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Contribution of Coulomb excitation techniques in quantitative analysis of materials
Nuclear Instruments and Methods in Physics Research B 213 (2004) 515–518 www.elsevier.com/locate/nimb
Contribution of Coulomb excitation techniques in quantitative analysis of materials L. Craciun *, E. Dragulescu, O. Muresan, A.T. Serban, P.M. Racolta National Institute for Physics and Nuclear Engineering, ‘‘Horia Hulubei’’, Bucharest, Romania
Abstract This contribution is an attempt to analyze the elemental composition of materials by prompt gamma-ray spectrometry with protons and heavy ions. The productions of gamma rays by Coulomb excitation method consists in bombardments with charged particles with energies below the Coulombian barrier, therefore the excitation of the nucleus is an electromagnetic phenomenon. Thin solid target of different materials were bombarded with 30 MeV 16 O, 55 MeV 35 Cl, 5 MeV 1 p at the Tandem accelerator of NIPNE – Bucharest. The experimental results allowed to set detection limits for quantitative analysis of various materials. These experiments showed that the detection limits strongly depend on the ion beam parameters (energy, intensity). Therefore an optimizing procedure of these parameters is essential for a successful output. Reliable results require the existence of standard samples. 2003 Elsevier B.V. All rights reserved. PACS: 25.70.D Keywords: Coulomb excitation; Elemental analysis
1. Introduction Coulomb excitation (Coulex) is a powerful method to study nuclear structure. As long as the energy of the incident beam is near or above the Coulomb barrier, nuclear reactions involving both incident particles and target occur with subsequent emission of the gamma rays due to the de-excitation of the product nuclei. Another possibility arises whenever the incident energy is below the Coulomb barrier. In this case by the proper selection of the beam energy assures that the interaction between the colliding beam and target particles is purely electromagnetic, thus allowing a model-indepen*
Corresponding author. E-mail address: cliviu@ifin.nipne.ro (L. Craciun).
dent description in terms of classical electrodynamics, free of the assumptions concerning the nuclear forces. The resulting reaction forms an unstable nucleus that then de-excites giving off a gamma ray at specific energies. The resulting gamma ray is then measured with a sensitive HPGe detector. Using heavy ions, only the second possibility is convenient for analytical purposes, while the second one is not, due to reactions with many exit channels giving numerous nuclear interferences and gamma-rays. 2. Theoretical aspects It is known [1] that the cross section r, of the Coulomb excitation process, in the first order perturbation approximation, is proportional to the
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01615-X
516
L. Craciun et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 515–518
transition electric moment BðEkÞ for the lowest kmultiplicity. The cross sections obtained when one uses incident heavy particles are significantly larger if compared with those of light ions. The incident particles lose energy mainly due to the electronic excitation process or ionization process of the target atoms. The energy loss by collision with the target nuclei is significant only at very low energy, where the Coulomb excitation is absent. The calculated c radiation efficiency is given by ðkÞ
Ycalc ðEi Þ ¼ 6:02217 104 rðkÞ ðEi Þ P
nk tmg=cm2 ; i n i Ai
target
Ion beam
Al window Shielding Lead HPGe Detector with cryostat
ð1Þ Fig. 1. The experimental set-up.
where r is in units of 1024 cm2 and the thickness t in units of mg/cm2 . Ai and ni are the atomic mass and the number of atoms of component i in the volume unit of the target, respectively. The radiation efficiency given by (1) can be brought to an equivalent form: ðkÞ
Ycalc ðEi Þ ¼
3:75870 109 nk rðkÞ ðEi Þ P tmg=cm2 : Zp i n i Ai ð2Þ
The experimental c emission efficiency Yexp at incident energy Ec , takes into account all possible c transitions (excitation and de-excitation) from the considered level Eex : 1 X ð1 þ aj Þ ðkÞ Yexp ðEex Þ ¼ kj ; ð3Þ Qf j ej Tj W j
celerator (8 MV) at NIPNE Bucharest. The experimental set-up consists of a reaction chamber placed in an extension of the tandem accelerator, and a semiconductor HPGe detector at 90 related to the incident beam. The detector energy resolution was 2.5 keV FWHM for the 1332 keV gamma-ray of 60 Co. The prompt gamma spectrum is recorded and analyzed using an integrated computerized system (Fig. 1). 4. Results A typical experimental result is shown in Fig. 2, where the gamma-ray spectrum is obtained
where Q is the collected charge from incident flux; f the isotopic abundance of the considered atom; kj the ()1), (+1) coefficient for transitions of the level of interest (kj ¼ 1 for the increase, kj ¼ þ1 for the decrease of population); Nj the number of pulses corresponding to the j photopeak; ej the absolute efficiency of the gamma transition for j; Tj the transition coefficient (absorption in thick target); W j the correction factor due to anisotropic transmission (if hc ¼ 55, W j ¼ 1).
3. Experimental procedure The protons and heavy ions (16 O, 35 Cl) of the incident beam were produced at the Tandem ac-
Fig. 2. Gamma-ray spectrum from with 30 MeV 16 O.
