- Email: [email protected]
Nondestructive imaging detectors for energetic particle beams
465
Nuclear Instruments and Methods in Physics Research B44 (1990) 465-472 North-Holland
NONDESTRUCTIVE Robert W. ODOM,
IMAGING Michael
Charles Evans and Associates,
DETECTORS
D. STRATHMAN,
FOR ENERGETIC S.E. BUTTRILL,
PARTICLE
Jr. and Scott
BEAMS
M. BAUMANN
301 Chesapeake Drive, Redwood City, CA 94063, USA
Received 22 May 1989 and in revised form 14 August 1989
This paper describes the design and discusses initial experimental results produced from a nondestructive, position sensitive particle beam detector. The detector measures the spatial and intensity distribution of a particle beam by determining the position and intensity of secondary electrons or ions produced by particle beam interactions with thin foils ( - 10 ug/cm*) or wire grids. These secondary charged particles are focused onto an imaging detector using stigmatic ion optics which provide a high degree of correlation between the position of the incident particle beam at the foil and the position of secondary ions at the image sensor. The imaging detector is nonintrusive to the particle beam flux since most of the incident beam passes through the thin foil or around the wire mesh. This detector can monitor particle beam intensities ranging from tens of microamperes down to several hundred particles per second. Image resolution on the order of 5 pm is observed for particle beams having diameters on the order of several millimeters.
1.
Introduction
Current state-of-the-art energetic beam detectors generally fall into two categories: 1) high intensity (> 10” events or particles per second) total beam monitors and single or multiwire position sensitive detectors (PSD) [l] and 2) low intensity (< 10’ events per second) semiconductor total beam monitors or PSD’s [2]. Although particle beams are generally consumed during the detection process and neither detector operates well outside of its optimum range, these devices have found extensive application in particle beam detection [3]. These detectors are limited primarily by the limited intensity range over which useful measurements can be obtained and the lack of high lateral resolution imaging capabilities. For example, neither detector performs routine and reliable beam monitoring at beam intensities between lo5 and approximately 10” particles per second (pps). In addition’neither of these detectors are capable of determining the lateral distribution of a particle beam at micrometer or submicrometer (pm) lateral resolutions. This paper presents results on the development of novel beam imaging techniques which characterize the intensity distributions within energetic particle beams with minimal beam attenuation. This nondestructive or nonintrusive detector converts a portion of the incident particle beam into a secondary electron or ion signal by passing the particle beam through a thin foil. The secondary electrons or ions produced from this beam/foil interaction are extracted from the foil surface and focused onto an imaging detector. The image detec0168-583X/90/%03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
tor described below is comprised of components which have been utilized individually in various particle beam detectors. For example, the production of secondary ions or electrons from collisions of energeticbeams with a foil “transducer” is a well established procedure in particle beam detection [4]. Similarly, microchannel plate intensifiers have been employed to detect the secondary ions or electrons produced in these beam/foil interactions [5]. A detector which coupled a foil transducer with proximity focusing electron optics, a microchannel plate intensifier and image sensor has also been briefly described in the literature [4]. The unique feature of the prototype detector described in this paper is the combination of the beam foil transducer with stigmatic ion optics which focus the secondary ion or electrons onto the imaging detector. The secondary electron or ion image can be magnified with these ion optics and thus increase the resolution in the particle beam image. The use of stigmatic electron optics also adds a wide range of design flexibility in the selection of the image detector. These optics should be adaptable to various detector configurations having a wide range of sensor areas and/or image magnifications. Several image detectors including dual microchannel plate (DMCP)/fluorescent screens, resistive anode encoders (RAE) [6] as well as a hybrid fluorescent screen/RAE detector were evaluated in this work. The beam transducers included thin carbon and metal oxide foils as well as wire meshes. A prototype detector having an active diameter of 2.5 mm was evaluated using 2.275 MeV He*+ beams. This detector produces useful secondary electron or ion images at particle beam intensities ranging from several nanoamperes to several hundred particles per second. The
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imaging detectors for energetic particle beams
resolution in many of these images is on the order of 5 pm which is close to the limit for this device based on the estimated transverse energy of secondary electrons produced in the beam/foil interaction.
2. Experimental The imaging detector measures the intensity and lateral distribution of secondary electrons (or ions) produced by the passage of an energetic beam through a thin foil. The correspondence between the intensity distribution within the secondary electron image and the incident particle beam is achieved with stigmatic imaging optics [7] which focus the secondaq electrons produced at the foil onto a position sensitive imaging detector with minimal image distortion. The design of this detector is illustrated schematically in figs. 1 and 2. Fig. 1 illustrates the assembled detector mounted in a vacuum housing which is attached to a National Electrostatics Corporation (NEC) Model 3SR-10 PelletronTM located at Charles Evans & Associates. This accelerator produced the 2.275 MeV He’+ beams used in all the experiments described in the next section. The detector is comprised of an electron emitting foil, an electron extraction and focusing optical system, the DMCP intensifier and a phosphor screen The beam foil and extraction electrodes are mounted on
Fig. 2. Schematic diagram of the foil/electron extraction optics of the detector. standoffs attached to a 6 in. conflat flange. The DNCP fluorescent screen assembly is a custom unit developed for us by Galileo Electra-Optics Corporation (Sturbridge, MA, USA). These microchannel plates have 8 p,rn diameter channels, an active area of 18 mm and are mounted in a chevron style at a distance of approximately 50 pm apart. The fluorescent screen is a thin layer of P-20 phosphor deposited onto a fiber optic face
Fig. 1. Schematic diagram of the complete detector housing.
