Uraniumis achemical elementwith symbolUandatomic number92. It is a silvery-whitemetalin theactinideseries of theperiodic table. A uranium atom has 92protonsand 92electrons, of which 6 arevalence electrons. Uranium is weaklyradioactivebecause all itsisotopesare unstable (with half-lives of the 6 naturally known isotopes, uranium-233 to uranium-238, varying between 69 years and 4.5 billion years).
The most common isotopes of uranium areuranium-238(which has 146neutronsand accounts for almost 99.3% of the uranium found in nature) anduranium-235(which has 143 neutrons, accounting for 0.7% of the element found naturally). Uranium has the second highestatomic weightof theprimordiallyoccurring elements, lighter only than plutonium.Itsdensityis about 70% higher than that oflead, but slightly lower than that ofgoldortungsten. It occurs naturally in low concentrations of a fewparts per millionin soil, rock and water, and is commerciallyextractedfrom uranium-bearingminerals such asuraninite.
In nature, uranium is found as uranium-238 (99.2739 – 99.2752%), uranium-235 (0.7198 – 0.7202%), and a very small amount of uranium-234(0.0050–0.0059%).Uranium decays slowly by emitting analpha particle. Thehalf-lifeof uranium-238 is about 4.47 billionyears and that of uranium-235 is 704millionyears,making them useful in dating theage of the Earth.
Isotope Uranium 238
Uranium-238 is the most stable isotope of uranium, with ahalf-lifeof about 4.468 × 109years, roughly theage of the Earth. Uranium-235 has a half-life of about 7.13 × 108years, and uranium-234 has a half-life of about 2.48 × 105years.For natural uranium, about 49% of its alpha rays are emitted by each of238U atom, and also 49% by234U (since the latter is formed from the former) and about 2.0% of them by the235U. When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of 234U was probably much lower than this.Uranium-238 is usually an α emitter (occasionally, it undergoes spontaneous fission), decaying through the“Uranium Series” of nuclear decay, which has 18 members, all of which eventually decay intolead-206, by a variety of different decay paths.
Uranium-238 is not fissile, but is a fertile isotope, because afterneutron activationit can produceplutonium-239, another fissile isotope. Indeed, the238U nucleus can absorb one neutron to produce the radioactive isotopeuranium-239.239U decays bybeta emissiontoneptunium-239, also a beta-emitter, that decays in its turn, within a few days into plutonium-239.239Pu was used as fissile material in the firstatomic bombdetonated in the “Trinity test” on 15 July 1945 inNew Mexico.
The 4n+2 chain of U-238 is called the “uranium series” or “uranium cascade”.
Beginning with naturally occurring uranium-238, this series includes the following elements:astatine,bismuth,lead,polonium,protactinium,radium, radon,thallium, andthorium. All are present, at least transiently, in any natural uranium-containing sample, whether metal, compound, or mineral. The series terminates with lead-206.
The total energy released from Uranium-238 to Lead-206, including the energy lost toneutrinos, is 51.7 MeV.
Isotope Uranium 235
Uranium-235is anisotope of uraniummaking up about 0.72% ofnatural uranium. Unlike the predominant isotopeuranium-238, it isfissile, i.e., it can sustain afissionchain reaction. It is the only fissile isotope that is aprimordial nuclideor found in significant quantity in nature.Thedecay series of235U, which is called theactinium serieshas 15 members, all of which eventually decay into lead-207.The constant rates of decay in these decay series makes the comparison of the ratios of parent to daughter elements useful inradiometric dating.
The 4n+3 chain ofUranium-235is commonly called the “actinium series” or “plutonium cascade”. Beginning with the naturally-occurring isotope U-235, this decay series includes the following elements : Actinium, astatine, bismuth, francium, lead, polonium, protactinium,radium,radon,thallium, andthorium. All are present, at least transiently, in any sample containing uranium-235, whether metal, compound, ore, or mineral. This series terminates with the stable isotopelead-207.
