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Preparation of graphite targets for radiocarbon dating by tandem accelarator mass spectrometer (TAMS)
Preparation of Graphite Targets for Radiocarbon Dating by Tandem Accelerator Mass Spectrometer (TAMS) DAVID
C. LOWE
Institute of Nuclear Sciences. DSIR, Private (Rrceitvd
Bag,
Lower Hutt. New Zealand
14 July 1983)
The introduction of TAMS has exciting implications for radiocarbon dating but improved sample preparation methods are needed. This paper describes a promising method for the conversion of a few milligrams of wood or charcoal into graphite targets for use in a caesium sputter ion source. Targets containing a large proportion of G-type graphite produced large C- currents, but those containing a high proportion of turbostatic Tn graphite were unsatisfactory; the type of graphite in the target is clearly of significance.
1. Introduction Since the appearance of the historic paper by Anderson er al.“’ on the feasibility of “C dating, many thousands of radiocarbon ages for a wide variety of archaeological and geological specimens have been reported. The majority of these dates have been determined by radioactive decay counting techniques. However. because the half-life of “C is relatively long, about 5700 years, sample activity is low, especially for old samples, and large samples-typically 0.5 to 10 g of elemental carbon, are required to achieve good counting statistics. In most cases samples older than 55,000 years may not be reliably dated, but miniature counters have lowered the carbon mass limit to about 10 mg, with a corresponding increase in counting time. In many cases months are required to count small samples containing less than 100 mg of carbon.“’ In the last five years, however, several laboratories have shown that particle accelerators may be used to “C date samples as small as 1 mg with ages up to 70,000 years’-‘-@ The technique uses a tandem Van de Graaff accelerator as an ultra-sensitive mass spectrometer to ion count the relative numbers of “C, ‘-‘C and “C atoms contained in carbon sample targets. However. a host of problems remain to be solved before radiocarbon dating using tandem accelerator mass spectrometry (TAMS) can be considered to be routine.“’ For example, the preparation of miniature carbon sample targets for use in conventional accelerator sputter ion sources poses several chemical and physical problems. Amongst the requirements for suitable sample targets are low isotopic fractionation and contamination in the sample preparation, ease of
preparation and high stable C- ion yields from the accelerator ion source. Of all the reported techniques for target preparation, none meets all of the required criteria. In this paper a promising method for the preparation of graphite targets from wood and charcoal specimens is described, and the results from initial tests of the targets in a caesium sputter ion source are discussed.
349
2. Carbon Target Requirements In lJC laboratories where radioactive decay counting techniques are used, the sample to be dated is initially converted into a gas, generally CO:. C2H2 or CH,. In TAIMS, because of low beam intensities and problems with cross contamination, gaseous targets have proved to be unsuitable.“’ Several workers have shown however, that carbon in the form of graphite or metal carbide targets may be used in conventional sputter ion sources to produce stable intense beams of C- ions suitable for TAMS. In Rochester, a mixture of amorphous carbon and finely divided silver or copper is pressed into an aluminium bucket mounted on an ion source wheel. These samples produce rather low C- ion yields compared to a target made from spectroscopic grade graphite and the sensitivity of the technique may thus be limited”‘In Oxford, CO: is converted into acetylene and the carbon is deposited onto a thin tantalum wire by thermal cracking. High beam currents have been produced but some difficulties with isotopic fractionation have been reported. (*JIn Seattle. at the University of Washington, graphite targets are produced by using a commercial graphitising process on a microscale. The carbonised sample is mixed with pitch and
