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The chlorine-36 measurement program at the Australian National University
114
Nuclear Instruments and Methods in Physics Research B29 (1987) 114-119 North-Holland. Amsterdam
THE CHLORYlE-36 MEASUREMENT PROGRAM AT THE AUSTRALIAN NATIONAL UNIVERSITY
L.K. FIFIELD ‘), T.R. OPHEL ‘), J.R. BIRD 2), G.E. CALF 2), G.B. ALLISON 3, and A.R. CHIVAS 4, ‘) Department of Nuclear Physics, Awtralian National University, GPO Box 4, Canberra, ACT 2601, Australia ‘) Australian Atomic Energy Commission, Lucas Heights, NS W 2232, Australia ‘) CSIRO Division of Soils, Adelaide, SA 5046, Australia 4, Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia
The chlorine-36 measurement system at the Australian National University is described and some early results are presented.
1. introduction
A chlorine-36 dating capability based on the 14UD pelletron accelerator was developed at the Australian National University during 1986 and is now entering the routine measurement phase. It involves a collaboration between the Department of Nuclear Physics, The Australian Atomic Energy Commission and CSIRO Division of Soils. The system is similar to that developed at the University of Rochester [l] but has some novel features. 2. The AMS system The essential components of the AMS system are shown in fig. 1, and are discussed individually below. 2.1. The ion source Beams of Cl- ions are obtained from silver chloride in a Hiconex 832 source and accelerated to 150 keV before mass analysis and injection into the accelerator. This source still operates in nonreflected geometry, and the AgCl is pressed into a copper holder with a conical former. A 1.6 mm diameter hole is drilled through the AgCl in the base of the cone. A minimum of 100 mg of material is required, but thus far this has not been a limitation. Source output is 15 /.tA after 1 h, but reaches 50% of this value after the first 10 min when measurements usually commence. Because of the large area of AgCl presented to the Cs beam, backgrounds from the copper holder are very low in this ion source. 2.2. Sample preparation The AgCl is purified to reduce its sulphur content in a similar manner to that described by Conard et al. [2]. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Before pressing into cones, it is dried in a vacuum oven for 12 h in order to minimize the counting rate of l*O’+ ions. 2.3. The low-energy beam chopper Because it is not possible to inject more. than a few PA of Cl- ions into the accelerator without beam-loading effects, some means of attenuating the stable chlorine beams, but not the flux of chlorine-36 ions, is required if the maximum capability of the ion source is to be utilized. This is accomplished by a beam chopper located just after the pre-acceleration section of the ion source. This chopper applies +400 V to two 5 cm long plates separated by 2 cm. The “volts on” and “volts off” periods are independently adjustable, and are typically 1.4 ms and 100 ~LSrespectively; i.e. the chopper transmits l/15 of the stable beams. The rise and fall times of the voltages on the plates are < 50 ns. The chopper is turned off when chlorine-36 is being counted. 2.4. The accelerator and beam transport The negative chlorine ions are stripped in the terminal of the accelerator by 2-3 pg/cm’ carbon foils. After the second stage of acceleration they pass around a 90 o analysing magnet and thence directly to the detection system comprising a retractable Faraday cup for the stable beams followed by an ionisation chamber for the chlorine-36 ions. Due to the high vacuum (- 4 x lo-* Torr) maintained in the tubes and beam line of the 14UD accelerator, it has not been necessary to incorporate any additional analysis by magnetic and/or electrostatic elements in the system. During a measurement sequence, the elements of the beam transport system after the accelerator are kept fixed, and only the terminal voltage, source inflection magnet and low-energy quadrupoles are adjusted in
L.K. Fifield et al. / “?I measurement program at ANV
BEAM
CHOPPER
115
*
r---INFLECTION
MAGNET
*
* TANK
-14UD
I= -FOIL
-HIGH
*
COMPUTER
CUP
ACCELERATOR
STRIPPER
VOLTAGE 12.34MV
%
12.OOMV
%I
11.66MV
37CI
TERMINAL*
CONTROLLED
ELEMENTS
L J.-HE
CUP
-SLITS
./
r-------
/r
---. r: Li L.___________J
SLITS -
ANALYSING
MPGNET
,I zz
i
tr STO;
CUP
Fig. 1. The AMS system based on the ANU 14UD pelletron accelerator. II. FACILITY
REPORTS
116
L. K. Fifield et al. / 36CI measurement program at A NU
order to change from 35C1 to 37C1 to 36C1. These adjustments are made under the control of a PDP11/24 computer, which also controls the operation of the Faraday cups and the beam chopper. The analysing magnet is set to transmit 132 MeV 36C110+ and hence terminal voltages of 12.00, 12.34 and 11.68 MV are required for 36C1, 35C1 and 37C1 respectively. A typical measurement sequence consists of 30 s measurements of the 35C1 and 37C1 beam currents, followed by a 10 min counting period for 36C1 and is repeated as many times as necessary. Both the 3’C1 and 37C1 beam currents are measured to allow the calculation of a correction for fractionation in the terminal stripper foil. The 35C1 : 37C1 ratio measured at the Faraday cup immediately upto the stream of the detector is - 4.5 : 1 compared isotopic abundance ratio of 3 : 1.
