Radioactivities in returned lunar materials and in meteorites

Carbon 14 terrestial ages were determined with low level minicomputers and accelerator mass spectrometry on 1 Yamato and 18 Allan Hills and nearby sited meteorites. Techniques for an accelerator mass spectrometer which make C(14) measurements on small samples were developed. Also Be(10) concentrations were measured in Byrd core and Allan Hills ice samples

160keV 26Al-AMS with a single-stage accelerator mass spectrometer

Proof-of-principle 26Al-AMS analysis is achieved with a single-stage accelerator mass spectrometer (SSAMS) utilising very low ion energy. The SSAMS operates by discriminating against atomic isobar interference in a negative ion source and suppressing molecules with thick gas stripper. Resulting 1+ ions counting is with a surface barrier detector. The NEC designed SSAMS for 14C analysis is a popular model accelerator mass spectrometer and the developed further capability might be a significant addition to established 26Al-AMS capacity. Measurements at these energies should also be sufficient for alternative 26Al positive-ion mass spectrometry (PIMS)

Radionuclide measurements by accelerator mass spectrometry at Arizona

Over the past years, Tandem Accelerator Mass Spectrometry (TAMS) has become established as an important method for radionuclide analysis. In the Arizona system the accelerator is operated at a thermal voltage of 1.8MV for C-14 analysis, and 1.6 to 2MV for Be-10. Samples are inserted into a cesium sputter ion source in solid form. Negative ions sputtered from the target are accelerated to about 25kV, and the injection magnet selects ions of a particular mass. Ions of the 3+ charge state, having an energy of about 9MeV are selected by an electrostatic deflector, surviving ions pass through two magnets, where only ions of the desired mass-energy product are selected. The final detector is a combination ionization chamber to measure energy loss (and hence, Z), and a silicon surface-barrier detector which measures residual energy. After counting the trace iosotope for a fixed time, the injected ions are switched to the major isotope used for normalization. These ions are deflected into a Faraday cup after the first high-energy magnet. Repeated measurements of the isotope ratio of both sample and standards results in a measurement of the concentration of the radionuclide. Recent improvements in sample preparation for C-14 make preparation of high-beam current graphite targets directly from CO2 feasible. Except for some measurements of standards and backgrounds for Be-10 measurements to date have been on C-14. Although most results have been in archaeology and quaternary geology, studies have been expanded to include cosmogenic C-14 in meteorites. The data obtained so far tend to confirm the antiquity of Antarctic meteorites from the Allan Hills site. Data on three samples of Yamato meteorites gave terrestrial ages of between about 3 and 22 thousand years

Determination of total I and 129I concentrations in freshwater of Argentina

Total iodine and129-iodine in rivers and lakes of Argentina were measured by means of inductively coupled plasma mass spectrometry and accelerator mass spectrometry respectively. The obtained isotopic ratios, higher than the natural level, are explained

Measurement of Ultra-Low Potassium Contaminations with Accelerator Mass Spectrometry

Levels of trace radiopurity in active detector materials is a subject of major concern in low-background experiments. Among the radio-isotopes, \k40 is one of the most abundant and yet whose signatures are difficult to reject. Procedures were devised to measure trace potassium concentrations in the inorganic salt CsI as well as in organic liquid scintillator (LS) with Accelerator Mass Spectrometry (AMS), giving, respectively, the \k40-contamination levels of $\sim 10^{-10}$ and $\sim 10^{-13}$ g/g. Measurement flexibilities and sensitivities are improved over conventional methods. The projected limiting sensitivities if no excess of potassium signals had been observed over background are $8 \times 10^{-13}$ g/g and $3 \times 10^{-17}$ g/g for the CsI and LS, respectively. Studies of the LS samples indicate that the radioactive contaminations come mainly in the dye solutes, while the base solvents are orders of magnitude cleaner. The work demonstrate the possibilities of measuring naturally-occurring isotopes with the AMS techniques.Comment: 18 pages, 4 figures, 3 table

Measurement of Trace I-129 Concentrations in CsI Powder and Organic Liquid Scintillator with Accelerator Mass Spectrometry

Levels of trace radiopurity in active detector materials is a subject of major concern in low-background experiments. Procedures were devised to measure trace concentrations of I-129 in the inorganic salt CsI as well as in organic liquid scintillator with Accelerator Mass Spectrometry (AMS) which leads to improvement in sensitivities by several orders of magnitude over other methods. No evidence of their existence in these materials were observed. Limits of < 6 X 10^{-13} g/g and < 2.6 X 10^{-17} g/g on the contaminations of I-129 in CsI and liquid scintillator, respectively, were derived.These are the first results in a research program whose goals are to develop techniques to measure trace radioactivity in detector materials by AMS.Comment: Proceedings of 10th International Conference on Accelerator Mass Spectrometr

Workshop on Cosmogenic Nuclides

Abstracts of papers presented at the Workshop on Cosmogenic Nuclides are compiled. The major topic areas covered include: new techniques for measuring nuclides such as tandem accelerator and resonance mass spectrometry; solar modulation of cosmic rays; pre-irradiation histories of extraterrestrial materials; terrestrial studies; simulations and cross sections; nuclide production rate calculations; and meteoritic nuclides

