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The high sensitivity of resonance ionization to elements of interest is matched by its high degree of insensitivity to atoms of other elements. This eliminates the need, in many applications, to perform extensive chemical separations of an element of interest. For example, Lugaro et al.

Likewise, Levine et al. Chemical separation of the ppm-level elements analyzed by Lugaro et al. In our experiment, the elemental selectivity of resonance ionization is particularly important. To give a recent example, de Mayer et al.

What are Isotopes?

They separated Rb from Sr by ion-exchange chromatography, and then measured the isotopic ratios of each element by thermal ionization mass spectrometry. Instead of chemically separating Rb from Sr, in our experiment we selectively ionize laser-ablated Sr and Rb at different times using resonance ionization. With this delay of the Rb photoionization, we observe well-separated peaks corresponding to 86 Sr, 87 Sr, 88 Sr, 85 Rb, and 87 Rb in our time-of-flight spectra Fig. A principal advantage of separation of isotopes by resonance ionization is that laser systems can be miniaturized for spaceflight e.

Time-of-flight spectra for GSD-1G standard glass. Lighter curve offset downwards by 3 mV for clarity is the mean of shots with first-step resonance lasers only, added to the mean of shots with second-step resonance and ionization lasers only. A small background under the 87 Sr peak allows us to quantify how much of that peak is due to accidental ionization of other species here, presumably molecules with mass 87, rather than to strontium.

For these shots, Rb ions were created 2. We excite two single-photon resonant transitions and then non-resonantly photoionize each element Fig. For Rb, ground-state i. For Sr, we excite atoms from the [Kr]5s 2 1 S 0 ground state to the [Kr]5s5p 1 P o 1 state at 21, cm —1 with Among the advantages of multiphoton processes is that we can quantitatively assess our unwanted backgrounds: by pulsing only the optical parametric oscillators that drive the first transitions in our resonance schemes, or only the dye lasers that drive the second transitions, and we can observe the extent of non-resonant ionization which is undesirable because it need not be elementally specific and any ionization due to accidental resonances with one of our lasers.

In contrast to these sources of background, ionization of Rb and Sr requires the simultaneous pulsing of all the lasers driving the relevant transitions. We show background spectra in Fig. Ions detected in the former case are unlikely to be Sr or Rb and, indeed, we see that a weak molecular interference is present under the 87 Sr peak , while the additional ions detected in the latter case almost certainly are.

Radioisotopes and the age of the Earth Volume 1 and 2

Resonance ionization schemes employed to excite and ionize Rb and Sr. Atoms of these elements are excited by successive absorption of three photons shown schematically as arrows with indicated wavelengths, passing through two intermediate electronically excited states denoted by horizontal lines with term and configuration before reaching the ionization continuum at 33, cm —1 or 4. We detect most isotopes of each element in approximately their natural proportions, but we more favorably detect Rb than Sr.

This fractionation could arise easily from any of a number of physical causes, such as a more efficient thermal ablation of Rb; b more efficient resonance ionization of Rb, e. We monitor the elemental fractionation by analyzing standards at regular intervals during a run, so that we can ultimately correct our data for this effect. We use the standard analyses to monitor isotopic fractionations as well, though most of these are small.

The exception is the routine over-detection of 87 Sr relative to other Sr isotopes, which presumably arises because of more efficient optical excitation of the odd isotope through excited states with multiple hyperfine sublevels. Such odd-even isotope effects are well known in resonance ionization spectroscopy e. The LARIMS instrument used for this experiment comprises a vacuum chamber with a triaxial sample-positioning stage and a multi-bounce time-of-flight mass spectrometer, an ablation laser, strontium ionization lasers, rubidium ionization lasers, laser attenuators, laser power meters, a wavelength meter, a digitizer, timing control electronics, a computer, and software.

We describe these components in the paragraphs below. First, we photographed the stub of Zagami and GSD-1G under a binocular microscope, so that we could compare the sample before and after ablation, and so that we might compare and correlate the acquired Rb and Sr spectra with the photomicrograph after ablation.

In this case, the sample was so fine grained that we were ultimately unable to recognize optical features that might correlate with variations in Rb and Sr content. Next we secured the stub with set-screws in a custom-designed jaw, and introduced the assembly into the vacuum chamber. There, we affixed the specimen jaw to the triaxial sample-positioning stage with a conical bayonet mount. The sample is driven into firm contact with the back of the sample electrode of the mass spectrometer Fig.

