Hydrogen standards in elastic recoil detection analysis

Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 115–124 www.elsevier.com/locate/nimb

Hydrogen standards in elastic recoil detection analysis Y.Q. Wang

*

IT Characterization Facility, University of Minnesota, Minneapolis, MN 55455, USA

Abstract Quantitative hydrogen analysis using elastic recoil detection (ERD) usually requires the use for a hydrogen standard. In this paper, we first discuss how to correctly use primary hydrogen standards (Kapton and H-implant in Si) through a real example in which Kapton is used to determine the absolute implant-dose in a H-implanted Si target. Second part of the paper is devoted to discussing secondary hydrogen standards. Two types of hydrogen-containing thin films are investigated: H- and D-loaded erbium thin film and ion implanted polymer thin film. The results indicate that both these thin films have high and stable hydrogen content and are suitable as hydrogen thin film standards in ERD analysis.
2004 Elsevier B.V. All rights reserved. Keywords: ERD; Hydrogen standard; Kapton; H-implant; Ion implanted polymer films

1. Introduction Ever since the first use of elastic recoil detection (ERD) concept by LÕEcuyer et al. [1], ERD has been increasingly used in light elements analyses and their depth profiling. Later inclusion of using common RBS beam (MeV 4 He ions) as ERD projectiles by Doyle and Peercy [2] further inspired ion beam analysis community to optimize ERD experimental setup [3–5] and to develop ERD data reduction software for hydrogen analysis [6–8]. The ERD with MeV 4 He beam, also referred to as forward recoil spectrometry, is now widely used to measure hydrogen isotopes in solid targets

* Present address: Division of Materials Science and Technology, Los Alamos National Laboratory, Mailstop K765, Los Alamos, NM 87545, USA. Tel.: +1-505-665-1596; fax: +1-505665-2992. E-mail address: [email protected] (Y.Q. Wang).

involving various applications. Several comprehensive review articles on ERD techniques have been published [9–11]. Although the principle behind ERD is similar to ion backscattering (RBS), obtaining hydrogen depth profiles from recoil proton spectra is much more complex because of the following extra factors to be considered: the less-known recoil cross section data for (4 He,p); the less direct energy-todepth conversion as a result of the mass-filtering range foil; and the poor energy resolution due to both the range foil and the ERD glancing incidence-exit geometry. The glancing incidence-exit geometry also increases charge measurement uncertainties from samples. To simplify these uncertainties, a known-hydrogen standard is often used in quantitative hydrogen analysis. Two types of H-standards are commonly used in ERD: thick target standard (known H-atoms/ cm3 ) and thin film standard (known H-atoms/ cm2 ). Kapton polyimide and H-implanted Si are

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2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.038

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the most frequently used primary H-standards in each category. Kapton polyimide [H10 C22 N2 O5 ]n has a well-known and high H-concentration (25.64 at.%, 2.25 · 1022 H-atoms/cm3 ) and is easily obtainable (DuPont). On the other hand, Himplanted silicon usually has a lower but also well-known H-content or H-implant dose (1 · 1016 –5 · 1016 atoms/cm2 ) and is radiation stable and re-usable. Besides, both the standards have uniform lateral H-distribution and smooth surface. However, there are certain concerns associated with the use of these two standards in ERD analysis. This paper will first address these concerns through a real example in which Kapton is used to determine the absolute implant-dose in a H-implanted Si target. As a primary H-standard, the implant dose in H-implanted Si is normally determined independently by the well-calibrated charge measurement system of the implanter. By cross examining the implant dose with Kapton standard, both experimental considerations and theoretical corrections involved in using these two standards are discussed. Then, we will look at other alternative H-standards (secondary) that do not generally have the shortcomings of these two primary standards. Two kinds of hydrogen thin film targets were studied for this purpose: a Hisotope thin film standard manufactured at Sandia National Laboratory for materials characterization community and an ion implanted polymer (IIP) thin film used by the author for electrical conductivity enhancements and sensing applications.