katcd io high: tetxperztxes in a mouid. The resulting targst yields intense. stabls C bezmj. “’ Ho~vever. thr process of diiuting the sample carbon with ‘-dead” cz+on in the form of pitch nssds 10 be examined. In Zcrich. s<\eral radiocarbon datss using T.QIS have been reported. Y” Tarpets arc prepared by con\-erring CO1 into CO over hot izinc. Cracking of the CO in a d.c. discharge results in deposition ofelemental carbon onto a copper disc for direct mounting in a sputter ion source. Initial results shou that ion currents from these samples are of the same order as those produced by spectroscopic grade graphite tars?%. but that relati\el> high backgrounds are present.“” in the sample preparation method described in this paper, solid sample carbon is directly con\,erted to graphite, to circumvent intermediate steps which might lead to background contamination and fractionation. The tnethod has the disadvantage that only carbonaceous solids liks bones. peats, woods. and charcoals may be treated: for CO+volving samples likt shells. corals. CO: dissolved in seaivater and atmospheric samples. another mrrhod must be devised. HORWW. at our decay counting radiocarbon laboratory. YY,, of the samples dated fall into the former category and the use of [he simple direct praphitisation method described here is justified. A parallel technique for producing yraphite targets irom CO: is under development at this laboratory. 3. Experimental
Pyrolysis is commonly used to retrieve elemental carbon from organic matter. The technique. which involves the thermal decomposition of organic matter in an oxygen-frrs atmosphere, is a complex process. For example. more than 200 products in addition to charcoal have been identified during the pyrolysis of wood at -1OO-C.““l At higher temperatures many pyrolysis products are thermally cracked to carbon and charcoal yields increase. Initial attempts made to vacuum pyrolyse finely ground whole wood sawdust. showed that typically 70”” of sample carbon was lost, presumably as volatile hydrocarbons pumped away by the vacuum system, and that the 6°C of the pyrolysis products fractionated by LIP to -3% when compared with the original sabvdust. No variation of the vacuum pyrolysis paramsters, including the application of a high voltage d.c. discharge to crack the volatiles, produced a significant increase in carbon yield. However, yields did significantly increase when the sawdust was Rams sealed inside evacuated Pyrex glass tubes and heated for 1 h at 600X. Inside a glass tube 50 mm long with an i.d. of 9 mm. 100 mg of 200 pm sawdust typically produced 60mg of carbon in the form of charcoal and tarry deposits. Assuming a formula for dry wood of (CH:O),. carbon yields ranged from about 60 to SO”,. similar to yields
(a) Non-graphitisable isotropic carbon
(b) Graphitisable semi ordered carbon
Cc) Turbostratic 20 ordered carbon Ts and Tn effect graphite
Cd) 30 ordered graphitic carbon G effect graphite
Fie. 1. Graphitising and non-graphitisin_g carbons, and possible graphite forms produced after sa:alytic graphitisation. from bomb pyrolysed white fir cellulose. reported by Leavitt er u/.‘!” On further heating under vacuum to 2000-C little or no loss of mass in the pyrolysis product was observed. Mass spectromrtric analysis showed oniy a small systematic decrease in 6°C of about O.l% indicating that the glass bomb pyrolysis products had not significantly fractionated. This material was thus considered a good starting point for graphitisation. 3.2. Catalytic
graphitisation
Carbon-containing substances can be placed into two broad categories: graphitisable and nongraphitisable (see Fig. 1). Materials from the former category show a large amount of crystalline graphite structure follov+ing heat treatment at temperatures as low as 1000-l 600-C.“” Those in the latter category, which includes most of the samples of interest to radiocarbon dating. do not produce nell defined graphite structures at heat treatment temperatures below 16OO’C. At higher temperatures and with the of certain catalysts, however. nonaddition graphitisable carbons can yield well ordered graphite structures: a phenomenon known as catal>-tic graphitisation. An excellent review of this complex subject has recently been published by Oya and Marsh.“S’ 3.3. Target graphitisation cussion
procedures, results and dis-
Various finely divided metals were tested as graphitisation catalysts. Carbonised samples were prepared from rimu (Dacrqdium cupressum) sa\vdust using the bomb pyrolpsis method described in Section 3.1. Using the apparatus shown in Fig. 2, various test mixtures of sample carbon and catalyst were heated under vacuum at different temperatures. The high temperature vacuum furnace used was capable of
Preparation
of _zzphite
351
targetS
22
(a)
26
23
28 ICuKa)
2i
(“)
G-effect
30
graphite
__.--__.---- ._._*.-._. ~
/
22
7
2i
x
28 (CuK,)
28
30
(“I
(b) To-effect
graphife
vacuum furnace. boron nitride boat and sample.