2.5. The heavy ion detector Fig. 2 shows the heavy ion detector used to detect the chlorine-36 ions. It is a double-gridded ionisation chamber filled with isobutane at a pressure of 112 Torr. A total energy signal is obtained from the Faraday cage formed by the cathode and first grid, while several energy loss signals are available from the segmented anode plane. A number of ion species are incident on the detector as shown in fig. 3. The 35C1, 37C1 and “C ions all arise from charge-exchange events in the high-energy section of the accelerator which produce ions of the correct rigidity to pass round the 90° analysing magnet. The “0 ions are injected into the accelerator as “0; molecular ions, and together with the 36S ions are
RESIDUAL ENERGY
36
c,
IO+
132 Me V
I 100 TORR ISOBUTANE ?!?dT l+m
MYLAR
cm.
Fig. 2. The heavy-ion detector and plan view of its anode plane.
117
L. K. Fifield et al. / 36CImeasurementprogram at ANU
I
,, :
36s10'
‘#2$+
:
?yc,* C 3+
ions are not on-axis, and enter the detector 2-3 mm lower than the 36C1 ions. Hence a measurement of the vertical height of ions entering the detector is sufficient to discriminate between the two. This is accomplished by the “sawtooth” AE, electrode in the vertically-oriented anode plane. Ions passing through the centre of the detector give equal “top” and “bottom” signals, whereas the “maverick” 37C1 ions give a larger “bottom” than “top” as illustrated in fig. 5. Resolution figures (fwhrn) for the various signals are 0.7% for E, 4% for AE, and AE,, and 3% for AE,. Typical counting rates in the detector are 5-100 s-l of 36S, lo-40 s-i of 37C1, 5-50 s-l of “0, with lower rates for other.ion species.
ENERGY
Fig. 3. Two-dimensional plot of energy loss versus total energy measured by the heavy-ion detector, showing the various ion species incident on the detector.
3. Measurements 3.1. Sensitivity and background
transmitted to the detector along with the 36C1 ions. The clearest separation between the 3aS’o+, 36C110t and 37C110+ ions is obtained from the combination of total energy and residual energy measurements as shown in fig. 4. The gas pressure is just sufficient to stop the 36S ions. Although not evident in figs. 3 and 4, there is one source of background which is not completely separated from the 36C1 ions by the total energy and the several energy loss signals. This arises from 132 MeV 37C110+ ions (the intense 37C1’o+ group corresponds to an energy of 128.4 MeV) which charge exchange to 11+ at the particular point in their trajectory round the 90’ analysing magnet which allows them to pass through the subsequent energy-defining slits. However, these
r
Two samples of Weeks Island halite processed in different laboratories have been measured in order to determine the sensitivity of the systems and the background of 36C1 from all sources. A sample was also prepared from Merck suprapure NaCl to determine whether it too could be used to determine system background. The results are presented in table 1. All three measurements represent counting times of 30 min. Evidently, a sensitivity of 1 x lo-l5 can be readily achieved in one hour and the system background is of the same order of magnitude. 3.2. Standards and reproducibility The measurements reported here consist of an absolute determination of the ratio of chlorine-36 to total chlorine which is derived from the observed currents of the stable beams and the counting rate of 36C1 ions in the detector. In order to test this approach, measurements have been made on a number of standards (obtained by tortuous paths from J. Fabryka-Martin, University of Arizona). The results are shown in table 2 and span a number of different runs over a period of six months. The agreement between the nominal and measured ratios is generally better than 5%.
