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A small rugged imaging X-ray spectrometer: A lixiscope with good energy resolution
Nuclear Instruments and Methods 172 (1980) 471--477 © North-Holland Publishing Company
A SMALL RUGGED IMAGING X-RAY SPECTROMETER: A LIXISCOPE WITH GOOD ENERGY RESOLUTION Lo I YIN, Jacob I. TROMBKA Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
and Stephen M. SELTZER Center for Radiation Research, National Bureau of Standards, Washington, DC 20234, USA
Received 13 December 1979
A new prototype Lixiscope (Low Intensity X-ray Imaging Scope) is described for operation in the 20-200 keV region. In addition to good spatial resolution, the new prototype is capable of providing simultaneous gamma-ray or X-ray single-photon counting, imaging, and energy resolution. The observed energy resolution determined from gamma-ray pulse-height spectra is only a factor of two poorer than that of a NaI(TI)-PMT (photomultipler tube) system. Taking into account the good spatial resolution, such a Lixiscope is thus equivalent to operating thousands of NaI(TI)-PMT systems in parallel with minimal degradation in overall energy resolution. These characteristics make the new prototype Lixiscope a compact and rugged device eminently suited for possible low-flux imaging applications.
1. Introduction
2. The Lixiscope
Previously we reported on a small portable X-ray imaging * device, shown in fig. 1 with the hybrid acronym: Lixiscope (Low Intensity X-ray Imaging Scope) [ 1 - 3 ] . The Lixiscope was originally conceived for use in X-ray astronomy where singlephoton imaging is required. Although the operating characteristics of the first prototypes [ 1 - 3 ] fell somewhat short of this goal, the rugged and fully portable device could be made immediately useful in terrestrial fluoroscopic or intensifier-assisted radiographic applications where the X-rays are orders of magnitude more intense by astronomical standards. In this paper we report on an improved Lixiscope whose characteristics are more compatible with those required in astronomy and other low-flux applications. Specifically, in addition to good spatial resolution in the 2 0 - 2 0 0 keV region, the improved prototype is capable of single-photon counting, imaging, and shows remarkably good energy resolution as well.
Briefly, the design of the Lixiscope as an X-ray imaging system is based on a modular approach where
* By imaging we mean the preservation of positional information of the incident X-ray in the detector plane.
Fig. 1. The first prototype Lixiscope including a radioisotope source in its container. 471
472
Lo I Yin et al. / A small rugged imaging X-ray spectrometer PHOTON TO ELECTRON CONVERSION °,
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FIBER OPTICS
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Fig. 2. Schematicdiagram of the Lixiscopesystem. the components are physically independent entities. This approach allows maximum versatility and ruggedness as well as capability for high detection efficiency through the judicious selection of the components. As will be seen, this feature served us well in the present study. Conceptually, the Lixiscope is extremely simple (see fig. 2). An X-ray or a gamma-ray image is first converted into a visible-light image by a scintillator. The visible-light image is then coupled via fiber optics to a microchannel plate (MCP) visible-light image intensifier [1]. The MCP image intensifier is a self-contained entity which is both compact and rugged. It is a highvacuum tube sealed by fiber-optic input and output face plates. Deposited on the vacuum side of the input fiber-optic face plate is a visible-light photocathode which converts the incident visible-light image produced in the scintillator into an electron image. The electrons are then multiplied by the microchannel plate (MCP) electron multiplier. Because electron multiplication is confined within the 12 tan diameter microchannels, positional information (i.e. image) is preserved. The output of the MCP is accelerated to about 5 kV to impinge on an aluminized phosphor deposited on the output fiberoptic face plate and provides a much intensified visible-light image. In the energy range between 20-200 keV, the advantages of the Lixiscope approach as compared with other attempts involving MCP detectors [4-11] are the following:
(a) high detection efficiency. Because a large number (>~10 a) of visible-light photons are produced in the scintillator by each absorbed X-ray or gamma-ray photon, the probability of information loss after the initial photoelectric absorption process is negligibly small. Therefore, the detection efficiency of the Lixiscope is essentially governed by the scintillator rather than by the MCP itself. Ultimately, a compromize must be reached between high quantum detection efficiency (thick sinctillator) and high spatial resolution (thin scintillator), For example, a 1 mm thick CsI scintillator would have nearly 100% detection efficiency for photon energies below 60 keV, and ~60% at 100 keV, ~28% at 150 keV, and ~16% at 200 keV. Using a thin rare-earth phosphor scintillator, the observed resolution of the first prototype Lixiscope at 28 keV is 4 line pairs (lp) per mm. (b) Ruggedness. In contrast to other attempts [6,7], the Lixiscope places the scintillator externally to, and physically independent of the visible-light image intensifier, thereby eliminating the need for any fragile vacuum window or attached vacuum pump. Consequently, the Lixiscope is very rugged ° and well suited for both space and terrestrial applications.
