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Precision in neon ion energy loss measurements with a silicon surface barrier detector
Nuclear Instruments and Methods in Physics Research BlO/ll North-Holland, Amsterdam
237
(1985) 237-240
PRECISION IN NEON ION ENERGY LOSS MEASUREMENTS SURFACE BARRIER DETECTOR * J.C. OVERLEY
and H.W.
Physics Department,
University of Oregon, Eugene, OR 97403, USA
WITH A SILICON
LEFEVRE
Neon ions were accelerated to energies between 1.5 and 3.0 MeV. Pulse height distributions were measured with a silicon surface barrier detector both for directly incident ions and for ions transmitted through carbon foils. Detector line shapes are skewed with widths of about 100 keV (fwhrn) and are not strongly dependent on energy or energy loss. The impact of line shapes on thickness resolution in energy-loss radiography is discussed. Information is developed for detector pulse height defect for neon ions and for neon ion stopping powers in carbon.
1. Introduction Fig. 1 is a “residual energy radiograph” of an insect’s wing. It was obtained [l] by rastering a focused 3 MeV *‘Ne+ beam across the wing and recording energies of individual transmitted ions with a silicon surface barrier detector. The image was constructed by photographing the screen of an oscilloscope with a similarly rastered beam intensified according to ion energy loss. In the paper describing this technique it was stated that depth resolutions approaching 100 A for materials of unit density might be achieved. In the present paper we investigate several parameters governing depth resolution when neon ions are transmitted through carbon. Concommitantly, information on the stopping power of carbon for neon ions and on the pulse height defect of silicon surface barrier detectors for neon ions is obtained.
1.0 X 1.0 mm’ on target or, alternatively, the object aperture was imaged at the target position to a square 10 I.rrn on a side. After beam size and position adjustments were made, beam intensity was reduced to about lo3 ions/s by defocusing at the ion source and by nearly closing the object slits. A silicon surface barrier detector was then inserted
2. Measurements A mixture duoplasmatron Graaff
of neon and helium gas was used in the ion source of the Oregon 5 MV Van de
accelerator.
Roughly
lo-20
uA
of
ions
were
extracted and accelerated to energies between 1.5 and 3.0 MeV. Beams were momentum analyzed and directed through the object slits of the scanning ion microprobe associated with the accelerator [2] and thence to the target position 6.8 m away. Targets were self-supporting carbon foils mounted over 0.8 cm diameter holes in 0.02 cm thick tantalum sheet. Beams were geometrically collimated to about
l
Work supported [PHY-83066831.
0168-583X/85/$03.30
(North-Holland
by the US National
Science
Foundation
0 Blsevier Science Publishers Physics Publishing Division)
B.V.
Fig. 1. Energy-loss radiograph of an insect’s wing obtained with 3 MeV incident neon ions. Lighter shades correspond to higher residual energies of transmitted ions. Residual energies are medians of five individual ion energy measurements at each of 256 x 256 raster positions. ‘The raster pattern covers an area 0.7 x 0.7 mm*. I. ATOMIC
PHYSICS/RELATED
PHENOMENA
238
J. C. Overley, H. W. Lefevre / Precision in neon ion energy loss
about 1 cm directly beyond the target position. The detector was constructed from 6 kO cm silicon by ORTEC. It had a 40 I-18cm-* gold entrance window and a 50 mm* active area. Depletion depth was 300 pm at 100 V bias. Fig. 2 is a composite of three pulse height spectra. The peak near channel 700 results from 2 MeV 4He+ ions passing directly through an empty foil holder to strike the detector. The spectrum is presented on a logarithmic scale to emphasize low energy tails caused by slit scattering and other processes. Overall system resolution for helium ions was about 20 keV, full width at half maximum (fwhm). The peak near channel 600 results from 2 MeV *‘Ne+ ions and, in conjunction with the 4He+ peak, illustrates the “pulse height defect” which exists when heavy ion energies are measured with solid state detectors. The defect is usually defined as the difference in energies of heavy and light ions which produce the same pulse height. It results from nuclear stopping, entrance window effects, and from electron-hole density dependent recombination along the ion track. The data of fig. 2 correspond to a defect of (240 f 20) keV for 2.0 MeV neon ions. A similar measurement at 1.5 MeV yielded a defect of (200 f 20) keV. These values are slightly higher than those reported by Potter and Campbell [3] and by Ipavich et al. [4]. The various values are probably consistent, however, within uncertainties imposed by variations in detector characteristics and precision of the measurements. The third peak in fig. 2 results from 2 MeV neon ions after passage through a 92 pg cm-* carbon foil. The few points shown above the He+ peak are also associated with this spectrum. They result partly from pulse pileup and partly from higher energy beams. At these low intensities, a continuum of energies (and
IO3
0
200
400 CHRNNEL
600
800
NUMBER
Fig. 2. Pulse height distributions measured with a silicon surface barrier detector. The uppermost peak is from 2 MeV 4He+ ions, the intermediate peak is from 2 MeV “Ne+ ions, and the lowest peak is from 2 MeV 20Ne+ ions after passage through 92 cg cmm2 of carbon.
