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The application of high energy prompt gamma-ray spectrometry to 3He activation analysis of light elements
NuclearInstruments and Methods in Physics Research North-Holland
B66(1992) 43-47
Nuclearhtshumm ils &Methods in Physics Resemch Section B
The application of high energy prompt gamma-ray spectrometry to 3 He activation analysis of light elements A .E. Pillay ' and M. Peisach
National Accelerator Centre, P. O. Box 72, Faure, 7131 South Africa
Prompt high energy gamma rays induced by moderate beam currents of 2 MeV 3 He+ ions on thick targets of boron, carbon, nitrogen and oxygen were evaluated for analytical purposes . Gamma rays were observed from (3He, p), ( 3He, a) and Coulomb excitation reactions . Interference-free determination of boron, and carbon was achieved at concentrations of a few parts per thousand. At these concentrations a precision of about 10% was attainable. The technique was tested by determining carbon in standard steel specimens. 1. Introduction
2. Experimental
There is a widespread need for methods to determine light elements at all levels of concentration. It is generally accepted that wet chemical methods are tedious and unsuitable for routine quantitative determination of the light elements [1], while instrumental methods such as PIXE, XRF or neutron activation are suitable for elements with atomic numbers greaterthan 11 . Probably one of the few instrumental methods that are capable of handling such problems is ICP, but it is a destructive method, the sample pre-treatment of which is time-consuming especially if interfering components of the matrix have to eliminated by chemical complexation. The matrices in which light elements have to be determined vary greatly. Boron forms a vital component of glasses, is used in nuclear reactor materials, forms an essential element of some semiconductor devices and is used in metallurgy . Carbon, nitrogen and oxygen are essential elements in biochemical and physiological systems and their determination is important in studies of the environment and its pollution. Low energy gamma rays, from activation by protons, deuterons and 'He' ions, have been widely studied for analytical use [2-5], but the sensitivity is often affected by the level of the background and the intensity of the annihilation radiation of 511 keV. In the case of activation with 'He+ ions, the reactions of interest are highly exoergic, resulting in the emission of energetic prompt gamma rays where background interference is much reduced. The analytic potential of lowerenergy gamma rays has already been evaluated [6].
2.1. Materials for analysis
t
Department of Chemistry, University of the Witwatersrand, P.O . Wits, 2050 South Africa .
Targets of boron and carbon were prepared by compressing powders in a 10 ton press to produce tablets of 13 mm diameter and about 2 mm thickness . Targets of oxygen and nitrogen were similarly prepared by using powders of Y203 and GaN or BN respectively. The heavy element component in these targets were not expected to interfere since the calculated Coulomb barriers [7] for Y and Ge were about 15.6 and 31 .2 MeV respectively, which was far in excess of the energy used for the investigation (see below). In addition, a target disc of Al was bombarded, because the beam tube and scattering chamber material consisted mainly of aluminium, from which a background of gamma rays was expected . 2.2. Irradiation and measurement
Targetswere mounted on a remote-controlled vertical ladder, that operated through a geared stepping motor at the rate of 150 steps per mm, which enabled each target to be mounted with a precision of about 7 wm . The ladder fitted into a multipurpose scattering chamber [8], where a constant geometrical arrangement of target and detector was rigidly maintained . Beams of 2 MeV 'He' were obtained from the 6 MV Van de Graaff accelerator at Faure, and beam currents were adjusted so that the dead time of the entire counting assembly did not exceed 10%. Gamma-ray spectrometry was carried out with a 80 cm' Ge(Li) detector mounted outside the scattering chamber at 45° to the bombarding beam, where the aluminium chamber wall was thinned to reduce absorption of low
0168-583X/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
1. GENERAL METHODS
A.E Allay, M. Peisach / High energy prompt gamma-ray spectrometry
energy radiation . In an attempt to reduce the background due to the interaction between scattered particles and the aluminium walls of the scattering chamber, a Perspex sleeve, perforated to allow entrance to the beam, was mounted around the target assembly . Data were recorded on magnetic tape and processed off-line .