R. W. Odom et al. / ~o~dest~tjue
imaging detectors
plate. A thin layer of indium tin oxide is coated onto the phosphor in order to eliminate surface charge buildup on the phosphor surface. The complete DMCP/ fluorescent screen assembly is mounted on a fiber optic faceplate which is seated into a 6 in. conflat flange. Fig. 2 is a full scale illustration of the beam/foil region and the emission lens assembly. The foils or grids used to produce secondary electrons or ions mount on a special holder which is inserted into the foil plate. The active foil diameter is approximately 2.5 mm in this device. The first plate above the foil is the electron extraction element. This plate is followed by two shaped electrodes which assist in focusing the secondary electron or ion signals onto the imaging detector. The particle beam passes through the foil or grid at 45 a angle and under typical conditions, more than 95% of the primary beam passes through the foil and arrives at a Faraday cup located at the end of the flight tube. Since the beam passes through the foil region virtually unattenuated, stray components of the beam could strike the focusing electrodes leading to spurious charge build up on the lens elements. In order to minimize this charge buildup, the focusing lens was designed so that the beam would not strike the lens element surfaces. This was accomp~sh~ by milling out segments of the two focusing elements within the beam flight path. Fig. 2 does not illustrate the Faraday cup used to monitor the incoming He” beam intensity. This Faraday cup mounts on a stainless steel rod and can be positioned in or out of the beam. Image magnification is set by the ratio of the image to object distances. The ion lens is equivalent to a thick lens in light optics and the approximate image and object distances are given by the distance between the extraction plate and image detector and the distance between the foil and the extractor plate, respectively. The image ma~fica~on for the ~nfiguration shown in figs. 1 and 2 is appro~mately 10. The lateral resolution in ion or electron images is given approximately by
R==A.E,‘~E~,
0)
where R is the lateral resolution in micrometers, A E is the transverse kinetic energy of the secondary species in eV and 1E 1 is the magnitude of the electric field between the foil and extractor plate in V/urn. This expression for the resolution assumes that the image resolution is not limited by the microchannel plates (MCP). The MCP resolution is on the order of two channel diameters and is thus approximately 16 urn for 8 pm channels. Assuming a 5 eV transverse kinetic energy for secondary electrons, the image resolution will be 5 pm at the electric field strength of (1 V/pm) used in this work. This 5 urn resolution corresponds to a 50 urn resolution at the MCP well within the spatial resolution of the plates calculated resolution agrees well with the measured values presented below.
forenergetic particle beams
467
3. Results and discussion A determination of the secondary emission characteristics from a number of foil targets was the first evaluation performed with this detector. The beam transducers included different thickness carbon films (6 and 50 nm), a 5 nm coating of TiO, on a 6 nm thick carbon film and a bare transmission electron microscopy (TEM) grid. The density of the 6 nm foils is on the order of 10 ug/cm*_ Electron and ion emission coefficients were determined from these foils. One goal of this work was to compare the emission characteristics of the continuous foils to those of bare (uncoated) structures. The TEM grids utilized in these measurements are ideal grid structures since the wire sizes are 20 urn, the patterns, and spacing of these wires are well defined and reproducible and the optical transmittance of the grids are high (2 80%). The emission coefficients were determined by measuring the incident beam current on the Faraday cup positioned in front of the foil and the secondary dectron current arriving at the front plate of the DMCP/fluorescent screen detector. The eIectron emission coefficient was defined as the ratio of these two currents. The electron extraction and transmission efficiency from the foil to the detector was assumed to be unity. The electron emission coefficients were measured using a -6 kV accelerating potential on the foil and a + 12.5 kV voltage on the first focusing electrode. The extraction plate and second focusing electrode were held at ground potential for these measurements. The secondary electron emission coefficients for the carbon foils, a TEM grid and the TiO, coated carbon foils ranged between 5 and 10 electrons per incident primary particle. Similar secondary ion or electron yields were observed for the copper wires of several other TEM grids structures. The electron emission values from the thin foil targets agree quite well with literature values for particle beam/foil interactions at 2 MeV energies
181.
Positive and negative ion emission coefficients were also determined from the 6 nm carbon foil. These coefficients range between 0.005 and 0.01 ions per incident particle and are thus approximately 0.1% of the secondary electron values. The positive ion values were obtained by reversing the polarities of the foil and focusing electrode in order to extract positive ions onto the detector. The negative ion measurements were performed by placing a 0.1 T magnetic field in the vicinity of the foil. This magnetic field deflected the electron signal away from the image detector but did not affect the negative secondary ion trajectories. Although the positive or negative ion emission coefficients are significantly less than the electron yields, the ion signals are essentially unaffected by stray magnetic fields. Thus, these ion signals could possibly be used to image par-
R. W. Odom et al. / Nondestmctive
468
Fig. 3. Secondary
electron
images produced
imaging detectors for energetic particle beams
from a 6 nm carbon
title beams produced in environments having stray or nonuniform magnetic fields. Examples of electron images produced from the interaction of the He’+ beam and the 6 nm carbon foil are illustrated in Figs. 3 through 6. All of these images were acquired using the Galileo Electra-Optics Corporation DMCP/fluorescent screen detector, a Pulnix CCD camera and a Recognition Concepts Inc. (Incline Village, NE) digital image processing system. The image in fig. 3a is a grey scale image of the secondary electron distribution produced from a defocused 20 pA He2+ beam passing through the 6 nm thick carbon foil. The bright areas in the photograph correspond to the open areas in the foil. The dark lines in this image are the 20 pm wide copper grids in the 200 mesh TEM grid used to support the carbon foil. This image is the average of 128 frames which corresponds to an acquisition time of approximately 4.3 s. The intensity in this image is represented by the linear grey scale shown in the figure and this grey scale spans an intensity range from 0 to 255 levels. The arrow near the center of the image is part of the structure of the TEM support grid. The beam diameter used to produce this image was approximately 1.5 mm and the beam intensity distribution contains some distortion in the lower portion of the image. There are several possible causes of this image distortion including a highly divergent He2+ beam, nonuniformities in the electron extraction optics or nonplanar carbon foils. Since the He2+ is collimated to within a few mrad at the foil position and the films are planar to within a few urn on the surface of the TEM grid, the most likely cause of this distortion is the nonuniformity in the electron extraction optics. The emission optics requires a uniform extraction field in order to produce high resolution secondary electron or ion images. The aperture defining the beam/foil interaction area is approximately 2.5 mm in diameter. The
foil at He *+ intensities
of (a) 20 pA and (b) 40000 particle/s.