magnetic rigidities) up to g-10 MeV is produced by neon ion charge changing in the residual gas of the accelerating tube and through other interactions. Neon ion stopping powers in carbon can be recovered from data like those in fig. 2. The neon ion energy scale was assumed to depend linearly on channel number and the zero intercept and channel width in energy units were determined by least squares fitting locations of peak maxima for incident energies of 1.5, 2.0, 2.5 and 3.0 MeV. At each of these energies, neon ion energy losses in foils of thickness (33.6 f 1.1) and (91.7 f 1.7) pg cm-* were determined. Foil thicknesses were obtained by applying Ziegler’s formula [S] for the slowing of helium in carbon to energy losses measured for 4He ions at several incident energies. Between 1.0 and 3.0 MeV, the electronic stopping powers, S, for neon in carbon presented by Northcliffe and Schilling [6] can be parameter&d in terms of energy E as S = ((0.192 f 0.006) keV”* cm* ug-‘) El/*. Contributions of nuclear stopping are well within these limits. We determine the average of eight values of the proportionality constant to be (0.194 f 0.012) keV’/* cm* t.tg-‘. Although this is consistent with Northchffe and Schilhng, our results are systematically higher than theirs by several per cent at high energies, and low by similar amounts of low energies. We disagree with earlier measurements [7] in this domain. Neon ion line shapes are asymmetric and different from helium ion line shapes. This has been noted previously [8]. Increased widths of the distributions are due to nuclear stopping and ion charge state fluctuations in the detector and tend to mask tails due to slit scattering. Calculations of line widths due to nuclear stopping yield values between 50 and 75 keV for neon ion energies between 1.0 and 3 MeV (see ref. [9], for example). The line widths we measure for directly incident neon ions increase from 90 to 100 keV (fwhm) as incident energy increases from 1.5 to 3.0 MeV. These line widths (and stopping powers also) were measured with the collimated beam. Focused unrastered beams can rapidly damage the detector, resulting in increased line width and decreased average pulse height. The data of fig. 2 were obtained with focused beams, however, and detector damage probably accounts for the 120 keV width of the direct neon ion peak. Fig. 2 also illustrates that energy straggling in the carbon foils does not increase line width appreciably. As another example, the 100 keV line width of 3 MeV ions increases only to 110 keV after passage through the 92 pg cm-* foil, a 0.9 MeV energy loss. Since line width and shape is governed mainly by detector response, precision in energy loss measurements is largely independent of ion energy or energy loss. For data similar to fig. 2, locations of peak maxima can easily be determined to f 5 keV. For 3.0 MeV neon ions suffering a 1 MeV loss in carbon, this corresponds
J. C. Overley, H. W. Lefevre / Precision in neon ion energy loss
to a depth resolution of f0.5 pg cm-’ or f23 A. Depth resolutions vary only by f 10% for energy losses ranging from 0 to 2 MeV for 3 MeV incident ions.