We observe that adjusting the ablation focus particularly influences the measured abundance of strontium, presumably because strontium is less volatile than rubidium, so that it requires tighter focus to affect more efficient ablation. We therefore adjust the focus to optimize strontium peak height before making hundreds of measurements; in general, the focus does not significantly change from run-to-run. Schematic diagram of the resonance ionization mass spectrometer, including sample stage, laser beam paths, ion optics, and detector. Electrodes pulsed to reject unwanted and elementally non-specific ions produced directly by the ablation process are noted.

The signals from the three materials Zagami, standard, and epoxy are generally easily distinguished. We use these data to bound the area for detailed measurements, which consist of spot analyses at hundreds of locations on a grid of points on Zagami and on the standard. Every spot analysis consists of — individual time-of-flight spectra, each from a single ablation pulse.

The first spot analysis, and every 5th spot analysis thereafter, is taken on the standard; however, the locations on both the sample and the standard are measured in a random order, to separate and identify potential spatial and time-dependent instrument biases. At each analysis spot we ablate the surface using a Big Sky Centurion diode-pumped solid state Nd:YAG laser, frequency quintupled to nm.

At this intensity, most samples produce plasma and an intense plume of energetic ions in addition to neutral atoms. However, we observed that after ablating for a period of 6 to 11 min i. We only implemented this last means of shielding the detector in time for one of the four data runs that we report below. The delay between Sr and Rb ionization separates the ion bunches in the mass spectrometer, allowing the independent measurement of the elements.

The strontium laser system produces the radiation at nm, nm, and nm needed to resonantly excite and ionize Sr atoms Fig. Similarly, the rubidium laser system produces the nm, nm, and nm light needed to resonantly excite and ionize Rb Fig. The ablation laser nm , the lasers exciting the first resonance steps for Sr and Rb nm and nm, respectively , and the lasers exciting the second resonance steps nm for Sr and nm for Rb are monitored with five GenTec EO M-LINK meters with QE-8 energy detector heads.

These sample partial reflections from each beam, and record a signal proportional to the energy of every laser pulse. It is important that every laser produce pulses of consistent energy over hours, since changes can lead to unexpected fractionation of Rb from Sr. We have not yet implemented monitoring of the nm lasers; however, because the nm light is frequency doubled to pump the dye lasers, we would detect degradation in pulse energy by monitoring the output of the dye lasers.

We control the outputs of the lasers driving the first and second resonance steps i. Both nm beams are attenuated using a Newport high power attenuator. Typical laser parameters for this experiment. Because of the lower pulse energies available to drive the nm and nm transitions in Rb, these beams were focused to a smaller diameter. Beam diameters are approximate because the beam profiles were irregularly shaped. We monitor the wavelengths of the tunable resonance lasers i.

Our time-of-flight mass spectrometer Fig. The first reflectron is off while photoions are created in the source and extracted into the flight tube, but is then energized so that it serves as an electrostatic mirror. The second reflectron is pulsed five times with a period of As the bunches of Rb and Sr ions bounce between the reflectrons, their separation increases and the temporal width of the bunches decreases, allowing us to clearly delineate the peaks for 84,86,87,88 Sr and 85,87 Rb.

After five bounces, the second reflectron is de-energized, we lower the electrode voltage that has protected the ETP MagneTOF hybrid discrete dynode detector from unwanted ions produced directly by ablation, and the resonance ions are allowed to reach the detector. The resulting pulses are directly digitized using a Gage Razor bit digitizer, sampling every 5 ns, with a bandwidth of MHz.

The entire sequence of ablation, resonant photoionization, multi-bounce flight of the photoions through the mass spectrometer, and detection is repeated at 20 Hz. A typical spot analysis includes — repetitions, and an equal number of backgrounds in which some or all of the resonance lasers are not fired. The timing and synchronization of all the components of the instrument are regulated using Highland Technologies T digital delay generators. The data that we present here include spot analyses on Zagami and spot analyses on the GSD-1G standard. These analyses were acquired in four separate measurement runs, each including Zagami and GSD-1G spots with the latter at intervals of every four Zagami spots from March to June The background-and-data acquisition typically lasted — ablation pulses in repeated groups of During the first 40 pulses in each group, the optical parametric oscillators that excite the first resonance transition for each element were fired, but the dye lasers and Nd:YAG lasers that then ionized the excited atoms were not.

During each of the next 40 pulses, the dye lasers and the Nd:YAG lasers were fired, but the first-step resonance lasers were not. The signals recorded from these pulses allowed us to assess backgrounds due to photoionization by our resonance lasers of species other than Rb and Sr Fig. These pulses were followed by 20 pulses without any resonance lasers firing at all; these were to determine our backgrounds due to ionization by the ablation process itself, as well as detector and digitizer noise.