Fig. 3. X-ray powder ditTYaction profiles (solid lines) of carbon catalytically graphitised by (a) the G-effect i2d = X5.5’), and (b) the Tn-effect (28 = 26.0.). The dashed !ines show the diffraction profile oi the carbon before graphitisation.
reaching temperatures in excess of 3OOO^Cand was specifically designed for the miniature samples used in the “C target preparation work.““ The reactants, typically 20 mg of elemental carbon and about IO mg of finely divided metal catalyst were placed in a piugged boron nitride (Caborundum Co. red grade) or graphite boat. A 0.5 mm diameter hole was drilled in the boat plug to allow excess gas. for example. vapourised metals. to be pumped away by the vacuum system. During the heating process the rate of heating was adjusted so that the pressure in the furnace tube always remained below 0.1 torr. Experience showed that, depending on sample type, too fast a heating rate resulted in the boat rupturing with subsequent loss of the sample. The graphitised samples were examined using conventional x-ray powder diffraction techniques. These showed that the most crystalline graphitic materials were formed at temperatures of up to 2000°C and the most effective catalyst was finely divided (< 100 j(m) chromium metal powder. Heating times were typically of the order of 30 min with temperatures above
15OO’C for at least 15 min. Experience showed that shorter heating times even at temperatures up to 25OO’C produced little or no graphite. Intense G-effect graphite peaks (see Fig. 3a) were produced by heating 301, by weight of chromium metal powder with bomb pyrolysed materials at temperatures above 1500-C for 15 min (see Table I). These samples produced intense, stable beams of Cions. up to 28 /LA. when tested in a caesium sputter ion source at University of Arizona, Physics Department (see Table I). The intensity and stability of the C- ion beams produced was comparable to those produced by spectroscopic grade graphite and the samples are hence excellent candidates for target material for “C dating purposes. Similar starting materials heated at lower temperatures, 11OO’C for most of the time, produced inferior target material with low intensity ion beams compared to spectroscopic grade graphite. X-ray diffraction tests of these samples showed that a significant quantity of Tn graphite (see Fig. 3bj was present. Spectroscopic graphite and the high yield samples listed in Table 1
Fig. 2. High
temperature
Table I. Tests of various
carbon target mat&&
with a caesium spurrer ion source (Cs ioniser temperature
170X) and x-ray powder yield of a spectroscopic grade standard graphite. The graphite x-ray peak heights are &ven in counts per second (cps). Because of the varying amounts of Cr catalyst used and the relatively low atomic weight of carbon, the wak heiahts eive onlv a rough indication of the relative amounts of G and Tn eraohite in the samolcj diffraction
(Cu K,). The negative
Sample tvpe _ _~ __.L___~. Pmus radiata
Pinus radiata Daqdium cupressum (New Zraland rimu) Dacrydium cupressum ~5~ Zealand rimu) Spect:oscopic grade zauhite
ion carbon
currents
are compared
Heat treatment vacuum CO.05 torr 1000 C for 60min 2SOU’C for I min IWO C for 6omin 2500 C for I mm 160&?040-C for 15 min ?_WO C for 1 min IHO-1600 C for Ijmin 2500-C for i min -
%i:h the current
Chromiun czalyst
Target
x-ray
performance cc- current)
diiTraction results
O”,
0.07 /IA
26 w-t eg
0.1 jL.A
initial material 31 Ui”, initial material 33 wt 9, initial material
23 #.A s!ablr 23 UA stable 19;l.A
-
G graphlte I _ I100 cps) Tn graphite i -2300 cps) G graphite i 5 5000 cps) Tn graphite t,. 2500 cps) G graphit? ( 5 2300 cps) No Tn graphtie G graphite I _ 10000 cps) No Tn graphire G graphite I.. 20000 cps) No Tn graphi:-
;i’ --_
DAVID C.
contain large amounts of Ci-effect graphite and little. if any, Tn graphite. For good target performance. the highlv. crvstaiiine G-effsct graphite thus appears to be ~ far more important that the Tn component.