IO”
IO2
239
Of greater interest, perhaps, is the precision when energies of single ions are measured. Pulse height spectra are then regarded as (unnormalized) probability distributions for obtaining a given measurement result for an assumed most probable energy. Since the width of all distributions is about 100 keV (fwhm), measurement of a single ion energy is known to be within f 50 keV of the most probable value with about 70% confidence. Equal upper and lower limits imply neglect of the skewing of the distributions. Several advantages can be gained by using the average or median values of several ion energy measurements. To illustrate, the pulse height distribution for transmitted neon ions in fig. 2 was regarded as typical. Energies were randomly selected according to this distribution and distributions of the means and medians of groups of n ions were constructed. Results for n = 5 are shown in fig. 3. Fig. 3a is the distribution of the mean of 10000 groups of ions. The noise associated with pile-up is still present although the energy contrast is decreased. The 93 keV width of the peak in the parent distribution is decreased to 43 keV (fwhm), almost as much as would be expected if the distributions were gaussian, but the peak location is somewhat lower than the most probable value for the parent distribution. Fig. 3b is the distribution of the median. In this case, the noise is effeo tively eliminated and the width of the distribution is only slightly greater. Fig. 3c is a compromise. A window of width 93 keV was adjusted to maxim& the number of selected events within the window and the distribution of the average of events within the window was constructed. Three or more events will fall within the window 84% of the time. In ambiguous cases, the highest energy window location was used. Noise is again effectively eliminated, the width of the distribution is about 48 keV (fwhm), and the location of the peak is nearly the same as the most probable value in the parent distribution.
3. Conclusions 10’
250
300
350 CHRNNEL
400
450
500
NUMBER
Fig. 3. Distributions of means and medians of five events selected randomly according to the (unnormalized) parent probability distribution shown by the solid line. Portions of the parent distribution are off-scale to the right. Fig. 3a is the distribution of the simple mean, 3b the distribution of the median. Fig. 3c is the distribution of the mean of those events within a window of width equal to the fwhm of the parent distribution. The window was positioned to maximize the number of events within it.
Energy losses of neon ions traversing carbon foils can be determined with a precision of f25 keV with 70% confidence by averaging several residual ion energy measurements made with a silicon surface barrier detector. This corresponds to a depth resolution of f2.5 pg cm-* or f 110 A in carbon. The figures do not depend strongly on ion energy or energy loss in the l-3 MeV incident energy range. The primary limitation is the 100 keV (fwhm) line width of the detector response to l-3 MeV neon ions. Since bolometric spectrometers should have much better heavy ion resolution [lo], they may eventually replace silicon detectors in this particular application. I. ATOMIC
PHYSICS/REXATED
PHENOMENA
240
J. C. Over&v, H. W. Lejevre / Precision in neon ion energy Ioss
References [l] J.C. Gverley, R.C. ChMOfiy, G.E. Sieger, J.D. MacDonald and H.W. Lefevre, Nucl. Instr. and Meth. 218 (1983) 43. [2] H.W. Lefevre, R.C. Connolly, G. Sieger and J.C. Overley, Nucl. Instr. and Meth. 218 (1983) 39. [3] D.W. Potter and RD. Campbell, Nucl. Instr. and Meth. 153 (1978) 525. [4] F.M. lpavich, R.A. Lundren, B.A. Lambird and G. GloekIer, Nucl. Instr. and Meth. 154 (1978) 291. [5] J.F. Ziegler, The Stopping and Ranges of Ions in Matter, Vol. 4, Helium: Stopping Powers and Ranges in AU Elemental Matter (Pergamon, New York, 1980).
(61 L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables A7 (1970) 23. (71 D.I. Porat and K. Ramavataram, Proc. Phys. Sot. 78 (1961) 1135. [8] A. L’Hoir, Nucl. Instr. and Meth. 223 (1984) 336. [9] E.C. Finch, M. Asghar, M. Forte, G. Siegert, J. Greif, R. Decker and the Lohengrin Collaboration, Nucl. Instr. and Meth. 142 (1977) 539. [lo] J.C. Overley, H.W. Lefevre, I.G. Nolt, J.V. Radostitz, S. Predko and P.A.R. Ade, these Proceedings (AARI’84) Nucl. Instr. and Meth. BlO/ll (1985) 928.