top half of the electron image in fig. 3 exhibits good resolution suggesting that the extraction optics maintain good image integrity over a region approximately 0.75 mm in diameter. This particular design of the emission lens therefore appears to maintain good image resolution over approximately 30% of the aperture diameter. The uniformity in the image resolution could be increased for larger areas (diameters) by increasing the spacing between the foil and extraction lenses. This increase in spacing would, however, require an increase in electric field strength in order to maintain image resolution. This detector can also monitor the intensity distribution of low intensity ion beams as is illustrated in fig. 3b. This image was produced by averaging the secondary electron signal produced by a 40000 pps He2+ beam. The image is the average of 512 video frames and was acquired in approximately 17 s. The intensity of this He’+ beam was measured using a Tennelec silicon surface barrier detector. Assuming an electron emission coefficient of 5, this image was produced by approximately 4 million electrons input onto the face of the microchannel plate assembly. The lateral resolution in this image is comparable to the resolution observed in fig. 3a. The detector is capable of producing relatively high lateral resolution images as is illustrated in fig. 4. This image was produced from an 8 pA He2+ beam which was focused to an approximate 0.5 mm diameter at the foil. The image in fig. 4a was acquired in half a second and illustrates excellent contrast and resolution. The lower portion of the figure is a plot of the intensity values of the picture elements (pixels) contained in the line drawn through the image. This line scan intensity profile shows that the approximately 8 pixels are contained within the 20 pm wide bars on the TEM grid. Each pixel therefore represents 2.5 urn at the foil. If one
R. W. Odom et al. / Nondestructiue
imaging detectors for energetic particle beams
469
the DMCP/fluorescent screen detector which was approximately a factor of 5 higher than that used for the electron image. As a consequence, the ion image exhibits higher noise levels compared to the electron image as would be expected from the higher gain condition of the DMCP/fluorescent screen detector. The resolution in the ion image is, however, essentially equivalent to that observed in the electron image. Similar images were observed in the analysis of positive ions emitted from the carbon foils. Since heavy ion trajectories are not adversely affected by stray magnetic fields. imaging the secondary ions could provide a convenient method of imaging energetic particles in variable magnetic field environments where the secondary electron trajectories would be severely distorted. The lower emission coefficients of secondary ions versus secondary electrons may limit their utility in low intensity particle beam applications. The distortion of secondary electron trajectories by
Fig. 4. High resolution electron image and line scan intensity profile produced from 8 pA He’* incident on 6 nm carbon foil.
requires at least 2 pixels to define a resolution element in the image, the detector can resolve features approximately 5 pm in size. This resolution agrees with the value calculated from eq. (1) in the previous section. Fig. 5a and b exhibit images produced from secondary electrons and ions, respectively. The secondary electron image exhibits good contrast and the image was acquired in a single frame (0.33 s). The ion image was produced from negative ions emitted from the carbon foil. A relatively strong magnetic field was placed close to the beam/foil interaction region in order to produce this negative ion image. A magnetic field of flux density on the order of 0.1 T deflected the electron signal to the side of the microchannel plate intensifier leaving the negative ion signal. Both the electron and negative ion images were produced from a 25 pA He’+ beam. The ion image is the average of 64 frames (2 + seconds integration time) and was acquired at a gain on
Fig. 5. (a) Secondary electron and (b) negative ion images produced from a 20 pA He’+ beam incident on a 6 nm carbon foil.
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t+---
l.wmm
imaging detectors for energetic particle beams
---+i
center of the beam illustrates that the beam is essentially Gaussian although there are several non-Gaussian asymmetries within the profile. Although most of the particle beam intensity is transmitted through a continuous foil transducer, charged particle beams will undergo charge stripping collisions in passing through a thin foil target [8]. In many applications these charge changing collisions are considered destructive of the beam quality. One possible way to minimize the affects of these stripping collisions is to employ thin filaments of carbon or polymeric materials such as polyester in place of the continuous carbon foil. An array of such filaments could be assembled which did not significantly attenuate the beam intensity (SO-95% transmittance). We evaluated the use of a bare TEM grids as an example of wire or grid type beam transducers. Figs. 7a and 7b are typical secondary electron images produced from a rectangular TEM grid structure at two different foil voltages. These images
b 255
GREY LEVEL
PIXEL
#
-
Fig. 6. (a) Pseudocolor image and (b) line scan intensity profile ofa7pAHe
** beam incident on a 50 nm carbon foil.
stray magnetic fields could possibly be eliminated by using Mu metal shielding around the imaging detector elements. The image in fig. 6 illustrates another method of presenting the image information acquired and stored on the image processing system. The intensity scale in this image is displayed in pseudo or false color in order to augment the various intensity regions in the image. The color-intensity mapping of this image is illustrated in the color bar at the right of the image. The image was produced from a 7 pA He 2+ beam focused to 300 pm diameter spot on a 50 nm thick carbon foil. The intensity profile illustrates four distinct intensity regions within the beam and the central (green) region is approximately 300 urn in diameter. Although the beam is nominally circular, the image illustrates obvious nonuniformities. The line scan intensity profile through the
Fig. 7. Secondary electron images produced from 2.5 nA He’+ beams incident on a TEM grid.
R. W. Odom et al. / Nondestructive
imaging detectors for energetic particle beams
were produced from a defocused, 2.5 nA He2+ beam. The TEM grids employed in this work were arrays of copper wires 20 pm in diameter spaced approximately 80 pm apart. The optical transmittance of this grid structure was approximately 80%. The secondary electron intensity in the figure exhibits a definite shading or gradation due to the orientation of the He beam with respect to the grid. The copper wires in the image are readily resolved suggesting that the image resolution is better than 20 pm. The image in fig. 7b was produced at a foil voltage which was 2.8% higher (6.17 vs 6.03 kV) than the image produced in fig. 7a. The image in fig. 7b shows higher resolution of the copper leads as well as a truer representation of the grid structure. The thin films used in these applications had relatively long operation times. For example, both 6 and 50 nm carbon foils were unaffected by nA He2+ beam currents for tens of hours of operation time. Similar results were obtained from the TiO, coated carbon foil. A single pair of dual microchannel plates were employed for all these initial measurements. We estimate that these plates were operated at intermediate gains (between lo4 and 105) for approximately 100 hours. There was no apparent degradation in the gain characteristics of the plates although the phosphor screen took on the appearance of a TEM grid after approximately 50 hours of operation. Apparently, the electron output from the DMCP assembly “burned” the grid pattern into the phosphor. By varying the gain of the microchannel plate assembly, the DMCP/fluorescent screen detector could be operated over a large dynamic range for these types of imaging applications. The minimum number of counts needed to produce a useful image on the detector was approximately 10000 secondary electrons while images produced from approximately 1 x 10” secondary electrons could also be
Fig. 8. Hybrid electrons
RAE produced
image of the distribution of secondary from a 10000 particle/s He*+ beam.