3. Results and discussion Energetically, all ('He, y), ('He, n), ('He, p), ('He, d) and ( 3 He, a) reactions on the stable isotopes of boron, carbon, nitrogen and oxygen are possible, with a bombarding beam of 2 MeV 3He ions, with the exception of 1Z C(3He, 3N, 16 0(3 He, 0 18 17 and 16 0(3 He, 7F which are endoergic with Q < -3 MeV. Because most reactions are highly exoergic, the reaction products are produced from highly excited states with the result that the gamma ray spectra are complex. The high energy prompt gamma rays which have analytical potential are those for which interference from other matrix components is unlikely. Since the Coulomb barrier [7] reaches about 3 MeV for Z = 10 (neon) the main interferences are likely to come from the light elements themselves, or from secondary nuclear effects generated by the very energetic protons and alpha particles produced in the reactions with the light elements under investigation.
0
0
3.1. Boron The high energy region of a typical gamma ray spectrum obtained from the bombardment of a boron target is shown in fig. 1. In this and subsequent figures, the gamma rays are labelled according to the Chemists' Convention, which lays most stress on the target nuclide [9]. The most prominent peak in the spectrum is due to the Doppler-broadened 4450 keV gamma ray from the Coulomb excitation of boron-11 . Since the Q-values for the (3 He, p) and (3 He, a) reactions on boron-11 are very high (13.185 and 9.122 MeV respectively) and the corresponding reactions on boron-10 have even higher Q-values, a flux of high energy particles is generated within the boron target . Under these conditions, Coulomb excitation occurs producing the 4450 keV gamma rays. Other relatively intense gamma rays are those from the reaction "B( 3 He, p)13C. The spectrum is complicated by the escape peaks, but even these may be used for analyses. 3.2 Carbon Fig. 2 shows the high energy region of the gamma ray spectrum obtained from the bombardment of graphite. The most prominent peaks are due to the 5106 keV 12 C p(4, 0) gamma rays . From the same reaction, the 4915 keV 12 C p(2, 0) gamma rays were detected, while the other prominent peak is due to the 2799 keV 12 C a(3, 1) gamma ray. The importance of
BORON
1200
1300
1400
1500
3600
1700
1800
1900
Channel number
2000
Fig. 1. The high energy portion of the gamma-ray spectrum obtained from a thick borontarget bombarded with
2 MeV 3He
ions.
45
A . E. Allay, M. Peisach / High energy prompt gamma-ray spectrometry CARBON
1200
1300
1400
1600
1500
1700
1800
. 1900 . 2000 , 2100 2200
2300
2400
Channel number
Fig. 2. The high energy portion of the gamma-ray spectrum obtained from a thick target of graphite bombarded with 2 MeV 3He + ions .
gen are those due to the 6130 keV t4N p(2, 0) gamma rays, as is shown in fig . 3. A very low intensity peak was observed from the 6917 keV '4N p(3, 0) gamma ray, which may be useful for analysis when high energy 3 He + ions are used . The importance of the 6130 keV gamma ray lies in the fact that the background against which it has to be measured is very low .
the relatively intense gamma rays lies in the fact that they can be measured without interference . 3.3. Nitrogen While several low energy gamma rays are suitable for analysis [6], the only prominent peaks due to nitro-
BORON NITRIDE
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
3400
Channel number
Fig . 3. The high energy portion of the gamma-ray spectrum obtained from a thick target of boron nitride bombarded with 2 MeV 3 He * ions. Peaks due to boron have not been labelled. I. GENERAL METHODS
A. E Pillay, M. Peisach / High energy prompt gamma-ray spectrometry
46
Table 1 Analytically useful 3 He-induced prompt high energy gamma rays Target element
E.,, [keVI
Assignment
Sensitivity [gMCI
Boron
3684 3854 4450
" B p(2,0) 11 B p(3, 0) "B (2, 0) -
37 mg 17 mg 1.8 mg
Carbon
2799 3948 4915 5106
12C a(3,1) 12C p(2, 0) 12C p(3, 0) 12C p(4, 0)
75 mg 5.2 mg 3.1 mg
Nitrogen
6130 6917
14N p(2, 0) 14N 03,0)
Oxygen
nil
a
Table 2 Carbon content of some standard steels Steel
Counts [WC]
Carbon content [%] Known Found Error
Relative error [%]
262 11263 1264
2.14±0.40 3.97±0.45 5.92±0.81
0.160 0.62 0.870
+14.4 -12.7 +5.9
0.183 0.541 0.920
+0 .023 -0 .079 +0 .050
10 mg
radiation was calculated as the weight of element which will produce a signal equivalent to about 3 .29 times the square root of the background value over that energy region [10]. Obviously the sensitivity will be a function of the integrated beam current . The values given in table 1 were calculated for 1 mC. Under such conditions, and using the gamma ray which gives the best sensitivity for each element, concentrations of B, C and N greater than a few parts per thousand can readily be determined without interference .
