41-Calcium RIMS

General

metallic Calcium

Calcium (Ca) is the fifth abundant element of the earths crust. It has six stable isotopes, whereby Ca-40 is the most dominant one with an abundance of 97%. The quasi-stable isotope Ca-41 is very interesting for trace analysis experiments. It is produced by neutron capture of cosmic neutrons in the atmosphere and it decays with a half-life period of 1.03E+5 years [Pau91]. Due to the balance of production and decay the isotope Ca-41 occurs in a natural isotopic ratio of 1E-14 to 1E-15 relative to the Ca-40 isotope [Rai79].

nuclid chart around calcium

Ca-41 decays under electron capture by gamma emission at 421 keV. Because of this weak decay and due to its long half-life, it can hardly be detected by standard radiometric measures. Therefore the ultra trace analysis is based on direct counting of the atoms in a sample instead of decay counting.

LARISSA diode laser system
(click for big version)

The detection method has to be selective on the other Ca isotopes (especially the neighbouring Ca-40) on the one hand and capable to sufficently separate more abundant isobars as the stable potassium isotope K-41 on the other hand.

Detection Method

excitation scheme

Presently there are three different physical approaches which ensure a sufficently high isotopic selectivity as well as a high isobaric supression to detect Ca-41. These methods are resonance ionization mass spectrometry (RIMS), the established accelerator mass spectrometry ( accelerator mass spectrometry = AMS) [Fin90a] and the atomic trap trace analysis (atomic trap trace analysis = ATTA) [Che99].

The setup of the LARISSA experiment is described on the "method" page. For excitation of the Ca atoms from the ground state the 4s2 1S0 -> 4s4p 1P1 -> 4s4d 1D2 -> 4s15f 1F3 excitation sheme turned out to be most efficent one to be excited with our laser system. These three step excited Ca atoms are then photo-ionized with a CO2-laser (wavelength 10.6 μm).

Applications

click f. scheme of
Ca metabolism

A large number of various applications opens up for isotopic ratio measurements of Ca-41:
At nuclear power facilities Ca-41 is produced in the concrete shielding around the reactors core due to the high neutron flux inside the nuclear reactor and by neutron capture in the abundant Ca-40 in the concrete. For deconstruction of these facilities the contribution of Ca-41 to the total radioactivity has to be measured for clearance of the materials. Furthermore a determination of the overall neutron flux of the nuclear reactor is possible by measuring the Ca-41 to Ca-40 ratios [Mue01].
In studies of the Ca metabolism inside the human body Ca-41 can be used as tracer with low background and a harmless radioactive load. These studies target at the better understanding of the metabolism mechanisms and the prevention or therapy of Osteoporoses. In this context we are participating in the OSTEODIET Project des 5th Framework Program of the EU.
For cosmophysical studies Ca-41 is one of the long-living isotopes from which informations about the origin, terrestrical age and the original size or shape of a meteorite can be obtained [Fin90b].

As an alternative to the well-established C-14 radiocarbon method Ca-41 could be used for dating of samples. The advantage of Ca-41 lies in the longer half-life (104´000 a versus 5´730 a of C-14), which opens up a longer time window for dating a sample up to the palaeolithic.

About this see the informative POSTER, presented on the LAP 2002 conference.

Literature List

• [Che99] Chen, C.Y., Li, K.M., Bailey, K., O`Connor, T.P., Young, L., Lu, Z.-T.:
"Ultrasensitive Isotope Trace Analyses with a Magneto-Optical Trap"
Science 286, 1139 (1999)
• [Fin90a] Fink, D., Middleton, R., Klein, J. and Sharma, P.:
"41Ca: Measurement by Accelerator Mass Spectrometry and Applications"
Nucl. Instr. Meth. Phys. Rev. B 47, 79 (1990)
• [Fin90b] Fink, D., Klein, J. and Middleton, R.:
"41Ca: Past, present and future"
Nucl. Instr. Meth. Phys. Rev. B 52, 572 (1990)
• [Mue01] Müller, P., Bushaw, B. A. , Blaum, K., Diel, S., Geppert, Ch., Nähler, A., Trautmann, N. and Wendt, K.:
"41Ca ultratrace determination with isotopic selectivity > 1012 by diode-laser-based RIMS"
Fresenius J Anal Chem 370, 508 (2001)
• [Pau91] Paul, M., Ahmad, J. and Kutschera, W.:
"Half life of 41Ca"
Z. Phys. A 152, 249 (1991)