www.elsevier.com/locate/nimb

Nuclear Instruments and Methods in Physics Research B 259 (2007) 282–287

NIMBBeam Interactions

with Materials & Atoms

Use of natural diamonds to monitor 14C AMS instrument backgrounds

R.E. Taylor a,b,*, John Southon c

a Department of Anthropology, University of California, Riverside, CA 92354, USAb Cotsen Institute of Archaeology, University of California, Los Angeles, CA 90024, USA

c Keck Accelerator Mass Spectrometry Laboratory, Department of Earth System Science, University of California, Irvine, CA 92697, USA

Available online 12 February 2007

Abstract

To examine one component of the instrument-based background in the University of California Keck Carbon Cycle AMS spectrom-eter, we have obtained measurements on a set of natural diamonds pressed into sample holders. Natural diamond samples (N = 14) fromdifferent sources within rock formations with geological ages greatly in excess of 100 Ma yielded a range of currents (�110–250 lA 12C�

where filamentous graphite typically yields �150 lA 12C�) and apparent 14C ages (64.9 ± 0.4 ka BP [0.00031 ± 0.00002 fm] to80.0 ± 1.1 ka BP [0.00005 ± 0.00001 fm]). Six fragments cut from a single diamond exhibited essentially identical 14C values –69.3 ± 0.5 ka–70.6 ± 0.5 ka BP. The oldest 14C age equivalents were measured on natural diamonds which exhibited the highest currentyields.� 2007 Elsevier B.V. All rights reserved.

1. Introduction

The initial anticipation [1,2] that AMS-based systemsmight achieve 14C-inferred age measurements of 105 years(�0.000004 fraction modern [fm]) on unknown age sampleshas been, to date, unrealized due to a variety of sampleprocessing and instrument-based experimental constraints.The typical situation is well illustrated by the early work ofthe University of Washington AMS group. An AMS-basedmeasurement of 69.0 ± 1.7 ka BP was obtained on a spe-cially prepared sample of geological graphite. However,graphite prepared from CO2 obtained from a sample ofmarble, which, like geologic graphite, should exhibit no14C activity due to its great geologic age, yielded an appar-ent age of 47.9 ± 0.7 ka BP [3]. A study carried out jointlythe University of California/Lawrence Livermore NationalLaboratory Center for Accelerator Mass Spectrometry andthe University of California, Riverside Radiocarbon Labo-ratory obtained an average apparent age of 64.5 ± 0.3 kaBP on samples of geologic graphite and an average

0168-583X/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2007.01.239

* Corresponding author. Address: Department of Anthropology, Uni-versity of California, Riverside, CA 92354, USA.

E-mail address: retaylor@ucr.edu (R.E. Taylor).

(N = 19) apparent age of 52.1 ± 0.4 ka BP on duplicate1 mg samples of catalytically-condensed graphitic carbonprepared from carefully-pretreated wood of reportedly Pli-ocene age. The lowest Pliocene wood blank value achievedby the LLNL/UCR laboratory collaboration was 60.5 ±0.6 ka BP [4–6].

There are a number potential sources of a 14C back-ground signal in an AMS-based system using catalyti-cally-condensed graphitic carbon produced from CO2

derived from the combustion or acidification of carbonifer-ous materials [7]. One of these categories is instrument ormachine background which involves the registration ofwhat is interpreted by the detector circuitry and/or soft-ware as a 14C-ion produced pulse when, in fact, anon-14C-ion mimics 14C or the detector counts 14C whichwas not originally present in the sample matrix when itwas introduced into the source.

To examine and monitor the level of machine back-ground in the University of California, Irvine Keck CarbonCycle AMS spectrometer [8], we have obtained a series ofmeasurements on a set of natural diamonds. Because oftheir great geologic age, we view it as a reasonable assump-tion that these gem-carbon samples contain no measurable14C and that their unique physical characteristics signifi-

Table 1Potential sources of pseudo-14C signal in AMS-based 14C measurements

(1) Pseudo14C-free sample background: 14C is present in carbon-iferous material that should not contain 14C because of itsgeologic age.(1) Non-14C-free sample: sample material erroneously

assumed to contain no detectable 14C.(2) Contaminated sample: 14C introduced into the 14C-

free sample material during chemical or physicalpretreatment.

(2) Combustion/acidification background: 14C introduced duringproduction of CO2 from sample.(1) Materials contamination: 14C introduced from mate-

rials in combustion/acidification tube other thansample (e.g. adsorbed CO2 on oxidizer).