claimed. Because of the shallow H-implant at 6 keV, the implant range of Rp ¼ 110 nm based on SRIM2000 simulation [13], a decent depth resolution is needed to separate the implanted bulk hydrogen from the surface hydrogen contamination. Various contributing factors to the total energy resolution and the details on how to improve ERD resolutions have been reported by others [4,5,8,14,15]. The following experimental conditions were used in our study: 1.3 MeV 4 Heþ beam with a beam current of 5 nA, 5 lm Mylar as a range foil, the incident angle between the beam and the target surface a ¼ 15, the scattering angle h ¼ 30, the solid angle subtended by the detector X ¼ 1:28 msr and the MCA channel width n ¼ 3:55 keV. The depth resolution was estimated to be near 40 nm at Si-surface by taking the FWHM of the H-surface peak measured from a bare silicon wafer as the system energy resolution (30 keV). At this beam energy, the maximum analysis depth for hydrogen in silicon is limited to 210 nm. Measured ERD spectra from Kapton (1-nm Pt-coated) and 6 keV H-implanted Si are shown in Fig. 1(up) and (down), respectively. A decent spectral separation between bulk hydrogen and surface hydrogen in the Himplant target was achieved with our experimental setup. For a uniform thick target (e.g. Kapton), the surface spectrum height, H ðEf 0 Þ, is determined by [8]

2. Primary hydrogen standards: Kapton versus Himplanted Si

where Ef 0 is the measured proton energy corresponding to the surface hydrogen in Kapton, X is the solid angle subtended by the detector, Q is the accumulated beam charge, C is the H-atomic density (atoms/cm3 ), rðE0 Þ is the recoil cross section at the incident beam energy of E0 , n is the MCA channel width, Sa is the stopping power of the recoil protons in Mylar range foil, fS0 g  kS1 ðE0 Þ= sin a þ S2 ðkE0 Þ= sinðh aÞ is the ERD stopping factor at the surface of the target, k  4M1 M2 =ðM1 þ M2 Þ2 cos2 h is the ERD kinematic factor, S1 and S2 are the stopping powers of

The goal is to measure the absolute implantdose in a H-implanted Si target. As a participant to this Round Robin study organized by University of Surrey [12], ERD with Kapton standard was used in our lab. The H-implant sample was from a 200-mm amorphised Si wafer implanted by Axcelis Technologies Inc. with 6-keV Hþ ions at a nominal dose exceeding 5 · 1016 atoms/cm2 . Two percent uniformity of the implant over the wafer is

H ðEf 0 Þ ¼ XQCrðE0 Þ

n Sa ðkE0 Þ 1 ; fS0 g Sa ðEf 0 Þ sin a

ð1Þ

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5000

Yield (counts/30uC)

320 cts (7.5%)

4000

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0 20

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þ

Fig. 1. Measured ERD spectra with 1.3 MeV He beam from: (up) Kapton thick target with 1-nm Pt-coating layer; and (down) 6 keV H-implant in Si.

4

He and 1 H in the target and M1 and M2 are the masses of 4 He and 1 H, respectively. For a thin film target like H-implant in Si, the area of the implanted hydrogen peak (A) is simply related to the H-implant dose (N ) by 1 : ð2Þ sin a Combining these two equations, we obtain that the implant dose of H-implanted Si is related to the surface spectrum height of Kapton by A ¼ XQN rðEÞ

N¼

A rðE0 Þ n Sa ðkE0 Þ C: H ðEf 0 Þ rðEÞ ½S0  Sa ðEf 0 Þ

ð3Þ

An important note in Eq. (3) is that E is the ‘‘effective’’ or ‘‘mean’’ beam energy in the Himplant target. A significant error can be introduced for the implanted target if the surface energy approximation (E ¼ E0 ) is assumed since the ‘‘apparent’’ depth of the implanted hydrogen atoms is significantly increased in the tilted ERD

