 |
| Titel |
Applications of New Synthetic Uranium Reference Materials for Research in Geochemistry |
| VerfasserIn |
Stephan Richter, Adolfo Alonso, Yetunde Aregbe, Roger Eykens, Ulf Jacobsson, Heinz Kuehn, Andre Verbruggen, Stefan Weyer |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250031863
|
|
|
|
|
|
| Zusammenfassung |
| For many applications in geochemistry research isotope ratio measurements play a significant
role. In geochronology isotope abundances of uranium and its daughter products thorium
and lead are being used to determine the age and history of various samples of
geological interest. For measuring the isotopic compositions of these elements by mass
spectrometry, suitable isotope reference materials are needed to validate measurement
procedures and to calibrate multi-collector and ion counting detector systems. IRMM is a
recognized provider for nuclear isotope reference materials to the nuclear industry and
nuclear safeguards authorities, which are also being applied widely for geochemical
applications.
Firstly, the double spike IRMM-3636 with a 233U/236U ratio of 1:1 was prepared which
allows internal mass fractionation correction for high precision 235U/238U ratio
measurements. The 234U abundance of this double spike material is low enough to allow an
accurate and precise correction of 234U/238U ratios, even for measurements of close
to equilibrium uranium samples. The double spike IRMM-3636 is offered in 3
concentrations: 1mg/g, 0.1mg/g and 0.005mg/g. Secondly, the 236U single spike
IRMM-3660 was prepared and is offered in 3 concentrations: 1mg/g, 0.1mg/g and
0.01mg/g. Thirdly, a "Quad"-isotope reference material, IRMM-3101, has been prepared
which is characterized by 233U/235U/236U/238U=1/1/1/1. This material is useful
for checking Faraday cup efficiencies and inter-calibration of MIC (multiple ion
counting) detectors. The quad-IRM is offered in 3 concentrations: 1mg/g, 0.1mg/g and
0.01mg/g.
As one example for the significant influence of synthetic reference materials for
geochemical research, the IRMM-074 series of gravimetrically prepared uranium mixtures
for linearity testing of secondary electron multipliers (SEMs) has been applied for the
redetermination of the secular equilibrium 234U/238U value and the 234U half-life by Cheng
et al (2009). Due to the use of IRMM-074, results with smaller uncertainties were obtained,
which are shifted by about 0.04% compared to the commonly used values published earlier
by Cheng et al. in 2000. This has a significant impact for U isotope measurements in
geochemistry.
As another example for a geochemical application, by using the new double spike
IRMM-3636, the 235U/238U ratios for several commonly used natural U standard materials
from NIST/NBL and IRMM, such as e.g. NBS960 (=NBL CRM-112a), NBS950a,b and
IRMM-184, have been re-measured at IRMM and other laboratories with improved precision
and accuracy. The (preliminary) new result of 137.839(24) for the 238U/235U ratio of NBL
CRM-112a is deviating by -0.030% from the well-known and widely used old consensus
value of 137.88. For this old consensus value no uncertainty has ever been assigned, but it is
outside the uncertainty limits of the new measurement result. The new result is based on
measurements made at several laboratories worldwide, such as University of Frankfurt
(Germany), National Taiwan University, NERC (University of Nottingham, UK), UNM
(University of Minnesota, US), Thermo Fisher Scientific, LLNL (Lawrence Livermore
National Laboratory, US.DOE), SAL/IAEA and IRMM. The (preliminary) new result of
137.839(24) can therefore be proposed as a new consensus value for the 238U/235U ratio
of NBL CRM-112a. In contrast to the older consensus value, this new result is
traceable to the common SI system of units and has an uncertainty assigned to
it.
For the close to natural standard IRMM-184, the re-measured 238U/235U ratio of
137.683(23) agrees quite well with the certified value of 137.697(41), the calculated
difference is only -0.010(35)% which is insignificant. As a conclusion, the IRMM-3636
Double Spike has been successfully applied for measurements of important uranium isotopic
standards like NBL CRM-112a and IRMM-184, with improved uncertainties at the level of
0.016% and traceability to the SI system. |
|
|
|
|
|
|