(2) Tube contamination: 14C introduced from walls ofcombustion/acidification tube (e.g. adsorbed CO2).

(3) Tube leakage: 14C introduced from leakage of atmo-spheric CO2 into combustion/acidification tube.

(3) Graphitization background: 14C introduced during graphiti-zation process.(1) Materials contamination: 14C introduced from mate-

rials in reaction tube (e.g. from Fe or Co powder asgraphitization catalyst).

(2) Tube contamination: 14C introduced from walls ofreaction tube.

(3) Vacuum line contamination: 14C introduced duringCO2 transfer in vacuum line.

(4) Transfer background: 14C introduced during graphite trans-fer to sample holder.(1) Target/cathode contamination: 14C introduced from

sample holder.(2) Manipulation contamination: 14C introduced during

physical transfer/packing of graphite into sampleholder.

(6) Storage background: 14C introduced at any point in the sam-ple processing sequence from any containment vessel.(1) Particulate contamination: 14C physically introduced

from carbon-containing particulates derived fromstorage containers.

(2) Atmospheric contamination: 14C introduced fromCO2 in air or from out gasing from storagecontainer.

(7) Instrument background: 14C signal registered in detector cir-cuitry when 14C-ion not present.(1) Detector anomaly: pseudo-14C pulse registered when

no ion of mass 14 is present at the detector (e.g. elec-tronic noise).

(2) Ion identification anomaly: mass 14 particle which isnot 14C reaches detector and is misidentified as 14C.

(3) Beam line anomaly: 14C internally derived from somecomponent of beam line reaches detector.

R.E. Taylor, J. Southon / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 282–287 283

cantly reduce or eliminate exogenous contamination frommore recent carbon sources. On this basis, we propose thatwe have eliminated the major sources of mass 14 ion withthe exception of that contributed from various componentsof instrument or machine background signal and perhapsthat contributed from the sample holder itself. Other back-grounds, such as 13CH� ! 13C+*! 13C+, from chargeexchange and scattering, can also contribute to producing‘‘14C’’ peaks if no dE/dx measurements are performed.However, since UCI backgrounds for processed (graphi-tized) samples are comparable with those on largermachines where 13C can be separated using dE/dx, 14Cfrom the actual sample is probably the dominant compo-nent of the ‘‘routine’’ background.

2. Sources of AMS background

Almost three decades of experience with AMS technol-ogy has yielded an appreciation of the difficulty of achiev-ing the initial optimistic projections of the potential ofachieving meaningful 14C-inferred ages to as much as 105

years. A review of the literature generated by AMS labora-tories dealing with background issues and our own experi-ence reveal a large number of factors that have beenidentified or suspected as sources of 14C from blanks orpseudo-14C-signals in the various instrumental systemsdeveloped for AMS 14C work [9–17].

Table 1 summarizes seven major potential sources ofpseudo-14C signal in AMS-based 14C measurements usingcatalytically-condensed graphitic carbon produced fromCO2 from combustion/acidification of carboniferous mate-rial. These factors operate singly or in various combina-tions to yield the apparent finite ages exhibited in 14C-free or presumed 14C-free sample materials used as back-grounds or blanks in AMS 14C systems.

To monitor one component of our instrument back-ground, ion source memory, which we view as the largestcomponent of the machine background in the UCI AMSsystem, we have taken advantage of several unique physicalproperties of naturally occurring diamonds. The mostimportant of these properties is the reported ability of dia-monds to exclude almost all molecular species from adher-ing to its surface and/or penetrating below the surface. Wewished to test the hypothesis that the surface of a naturaldiamond would eliminate or significantly reduce the adhe-sion of exogenous CO2 and other-carbon-containingmolecules.

3. Natural diamonds as AMS targets

Except for trace impurities such as boron, oxygen, sulfurand nitrogen, diamond, a natural crystalline mineral sub-stance, is the transparent species of a concentrated formof pure carbon. Its surface property of extreme harness –the hardest surface known with a resistance to physicalabrasion more than four times that of the next most resis-tant natural material – is the result of covalent chemical

bonding forming a tetrahedral structural unit. This charac-teristic makes diamond highly resistant to chemical corro-sion. This is related to the ability of diamonds to repel andexclude water from adhering to its surface, a very unusualproperty for a mineral [18,19]. It was this unique physicalcharacteristic of diamond that was the basis of our hypoth-esis that this surface would eliminate or significant reducethe adhesion of carbon or carbon-containing molecules

284 R.E. Taylor, J. Southon / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 282–287

from the ion source of an AMS spectrometer that wouldcontribute to a trace memory or sample cross talk effect.