20 mg 47 mg
Coulomb excitation by secondary radiation .
3.4. Oxygen No high energy gamma rays were observed from the bombardment of yttrium oxide. However, low energy gamma rays, mainly from the reaction ,6(3He, p)ísF have been suggested as of use for analysis [6] . 3.5. Sensitivities Table 1 lists the main high energy gamma rays observed from thick targets bombarded with 2 MeV 3 He + ions. The sensitivity attainable with each listed
3.6. Steel analysis Standard reference materials Low-Alloy Steels 1262, 1263 and 1264 obtained from the U.S. National Bureau of Standards, Washington, DC, were analysed for their carbon content . Because the matrix elements had high Coulomb barriers towards the low energy 'He' beam, the gamma-ray yield from the samples was low and beam currents of up to 100 nA could readily be used. Fig . 4 shows a typical spectrum from a steel sample, in which the carbon lines were observed at a level of
STEEL 1263
5
wc cm uU
N CC O U
i
1200
o - i,~ ." ~ ~ o 1'I~n[~r 1 i n~ I J r ~1i1'l,
1300
1400
1500
1600
~,~,
I
1700
1800
1900 2000
Channel number
all
y
2100 2200
2300 2400
Fig. 4. The high energy portion of the gamma-ray spectrum obtained from the reference standard Low-Alloy Steel 1263 containing 0.62% C by mass, showing the prominent peaks due to carbon.
A.E. Pillay, M. Peisach / High energy prompt gamma-ray spectrometry 0.62% by mass . The results of analyses of the steels, using the combined areas under the three peaks from the 5106 keV gamma ray, are given in table 2. The error ranges refer to 2o" errors based on the counting statistics only. The analyses were acceptably accurate, with a relative precision of about 11% as given by the relative root mean square error. 4. Conclusion The potential for analysis of the high energy prompt gamma ray excited by 2 MeV 3 He + ions has been evaluated for the light elements B, C, N and O. Boron, carbon and nitrogen yielded gamma rays of practical importance . The application of the technique to high purity metallurgical matrices has shown that the method is adequate for quantitative analysis at minor concentration levels, in thick samples containing a homogeneous distribution of the element under consideration. The sensitivity could be improved through the use of a more efficient detector.
47
References [1) H . Willard, L . Merritt, J . Dean and F. Settle, Instrumental Methods of Analysis (Wadsworth, Belmont, California, 1988) p . 226. [21 C. Olivier and M. Peisach, J . Radioanal. Chem. 49 (1979) 265. [3j I .S . Giles and M . Peisach, J. Radioanal . Chem. 50 (1979) 307. [4j D . Gihwala, Analytical Application of Proton-Induced Prompt Photo Spectrometry, Ph .D . Thesis, University of Cape Town (1983). [51 A .E. Pillay, Application of Nuclear Methods to Chemical and Isotopic Analysis, Ph.D. Thesis, University of London (1982). [6] A.E. Pillay and M . Peisach, Proc. Int. Conf. on Activation Analysis and its Applications, Beijing, China, October 1990, to be published in J. Radioanal . Nucl. Chem. (1992) . [71 J .B. Marion and F.C. Young, Nuclear Reaction Analysis (North-Holland, Amsterdam, 1968) p. 108. [81 I.S. Giles and M. Peisach, J. Radioanal . Chem. 32 (1976) 105 . [9) M . Peisach, J . Radioanal . Chem. 12 (1972) 251. [101 L.A. Currie, Anal . Chem . 40 (1968) 586.
