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36Cl in a new light: AMS measurements assisted by ion-laser interaction
Nuclear Inst. and Methods in Physics Research B 456 (2019) 163–168
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36
Cl in a new light: AMS measurements assisted by ion-laser interaction
Johannes Lachner , Christoph Marek, Martin Martschini, Alfred Priller, Peter Steier, Robin Golser ⁎
T
University of Vienna, Isotope Physics, Währinger Str. 17, 1090 Wien, Austria
ARTICLE INFO
ABSTRACT
Keywords: 36 Cl ILIAMS AMS VERA Ion-laser interaction Ion beam cooler Selective laser photodetachment Isobar suppression
The experimental setup at the Vienna Environmental Research Accelerator (VERA) was extended with an ion beam cooler and a 18 W laser to perform Ion-Laser InterAction Mass Spectrometry (ILIAMS). Gas collisions of the decelerated negative ion beam with He buffer gas inside a radio-frequency quadrupole slow down the ions, which facilitates element selective photodetachment to remove isobaric interferences. Here we present the first successful implementation of a suppression of the isobar of 36Cl via laser-photodetachment into an AMS measurement. Whereas conventional AMS measurements require separation of the isobar 36S at high beam energy, ILIAMS takes advantage of the higher electron affinities of Cl− (3.6 eV) compared to S− (2.1 eV). Using 532 nm photons (Eph = 2.3 eV) the isobar 36S is suppressed by 10 orders of magnitude. A high transmission of the Cl− beam through the ion cooler (up to 80%) and a quantitative suppression of the isobaric interference allows for competitive measurements at low accelerator voltages and with lower charge states. This results in high yields for 36Cl transport to the detector. The ILIAMS system is stable over several days of beamtime and blank levels below 36Cl/Cl = 10−15 are reached. The accomplishment of a 36Cl AMS measurement at a final beam energy as low as 5.4 MeV with full separation of isobars and m/q interferences shows the potential of the new isobar suppression method to widen the capabilities of smaller and middle-sized AMS facilities.
1. Introduction Within the six classical accelerator mass spectrometry (AMS) nuclides (10Be, 14C, 26Al, 36Cl, 41Ca, 129I) 36Cl and 41Ca are the last of those to require a beam energy of several 10s MeV to suppress the atomic isobars via an element specific energy loss. For measurements of 36Cl 6,MV), sewith middle-sized AMS facilities (acceleration voltage paration of the isobars 36Cl and 36S can be achieved in split anode gas ionization chambers (e.g. [1–3]) and in some cases with additional assistance from a post-stripping absorber foil (e.g. [4,5]). In addition to a thorough chemical suppression of Sulfur during the extraction of Chlorine from the original sample material, a number of laboratories undertake special efforts in the chemical handling of the samples and targets using AgBr backing [6] in order to reduce the 36S counting rate originating from the target holder. From this viewpoint presently 36Cl is among the most costly AMS nuclides in terms of equipment and labor. Laser photodetachment for suppression of S−[7] was documented early to reduce the S interference by 3 orders of magnitude showing potential for accelerator mass spectrometry (AMS). The element selective isobar suppression takes advantage of the different binding energies for different negative ions or molecules. In the interaction between photons and negative ions the additional electron is removed and
⁎
the ion is neutralized if the photon energy exceeds the binding energy, i.e. the electron affinity (EA). A high flux of photons with a suitable energy between the EAs of two isobars removes one of the two isobars from the ion beam. Laser photodetachment is one of two presently explored ways to suppress the isobaric background before the acceleration process. In both cases the ions are slowed down electrostatically before interacting with an appropriate reaction gas or with an intense photon beam. Chemical reaction cells filled with NO2 have demonstrated a maximum suppression of S− by 7 orders of magnitude [8,9]. Ion-laser-interaction mass spectrometry (ILIAMS) accomplishes an efficient neutralization of the interfering S− by an effective overlap of the ion beam with photons of a suitable energy. The suppression factor fsupp is given by the intensity I of the remaining ion beam after the interaction with the photons relative to the intensity I0 transmitted through the system I without photon flux via fsupp = I0 = e t , with the cross section for photodetachment, the photon flux and t the interaction time between the photon and anion fluxes. In order to keep the applied photon flux technically feasible, the interaction time has to be increased as much as possible. To study the practicality of laser photodetachment in AMS a test-bench named ILIAS was installed, where the ions were first decelerated electrostatically and then further cooled down in collisions
Corresponding author. E-mail address: [email protected] (J. Lachner).
https://doi.org/10.1016/j.nimb.2019.05.061 Received 8 January 2018; Received in revised form 21 May 2019; Accepted 23 May 2019 Available online 05 June 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
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with a He buffer gas inside a radio-frequency quadrupole (RFQ) ion cooler. Also first tests on the survival and detachment of negative ions were conducted [10,11]. Here, we present the performance of ILIAMS for 36Cl measurements at VERA. The focus is on the intensity of the transmitted Cl− beam, suppression of the S− isobar, accessible blank levels, the stability of the whole system in a 36Cl/37Cl AMS measurement and its applicability for routine 36Cl measurements. 2. ILIAMS at VERA A general description of the present ILIAMS setup is found in [11,12]. A 30 keV Cl− ion beam is extracted from a MC-SNICS ion source using Cs or Rb material with typical outputs of few µ A [13] for each of the stable Cl isotopes. Targets consist of AgCl powder pressed into regular Cu target holders without special pretreatment. After the ion source a perforated metal sheet acts as an attenuator that can be inserted within a few seconds. It reduces the beam intensity by a factor of 60 but should not significantly affect the phase space of the beam from the ion source as the attenuators dimensions (ca. 5 cm × 5 cm) are much larger than the size of the beam at that position (beam diameter ca. 1 cm). Thereby, the attenuator adjusts the intensity of the stable isotopes (35Cl, 37Cl) such that their beam transport through the ion cooler and through the accelerator is more similar to the one of the mass of interest. The 30 keV negative ions are then mass filtered by a 90° bending magnet. The switching time of this magnet between neighboring masses is a few seconds. The measurement sequence via slow-bouncing consists of three segments injecting A) the non-attenuated mass 37 beam, B) the attenuated mass 37 beam, and C) the nonattenuated mass 36 beam into the cooler. The intensity of the mass 37 beams can be measured in Faraday cups before the cooler, after the cooler, and after the accelerator. The negative ions are decelerated to a kinetic energy < 100 eV and enter the ion cooler through a 3 mm entrance aperture. Inside the ion beam cooler the ions collide with He buffer gas and are trapped by the potential generated from the RFQ electrodes (inscribed radius r0 = 4.35 mm, length l = 95.1 cm, electrode radius r = 5.00 mm, [11], peak-to-peak voltage VRF , p2p 400 V). The decelerated ions are guided towards the exit by a small linear electric field gradient (typically 0–4 V/m). The negative ions remain inside the ion cooler for 0.5 ms to few tens of ms. This can be controlled in partial by the electric field gradient and the buffer gas pressure but is largely influenced by the intensity of the ion beam transported through the system [11]. The buffer gas pressure is chosen at the highest possible value for optimal transmission of the Cl− ion beam. This way, the residence time of the ions remains long, which is beneficial for the suppression of the isobar. Following extraction from the ion cooler and reacceleration to 30 keV the beam is energy-analyzed by a 45° electrostatic filter. Between the 90° bending magnet and the 45° electrostatic analyzer (ESA) the ion beam is overlapped with 532 nm photons emitted by a VERDI V-18 (Coherent Inc., Santa Clara, CA, USA) laser with a maximum cw output of 18 W, of which up to 12 W can be fully transmitted through the system to a powermeter placed behind a viewport after the ESA. The losses are mainly caused by reflection and absorption at the MgF2 viewports. In the interaction zone the laser spot is elliptical (a = 1.5 mm, b = 1 mm) with a Gaussian spatial profile. The main interaction zone of ions and laser, which is in between the two 3 mm apertures of the cooler, therefore is not uniformly illuminated. In the actual experiment the mirrors and lenses of the optical setup are not tuned for optimal optical transmission, but for best suppression of the isobar. This can decrease the transmitted power of the laser from the optimum optical value by 5–10%. A further mass separation on the low-energy (LE) side of the accelerator takes place at the injection magnet. During measurement segments A and B, while the mass 37 amu beam is injected into the ion cooler, the electric system on the injection magnet to the accelerator is
Fig. 1. The transmission through the RFQ ion beam cooler with a He buffer gas pressure of 0.31 mbar and a guiding field of 2.4 V/m is stable for an attenuated ion beam. The data are taken on a set of samples that are remeasured in every turn of the sample wheel and thus also document a temporal stability over ≈48 h in total.
