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A review of ion sources for accelerator mass spectrometry
193
Nuclear Instruments and Methods in Physics Research I35 (1984) 193-199 North-Holland. Amsterdam
A REVIEW OF ION SOURCES
FOR ACCELERATOR
lMASS SPE~~~E~Y
*
Accelerator mass spectrometry places unusually difficult demands on ion source design and possible ways of meeting these are discussed. A brief review of the state-of-the-art methods of generating negative ion beams of beryllium, carbon, aluminum, chlorine and calcium are presented. A new method of producing B-25 pA of r*C- ions from CO, gas is described in detail with emphasis on accelerator 14C dating. Ionization efficiency is high ( - 7.7%) and it may be possibfe to date samples ~nta~ning 100 pg of rzC (i.e. - 0.2 at. ems of CO_& Accelerator measurements with contemporary CO, followed by CO, from anthracite enabled the age of the latter to be determined to be > 39000 years without background subtraction.
f. In&oduction
As the name implies, accelerator mass spectrometry (AMS) requires the use of a particle accelerator. Almost any type capable of awlerating heavy ions to some fraction of a MeV/nu&on is suitabfe but the tandem accelerator has important advantages. Because of these and since most measurements have been made with tandems this review is confined to negative ion sources. In spite of present interests being limited to a few isotopes, AMS places extremely stringent requirements on the ion source. Some of these are as follows: (I) Czmmt. To exploit the prime advantage of AMS, namely the ability to measure extremely small isotopic ratios, requires a large current. For example, for an isotope present at the level of 10-14, the count rate is 425 per hour; assuming a negative ion current of 10 @A and an accelerator transmission of 20%. Thus a 5% stati~~~~y limited measurement can be made in about one hour but a I% measurement requires a prohibitively long time. (2) Ernitt~~~e. Since the goals of AMS are to measure small isotopic ratios with high precision, the accelerator transmission should be high and reproducible, This can be achieved only if the emittance of the source and injector is less than the acceptance of the acceierator. (3) Source material. An unusual but important requirement is that the source should produce the desired negative ion (and current) from a sample which is convenient to chemically separate e.g. AI- ions from aluminum oxide.
* Work supported by the National Science Foundation.
0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (Norm-Holland Physics Publishing Division)
(4) Memory eJ&fs. It is not unusual. for a discharge source to continue to give an appreciable yield long after the support gas is turned off or changed. In AMS this is intolerable and it is highly desirable that within 30 min of sample changing the current from the previous sample should be attenuated by a factor of between lo3 and 10’. (5) Ionization efficiency. Since samples are frequently small (e.g. a single cosmic spherule) the ionization efficiency should be high - preferabIy greater than 1%. (6) Stability. Although it is common practice to monitor the source output during measurement it is desirable that the negative ion current shouId not be subject to short term fluctuations or long term drifts. (7) Sample changing. It is highly desirable that ion source samples can be changed quickly and after changing the current should stabilize within 5 or 10 min. For the most precise work it might be desirable to interchange a sampie and a standard at 1 or 2 min intervals. 2. Available negative ion sources It is not easy to classify negative ion sources since, in final analysis, the operating principles blend together in an all but continuous spectrum. Generally speaking the main types are: (1) direct extraction from a ptasma, (2) cesium ion sputtering, (3) surface ionization and (4) charge exchange of positive ions. None are ideally suited to all of the needs of AMS but some are closer than others and a few general conclusions are possible. Sources of the direct extraction type usually require the sample to be in the form of a gas or a vapor and their applicability is limited. Also they have notoriously long memories which may all but preclude their usefulness to AMS. III. ION SOURCES ,’ INSTRUMENTATION
R. Middleton / Ion sources for A MS
194
The cesium sputter source has many advantages and probably comes closest to satisfying AMS requirements. It is versatile, can be used with solids and gases, sample changing is rapid and memory effects are small. However, even the latest high intensity versions are unable to produce the desirable 10 /LA minimum in all cases. Surface ionization sources, such as those using lanthanum hexaboride, are probably restricted to the halogens but have the potential of large currents and a high degree of selectivity. It is possible that they may find application in the selective ionization of 36C1 in the presence of sulfur. Since charge exchange in an alkali metal vapor frequently enables 10% or more conversion of positive to negative ions and there is available a wide range of intense positive ion sources, it provides an important method of generating intense negative ion beams. In practice such sources are difficult to operate stably over prolonged periods, the negative ion spectrum is often complex because of molecular dissociation and the emittance may vary with the density of the donor vapor. It should also be remembered that the requirements numerated in sec. 1 now pertain to the positive ion source. 3. Generation of specific negative ions 3.1. Beryllium For 3 MV or more tandems, BeO- is a satisfactory negative ion; the only disadvantage is that some loss of
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transmission is likely to occur from the Coulomb explosion if a stripper foil is used. Cesium sputter sources such as UNIS [l] and General Ionex 834 [2], with reflected geometry, typically yield about 0.5 PA from sputter targets of compressed beryllium oxide and have an ionization efficiency of between 0.1 and 0.2% [3]. Although 0.5 pA is adequate for many purposes higher currents are needed for samples with ‘*Be : 9Be ratios less than lo-“. The high intensity sputter source described in ref. [4] produces 10 to 20 times more current, has somewhat improved emittance and a higher ioni~tion efficiency. Fig. 1 shows the results of some measurements made with a high intensity source comparing cathodes containing pure Be0 and a 1: 5 mixture (by weight) with silver powder. The cathodes were made from copper and the powder was compacted into a 1.6 mm diameter by 1.6 mm deep hole. It is noteworthy that although higher currents were obtained from the pure sample, the current was slow in rising and took about 40 min to attain its maximum value. Also after about 100 min the current began to fall and at the end of 180 min was only 5 PA. The run was terminated at this time and the cathode was removed. It came as a surprise to find at least half of the Be0 still present and the reason for the drop in current is not understood. The sample with silver behaved quite differently. It attained its maximum current almost instantaneously, very rapidly fell to about 8 PA and then very slowly declined to 5 PIA over a 10 h period. The ionization efficiency for the sample with silver was high and about 1.8%.