* A Lixiscopewas accidentallydropped from about 1 m onto a concrete floor. While the Lixiscope suffered no physical damage, the instantaneousemotional distress to the authors was considerable.
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
3. New prototype Lixiscope The relatively low electron gain (103-104 ) and the problem of ion feedback in conventional singleMCP image intensifiers precluded single-photon X-ray imaging with our previous prototype [1,2]. Subsequently, using an experimental tube incorporating a curved MCP, we were able to achieve single X-ray and gamma-ray photon counting including a suggestive energy sensivitity [3]. The new prototype reported here consists of a dual-MCP, also known as chevron-MCP, image intensifier tube purchased from Varian Associates" The microchannels are 12/am in diameter with length-to-diameter ratio of 80 : 1. For gamma-ray single-photon counting and imaging, thin NaI(TI) scintillators were used in conjunction with the chevron-MCP tube. Before presenting the experimental results it is necessary to discuss the rationale behind our approach, and to indicate how energy resolution in the 2 0 - 2 0 0 keV range can be achieved with this new device.
4. Single gamma-ray imaging and energy resolution As in well known, when a gamma-ray is absorbed in a scintillator the number of visible-light photons generated by the primary photoelectron is essentially proportional to its kinetic energy and hence proportional to the gamma-ray energy. Consequently, the number of electrons generated at the photocathode of the Lixiscope is also proportional to the energy of the incident gamma-ray, in complete analogy with a scintillation-photomultiplier tube (PMT) system. The gain for single-electron inputs in the present chevron-MCP intensifier can be driven to saturation resulting in a Gaussian-shaped pulse-height distribution and a mean gain of ~107. Such a saturated gain distribution greatly decreases the gain variance in individual microchannels as well as among many microchannels. Thus, in the gain-saturated mode, the MCP output can be made proportional to the number of simultaneous input electrons. This has been demonstrated, for instance, with high-intensity visible light where the output pulse height was shown [12] to be indeed proportional to single, double, and * Manufacturers are specified only to uniquely identify the components and not as an endorsement by the National Aeronautical and Space Administration or the National Bureau of Standards.
473
triple photoelectron inputs. It should be noted that in the high-intensity visible-light case, the multiple electrons simultaneously trigger microchannels which are randomly distributed in location. Those simultaneously triggered by an absorbed gamma ray are most likely to occur in a bundle, i.e. immediately adjacent to each other. In fact, in order to preserve good spatial resolution, it is desirbale to design the scintillator-fiber-optic system to make the bundle as small as possible per absorbed gamma ray. This implies that many single microchannels in the bundle will be accepting multiple electrons as well. Since in the gain-saturated mode the charge output of the single microchannel is clamped at the saturated value, the proportionality between output pulse height and input number will be degraded in such cases. Furthermore, due to the inherently high resistivity and small diameter (~ 10 ~m) of the microchannels, we are also limited by the total charge deliverable by the MCP. Therefore the gain-saturated mode of the MCP appears to be unsuitable for the intended application as a gamma-ray spectrometer where thousands of simultaneous photocathode electrons may be produced by an absorbed gamma ray. On the other hand, by utilizing the high gain capability of the chevron MCP but not operating exclusively in the gain-saturated mode, we can decrease the overall statistical variance in the gain by taking advantage of another familair mechanism in the following manner. When the MCP is far from gain saturation, its single-electron gain distribution is exponential (Poisson) in shape. Such a distribution is a consequence of the statistical fluctuations in the electron multiplication process within each microchannel as well as from microchannel to microchannel. Nevertheless, by analogy with the PMT case, as the number of simultaneous input electrons become large, say >10, the pulse height distribution should become nearly Gaussian in accordance with the central-limit theorem, regardless of the source of the statistical fluctuations. In this manner, the Lixiscope could also be made to provide energy resolution. Fig. 3 demonstrates the peaking of an exponential pulse-height distribution as the number of simultaneous events is increased. The central-limit theorem should apply whether the input electrons are spread over a large number of microchannels, or concentrated in one microchannel, or a combination of both. Based on the discussion above it is preferable to use the high gain capability of the saturable MCP while operating it far below saturation. Energy
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
474
5. Experimental procedure and results
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Fig. 3. Illustration o f the central-limit theorem. A n exponential gain or pulse-height distribution from single-electron inputs is seen to approach a Gaussian as the n u m b e r o f simult a n e o u s electrons b e c o m e s large. The curves are normalized to unit area and relative m e a n gain.