4. Materials and methods

The natural diamonds employed in this study wereobtained from alluvial deposits in the state of Minas Geraisin Brazil [20]. The youngest geological contexts of Brazilianalluvial diamonds are early Paleozoic, thus we haveassumed an age for these materials greatly in excess of100 my. The diamonds obtained were less than one caretin size (1 caret = 200 mg), but of gem quality.

Nine diamonds were selected for measurement with onebeing cut into six segments. Under magnification, each dia-mond fragment was appropriately cut, sized and pressedinto a 1.6 mm hole drilled into the center of aluminumcathode sample holder. Possible surface contaminationintroduced by the cutting and manipulation required tomount a diamond in a sample holder was monitored byobserving the initial count rates coming from samples.All diamond samples exhibited somewhat higher initial14C count rates from surface contamination which then sta-bilized and plateaued as the surface was penetrated bycesium sputtering.

Fig. 1. Microphotograph of UCIAMS-15453 diamond mounted in cathode. DSantos (UCI).

Fig. 1 is a microphotograph of one of the mounted sam-ples. In an effort to increase the thermal conductivity of fiveof the diamonds (UCIAMS-15443 to UCIAMS-15457)with respect to the metal surface of each of their cathodes,silver powder was placed in the bottom of the target holeprior to the mounting of the diamond fragment. We sawno increase in the maximum current exhibited by thesesamples, but they showed slightly higher 14C content thanUCIAMS-9638 to 9640. Previous tests showed that 14Ccount rates from silver powder cathodes were comparablewith those from diamonds. Because of this and becausethe silver packing was largely shielded from the Cs beamby the diamonds themselves, the excess 14C was thereforeprobably due to differences between diamonds or run torun changes in the spectrometer, not from carbon in the sil-ver powder.

5. Results

Table 2 lists the results we have obtained on the six seg-ments cut from the same diamond (Section A) and the mea-surement on eight other individual natural diamonds(Section B). In addition to the 14C content, we report alsothe maximum 12C� currents achieved on each sample and

iameter of diamond = 1.6 mm. Microphotograph taken by Guaciara M.

Table 2Carbon isotope and 12C� current measurements on (A) six splits from of a single natural diamond and (B) individual natural diamonds (N = 8) with associated analysis of Ceylon graphite made duringdiamond measurements

Sample no. Sample materiala Maximum current (lA12C�) d13Cb (&) 14C Content fraction modern (fm) Conventional 14C age (ka yrs. BP)

A. Splits from single natural diamond and associated Ceylon geological graphite

UCIAMS-12677 D 131 �0.3 0.00018 ± 0.00001 69.4 ± 0.5UCIAMS-12678 D 127 �5.6 0.00017 ± 0.00002 69.6 ± 0.8UCIAMS-12676 D 145 4.2 0.00017 ± 0.00001 70,0 ± 0.5UCIAMS-12679 D 168 �19.3 0.00016 ± 0.00001 70.3 ± 0.5UCIAMS-12674 D 122 3.2 0.00015 ± 0.00001 70.6 ± 0.7UCIAMS-12675 D 184 � 0.5 0.00015 ± 0.00001 70.6 ± 0.6

UCIAMS-12680 G 81 �4.9 0.00025 ± 0.00002 66.5 ± 0.6UCIAMS-12681 G 70 �11.0 0.00020 ± 0.00003 68.3 ± 1.1.

B. Individual natural diamonds and associated Ceylon geological graphite

UCIAMS-15445 D 136 �14.1 0.00021 ± 0.00003 68.1 ± 1.2c

UCIAMS-15444 D 125 �15.4 0.00018 ± 0.00002 69.4 ± 1.0c

UCIAMS-15443 D 125 �20.5 0.00015 ± 0.00002 70.9 ± 1.0c

UCIAMS-15446 D 127 �23.1 0.00013 ± 0.00002 71.7 ± 1.0c

UCIAMS-15447 D 127 �19.8 0.00011 ± 0.00002 3.3 ± 1.6c

UCIAMS-9638 D 250 �20.5 0.00008 ± 0.00001 75.7 ± 0.8d

UCIAMS-9640 D 240 � 3.1 0.00006 ± 0.00001 78.4 ± 0.9d

UCIAMS-9639 D 197 4.7 0.00005 ± 0.00001 80.0 ± 1.1d

UCIAMS-15440 G 131 �11.5 0.00069 ± 0.0004 58.4 ± 0.5UCIAMS-15441 G 122 �10.7 0.00035 ± 0.00004 64.0 ± 0.5UCIAMS-15442 G 95 �9.4 0.00032 ± 0.00003 64.7 ± 0.8