LiWE
Espkt! hiiks iire due to b’. J. Judd for expert technical assistant e during aii stages of the work. The assistamx of X. J. T. J&l To; testing graphite spwlmens in n sputter ion source is ~~teTull\- acknowledged. Thanks are also due to I. Brown. Sf. Bowden and R. Berezowski Ior u-ray diffraction analysts oi carbon samp!es. .4 L',:-iZ,J~t_IEii~en!rr2i 5-
4. Conclusions Bomb pyrolysis of organic sample material followed by catalytic graphitisation appears to be a very promising method for producing suitable targets for “C dating by TAMS. T’ne method is simple. relatively fast. up to 10 samples per day per operator. and could be easily automated using a microprocessor to control the catalytic graphitisation step. A high proportion of the highly crystalline G-etfect graphite appears to be essential for targets capable of producing intense, stable C- beams. The Tn form of graphite on its own does not produce these beams and may even be detrimental in targets which contain G-effect graphite. To form G-effect graphite from carbonised samples, the temperature. catalyst and period of heat treatment (soak time) are important parameters. Excellent carbon target material has been made by adding 309, by weight of finely divided ( < 100 pm) chromium metal powder to bomb-pyrolysed sawdust and heating for at least 15 min above 15OO’C under vacuum. More work is needed to assess the significance of the various forms of graphite on the C Field from graphite targets in a caesium sputter ion source, as ~vell as the possible effects of trace impurities. Investigations are continuing on the possible eifscts of fractionation and contamination during the preparation of solid graphite targets from organic starting materials.
References I. Anderson E. C.. Libby- W. F.. Weinhouse S. ef al. Ph~s. Rec. 72, 931-936
C. LOWE
Institute of Nuclear Sciences. DSIR, Private (Rrceitvd
Bag,
Lower Hutt. New Zealand
14 July 1983)
The introduction of TAMS has exciting implications for radiocarbon dating but improved sample preparation methods are needed. This paper describes a promising method for the conversion of a few milligrams of wood or charcoal into graphite targets for use in a caesium sputter ion source. Targets containing a large proportion of G-type graphite produced large C- currents, but those containing a high proportion of turbostatic Tn graphite were unsatisfactory; the type of graphite in the target is clearly of significance.
1. Introduction Since the appearance of the historic paper by Anderson er al.“’ on the feasibility of “C dating, many thousands of radiocarbon ages for a wide variety of archaeological and geological specimens have been reported. The majority of these dates have been determined by radioactive decay counting techniques. However. because the half-life of “C is relatively long, about 5700 years, sample activity is low, especially for old samples, and large samples-typically 0.5 to 10 g of elemental carbon, are required to achieve good counting statistics. In most cases samples older than 55,000 years may not be reliably dated, but miniature counters have lowered the carbon mass limit to about 10 mg, with a corresponding increase in counting time. In many cases months are required to count small samples containing less than 100 mg of carbon.“’ In the last five years, however, several laboratories have shown that particle accelerators may be used to “C date samples as small as 1 mg with ages up to 70,000 years’-‘-@ The technique uses a tandem Van de Graaff accelerator as an ultra-sensitive mass spectrometer to ion count the relative numbers of “C, ‘-‘C and “C atoms contained in carbon sample targets. However. a host of problems remain to be solved before radiocarbon dating using tandem accelerator mass spectrometry (TAMS) can be considered to be routine.“’ For example, the preparation of miniature carbon sample targets for use in conventional accelerator sputter ion sources poses several chemical and physical problems. Amongst the requirements for suitable sample targets are low isotopic fractionation and contamination in the sample preparation, ease of
preparation and high stable C- ion yields from the accelerator ion source. Of all the reported techniques for target preparation, none meets all of the required criteria. In this paper a promising method for the preparation of graphite targets from wood and charcoal specimens is described, and the results from initial tests of the targets in a caesium sputter ion source are discussed.
