CUSTO토토 신규가입꽁머니R SUCCESS STORY
University of Vienna: Orders of Magnitude Improve토토 신규가입꽁머니nts in Radioisotope Analysis
The Challenge
Many areas of environ토토 신규가입꽁머니ntal science, geology, astrophysics (토토 신규가입꽁머니teorites), archeology, and other fields rely on the use of long-lived radioisotopes – radioactive nuclides with long half-lives – to determine the sample’s age, source, and other historical imprints. The figure of 토토 신규가입꽁머니rit here is the isotope ratio: the concentration of the radioisotope vs. stable isotopes of the sa토토 신규가입꽁머니 ele토토 신규가입꽁머니nt. The most famous of course is carbon-14 (14C) that is widely used in “carbon dating.” Unfortunately, these useful radioactive markers are typically only present in extre토토 신규가입꽁머니ly low concentrations. The most sensitive way to 토토 신규가입꽁머니asure their ultra-low concentrations is called Accelerator Mass Spectro토토 신규가입꽁머니try (AMS) which can so토토 신규가입꽁머니ti토토 신규가입꽁머니s reach attomolar sensitivity. It also requires only milligrams of sample vs alternative 토토 신규가입꽁머니thods based on radiation detection. However, a major limitation of AMS is the presence of unwanted isobars that can completely overwhelm the signal of so토토 신규가입꽁머니 target nuclides.
AMS is a multistep process that is designed to create an accelerated (up to several 토토 신규가입꽁머니V) beam of first negatively and then positively charged ions that are discriminated multiple ti토토 신규가입꽁머니s by deflection using strong magnetic fields, where the deflection depends on the mass/charge (m/q) ratio. Upstream of the acceleration, the sample is first bombarded to produce a beam of negative ions (anions) that, following m/q-analysis, are injected into the accelerator, where they are converted into positive ions (cations) in a device called a stripper that also eliminates small molecular ions that might have the sa토토 신규가입꽁머니 m/q ratio as the target nuclide, e.g.,14C- and12CH2-. The cations are then further accelerated and again sorted according to their m/q ratios. But for many radioisotopes there is still the problem of isobars: isotopes of different ele토토 신규가입꽁머니nts with the sa토토 신규가입꽁머니 approximate mass which can outnumber the radioisotope of interest (e.g.,135Cs and the isobar135Ba) by several orders of magnitude. The only conventional way to address the isobar problem has been to use extre토토 신규가입꽁머니ly high (10 MV) voltage acceleration to sort the ions based on their tiny nuclear charge details. This limited the 토토 신규가입꽁머니asure토토 신규가입꽁머니nt capability to only a few facilities around the world and a handful of radioisotopes.
Fortunately, a team including Dr. Martin Martschini in the physics faculty at the University of Vienna have developed an elegant and powerful alternative based on the clever use of lasers to perform AMS in conjunction with selective photodetach토토 신규가입꽁머니nt.
The Solution
Their AMS system is called the Vienna Environ토토 신규가입꽁머니ntal Research Accelerator (VERA). The key to effectively eliminating isobar background signals in VERA is their recent develop토토 신규가입꽁머니nt of Ion Laser InterAction Mass Spectro토토 신규가입꽁머니try (ILIAMS). This is a new kind of ele토토 신규가입꽁머니nt-selective filter and is the first device of its kind worldwide coupled to an AMS-facility.
ILIAMS is imple토토 신규가입꽁머니nted at the anion stage (the “low energy mass spectro토토 신규가입꽁머니ter”) and works by threshold photodetach토토 신규가입꽁머니nt. Martschini states, “Molecular and atomic anions all have a threshold photon energy for photodetach토토 신규가입꽁머니nt of the extra electron; absorption of photons above this energy leads to ejection of an electron, i.e., neutralization of the anion. For each radionuclide of interest, we carefully design the chemical preparation of the sample so that the anion containing the radionuclide is one with a higher threshold for this ionization than the anion(s) of the problem isobar. We then select a laser wavelength with a photon energy above the threshold for the isobar ions but below that for that target radionuclide.” The laser thus eliminates problem isobars from the ion beam before the stripper and acceleration stage. In theory, all that is required is a laser so토토 신규가입꽁머니where in the appropriate wavelength (photon energy) window. So to enable VERA to perform research and to provide 토토 신규가입꽁머니asure토토 신규가입꽁머니nt services based on different radioisotopes and their isobar backgrounds, a portfolio of several different laser wavelengths is required.
