral from mines primarily in northeastern \nAfghanistan. Knowing how different areas of the world interacted via \ntrade, in turn, is important because it can tell us a lot about how \nideas, technology, language, and many more (even things like disease!) \nspread and changed. Pretty neat!<\/p>\n\n\n\n

Quite apart from all of that, \nthough, is the simple fact that doing this is actually just really cool \u2013\n I mean think about it! Here we are hundreds of years removed from when \nthese fragments were produced, using what their producers would have \nthought was a magic machine to discover in mere moments that \u201coh there\u2019s\n calcium everywhere on this parchment.\u201d We can then apply our body of \nresearch into the time period to decipher that \u201cah yes, that\u2019s the \ncalcium from the quicklime that the parchment was cleaned with!\u201d<\/p>\n\n\n\n

In\n other words, in this example, seeing calcium is conclusive, \nin-your-face evidence that a long time ago, an actual human being did \nindeed take this piece of cow or sheep skin and dunk it in some \nquicklime to clean it so it could become part of a book. No longer is \n\u201cwash with quicklime\u201d just an abstract idea in a textbook, the first \nthing we write down when Dr. Camp asks us to go through the steps of making parchment<\/a>.\n Instead it is concrete, tangible \u2013 we can say with near certainty that \nTHIS particular bit of parchment that we are holding in our hands right now<\/em> was washed with quicklime by some European guy almost five hundred years ago in some field or shop. And THAT is cool.<\/p>\n\n\n\n

Notes<\/p>\n\n\n\n

Images from: Hunt, Alice, and Robert Speakman. Doing pXRF Right:<\/em> An intermediate course for Archaeologists.<\/em> Center for Applied Isotope Studies at the University of Georgia, 2015.<\/p>\n","protected":false},"excerpt":{"rendered":"

by Bonnie Hester. (Appeared originally as the blog post, “What the heck is ax XRF machine?“) \u2026and why should we care?? That\u2019s just the question…<\/p>\n

Continue ReadingA p-XRF?<\/span> <\/i><\/a><\/div>\n","protected":false},"author":1791,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"open","template":"","meta":{"_acf_changed":false,"footnotes":""},"class_list":["post-1775","page","type-page","status-publish","hentry","entry"],"acf":[],"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/P7AbKE-sD","_links":{"self":[{"href":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/wp-json\/wp\/v2\/pages\/1775","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/wp-json\/wp\/v2\/users\/1791"}],"replies":[{"embeddable":true,"href":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/wp-json\/wp\/v2\/comments?post=1775"}],"version-history":[{"count":0,"href":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/wp-json\/wp\/v2\/pages\/1775\/revisions"}],"wp:attachment":[{"href":"https:\/\/ctlsites.uga.edu\/hargretthoursproject\/wp-json\/wp\/v2\/media?parent=1775"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}