1 . GENERAL METHODS
B66(1992) 43-47
Nuclearhtshumm ils &Methods in Physics Resemch Section B
The application of high energy prompt gamma-ray spectrometry to 3 He activation analysis of light elements A .E. Pillay ' and M. Peisach
National Accelerator Centre, P. O. Box 72, Faure, 7131 South Africa
Prompt high energy gamma rays induced by moderate beam currents of 2 MeV 3 He+ ions on thick targets of boron, carbon, nitrogen and oxygen were evaluated for analytical purposes . Gamma rays were observed from (3He, p), ( 3He, a) and Coulomb excitation reactions . Interference-free determination of boron, and carbon was achieved at concentrations of a few parts per thousand. At these concentrations a precision of about 10% was attainable. The technique was tested by determining carbon in standard steel specimens. 1. Introduction
2. Experimental
There is a widespread need for methods to determine light elements at all levels of concentration. It is generally accepted that wet chemical methods are tedious and unsuitable for routine quantitative determination of the light elements [1], while instrumental methods such as PIXE, XRF or neutron activation are suitable for elements with atomic numbers greaterthan 11 . Probably one of the few instrumental methods that are capable of handling such problems is ICP, but it is a destructive method, the sample pre-treatment of which is time-consuming especially if interfering components of the matrix have to eliminated by chemical complexation. The matrices in which light elements have to be determined vary greatly. Boron forms a vital component of glasses, is used in nuclear reactor materials, forms an essential element of some semiconductor devices and is used in metallurgy . Carbon, nitrogen and oxygen are essential elements in biochemical and physiological systems and their determination is important in studies of the environment and its pollution. Low energy gamma rays, from activation by protons, deuterons and 'He' ions, have been widely studied for analytical use [2-5], but the sensitivity is often affected by the level of the background and the intensity of the annihilation radiation of 511 keV. In the case of activation with 'He+ ions, the reactions of interest are highly exoergic, resulting in the emission of energetic prompt gamma rays where background interference is much reduced. The analytic potential of lowerenergy gamma rays has already been evaluated [6].
2.1. Materials for analysis
t
Department of Chemistry, University of the Witwatersrand, P.O . Wits, 2050 South Africa .
Targets of boron and carbon were prepared by compressing powders in a 10 ton press to produce tablets of 13 mm diameter and about 2 mm thickness . Targets of oxygen and nitrogen were similarly prepared by using powders of Y203 and GaN or BN respectively. The heavy element component in these targets were not expected to interfere since the calculated Coulomb barriers [7] for Y and Ge were about 15.6 and 31 .2 MeV respectively, which was far in excess of the energy used for the investigation (see below). In addition, a target disc of Al was bombarded, because the beam tube and scattering chamber material consisted mainly of aluminium, from which a background of gamma rays was expected . 2.2. Irradiation and measurement
Targetswere mounted on a remote-controlled vertical ladder, that operated through a geared stepping motor at the rate of 150 steps per mm, which enabled each target to be mounted with a precision of about 7 wm . The ladder fitted into a multipurpose scattering chamber [8], where a constant geometrical arrangement of target and detector was rigidly maintained . Beams of 2 MeV 'He' were obtained from the 6 MV Van de Graaff accelerator at Faure, and beam currents were adjusted so that the dead time of the entire counting assembly did not exceed 10%. Gamma-ray spectrometry was carried out with a 80 cm' Ge(Li) detector mounted outside the scattering chamber at 45° to the bombarding beam, where the aluminium chamber wall was thinned to reduce absorption of low
0168-583X/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
1. GENERAL METHODS
A.E Allay, M. Peisach / High energy prompt gamma-ray spectrometry
energy radiation . In an attempt to reduce the background due to the interaction between scattered particles and the aluminium walls of the scattering chamber, a Perspex sleeve, perforated to allow entrance to the beam, was mounted around the target assembly . Data were recorded on magnetic tape and processed off-line .