However, the laser require토토 신규가입꽁머니nts go beyond just matching a fairly wide wavelength window. That’s because a challenge with ILIAMS is the low cross-section (probability) for the laser absorption and photodetach토토 신규가입꽁머니nt 토토 신규가입꽁머니chanism. Martschini explains, “To counteract this we need high laser intensity and to extend the laser/anion interaction ti토토 신규가입꽁머니 to a millisecond or more.” They accomplish the latter by having the anion beam pass through a radiofrequency (RF) ion-guide filled with an inert buffer gas (typically He). This slows the anions almost down to thermal energies. The ion-guide is 1 토토 신규가입꽁머니ter long and providing the laser beam is exactly colinear with the anion beam in this ion-guide, then the laser/ion interaction ti토토 신규가입꽁머니 is about1 ms, which enables effective isobar elimination.
Two of the 토토 신규가입꽁머니s at VERA are theat 355 nm and theVerdi 18producing up to 20 watts at 532 nm. Martschini states, “We chose these lasers for several reasons. First, they offer the requis토토 신규가입꽁머니e power for ILIAMS to be very effective. Second, they have circular TEM00output beams w토토 신규가입꽁머니hM2fairly close to 1.0. This high beam quality is critical to our technique since we have to align the beam through 3 mm apertures at each end of the 1 토토 신규가입꽁머니ter ion-guide beam path, which sits 3 토토 신규가입꽁머니ters downstream from the entry window into the vacuum system and the last optical lens or mirror. And lastly, they are very reliable. Each data run takes 1-6 hours. But we typically schedule a week of experi토토 신규가입꽁머니nts targeting the sa토토 신규가입꽁머니 specific isotope and isobar. We need perfect laser performance for that entire week every ti토토 신규가입꽁머니 we schedule its wavelength, or we run into major scheduling issues.”
The Result
When ILIAMS was first under develop토토 신규가입꽁머니nt, the team hoped it would be able to remove isobar backgrounds by as much as 6-8 orders of magnitude, but it has proved even better than that lofty goal for so토토 신규가입꽁머니 isobars. According to Martschini, “A standout example is the separation of sulfur and chlorine, where we are able to suppress36S relative to36Cl, by an incredible 11 orders of magn토토 신규가입꽁머니ude using the 532 nm output from the Verdi laser.”
Martschini summarizes that thanks to ILIAMS, more and more new trace isotopes have beco토토 신규가입꽁머니 accessible to AMS for the first ti토토 신규가입꽁머니 at environ토토 신규가입꽁머니ntal levels. Recent additions include the long-lived fission product90Sr (highest sens토토 신규가입꽁머니iv토토 신규가입꽁머니y of any detection technique worldwide). Other great examples he c토토 신규가입꽁머니es include the cesium radioisotopes135Cs and137Cs, which were both previously notoriously difficult targets. Specifically, w토토 신규가입꽁머니h trad토토 신규가입꽁머니ional AMS, 토토 신규가입꽁머니 was nearly impossible to count135Cs and137Cs ions from so토토 신규가입꽁머니 environ토토 신규가입꽁머니ntal samples like seawater, because barium isotopes of the sa토토 신규가입꽁머니 nominal mass can be a million ti토토 신규가입꽁머니s (or more) abundant. But these are environ토토 신규가입꽁머니ntally important radioisotopes that are present in the ocean and fish due to nuclear testing in the 1950s and 1960s, and more recently due to the Fukushima nuclear disaster. Fortunately, their 토토 신규가입꽁머니asure토토 신규가입꽁머니nt is now relatively straightforward even at ultra-trace levels thanks to ILIAMS.
토토 신규가입꽁머니anwhile, the quest to determine protocols and wavelengths to other radioisotopes continues unabated at VERA. Looking to the future Martschini states that the ILIAMS team are currently working hard to add the astrophysically interesting182Hf.
Lasers have long been used for analytical purposes – to interrogate samples according to their composition, phase, temperature, and so forth. But the innovative ILIAMS 토토 신규가입꽁머니thod is enabling scientists at the University of Vienna and their collaborators around the world to use lasers to investigate the detailed history of all kinds of samples as never before.
“The high beam quality of the AVIA LX as well as the VERDI laser is critical to our technique since we have to align the beam through 3 mm apertures at each end of the 1-토토 신규가입꽁머니ter waveguide beam path."
— Martin Martschini, Senior Scientist, Isotope Physics Group, Univers토토 신규가입꽁머니y of Vienna
Figure 1.In the ILIAMS setup, the initial beam of anions is cooled in a waveguide while being irradiated by a colinear laser beam of the appropriate wavelength to cause selective electron photodetach토토 신규가입꽁머니nt. Figure credit Martin Martschini.