operated in the fast-bouncing mode. During segment C the 36 amu beam is continuously injected into the ion cooler and the electric system on the injection magnet is set to constant voltage so that the 36 amu beam is also injected into the accelerator without interruption. Data are collected into separate spectra for each segment. Charge exchange in the accelerator was performed with graphite foils for the charge state 7+ and with He or Ar stripper gas for charge state 2+. On the highenergy (HE) side the mass 36 beam was directed to two different gas ionization chambers (GIC). In the case of the 7+ beam (E = 24 MeV) a GIC with three anodes was used [14], the 2+ events were measured in a dual-anode GIC [15] developed at ETH [16]. 3. Results & discussion 3.1. Beam transport The transport of the ions through the AMS system depends on the beam intensity (Figs. 1, 2 and [12]) Also during the tuning procedure we observe that different beam intensities (attenuated vs. non-attenuated) require adjusted settings for the injection/extraction lenses and the beam guiding electrodes for optimal transmission through the gasfilled RFQ. This can be related to space charge arising from the
Fig. 2. The transmission of Cl− to Cl2+ through the accelerator with He as stripper medium for attenuated (2–40 nA) and non-attenuated (> 90 nA) beams from the same targets over a measurement including two different tunings of the AMS facility (I: runtime 60 h, II: runtime 16 h) shows a current dependency. A condition is set on the transmission of the attenuated beam to prevent instabilities of the system from affecting the results. Less than 10% of the data does not fulfill that condition and is discarded from further evaluation. 164
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decelerated negative ions. These effects are greatest at the entrance and exit of the ion cooler, where the slow particles are affected most by their repulsive Coulomb forces and are not focused by the potential of the RFQ. Therefore, high beam intensities and space charge in front of the ion cooler lead to a bad transmission through the ion cooler. At the exit of the ion cooler an intense beam develops a larger divergence and therefore its transport efficiency through the following system is worsened compared to a weak beam (Fig. 2). For the measurement of a 36Cl/Cl ratio and its normalization such differences in transport efficiency depending on the beam intensity are very critical. Our measurement shows a sufficiently constant behavior for the transport of ions if the beam intensity is in the range of few tens of nA or below (Fig. 1). Therefore we choose to normalize the 36Cl rate to the attenuated 37Cl beam current on the HE side. The 36Cl/Cl ratios of the samples are calculated from the measured 36Cl/37Cl ratios by dividing through the natural 37Cl/Cl ratio of 0.2423. Automatic tuning of the beam guiding and shaping parameters on the LE side including the ion cooler therefore is conducted with the attenuated beam (37Cl), then the setup is scaled to 36Cl.
Fig. 4. The spectrum of a AgCl blank sample with 10% Ag2S recorded in a compact dual-anode GIC shows only 36Cl events when all components are set for mass 36 (segment C). Switching the ion cooler injection to mass 37 (bouncing segment A) while still transporting 36 amu towards accelerator and detector allows monitoring the GIC and the sulfur peak position via background events of 36S and 18O.
3.2. Single ion detection in gas ionization chamber
3.3. Molecular interferences & appearances
36 Cl counts were recorded in two different gas ionization chambers. The multi-anode ionization chamber (MAIC, [14]) separates 36Cl7+ and 36 7+ S at a beam energy of 24 MeV on three anodes [17]. 36 7+ Cl and 36S7+ are distinguished in their energy loss signals on the three anodes (Fig. 3). This documents that indeed no 36S reaches the detector when the laser is turned on. The use of the high charge state (7+) requires foil stripping in the accelerator and limits the accelerator transmission to a maximum of 14%. When using ILIAMS on the lowenergy side of the AMS facility, the high energy and clear separation in the detector is not needed for the S/Cl separation any more. Further measurements were conducted using lower charge states (2+,3+) to gain higher efficiency in the beam transport. Higher transport yields, e.g. up to 35% for the 2+ (Fig. 2), can be achieved using a gas stripper and a lower charge state. Because of the lower beam energy a more compact dual-anode GIC is utilized. Still, a separation of the center of mass of 36Cl2+ from 36S2+ is accomplished in the measurement of energy loss E and residual energy Eres (Fig. 4). In normal operation all 36S events are suppressed via photodetachment in the ion cooler. Any malfunction of the photodetachment, e.g. caused by an interruption of the laser, and sudden survival of S− ions could thus still be recognized via an increase of events in the respective 36S region.