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geometry. Let us discuss the quantities in Eq. (3) in more detail. 2.1. Surface spectrum height of Kapton, H (Ef 0 ) To accurately determine the surface spectrum height for Kapton, several precautions need to be carefully considered. As shown in Fig. 1(up), the surface spectrum height could be easily underestimated by 7–8% if one neglects the surface spectrum broadening as a result of the poor energy resolution and carelessly reads the Kapton spectrum. On the other hand, the spectrum height could be overestimated by a noticeable amount (6%) because of the spectral shift due to beam charging on Kapton. It is found in our measurements that the surface front edge of H-spectrum from a bare Kapton sheet (20 lm thickness) was shifted approximately six channels (20 keV) higher than the expected position from a more conductive target (e.g. a surface hydrogen peak position from a silicon wafer). A similar shift was also seen from the RBS spectrum of the Kapton. Beam charging also introduces more noise in detection system and affects the charge integration accuracy. It is also found that the pre-coating of 1 nm Pt on Kapton adequately eliminated these charging related problems. At the same time, the concurrently measured Pt-signal by a RBS detector at 165 also provided a more reliable reference for the charge measurement in ERD, as compared with the RBS oxygen spectrum height used for the bare Kapton. The correlations between the directly measured charge on targets and the concurrently measured RBS-signal from the targets are plotted in Fig. 2(up), where Si-surface height was used for the H-implanted Si. Hydrogen loss (H-mobility) during ERD measurement as a result of the beam irradiation and/or thermal heating, as shown in Fig. 2(down), would underestimate the surface spectrum height if not properly corrected. Fig. 2(down) indicates that the Pt-coating also helped reduce the H-loss in Kapton, possibly by reducing thermal spikes on Kapton surface which can break C–H covalent bonds and release H2 from the Kapton matrix. As expected, the implanted bulk hydrogen in the H-implanted Si is found to be very stable, whereas the

surface hydrogen contamination decreases significantly with increasing the beam dose. To correct for the hydrogen loss, the H-loss versus 4 He-fluence curve is simply extrapolated to zero 4 He-fluence. 2.2. Peak area of the implanted hydrogen, A The ERD spectrum of the H-implanted Si as shown in Fig. 1(down) still indicates a noticeable spectral overlap between the H-surface and Himplant. To accurately determine the peak area of the H-implant, two Gaussian functions were used to do a nonlinear least square fit to the overlapped spectrum. The resulting two Gaussian lines and their sum curve are also shown in Fig. 1(down). 2.3. Theoretical correction terms Two theoretical correction terms in Eq. (3), rðE0 Þ=rðEÞ and Sa ðkE0 Þ=Sa ðEf 0 Þ, can sometimes be overlooked since they are either insignificant or non-existing in RBS relative analysis which we feel more familiar. In fact, these corrections can be very important in ERD relative analysis. Using SRIM2000 stopping data [13], the surface recoil proton energy kE0 ¼ 0:624 MeV decreases to Ef 0 ¼ 0:336 MeV after passing through 5 lm Mylar foil, and thus the stopping power ratio before and after the Mylar foil, Sa ðkE0 Þ=Sa ðEf 0 Þ, is estimated to be 0.679. On the other hand, seen by the incoming beam of E0 ¼ 1:3 MeV, the H-implant peak is ‘‘apparently’’ located 425 nm below the Si-surface in our ERD geometry. The mean beam energy decreases to E ¼ 1:175 MeV when the beam reaches to such a depth. Considering Rutherford recoil cross sections for such a low energy region [16], the term rðE0 Þ=rðEÞ becomes 0.817, significantly smaller than the unity. In other words, one could simply overestimate the implant dose by nearly 18% if rðE0 Þ=rðEÞ term is neglected, and in a much worse case could even inflate the implant dose by as much as 55% if both terms were simply treated as unity. The stopping factor fS0 g at the Kapton surface used in our calculation was 69 eV/A based on SRIM2000 stopping data [13]. After taking careful considerations of both measurement and calculation, the implant dose of 6 keV H-implanted Si is determined to be

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1.08

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1.02

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0.98

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0.8 Kapton Kapton (Pt-coated) 0.7

H-implant surface H-implant bulk H-implant total

0.6 0

2

4

6

8

Accumulated He-beam Charge (uC) Fig. 2. (Up) RBS-normalization for accurate charge measurement in ERD; and (down) H-stability in ERD measurement with 1.3 MeV 4 Heþ beam and 5 nA beam current.