Gamma Spectra of Materials containing Uranium
The uranium contained in the glaze and in the glass has been refined, so it does not contains all the products of the decay : from radio to the lead and bismuth. For this reason there is only the first isotope of the decay: thorium-234.
It should be noted in the spectra of the glaze the presence of isotope uranium-235. These photo peaks are sign that for these artifacts were used refined natural uranium and not depleted uranium. We can therefore deduce that the manufacturing period is probably before the Second World War because subsequently, for this kind of products, predominantly depleted uranium has been used.
Gamma Spectra of Uranium Ores
Autunite – First Sample
Autunite(hydrated calcium uranyl phosphate) with formula : Ca(UO2)2(PO4)2·10-12H2O.
Is a yellow – greenishfluorescentmineralwith ahardnessof 2 – 2½. Autunite crystallizes in theorthorhombicsystem and often occurs as tabular squarecrystals. Due to the moderateuraniumcontent of 48.27% it isradioactiveand also used as uranium ore. If the mineral dries out, it converts to meta-autunite-I, which can turn into meta-autunite-II after heating. These two subsequent minerals are very rare in nature. For scientific studies it is recommended to store the mineral in a sealed container to minimize the water loss. Museums are known to have covered the mineral withlacquerto avoid drying of the mineral.
Autunite was discovered in 1852 nearAutun,France. It occurs as anoxidationproduct of uranium minerals ingranitepegmatitesand hydrothermaldeposits.Associate minerals include metaautunite,torbernite,phosphuranylite,saleeite,uranophaneand sabugalite.
Autunite – Second Sample
Uraninite
Uraniniteis aradioactive, uranium-richmineralandorewith achemical compositionthat is largelyUO2, but due tooxidationthe mineral typically contains variable proportions of U3O8. Additionally, due toradioactive decay, the ore also containsoxidesoflead and trace amounts ofhelium. It may also containthorium, andrare earth elements. It used to be known aspitchblende.
All uraninite minerals contain a small amount ofradiumas aradioactive decayproduct of uranium. Uraninite also always contains small amounts of theleadisotopes206Pb and207Pb, the end products of the decay series of the uranium isotopes238U and235U respectively.
Pitchblende
We acquired the gamma spectrum of a beautiful specimen of pitchblende from Val Vedello. The ore is rather large so it was not possible to putit in the shielded measuring well. The sample is still quite active for which the environmental level does not significantly alter the measurement. In the images below you can see the sample, the measurement setup and the obtained gammaspectra.
Saleeite
Saleeite is a secondary uranium mineral occurring in the oxidized zones of uranium deposits, or as disseminations in carnotite-bearing sandstones. Its chemical formula is Mg(UO2)2(PO4)2·10(H2O).
Polycrase (Uranium and Thorium)
Uranium 238 Decay
The decay of uranium-238 could be divided in two parts :
- U-238 -> .. -> Ra-226 : Uranium-238 has half-life of millions of years
- Ra-226 -> .. -> Pb-206 : Radium-226 has half-life of 1600 years
In uranium ores all the decay chain isotopes are present because, during time, an equilibrium has been achieved, thus the activity is the same for all the isotopes.In “modern” objects built with refined uranium the isotopes which comes from radium are absent because they had not enough time to be generated, particularly the following isotopes are absent : Ra-226, Pb-214 e Bi-214.This feature is evident in the spectra shown in the image above.
The red line is the gamma spectrum of a sample of the uranium ore uraninite, all the uranium decay chain isotopes shows their gamma photo peaks (at least the ones which are gamma emitters). The descendants of radium Ra-226 : Pb-214 and Bi-214 are present.
The green line is the gamma spectrum of uranium glaze (from fiestaware). The photo peaks of thorium-234 and uranium-235 are present, while the photo peaks of radium and its descendants are absent.
The photo peak near Ra-226 in the gamma spectrum of Uranium glaze is due to U-235.
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