The negative mass 36 beam injected into the ion cooler mainly consists of 36S− and molecular isobars. Intense molecular interference may arise from 12C3 ,18 O2 , combinations of 16,17,18O and 1H, or 19 17 − F O , followed by intense m/q interferences of 12C+ to 36Cl3+ or 18 + O to 36Cl2+. Molecular background can be removed in the stripping process in the accelerator, but those primary molecules may already be suppressed by means of photons with a suitable energy. However, a breakup or transformation of molecules inside the ion cooler may result in the appearance of new negative beams. Following an excitation of the original negative molecule, e.g. via a collision, the excited state generally may decay via different channels. This de-excitation can result in a repopulation of the original state, a separation into a neutral and a negative fragment, or a separation into the neutral molecule and a free electron, i.e. collisional detachment. We could observe the transformation into a neutral and a negative fragment in several instances: One case is the appearance of 12C2 after the ion cooler when a 12C beam is injected into the ion cooler (Fig. 5). Other examples of 3 such molecular transitions in the vicinity of the cooler are documented in [12] or discussed below (SH− S−, Fig. 4). The intensity of the 12C2 beam appearance after the ILIAMS can be influenced by the settings of the ion cooler. If slightly less deceleration than required for an optimal transport of C3 is applied, a breakup of this molecule is favored and the
Fig. 3. Spectra recorded in the triple anode MAIC detector of a sample (36Cl/Cl = 4.5·10 13 ) during a measurement in the charge state 7+ show 36Cl and 36S events reaching the detector when the laser is turned off and only 36Cl when it is on with transmitted power 2 W measured in the post-cooler powermeter. Part a) depicts the energy loss signals E1 and E2 on the first and second anode, part b) the energy loss signal E1 and residual energy Eres derived from the signal on anode 3. Best separation of 36Cl7+ and 36S7+ is visible in the 2D-plot of anode 3 vs. anode 1. 165
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photodetachment into a negative 36 amu molecule or ion during (or after) the passage of the beam through the cooler, which is then converted into an m/q = 36/2 interference in the stripping process. If the molecular transformation happens in a late stage of the cooler passage or during the extraction, the overlap of the photon beam and particle beam may not be sufficient to detach all new ions and molecules coming into existence. Additionally, if the applied photon energy of 2.33 eV is only slightly above threshold, as it is the case for the SH− molecule (EA(SH) = 2.32 eV, [22]), cross-sections for detachment may yet be too low for a significant neutralization. Electron affinities for many potential precursor molecules at mass 37 are unknown, so the reasons for the appearance of 18O1+ in the detector are subject to speculation. Further backgrounds might originate from molecules such as (18O +19F)− being converted into O2 at the end of the ion cooler and finally to 18O1+ or 18O22+ in the accelerator.
Fig. 5. The mass scan of the magnet after the cooler on a graphite target covers the mass range from 23 to 37 amu, with mass 36 injected into the cooler. With suitable settings for a hard injection of the beam into the cooler some of the C3 molecules are transformed to C2 in collisions. Laser photodetachment only has an impact on the weaker bound C3 while the intensity of the C2 beam is not affected.
3.4. S suppression factor The factor by which the S− ion can be suppressed in the photodetachment process was measured in mixed AgCl/Ag2S targets with a nominal atomic ratio of Cl:S = 10:1. Switching to a mass 34 beam in order to determine the suppression factor has two major advantages: First, the isotopic abundance of 34S is 4.25% and therefore a 34S current of the order of 100 pA can be measured in a Faraday cup directly in front of the detector if the laser is turned off. By normalizing the counting rate in the detector to the 34S2+ current the determined suppression factor of the ILIAMS is independent of the S− ionization yield. Second, suppression factors derived on mass 36 were finally limited by counting rates identified as real 36Cl, but there is no isobar to 34 S. Therefore, the 2+ events registered in the detector could be attributed unambiguously to surviving 34S. For laser powers between 3 W and 12 W the 34S intensity is reduced by more than 10 orders of magnitude (Fig. 6). Under identical conditions not the slightest reduction in Cl intensities is observed. There is a minor drawback of this experiment: When the negative ions are in large part neutralized in the ion beam cooler, the transmission of a rare beam depends on the injected beam intensity as space charge created in the injection area pushes the ions faster through the cooler and reduces the mean residence time of the ions by ca. a factor of 5 for an intensity increase by a factor of 100 [11]. The interaction time of the ions in a less intense beam with the laser is extended. Thus, the more intense 34 − S beam is not a real equivalent to the 36S− beam and the real suppression factors for 36S− may even be higher. For realistic S contents in the sub-% level of an AgCl sample, a transmitted power of only 1 W
formation of C2 is increased. The transformation and destruction of C3 in collisions can also be observed by using the selectivity of the ion-laser interaction: With the electron affinities of C3 and C2 being 2.00 eV and 3.27 eV [18], respectively, the 2.33 eV photons neutralize the C3 molecules. This behavior is one further indication that the actual transformation happens before the injected C3 molecule is effectively cooled down inside the ion cooler and can be neutralized by the photons as else no C2 beam could appear after the cooler. A mass scan of the transmitted beam (Fig. 5) therefore only shows C2 but no C3 when the laser is turned on. A direct reaction neutralizing the C3 in the collision is also possible, but cannot be observed. The suppression of C3 inside the cooler and separation of C2 by means of the mass analysis before the injection into the accelerator is sufficient to avoid an intense 12C+ interference in the detector when choosing the 3 + charge state for Cl. Choosing the charge state 2+ for transport to the detector results in a high yield in the stripper medium but gives rise to potentially surviving 2+ molecules or an m/q interference with 18 amu (atomic mass units) in the 1+ charge state. In fact, when the system is tuned to 36 amu and the laser works at a transmitted power of 2 W or more, no events appear in the two-dimensional spectrum in the GIC except those that can be identified as 36Cl2+ (Fig. 4). Potentially intense m/q backgrounds from O2 (EA(O2 ) = 0.45 eV, [19]) or C3 are effectively removed in the ILIAMS setup. A background contribution on the HE side of the AMS system from injected (17O + 19F)− (EA(FO) = 2.27 eV, [20]) can be observed in the 36 amu beam but can be completely removed with the additional good separation of the HE mass spectrometer. Other molecular background that can not be detached (e.g. EA(B2N) = 3.1 eV, [21]) may survive but is too rare to be observed in the beam. During the measurement the magnet injecting the beam to the ion cooler is operated in a slow-sequencing mode (periods for segments of m = 36 amu 1–10 min and for m = 37 amu segments 1 min) while the injection into the accelerator is in a fast-sequencing mode (period for a cycle of m = 36 amu 200 ms and m = 37 amu injection < 100 ms). This means that during the injection of mass 37 (segments A and B) into the ion cooler there is a subsequent transport of a 36 amu beam to the accelerator. For this reason, the appearance of 36S2+ and 18O1+ ions can be observed in the detector while injecting mass 37 instead of mass 36 into the ion cooler (Fig. 4). While this does not affect the quality of the 36Cl AMS measurement it documents the potential to study the physical properties of negative molecules and ions in such a setup. This appearance is explained by a transformation of an initial negative 37 amu molecule (e.g. SH−) that is not subject to
Fig. 6. Suppression factor fsupp recorded from a
34 2+
S
beam depending on the
transmitted laser power ( = 532 nm) through the RFQ ion cooler with a He buffer gas pressure of 0.31 mbar and a guiding field of 2.4 V/m. The counting rate limiting the suppression factor to a value of 3·1010 at high laser intensities (> 5 W) stems from surviving 34S2+.