237
(1985) 237-240
PRECISION IN NEON ION ENERGY LOSS MEASUREMENTS SURFACE BARRIER DETECTOR * J.C. OVERLEY
and H.W.
Physics Department,
University of Oregon, Eugene, OR 97403, USA
WITH A SILICON
LEFEVRE
Neon ions were accelerated to energies between 1.5 and 3.0 MeV. Pulse height distributions were measured with a silicon surface barrier detector both for directly incident ions and for ions transmitted through carbon foils. Detector line shapes are skewed with widths of about 100 keV (fwhrn) and are not strongly dependent on energy or energy loss. The impact of line shapes on thickness resolution in energy-loss radiography is discussed. Information is developed for detector pulse height defect for neon ions and for neon ion stopping powers in carbon.
1. Introduction Fig. 1 is a “residual energy radiograph” of an insect’s wing. It was obtained [l] by rastering a focused 3 MeV *‘Ne+ beam across the wing and recording energies of individual transmitted ions with a silicon surface barrier detector. The image was constructed by photographing the screen of an oscilloscope with a similarly rastered beam intensified according to ion energy loss. In the paper describing this technique it was stated that depth resolutions approaching 100 A for materials of unit density might be achieved. In the present paper we investigate several parameters governing depth resolution when neon ions are transmitted through carbon. Concommitantly, information on the stopping power of carbon for neon ions and on the pulse height defect of silicon surface barrier detectors for neon ions is obtained.
1.0 X 1.0 mm’ on target or, alternatively, the object aperture was imaged at the target position to a square 10 I.rrn on a side. After beam size and position adjustments were made, beam intensity was reduced to about lo3 ions/s by defocusing at the ion source and by nearly closing the object slits. A silicon surface barrier detector was then inserted
2. Measurements A mixture duoplasmatron Graaff
of neon and helium gas was used in the ion source of the Oregon 5 MV Van de
accelerator.
Roughly
lo-20
uA
of
ions
were
extracted and accelerated to energies between 1.5 and 3.0 MeV. Beams were momentum analyzed and directed through the object slits of the scanning ion microprobe associated with the accelerator [2] and thence to the target position 6.8 m away. Targets were self-supporting carbon foils mounted over 0.8 cm diameter holes in 0.02 cm thick tantalum sheet. Beams were geometrically collimated to about
l
Work supported [PHY-83066831.
0168-583X/85/$03.30
(North-Holland
by the US National
Science
Foundation
0 Blsevier Science Publishers Physics Publishing Division)
B.V.
Fig. 1. Energy-loss radiograph of an insect’s wing obtained with 3 MeV incident neon ions. Lighter shades correspond to higher residual energies of transmitted ions. Residual energies are medians of five individual ion energy measurements at each of 256 x 256 raster positions. ‘The raster pattern covers an area 0.7 x 0.7 mm*. I. ATOMIC
PHYSICS/RELATED
PHENOMENA
238
J. C. Overley, H. W. Lefevre / Precision in neon ion energy loss
about 1 cm directly beyond the target position. The detector was constructed from 6 kO cm silicon by ORTEC. It had a 40 I-18cm-* gold entrance window and a 50 mm* active area. Depletion depth was 300 pm at 100 V bias. Fig. 2 is a composite of three pulse height spectra. The peak near channel 700 results from 2 MeV 4He+ ions passing directly through an empty foil holder to strike the detector. The spectrum is presented on a logarithmic scale to emphasize low energy tails caused by slit scattering and other processes. Overall system resolution for helium ions was about 20 keV, full width at half maximum (fwhm). The peak near channel 600 results from 2 MeV *‘Ne+ ions and, in conjunction with the 4He+ peak, illustrates the “pulse height defect” which exists when heavy ion energies are measured with solid state detectors. The defect is usually defined as the difference in energies of heavy and light ions which produce the same pulse height. It results from nuclear stopping, entrance window effects, and from electron-hole density dependent recombination along the ion track. The data of fig. 2 correspond to a defect of (240 f 20) keV for 2.0 MeV neon ions. A similar measurement at 1.5 MeV yielded a defect of (200 f 20) keV. These values are slightly higher than those reported by Potter and Campbell [3] and by Ipavich et al. [4]. The various values are probably consistent, however, within uncertainties imposed by variations in detector characteristics and precision of the measurements. The third peak in fig. 2 results from 2 MeV neon ions after passage through a 92 pg cm-* carbon foil. The few points shown above the He+ peak are also associated with this spectrum. They result partly from pulse pileup and partly from higher energy beams. At these low intensities, a continuum of energies (and
IO3
0
200
400 CHRNNEL
600
800
NUMBER
Fig. 2. Pulse height distributions measured with a silicon surface barrier detector. The uppermost peak is from 2 MeV 4He+ ions, the intermediate peak is from 2 MeV “Ne+ ions, and the lowest peak is from 2 MeV 20Ne+ ions after passage through 92 cg cmm2 of carbon.