-T----e--
I
12f ‘e’*e.*e
ll10 -t 9-
5.9mg .*
4.
l*
Be0
EFFIC. > 0.42 %
l*
?!
le
3.2mg EFFIC.-
BeO+ 16.3mg 1.8%
Ag
TERMINATED
Fig. 1.A comparison Be0 &ed
TIME
(MIN.)
of the negative ion Currents from a high intensity sputter source [4] obtained with cathodes containing Be0 and with silver powder (1: 5 by wt.). The reason for the decline in current from the pure Be0 is not understood and was not
due to sample exhaustion.
R. Middleton / Ion sources for AMS
Source memory effects are generally expected to be small for sputter sources, particularly when the target has low volatility. This indeed appears to be the case and Klein et al. [3] report observing a “Be : 9Be ratio of less than 2 x lo-” from a Be0 blank in a reflected geometry UNIS source. The effects are likely to be larger with high intensity sources [4] since the ionizer and sputter target are confined in a common chamber and the latter is at an elevated temperature. Because of this we have made many measurements on blanks containing commercially available Be0 and with these we typically observe a “Be: 9Be ratio of 6 X lo-r5. Since this is higher than expected from earlier studies made with cathodes containing large amounts of 26Al we recently measured the ‘(‘Be background from cathodes containing MgO and Ga,O,. In both cases the 9Be current was very small (< 1 nA with Ga,O,) and no “Be counts were obtained. This strongly suggests that the “Be observed from the commercial Be0 is not the result of source memory but is due to the oxide being contaminated. Fig. 2 shows a comparison of the 2-dimensional spectra obtained with blanks of Be0 and Ga,O,. Since the elemental ion 9Be- either has a small electron affinity or is metastable, it is only weakly formed in sputter sources. From the high intensity source the current is about 2.5% that of BeO- i.e. 0.2-0.25 PA (the yield is about the same from beryllium metal).
195
Larger currents have been obtained from charge exchange sources and Heinemeier and Tykesson [S]) report obtaining 2.3 PA from the charge exchange of 100 PA of positive ions in sodium vapor. 3.2. Carbon Although sputter sources generate between 30 and 300 PA of 12C- ions from graphite, the yield from less dense forms of carbon is disappointing and frequently is more than an order of magnitude less. The problems of converting 14C dating samples into high yield sputter targets, while minimizing isotopic fractionation and contemporary contamination, have been the subject of much study and the reader is referred to refs. [6] and [7] and those contained therein. The most attractive source material is CO, and the generation of “C- ions directly from the gas is discussed in sect. 4. Failing this, CO, can be reduced to an amorphous carbon and its density increased by dissolving in iron or by graphitization. Alternatively the CO, can be converted into acetylene which can be cracked and deposited as a type of graphite. Since the steps involved in increasing the density increase the risk of contemporary contamination it was considered worthwhile testing pressed amorphous samples, with and without silver, in the high intensity source. Two cathodes were prepared from magnesium
1.2
1.0
5
0.8
r” w
0.6
a 0.4
Fig. mm Be0 that
2. Two dimensional energy spectra obtained with commercial Be0 (on left) and a blank containing gallium oxide. Both are 20 measurements and for the Be0 sample the BeO- current was about 11 aA. The 41 counts observed in the “‘Be window from the corresponds to a ‘background’ “Be: 9Be ratio of 6X 10-r’; no “Be counts were observed from the gallium oxide. It is thought the “Be is not due to ion source memory effects. III. ION SOURCES ,’ INSTRUMENTATION
R. Middleton / Ion sources for AMS
196
1
‘0
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1
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I
10
20
30
40
50
60
70
80
90
TIME
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100
I
I 1
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I
110
120
1
(MIN.)
amorphous carbon and amorphous Fig. 3. The l*C- currents measured as a function of time with cathodes containing compressed carbon mixed with silver powder. The amorphous carbon was prepared by reducing CO, gas with magnesium; the source was of the high intensity type described in ref. [4].