Table 1 Low level samples
_ RESIDUAL
Sample
36Cl/Cl (X 10’5)
Weeks Island halite, processed at CSIRO, Adelaide Weeks Island halite, processed at RSES, ANU Merck suprapure NaCl processed at RSES, ANU
1.3 * 0.6 6 *2 3 f1.5
ENERGY
Fig. 4. Two-dimensional plot of total energy versus residual energy measured by the detector, showing the separation of ‘6C1 from ‘% and 37C1ions.
II. FACILITY REPORTS
118
L K. Fifield et al. / “Cl measurement program at ANU
w
a 1
AE3
“TOP”
Fig. 5. Two-dimensional plot of the “bottom” versus the “top” signals from the sawtooth A& electrode. The box to the right duplicates the box within the main body of the figure, but contains only those events that satisfy a “36Clr, requirement in residual energy and total energy (see fig. 4).
Measurements have also been made in four separate runs on a sample of halite from the Meadowbank mine in Cheshire, and yielded values of 32 f 8,34 _t 7,45 f 18 and 32 f 7 (all ~10~r5) for the ratio of chlorine-36 to total chlorine. 3.3. Australian salt lakes Samples of chloride from a number of salt lakes, mainly in Western Australia, have been studied in order to assist in the determination of the origin of the salt. The results of the measurements are shown in table 3. There is little variation in the chlorine-36 ratio from lake to lake, and the values obtained are close to the mean value for granite, suggesting that the predominant source of the salt is the weathering of surface rocks.
3.4. SoilprofiIe from a semi-arid region of South Australia Preliminary measurements have been made on samples of chloride from the soil at various depths beneath
a vegetated dune in a semi-arid region of South Australia. The motivation for these measurements is to determine the rate at which salt travels down through the unsaturated zone and to measure changes in this Table 3 Australian salt lakes NO.
Table 2 Standards 36c1/c1 (X
2490 10000
36Cl/Cl (X 10’5)
L. Amadeus, Northern Territory L. Walpamunda, NW Victoria L. Ballard, Western Australia L. Lefroy, Western Australia L. Deborah E., Western Australia L. Koolkooldine, Western Australia L. Campion, Western Australia L. near Cunderdin, Western Australia
53*10 59* 7 51* 6 30* 5 57f 8 45* 8 29* 6 31* 5
10’5)
Nominal value a) 500
Name
Measured vahte b, 605f 90 530& 60 2350*150 2470 f 100 9700 f 250 9650f250 9540*200
a) Standards from J. Fabryka-Martin, University of Arizona. b, Individual measurements made during different runs.
Table 4 Soil profile from a vegetated dune in a semi-arid region of South Australia
Depth (ml
36Cl/Cl( x 10’5)
l-l.5 12 20 > 28
350 f 20 70* 9 49* 5 39f 7
L. K. FifieId et al. / %I
measuremeni program at ANU
119
rate due to changes in land use. The results to date are presented in table 4, and show clearly the effect of the bomb spike in the 1-1.5 m zone.
(b) Studies of water from a number of underground aquifers, including the Great Artesian Basin. (c) Study of a core from Lake Eyre.
4. Future plans
References
The system has been developed to is possible to measure 24 samples in the next year it will be employed projects, including the following. (a) Further studies of soil profiles in basin, which have relevance to salinity in this basin.
the point where it a 3-day run. Over in a number of the Murray River the problems of
111 D. Elmore, B.R. Fulton, M.R. Clover, J.R. Marsden, H.E.
Gove, H. Naylor, K.H. Purser, L.R. Kilius, R.P. Beukens and A.E. Litherland, Nature 277 (1979) 22. PI N.J. Conard, D. Elmore, P.W. Kubik, H.E. Gove, L.E. Tubbs, B.A. Chrunyk and M. Wahlen, Radiocarbon 28 (1986) 556.