471
obtained. Thus, this detection system has a dynamic range on the order of 107. The relative advantages and disadvantages of the DIP technique for beam monitoring were also evaluated in this research. The coupling of the DMCP/fluorescent screen detector to a video camera and a digital image processing system provides a very convenient method for acquiring and storing a variety of particle beam images. This system provides a routine method for tuning energetic particle beams with little or no attenuation of the beam intensity and is capable of handling a wide range of signal intensities. The DIP technique does not, however, lend itself to the measurement of low intensity signals (< 1000 pps) nor does it provide a direct quantitative correlation between image intensity and the secondary ion or electron intensity. The camera-based system can theoretically perform this correlation but the process is fraught with difficulties which include the limited dynamic range of the camera and the variability in microchannel plate gain with plate age [91. The measurement of the intensity in various pixels in the image is directly related t-o the incident beam intensity. Thus, any technique which can quantitatively measure the image intensity will provide a direct measure of the particle beam intensity. One method of performing this correlation is to use a conventional or hybrid resistive anode encoder (RAE) in place of the fluorescent screen detector. We have utilized RAE detectors for imaging the lateral distributions of secondary ions produced by stigmatic imaging secondary ion mass spectrometers [lo]. We have also developed a hybrid RAE detector (patent pending) which is comprised of an optically transparent resistive anode coated with a phosphor screen. This hybrid detector combines the pulse counting, image acquisition capability of conventional resistive anode encoders with the high dynamic range capability of a conventional phosphor screen. Thus, the hybrid RAE can acquire a quantitative two dimensional image of intensity distribution of incident radiation (ions or electrons) up to the count rate limit of the detection electronics (- lo5 counts/s). The detector simultaneously produces a qualitative light image of the intensity distribution in the image much like a conventional fluorescent screen detector and thus has the extended dynamic range capability of the DMCP/fluorescent screen devices. As an illustration of the type of images produced from a hybrid detector, fig. 8 is a photograph of a computer monitor displaying a grey scale image acquired using this detector. This image was produced by a He*+ beam passing through a 6 nm carbon foil mounted on a TEM grid. The intensity of the particle beam was 10000 pps and the image was acquired in 30 s. The image was acquired and stored on a personal computer (PC)-based image processing system [ll]. Although there is some
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distortion in the image due primarily to improper film resistance in certain regions of the hybrid anode, this image does illustrate good image resolution. Equally important is the fact that signal intensity in each pixel of the image has been quantitatively measured. For example, the total number of electrons detected in this image is 1.5 million. Once these quantitative images are acquired and stored on the computer system, a variety of image processing functions such as quantitative line scan profiles, selected area analysis and imaging ratios can be determined.
4. Conclusions The secondary electron or ion image resolution for this prototype system is on the order of 5 pm for beam diameters of approximately 1.5 mm. Since the focusing properties of the emission lens can be modified to accommodate a range of beam and image sensor geometries, there are a number of possible detector sizes, beam image resolution and/or image magnifications possible with the basic emission lens design. The development of detectors for large area beams (> 10 cm in diameter) appears feasible although there are obvious image size/resolution compromises which have to be considered. Large area imaging detectors could find extensive utilization as beam monitors in a variety of applications including use on ion beam radiotherapy instruments [12]. A series combination of two quantitative position sensitive detectors each comprised of a foil transducer, stigmatic optics and resistive anode encoder image sensor could provide a straightforward method of nonintrusively determining particle beam emittance and/or energy. We are currently exploring various designs of this type of beam emittance detector.
The authors gratefully acknowledge support for this research provided by DOE Small Business Innovative Research (SBIR) Grant #DE-AC03-88ER8066. The assistance of Dr. David A. Reed in the design of the emission lens assembly is also gratefully acknowledged.
References [l] H.W. Fulbright,
Nucl. Instr. and Meth. 162 (1979) 21. [2] J.T. Walton, Nucl. Instr. and Meth. 226 (1984) 1. [3] E. Laegsgaard, Nucl. Instr. and Meth. 162 (1979) 93. [4] F.S. Goulding and B.G. Harvey, in: Ann. Rev. Nucl. Sci., 25 (1975) 167. [5] M.L. Muga, Nucl. Instr. and Meth. 105 (1972) 61. [6] M. Lampton and F. Paresce, Rev. Sci. Instrum. 45 (1974) 1098. [7] R. Castaing and G. Slodzian, J. Phys. El4 (1981) 1119. [8] H.J. Frischkorn and K.O. Groeneveld, Phys. Scripta T6 (1983) 89. [9] H.G. Boettger, C.E. Giffin and D.D. Norris, in: Multichannel Image detectors, ed. Y. Talmi, ACS Symposium Series 102 (Amer. Chem. Sot., Washington, DC, 1979). [lo] R.W. Odom, B.K. Furman, C.A. Evans, Jr., C.E. Bryson, W.A. Petersen, M.K. Kelley and D.H. Wayne, Anal. Chem. 55 (1983) 574. [ll] R.W. Odom, D.H. Wayne and C.A. Evans Jr., in: Secondary Ion Mass Spectroscopy SIMS IV, eds. A. Benninghoven, J. Okano, R. Schimizu and H.W. Werner, Springer Verlag Chemical Physics Series 36 (Springer, New York, 1984) p. 186. [12] W. Scharf, Particle Accelerators and Their Uses (Harwood Academic Publishers, London, 1986) vol. 2.