reduced CO,, one containing 10 parts by weight of silver. Both were pressed and hammered into 1.6 mm diameter by 1.6 mm deep cavities drilled into copper cathode blanks. Fig. 3 shows the observed currents as a function of time. That from the sample without silver climbed very slowly and was unsteady for the first 45 min; during this time it glowed very brightly. As the glow subsided the current became steadier but even after two hours it was still slowly rising. As expected the current from the sample with silver rose more quickly and stabilized earlier but the steady current was less. The currents from both are more than adequate and apart from the delay in attaining stability either would be satisfactory for accelerator t4C dating. 3.3. Aluminum Since aluminum has a small electron affinity (0.46 eV) it is a difficult negative ion to form by sputtering; Middleton [8] reports obtaining about 0.1 IJ.A from the metal in a UNIS source. Larger molecular beams can be obtained but these are less attractive for AMS since it is then no longer possible to capitalize on the fact that the stable isobar, 26Mg, does not form a stable negative ion. The difficulties of negative ion formation are further compounded by the compelling need to use aluminum oxide as a source material. A few years ago it was discovered [9] that 2-3 I_IA of *‘Al- ions can be obtained from aluminum oxide targets in a high intensity sputter source [4] with an ionization
efficiency of about 0.25%. Currents, like those from beryllium oxide and amorphous carbon, were observed to rise slowly and required about 40 min to reach maximum. Mixing with silver powder significantly reduced the rise time but resulted in about one-half the negative ion current. It was also observed that samples which were baked for about 30 min at 1200°C gave larger currents. Since terrestrial samples typically have 26A1: 27Al ratios of about lo-i4 it is desirable that negative ion currents should be 10 PA or larger. The prospects of obtaining such currents from a sputter source are not good and charge exchange might be a better approach. Preliminary experiments at the University of toronto [lo] have resulted in currents of 0.18 pA and with an ionization efficiency of 0.17%. It is anticipated that significantly larger currents will be attained with an improved positive ion source (the present source produces about 4 PA) and with the optimum choice of donor material (lithium vapor was used). 3.4. Chlorine Chlorine has the largest electron affinity of all the elements (3.62 eV) and intense beams can be formed by a variety of ways. However, since most samples for AMS are in the form of a solid, cesium ion sputtering is particularly convenient. The largest difficulty confronting the experimenter is reducing sulfur content since the isobar interfering with the detection of 36C1 is 36S.
R. Middleton / Ion sourcesfor AM.5 Fortunately the natural abundance of MEGis only 0.02% and chemical purification often suffices. An alternate and attractive approach is to form chlorine negative ions by surface ionization on a low work function surface such as lanthanum hexaboride. Since the electron affinity of sulfur (2.08 eV) is considerably less than that of chlorine the process should favor chlorine negative ion formation by several orders of magnitude. Although the possibilities are well-known, as far as the author is aware, no results of experimental studies have been published. 3.5. Calcium The negative ion of calcium, like that of beryllium, is thought to be metastable and it is extremely weakly formed by the sputter process. However, it has been shown [ll] that about 1 I_LAof CaH- and CaH; ions (CaH; is very weak) can be produced by sputtering a metallic surface sprayed with ammonia gas. Raisbeck et al. [12] suggest using CaH; ions since the interference caused by 41K while detecting 41Ca should be minimal. They report KH; to be an improbable negative ion and the ratio KH;/Kis 2 10W8. The disadvantage of the above is that it requires metallic calcium whereas most samples are likely to be oxides. Some recent experiments at the University of Rochester [13], where CaO- ions were accelerated, suggest that the difficulties caused by 4’K might have been overestimated. In view of this we have measured the yield of CaOions from compressed calcium oxide
191
targets in a high intensity source. The negative ion is certainly much more difficult to form than BeO- and it was only with difficulty that 1 pA could be obtained.
4. Formation of 12C - ions from CO, gas Since it is relatively simple to convert most 14C dating samples into CO,, with minimal contamination risk, the gas is an ideal source material for “C- formation. This was appreciated by Heinemeier and Andersen [14] and they measured the yield of 12C- ions from an ANIS source operated on CO,. They obtained between 2 and 4 PA and determined the ionization efficiency to be about 0.15%. Not too surprisingly, since the source has a gas discharge, memory effects were observed to be relatively severe. The high intensity sputter source, described in ref. [4], appears at first sight not to be amenable to operation with gases. This is because the ionizer and sputter target are contained in the same chamber into which cesium vapor is admitted. It was thought that chemical interaction with the gas might deplete the cesium supply and impair the operation of the ionizer. Recent experiments with a variety of gases have shown this not to be a major difficulty. A sectional drawing of the cathode stem used to admit gases is shown in fig. 4. Gas from an external needle valve flows down a 1 mm bore stainless steel tube and enters the source through a 0.5 mm diameter hole drilled through the center of the cathode. The
FREON COOLING lmm BORE S.S. TUBE
GAS INLET FROM NEEDLE VALVE
COOLING
-
LTITANIUM Fig. 4. The modified freon cooled cathode through a typical sputter cathode.