UCIAMS-9641 G 106 �3.1 0.00024 ± 0.00002 70.1 ± 0.8e

UCIAMS-9642 G 111 0.2 0.00024 ± 0.00002 67.0 ± 0.7f

a D = Diamond, G = Ceylon geological graphite.b d13C Values measured using the AMS spectrometer. These values can differ, typically by 1–3&, from that measured on a conventional mass spectrometer.c Associated Ceylon geologic graphite measurements: UCIAMS-15440, 15441 and 14440.d Associated Ceylon geologic graphite measurements: UCIAMS-9641 and 9642.e Graphite baked in air prior to analyses.f Graphite baked in hydrogen prior to analyses.

R.E

.T

ay

lor,

J.

So

uth

on

/N

ucl.

Instr.

an

dM

eth.

inP

hy

s.R

es.B

25

9(

20

07

)2

82

–2

87

285

286 R.E. Taylor, J. Southon / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 282–287

the d13C values measured during the AMS analysis. Forcomparison, the 12C� currents and apparent age exhibitedby Ceylon geological graphite measured at the same time asthe diamonds are also reported.

Our natural diamond samples yielded a range ofcurrents (�120–250 lA 12C� where catalytically-condensedgraphitic carbon in this instrument typically yields�150 lA 12C�) and apparent 14C ages (64.9 ± 0.4 ka BP[0.00031 ± 0.00002 fm] to 80.0 ± 1.1 ka BP [0.00005 ±0.00001 fm]). The six fragments cut from a single diamondexhibited essentially identical 14C values – 69.4 ± 0.6 ka to70.6 ± 0.5 ka BP. The oldest 14C age equivalents were mea-sured on natural diamonds which exhibited the highest cur-rent yields. Conditions varied somewhat from run to runbut, in general, the currents exhibited in the diamondscame up as rapidly as those in the Ceylon geologic graphitesamples and were stable within ±10% for periods of 1.5–3 h. Earlier investigators have reported wide variations ind13C values in diamonds including variations within singlestones [21–23]. As indicated in Table 2, we have observedsimilar d13C variations.

Our measurements have confirmed our hypothesis thatdiamonds represent a much ‘‘cleaner’’ surface with respectto adhesion of carbon-containing molecules from the ionsource that contribute to a trace memory or sample ‘‘crosstalk’’ effect. At this time, it is not clear to us what factorsmight be involved in the greater variability in the apparent14C concentrations exhibited in individual diamonds (Sec-tion B) as opposed to splits from a single natural diamond(Section A). Possible factors suggested to us are greatervariability in the orientation of the crystal facies and micro-fractures in individual diamonds.

6. Future studies

The demonstration that some natural diamonds can beused to characterize one component of machine back-ground in an AMS system would be of limited generalapplicability unless there was the interest in exploiting thisobservation as a means of extending the 14C time frame. Totake advantage of the lowered background blanks and thusthe potential of an extended AMS 14C dating range, forroutine AMS 14C measurements it would be necessary todemonstrate a routine capability to synthesize a diamondmatrix through some intermediate chemical form fromCO2 derived from the combustion or acidification of vari-ous types of carbonaceous samples. Beginning in the late1960s, studies were initiated to accomplish this directlyfrom a gas phase at relatively low pressures and tempera-tures. After several decades of experimentation, severaltypes of chemical vapor deposition (CVD) methods weredemonstrated to be practical and effective in producing dia-mond films [19,24].

CVD techniques involve a gas phase chemical reactionoccurring above a solid which causes a diamond film depo-sition on the solid substrate. Contemporary techniques

require a means of synthesizing or activating in gas phasecarbon-containing precursor molecules. This is currentlyaccomplished by thermal (e.g. hot filament), electric dis-charge (e.g. DC, RF or microwave), or in a combustionenvironment (e.g. as in an oxyacetylene torch). Typically,the precursor gas (usually CH4,but the use of CO2 is cur-rently being explored by several groups) is diluted in anexcess of hydrogen and the temperature of the substrate(e.g. Si) is usually greater than 700 �C to avoid the formationof amorphous carbon rather than diamond. Various exper-imental arrangements have been refined to optimize for var-ious desired outcomes such as high growth rate or large areadeposition and a large scientific and commercial literaturenow exists which describes the varying applications [25].