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of 532 nm photons is largely sufficient to remove any 3.5.
36
S interference.
36
Cl/Cl blank level
The use of low-level standards (36Cl/Cl<1.5·10−12) and long sputtering times for single runs on the targets significantly reduces the blank levels resulting in a present value for targets of AgCl or AgCl/ Ag2S of 36Cl/Cl 8·10 16 . The remaining events are identified as 36Cl (Fig. 4). The same count rate as on the AgCl blanks is found on press blanks constituted solely of Nb powder pressed into Cu target holders. Therefore, it is assumed that the blank level may be further reduced if a stronger focus is set on reducing the cross-contamination during sample handling and in the ion source. An upper limit for the remaining 36S contribution to the 36X/Cl background level of < 10−16 can be derived from detecting single events in the 36S/36Cl overlap region where one cannot fully distinguish between 36S2+ and 36Cl2+. 3.6. Measurement stability A small temporal variability of transmission through the ion cooler or accelerator is observed on top of the beam current’s influences on the transmission (Figs. 1, 2). This demands for a careful control of a) the beam currents and b) temporal variations of the beam transmission through the system, especially concerning diverging transmissions of the 36Cl and the 37Cl beam. Providing that standard targets with similar beam currents to the unknown samples are used, the stability of reference materials over time is the critical parameter determining the quality of the measurements. A remaining temporal variability with drifts over few hours or immediate stepwise changes then can be corrected in large part by turnwise normalization, i.e. estimating and applying a separate normalization factor for each turn of the 40 cathode sample wheel. A good tuning of the ILIAMS and AMS system results in stable measurements over many hours with a measurement uncertainty for the average normalization factor of 1% (Fig. 7). In order to maintain a high reproducibility, typically after 2 days a retuning of the electrostatic components is necessary to compensate larger drifts of power supplies and changing phase space from the ion source due to the cratering of the sputter target. For these experiments a set of internal reference materials calibrated against K-381/4N was used [23]. Unknown samples with 36Cl/Cl ratios of the order of 10−13 or higher are measured to a relative uncertainty of 5%. Targets of unknown samples created from the same material and reference materials with 36Cl/Cl ratios ranging from 10−14 to 10−11 were measured during
Fig. 8. 36Cl/Cl ratios of different materials (internal reference materials and unknowns) measured in beamtimes applying ILIAMS are compared to results obtained in conventional 36Cl measurements at VERA.
different beamtimes and could be reproduced within the uncertainty margins (Fig. 8). 4. Conclusion & outlook The installation of ILIAMS at VERA proves the general feasibility of Cl measurements at AMS facilities independent of their terminal voltages. Automatic measurements of unknown samples have been conducted. The setup is now fully usable for routine operation. A laser power transmitted through the ILIAMS system of 1 W is sufficient to suppress the S extracted from an AgCl cathode by 7 orders of magnitude. For typical S contents in an AgCl sample the 36S induced background is certainly reduced down to 36S/Cl levels below 10−16 at only 25% loss of 36Cl ions. The events leading to a present blank ratio at the level of 8·10 16 are identified as real 36Cl and has also been observed in classical AMS-measurements of 36Cl at VERA before [17]. This remaining background has to be attributed to a cross-contamination in the ion source or in the handling of the sample materials. These performance values are unprecedented for AMS systems using a final beam energy below 40 MeV, which lose or discard 50% of the 36 Cl in the detection system in order to achieve similar blank levels (e.g. [24,5,25,3]). Although introducing a new element on the LE side of the AMS system, ILIAMS reduces the complexity of 36Cl measurements with a) less requirements on the sample preparation and b) simple detection on the HE side allowing for higher 36Cl detection efficiencies at low ion beam energies in the single MeV range. Thus, 36Cl detection should be feasible also at very compact AMS systems. There is potential for a full suppression of all interferences on the LE side but this is critically affected by rare molecular interferences at mass 36. In this context the high electron affinity of Cl may play a key role as only few molecular systems have electron affinities higher than 3 eV. With the proper photon wavelength, corresponding to an energy just below 3.6 eV, thus the beam could be cleaned completely. The successful measurement of 36Cl with ILIAMS also points out that new isotopes (e.g. 135Cs and 182Hf) are in reach for AMS now. In the future, the limiting factor for a multitude of AMS nuclides or applications in order to achieve the necessary suppression of the isobars may not be the available beam energy of the AMS system but the feasibility to form suitable negative ions and molecules allowing for efficient photodetachment of the interferences. 36
Fig. 7. A stable normalization can be obtained from a set of internal standards (11 cathodes with nominal 36Cl/Cl ratios ranging from 1.5·10−13 to 1.5·10−12) over a period of two days. The depicted standard deviation of the single measurement amounts to 10% of the inverse normalization factor. (1/ fn =
(36Cl / Cl)measured ); ( 36Cl / Cl)nominal
the uncertainty of the average is 1.0%.