stem used to leak gases into a high intensity
INSERT
source. The inset drawing
III. ION SOURCES
shows a cross section
/ INSTRUMENTATION
198
R. Middleton
/ Ion sources for AMS
cathodes were made from copper and usually contained a hard pressed insert, similar to the titanium one shown in fig. 4. The i2C- current from CO, gas has been measured with several different cathode inserts and roughly correlated with the getter ability of the material. For example, titanium (55 PA), zirconium (40 PA), scandium (35 PA), tantalum (30 PA), nickel (13 PA), magnesium (8 PA), copper (3 PA) and gold (1 PA). Measurements were made with fairly nominal source conditions, the cathode voltage was 6 kV and the cathode current 5-7 mA. Typically gas consumption was about 2-3 at. cm3/h. Some measurements were also made with carbon monoxide and acetylene. The carbon monoxide gave marginally more current but not sufficient to warrant the conversion of CO, into CO. Acetylene typically produced 2 or 3 times the current but the mass 13 current was about 20% that of mass 12, indicating an undesirably large i2CHcomponent. Acetylene also appeared to be cracking and depositing on the ionizer and posed a serious memory threat. Measurements on the tandem accelerator revealed that the transmission of carbon ions from CO, on titanium was sometimes less than that from graphite cathodes, particularly when the gas flow was high. It is thought that this was due to the large size of the titanium insert (- 5 mm diameter) and disproportionately more negative ions coming from the cesium beam halo than from the intense core. This difficulty was overcome by reducing the diameter of the inserts to
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I
I
1
I
2 k
10
GAS
lx
2
y
CONSUMPTION
0.25cclhr 48T,
5
C-
H/
MASS
(a.m.u.1
Fig. 5. A negative ion spectrum obtained with CO, gas and a sputter cathode containing 100 mesh titanium powder similar to that shown in the inset sketch. It is noteworthy that the C; peak is relatively much weaker than is observed from graphite and the mass 13 to 12 ratio is about 1.6% - indicating the presence of some ‘*CH- ions.
1.6 mm and making them from pressed 100 mesh titanium powder as shown in the inset drawing in fig. 5. The i*C- current from these is less than that from a 5 mm diameter solid titanium insert but the maximum current of about 30 PA occurs at a much lower rate of gas flow. It was found that a gas flow which produced an almost imperceptible rise in the source base vacuum (- 3 x lo-’ Torr) produced an acceptable 23-25 PA. One such cathode was operated for 4 hours with a steady current of 24 /.tA on 1 at. cm3; this corresponds to an ionization efficiency of about 7.7%. Fig. 5 shows a typical negative ion spectrum obtained with such a cathode. Most titanium powder cathodes give a i2C- current of several microamps without gas when first put in the source and after lo-15 min of decline the current usually stabilized between 0.5 and 0.7 PA. The origin of this residual current is unknown and may be the result of a combination of gettering of residual hydrocarbons in the vacuum and carbon impurities in the titanium (99% purity). When gas is added the current almost instantaneously climbs to its final value and when shut off the fall is equally dramatic down to about 3 PA. Thereafter the fall is very slow and it is evident that some carbon is retained and a new cathode should be used with each gas sample. The extent of source memory effects have yet to be fully determined but some preliminary measurements have been made on the accelerator with contemporary (1850) CO, and some made from anthracite. Because of limited experience with 14C detection, the accelerator, beam transport system and detector were set up using a graphite cathode followed by measurements with the contemporary CO,. At a terminal voltage of -5.5 MV and with charge state 4 ions, the 14C count rate with the latter was about 20/s. This is a little less than expected from the 16 PA i2C- current and the measured 13C transmission of 20% and corresponds to an overall transmission of about 17% for 14C ions. After about one hour of measurements the titanium powder cathode was replaced by a new one and after about 15 min of operation the 14C count rate was again measured. With a 12C- current of about 0.4 PA this was 0.2/s. Assuming that the transmission was still 17% for 14C ions this corresponds to a surprisingly young age of 7600 years for the background carbon. Since the i4C background was not measured prior to admitting the contemporary CO, it is not known whether this is due to source memory. CO, prepared from the anthracite was then admitted and after the 12C- current had stabilized around 18 PA the 14C was counted. The count rate was unchanged and remained 0.2/s. Assuming 17% transmission for the 14C ions, this corresponds to a sample age of 39 000 years. To summarize, it has been demonstrated that ‘*C-
R. Middleton / Ion sources for AMS
currents of between 20 and 25 PA can be obtained from CO, gas. The gas consumption is low (- 0.25 at. cm3/h) and the ionization efficiency is high (- 7.7%). Of particular importance is the fact that the 14C background can be measured in a meaningful way before the sample gas is admitted - and if necessary corrected for. The complete absence of ion source memory has not been demonstrated but it has at least been shown to be small. Note added in proof: Since the Conference, we have definitely established that beryllium oxide from four apparently independent suppliers contains “Be at a concentration of about lo-r4 [15]. In particular we have shown that Be0 extracted from beryl crystals from a mine does not contain “Be.
References [l] R. Middleton and C.T. Adams, Nucl. Instr. and Meth. 118 (1974) 329. [2] G. Braun-Elwert, J. Huber, G. Korschinek and Kutschera, Nucl. Instr. and Meth. 146 (1977) 121.
W.