5.57 · 1016 atoms/cm2 . The 5% uncertainty in our analysis is mainly due to the uncertainty of stopping data in Kapton. If the newer stopping data in Si by Konac et al. [17] is used, the implant dose changes to 5.75 · 1016 atoms/cm2 since the H-implant peak is slightly shallower (94 nm) as determined by Quarkerd simulation package [18], compared with that predicted by SRIM (110 nm). For comparison, the average H-implant dose for this Round Robin H-analysis provided by all other participating ion beam labs is (5.71 ± 0.13) · 1016 atoms/cm2 [12]. The surface

hydrogen content on the implanted Si varied from 5 · 1015 to 1.4 · 1016 atoms/cm2 , presumably more depending on the vacuum pressure of each labÕs analysis chamber.

3. Secondary hydrogen standards As discussed so far, the proper use of these two primary hydrogen standards in ERD requires considerable care in both experimental measurements and theoretical corrections. For example,

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the use of Kapton thick standard requires doing H-stability correction and eliminating the surface charging problem. It also requires reliable stopping data for 4 He ions and protons in Kapton and the range foil (e.g. Mylar). On the other hand, H-implant targets usually do not have very high H-content (<5 at.%) and thus often require longer time for data acquisition. It also requires the correction for surface hydrogen contamination and the correction for recoil cross sections with mean energy approximations. Next we will discuss the use of other alternative hydrogen standards (secondary hydrogen standards) that may overcome the shortcomings of Kapton and/or H-implanted Si. Two kinds of hydrogen thin film targets were investigated for this purpose: a H-isotope thin film manufactured at Sandia National Laboratory for materials characterization community and an IIP thin film used by the author for electrical conductivity enhancements and sensing applications. 3.1. Sandia H-isotope thin film standard Several considerations were taken into account during the manufacture of the so-called ErHD/ Mo thin film standard, with accuracy and stability being the most important. The target size is approximately 5 mm in diameter and the thickness of the ErHD film is claimed to be 500 nm. To use this film as an ERD standard, the absolute amounts of Er, H and D in the film need to be accurately determined. As a participant to this Round Robin study [19], ERD/RBS techniques were used in our lab. The same ERD and RBS geometries as above were used here, but a much higher 4 Heþþ beam energy (E0 ¼ 5 MeV) was used to minimize the H- and D-spectral overlap in ERD and also the Er- and Mo-spectral overlap in RBS. For 5 MeV incident 4 Heþþ beam, 28 lm thick Mylar was used as a range foil and the solid angle subtended by the ERD detector was 1.18 msr. Fig. 3 show the measured ERD (up) and RBS (down) spectra from the ErHD/Mo thin film target, respectively. To accurately determine the peak areas for H and D, each peak was described by a combined function of (semi-Gaussian + linear + semi-Gaussian), and then a nonlinear least