3. Results and discussion Energetically, all ('He, y), ('He, n), ('He, p), ('He, d) and ( 3 He, a) reactions on the stable isotopes of boron, carbon, nitrogen and oxygen are possible, with a bombarding beam of 2 MeV 3He ions, with the exception of 1Z C(3He, 3N, 16 0(3 He, 0 18 17 and 16 0(3 He, 7F which are endoergic with Q < -3 MeV. Because most reactions are highly exoergic, the reaction products are produced from highly excited states with the result that the gamma ray spectra are complex. The high energy prompt gamma rays which have analytical potential are those for which interference from other matrix components is unlikely. Since the Coulomb barrier [7] reaches about 3 MeV for Z = 10 (neon) the main interferences are likely to come from the light elements themselves, or from secondary nuclear effects generated by the very energetic protons and alpha particles produced in the reactions with the light elements under investigation.
0
0
3.1. Boron The high energy region of a typical gamma ray spectrum obtained from the bombardment of a boron target is shown in fig. 1. In this and subsequent figures, the gamma rays are labelled according to the Chemists' Convention, which lays most stress on the target nuclide [9]. The most prominent peak in the spectrum is due to the Doppler-broadened 4450 keV gamma ray from the Coulomb excitation of boron-11 . Since the Q-values for the (3 He, p) and (3 He, a) reactions on boron-11 are very high (13.185 and 9.122 MeV respectively) and the corresponding reactions on boron-10 have even higher Q-values, a flux of high energy particles is generated within the boron target . Under these conditions, Coulomb excitation occurs producing the 4450 keV gamma rays. Other relatively intense gamma rays are those from the reaction "B( 3 He, p)13C. The spectrum is complicated by the escape peaks, but even these may be used for analyses. 3.2 Carbon Fig. 2 shows the high energy region of the gamma ray spectrum obtained from the bombardment of graphite. The most prominent peaks are due to the 5106 keV 12 C p(4, 0) gamma rays . From the same reaction, the 4915 keV 12 C p(2, 0) gamma rays were detected, while the other prominent peak is due to the 2799 keV 12 C a(3, 1) gamma ray. The importance of
BORON
1200
1300
1400
1500
3600
1700
1800
1900
Channel number
2000
Fig. 1. The high energy portion of the gamma-ray spectrum obtained from a thick borontarget bombarded with
2 MeV 3He
ions.
45
A . E. Allay, M. Peisach / High energy prompt gamma-ray spectrometry CARBON
1200
1300
1400
1600
1500
1700
1800
. 1900 . 2000 , 2100 2200
2300
2400
Channel number
Fig. 2. The high energy portion of the gamma-ray spectrum obtained from a thick target of graphite bombarded with 2 MeV 3He + ions .
gen are those due to the 6130 keV t4N p(2, 0) gamma rays, as is shown in fig . 3. A very low intensity peak was observed from the 6917 keV '4N p(3, 0) gamma ray, which may be useful for analysis when high energy 3 He + ions are used . The importance of the 6130 keV gamma ray lies in the fact that the background against which it has to be measured is very low .
the relatively intense gamma rays lies in the fact that they can be measured without interference . 3.3. Nitrogen While several low energy gamma rays are suitable for analysis [6], the only prominent peaks due to nitro-
BORON NITRIDE
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
3400
Channel number
Fig . 3. The high energy portion of the gamma-ray spectrum obtained from a thick target of boron nitride bombarded with 2 MeV 3 He * ions. Peaks due to boron have not been labelled. I. GENERAL METHODS
A. E Pillay, M. Peisach / High energy prompt gamma-ray spectrometry
46
Table 1 Analytically useful 3 He-induced prompt high energy gamma rays Target element
E.,, [keVI
Assignment
Sensitivity [gMCI
Boron
3684 3854 4450
" B p(2,0) 11 B p(3, 0) "B (2, 0) -
37 mg 17 mg 1.8 mg
Carbon
2799 3948 4915 5106
12C a(3,1) 12C p(2, 0) 12C p(3, 0) 12C p(4, 0)
75 mg 5.2 mg 3.1 mg
Nitrogen
6130 6917
14N p(2, 0) 14N 03,0)
Oxygen
nil
a
Table 2 Carbon content of some standard steels Steel
Counts [WC]
Carbon content [%] Known Found Error
Relative error [%]
262 11263 1264
2.14±0.40 3.97±0.45 5.92±0.81
0.160 0.62 0.870
+14.4 -12.7 +5.9
0.183 0.541 0.920
+0 .023 -0 .079 +0 .050
10 mg
radiation was calculated as the weight of element which will produce a signal equivalent to about 3 .29 times the square root of the background value over that energy region [10]. Obviously the sensitivity will be a function of the integrated beam current . The values given in table 1 were calculated for 1 mC. Under such conditions, and using the gamma ray which gives the best sensitivity for each element, concentrations of B, C and N greater than a few parts per thousand can readily be determined without interference .
