The RELAX/RIMSKI laboratory houses two unique, ultra sensitive, time of flight, resonance ionisation mass spectrometers specifically designed for analysing isotopic ratios of small quantities of xenon and krypton.
RELAX (Refrigerator Enhanced Laser Analyser for Xenon – see Figure 1) is routinely used to analyse small quantities of xenon in a variety of terrestrial and extraterrestrial samples. Xenon is of particular interest among the noble gases. It has nine isotopes, some of which are uniquely produced by each of the known nucleosynthesis processes (s-, r- and p-processes). The geochemically or cosmochemically significant nuclei 128Te, 130Te, 238U, 232Th, 244Pu and 129I each decay to produce a distinct xenon isotopic signature. Xenon is usually extremely depleted in solid materials, therefore the magnitude of isotope anomalies produced by radioactive decay can be readily observed. Cosmic ray exposure shows up as relative excesses of low mass stable xenon isotopes in minerals rich in target elements (Ba, light rare earth elements) and after artificial neutron irradiation iodine, barium, tellurium and uranium concentrations can be derived from abundances of xenon isotopes produced via neutron capture (or neutron induced fission in the case of uranium) followed by beta decay. The contribution to our understanding of our solar system from the xenon isotope record is limited only by our ability to measure the minute quantities (few thousand atoms) of sample available.
Using the same fundamental principles as are found in the RELAX spectrometer, RIMSKI (Resonance Ionisation Mass Spectrometer for Krypton Isotopes - – see Figure 2) is used to measure all naturally occurring isotopes of krypton, including the isotope 81Kr which is only produced in very low quantities in cosmogenic spallation reactions. Primary applications of this instrument are currently to assess the cosmic ray exposure age of individual grains from meteoritic material. RIMSKI has been used to measure 81Kr in samples ranging from Eucrites and Howardites, to individual chondrules as well as CAIs and pieces of the recent Chelyabinsk LL5 meteorite.
The detection limits of both spectrometers are ~1000 atoms, a value that is limited only by the blank (around 1000 atoms). A number of features combine to contribute to the high sensitivity:
- a resonance ionisation ion source,
- a cryogenic sample concentrator,
- a time of flight mass analyser.
Resonance ionisation is a selective ionisation technique: only atoms or molecules that have a resonance at the wavelength(s) generate by one of more lasers will be efficiently ionised. Other atoms/molecules are only ionized by non-resonant processes, which are inherently less efficient. This allows for the detection of very small quantities of a given species in the presence of much larger quantities of other species, and thus measurement of isotopic ratios without isobaric interferences is possible.
The ionisation scheme for RELAX uses 2 photons at 249.6 nm to excite xenon atoms, which are subsequently ionised by a 3rd photon at the same wavelength (Figure 3).
The krypton ionisation scheme employed by RIMSKI is more complicated (Figure 4). Ionisation of krypton is most easily achieved using a 3-step process: a 116.48 nm photon initially excites the atoms from the ground state, a 558.04 nm photon further excited the atoms to a higher energy level, and finally the excited atoms are ionised with a 1064 nm photon. The VUV 116.48 nm photon is produced via a four-wave mixing scheme, in which a partial pressure of xenon is used as a negatively dispersive medium and a partial pressure of argon is used as a positively dispersive medium. This gas cell allows for two 252.5 nm photons and one 1507.3 nm photon to be combined within a xenon atom before the excitation decays to release a 116.48 nm VUV photon.
The cryogenic sample concentrator makes use of a localised cold spot in the ion source. Gas in the mass spectrometer condenses on the cold spot before being released by a pulse from a heating laser. This created a concentrated plume of gas, and the ionising lasers are fired through this plume when it reaches a maximum.
The time of flight mass analyser separates out the isotopes according to their mass, and allows for the detection of all isotopes from each pulse of the ionising laser.
Further details about the spectrometers can be found in our publications:
- Characteristics and applications of RELAX, an ultrasensitive resonance ionization mass spectrometer for xenon. S. A. Crowther et al., (2008) Journal of Analytical atomic Spectrometry 23, 938-947
- A resonance ionization time of flight mass spectrometer with a cryogenic sample concentrator for isotopic analysis of krypton from extraterrestrial samples. I. Strashnov et al. (2011) Journal of Analytical atomic Spectrometry 26, 1763-1772
The high sensitivity of RELAX and RIMSKI, and their ability to analyse the isotopic composition of xenon and krypton is small quantities of extraterrestrial samples has lead to our involvement with recent sample return missions, where material is severely limited. We produced the first xenon isotopic analysis of the present day solar wind as sampled by NASA’s mission, and have recently published our results of this study:
- The Genesis solar xenon composition and its relationship to planetary xenon signatures. S. A. Crowther & J. D. Gilmour (2013) Geochimica et Cosmochimica Acta 123, 17-34.
We are collaborating with leading scientists worldwide to study material from the asteroid Itokawa collected by the JAXA’s mission, and have recently made the first xenon analysis of individual grains collected from the asteroid by this mission. And we are currently in the process of combining Closed System Stepped Etching with RELAX to analyse samples of comet Wild 2 from NASA’s Stardust mission.
In addition to our involvement with these sample return missions, RELAX and RIMSKI are routinely used to study a variety of terrestrial and extraterrestrial materials. Recent analytical highlights include:
Demonstrating that the oldest samples of the Earth’s crust incorporated 244Pu when the crystallised 4.3 billion years ago, from which we inferred the initial Pu/U ratio.
- Extinct Pu-244 in ancient zircons. G. Turner et al. (2004) Science 306, 89-91.
- Pu-Xe, U-Xe, U-Pb chronology and isotope systematics of ancient zircons from western Australia. G. Turner et al. (2007) Earth and Planetary Science Letters 261, 491-499.
A study of I-Xe ages of material of different metamorphic grade from Rumuruti chondrites NWA 6492, NWA 830 and NWA 3364 suggest a link between the time of closure to Xe-loss and extent of metamorphism on the R-chondrite parent body.
- First I-Xe ages of Rumuruti chondrites and the thermal history of their parent body. J. L. Claydon et al. (2013) 44th Lunar and Planetary Science Conference, Abstract # 2211.
Comparisons of I-Xe systematics in “normal” and anomalous eucrites shows lower 129I/244Pu ratios in the “normal” eucrites, reflecting extended loss of Xe on the asteroid 4 Vesta
- The I-Pu-Xe system in anomalous and Vestan eucrites: was Vesta unusually large? J. L. Claydon et al. (2013) 44th Lunar and Planetary Science Conference, Abstract # 2173.
Determination of 81Kr-Kr cosmic ray exposure ages for a suite of eucrites. Samples analysed were 1-4 mg, and the precision of the ages determined was comparable to those reported by other groups analysing samples >100 times larger.
- Times of impacts that deliver samples of Vesta to Earth derived from ultrasensitive 81Kr–Kr cosmic ray exposure age analysis of Eucrites. I. Strashnovet al.(2013) Geochimica et Cosmochimica Acta 106, 71-83.
Alongside our programme of sample analysis, we strive to continually develop and improve the spectrometers. RIMSKI is currently undergoing redevelopments to enable simultaneous analysis of xenon and krypton from a single sample. We have begun adding a xenon ionisation line to the spectrometer and are working towards detecting a xenon signal.
Analysis of different types of samples requires different techniques for extracting the gas from them. Recent developments in this area include developing a method to extract gas from small, sealed gold tube inside the spectrometer extraction line. This technique will be applied to tubes containing xenon and krypton extracted from samples collected by the Japanese mission.