square fit was done to the entire overlapped spectrum. The resulting H- and D-peak distributions and their sum distribution are also shown in Fig. 3(up). The curve fit indicates that H- and Dpeak areas are 57.3% and 42.7%, respectively. Although a significant signal overlap between Er and Mo was observed in the concurrently measured RBS spectrum at the ERD incident angle, little overlap was seen when the normal incident angle was used in the RBS measurement as shown in Fig. 3(down). A noticeable amount of oxygen (and carbon) was also observed near the surface of the film. A Bi-implant standard was used in RBS to determine Er-content in the film. This Bi-standard was carefully calibrated against the original Harwell Bi-standard and contains (4.78 ± 0.09) · 1015 209 Bi atoms/cm2 that are mainly located at a depth of 24 nm below the Si-surface. Rutherford cross section data with the electronic screening corrections were used for Er and Bi. While the surface energy approximation introduces negligible errors in calculating scattering cross section for Bi, it is not true for Er target since the ErHD film is much thicker (500 nm). In fact, if the mean energy approximation were not considered in calculating Er scattering cross section, the Er-amount in the film would be overestimated by nearly 3%. To determine H- and D-amounts in the ErHD film, a special H and D containing reference standard was made in this work. Known amount of polystyrene ([C8 H8 ]n ) and deuterated polystyrene ([D8 H8 ]n ) powders were pre-mixed and dissolved in Toluene, then the solution was spincoated on Si-wafer to form a hPS + dPS thin film. The thickness of the dried hPS + dPS film was carefully measured to be (315 ± 5) nm by a surface profilometer. The H and D content in the film was then calculated to be 7.85 · 1017 (H-atoms/cm2 ) and 7.62 · 1017 (D-atoms/cm2 ), respectively. The actual H-amount in the film measured using the Kapton standard was found to be within ±5% uncertainty of the calculated value. Thus, we can reasonably assume that the calculated D-amount in the film is also accurate with an uncertainty of less than ±5%. Hydrogen and deuterium in both ErHD/Mo target and hPS + dPS reference were found rather

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Fig. 3. Measured ERD (up) and RBS (down) spectra from Sandia ErHD/Mo thin film target with 5 MeV 4 Heþþ beam and 5 nA beam current.

stable under the ERD measurement conditions that we have used (5 MeV 4 Heþþ beam to 5plC@5pnA). Both the sample and the reference standard are thin films, thus a rather simple equation is used in this relative analysis: Ns ¼

As rðEst Þ Nst ; Ast rðEs Þ

ð4Þ

where the subscripts (s and st) refer to sample and standard, respectively. The cross section ratio term is included in Eq. (4) due to the fact that the mean

beam energy (Es ) to recoil hydrogen in the sample and in the standard (Est ) may be quite different depending on the matrix and thickness of the sample and the standard. Using SRIM2000 stopping data [13], the mean beam energies in hPS + dPS reference film and ErHD film are estimated to be 4.94 and 4.74 MeV, respectively. Taking measured rðEÞ versus E data from [20] for E near 4–5 MeV region, one finds that for our reference film, the surface energy approximation on rðEÞ, that is E ¼ E0 ¼ 5 MeV, would introduce

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a systematic error of 1%. However, for the ErHD film target, the surface energy approximation on rðEÞ would introduce an unnecessary systematic error of 3.5%. Therefore, the final Hand D-amount measured in the ErHD/Mo film target would be overestimated by nearly 3% if the rðEÞ correction term in Eq. (4) were not included. In summary, our RBS/ERD measurements with 5 MeV 4 Heþþ beam found that the ErHD/Mo thin film target contains 1.61 · 1018 Er-atoms/cm2 ), 1.66 · 1018 (H-atoms/cm2 ) and 1.37 · 1018 (Datoms/cm2 ). The uncertainty of 5% in our analysis is mainly due to the uncertainties of H and D content in our reference standard. For comparison, the mean results provided by all the Round Robin participating labs for this thin film standard are (1.60 ± 0.04) · 1018 (Er-atoms/cm2 ), (1.70 ± 0.06) · 1018 ) (H-atoms/cm2 ) and (1.40 ± 0.02) · 1018 (Datoms/cm2 ) [19]. As a comment, we believe that a larger (>10 mm in diameter), thinner (200 nm) and smoother ErHD film on a lighter substrate would make an even better hydrogen standard for ERD. 3.2. Ion implanted polymer thin films Another H-containing thin film target we have investigated is IIP thin films (21,22). IIP thin films have several unique characteristics that make them good candidates as hydrogen thin film standards: (1) film thickness can be easily tuned by spincoating process; (2) remaining hydrogen in the film after ion implantation/irradiation is still considerably high and rather stable; (3) ion implantation/ irradiation makes polymer films electrically conductive (no charging problem) and mechanically strong (reusable target); and (4) RBS signals from the implanted ion species and/or the film substrate elements can be used to normalize the charge measurement in ERD. For this study, solution of poly(styrene-coacrylonitrile) (PSA) was spin-coated onto a glass substrate to form a uniform thin film with a thickness of 200 nm. The thin film was then implanted with 50 keV Nþ ions to a dose of 5 · 1016 ions/cm2 under a dose rate of 0.5 lA/cm2 . The ion beam energy deposition in the polymer thin film broke the covalent C–H bonds, released