20 mg 47 mg
Coulomb excitation by secondary radiation .
3.4. Oxygen No high energy gamma rays were observed from the bombardment of yttrium oxide. However, low energy gamma rays, mainly from the reaction ,6(3He, p)ísF have been suggested as of use for analysis [6] . 3.5. Sensitivities Table 1 lists the main high energy gamma rays observed from thick targets bombarded with 2 MeV 3 He + ions. The sensitivity attainable with each listed
3.6. Steel analysis Standard reference materials Low-Alloy Steels 1262, 1263 and 1264 obtained from the U.S. National Bureau of Standards, Washington, DC, were analysed for their carbon content . Because the matrix elements had high Coulomb barriers towards the low energy 'He' beam, the gamma-ray yield from the samples was low and beam currents of up to 100 nA could readily be used. Fig . 4 shows a typical spectrum from a steel sample, in which the carbon lines were observed at a level of
STEEL 1263
5
wc cm uU
N CC O U
i
1200
o - i,~ ." ~ ~ o 1'I~n[~r 1 i n~ I J r ~1i1'l,
1300
1400
1500
1600
~,~,
I
1700
1800
1900 2000
Channel number
all
y
2100 2200
2300 2400
Fig. 4. The high energy portion of the gamma-ray spectrum obtained from the reference standard Low-Alloy Steel 1263 containing 0.62% C by mass, showing the prominent peaks due to carbon.
A.E. Pillay, M. Peisach / High energy prompt gamma-ray spectrometry 0.62% by mass . The results of analyses of the steels, using the combined areas under the three peaks from the 5106 keV gamma ray, are given in table 2. The error ranges refer to 2o" errors based on the counting statistics only. The analyses were acceptably accurate, with a relative precision of about 11% as given by the relative root mean square error. 4. Conclusion The potential for analysis of the high energy prompt gamma ray excited by 2 MeV 3 He + ions has been evaluated for the light elements B, C, N and O. Boron, carbon and nitrogen yielded gamma rays of practical importance . The application of the technique to high purity metallurgical matrices has shown that the method is adequate for quantitative analysis at minor concentration levels, in thick samples containing a homogeneous distribution of the element under consideration. The sensitivity could be improved through the use of a more efficient detector.
47
References [1) H . Willard, L . Merritt, J . Dean and F. Settle, Instrumental Methods of Analysis (Wadsworth, Belmont, California, 1988) p . 226. [21 C. Olivier and M. Peisach, J . Radioanal. Chem. 49 (1979) 265. [3j I .S . Giles and M . Peisach, J. Radioanal . Chem. 50 (1979) 307. [4j D . Gihwala, Analytical Application of Proton-Induced Prompt Photo Spectrometry, Ph .D . Thesis, University of Cape Town (1983). [51 A .E. Pillay, Application of Nuclear Methods to Chemical and Isotopic Analysis, Ph.D. Thesis, University of London (1982). [6] A.E. Pillay and M . Peisach, Proc. Int. Conf. on Activation Analysis and its Applications, Beijing, China, October 1990, to be published in J. Radioanal . Nucl. Chem. (1992) . [71 J .B. Marion and F.C. Young, Nuclear Reaction Analysis (North-Holland, Amsterdam, 1968) p. 108. [81 I.S. Giles and M. Peisach, J. Radioanal . Chem. 32 (1976) 105 . [9) M . Peisach, J . Radioanal . Chem. 12 (1972) 251. [101 L.A. Currie, Anal . Chem . 40 (1968) 586.
1 . GENERAL METHODS