resolution and linearity are then achieved via the central-limit theorem, just as in a scintillator-PMT system. In addition, recalling the high spatial resolution of the Lixiscope, this new device, which is now capable of single gamma-ray counting, imaging, and energy resolution, is equivalent to having thousands of scintillator-PMT systems operating in parallel. These characteristics, coupled with the compactness and ruggedness of the Lixiscope, make such a device unique in the 2 0 - 2 0 0 keV region and ideally suited for use at the flux levels encountered in astronomy and solar physics. It should be mentioned that energy resolution is not likely to occur if the MCP itself is used as a detector. In this case, a gamma-ray is absorbed at some depth in the wall of a microchannel. The primary photoelectron has indeed a kinetic energy proportional to the energy of the gamma-ray. However, subsequent energy loss and stopping of this electron and its secondaries in the wall material prior to entering the microchannel for detection destroys the energy information.
Taking advantage of the modular approach of the Lixiscope, three forms of scintillators were used in conjunction with the chevron-MCP tube: a rare-earth phosphor screen (Kodak Lanex regular), 0.8 mm thick NaI(T1) on plate glass, and 0.3 mm thick NaI(T1) on a fiber-optic plate. The Lanex screen was used to measure the spatial resolution at 28 keV with high-flux input as well as to observe output scintillations due to single gamma-ray photons. Because the decay time of the Lanex screen is in the millisecond range it is not suitable for pulse counting and pulse-height analysis. The 0.8 mm thick NaI(T1) crystal (Harshaw type K968HG32K) is 25 mm in diameter, mounted on a 1.6 mm thick plate glass with 0.13 mm thick Be entrance window. The detection efficiency-of this crystal for 124 keV (STCo) gamma rays is about 25%, and is therefore suitable for assessing the feasibility of both single-photon counting and energy resolution. However, the dispersion of the visible-light in the plate glass before reaching the fiber-optic input of the chevron-MCP tube precludes good spatial resolution in this case. The third scintillator, 0.3 mm thick NaI(TI), has a detection efficiency of only about 10% at 124 keV. But it was custom-mounted by the Harshaw Chemical Co. on a 25 nun diameter fiber-optic plate in order to improve spatial resolution. A thin Be entrance window was also provided. This NaI(T1)-fiber-optic combination was used for high-flux imaging and spatial resolution measurements as well as single gamma-ray counting, imaging, and pulse-height analyses. For visual observation and measurements of spatial resolution, the output phosphor screen of the chevron-MCP tube was placed at 6 kV with respect to the output of the MCP. For pulse-height analysis, this voltage was decreased to 1 kV and the output phosphor screen served as an anode to collect the output charges of the MCP. The charge pulses were capacitively coupled to a charge-sensitive preamplifier whose output was then shaped and amplified before being fed into a multichannel analyzer for pulseheight analysis. In fig. 4 we show the spatial resolution of the new prototype Lixiscope using a Pb resolution chart and high-flux input of 28 keV X-rays. The image in fig. 4A was obtained with the Lanex screen scintillator and that in fig. 4B with the 0.3 mm thick NaI(T1)fiber-optic-plate combination. The observed resolution is respectively 4 lp mm -1 and 2 lp mm -1 (lp =
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
475
Fig. 5. Negative photograph of Lixiscope output scintillations due to single incident gamma-ray photons from a weak 241Am source (27 keV and 60 keV). The input scintillator was a Lanex screen. The black line is due to a flaw in the MCP. The uniformly greyish background is not from noise in the Lixiscope.