It is not clear whether appropriate experimentalarrangements necessary to convert CO2 to a diamond filmcan be designed that will permit efficient and reproducibledeposition on a appropriate substrate while avoiding sig-nificant contamination processes with respect to 14C. Oneof the first future experiments that will need to be under-taken is to determine the degree of variability in 12C� cur-rents exhibited by thin diamond films in contrast to naturaldiamonds. We intend to carry out these experiments in thenear future.

Acknowledgements

The authors thank the Dean of Physical Sciences andVice Chancellor for Research, University of California, Ir-vine, the National Science Foundation, and the GabrielleO. Vierra Memorial Fund for financial support and Guac-iara M. Santos (UCI) for the photomicrograph. We alsoacknowledge the assistance of Emanuel Jacobson, OzelFine Jewelry, Redlands, California who obtained the dia-monds and cut and mounted them into our sample holdersand the helpful comments of reviewers.

References

[1] R.A. Muller, Science 196 (1977) 489.[2] R.A. Muller, Physics Today 32 (1979) 23.[3] F.H. Schmidt, D.R. Balsley, D.D. Leach, in: H.E. Gove et al. (Eds.),

Accelerator Mass Spectrometry, Amsterdam, 1987.[4] D. Kirner, R.E. Taylor, J.R. Southon, Radiocarbon 37 (1995) 697.[5] D. Kirner, J.R. Southon, P.E. Hare, R.E. Taylor, in: M.V. Orna

(Ed.), Archaeological Chemistry Organic, Inorganic and BiochemicalAnalysis, Washington, DC 1996.

[6] D. Kirner, R. Burky, R.E. Taylor, J.R. Southon, Nucl. Instr. andMeth. B 123 (1997) 214.

[7] R.E. Taylor, D.J. Donahue, T.H. Sable, P.E. Damon, A.T.J. Jull, in:J.B. Lambert (Ed.), Archaeological Chemistry III, Washington, DC1984.

[8] J. Southon, G. Santos, K. Druffel-Rodriguez, E. Druffel, S. Trum-bore, X. Su, S. Griffin, S. Ali, M. Mazon, Radiocarbon 46 (2004) 41.

[9] R. Gillespie, R.E.M. Hedges, Nucl. Instr. and Meth. B 5 (1984) 294.[10] J.S. Vogel, D.E. Nelson, J.R. Southon, Radiocarbon 29 (1987) 323.[11] R.P. Beukens, in: R.E. Taylor et al. (Eds.), Radiocarbon After Four

Decades: An Interdisciplinary Perspective, New York, 1992.[12] P.M. Gulliksen, M.S. Thomsen, Radiocarbon 34 (1992) 312.[13] R.P. Beukens, Nucl. Instr. and Meth. B 79 (1993) 620.

R.E. Taylor, J. Southon / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 282–287 287

[14] M. Schleicher, M. Grootes, M.J. Nadeau, A. Schoon, Radiocarbon40 (1998) 85.

[15] M.J. Madeau, P.M. Grootes, A. Voelker, F. Bruhn, A. Duhr, A.Oriwall, Radiocarbon 43 (2001) 169.

[16] S. Freeman, P. Bishop, C. Bryant, G. Cook, D. Dugans, T. Ertunc, A.Fallick, R. Ganeshram, C. Maden, P. Naysmith, C. Schnabel, M.Scott, M. Summerfield, S. Xu, Nucl. Instr. and Meth. B, theseProceedings, doi:10.1016/j.nimb.2007.01.312.

[17] A. Galindo-Uribarri, J. Gomez del Campo, P. Hausladen, J. Beene,B. Fuentes, F. Liang, Y. Liu, E. Padilla, D. Stracener, A.E.Litherland, J. Doupe, Nucl. Instr. and Meth. B, these Proceedings,doi:10.1016/j.nimb.2007.01.295.

[18] T. Stachel, G.P. Brey, J.W. Harris, Elements 1 (2005) 73.[19] J.E. Field, The Properties of Natural and Synthetic Diamond, New

York, 1992.[20] E. Jacobson, Ozel Fine Jewelry, Redlands, Calif., per. comm. 2004.[21] M.B. Kirkley, J.J. Burney, M.L. Otter, S.J. Hill, L.R. Daniels, Appl.

Geochem. 6 (1991) 477.[22] P. Cartigny, J.W. Harris, M. Javoy, Earth Planet. Sci. Lett. 185 (2001)

85.[23] P. Cartigny, Elements 1 (2005) 79.[24] B. Dishler, C. Wild (Eds.), Low-Pressure Synthetic Diamond,

Amsterdam, 1998.[25] P. May, Phil. Trans R. Soc. Lond. A 358 (2000) 473.