II. FACILITY
REPORTS
Nuclear Instruments and Methods in Physics Research B29 (1987) 114-119 North-Holland. Amsterdam
THE CHLORYlE-36 MEASUREMENT PROGRAM AT THE AUSTRALIAN NATIONAL UNIVERSITY
L.K. FIFIELD ‘), T.R. OPHEL ‘), J.R. BIRD 2), G.E. CALF 2), G.B. ALLISON 3, and A.R. CHIVAS 4, ‘) Department of Nuclear Physics, Awtralian National University, GPO Box 4, Canberra, ACT 2601, Australia ‘) Australian Atomic Energy Commission, Lucas Heights, NS W 2232, Australia ‘) CSIRO Division of Soils, Adelaide, SA 5046, Australia 4, Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia
The chlorine-36 measurement system at the Australian National University is described and some early results are presented.
1. introduction
A chlorine-36 dating capability based on the 14UD pelletron accelerator was developed at the Australian National University during 1986 and is now entering the routine measurement phase. It involves a collaboration between the Department of Nuclear Physics, The Australian Atomic Energy Commission and CSIRO Division of Soils. The system is similar to that developed at the University of Rochester [l] but has some novel features. 2. The AMS system The essential components of the AMS system are shown in fig. 1, and are discussed individually below. 2.1. The ion source Beams of Cl- ions are obtained from silver chloride in a Hiconex 832 source and accelerated to 150 keV before mass analysis and injection into the accelerator. This source still operates in nonreflected geometry, and the AgCl is pressed into a copper holder with a conical former. A 1.6 mm diameter hole is drilled through the AgCl in the base of the cone. A minimum of 100 mg of material is required, but thus far this has not been a limitation. Source output is 15 /.tA after 1 h, but reaches 50% of this value after the first 10 min when measurements usually commence. Because of the large area of AgCl presented to the Cs beam, backgrounds from the copper holder are very low in this ion source. 2.2. Sample preparation The AgCl is purified to reduce its sulphur content in a similar manner to that described by Conard et al. [2]. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Before pressing into cones, it is dried in a vacuum oven for 12 h in order to minimize the counting rate of l*O’+ ions. 2.3. The low-energy beam chopper Because it is not possible to inject more. than a few PA of Cl- ions into the accelerator without beam-loading effects, some means of attenuating the stable chlorine beams, but not the flux of chlorine-36 ions, is required if the maximum capability of the ion source is to be utilized. This is accomplished by a beam chopper located just after the pre-acceleration section of the ion source. This chopper applies +400 V to two 5 cm long plates separated by 2 cm. The “volts on” and “volts off” periods are independently adjustable, and are typically 1.4 ms and 100 ~LSrespectively; i.e. the chopper transmits l/15 of the stable beams. The rise and fall times of the voltages on the plates are < 50 ns. The chopper is turned off when chlorine-36 is being counted. 2.4. The accelerator and beam transport The negative chlorine ions are stripped in the terminal of the accelerator by 2-3 pg/cm’ carbon foils. After the second stage of acceleration they pass around a 90 o analysing magnet and thence directly to the detection system comprising a retractable Faraday cup for the stable beams followed by an ionisation chamber for the chlorine-36 ions. Due to the high vacuum (- 4 x lo-* Torr) maintained in the tubes and beam line of the 14UD accelerator, it has not been necessary to incorporate any additional analysis by magnetic and/or electrostatic elements in the system. During a measurement sequence, the elements of the beam transport system after the accelerator are kept fixed, and only the terminal voltage, source inflection magnet and low-energy quadrupoles are adjusted in
L.K. Fifield et al. / “?I measurement program at ANV
BEAM
CHOPPER
115
*
r---INFLECTION
MAGNET
*
* TANK
-14UD
I= -FOIL
-HIGH
*
COMPUTER
CUP
ACCELERATOR
STRIPPER
VOLTAGE 12.34MV
%
12.OOMV
%I
11.66MV
37CI
TERMINAL*
CONTROLLED
ELEMENTS
L J.-HE
CUP
-SLITS
./
r-------
/r
---. r: Li L.___________J
SLITS -
ANALYSING
MPGNET
,I zz
i
tr STO;
CUP
Fig. 1. The AMS system based on the ANU 14UD pelletron accelerator. II. FACILITY
REPORTS
116
L. K. Fifield et al. / 36CI measurement program at A NU
order to change from 35C1 to 37C1 to 36C1. These adjustments are made under the control of a PDP11/24 computer, which also controls the operation of the Faraday cups and the beam chopper. The analysing magnet is set to transmit 132 MeV 36C110+ and hence terminal voltages of 12.00, 12.34 and 11.68 MV are required for 36C1, 35C1 and 37C1 respectively. A typical measurement sequence consists of 30 s measurements of the 35C1 and 37C1 beam currents, followed by a 10 min counting period for 36C1 and is repeated as many times as necessary. Both the 3’C1 and 37C1 beam currents are measured to allow the calculation of a correction for fractionation in the terminal stripper foil. The 35C1 : 37C1 ratio measured at the Faraday cup immediately upto the stream of the detector is - 4.5 : 1 compared isotopic abundance ratio of 3 : 1.