Nuclear Instruments and Methods in Physics Research B44 (1990) 465-472 North-Holland
NONDESTRUCTIVE Robert W. ODOM,
IMAGING Michael
Charles Evans and Associates,
DETECTORS
D. STRATHMAN,
FOR ENERGETIC S.E. BUTTRILL,
PARTICLE
Jr. and Scott
BEAMS
M. BAUMANN
301 Chesapeake Drive, Redwood City, CA 94063, USA
Received 22 May 1989 and in revised form 14 August 1989
This paper describes the design and discusses initial experimental results produced from a nondestructive, position sensitive particle beam detector. The detector measures the spatial and intensity distribution of a particle beam by determining the position and intensity of secondary electrons or ions produced by particle beam interactions with thin foils ( - 10 ug/cm*) or wire grids. These secondary charged particles are focused onto an imaging detector using stigmatic ion optics which provide a high degree of correlation between the position of the incident particle beam at the foil and the position of secondary ions at the image sensor. The imaging detector is nonintrusive to the particle beam flux since most of the incident beam passes through the thin foil or around the wire mesh. This detector can monitor particle beam intensities ranging from tens of microamperes down to several hundred particles per second. Image resolution on the order of 5 pm is observed for particle beams having diameters on the order of several millimeters.
1.
Introduction
Current state-of-the-art energetic beam detectors generally fall into two categories: 1) high intensity (> 10” events or particles per second) total beam monitors and single or multiwire position sensitive detectors (PSD) [l] and 2) low intensity (< 10’ events per second) semiconductor total beam monitors or PSD’s [2]. Although particle beams are generally consumed during the detection process and neither detector operates well outside of its optimum range, these devices have found extensive application in particle beam detection [3]. These detectors are limited primarily by the limited intensity range over which useful measurements can be obtained and the lack of high lateral resolution imaging capabilities. For example, neither detector performs routine and reliable beam monitoring at beam intensities between lo5 and approximately 10” particles per second (pps). In addition’neither of these detectors are capable of determining the lateral distribution of a particle beam at micrometer or submicrometer (pm) lateral resolutions. This paper presents results on the development of novel beam imaging techniques which characterize the intensity distributions within energetic particle beams with minimal beam attenuation. This nondestructive or nonintrusive detector converts a portion of the incident particle beam into a secondary electron or ion signal by passing the particle beam through a thin foil. The secondary electrons or ions produced from this beam/foil interaction are extracted from the foil surface and focused onto an imaging detector. The image detec0168-583X/90/%03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
tor described below is comprised of components which have been utilized individually in various particle beam detectors. For example, the production of secondary ions or electrons from collisions of energeticbeams with a foil “transducer” is a well established procedure in particle beam detection [4]. Similarly, microchannel plate intensifiers have been employed to detect the secondary ions or electrons produced in these beam/foil interactions [5]. A detector which coupled a foil transducer with proximity focusing electron optics, a microchannel plate intensifier and image sensor has also been briefly described in the literature [4]. The unique feature of the prototype detector described in this paper is the combination of the beam foil transducer with stigmatic ion optics which focus the secondary ion or electrons onto the imaging detector. The secondary electron or ion image can be magnified with these ion optics and thus increase the resolution in the particle beam image. The use of stigmatic electron optics also adds a wide range of design flexibility in the selection of the image detector. These optics should be adaptable to various detector configurations having a wide range of sensor areas and/or image magnifications. Several image detectors including dual microchannel plate (DMCP)/fluorescent screens, resistive anode encoders (RAE) [6] as well as a hybrid fluorescent screen/RAE detector were evaluated in this work. The beam transducers included thin carbon and metal oxide foils as well as wire meshes. A prototype detector having an active diameter of 2.5 mm was evaluated using 2.275 MeV He*+ beams. This detector produces useful secondary electron or ion images at particle beam intensities ranging from several nanoamperes to several hundred particles per second. The
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R. W. Odom et al. / Nondestructive
imaging detectors for energetic particle beams
resolution in many of these images is on the order of 5 pm which is close to the limit for this device based on the estimated transverse energy of secondary electrons produced in the beam/foil interaction.
2. Experimental The imaging detector measures the intensity and lateral distribution of secondary electrons (or ions) produced by the passage of an energetic beam through a thin foil. The correspondence between the intensity distribution within the secondary electron image and the incident particle beam is achieved with stigmatic imaging optics [7] which focus the secondaq electrons produced at the foil onto a position sensitive imaging detector with minimal image distortion. The design of this detector is illustrated schematically in figs. 1 and 2. Fig. 1 illustrates the assembled detector mounted in a vacuum housing which is attached to a National Electrostatics Corporation (NEC) Model 3SR-10 PelletronTM located at Charles Evans & Associates. This accelerator produced the 2.275 MeV He’+ beams used in all the experiments described in the next section. The detector is comprised of an electron emitting foil, an electron extraction and focusing optical system, the DMCP intensifier and a phosphor screen The beam foil and extraction electrodes are mounted on
Fig. 2. Schematic diagram of the foil/electron extraction optics of the detector. standoffs attached to a 6 in. conflat flange. The DNCP fluorescent screen assembly is a custom unit developed for us by Galileo Electra-Optics Corporation (Sturbridge, MA, USA). These microchannel plates have 8 p,rn diameter channels, an active area of 18 mm and are mounted in a chevron style at a distance of approximately 50 pm apart. The fluorescent screen is a thin layer of P-20 phosphor deposited onto a fiber optic face
Fig. 1. Schematic diagram of the complete detector housing.