349
2. Carbon Target Requirements In lJC laboratories where radioactive decay counting techniques are used, the sample to be dated is initially converted into a gas, generally CO:. C2H2 or CH,. In TAIMS, because of low beam intensities and problems with cross contamination, gaseous targets have proved to be unsuitable.“’ Several workers have shown however, that carbon in the form of graphite or metal carbide targets may be used in conventional sputter ion sources to produce stable intense beams of C- ions suitable for TAMS. In Rochester, a mixture of amorphous carbon and finely divided silver or copper is pressed into an aluminium bucket mounted on an ion source wheel. These samples produce rather low C- ion yields compared to a target made from spectroscopic grade graphite and the sensitivity of the technique may thus be limited”‘In Oxford, CO: is converted into acetylene and the carbon is deposited onto a thin tantalum wire by thermal cracking. High beam currents have been produced but some difficulties with isotopic fractionation have been reported. (*JIn Seattle. at the University of Washington, graphite targets are produced by using a commercial graphitising process on a microscale. The carbonised sample is mixed with pitch and
katcd io high: tetxperztxes in a mouid. The resulting targst yields intense. stabls C bezmj. “’ Ho~vever. thr process of diiuting the sample carbon with ‘-dead” cz+on in the form of pitch nssds 10 be examined. In Zcrich. s<\eral radiocarbon datss using T.QIS have been reported. Y” Tarpets arc prepared by con\-erring CO1 into CO over hot izinc. Cracking of the CO in a d.c. discharge results in deposition ofelemental carbon onto a copper disc for direct mounting in a sputter ion source. Initial results shou that ion currents from these samples are of the same order as those produced by spectroscopic grade graphite tars?%. but that relati\el> high backgrounds are present.“” in the sample preparation method described in this paper, solid sample carbon is directly con\,erted to graphite, to circumvent intermediate steps which might lead to background contamination and fractionation. The tnethod has the disadvantage that only carbonaceous solids liks bones. peats, woods. and charcoals may be treated: for CO+volving samples likt shells. corals. CO: dissolved in seaivater and atmospheric samples. another mrrhod must be devised. HORWW. at our decay counting radiocarbon laboratory. YY,, of the samples dated fall into the former category and the use of [he simple direct praphitisation method described here is justified. A parallel technique for producing yraphite targets irom CO: is under development at this laboratory. 3. Experimental
Pyrolysis is commonly used to retrieve elemental carbon from organic matter. The technique. which involves the thermal decomposition of organic matter in an oxygen-frrs atmosphere, is a complex process. For example. more than 200 products in addition to charcoal have been identified during the pyrolysis of wood at -1OO-C.““l At higher temperatures many pyrolysis products are thermally cracked to carbon and charcoal yields increase. Initial attempts made to vacuum pyrolyse finely ground whole wood sawdust. showed that typically 70”” of sample carbon was lost, presumably as volatile hydrocarbons pumped away by the vacuum system, and that the 6°C of the pyrolysis products fractionated by LIP to -3% when compared with the original sabvdust. No variation of the vacuum pyrolysis paramsters, including the application of a high voltage d.c. discharge to crack the volatiles, produced a significant increase in carbon yield. However, yields did significantly increase when the sawdust was Rams sealed inside evacuated Pyrex glass tubes and heated for 1 h at 600X. Inside a glass tube 50 mm long with an i.d. of 9 mm. 100 mg of 200 pm sawdust typically produced 60mg of carbon in the form of charcoal and tarry deposits. Assuming a formula for dry wood of (CH:O),. carbon yields ranged from about 60 to SO”,. similar to yields