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This project was supported the University of Vienna via funding through “Investitionsprojekt der Universität Wien 2017”. References [1] P. Steier, O. Forstner, R. Golser, W. Kutschera, M. Martschini, S. Merchel, T. Orlowski, A. Priller, C. Vockenhuber, A. Wallner, 36Cl exposure dating with a 3MV tandem, Nucl. Instrum. Meth. B 268 (7) (2010) 744–747. [2] K. Wilcken, S. Freeman, A. Dougans, S. Xu, R. Loger, C. Schnabel, Improved 36Cl AMS at 5MV, Nucl. Instrum. Meth. B 268 (7) (2010) 748–751. [3] K. Wilcken, D. Fink, M. Hotchkis, D. Garton, D. Button, M. Mann, R. Kitchen, T. Hauser, A. O’Connor, Accelerator mass spectrometry on SIRIUS: new 6MV spectrometer at ANSTO, Nucl. Instrum. Meth. B 406 (2017) 278–282. [4] S. Akhmadaliev, R. Heller, D. Hanf, G. Rugel, S. Merchel, The new 6MV AMS-facility DREAMS at Dresden, Nucl. Instrum. Meth. B 294 (2013) 5–10. [5] R. Finkel, M. Arnold, G. Aumaître, L. Benedetti, D. Bourlès, K. Keddadouche, S. Merchel, Improved 36Cl performance at the ASTER HVE 5MV accelerator mass spectrometer national facility, Nucl. Instrum. Meth. B 294 (2013) 121–125. [6] S. Merchel, W. Bremser, V. Alfimov, M. Arnold, G. Aumaître, L. Benedetti, D.L. Bourlès, M. Caffee, L.K. Fifield, R.C. Finkel, S.P.H.T. Freeman, M. Martschini, Y. Matsushi, D.H. Rood, K. Sasa, P. Steier, T. Takahashi, M. Tamari, S.G. Tims, Y. Tosaki, K.M. Wilcken, S. Xu, Ultra-trace analysis of 36Cl by accelerator mass spectrometry: an interlaboratory study, Anal. Bioanal. Chem. 400 (2011) 3125–3132. [7] D. Berkovits, E. Boaretto, G. Hollos, W. Kutschera, R. Naaman, M. Paul, Z. Vager, Selective suppression of negative ions by lasers, Nucl. Instrum. Meth. A 281 (3) (1989) 663–666. [8] A. Litherland, I. Tomski, X.-L. Zhao, L.M. Cousins, J. Doupé, G. Javahery, W. Kieser, Isobar separation at very low energy for AMS, Nucl. Instrum. Meth. B 259 (1) (2007) 230–235. [9] J. Eliades, X.-L. Zhao, A. Litherland, W. Kieser, Negative ion-gas reaction studies using ion guides and accelerator mass spectrometry II: S-, SO- and Cl- with NO2 and N2O, Nucl. Instrum. Meth. B 361 (2015) 300–306. [10] O. Forstner, P. Andersson, D. Hanstorp, J. Lahner, M. Martschini, J. Pitters, A. Priller, P. Steier, R. Golser, The ILIAS project for selective isobar suppression by laser photodetachment, Nucl. Instrum. Meth. B 361 (2015) 217–221. [11] M. Martschini, J. Pitters, T. Moreau, P. Andersson, O. Forstner, D. Hanstorp, J. Lachner, Y. Liu, A. Priller, P. Steier, R. Golser, Selective laser photodetachment of intense atomic and molecular negative ion beams with the ILIAS RFQ ion beam
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36
Cl in a new light: AMS measurements assisted by ion-laser interaction
Johannes Lachner , Christoph Marek, Martin Martschini, Alfred Priller, Peter Steier, Robin Golser ⁎
T
University of Vienna, Isotope Physics, Währinger Str. 17, 1090 Wien, Austria
ARTICLE INFO
ABSTRACT
Keywords: 36 Cl ILIAMS AMS VERA Ion-laser interaction Ion beam cooler Selective laser photodetachment Isobar suppression
The experimental setup at the Vienna Environmental Research Accelerator (VERA) was extended with an ion beam cooler and a 18 W laser to perform Ion-Laser InterAction Mass Spectrometry (ILIAMS). Gas collisions of the decelerated negative ion beam with He buffer gas inside a radio-frequency quadrupole slow down the ions, which facilitates element selective photodetachment to remove isobaric interferences. Here we present the first successful implementation of a suppression of the isobar of 36Cl via laser-photodetachment into an AMS measurement. Whereas conventional AMS measurements require separation of the isobar 36S at high beam energy, ILIAMS takes advantage of the higher electron affinities of Cl− (3.6 eV) compared to S− (2.1 eV). Using 532 nm photons (Eph = 2.3 eV) the isobar 36S is suppressed by 10 orders of magnitude. A high transmission of the Cl− beam through the ion cooler (up to 80%) and a quantitative suppression of the isobaric interference allows for competitive measurements at low accelerator voltages and with lower charge states. This results in high yields for 36Cl transport to the detector. The ILIAMS system is stable over several days of beamtime and blank levels below 36Cl/Cl = 10−15 are reached. The accomplishment of a 36Cl AMS measurement at a final beam energy as low as 5.4 MeV with full separation of isobars and m/q interferences shows the potential of the new isobar suppression method to widen the capabilities of smaller and middle-sized AMS facilities.
1. Introduction Within the six classical accelerator mass spectrometry (AMS) nuclides (10Be, 14C, 26Al, 36Cl, 41Ca, 129I) 36Cl and 41Ca are the last of those to require a beam energy of several 10s MeV to suppress the atomic isobars via an element specific energy loss. For measurements of 36Cl 6,MV), sewith middle-sized AMS facilities (acceleration voltage paration of the isobars 36Cl and 36S can be achieved in split anode gas ionization chambers (e.g. [1–3]) and in some cases with additional assistance from a post-stripping absorber foil (e.g. [4,5]). In addition to a thorough chemical suppression of Sulfur during the extraction of Chlorine from the original sample material, a number of laboratories undertake special efforts in the chemical handling of the samples and targets using AgBr backing [6] in order to reduce the 36S counting rate originating from the target holder. From this viewpoint presently 36Cl is among the most costly AMS nuclides in terms of equipment and labor. Laser photodetachment for suppression of S−[7] was documented early to reduce the S interference by 3 orders of magnitude showing potential for accelerator mass spectrometry (AMS). The element selective isobar suppression takes advantage of the different binding energies for different negative ions or molecules. In the interaction between photons and negative ions the additional electron is removed and
⁎
the ion is neutralized if the photon energy exceeds the binding energy, i.e. the electron affinity (EA). A high flux of photons with a suitable energy between the EAs of two isobars removes one of the two isobars from the ion beam. Laser photodetachment is one of two presently explored ways to suppress the isobaric background before the acceleration process. In both cases the ions are slowed down electrostatically before interacting with an appropriate reaction gas or with an intense photon beam. Chemical reaction cells filled with NO2 have demonstrated a maximum suppression of S− by 7 orders of magnitude [8,9]. Ion-laser-interaction mass spectrometry (ILIAMS) accomplishes an efficient neutralization of the interfering S− by an effective overlap of the ion beam with photons of a suitable energy. The suppression factor fsupp is given by the intensity I of the remaining ion beam after the interaction with the photons relative to the intensity I0 transmitted through the system I without photon flux via fsupp = I0 = e t , with the cross section for photodetachment, the photon flux and t the interaction time between the photon and anion fluxes. In order to keep the applied photon flux technically feasible, the interaction time has to be increased as much as possible. To study the practicality of laser photodetachment in AMS a test-bench named ILIAS was installed, where the ions were first decelerated electrostatically and then further cooled down in collisions
Corresponding author. E-mail address: [email protected] (J. Lachner).