R. W. Odom et al. / ~o~dest~tjue
imaging detectors
plate. A thin layer of indium tin oxide is coated onto the phosphor in order to eliminate surface charge buildup on the phosphor surface. The complete DMCP/ fluorescent screen assembly is mounted on a fiber optic faceplate which is seated into a 6 in. conflat flange. Fig. 2 is a full scale illustration of the beam/foil region and the emission lens assembly. The foils or grids used to produce secondary electrons or ions mount on a special holder which is inserted into the foil plate. The active foil diameter is approximately 2.5 mm in this device. The first plate above the foil is the electron extraction element. This plate is followed by two shaped electrodes which assist in focusing the secondary electron or ion signals onto the imaging detector. The particle beam passes through the foil or grid at 45 a angle and under typical conditions, more than 95% of the primary beam passes through the foil and arrives at a Faraday cup located at the end of the flight tube. Since the beam passes through the foil region virtually unattenuated, stray components of the beam could strike the focusing electrodes leading to spurious charge build up on the lens elements. In order to minimize this charge buildup, the focusing lens was designed so that the beam would not strike the lens element surfaces. This was accomp~sh~ by milling out segments of the two focusing elements within the beam flight path. Fig. 2 does not illustrate the Faraday cup used to monitor the incoming He” beam intensity. This Faraday cup mounts on a stainless steel rod and can be positioned in or out of the beam. Image magnification is set by the ratio of the image to object distances. The ion lens is equivalent to a thick lens in light optics and the approximate image and object distances are given by the distance between the extraction plate and image detector and the distance between the foil and the extractor plate, respectively. The image ma~fica~on for the ~nfiguration shown in figs. 1 and 2 is appro~mately 10. The lateral resolution in ion or electron images is given approximately by
R==A.E,‘~E~,
0)
where R is the lateral resolution in micrometers, A E is the transverse kinetic energy of the secondary species in eV and 1E 1 is the magnitude of the electric field between the foil and extractor plate in V/urn. This expression for the resolution assumes that the image resolution is not limited by the microchannel plates (MCP). The MCP resolution is on the order of two channel diameters and is thus approximately 16 urn for 8 pm channels. Assuming a 5 eV transverse kinetic energy for secondary electrons, the image resolution will be 5 pm at the electric field strength of (1 V/pm) used in this work. This 5 urn resolution corresponds to a 50 urn resolution at the MCP well within the spatial resolution of the plates calculated resolution agrees well with the measured values presented below.
forenergetic particle beams
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3. Results and discussion A determination of the secondary emission characteristics from a number of foil targets was the first evaluation performed with this detector. The beam transducers included different thickness carbon films (6 and 50 nm), a 5 nm coating of TiO, on a 6 nm thick carbon film and a bare transmission electron microscopy (TEM) grid. The density of the 6 nm foils is on the order of 10 ug/cm*_ Electron and ion emission coefficients were determined from these foils. One goal of this work was to compare the emission characteristics of the continuous foils to those of bare (uncoated) structures. The TEM grids utilized in these measurements are ideal grid structures since the wire sizes are 20 urn, the patterns, and spacing of these wires are well defined and reproducible and the optical transmittance of the grids are high (2 80%). The emission coefficients were determined by measuring the incident beam current on the Faraday cup positioned in front of the foil and the secondary dectron current arriving at the front plate of the DMCP/fluorescent screen detector. The eIectron emission coefficient was defined as the ratio of these two currents. The electron extraction and transmission efficiency from the foil to the detector was assumed to be unity. The electron emission coefficients were measured using a -6 kV accelerating potential on the foil and a + 12.5 kV voltage on the first focusing electrode. The extraction plate and second focusing electrode were held at ground potential for these measurements. The secondary electron emission coefficients for the carbon foils, a TEM grid and the TiO, coated carbon foils ranged between 5 and 10 electrons per incident primary particle. Similar secondary ion or electron yields were observed for the copper wires of several other TEM grids structures. The electron emission values from the thin foil targets agree quite well with literature values for particle beam/foil interactions at 2 MeV energies
181.
Positive and negative ion emission coefficients were also determined from the 6 nm carbon foil. These coefficients range between 0.005 and 0.01 ions per incident particle and are thus approximately 0.1% of the secondary electron values. The positive ion values were obtained by reversing the polarities of the foil and focusing electrode in order to extract positive ions onto the detector. The negative ion measurements were performed by placing a 0.1 T magnetic field in the vicinity of the foil. This magnetic field deflected the electron signal away from the image detector but did not affect the negative secondary ion trajectories. Although the positive or negative ion emission coefficients are significantly less than the electron yields, the ion signals are essentially unaffected by stray magnetic fields. Thus, these ion signals could possibly be used to image par-
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Fig. 3. Secondary
electron
images produced
imaging detectors for energetic particle beams
from a 6 nm carbon
title beams produced in environments having stray or nonuniform magnetic fields. Examples of electron images produced from the interaction of the He’+ beam and the 6 nm carbon foil are illustrated in Figs. 3 through 6. All of these images were acquired using the Galileo Electra-Optics Corporation DMCP/fluorescent screen detector, a Pulnix CCD camera and a Recognition Concepts Inc. (Incline Village, NE) digital image processing system. The image in fig. 3a is a grey scale image of the secondary electron distribution produced from a defocused 20 pA He2+ beam passing through the 6 nm thick carbon foil. The bright areas in the photograph correspond to the open areas in the foil. The dark lines in this image are the 20 pm wide copper grids in the 200 mesh TEM grid used to support the carbon foil. This image is the average of 128 frames which corresponds to an acquisition time of approximately 4.3 s. The intensity in this image is represented by the linear grey scale shown in the figure and this grey scale spans an intensity range from 0 to 255 levels. The arrow near the center of the image is part of the structure of the TEM support grid. The beam diameter used to produce this image was approximately 1.5 mm and the beam intensity distribution contains some distortion in the lower portion of the image. There are several possible causes of this image distortion including a highly divergent He2+ beam, nonuniformities in the electron extraction optics or nonplanar carbon foils. Since the He2+ is collimated to within a few mrad at the foil position and the films are planar to within a few urn on the surface of the TEM grid, the most likely cause of this distortion is the nonuniformity in the electron extraction optics. The emission optics requires a uniform extraction field in order to produce high resolution secondary electron or ion images. The aperture defining the beam/foil interaction area is approximately 2.5 mm in diameter. The
foil at He *+ intensities
of (a) 20 pA and (b) 40000 particle/s.