42
Mo sample bombarded
L. Craciun et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 515–518 Table 1 Detection limits of various elements bombarded with 30 MeV
16
517
O
Element
Nuclear reaction
Emitting nucleus
c-ray energy (KeV)
Calculated detection limit (Nb, ppm)
Interference
Al
CE
27
Al
844 1014
310 470
Fe
Ti
CE
47
Ti Ti 48 Ti
159 889 983
105 – –
Sb, Hf, Re
CE
51
V
320
35
CE
53
Cr
564
–
Ge
CE
57
Fe Fe
122 352 846
460 – 60
Sm, Gd, Tb, Yb, Hf, W Al
Cu
669 963
– –
98 325 423
–
Ga
319
–
V, Pd
Mo Mo 100 Mo
204 481 535
260 – 330
I, Hf
CE
123
Sb
160
1100
Ti, Hf, Re, Se
CE
163
Dy
73
32
Hf
CE
176
Hf
88 202
– –
Sc, Gd, Dy Sc, Mo
Ta
CE
181
Ta
136 165 301
15 150 –
Tb, Hf
W
CE
182
W W 186 W
100 111 122
40 35 50
197
191 279 547
– 170 –
46
V Cr Fe
56
Cu
CE
63
Ag
CE
107
Ga Mo
CE
69
CE
95
Ag
97
Sb Dy
184
Au
CE
Au
from a molybdenum sample bombarded with 30 MeV 16 O. Table 1 summarizes the detection limits for several elements. It indicates that the method is fairly well suited for rare earth determination in many matrices at a level of about 100 ppm.
Ga, Pd
Pt 40 90
Ru
Er, Yb, Dy Fe Fe, Sm, Gd Eu As, Pd
5. Conclusions • The Coulombian excitation can be used as spectroscopic instrument for quantitative as well as qualitative detection of various impurities in metals or organic compounds.
518
L. Craciun et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 515–518
• The impurity detection limit depends on the irradiation conditions (energy, intensity). • For detection of the impurities in the range Z ¼ 21–62 one may use HPGe-type detectors, whereas for Z ¼ 63–72 one should use planar detectors which have a high efficiency for gamma radiation below 100 keV. • As long as the target is strongly oxidized or is a powder target, a Compton suppression spectrometer is recomanded in order to reduce the background. • Development of the method depends on the standard sample compounds with known concentrations of different elements.
Acknowledgements This work was partially supported from contract no. 1345/2001, RELANSIN project and from IDRANAP contract.
Reference [1] K. Alder, A. Winther, Electromagnetic Excitation – Theory of Coulomb Excitation with Heavy Ions, North Holland, Amsterdam, 1975.
Contribution of Coulomb excitation techniques in quantitative analysis of materials L. Craciun *, E. Dragulescu, O. Muresan, A.T. Serban, P.M. Racolta National Institute for Physics and Nuclear Engineering, ‘‘Horia Hulubei’’, Bucharest, Romania
Abstract This contribution is an attempt to analyze the elemental composition of materials by prompt gamma-ray spectrometry with protons and heavy ions. The productions of gamma rays by Coulomb excitation method consists in bombardments with charged particles with energies below the Coulombian barrier, therefore the excitation of the nucleus is an electromagnetic phenomenon. Thin solid target of different materials were bombarded with 30 MeV 16 O, 55 MeV 35 Cl, 5 MeV 1 p at the Tandem accelerator of NIPNE – Bucharest. The experimental results allowed to set detection limits for quantitative analysis of various materials. These experiments showed that the detection limits strongly depend on the ion beam parameters (energy, intensity). Therefore an optimizing procedure of these parameters is essential for a successful output. Reliable results require the existence of standard samples. 2003 Elsevier B.V. All rights reserved. PACS: 25.70.D Keywords: Coulomb excitation; Elemental analysis
1. Introduction Coulomb excitation (Coulex) is a powerful method to study nuclear structure. As long as the energy of the incident beam is near or above the Coulomb barrier, nuclear reactions involving both incident particles and target occur with subsequent emission of the gamma rays due to the de-excitation of the product nuclei. Another possibility arises whenever the incident energy is below the Coulomb barrier. In this case by the proper selection of the beam energy assures that the interaction between the colliding beam and target particles is purely electromagnetic, thus allowing a model-indepen*
Corresponding author. E-mail address: cliviu@ifin.nipne.ro (L. Craciun).
dent description in terms of classical electrodynamics, free of the assumptions concerning the nuclear forces. The resulting reaction forms an unstable nucleus that then de-excites giving off a gamma ray at specific energies. The resulting gamma ray is then measured with a sensitive HPGe detector. Using heavy ions, only the second possibility is convenient for analytical purposes, while the second one is not, due to reactions with many exit channels giving numerous nuclear interferences and gamma-rays. 2. Theoretical aspects It is known [1] that the cross section r, of the Coulomb excitation process, in the first order perturbation approximation, is proportional to the
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01615-X
516
L. Craciun et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 515–518
transition electric moment BðEkÞ for the lowest kmultiplicity. The cross sections obtained when one uses incident heavy particles are significantly larger if compared with those of light ions. The incident particles lose energy mainly due to the electronic excitation process or ionization process of the target atoms. The energy loss by collision with the target nuclei is significant only at very low energy, where the Coulomb excitation is absent. The calculated c radiation efficiency is given by ðkÞ
Ycalc ðEi Þ ¼ 6:02217 104 rðkÞ ðEi Þ P
nk tmg=cm2 ; i n i Ai
target
Ion beam
Al window Shielding Lead HPGe Detector with cryostat
ð1Þ Fig. 1. The experimental set-up.