https://doi.org/10.1016/j.nimb.2019.05.061 Received 8 January 2018; Received in revised form 21 May 2019; Accepted 23 May 2019 Available online 05 June 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
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with a He buffer gas inside a radio-frequency quadrupole (RFQ) ion cooler. Also first tests on the survival and detachment of negative ions were conducted [10,11]. Here, we present the performance of ILIAMS for 36Cl measurements at VERA. The focus is on the intensity of the transmitted Cl− beam, suppression of the S− isobar, accessible blank levels, the stability of the whole system in a 36Cl/37Cl AMS measurement and its applicability for routine 36Cl measurements. 2. ILIAMS at VERA A general description of the present ILIAMS setup is found in [11,12]. A 30 keV Cl− ion beam is extracted from a MC-SNICS ion source using Cs or Rb material with typical outputs of few µ A [13] for each of the stable Cl isotopes. Targets consist of AgCl powder pressed into regular Cu target holders without special pretreatment. After the ion source a perforated metal sheet acts as an attenuator that can be inserted within a few seconds. It reduces the beam intensity by a factor of 60 but should not significantly affect the phase space of the beam from the ion source as the attenuators dimensions (ca. 5 cm × 5 cm) are much larger than the size of the beam at that position (beam diameter ca. 1 cm). Thereby, the attenuator adjusts the intensity of the stable isotopes (35Cl, 37Cl) such that their beam transport through the ion cooler and through the accelerator is more similar to the one of the mass of interest. The 30 keV negative ions are then mass filtered by a 90° bending magnet. The switching time of this magnet between neighboring masses is a few seconds. The measurement sequence via slow-bouncing consists of three segments injecting A) the non-attenuated mass 37 beam, B) the attenuated mass 37 beam, and C) the nonattenuated mass 36 beam into the cooler. The intensity of the mass 37 beams can be measured in Faraday cups before the cooler, after the cooler, and after the accelerator. The negative ions are decelerated to a kinetic energy < 100 eV and enter the ion cooler through a 3 mm entrance aperture. Inside the ion beam cooler the ions collide with He buffer gas and are trapped by the potential generated from the RFQ electrodes (inscribed radius r0 = 4.35 mm, length l = 95.1 cm, electrode radius r = 5.00 mm, [11], peak-to-peak voltage VRF , p2p 400 V). The decelerated ions are guided towards the exit by a small linear electric field gradient (typically 0–4 V/m). The negative ions remain inside the ion cooler for 0.5 ms to few tens of ms. This can be controlled in partial by the electric field gradient and the buffer gas pressure but is largely influenced by the intensity of the ion beam transported through the system [11]. The buffer gas pressure is chosen at the highest possible value for optimal transmission of the Cl− ion beam. This way, the residence time of the ions remains long, which is beneficial for the suppression of the isobar. Following extraction from the ion cooler and reacceleration to 30 keV the beam is energy-analyzed by a 45° electrostatic filter. Between the 90° bending magnet and the 45° electrostatic analyzer (ESA) the ion beam is overlapped with 532 nm photons emitted by a VERDI V-18 (Coherent Inc., Santa Clara, CA, USA) laser with a maximum cw output of 18 W, of which up to 12 W can be fully transmitted through the system to a powermeter placed behind a viewport after the ESA. The losses are mainly caused by reflection and absorption at the MgF2 viewports. In the interaction zone the laser spot is elliptical (a = 1.5 mm, b = 1 mm) with a Gaussian spatial profile. The main interaction zone of ions and laser, which is in between the two 3 mm apertures of the cooler, therefore is not uniformly illuminated. In the actual experiment the mirrors and lenses of the optical setup are not tuned for optimal optical transmission, but for best suppression of the isobar. This can decrease the transmitted power of the laser from the optimum optical value by 5–10%. A further mass separation on the low-energy (LE) side of the accelerator takes place at the injection magnet. During measurement segments A and B, while the mass 37 amu beam is injected into the ion cooler, the electric system on the injection magnet to the accelerator is
Fig. 1. The transmission through the RFQ ion beam cooler with a He buffer gas pressure of 0.31 mbar and a guiding field of 2.4 V/m is stable for an attenuated ion beam. The data are taken on a set of samples that are remeasured in every turn of the sample wheel and thus also document a temporal stability over ≈48 h in total.
operated in the fast-bouncing mode. During segment C the 36 amu beam is continuously injected into the ion cooler and the electric system on the injection magnet is set to constant voltage so that the 36 amu beam is also injected into the accelerator without interruption. Data are collected into separate spectra for each segment. Charge exchange in the accelerator was performed with graphite foils for the charge state 7+ and with He or Ar stripper gas for charge state 2+. On the highenergy (HE) side the mass 36 beam was directed to two different gas ionization chambers (GIC). In the case of the 7+ beam (E = 24 MeV) a GIC with three anodes was used [14], the 2+ events were measured in a dual-anode GIC [15] developed at ETH [16]. 3. Results & discussion 3.1. Beam transport The transport of the ions through the AMS system depends on the beam intensity (Figs. 1, 2 and [12]) Also during the tuning procedure we observe that different beam intensities (attenuated vs. non-attenuated) require adjusted settings for the injection/extraction lenses and the beam guiding electrodes for optimal transmission through the gasfilled RFQ. This can be related to space charge arising from the
Fig. 2. The transmission of Cl− to Cl2+ through the accelerator with He as stripper medium for attenuated (2–40 nA) and non-attenuated (> 90 nA) beams from the same targets over a measurement including two different tunings of the AMS facility (I: runtime 60 h, II: runtime 16 h) shows a current dependency. A condition is set on the transmission of the attenuated beam to prevent instabilities of the system from affecting the results. Less than 10% of the data does not fulfill that condition and is discarded from further evaluation. 164
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decelerated negative ions. These effects are greatest at the entrance and exit of the ion cooler, where the slow particles are affected most by their repulsive Coulomb forces and are not focused by the potential of the RFQ. Therefore, high beam intensities and space charge in front of the ion cooler lead to a bad transmission through the ion cooler. At the exit of the ion cooler an intense beam develops a larger divergence and therefore its transport efficiency through the following system is worsened compared to a weak beam (Fig. 2). For the measurement of a 36Cl/Cl ratio and its normalization such differences in transport efficiency depending on the beam intensity are very critical. Our measurement shows a sufficiently constant behavior for the transport of ions if the beam intensity is in the range of few tens of nA or below (Fig. 1). Therefore we choose to normalize the 36Cl rate to the attenuated 37Cl beam current on the HE side. The 36Cl/Cl ratios of the samples are calculated from the measured 36Cl/37Cl ratios by dividing through the natural 37Cl/Cl ratio of 0.2423. Automatic tuning of the beam guiding and shaping parameters on the LE side including the ion cooler therefore is conducted with the attenuated beam (37Cl), then the setup is scaled to 36Cl.
Fig. 4. The spectrum of a AgCl blank sample with 10% Ag2S recorded in a compact dual-anode GIC shows only 36Cl events when all components are set for mass 36 (segment C). Switching the ion cooler injection to mass 37 (bouncing segment A) while still transporting 36 amu towards accelerator and detector allows monitoring the GIC and the sulfur peak position via background events of 36S and 18O.