top half of the electron image in fig. 3 exhibits good resolution suggesting that the extraction optics maintain good image integrity over a region approximately 0.75 mm in diameter. This particular design of the emission lens therefore appears to maintain good image resolution over approximately 30% of the aperture diameter. The uniformity in the image resolution could be increased for larger areas (diameters) by increasing the spacing between the foil and extraction lenses. This increase in spacing would, however, require an increase in electric field strength in order to maintain image resolution. This detector can also monitor the intensity distribution of low intensity ion beams as is illustrated in fig. 3b. This image was produced by averaging the secondary electron signal produced by a 40000 pps He2+ beam. The image is the average of 512 video frames and was acquired in approximately 17 s. The intensity of this He’+ beam was measured using a Tennelec silicon surface barrier detector. Assuming an electron emission coefficient of 5, this image was produced by approximately 4 million electrons input onto the face of the microchannel plate assembly. The lateral resolution in this image is comparable to the resolution observed in fig. 3a. The detector is capable of producing relatively high lateral resolution images as is illustrated in fig. 4. This image was produced from an 8 pA He2+ beam which was focused to an approximate 0.5 mm diameter at the foil. The image in fig. 4a was acquired in half a second and illustrates excellent contrast and resolution. The lower portion of the figure is a plot of the intensity values of the picture elements (pixels) contained in the line drawn through the image. This line scan intensity profile shows that the approximately 8 pixels are contained within the 20 pm wide bars on the TEM grid. Each pixel therefore represents 2.5 urn at the foil. If one
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the DMCP/fluorescent screen detector which was approximately a factor of 5 higher than that used for the electron image. As a consequence, the ion image exhibits higher noise levels compared to the electron image as would be expected from the higher gain condition of the DMCP/fluorescent screen detector. The resolution in the ion image is, however, essentially equivalent to that observed in the electron image. Similar images were observed in the analysis of positive ions emitted from the carbon foils. Since heavy ion trajectories are not adversely affected by stray magnetic fields. imaging the secondary ions could provide a convenient method of imaging energetic particles in variable magnetic field environments where the secondary electron trajectories would be severely distorted. The lower emission coefficients of secondary ions versus secondary electrons may limit their utility in low intensity particle beam applications. The distortion of secondary electron trajectories by
Fig. 4. High resolution electron image and line scan intensity profile produced from 8 pA He’* incident on 6 nm carbon foil.
requires at least 2 pixels to define a resolution element in the image, the detector can resolve features approximately 5 pm in size. This resolution agrees with the value calculated from eq. (1) in the previous section. Fig. 5a and b exhibit images produced from secondary electrons and ions, respectively. The secondary electron image exhibits good contrast and the image was acquired in a single frame (0.33 s). The ion image was produced from negative ions emitted from the carbon foil. A relatively strong magnetic field was placed close to the beam/foil interaction region in order to produce this negative ion image. A magnetic field of flux density on the order of 0.1 T deflected the electron signal to the side of the microchannel plate intensifier leaving the negative ion signal. Both the electron and negative ion images were produced from a 25 pA He’+ beam. The ion image is the average of 64 frames (2 + seconds integration time) and was acquired at a gain on
Fig. 5. (a) Secondary electron and (b) negative ion images produced from a 20 pA He’+ beam incident on a 6 nm carbon foil.
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t+---
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center of the beam illustrates that the beam is essentially Gaussian although there are several non-Gaussian asymmetries within the profile. Although most of the particle beam intensity is transmitted through a continuous foil transducer, charged particle beams will undergo charge stripping collisions in passing through a thin foil target [8]. In many applications these charge changing collisions are considered destructive of the beam quality. One possible way to minimize the affects of these stripping collisions is to employ thin filaments of carbon or polymeric materials such as polyester in place of the continuous carbon foil. An array of such filaments could be assembled which did not significantly attenuate the beam intensity (SO-95% transmittance). We evaluated the use of a bare TEM grids as an example of wire or grid type beam transducers. Figs. 7a and 7b are typical secondary electron images produced from a rectangular TEM grid structure at two different foil voltages. These images
b 255
GREY LEVEL
PIXEL
#
-
Fig. 6. (a) Pseudocolor image and (b) line scan intensity profile ofa7pAHe
** beam incident on a 50 nm carbon foil.
stray magnetic fields could possibly be eliminated by using Mu metal shielding around the imaging detector elements. The image in fig. 6 illustrates another method of presenting the image information acquired and stored on the image processing system. The intensity scale in this image is displayed in pseudo or false color in order to augment the various intensity regions in the image. The color-intensity mapping of this image is illustrated in the color bar at the right of the image. The image was produced from a 7 pA He 2+ beam focused to 300 pm diameter spot on a 50 nm thick carbon foil. The intensity profile illustrates four distinct intensity regions within the beam and the central (green) region is approximately 300 urn in diameter. Although the beam is nominally circular, the image illustrates obvious nonuniformities. The line scan intensity profile through the
Fig. 7. Secondary electron images produced from 2.5 nA He’+ beams incident on a TEM grid.
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imaging detectors for energetic particle beams
were produced from a defocused, 2.5 nA He2+ beam. The TEM grids employed in this work were arrays of copper wires 20 pm in diameter spaced approximately 80 pm apart. The optical transmittance of this grid structure was approximately 80%. The secondary electron intensity in the figure exhibits a definite shading or gradation due to the orientation of the He beam with respect to the grid. The copper wires in the image are readily resolved suggesting that the image resolution is better than 20 pm. The image in fig. 7b was produced at a foil voltage which was 2.8% higher (6.17 vs 6.03 kV) than the image produced in fig. 7a. The image in fig. 7b shows higher resolution of the copper leads as well as a truer representation of the grid structure. The thin films used in these applications had relatively long operation times. For example, both 6 and 50 nm carbon foils were unaffected by nA He2+ beam currents for tens of hours of operation time. Similar results were obtained from the TiO, coated carbon foil. A single pair of dual microchannel plates were employed for all these initial measurements. We estimate that these plates were operated at intermediate gains (between lo4 and 105) for approximately 100 hours. There was no apparent degradation in the gain characteristics of the plates although the phosphor screen took on the appearance of a TEM grid after approximately 50 hours of operation. Apparently, the electron output from the DMCP assembly “burned” the grid pattern into the phosphor. By varying the gain of the microchannel plate assembly, the DMCP/fluorescent screen detector could be operated over a large dynamic range for these types of imaging applications. The minimum number of counts needed to produce a useful image on the detector was approximately 10000 secondary electrons while images produced from approximately 1 x 10” secondary electrons could also be
Fig. 8. Hybrid electrons
RAE produced
image of the distribution of secondary from a 10000 particle/s He*+ beam.