199
[3] J. Klein, R. Middleton and Hongqing Tang, Nucl. Instr. and Meth. 193 (1982) 601. [4] R. Middleton, Nucl. Instr. and Meth. 214 (1983) 139. [S] J. Heinemeier and P. Tykesson, Nucl. Instr. and Meth. 141 (1977) 183. (61 R. Gillespie and R.E.M. Hedges, Radiocarbon 25 (1983) 771. [7] J.S. Vogel, LG. Nowikow, J.R. Southon and D.E. Nelson, Radiocarbon 25 (1983) 775. [8] R. Middleton, Nucl. Instr. and Meth. 144 (1977) 373. 191 R. Middleton, J. Klein, G.M. Raisbeck and F. Yiou, Nucl. Instr. and Meth. 218 (1983) 430. [lo] G.E. Aardsma, Ph.D. Thesis, University of Toronto, Canada (1984). [ll] R. Middleton, Nucl. Instr. and Meth. 141 (1977) 373. [12] G.M. Raisbeck, F. Yiou, A. Peghaire, J. Guillot and J. Uzureau, Proc. Symp. on Accelerator mass spectrometry, Argonne National Lab. (1981) p. 426. [13] D. Elmore, University of Rochester, private communication. [14] J. Heinemeier and H.H. Andersen, Radiocarbon 25 (1983) 761. [15] R. Middleton, J. Klein, L. Brown and F. Tera, Nucl. Instr. and Meth. B5 (1984) in press.
III. ION SOURCES
/ INSTRUMENTATION
Nuclear Instruments and Methods in Physics Research I35 (1984) 193-199 North-Holland. Amsterdam
A REVIEW OF ION SOURCES
FOR ACCELERATOR
lMASS SPE~~~E~Y
*
Accelerator mass spectrometry places unusually difficult demands on ion source design and possible ways of meeting these are discussed. A brief review of the state-of-the-art methods of generating negative ion beams of beryllium, carbon, aluminum, chlorine and calcium are presented. A new method of producing B-25 pA of r*C- ions from CO, gas is described in detail with emphasis on accelerator 14C dating. Ionization efficiency is high ( - 7.7%) and it may be possibfe to date samples ~nta~ning 100 pg of rzC (i.e. - 0.2 at. ems of CO_& Accelerator measurements with contemporary CO, followed by CO, from anthracite enabled the age of the latter to be determined to be > 39000 years without background subtraction.
f. In&oduction
As the name implies, accelerator mass spectrometry (AMS) requires the use of a particle accelerator. Almost any type capable of awlerating heavy ions to some fraction of a MeV/nu&on is suitabfe but the tandem accelerator has important advantages. Because of these and since most measurements have been made with tandems this review is confined to negative ion sources. In spite of present interests being limited to a few isotopes, AMS places extremely stringent requirements on the ion source. Some of these are as follows: (I) Czmmt. To exploit the prime advantage of AMS, namely the ability to measure extremely small isotopic ratios, requires a large current. For example, for an isotope present at the level of 10-14, the count rate is 425 per hour; assuming a negative ion current of 10 @A and an accelerator transmission of 20%. Thus a 5% stati~~~~y limited measurement can be made in about one hour but a I% measurement requires a prohibitively long time. (2) Ernitt~~~e. Since the goals of AMS are to measure small isotopic ratios with high precision, the accelerator transmission should be high and reproducible, This can be achieved only if the emittance of the source and injector is less than the acceptance of the acceierator. (3) Source material. An unusual but important requirement is that the source should produce the desired negative ion (and current) from a sample which is convenient to chemically separate e.g. AI- ions from aluminum oxide.
* Work supported by the National Science Foundation.
0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (Norm-Holland Physics Publishing Division)
(4) Memory eJ&fs. It is not unusual. for a discharge source to continue to give an appreciable yield long after the support gas is turned off or changed. In AMS this is intolerable and it is highly desirable that within 30 min of sample changing the current from the previous sample should be attenuated by a factor of between lo3 and 10’. (5) Ionization efficiency. Since samples are frequently small (e.g. a single cosmic spherule) the ionization efficiency should be high - preferabIy greater than 1%. (6) Stability. Although it is common practice to monitor the source output during measurement it is desirable that the negative ion current shouId not be subject to short term fluctuations or long term drifts. (7) Sample changing. It is highly desirable that ion source samples can be changed quickly and after changing the current should stabilize within 5 or 10 min. For the most precise work it might be desirable to interchange a sampie and a standard at 1 or 2 min intervals. 2. Available negative ion sources It is not easy to classify negative ion sources since, in final analysis, the operating principles blend together in an all but continuous spectrum. Generally speaking the main types are: (1) direct extraction from a ptasma, (2) cesium ion sputtering, (3) surface ionization and (4) charge exchange of positive ions. None are ideally suited to all of the needs of AMS but some are closer than others and a few general conclusions are possible. Sources of the direct extraction type usually require the sample to be in the form of a gas or a vapor and their applicability is limited. Also they have notoriously long memories which may all but preclude their usefulness to AMS. III. ION SOURCES ,’ INSTRUMENTATION
R. Middleton / Ion sources for A MS
194
The cesium sputter source has many advantages and probably comes closest to satisfying AMS requirements. It is versatile, can be used with solids and gases, sample changing is rapid and memory effects are small. However, even the latest high intensity versions are unable to produce the desirable 10 /LA minimum in all cases. Surface ionization sources, such as those using lanthanum hexaboride, are probably restricted to the halogens but have the potential of large currents and a high degree of selectivity. It is possible that they may find application in the selective ionization of 36C1 in the presence of sulfur. Since charge exchange in an alkali metal vapor frequently enables 10% or more conversion of positive to negative ions and there is available a wide range of intense positive ion sources, it provides an important method of generating intense negative ion beams. In practice such sources are difficult to operate stably over prolonged periods, the negative ion spectrum is often complex because of molecular dissociation and the emittance may vary with the density of the donor vapor. It should also be remembered that the requirements numerated in sec. 1 now pertain to the positive ion source. 3. Generation of specific negative ions 3.1. Beryllium For 3 MV or more tandems, BeO- is a satisfactory negative ion; the only disadvantage is that some loss of