3.2. Single ion detection in gas ionization chamber
3.3. Molecular interferences & appearances
36 Cl counts were recorded in two different gas ionization chambers. The multi-anode ionization chamber (MAIC, [14]) separates 36Cl7+ and 36 7+ S at a beam energy of 24 MeV on three anodes [17]. 36 7+ Cl and 36S7+ are distinguished in their energy loss signals on the three anodes (Fig. 3). This documents that indeed no 36S reaches the detector when the laser is turned on. The use of the high charge state (7+) requires foil stripping in the accelerator and limits the accelerator transmission to a maximum of 14%. When using ILIAMS on the lowenergy side of the AMS facility, the high energy and clear separation in the detector is not needed for the S/Cl separation any more. Further measurements were conducted using lower charge states (2+,3+) to gain higher efficiency in the beam transport. Higher transport yields, e.g. up to 35% for the 2+ (Fig. 2), can be achieved using a gas stripper and a lower charge state. Because of the lower beam energy a more compact dual-anode GIC is utilized. Still, a separation of the center of mass of 36Cl2+ from 36S2+ is accomplished in the measurement of energy loss E and residual energy Eres (Fig. 4). In normal operation all 36S events are suppressed via photodetachment in the ion cooler. Any malfunction of the photodetachment, e.g. caused by an interruption of the laser, and sudden survival of S− ions could thus still be recognized via an increase of events in the respective 36S region.
The negative mass 36 beam injected into the ion cooler mainly consists of 36S− and molecular isobars. Intense molecular interference may arise from 12C3 ,18 O2 , combinations of 16,17,18O and 1H, or 19 17 − F O , followed by intense m/q interferences of 12C+ to 36Cl3+ or 18 + O to 36Cl2+. Molecular background can be removed in the stripping process in the accelerator, but those primary molecules may already be suppressed by means of photons with a suitable energy. However, a breakup or transformation of molecules inside the ion cooler may result in the appearance of new negative beams. Following an excitation of the original negative molecule, e.g. via a collision, the excited state generally may decay via different channels. This de-excitation can result in a repopulation of the original state, a separation into a neutral and a negative fragment, or a separation into the neutral molecule and a free electron, i.e. collisional detachment. We could observe the transformation into a neutral and a negative fragment in several instances: One case is the appearance of 12C2 after the ion cooler when a 12C beam is injected into the ion cooler (Fig. 5). Other examples of 3 such molecular transitions in the vicinity of the cooler are documented in [12] or discussed below (SH− S−, Fig. 4). The intensity of the 12C2 beam appearance after the ILIAMS can be influenced by the settings of the ion cooler. If slightly less deceleration than required for an optimal transport of C3 is applied, a breakup of this molecule is favored and the
Fig. 3. Spectra recorded in the triple anode MAIC detector of a sample (36Cl/Cl = 4.5·10 13 ) during a measurement in the charge state 7+ show 36Cl and 36S events reaching the detector when the laser is turned off and only 36Cl when it is on with transmitted power 2 W measured in the post-cooler powermeter. Part a) depicts the energy loss signals E1 and E2 on the first and second anode, part b) the energy loss signal E1 and residual energy Eres derived from the signal on anode 3. Best separation of 36Cl7+ and 36S7+ is visible in the 2D-plot of anode 3 vs. anode 1. 165
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photodetachment into a negative 36 amu molecule or ion during (or after) the passage of the beam through the cooler, which is then converted into an m/q = 36/2 interference in the stripping process. If the molecular transformation happens in a late stage of the cooler passage or during the extraction, the overlap of the photon beam and particle beam may not be sufficient to detach all new ions and molecules coming into existence. Additionally, if the applied photon energy of 2.33 eV is only slightly above threshold, as it is the case for the SH− molecule (EA(SH) = 2.32 eV, [22]), cross-sections for detachment may yet be too low for a significant neutralization. Electron affinities for many potential precursor molecules at mass 37 are unknown, so the reasons for the appearance of 18O1+ in the detector are subject to speculation. Further backgrounds might originate from molecules such as (18O +19F)− being converted into O2 at the end of the ion cooler and finally to 18O1+ or 18O22+ in the accelerator.
Fig. 5. The mass scan of the magnet after the cooler on a graphite target covers the mass range from 23 to 37 amu, with mass 36 injected into the cooler. With suitable settings for a hard injection of the beam into the cooler some of the C3 molecules are transformed to C2 in collisions. Laser photodetachment only has an impact on the weaker bound C3 while the intensity of the C2 beam is not affected.
3.4. S suppression factor The factor by which the S− ion can be suppressed in the photodetachment process was measured in mixed AgCl/Ag2S targets with a nominal atomic ratio of Cl:S = 10:1. Switching to a mass 34 beam in order to determine the suppression factor has two major advantages: First, the isotopic abundance of 34S is 4.25% and therefore a 34S current of the order of 100 pA can be measured in a Faraday cup directly in front of the detector if the laser is turned off. By normalizing the counting rate in the detector to the 34S2+ current the determined suppression factor of the ILIAMS is independent of the S− ionization yield. Second, suppression factors derived on mass 36 were finally limited by counting rates identified as real 36Cl, but there is no isobar to 34 S. Therefore, the 2+ events registered in the detector could be attributed unambiguously to surviving 34S. For laser powers between 3 W and 12 W the 34S intensity is reduced by more than 10 orders of magnitude (Fig. 6). Under identical conditions not the slightest reduction in Cl intensities is observed. There is a minor drawback of this experiment: When the negative ions are in large part neutralized in the ion beam cooler, the transmission of a rare beam depends on the injected beam intensity as space charge created in the injection area pushes the ions faster through the cooler and reduces the mean residence time of the ions by ca. a factor of 5 for an intensity increase by a factor of 100 [11]. The interaction time of the ions in a less intense beam with the laser is extended. Thus, the more intense 34 − S beam is not a real equivalent to the 36S− beam and the real suppression factors for 36S− may even be higher. For realistic S contents in the sub-% level of an AgCl sample, a transmitted power of only 1 W
formation of C2 is increased. The transformation and destruction of C3 in collisions can also be observed by using the selectivity of the ion-laser interaction: With the electron affinities of C3 and C2 being 2.00 eV and 3.27 eV [18], respectively, the 2.33 eV photons neutralize the C3 molecules. This behavior is one further indication that the actual transformation happens before the injected C3 molecule is effectively cooled down inside the ion cooler and can be neutralized by the photons as else no C2 beam could appear after the cooler. A mass scan of the transmitted beam (Fig. 5) therefore only shows C2 but no C3 when the laser is turned on. A direct reaction neutralizing the C3 in the collision is also possible, but cannot be observed. The suppression of C3 inside the cooler and separation of C2 by means of the mass analysis before the injection into the accelerator is sufficient to avoid an intense 12C+ interference in the detector when choosing the 3 + charge state for Cl. Choosing the charge state 2+ for transport to the detector results in a high yield in the stripper medium but gives rise to potentially surviving 2+ molecules or an m/q interference with 18 amu (atomic mass units) in the 1+ charge state. In fact, when the system is tuned to 36 amu and the laser works at a transmitted power of 2 W or more, no events appear in the two-dimensional spectrum in the GIC except those that can be identified as 36Cl2+ (Fig. 4). Potentially intense m/q backgrounds from O2 (EA(O2 ) = 0.45 eV, [19]) or C3 are effectively removed in the ILIAMS setup. A background contribution on the HE side of the AMS system from injected (17O + 19F)− (EA(FO) = 2.27 eV, [20]) can be observed in the 36 amu beam but can be completely removed with the additional good separation of the HE mass spectrometer. Other molecular background that can not be detached (e.g. EA(B2N) = 3.1 eV, [21]) may survive but is too rare to be observed in the beam. During the measurement the magnet injecting the beam to the ion cooler is operated in a slow-sequencing mode (periods for segments of m = 36 amu 1–10 min and for m = 37 amu segments 1 min) while the injection into the accelerator is in a fast-sequencing mode (period for a cycle of m = 36 amu 200 ms and m = 37 amu injection < 100 ms). This means that during the injection of mass 37 (segments A and B) into the ion cooler there is a subsequent transport of a 36 amu beam to the accelerator. For this reason, the appearance of 36S2+ and 18O1+ ions can be observed in the detector while injecting mass 37 instead of mass 36 into the ion cooler (Fig. 4). While this does not affect the quality of the 36Cl AMS measurement it documents the potential to study the physical properties of negative molecules and ions in such a setup. This appearance is explained by a transformation of an initial negative 37 amu molecule (e.g. SH−) that is not subject to
Fig. 6. Suppression factor fsupp recorded from a
34 2+
S
beam depending on the
transmitted laser power ( = 532 nm) through the RFQ ion cooler with a He buffer gas pressure of 0.31 mbar and a guiding field of 2.4 V/m. The counting rate limiting the suppression factor to a value of 3·1010 at high laser intensities (> 5 W) stems from surviving 34S2+.