2.5. The heavy ion detector Fig. 2 shows the heavy ion detector used to detect the chlorine-36 ions. It is a double-gridded ionisation chamber filled with isobutane at a pressure of 112 Torr. A total energy signal is obtained from the Faraday cage formed by the cathode and first grid, while several energy loss signals are available from the segmented anode plane. A number of ion species are incident on the detector as shown in fig. 3. The 35C1, 37C1 and “C ions all arise from charge-exchange events in the high-energy section of the accelerator which produce ions of the correct rigidity to pass round the 90° analysing magnet. The “0 ions are injected into the accelerator as “0; molecular ions, and together with the 36S ions are
RESIDUAL ENERGY
36
c,
IO+
132 Me V
I 100 TORR ISOBUTANE ?!?dT l+m
MYLAR
cm.
Fig. 2. The heavy-ion detector and plan view of its anode plane.
117
L. K. Fifield et al. / 36CImeasurementprogram at ANU
I
,, :
36s10'
‘#2$+
:
?yc,* C 3+
ions are not on-axis, and enter the detector 2-3 mm lower than the 36C1 ions. Hence a measurement of the vertical height of ions entering the detector is sufficient to discriminate between the two. This is accomplished by the “sawtooth” AE, electrode in the vertically-oriented anode plane. Ions passing through the centre of the detector give equal “top” and “bottom” signals, whereas the “maverick” 37C1 ions give a larger “bottom” than “top” as illustrated in fig. 5. Resolution figures (fwhrn) for the various signals are 0.7% for E, 4% for AE, and AE,, and 3% for AE,. Typical counting rates in the detector are 5-100 s-l of 36S, lo-40 s-i of 37C1, 5-50 s-l of “0, with lower rates for other.ion species.
ENERGY
Fig. 3. Two-dimensional plot of energy loss versus total energy measured by the heavy-ion detector, showing the various ion species incident on the detector.
3. Measurements 3.1. Sensitivity and background
transmitted to the detector along with the 36C1 ions. The clearest separation between the 3aS’o+, 36C110t and 37C110+ ions is obtained from the combination of total energy and residual energy measurements as shown in fig. 4. The gas pressure is just sufficient to stop the 36S ions. Although not evident in figs. 3 and 4, there is one source of background which is not completely separated from the 36C1 ions by the total energy and the several energy loss signals. This arises from 132 MeV 37C110+ ions (the intense 37C1’o+ group corresponds to an energy of 128.4 MeV) which charge exchange to 11+ at the particular point in their trajectory round the 90’ analysing magnet which allows them to pass through the subsequent energy-defining slits. However, these
r
Two samples of Weeks Island halite processed in different laboratories have been measured in order to determine the sensitivity of the systems and the background of 36C1 from all sources. A sample was also prepared from Merck suprapure NaCl to determine whether it too could be used to determine system background. The results are presented in table 1. All three measurements represent counting times of 30 min. Evidently, a sensitivity of 1 x lo-l5 can be readily achieved in one hour and the system background is of the same order of magnitude. 3.2. Standards and reproducibility The measurements reported here consist of an absolute determination of the ratio of chlorine-36 to total chlorine which is derived from the observed currents of the stable beams and the counting rate of 36C1 ions in the detector. In order to test this approach, measurements have been made on a number of standards (obtained by tortuous paths from J. Fabryka-Martin, University of Arizona). The results are shown in table 2 and span a number of different runs over a period of six months. The agreement between the nominal and measured ratios is generally better than 5%.