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transmission is likely to occur from the Coulomb explosion if a stripper foil is used. Cesium sputter sources such as UNIS [l] and General Ionex 834 [2], with reflected geometry, typically yield about 0.5 PA from sputter targets of compressed beryllium oxide and have an ionization efficiency of between 0.1 and 0.2% [3]. Although 0.5 pA is adequate for many purposes higher currents are needed for samples with ‘*Be : 9Be ratios less than lo-“. The high intensity sputter source described in ref. [4] produces 10 to 20 times more current, has somewhat improved emittance and a higher ioni~tion efficiency. Fig. 1 shows the results of some measurements made with a high intensity source comparing cathodes containing pure Be0 and a 1: 5 mixture (by weight) with silver powder. The cathodes were made from copper and the powder was compacted into a 1.6 mm diameter by 1.6 mm deep hole. It is noteworthy that although higher currents were obtained from the pure sample, the current was slow in rising and took about 40 min to attain its maximum value. Also after about 100 min the current began to fall and at the end of 180 min was only 5 PA. The run was terminated at this time and the cathode was removed. It came as a surprise to find at least half of the Be0 still present and the reason for the drop in current is not understood. The sample with silver behaved quite differently. It attained its maximum current almost instantaneously, very rapidly fell to about 8 PA and then very slowly declined to 5 PIA over a 10 h period. The ionization efficiency for the sample with silver was high and about 1.8%.
-T----e--
I
12f ‘e’*e.*e
ll10 -t 9-
5.9mg .*
4.
l*
Be0
EFFIC. > 0.42 %
l*
?!
le
3.2mg EFFIC.-
BeO+ 16.3mg 1.8%
Ag
TERMINATED
Fig. 1.A comparison Be0 &ed
TIME
(MIN.)
of the negative ion Currents from a high intensity sputter source [4] obtained with cathodes containing Be0 and with silver powder (1: 5 by wt.). The reason for the decline in current from the pure Be0 is not understood and was not
due to sample exhaustion.
R. Middleton / Ion sources for AMS
Source memory effects are generally expected to be small for sputter sources, particularly when the target has low volatility. This indeed appears to be the case and Klein et al. [3] report observing a “Be : 9Be ratio of less than 2 x lo-” from a Be0 blank in a reflected geometry UNIS source. The effects are likely to be larger with high intensity sources [4] since the ionizer and sputter target are confined in a common chamber and the latter is at an elevated temperature. Because of this we have made many measurements on blanks containing commercially available Be0 and with these we typically observe a “Be: 9Be ratio of 6 X lo-r5. Since this is higher than expected from earlier studies made with cathodes containing large amounts of 26Al we recently measured the ‘(‘Be background from cathodes containing MgO and Ga,O,. In both cases the 9Be current was very small (< 1 nA with Ga,O,) and no “Be counts were obtained. This strongly suggests that the “Be observed from the commercial Be0 is not the result of source memory but is due to the oxide being contaminated. Fig. 2 shows a comparison of the 2-dimensional spectra obtained with blanks of Be0 and Ga,O,. Since the elemental ion 9Be- either has a small electron affinity or is metastable, it is only weakly formed in sputter sources. From the high intensity source the current is about 2.5% that of BeO- i.e. 0.2-0.25 PA (the yield is about the same from beryllium metal).
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Larger currents have been obtained from charge exchange sources and Heinemeier and Tykesson [S]) report obtaining 2.3 PA from the charge exchange of 100 PA of positive ions in sodium vapor. 3.2. Carbon Although sputter sources generate between 30 and 300 PA of 12C- ions from graphite, the yield from less dense forms of carbon is disappointing and frequently is more than an order of magnitude less. The problems of converting 14C dating samples into high yield sputter targets, while minimizing isotopic fractionation and contemporary contamination, have been the subject of much study and the reader is referred to refs. [6] and [7] and those contained therein. The most attractive source material is CO, and the generation of “C- ions directly from the gas is discussed in sect. 4. Failing this, CO, can be reduced to an amorphous carbon and its density increased by dissolving in iron or by graphitization. Alternatively the CO, can be converted into acetylene which can be cracked and deposited as a type of graphite. Since the steps involved in increasing the density increase the risk of contemporary contamination it was considered worthwhile testing pressed amorphous samples, with and without silver, in the high intensity source. Two cathodes were prepared from magnesium
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reduced CO,, one containing 10 parts by weight of silver. Both were pressed and hammered into 1.6 mm diameter by 1.6 mm deep cavities drilled into copper cathode blanks. Fig. 3 shows the observed currents as a function of time. That from the sample without silver climbed very slowly and was unsteady for the first 45 min; during this time it glowed very brightly. As the glow subsided the current became steadier but even after two hours it was still slowly rising. As expected the current from the sample with silver rose more quickly and stabilized earlier but the steady current was less. The currents from both are more than adequate and apart from the delay in attaining stability either would be satisfactory for accelerator t4C dating. 3.3. Aluminum Since aluminum has a small electron affinity (0.46 eV) it is a difficult negative ion to form by sputtering; Middleton [8] reports obtaining about 0.1 IJ.A from the metal in a UNIS source. Larger molecular beams can be obtained but these are less attractive for AMS since it is then no longer possible to capitalize on the fact that the stable isobar, 26Mg, does not form a stable negative ion. The difficulties of negative ion formation are further compounded by the compelling need to use aluminum oxide as a source material. A few years ago it was discovered [9] that 2-3 I_IA of *‘Al- ions can be obtained from aluminum oxide targets in a high intensity sputter source [4] with an ionization