Fig. 4. Spatial resolution of the new prototype Lixiscope at 28 keV measured with a Pb resolution chart. The black line is due to a flaw in the microchannel plate. (a). Lanex screen scintillator. Resolution: 4 line pairs mm-1 . (b) 0.3 mm thick NaI(T1)-fiber-optic-plate combination. The NaI(T1) crystal was accidentally cracked during mounting. Resolution: 2 lp mm-1.
line pairs). Unfortunately, the NaI(T1) was accidentally cracked during mounting, which is readily apparent in fig. 4B. In addition, the black line present in both fig. 4A and 4B is due to a flaw in the MCP. In fig. 5 we demonstrate the capability of the new prototype to image single gamma-ray photons. This negative photograph shows the individual scintillations (dark spots) from the Lixiscope output due to single incident gamma-ray photons from a weak 241Am source (27 keV and 60 keV). The input conversion scintillator in this case was the Lanex screen. The thick black line in the photograph is due to a flaw in the MCP. The overall greyish background resulted from reproducing the negative from a posi-
tive Polaroid print. It is not a background noise in the intensifier as is clear from the fact that it is uniformly present throughout the photograph rather than just in the intensifier output area (center circle). Similar results were obtained with the 0.3 mm thick NaI(Tl)-fiber-optic-plate combination. As was expected, the 0.8 mm thick NaI(T1) on the plate glass produced single gamma-ray scintillations which were large disks of light about 5 mm in diameter. Because the present prototype Lixiscope is capable of energy resolution, as will be shown below, the observed brightness of the output scintillations due to single incident gamma-ray photons is also proportional to the energy o f the gamma-rays. In fig. 6 we show the pulse height spectra for various gamma-ray sources obtained with the 0.3 mm thick NaI(T1)-fiber-optic-plate combination. Fig. 7 illustrates the measured linearity of the Lixiscope with respect to gamma-ray energy as derived from the pulse-height spectra of fig. 6. In fig. 8 we compare the energy resolution at 23 keV (l°gCd) for the Lixiscope using the 0.3 mm NaI(T1)-fiber-optic-plate combination (72%), with that o f the same crystal-fiber-opticplate combination mounted on the center of a 75 mm (3") diameter photomultiplier tube (38%). Clearly, figs. 6, 7, and 8 demonstrate not only single gammaray counting but also the energy resolving capability
476
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
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of the new prototype• From fig. 8 it is seen that the energy resolution of the Lixiscope is only a factor of two worse than that of the same crystal coupled to a large 75 mm PMT. Considering the spatial resolution of about 2 lp m m -1 achieved by the Lixiscope with the crystal (fig. 4B), this is equivalent to having thousands o f NaI(T1)-PMT systems operating in tandem with only a factor o f two degradation in overall energy resolution• In the present phototype, the output is derived from a visible-light phosphor screen• In the pulsecounting and single-photon imaging mode, the output needs to be digitized. This can be accomplished by replacing the output phosphor screen with a position-sensing anode such as a grid or a resistive anode• In this fashion, for low input fluxes, the output of the device will be able to provide simultaneous information concerning the energy, counting rate, position and time variation of the incident gamma-rays. For visual examination, the image can then be integrated in a storage oscilloscope. The time response of the MCP is known to be of the order of a nanosecond. Therefore, the time resolution of such a device is primarily determined by the decay time of the front scintillator, e.g. 0.25/as for NaI(TI), and by counting statistics•
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6. Summary We have presented experimental data obtained from a new prototype Lixiscope which demonstrate
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
its capabilities in simultaneous single gamma-ray p h o t o n counting, imaging, and energy resolution in the 2 0 - 2 0 0 keV range. Using a 0.3 m m thick NaI(T1)-fiber-optic-plate combination, the observed spatial resolution at 28 keV is 2 lp mm -1 ; the energy resolution at 23 keV is 72% fwhm as compared with 38% for the same crystal-fiber-optic-plate combination mounted on a 75 m m diameter PMT. Such operating characteristics make the Lixiscope ideally suited for low-flux imaging in the hard X-ray/soft gamma-ray regions where such an imaging spectrometer is greatly needed. The temporal resolution o f the device is essentially limited b y input statistics. The energy resolving capability can be utilized to advantage in selecting the operating energy ranges of a second-stage Lixiscope. We gratefully acknowledge the financial support of the Goddard Space Flight Center Director's Discretionary F u n d in performing this research. We thank U.J. Waiters and P.M. BaltzeU for obtaining the negative photograph o f single-photon scintillations shown in fig. 5. We also are indebted to M.R. Farukhi of Harshaw Chemical Co. for his efforts in fabricating the special NaI(T1)-fiber-optic-plate combination.