Table 1 Low level samples
_ RESIDUAL
Sample
36Cl/Cl (X 10’5)
Weeks Island halite, processed at CSIRO, Adelaide Weeks Island halite, processed at RSES, ANU Merck suprapure NaCl processed at RSES, ANU
1.3 * 0.6 6 *2 3 f1.5
ENERGY
Fig. 4. Two-dimensional plot of total energy versus residual energy measured by the detector, showing the separation of ‘6C1 from ‘% and 37C1ions.
II. FACILITY REPORTS
118
L K. Fifield et al. / “Cl measurement program at ANU
w
a 1
AE3
“TOP”
Fig. 5. Two-dimensional plot of the “bottom” versus the “top” signals from the sawtooth A& electrode. The box to the right duplicates the box within the main body of the figure, but contains only those events that satisfy a “36Clr, requirement in residual energy and total energy (see fig. 4).
Measurements have also been made in four separate runs on a sample of halite from the Meadowbank mine in Cheshire, and yielded values of 32 f 8,34 _t 7,45 f 18 and 32 f 7 (all ~10~r5) for the ratio of chlorine-36 to total chlorine. 3.3. Australian salt lakes Samples of chloride from a number of salt lakes, mainly in Western Australia, have been studied in order to assist in the determination of the origin of the salt. The results of the measurements are shown in table 3. There is little variation in the chlorine-36 ratio from lake to lake, and the values obtained are close to the mean value for granite, suggesting that the predominant source of the salt is the weathering of surface rocks.
3.4. SoilprofiIe from a semi-arid region of South Australia Preliminary measurements have been made on samples of chloride from the soil at various depths beneath
a vegetated dune in a semi-arid region of South Australia. The motivation for these measurements is to determine the rate at which salt travels down through the unsaturated zone and to measure changes in this Table 3 Australian salt lakes NO.
Table 2 Standards 36c1/c1 (X
2490 10000
36Cl/Cl (X 10’5)
L. Amadeus, Northern Territory L. Walpamunda, NW Victoria L. Ballard, Western Australia L. Lefroy, Western Australia L. Deborah E., Western Australia L. Koolkooldine, Western Australia L. Campion, Western Australia L. near Cunderdin, Western Australia
53*10 59* 7 51* 6 30* 5 57f 8 45* 8 29* 6 31* 5
10’5)
Nominal value a) 500
Name
Measured vahte b, 605f 90 530& 60 2350*150 2470 f 100 9700 f 250 9650f250 9540*200
a) Standards from J. Fabryka-Martin, University of Arizona. b, Individual measurements made during different runs.
Table 4 Soil profile from a vegetated dune in a semi-arid region of South Australia
Depth (ml
36Cl/Cl( x 10’5)
l-l.5 12 20 > 28
350 f 20 70* 9 49* 5 39f 7
L. K. FifieId et al. / %I
measuremeni program at ANU
119
rate due to changes in land use. The results to date are presented in table 4, and show clearly the effect of the bomb spike in the 1-1.5 m zone.
(b) Studies of water from a number of underground aquifers, including the Great Artesian Basin. (c) Study of a core from Lake Eyre.
4. Future plans
References
The system has been developed to is possible to measure 24 samples in the next year it will be employed projects, including the following. (a) Further studies of soil profiles in basin, which have relevance to salinity in this basin.
the point where it a 3-day run. Over in a number of the Murray River the problems of
111 D. Elmore, B.R. Fulton, M.R. Clover, J.R. Marsden, H.E.
Gove, H. Naylor, K.H. Purser, L.R. Kilius, R.P. Beukens and A.E. Litherland, Nature 277 (1979) 22. PI N.J. Conard, D. Elmore, P.W. Kubik, H.E. Gove, L.E. Tubbs, B.A. Chrunyk and M. Wahlen, Radiocarbon 28 (1986) 556.
II. FACILITY
REPORTS