efficiency of about 0.25%. Currents, like those from beryllium oxide and amorphous carbon, were observed to rise slowly and required about 40 min to reach maximum. Mixing with silver powder significantly reduced the rise time but resulted in about one-half the negative ion current. It was also observed that samples which were baked for about 30 min at 1200°C gave larger currents. Since terrestrial samples typically have 26A1: 27Al ratios of about lo-i4 it is desirable that negative ion currents should be 10 PA or larger. The prospects of obtaining such currents from a sputter source are not good and charge exchange might be a better approach. Preliminary experiments at the University of toronto [lo] have resulted in currents of 0.18 pA and with an ionization efficiency of 0.17%. It is anticipated that significantly larger currents will be attained with an improved positive ion source (the present source produces about 4 PA) and with the optimum choice of donor material (lithium vapor was used). 3.4. Chlorine Chlorine has the largest electron affinity of all the elements (3.62 eV) and intense beams can be formed by a variety of ways. However, since most samples for AMS are in the form of a solid, cesium ion sputtering is particularly convenient. The largest difficulty confronting the experimenter is reducing sulfur content since the isobar interfering with the detection of 36C1 is 36S.
R. Middleton / Ion sourcesfor AM.5 Fortunately the natural abundance of MEGis only 0.02% and chemical purification often suffices. An alternate and attractive approach is to form chlorine negative ions by surface ionization on a low work function surface such as lanthanum hexaboride. Since the electron affinity of sulfur (2.08 eV) is considerably less than that of chlorine the process should favor chlorine negative ion formation by several orders of magnitude. Although the possibilities are well-known, as far as the author is aware, no results of experimental studies have been published. 3.5. Calcium The negative ion of calcium, like that of beryllium, is thought to be metastable and it is extremely weakly formed by the sputter process. However, it has been shown [ll] that about 1 I_LAof CaH- and CaH; ions (CaH; is very weak) can be produced by sputtering a metallic surface sprayed with ammonia gas. Raisbeck et al. [12] suggest using CaH; ions since the interference caused by 41K while detecting 41Ca should be minimal. They report KH; to be an improbable negative ion and the ratio KH;/Kis 2 10W8. The disadvantage of the above is that it requires metallic calcium whereas most samples are likely to be oxides. Some recent experiments at the University of Rochester [13], where CaO- ions were accelerated, suggest that the difficulties caused by 4’K might have been overestimated. In view of this we have measured the yield of CaOions from compressed calcium oxide
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targets in a high intensity source. The negative ion is certainly much more difficult to form than BeO- and it was only with difficulty that 1 pA could be obtained.
4. Formation of 12C - ions from CO, gas Since it is relatively simple to convert most 14C dating samples into CO,, with minimal contamination risk, the gas is an ideal source material for “C- formation. This was appreciated by Heinemeier and Andersen [14] and they measured the yield of 12C- ions from an ANIS source operated on CO,. They obtained between 2 and 4 PA and determined the ionization efficiency to be about 0.15%. Not too surprisingly, since the source has a gas discharge, memory effects were observed to be relatively severe. The high intensity sputter source, described in ref. [4], appears at first sight not to be amenable to operation with gases. This is because the ionizer and sputter target are contained in the same chamber into which cesium vapor is admitted. It was thought that chemical interaction with the gas might deplete the cesium supply and impair the operation of the ionizer. Recent experiments with a variety of gases have shown this not to be a major difficulty. A sectional drawing of the cathode stem used to admit gases is shown in fig. 4. Gas from an external needle valve flows down a 1 mm bore stainless steel tube and enters the source through a 0.5 mm diameter hole drilled through the center of the cathode. The
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III. ION SOURCES
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/ Ion sources for AMS
cathodes were made from copper and usually contained a hard pressed insert, similar to the titanium one shown in fig. 4. The i2C- current from CO, gas has been measured with several different cathode inserts and roughly correlated with the getter ability of the material. For example, titanium (55 PA), zirconium (40 PA), scandium (35 PA), tantalum (30 PA), nickel (13 PA), magnesium (8 PA), copper (3 PA) and gold (1 PA). Measurements were made with fairly nominal source conditions, the cathode voltage was 6 kV and the cathode current 5-7 mA. Typically gas consumption was about 2-3 at. cm3/h. Some measurements were also made with carbon monoxide and acetylene. The carbon monoxide gave marginally more current but not sufficient to warrant the conversion of CO, into CO. Acetylene typically produced 2 or 3 times the current but the mass 13 current was about 20% that of mass 12, indicating an undesirably large i2CHcomponent. Acetylene also appeared to be cracking and depositing on the ionizer and posed a serious memory threat. Measurements on the tandem accelerator revealed that the transmission of carbon ions from CO, on titanium was sometimes less than that from graphite cathodes, particularly when the gas flow was high. It is thought that this was due to the large size of the titanium insert (- 5 mm diameter) and disproportionately more negative ions coming from the cesium beam halo than from the intense core. This difficulty was overcome by reducing the diameter of the inserts to
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Fig. 5. A negative ion spectrum obtained with CO, gas and a sputter cathode containing 100 mesh titanium powder similar to that shown in the inset sketch. It is noteworthy that the C; peak is relatively much weaker than is observed from graphite and the mass 13 to 12 ratio is about 1.6% - indicating the presence of some ‘*CH- ions.