Accelerator Mass Spectrometry and Bayesian Data Analysis

Accelerator mass spectrometry (AMS) is an ultrasensitive technique for measuring the concentration of a single isotope. The electric and magnetic fields of an electrostatic accelerator system are used to filter out other isotopes from the ion beam. The high velocity means that molecules can be destroyed and removed from the measurement background. As a result, concentrations down to one atom in 10^16 atoms are measurable. This thesis describes the construction of the new AMS system in the Accelerator Laboratory of the University of Helsinki. The system is described in detail along with the relevant ion optics. System performance and some of the 14C measurements done with the system are described. In a second part of the thesis, a novel statistical model for the analysis of AMS data is presented. Bayesian methods are used in order to make the best use of the available information. In the new model, instrumental drift is modelled with a continuous first-order autoregressive process. This enables rigorous normalization to standards measured at different times. The Poisson statistical nature of a 14C measurement is also taken into account properly, so that uncertainty estimates are much more stable. It is shown that, overall, the new model improves both the accuracy and the precision of AMS measurements. In particular, the results can be improved for samples with very low 14C concentrations or measured only a few times.Luonnossa useat prosessit muuttavat alkuaineen isotooppien runsaussuhteita. Näin ollen isotooppien määrän mittaaminen auttaa osaltaan ymmärtämään luonnon prosesseja ja tutkittavan kohteen historiaa. Yksi isotooppien runsaussuhteita muuttava prosessi on radioaktiivisuus. Esimerkiksi radioaktiivisen hiili-14 isotoopin määrä kertoo kuinka kauan eloperäinen näyte on ollut kuolleena tai eristettynä hiilen normaalista kiertokulusta. Luukappaleen, vuosikertaviinin tai vaikkapa ilmakehän aerosolien alkuperää voidaan siis ymmärtää paremmin mittaamalla kohteen hiili-14 pitoisuus. Koska saman alkuaineen eri isotoopit käyttäytyvät kemiallisesti samalla tavoin, harvinaisia isotooppeja voidaan myös käyttää leima-aineina tutkittaville prosesseille kuten lääkeaineiden kulkeutumiselle kehossa. Kiihdytinpohjaisessa massaspektrometriassa hiukkaskiihdytintä käytetään harvinaisten alkuaine-isotooppien runsaussuhteiden mittaamiseen. Kiihdyttimellä saavutettava hiukkasten suuri nopeus mahdollistaa häiritsevien molekyylien poistamisen hiukkassuihkusta törmäyksien avulla. Ei-halutut ionit erotetaan hiukkassuihkusta magneetti- ja sähkökentillä. Tavalliseen massaspektrometriin verrattuna kiihdytinmassaspektrometrillä voidaan mitata yli viisi kertaluokkaa pienempiä pitoisuuksia. Esimerkiksi hiili-14-isotoopin ilmaisukynnys on alle 1:10^15, joka vastaa yhtä milligrammaa hiili-14-isotooppia miljoonassa tonnissa tavallista hiiltä. Tässä väitöskirjatyössä esitellään Helsingin yliopiston fysikaalisten tieteiden laitoksen kiihdytinmassaspektrometri, joka on ensimmäinen laite Suomessa. Nykyinen laitteisto, ionioptiset laskut, kiihdyttimen vaatimat merkittävät muutostyöt sekä mittausten läpivienti kuvataan pääpiirteissään. Lisäksi käydään läpi laitteiston olennaiset ominaisuudet, suorituskyky sekä näiden toteamiseksi suoritetut mittaukset. Laitteen nykyinen tarkkuus on alle prosentin, ja pienin laitteella mitattu hiili-14 runsaussuhde on 4×10^-17, joka vastaa kansainvälisesti parasta tausta-arvoa. Kiihdytinmassaspektrometrian sovellusalueilla luotettavat tulokset ja mittausepävarmuudet ovat ensiarvoisen tärkeitä. Tässä työssä kehitetty bayeslainen tulostenkäsittelymenetelmä parantaa kiihdytinmassaspektrometrian tuloksia merkittävästi keskiarvoistukseen perustuviin menetelmiin verrattuna. Uudessa menetelmässä laitteesta itsestään johtuvaa virhettä kuvataan mukautuvalla autoregressiivisellä prosessilla. Mukautuva prosessi etsii parhaan tavan, jolla mittaustulokset normitetaan standardinäytteiden tuloksiin. Yksittäisten mittaustulosten tiedetään noudattavan Poisson-jakaumaa, joka myös huomioidaan uudessa menetelmässä. Siinä missä otos-pohjaisten menetelmien mittausepävarmuudet heittelevät aiheuttaen virherajojen moninkertaisia ylityksiä, uudessa menetelmässä esitieto virhelähteistä vakauttaa mittausepävarmuudet. Näin tulosten luotettavuus paranee merkittävästi. Myös varsinaisen tuloksen tarkkuus paranee hieman aikaisempiin menetelmiin verrattuna. Lisäksi menetelmä antaa erityisen tarkkoja ja luotettavia tuloksia 1) pienille konsentraatioille, 2) mittauksille joiden välillä on korrelaatioita sekä 3) tapauksille, joissa yhdestä näytteestä voidaan ottaa vain muutamia mittauksia