H2 and other small H–C molecules, and finally changed the PSA matrix into something similar to a hydrogenated amorphous carbon structure [21,22]. Because of the hydrogen depletion, shrinkage and densification of the polymer during the implantation, the film thickness decreased to only 115 nm after the implantation as measured by a surface profilometer. At the same time, hydrogen measurement by ERD with 2 MeV 4 Heþ beam and 8 lm Mylar range foil indicated that the H-content in the film decreased from 1 · 1018 atoms/cm2 to 3.38 · 1017 atoms/cm2 as a result of the Nþ ion implantation. Fig. 4 shows the H-stability in the IIP film as a function of the accumulated 4 Heþ beam charge during the ERD analysis. A relatively large beam current of 35 nA was used in the measurement and the maximum accumulated charge of 20 lC is equivalent to a beam dose of 4 · 1015 He-ions/cm2 for a beam spot size of 2 mm in diameter. Even with such ‘‘unfriendly’’ beam conditions (the normal beam conditions for ERD: Q 6 5 lC and I 6 5 nA), the H-content in the IIP thin film remains remarkably stable considering the fact that the statistical uncertainties of the data themselves are nearly ±2%. The uncertainties related to the charge measurement in Fig. 4 were already corrected using the concurrently measured RBS Sispectrum height from the glass substrate. Such a remarkable H-stability is most likely due to the fact that the remaining hydrogen in the IIP thin ERD-Measured H-content (at/cm2)

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3.6E+17

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Accumulated He Charge (uC) Fig. 4. H-content variation in an IIP thin film during ERD measurement with 2 MeV 4 Heþ beam and 35 nA beam current.

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film is defect-trapped and remains immobile during ERD analysis. As a potential hydrogen thin film standard, H-lateral uniformity in the IIP thin film is another important factor to consider. Presumably, the primary factor to affect H-uniformity in IIP thin films is the film thickness uniformity during the spin-coating and drying/curing processes. While much larger scale H-uniformity data are not currently available, the hydrogen uniformity within a 15 mm by 15 mm scanning area on the IIP film was found better than 5%. Therefore, IIP thin films are proved to be another suitable hydrogen thin film standard in ERD analysis. 4. Conclusions Two commonly used primary hydrogen standards in ERD analysis were reexamined: Kapton as a known H-concentration reference (atoms/cm3 ) and H-implanted silicon as a known H-amount reference (atoms/cm2 ). A low energy ERD with Kapton standard was used to determine the absolute implant-dose in a 6 keV H-implanted Si target. After careful considerations in both ERD measurement and spectrum analysis, the implant dose we obtained is in excellent agreement with that provided by other Round Robin participating IBA groups. Two secondary hydrogen thin film standards are also investigated. First, high energy ERD/RBS were used to measure the absolute amounts of H, D and Er in a Sandia-made ErHD/Mo thin film standard. As a participant to this Round Robin analysis, our lab once again produced the results that agree very well with those obtained by other participating IBA labs. Then an IIP thin film was investigated. The high and stable H-content in the IIP thin film plus its other unique characteristics makes it another suitable hydrogen thin film standard for ion beam analysis community.

Acknowledgements The author would like to thank Dr. William Lennard of the University of Western Ontario for

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providing the Bi-implant standard, the 6 keV H-implant standard, and Quarkerd simulation software, Dr. Daniel Savin of the University of Minnesota for providing the deuterated polystyrene thin film standard, and Dr. James Banks of Sandia National Laboratory for providing erbium hydrogen isotope thin film standard. The IIP thin film target was prepared in collaboration with Dr. Ryan Giedd at Southwest Missouri State University and Dr. Mary Moss of Brewer Science Inc. in Rolla, Missouri.

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