477
References [1] L.I Yin, J.I. Trombka and S.M. Seltzer, Nucl. Instr. and Meth. 158 (1979) 175. [2] L.I Yin and S.M. Seltzer, Phys. Med. Biol. 23 (1978) 993. [3] L.I. Yin, J.I. Trombka and S.M. Seltzer, Space Sci. Instr. 4 (1979) 321. [41 J. Adams and I.C.P. MiUar, Acta Electron. 14 (1971) 237. [5] J. Adams, Adv. Electron. Electr. Phys. 22A (1966) 139. [6] I.C.P. Millar, D.L. Lamport and A.W. Woodhead, IEEE Trans. Electron. Dev. ED 18 (197 l) 1101 ; I.C.P. Miller, D. Washington and D.L. Lamport, Adv. Electron. Electr. Phys. 33A (1972) 153; A.W. Woodhead and G. Eschard, Acta Electr. 14 (1971) 181. [7] W. Kuhl, in: Small vessel angiography, ed. S.K. Hilal (C.V. Mosby Co., St. Louis, 1973) ch. 9, p. 68. [81 G.R. Reigler and K.A. More, IEEE Trans. Nucl. Sci. NS 20 (1973) 102. [9] S. Baiter, J.S. Laughlin, L.N. Rothenburg, N.S. Thomas and J. Zandmanis, Radiology 110 (1974) 667. [10] R.G. Gould, P.F. Judy, J.C. Klopping and B.E. Bjarngard, Nucl. Instr. and Meth. 144 (1977) 493. [111 K.W. Dolan and J. Chang. Proc. SPIE 106 (1977) 178. [12] G. Pi6tri, IEEE trans. Nucl. Sci. NS 24 (1977) 228.
A SMALL RUGGED IMAGING X-RAY SPECTROMETER: A LIXISCOPE WITH GOOD ENERGY RESOLUTION Lo I YIN, Jacob I. TROMBKA Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
and Stephen M. SELTZER Center for Radiation Research, National Bureau of Standards, Washington, DC 20234, USA
Received 13 December 1979
A new prototype Lixiscope (Low Intensity X-ray Imaging Scope) is described for operation in the 20-200 keV region. In addition to good spatial resolution, the new prototype is capable of providing simultaneous gamma-ray or X-ray single-photon counting, imaging, and energy resolution. The observed energy resolution determined from gamma-ray pulse-height spectra is only a factor of two poorer than that of a NaI(TI)-PMT (photomultipler tube) system. Taking into account the good spatial resolution, such a Lixiscope is thus equivalent to operating thousands of NaI(TI)-PMT systems in parallel with minimal degradation in overall energy resolution. These characteristics make the new prototype Lixiscope a compact and rugged device eminently suited for possible low-flux imaging applications.
1. Introduction
2. The Lixiscope
Previously we reported on a small portable X-ray imaging * device, shown in fig. 1 with the hybrid acronym: Lixiscope (Low Intensity X-ray Imaging Scope) [ 1 - 3 ] . The Lixiscope was originally conceived for use in X-ray astronomy where singlephoton imaging is required. Although the operating characteristics of the first prototypes [ 1 - 3 ] fell somewhat short of this goal, the rugged and fully portable device could be made immediately useful in terrestrial fluoroscopic or intensifier-assisted radiographic applications where the X-rays are orders of magnitude more intense by astronomical standards. In this paper we report on an improved Lixiscope whose characteristics are more compatible with those required in astronomy and other low-flux applications. Specifically, in addition to good spatial resolution in the 2 0 - 2 0 0 keV region, the improved prototype is capable of single-photon counting, imaging, and shows remarkably good energy resolution as well.
Briefly, the design of the Lixiscope as an X-ray imaging system is based on a modular approach where
* By imaging we mean the preservation of positional information of the incident X-ray in the detector plane.