magnetic rigidities) up to g-10 MeV is produced by neon ion charge changing in the residual gas of the accelerating tube and through other interactions. Neon ion stopping powers in carbon can be recovered from data like those in fig. 2. The neon ion energy scale was assumed to depend linearly on channel number and the zero intercept and channel width in energy units were determined by least squares fitting locations of peak maxima for incident energies of 1.5, 2.0, 2.5 and 3.0 MeV. At each of these energies, neon ion energy losses in foils of thickness (33.6 f 1.1) and (91.7 f 1.7) pg cm-* were determined. Foil thicknesses were obtained by applying Ziegler’s formula [S] for the slowing of helium in carbon to energy losses measured for 4He ions at several incident energies. Between 1.0 and 3.0 MeV, the electronic stopping powers, S, for neon in carbon presented by Northcliffe and Schilling [6] can be parameter&d in terms of energy E as S = ((0.192 f 0.006) keV”* cm* ug-‘) El/*. Contributions of nuclear stopping are well within these limits. We determine the average of eight values of the proportionality constant to be (0.194 f 0.012) keV’/* cm* t.tg-‘. Although this is consistent with Northchffe and Schilhng, our results are systematically higher than theirs by several per cent at high energies, and low by similar amounts of low energies. We disagree with earlier measurements [7] in this domain. Neon ion line shapes are asymmetric and different from helium ion line shapes. This has been noted previously [8]. Increased widths of the distributions are due to nuclear stopping and ion charge state fluctuations in the detector and tend to mask tails due to slit scattering. Calculations of line widths due to nuclear stopping yield values between 50 and 75 keV for neon ion energies between 1.0 and 3 MeV (see ref. [9], for example). The line widths we measure for directly incident neon ions increase from 90 to 100 keV (fwhm) as incident energy increases from 1.5 to 3.0 MeV. These line widths (and stopping powers also) were measured with the collimated beam. Focused unrastered beams can rapidly damage the detector, resulting in increased line width and decreased average pulse height. The data of fig. 2 were obtained with focused beams, however, and detector damage probably accounts for the 120 keV width of the direct neon ion peak. Fig. 2 also illustrates that energy straggling in the carbon foils does not increase line width appreciably. As another example, the 100 keV line width of 3 MeV ions increases only to 110 keV after passage through the 92 pg cm-* foil, a 0.9 MeV energy loss. Since line width and shape is governed mainly by detector response, precision in energy loss measurements is largely independent of ion energy or energy loss. For data similar to fig. 2, locations of peak maxima can easily be determined to f 5 keV. For 3.0 MeV neon ions suffering a 1 MeV loss in carbon, this corresponds
J. C. Overley, H. W. Lefevre / Precision in neon ion energy loss
to a depth resolution of f0.5 pg cm-’ or f23 A. Depth resolutions vary only by f 10% for energy losses ranging from 0 to 2 MeV for 3 MeV incident ions.