(a) Non-graphitisable isotropic carbon
(b) Graphitisable semi ordered carbon
Cc) Turbostratic 20 ordered carbon Ts and Tn effect graphite
Cd) 30 ordered graphitic carbon G effect graphite
Fie. 1. Graphitising and non-graphitisin_g carbons, and possible graphite forms produced after sa:alytic graphitisation. from bomb pyrolysed white fir cellulose. reported by Leavitt er u/.‘!” On further heating under vacuum to 2000-C little or no loss of mass in the pyrolysis product was observed. Mass spectromrtric analysis showed oniy a small systematic decrease in 6°C of about O.l% indicating that the glass bomb pyrolysis products had not significantly fractionated. This material was thus considered a good starting point for graphitisation. 3.2. Catalytic
graphitisation
Carbon-containing substances can be placed into two broad categories: graphitisable and nongraphitisable (see Fig. 1). Materials from the former category show a large amount of crystalline graphite structure follov+ing heat treatment at temperatures as low as 1000-l 600-C.“” Those in the latter category, which includes most of the samples of interest to radiocarbon dating. do not produce nell defined graphite structures at heat treatment temperatures below 16OO’C. At higher temperatures and with the of certain catalysts, however. nonaddition graphitisable carbons can yield well ordered graphite structures: a phenomenon known as catal>-tic graphitisation. An excellent review of this complex subject has recently been published by Oya and Marsh.“S’ 3.3. Target graphitisation cussion
procedures, results and dis-
Various finely divided metals were tested as graphitisation catalysts. Carbonised samples were prepared from rimu (Dacrqdium cupressum) sa\vdust using the bomb pyrolpsis method described in Section 3.1. Using the apparatus shown in Fig. 2, various test mixtures of sample carbon and catalyst were heated under vacuum at different temperatures. The high temperature vacuum furnace used was capable of
Preparation
of _zzphite
351
targetS
22
(a)
26
23
28 ICuKa)
2i
(“)
G-effect
30
graphite
__.--__.---- ._._*.-._. ~
/
22
7
2i
x
28 (CuK,)
28
30
(“I
(b) To-effect
graphife
vacuum furnace. boron nitride boat and sample.
Fig. 3. X-ray powder ditTYaction profiles (solid lines) of carbon catalytically graphitised by (a) the G-effect i2d = X5.5’), and (b) the Tn-effect (28 = 26.0.). The dashed !ines show the diffraction profile oi the carbon before graphitisation.
reaching temperatures in excess of 3OOO^Cand was specifically designed for the miniature samples used in the “C target preparation work.““ The reactants, typically 20 mg of elemental carbon and about IO mg of finely divided metal catalyst were placed in a piugged boron nitride (Caborundum Co. red grade) or graphite boat. A 0.5 mm diameter hole was drilled in the boat plug to allow excess gas. for example. vapourised metals. to be pumped away by the vacuum system. During the heating process the rate of heating was adjusted so that the pressure in the furnace tube always remained below 0.1 torr. Experience showed that, depending on sample type, too fast a heating rate resulted in the boat rupturing with subsequent loss of the sample. The graphitised samples were examined using conventional x-ray powder diffraction techniques. These showed that the most crystalline graphitic materials were formed at temperatures of up to 2000°C and the most effective catalyst was finely divided (< 100 j(m) chromium metal powder. Heating times were typically of the order of 30 min with temperatures above