Fig. 1. The first prototype Lixiscope including a radioisotope source in its container. 471
472
Lo I Yin et al. / A small rugged imaging X-ray spectrometer PHOTON TO ELECTRON CONVERSION °,
PHOTON GAIN
X RAY
ELECTRON GAIN
ELECTRON TO PHOTON CONVERSION
\\
VISIBLE LIGHT
\\ \\ l
FIBER OPTICS
SCINTILLATOR
FIBER OPTICS MICROCHANNEL PLATE
PHOTOCATHODE
OUTPUT PHOSPHOR
Fig. 2. Schematicdiagram of the Lixiscopesystem. the components are physically independent entities. This approach allows maximum versatility and ruggedness as well as capability for high detection efficiency through the judicious selection of the components. As will be seen, this feature served us well in the present study. Conceptually, the Lixiscope is extremely simple (see fig. 2). An X-ray or a gamma-ray image is first converted into a visible-light image by a scintillator. The visible-light image is then coupled via fiber optics to a microchannel plate (MCP) visible-light image intensifier [1]. The MCP image intensifier is a self-contained entity which is both compact and rugged. It is a highvacuum tube sealed by fiber-optic input and output face plates. Deposited on the vacuum side of the input fiber-optic face plate is a visible-light photocathode which converts the incident visible-light image produced in the scintillator into an electron image. The electrons are then multiplied by the microchannel plate (MCP) electron multiplier. Because electron multiplication is confined within the 12 tan diameter microchannels, positional information (i.e. image) is preserved. The output of the MCP is accelerated to about 5 kV to impinge on an aluminized phosphor deposited on the output fiberoptic face plate and provides a much intensified visible-light image. In the energy range between 20-200 keV, the advantages of the Lixiscope approach as compared with other attempts involving MCP detectors [4-11] are the following:
(a) high detection efficiency. Because a large number (>~10 a) of visible-light photons are produced in the scintillator by each absorbed X-ray or gamma-ray photon, the probability of information loss after the initial photoelectric absorption process is negligibly small. Therefore, the detection efficiency of the Lixiscope is essentially governed by the scintillator rather than by the MCP itself. Ultimately, a compromize must be reached between high quantum detection efficiency (thick sinctillator) and high spatial resolution (thin scintillator), For example, a 1 mm thick CsI scintillator would have nearly 100% detection efficiency for photon energies below 60 keV, and ~60% at 100 keV, ~28% at 150 keV, and ~16% at 200 keV. Using a thin rare-earth phosphor scintillator, the observed resolution of the first prototype Lixiscope at 28 keV is 4 line pairs (lp) per mm. (b) Ruggedness. In contrast to other attempts [6,7], the Lixiscope places the scintillator externally to, and physically independent of the visible-light image intensifier, thereby eliminating the need for any fragile vacuum window or attached vacuum pump. Consequently, the Lixiscope is very rugged ° and well suited for both space and terrestrial applications.
* A Lixiscopewas accidentallydropped from about 1 m onto a concrete floor. While the Lixiscope suffered no physical damage, the instantaneousemotional distress to the authors was considerable.
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
3. New prototype Lixiscope The relatively low electron gain (103-104 ) and the problem of ion feedback in conventional singleMCP image intensifiers precluded single-photon X-ray imaging with our previous prototype [1,2]. Subsequently, using an experimental tube incorporating a curved MCP, we were able to achieve single X-ray and gamma-ray photon counting including a suggestive energy sensivitity [3]. The new prototype reported here consists of a dual-MCP, also known as chevron-MCP, image intensifier tube purchased from Varian Associates" The microchannels are 12/am in diameter with length-to-diameter ratio of 80 : 1. For gamma-ray single-photon counting and imaging, thin NaI(TI) scintillators were used in conjunction with the chevron-MCP tube. Before presenting the experimental results it is necessary to discuss the rationale behind our approach, and to indicate how energy resolution in the 2 0 - 2 0 0 keV range can be achieved with this new device.
4. Single gamma-ray imaging and energy resolution As in well known, when a gamma-ray is absorbed in a scintillator the number of visible-light photons generated by the primary photoelectron is essentially proportional to its kinetic energy and hence proportional to the gamma-ray energy. Consequently, the number of electrons generated at the photocathode of the Lixiscope is also proportional to the energy of the incident gamma-ray, in complete analogy with a scintillation-photomultiplier tube (PMT) system. The gain for single-electron inputs in the present chevron-MCP intensifier can be driven to saturation resulting in a Gaussian-shaped pulse-height distribution and a mean gain of ~107. Such a saturated gain distribution greatly decreases the gain variance in individual microchannels as well as among many microchannels. Thus, in the gain-saturated mode, the MCP output can be made proportional to the number of simultaneous input electrons. This has been demonstrated, for instance, with high-intensity visible light where the output pulse height was shown [12] to be indeed proportional to single, double, and * Manufacturers are specified only to uniquely identify the components and not as an endorsement by the National Aeronautical and Space Administration or the National Bureau of Standards.