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of 532 nm photons is largely sufficient to remove any 3.5.
36
S interference.
36
Cl/Cl blank level
The use of low-level standards (36Cl/Cl<1.5·10−12) and long sputtering times for single runs on the targets significantly reduces the blank levels resulting in a present value for targets of AgCl or AgCl/ Ag2S of 36Cl/Cl 8·10 16 . The remaining events are identified as 36Cl (Fig. 4). The same count rate as on the AgCl blanks is found on press blanks constituted solely of Nb powder pressed into Cu target holders. Therefore, it is assumed that the blank level may be further reduced if a stronger focus is set on reducing the cross-contamination during sample handling and in the ion source. An upper limit for the remaining 36S contribution to the 36X/Cl background level of < 10−16 can be derived from detecting single events in the 36S/36Cl overlap region where one cannot fully distinguish between 36S2+ and 36Cl2+. 3.6. Measurement stability A small temporal variability of transmission through the ion cooler or accelerator is observed on top of the beam current’s influences on the transmission (Figs. 1, 2). This demands for a careful control of a) the beam currents and b) temporal variations of the beam transmission through the system, especially concerning diverging transmissions of the 36Cl and the 37Cl beam. Providing that standard targets with similar beam currents to the unknown samples are used, the stability of reference materials over time is the critical parameter determining the quality of the measurements. A remaining temporal variability with drifts over few hours or immediate stepwise changes then can be corrected in large part by turnwise normalization, i.e. estimating and applying a separate normalization factor for each turn of the 40 cathode sample wheel. A good tuning of the ILIAMS and AMS system results in stable measurements over many hours with a measurement uncertainty for the average normalization factor of 1% (Fig. 7). In order to maintain a high reproducibility, typically after 2 days a retuning of the electrostatic components is necessary to compensate larger drifts of power supplies and changing phase space from the ion source due to the cratering of the sputter target. For these experiments a set of internal reference materials calibrated against K-381/4N was used [23]. Unknown samples with 36Cl/Cl ratios of the order of 10−13 or higher are measured to a relative uncertainty of 5%. Targets of unknown samples created from the same material and reference materials with 36Cl/Cl ratios ranging from 10−14 to 10−11 were measured during
Fig. 8. 36Cl/Cl ratios of different materials (internal reference materials and unknowns) measured in beamtimes applying ILIAMS are compared to results obtained in conventional 36Cl measurements at VERA.
different beamtimes and could be reproduced within the uncertainty margins (Fig. 8). 4. Conclusion & outlook The installation of ILIAMS at VERA proves the general feasibility of Cl measurements at AMS facilities independent of their terminal voltages. Automatic measurements of unknown samples have been conducted. The setup is now fully usable for routine operation. A laser power transmitted through the ILIAMS system of 1 W is sufficient to suppress the S extracted from an AgCl cathode by 7 orders of magnitude. For typical S contents in an AgCl sample the 36S induced background is certainly reduced down to 36S/Cl levels below 10−16 at only 25% loss of 36Cl ions. The events leading to a present blank ratio at the level of 8·10 16 are identified as real 36Cl and has also been observed in classical AMS-measurements of 36Cl at VERA before [17]. This remaining background has to be attributed to a cross-contamination in the ion source or in the handling of the sample materials. These performance values are unprecedented for AMS systems using a final beam energy below 40 MeV, which lose or discard 50% of the 36 Cl in the detection system in order to achieve similar blank levels (e.g. [24,5,25,3]). Although introducing a new element on the LE side of the AMS system, ILIAMS reduces the complexity of 36Cl measurements with a) less requirements on the sample preparation and b) simple detection on the HE side allowing for higher 36Cl detection efficiencies at low ion beam energies in the single MeV range. Thus, 36Cl detection should be feasible also at very compact AMS systems. There is potential for a full suppression of all interferences on the LE side but this is critically affected by rare molecular interferences at mass 36. In this context the high electron affinity of Cl may play a key role as only few molecular systems have electron affinities higher than 3 eV. With the proper photon wavelength, corresponding to an energy just below 3.6 eV, thus the beam could be cleaned completely. The successful measurement of 36Cl with ILIAMS also points out that new isotopes (e.g. 135Cs and 182Hf) are in reach for AMS now. In the future, the limiting factor for a multitude of AMS nuclides or applications in order to achieve the necessary suppression of the isobars may not be the available beam energy of the AMS system but the feasibility to form suitable negative ions and molecules allowing for efficient photodetachment of the interferences. 36
Fig. 7. A stable normalization can be obtained from a set of internal standards (11 cathodes with nominal 36Cl/Cl ratios ranging from 1.5·10−13 to 1.5·10−12) over a period of two days. The depicted standard deviation of the single measurement amounts to 10% of the inverse normalization factor. (1/ fn =
(36Cl / Cl)measured ); ( 36Cl / Cl)nominal
the uncertainty of the average is 1.0%.
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Acknowledgements
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