471
obtained. Thus, this detection system has a dynamic range on the order of 107. The relative advantages and disadvantages of the DIP technique for beam monitoring were also evaluated in this research. The coupling of the DMCP/fluorescent screen detector to a video camera and a digital image processing system provides a very convenient method for acquiring and storing a variety of particle beam images. This system provides a routine method for tuning energetic particle beams with little or no attenuation of the beam intensity and is capable of handling a wide range of signal intensities. The DIP technique does not, however, lend itself to the measurement of low intensity signals (< 1000 pps) nor does it provide a direct quantitative correlation between image intensity and the secondary ion or electron intensity. The camera-based system can theoretically perform this correlation but the process is fraught with difficulties which include the limited dynamic range of the camera and the variability in microchannel plate gain with plate age [91. The measurement of the intensity in various pixels in the image is directly related t-o the incident beam intensity. Thus, any technique which can quantitatively measure the image intensity will provide a direct measure of the particle beam intensity. One method of performing this correlation is to use a conventional or hybrid resistive anode encoder (RAE) in place of the fluorescent screen detector. We have utilized RAE detectors for imaging the lateral distributions of secondary ions produced by stigmatic imaging secondary ion mass spectrometers [lo]. We have also developed a hybrid RAE detector (patent pending) which is comprised of an optically transparent resistive anode coated with a phosphor screen. This hybrid detector combines the pulse counting, image acquisition capability of conventional resistive anode encoders with the high dynamic range capability of a conventional phosphor screen. Thus, the hybrid RAE can acquire a quantitative two dimensional image of intensity distribution of incident radiation (ions or electrons) up to the count rate limit of the detection electronics (- lo5 counts/s). The detector simultaneously produces a qualitative light image of the intensity distribution in the image much like a conventional fluorescent screen detector and thus has the extended dynamic range capability of the DMCP/fluorescent screen devices. As an illustration of the type of images produced from a hybrid detector, fig. 8 is a photograph of a computer monitor displaying a grey scale image acquired using this detector. This image was produced by a He*+ beam passing through a 6 nm carbon foil mounted on a TEM grid. The intensity of the particle beam was 10000 pps and the image was acquired in 30 s. The image was acquired and stored on a personal computer (PC)-based image processing system [ll]. Although there is some
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distortion in the image due primarily to improper film resistance in certain regions of the hybrid anode, this image does illustrate good image resolution. Equally important is the fact that signal intensity in each pixel of the image has been quantitatively measured. For example, the total number of electrons detected in this image is 1.5 million. Once these quantitative images are acquired and stored on the computer system, a variety of image processing functions such as quantitative line scan profiles, selected area analysis and imaging ratios can be determined.
4. Conclusions The secondary electron or ion image resolution for this prototype system is on the order of 5 pm for beam diameters of approximately 1.5 mm. Since the focusing properties of the emission lens can be modified to accommodate a range of beam and image sensor geometries, there are a number of possible detector sizes, beam image resolution and/or image magnifications possible with the basic emission lens design. The development of detectors for large area beams (> 10 cm in diameter) appears feasible although there are obvious image size/resolution compromises which have to be considered. Large area imaging detectors could find extensive utilization as beam monitors in a variety of applications including use on ion beam radiotherapy instruments [12]. A series combination of two quantitative position sensitive detectors each comprised of a foil transducer, stigmatic optics and resistive anode encoder image sensor could provide a straightforward method of nonintrusively determining particle beam emittance and/or energy. We are currently exploring various designs of this type of beam emittance detector.
The authors gratefully acknowledge support for this research provided by DOE Small Business Innovative Research (SBIR) Grant #DE-AC03-88ER8066. The assistance of Dr. David A. Reed in the design of the emission lens assembly is also gratefully acknowledged.
References [l] H.W. Fulbright,
Nucl. Instr. and Meth. 162 (1979) 21. [2] J.T. Walton, Nucl. Instr. and Meth. 226 (1984) 1. [3] E. Laegsgaard, Nucl. Instr. and Meth. 162 (1979) 93. [4] F.S. Goulding and B.G. Harvey, in: Ann. Rev. Nucl. Sci., 25 (1975) 167. [5] M.L. Muga, Nucl. Instr. and Meth. 105 (1972) 61. [6] M. Lampton and F. Paresce, Rev. Sci. Instrum. 45 (1974) 1098. [7] R. Castaing and G. Slodzian, J. Phys. El4 (1981) 1119. [8] H.J. Frischkorn and K.O. Groeneveld, Phys. Scripta T6 (1983) 89. [9] H.G. Boettger, C.E. Giffin and D.D. Norris, in: Multichannel Image detectors, ed. Y. Talmi, ACS Symposium Series 102 (Amer. Chem. Sot., Washington, DC, 1979). [lo] R.W. Odom, B.K. Furman, C.A. Evans, Jr., C.E. Bryson, W.A. Petersen, M.K. Kelley and D.H. Wayne, Anal. Chem. 55 (1983) 574. [ll] R.W. Odom, D.H. Wayne and C.A. Evans Jr., in: Secondary Ion Mass Spectroscopy SIMS IV, eds. A. Benninghoven, J. Okano, R. Schimizu and H.W. Werner, Springer Verlag Chemical Physics Series 36 (Springer, New York, 1984) p. 186. [12] W. Scharf, Particle Accelerators and Their Uses (Harwood Academic Publishers, London, 1986) vol. 2.