15OO’C for at least 15 min. Experience showed that shorter heating times even at temperatures up to 25OO’C produced little or no graphite. Intense G-effect graphite peaks (see Fig. 3a) were produced by heating 301, by weight of chromium metal powder with bomb pyrolysed materials at temperatures above 1500-C for 15 min (see Table I). These samples produced intense, stable beams of Cions. up to 28 /LA. when tested in a caesium sputter ion source at University of Arizona, Physics Department (see Table I). The intensity and stability of the C- ion beams produced was comparable to those produced by spectroscopic grade graphite and the samples are hence excellent candidates for target material for “C dating purposes. Similar starting materials heated at lower temperatures, 11OO’C for most of the time, produced inferior target material with low intensity ion beams compared to spectroscopic grade graphite. X-ray diffraction tests of these samples showed that a significant quantity of Tn graphite (see Fig. 3bj was present. Spectroscopic graphite and the high yield samples listed in Table 1
Fig. 2. High
temperature
Table I. Tests of various
carbon target mat&&
with a caesium spurrer ion source (Cs ioniser temperature
170X) and x-ray powder yield of a spectroscopic grade standard graphite. The graphite x-ray peak heights are &ven in counts per second (cps). Because of the varying amounts of Cr catalyst used and the relatively low atomic weight of carbon, the wak heiahts eive onlv a rough indication of the relative amounts of G and Tn eraohite in the samolcj diffraction
(Cu K,). The negative
Sample tvpe _ _~ __.L___~. Pmus radiata
Pinus radiata Daqdium cupressum (New Zraland rimu) Dacrydium cupressum ~5~ Zealand rimu) Spect:oscopic grade zauhite
ion carbon
currents
are compared
Heat treatment vacuum CO.05 torr 1000 C for 60min 2SOU’C for I min IWO C for 6omin 2500 C for I mm 160&?040-C for 15 min ?_WO C for 1 min IHO-1600 C for Ijmin 2500-C for i min -
%i:h the current
Chromiun czalyst
Target
x-ray
performance cc- current)
diiTraction results
O”,
0.07 /IA
26 w-t eg
0.1 jL.A
initial material 31 Ui”, initial material 33 wt 9, initial material
23 #.A s!ablr 23 UA stable 19;l.A
-
G graphlte I _ I100 cps) Tn graphite i -2300 cps) G graphite i 5 5000 cps) Tn graphite t,. 2500 cps) G graphit? ( 5 2300 cps) No Tn graphtie G graphite I _ 10000 cps) No Tn graphire G graphite I.. 20000 cps) No Tn graphi:-
;i’ --_
DAVID C.
contain large amounts of Ci-effect graphite and little. if any, Tn graphite. For good target performance. the highlv. crvstaiiine G-effsct graphite thus appears to be ~ far more important that the Tn component.
LiWE
Espkt! hiiks iire due to b’. J. Judd for expert technical assistant e during aii stages of the work. The assistamx of X. J. T. J&l To; testing graphite spwlmens in n sputter ion source is ~~teTull\- acknowledged. Thanks are also due to I. Brown. Sf. Bowden and R. Berezowski Ior u-ray diffraction analysts oi carbon samp!es. .4 L',:-iZ,J~t_IEii~en!rr2i 5-
4. Conclusions Bomb pyrolysis of organic sample material followed by catalytic graphitisation appears to be a very promising method for producing suitable targets for “C dating by TAMS. T’ne method is simple. relatively fast. up to 10 samples per day per operator. and could be easily automated using a microprocessor to control the catalytic graphitisation step. A high proportion of the highly crystalline G-etfect graphite appears to be essential for targets capable of producing intense, stable C- beams. The Tn form of graphite on its own does not produce these beams and may even be detrimental in targets which contain G-effect graphite. To form G-effect graphite from carbonised samples, the temperature. catalyst and period of heat treatment (soak time) are important parameters. Excellent carbon target material has been made by adding 309, by weight of finely divided ( < 100 pm) chromium metal powder to bomb-pyrolysed sawdust and heating for at least 15 min above 15OO’C under vacuum. More work is needed to assess the significance of the various forms of graphite on the C Field from graphite targets in a caesium sputter ion source, as ~vell as the possible effects of trace impurities. Investigations are continuing on the possible eifscts of fractionation and contamination during the preparation of solid graphite targets from organic starting materials.
References I. Anderson E. C.. Libby- W. F.. Weinhouse S. ef al. Ph~s. Rec. 72, 931-936