1.6 mm and making them from pressed 100 mesh titanium powder as shown in the inset drawing in fig. 5. The i*C- current from these is less than that from a 5 mm diameter solid titanium insert but the maximum current of about 30 PA occurs at a much lower rate of gas flow. It was found that a gas flow which produced an almost imperceptible rise in the source base vacuum (- 3 x lo-’ Torr) produced an acceptable 23-25 PA. One such cathode was operated for 4 hours with a steady current of 24 /.tA on 1 at. cm3; this corresponds to an ionization efficiency of about 7.7%. Fig. 5 shows a typical negative ion spectrum obtained with such a cathode. Most titanium powder cathodes give a i2C- current of several microamps without gas when first put in the source and after lo-15 min of decline the current usually stabilized between 0.5 and 0.7 PA. The origin of this residual current is unknown and may be the result of a combination of gettering of residual hydrocarbons in the vacuum and carbon impurities in the titanium (99% purity). When gas is added the current almost instantaneously climbs to its final value and when shut off the fall is equally dramatic down to about 3 PA. Thereafter the fall is very slow and it is evident that some carbon is retained and a new cathode should be used with each gas sample. The extent of source memory effects have yet to be fully determined but some preliminary measurements have been made on the accelerator with contemporary (1850) CO, and some made from anthracite. Because of limited experience with 14C detection, the accelerator, beam transport system and detector were set up using a graphite cathode followed by measurements with the contemporary CO,. At a terminal voltage of -5.5 MV and with charge state 4 ions, the 14C count rate with the latter was about 20/s. This is a little less than expected from the 16 PA i2C- current and the measured 13C transmission of 20% and corresponds to an overall transmission of about 17% for 14C ions. After about one hour of measurements the titanium powder cathode was replaced by a new one and after about 15 min of operation the 14C count rate was again measured. With a 12C- current of about 0.4 PA this was 0.2/s. Assuming that the transmission was still 17% for 14C ions this corresponds to a surprisingly young age of 7600 years for the background carbon. Since the i4C background was not measured prior to admitting the contemporary CO, it is not known whether this is due to source memory. CO, prepared from the anthracite was then admitted and after the 12C- current had stabilized around 18 PA the 14C was counted. The count rate was unchanged and remained 0.2/s. Assuming 17% transmission for the 14C ions, this corresponds to a sample age of 39 000 years. To summarize, it has been demonstrated that ‘*C-
R. Middleton / Ion sources for AMS
currents of between 20 and 25 PA can be obtained from CO, gas. The gas consumption is low (- 0.25 at. cm3/h) and the ionization efficiency is high (- 7.7%). Of particular importance is the fact that the 14C background can be measured in a meaningful way before the sample gas is admitted - and if necessary corrected for. The complete absence of ion source memory has not been demonstrated but it has at least been shown to be small. Note added in proof: Since the Conference, we have definitely established that beryllium oxide from four apparently independent suppliers contains “Be at a concentration of about lo-r4 [15]. In particular we have shown that Be0 extracted from beryl crystals from a mine does not contain “Be.
References [l] R. Middleton and C.T. Adams, Nucl. Instr. and Meth. 118 (1974) 329. [2] G. Braun-Elwert, J. Huber, G. Korschinek and Kutschera, Nucl. Instr. and Meth. 146 (1977) 121.
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[3] J. Klein, R. Middleton and Hongqing Tang, Nucl. Instr. and Meth. 193 (1982) 601. [4] R. Middleton, Nucl. Instr. and Meth. 214 (1983) 139. [S] J. Heinemeier and P. Tykesson, Nucl. Instr. and Meth. 141 (1977) 183. (61 R. Gillespie and R.E.M. Hedges, Radiocarbon 25 (1983) 771. [7] J.S. Vogel, LG. Nowikow, J.R. Southon and D.E. Nelson, Radiocarbon 25 (1983) 775. [8] R. Middleton, Nucl. Instr. and Meth. 144 (1977) 373. 191 R. Middleton, J. Klein, G.M. Raisbeck and F. Yiou, Nucl. Instr. and Meth. 218 (1983) 430. [lo] G.E. Aardsma, Ph.D. Thesis, University of Toronto, Canada (1984). [ll] R. Middleton, Nucl. Instr. and Meth. 141 (1977) 373. [12] G.M. Raisbeck, F. Yiou, A. Peghaire, J. Guillot and J. Uzureau, Proc. Symp. on Accelerator mass spectrometry, Argonne National Lab. (1981) p. 426. [13] D. Elmore, University of Rochester, private communication. [14] J. Heinemeier and H.H. Andersen, Radiocarbon 25 (1983) 761. [15] R. Middleton, J. Klein, L. Brown and F. Tera, Nucl. Instr. and Meth. B5 (1984) in press.
III. ION SOURCES
/ INSTRUMENTATION