Brief history and introduction

These pulses were followed by ablation pulses that were each accompanied by the full complement of resonance lasers, in which we obtained our Rb and Sr signals. The data that we report here were all acquired with relatively high ablation laser intensity. Like others e. However, work by Poitrasson et al. Russo et al. Specifically, reproducibility increases with ablation laser intensity, and R.

We are planning a future suite of experiments with a femtosecond ablation laser. The time-of-flight spectra that we obtain under these conditions are qualitatively different from what we formerly observed e. The most important difference is that, during the sample conditioning period, the Sr isotope peaks are absent from our time-of-flight spectra, replaced by a low, broad smear. We attribute this effect to the development of a plasma of ablated material in front of the sample. The plasma shields the photoions from the extraction field of the mass spectrometer's ion optics, leading to smearing of the isotopic peaks.

Unfortunately, we could not avoid waiting for the plasma to dissipate more quickly by delaying the photoionization of Sr by more than about 1. Even after waiting for several thousand pulses, occasional pulses still show the effects of the plasma. To avoid biases from these data, we excluded spectra with We integrated peaks for each of the isotopes 86,87,88 Sr and 85,87 Rb from the time-of-flight spectrum for each pulse, after defining time intervals of interest for each isotope based on the spectra of the GSD-1G standard.

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We evaluated the mean signal for each isotope by computing the mean of its integrated peak from the pulses with all the resonance lasers firing, and correcting for the mean integrated peak in each type of background pulse the background integrals taken with some of the resonance lasers firing were themselves first corrected for the background integrals that assessed the direct ionization from ablation. We assessed our statistical confidence in these mean signals by computing the standard error, summing the variances among the integrals measured in each type of pulse.

Although the integrals from individual pulses are distributed in a strongly non-Gaussian way, the standard error of the mean is still a valid estimate of confidence, as we could assemble many populations of integrals at random from their actual distribution, and find that the means of each population cluster around the mean of the actual population of integrals with a spread given by the standard error.

Isotope integrals vary greatly from one spot on Zagami to another, which is to be expected since the meteorite, although fine-grained, is mineralogically and chemically heterogeneous. This removes data with high uncertainty that would poorly constrain the age of the specimen.

Quick Overview

We stress that this is the only criterion that we employ for excluding spots of Zagami from our analysis; we are not otherwise picking the 'best' or 'cleanest' signals in any way. In the present experiment, this low-signal exclusion criterion removes spot analyses out of attempted, leaving us with data points with robust detection of both Rb and Sr. The integrals of the five isotopes exhibit significant covariances. Likewise, integrals of isotopes of the same element also fluctuate together because of fluctuations in their respective resonance lasers.

We calculated and accounted for all the covariances when we computed isotope ratios. We used the analyses of GSD-1G reference glass to determine the instrumental fractionation among the isotopes as a function of time during each run. On some days we observe the elemental fractionation to vary only slightly, whereas, on other days, we see large changes as a function of time Fig. Ultimately, we do not know the causes of these changes, although we have seen some correlations between greater overdetection of Rb relative to Sr and cooler laboratory temperatures.

Since that time, we have taken great pains to ensure a nearly constant temperature in our laboratory, including the installation of a high-capacity air-handling system that runs continually, and circulating chilled water to stabilize the temperature of laser dyes and the OPO laser heads.

While we continue to search for root causes of the fluctuations in instrumental fractionation, for the moment we are limited to monitoring them and correcting for them. We estimate the fractionation of each isotope relative to 86 Sr at the time that each Zagami analysis was performed by interpolating between the calculated fractionations at the time that GSD-1G was analyzed. Instrumental fractionation factors determined from GSD-1G analyses during two 16 h periods of our runs. Fractionation factors are expressed as the measured abundance ratios in the GSD-1G reference glass divided by their published abundance ratios.

The isochron diagrams shown in Fig. Zagami isochron diagrams for the four data runs described in the text. The age is calculated from the slope of the best-fitting line, using the 87 Rb half-life determined by Rotenberg et al. These four runs differed in certain respects, as we continued to modify and upgrade our instrument during this period. For example, during the run beginning on 7 March, the ablation beam was passed through an attenuator that we had previously used, although the attenuator was set for maximum transmission.

Before the 15 May run, we replaced our aging detector with a new unit of the same model, and before the 28 May run we strengthened the rejection scheme in our time-of-flight mass spectrometer for ions produced directly by ablation by pulsing the voltage on the electrode in front of the detector. Because data from these four runs were not acquired under identical conditions, it may be best to evaluate their results separately. On the other hand, since each run included analyses of the GSD-1G standard, it also seems reasonable to combine the standard-corrected data from all the runs into a single analysis.