where r is in units of 1024 cm2 and the thickness t in units of mg/cm2 . Ai and ni are the atomic mass and the number of atoms of component i in the volume unit of the target, respectively. The radiation efficiency given by (1) can be brought to an equivalent form: ðkÞ
Ycalc ðEi Þ ¼
3:75870 109 nk rðkÞ ðEi Þ P tmg=cm2 : Zp i n i Ai ð2Þ
The experimental c emission efficiency Yexp at incident energy Ec , takes into account all possible c transitions (excitation and de-excitation) from the considered level Eex : 1 X ð1 þ aj Þ ðkÞ Yexp ðEex Þ ¼ kj ; ð3Þ Qf j ej Tj W j
celerator (8 MV) at NIPNE Bucharest. The experimental set-up consists of a reaction chamber placed in an extension of the tandem accelerator, and a semiconductor HPGe detector at 90 related to the incident beam. The detector energy resolution was 2.5 keV FWHM for the 1332 keV gamma-ray of 60 Co. The prompt gamma spectrum is recorded and analyzed using an integrated computerized system (Fig. 1). 4. Results A typical experimental result is shown in Fig. 2, where the gamma-ray spectrum is obtained
where Q is the collected charge from incident flux; f the isotopic abundance of the considered atom; kj the ()1), (+1) coefficient for transitions of the level of interest (kj ¼ 1 for the increase, kj ¼ þ1 for the decrease of population); Nj the number of pulses corresponding to the j photopeak; ej the absolute efficiency of the gamma transition for j; Tj the transition coefficient (absorption in thick target); W j the correction factor due to anisotropic transmission (if hc ¼ 55, W j ¼ 1).
3. Experimental procedure The protons and heavy ions (16 O, 35 Cl) of the incident beam were produced at the Tandem ac-
Fig. 2. Gamma-ray spectrum from with 30 MeV 16 O.
42
Mo sample bombarded
L. Craciun et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 515–518 Table 1 Detection limits of various elements bombarded with 30 MeV
16
517
O
Element
Nuclear reaction
Emitting nucleus
c-ray energy (KeV)
Calculated detection limit (Nb, ppm)
Interference
Al
CE
27
Al
844 1014
310 470
Fe
Ti
CE
47
Ti Ti 48 Ti
159 889 983
105 – –
Sb, Hf, Re
CE
51
V
320
35
CE
53
Cr
564
–
Ge
CE
57
Fe Fe
122 352 846
460 – 60
Sm, Gd, Tb, Yb, Hf, W Al
Cu
669 963
– –
98 325 423
–
Ga
319
–
V, Pd
Mo Mo 100 Mo
204 481 535
260 – 330
I, Hf
CE
123
Sb
160
1100
Ti, Hf, Re, Se
CE
163
Dy
73
32
Hf
CE
176
Hf
88 202
– –
Sc, Gd, Dy Sc, Mo
Ta
CE
181
Ta
136 165 301
15 150 –
Tb, Hf
W
CE
182
W W 186 W
100 111 122
40 35 50
197
191 279 547
– 170 –
46
V Cr Fe
56
Cu
CE
63
Ag
CE
107
Ga Mo
CE
69
CE
95
Ag
97
Sb Dy
184
Au
CE
Au
from a molybdenum sample bombarded with 30 MeV 16 O. Table 1 summarizes the detection limits for several elements. It indicates that the method is fairly well suited for rare earth determination in many matrices at a level of about 100 ppm.
Ga, Pd
Pt 40 90
Ru
Er, Yb, Dy Fe Fe, Sm, Gd Eu As, Pd
5. Conclusions • The Coulombian excitation can be used as spectroscopic instrument for quantitative as well as qualitative detection of various impurities in metals or organic compounds.
518
L. Craciun et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 515–518
• The impurity detection limit depends on the irradiation conditions (energy, intensity). • For detection of the impurities in the range Z ¼ 21–62 one may use HPGe-type detectors, whereas for Z ¼ 63–72 one should use planar detectors which have a high efficiency for gamma radiation below 100 keV. • As long as the target is strongly oxidized or is a powder target, a Compton suppression spectrometer is recomanded in order to reduce the background. • Development of the method depends on the standard sample compounds with known concentrations of different elements.
Acknowledgements This work was partially supported from contract no. 1345/2001, RELANSIN project and from IDRANAP contract.
Reference [1] K. Alder, A. Winther, Electromagnetic Excitation – Theory of Coulomb Excitation with Heavy Ions, North Holland, Amsterdam, 1975.