473
triple photoelectron inputs. It should be noted that in the high-intensity visible-light case, the multiple electrons simultaneously trigger microchannels which are randomly distributed in location. Those simultaneously triggered by an absorbed gamma ray are most likely to occur in a bundle, i.e. immediately adjacent to each other. In fact, in order to preserve good spatial resolution, it is desirbale to design the scintillator-fiber-optic system to make the bundle as small as possible per absorbed gamma ray. This implies that many single microchannels in the bundle will be accepting multiple electrons as well. Since in the gain-saturated mode the charge output of the single microchannel is clamped at the saturated value, the proportionality between output pulse height and input number will be degraded in such cases. Furthermore, due to the inherently high resistivity and small diameter (~ 10 ~m) of the microchannels, we are also limited by the total charge deliverable by the MCP. Therefore the gain-saturated mode of the MCP appears to be unsuitable for the intended application as a gamma-ray spectrometer where thousands of simultaneous photocathode electrons may be produced by an absorbed gamma ray. On the other hand, by utilizing the high gain capability of the chevron MCP but not operating exclusively in the gain-saturated mode, we can decrease the overall statistical variance in the gain by taking advantage of another familair mechanism in the following manner. When the MCP is far from gain saturation, its single-electron gain distribution is exponential (Poisson) in shape. Such a distribution is a consequence of the statistical fluctuations in the electron multiplication process within each microchannel as well as from microchannel to microchannel. Nevertheless, by analogy with the PMT case, as the number of simultaneous input electrons become large, say >10, the pulse height distribution should become nearly Gaussian in accordance with the central-limit theorem, regardless of the source of the statistical fluctuations. In this manner, the Lixiscope could also be made to provide energy resolution. Fig. 3 demonstrates the peaking of an exponential pulse-height distribution as the number of simultaneous events is increased. The central-limit theorem should apply whether the input electrons are spread over a large number of microchannels, or concentrated in one microchannel, or a combination of both. Based on the discussion above it is preferable to use the high gain capability of the saturable MCP while operating it far below saturation. Energy
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
474
5. Experimental procedure and results
2.s n=32
2.0 le 1.S (n tkl
l-
_z 1.0
0.5
0
1
2
RELATIVE PULSE HEIGHT
Fig. 3. Illustration o f the central-limit theorem. A n exponential gain or pulse-height distribution from single-electron inputs is seen to approach a Gaussian as the n u m b e r o f simult a n e o u s electrons b e c o m e s large. The curves are normalized to unit area and relative m e a n gain.
resolution and linearity are then achieved via the central-limit theorem, just as in a scintillator-PMT system. In addition, recalling the high spatial resolution of the Lixiscope, this new device, which is now capable of single gamma-ray counting, imaging, and energy resolution, is equivalent to having thousands of scintillator-PMT systems operating in parallel. These characteristics, coupled with the compactness and ruggedness of the Lixiscope, make such a device unique in the 2 0 - 2 0 0 keV region and ideally suited for use at the flux levels encountered in astronomy and solar physics. It should be mentioned that energy resolution is not likely to occur if the MCP itself is used as a detector. In this case, a gamma-ray is absorbed at some depth in the wall of a microchannel. The primary photoelectron has indeed a kinetic energy proportional to the energy of the gamma-ray. However, subsequent energy loss and stopping of this electron and its secondaries in the wall material prior to entering the microchannel for detection destroys the energy information.