We therefore present both separate and combined analyses. The four runs include two very long runs and two that were aborted after a relatively small number of spots were analyzed. Specifically, for the 7 March run we analyzed 85 spots on Zagami and 22 spots of GSD-1G before the water-cooling system for one of the rubidium resonance lasers failed, 18 h into the run. This was followed by a run beginning on 15 May during which we analyzed 56 spots on Zagami and 15 on GSD-1G before aborting after 20 h out of fear for the integrity of the new detector that we had just installed.

The two aborted runs highlight failure modes for our experiment that we are working to eliminate, and, because of the runs' smaller number of spot analyses, they have poorer precision than we have otherwise obtained. Our hope is that ultimately replacing our ablation laser with a femtosecond laser, or even a nanosecond laser with a higher repetition rate, will allow us to relax the requirement of such lengthy conditioning times. The ages are calculated from the slopes of the best fitting lines using the value of the 87 Rb half-life determined by Rotenberg et al.

We stress that the negative age determinations are completely consistent with what one expects when analyzing a very young specimen with low precision. Our uncertainty in each run represents quite a large fraction of the age that we determine for Zagami.


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We are presently beginning a suite of analyses of the Boulder Creek Granite, for which Peterman et al. We therefore believe that our precision is better understood in absolute terms rather than in fractional terms; it appears that our precision depends more on the abundances of Rb and Sr in our analyses than on the age of the specimen. For the Boulder Creek Granite, Peterman et al.

All four isochron ages agree broadly with the more precise ages determined for Zagami by Shih et al.

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The disparity between the precision obtained in these experiments and the precision that we achieve of several hundreds of millions of years arises from two differences between our respective techniques. First, unlike in a typical thermal ionization mass spectrometry experiment, we did not make any effort to select pristine mineral separates or spots for analysis. On the contrary, the ensemble of spots that we analyzed was simply a rectangular grid of points on the polished surface of the meteorite specimen. Therefore, many of the spots that we analyzed could have been slightly altered by grain-grain interactions or grain-fluid interactions in ways that would have altered their abundances of Rb or Sr.

An effort to date the meteorite with the highest achievable precision would tend to exclude such mineral grains after thorough inspection and characterization, but we have avoided these steps to mimic the operation of an instrument like ours in a spaceflight context, where such careful spot selection is probably impractical. Second, whereas isochrons obtained with thermal ionization mass spectrometry have a relatively small number of individual analyses, each performed with high precision, our isochron consists of a very large number of lower-precision spot analyses limited ultimately by the number of atoms that we ablate, resonantly ionize, and detect.

For the sake of comparison, all the Zagami analyses of Borg et al. Nevertheless, we are looking closely at this question in our ongoing measurements. The mean square weighted deviation MSWD; i. The probability that our data can be explained as a statistical line subject to random jitter of a size given by our uncertainty is even smaller for the other runs: a point line as in the 12 March run yields an MSWD of 1.

Quite possibly both these explanations are true, at least to some extent. A synthesized isochron diagram showing all spot analyzes from our four runs together is shown in Fig. Zagami isochron diagram with data from all four runs with bright ablation laser pulses. Symbols are as in Fig. Although the four runs were performed with slightly different experimental conditions, it may be appropriate to combine their data in this way, because the data from each run was standardized using GSD-1G analyses.

In the two runs that were completed, we achieved precision of Ma or better, meeting the criterion that NASA has established for in situ dating. One of the key reasons that the intense ablation pulses were necessary was to minimize elemental fractionation during the laser ablation process.

Fractionation prior to photoionization is an atypical concern for a resonance ionization mass spectrometry experiment; one of the advantages of this technique for many applications is that, since atoms are ionized from the vapor phase rather than directly from a solid sample, there is no matrix-induced isotopic fractionation that differs between a sample and a standard.

There are, of course, isotope shifts and odd-even effects that lead to spectroscopic fractionations, but these generally affect samples and standards to the same extent. In our work, because we need to analyze two elements, we found ourselves once more subject to often irregular and unpredictable fractionation between the elements, even when comparing replicate analyses of standards.

Operating the ablation laser at high intensity has allowed us to obtain accurate and reproducible results. But, have we been misled about the reliability of radioactive dating methods? The RATE group believes we have. The RATE group, consisting of six young-earth creationist geologists, geochemists, and physicists, is cooperating to research the issue of Radioisotopes and the Age of the Earth. They have dared to ask the tough questions and are searching for an alternative explanation for the billions of years found in rocks.

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