IO”
IO2
239
Of greater interest, perhaps, is the precision when energies of single ions are measured. Pulse height spectra are then regarded as (unnormalized) probability distributions for obtaining a given measurement result for an assumed most probable energy. Since the width of all distributions is about 100 keV (fwhm), measurement of a single ion energy is known to be within f 50 keV of the most probable value with about 70% confidence. Equal upper and lower limits imply neglect of the skewing of the distributions. Several advantages can be gained by using the average or median values of several ion energy measurements. To illustrate, the pulse height distribution for transmitted neon ions in fig. 2 was regarded as typical. Energies were randomly selected according to this distribution and distributions of the means and medians of groups of n ions were constructed. Results for n = 5 are shown in fig. 3. Fig. 3a is the distribution of the mean of 10000 groups of ions. The noise associated with pile-up is still present although the energy contrast is decreased. The 93 keV width of the peak in the parent distribution is decreased to 43 keV (fwhm), almost as much as would be expected if the distributions were gaussian, but the peak location is somewhat lower than the most probable value for the parent distribution. Fig. 3b is the distribution of the median. In this case, the noise is effeo tively eliminated and the width of the distribution is only slightly greater. Fig. 3c is a compromise. A window of width 93 keV was adjusted to maxim& the number of selected events within the window and the distribution of the average of events within the window was constructed. Three or more events will fall within the window 84% of the time. In ambiguous cases, the highest energy window location was used. Noise is again effectively eliminated, the width of the distribution is about 48 keV (fwhm), and the location of the peak is nearly the same as the most probable value in the parent distribution.
3. Conclusions 10’
250
300
350 CHRNNEL
400
450
500
NUMBER
Fig. 3. Distributions of means and medians of five events selected randomly according to the (unnormalized) parent probability distribution shown by the solid line. Portions of the parent distribution are off-scale to the right. Fig. 3a is the distribution of the simple mean, 3b the distribution of the median. Fig. 3c is the distribution of the mean of those events within a window of width equal to the fwhm of the parent distribution. The window was positioned to maximize the number of events within it.
Energy losses of neon ions traversing carbon foils can be determined with a precision of f25 keV with 70% confidence by averaging several residual ion energy measurements made with a silicon surface barrier detector. This corresponds to a depth resolution of f2.5 pg cm-* or f 110 A in carbon. The figures do not depend strongly on ion energy or energy loss in the l-3 MeV incident energy range. The primary limitation is the 100 keV (fwhm) line width of the detector response to l-3 MeV neon ions. Since bolometric spectrometers should have much better heavy ion resolution [lo], they may eventually replace silicon detectors in this particular application. I. ATOMIC
PHYSICS/REXATED
PHENOMENA
240
J. C. Over&v, H. W. Lejevre / Precision in neon ion energy Ioss
References [l] J.C. Gverley, R.C. ChMOfiy, G.E. Sieger, J.D. MacDonald and H.W. Lefevre, Nucl. Instr. and Meth. 218 (1983) 43. [2] H.W. Lefevre, R.C. Connolly, G. Sieger and J.C. Overley, Nucl. Instr. and Meth. 218 (1983) 39. [3] D.W. Potter and RD. Campbell, Nucl. Instr. and Meth. 153 (1978) 525. [4] F.M. lpavich, R.A. Lundren, B.A. Lambird and G. GloekIer, Nucl. Instr. and Meth. 154 (1978) 291. [5] J.F. Ziegler, The Stopping and Ranges of Ions in Matter, Vol. 4, Helium: Stopping Powers and Ranges in AU Elemental Matter (Pergamon, New York, 1980).
(61 L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables A7 (1970) 23. (71 D.I. Porat and K. Ramavataram, Proc. Phys. Sot. 78 (1961) 1135. [8] A. L’Hoir, Nucl. Instr. and Meth. 223 (1984) 336. [9] E.C. Finch, M. Asghar, M. Forte, G. Siegert, J. Greif, R. Decker and the Lohengrin Collaboration, Nucl. Instr. and Meth. 142 (1977) 539. [lo] J.C. Overley, H.W. Lefevre, I.G. Nolt, J.V. Radostitz, S. Predko and P.A.R. Ade, these Proceedings (AARI’84) Nucl. Instr. and Meth. BlO/ll (1985) 928.