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Re: The case of the missing elements
¿ªÔÆÌåÓýI use a Rock crusher for big rock samples.? Most are just done in using a cloth covered hammer and then a ceramic mortar and pestle then sieving the powder down to a uniform and appropriate size.? Yeah, I don¡¯t crush my nice specimens but I do when I have a bulk prospecting ?sample. Using a rock saw to get a flat surface cut to look at the homogeneity of the rock is useful as well then selecting the target area to shoot The biggest issue I see with trying to determine what a mineral is, is using the elemental composition to justify what Mindat says should be in it.? If it¡¯s a single crystal that¡¯s ok, but to shoot a whole rock and make a definitive statement that its¡± leaverite¡± is wrong as you have no idea what is fluorescing, a xtal? the matrix? or the host rock¡¯s body - anyone of which will introduce its own elements.? ?Dud From: [email protected]
[mailto:[email protected]] On Behalf Of WILLIAM S Dubyk
Sent: Friday, February 7, 2020 9:02 PM To: [email protected] Subject: Re: [XRF] The case of the missing elements ? Powdering a specimen is a common and practical way of getting a uniform xrf, but like you mentioned Dud, us collectors do cringe at that especially if it is unusual, expensive, and/or hard to replace. Microprobe analysis of specimens is maybe a little worse, where a chunk has to be taken out of the mineral. ? How would you prepare a sample like that? Taking a piece off with a Dremel or even a cheese grater might introduce contamination. Do you know how labs do it, and can we do this in the home lab? I have some samples I can use so I am willing to give it a try. ? Steve ? From: [email protected] <[email protected]> on behalf of Dude
<dfemer@...> ? Good points here Geo. However we¡¯re all doing this all wrong and comparing apples to oranges.? The excitation sources have widely different energies and source strength and exposure is from within or on the surface.? Worse yet we have no idea of the elemental distribution in the matrix which is going to determine what gets excited and what gets absorbed. ?What is the gamma activity of the Cerite? What¡¯s the mass. Is this a big boy or a chip? The excitation energy from the Th is going to be very high while the Am is lower in energy and has the Np x-rays which are closer and stronger to excite the absorption edges of both the Fe and Y.? But the biggest problem is just what are you exciting and where is it ¨C on the surface, deep inside just a small xtal? To properly compare these measurements the rock should be pulverized down to 200 u or so (Charles just cringed) ?mixed and homogenized and put into a XRF cup with a mylar cover and then measured.? It makes for a homogenous mixture removing the ambiguity of what the matrix effects are. I think Geo is right in the density- thickness problem and the different source ?strengths. Charles, I have an external ?higher activity natural ?source that I think you might like to try, give me a call Dud ? ? ? From: [email protected]
[mailto:[email protected]] On Behalf Of GEOelectronics@... ? Hi Charles, cool observation. If I may guess at it: ? We know from experience that an atom will make XRF if excited by energy from alpha, beta (or electron stream) Gamma, X-Rays etc. Probably many other ways too. ? So if you rely on internal energy to do this, it will do so in response to the available radiation from the rock. Some have LOTS, some have very little, some almost none. ? One advantage to that way is all the atoms, even those deep within a big rock are being excited. If those XRF rays can get out depends only on the energy of the ray and the overburden. ? Rays above 50 keV have little problem penetrating silicon-calcium-oxygen atoms. This goes for adding external energy or gathering internally generated XRF. ? So your method might show up something that an external exciter misses. ? But a good exciter will work on a rock that has no discernable radioactivity, the thing is how radioactive is the rock and how radioactive is does the external exciter have to be to equal or better it..... ? The pro units have a need for speed so they have X-Ray tubes, blasting the sample with as much as the law allows. We don't actually have a need for speed so we can use tiny little exciters, but as far as the elements at the surface and a little further inside are concerned, it is much more "exciting" than the stuff coming from deep inside and spread over the entire surface of the rock. ? In other words it may be small, but it is concentrated where it counts, we substitute long run times to get the results we need. ? Say on another note, do you have enough Trinitite to do some Gamma Spec runs on it with your Si-PIN? You could precisely calibrate the detector with Am, the see if the X-Rays coming from Trinitite are from Np or U atoms, or both. ? Geo ? ? -----
Original Message ----- ? I recently picked up a specimen of melanocerite-Ce at the mineral show and did a scan without Am241 by depending on the internal radiation.? I was not surprised to see Th La Ce and Nd in the blue plot. ? Then I wanted to see if I got the same results using Am241 as an additional exciter.? So I carefully moved the specimen just far enough from the detector to allow me to slide in the Am241 jig.? I was careful to keep the same spot on the rock even though it looked quite uniform. ? Curiously I am now getting additional strong Fe and Y peaks in the red plot.? What could account for this?? I am not surprised that the La Ce Nd are lit up as they are. ? I have also attached both .mca files in case one wishes to analyze them.? However, I am not really asking for that.? I would just like to understand why the Am241 lights up the Fe and Y so strongly compared to the internal radiation.? It is kind of a pain to weed through the Am241 background peaks but it appears that I may not get a complete picture with the internal radiation alone. ? Charles ? ? ? |