Taking advantage of the modular approach of the Lixiscope, three forms of scintillators were used in conjunction with the chevron-MCP tube: a rare-earth phosphor screen (Kodak Lanex regular), 0.8 mm thick NaI(T1) on plate glass, and 0.3 mm thick NaI(T1) on a fiber-optic plate. The Lanex screen was used to measure the spatial resolution at 28 keV with high-flux input as well as to observe output scintillations due to single gamma-ray photons. Because the decay time of the Lanex screen is in the millisecond range it is not suitable for pulse counting and pulse-height analysis. The 0.8 mm thick NaI(T1) crystal (Harshaw type K968HG32K) is 25 mm in diameter, mounted on a 1.6 mm thick plate glass with 0.13 mm thick Be entrance window. The detection efficiency-of this crystal for 124 keV (STCo) gamma rays is about 25%, and is therefore suitable for assessing the feasibility of both single-photon counting and energy resolution. However, the dispersion of the visible-light in the plate glass before reaching the fiber-optic input of the chevron-MCP tube precludes good spatial resolution in this case. The third scintillator, 0.3 mm thick NaI(TI), has a detection efficiency of only about 10% at 124 keV. But it was custom-mounted by the Harshaw Chemical Co. on a 25 nun diameter fiber-optic plate in order to improve spatial resolution. A thin Be entrance window was also provided. This NaI(T1)-fiber-optic combination was used for high-flux imaging and spatial resolution measurements as well as single gamma-ray counting, imaging, and pulse-height analyses. For visual observation and measurements of spatial resolution, the output phosphor screen of the chevron-MCP tube was placed at 6 kV with respect to the output of the MCP. For pulse-height analysis, this voltage was decreased to 1 kV and the output phosphor screen served as an anode to collect the output charges of the MCP. The charge pulses were capacitively coupled to a charge-sensitive preamplifier whose output was then shaped and amplified before being fed into a multichannel analyzer for pulseheight analysis. In fig. 4 we show the spatial resolution of the new prototype Lixiscope using a Pb resolution chart and high-flux input of 28 keV X-rays. The image in fig. 4A was obtained with the Lanex screen scintillator and that in fig. 4B with the 0.3 mm thick NaI(T1)fiber-optic-plate combination. The observed resolution is respectively 4 lp mm -1 and 2 lp mm -1 (lp =
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
475
Fig. 5. Negative photograph of Lixiscope output scintillations due to single incident gamma-ray photons from a weak 241Am source (27 keV and 60 keV). The input scintillator was a Lanex screen. The black line is due to a flaw in the MCP. The uniformly greyish background is not from noise in the Lixiscope.
Fig. 4. Spatial resolution of the new prototype Lixiscope at 28 keV measured with a Pb resolution chart. The black line is due to a flaw in the microchannel plate. (a). Lanex screen scintillator. Resolution: 4 line pairs mm-1 . (b) 0.3 mm thick NaI(T1)-fiber-optic-plate combination. The NaI(T1) crystal was accidentally cracked during mounting. Resolution: 2 lp mm-1.
line pairs). Unfortunately, the NaI(T1) was accidentally cracked during mounting, which is readily apparent in fig. 4B. In addition, the black line present in both fig. 4A and 4B is due to a flaw in the MCP. In fig. 5 we demonstrate the capability of the new prototype to image single gamma-ray photons. This negative photograph shows the individual scintillations (dark spots) from the Lixiscope output due to single incident gamma-ray photons from a weak 241Am source (27 keV and 60 keV). The input conversion scintillator in this case was the Lanex screen. The thick black line in the photograph is due to a flaw in the MCP. The overall greyish background resulted from reproducing the negative from a posi-
tive Polaroid print. It is not a background noise in the intensifier as is clear from the fact that it is uniformly present throughout the photograph rather than just in the intensifier output area (center circle). Similar results were obtained with the 0.3 mm thick NaI(Tl)-fiber-optic-plate combination. As was expected, the 0.8 mm thick NaI(T1) on the plate glass produced single gamma-ray scintillations which were large disks of light about 5 mm in diameter. Because the present prototype Lixiscope is capable of energy resolution, as will be shown below, the observed brightness of the output scintillations due to single incident gamma-ray photons is also proportional to the energy o f the gamma-rays. In fig. 6 we show the pulse height spectra for various gamma-ray sources obtained with the 0.3 mm thick NaI(T1)-fiber-optic-plate combination. Fig. 7 illustrates the measured linearity of the Lixiscope with respect to gamma-ray energy as derived from the pulse-height spectra of fig. 6. In fig. 8 we compare the energy resolution at 23 keV (l°gCd) for the Lixiscope using the 0.3 mm NaI(T1)-fiber-optic-plate combination (72%), with that o f the same crystal-fiber-opticplate combination mounted on the center of a 75 mm (3") diameter photomultiplier tube (38%). Clearly, figs. 6, 7, and 8 demonstrate not only single gammaray counting but also the energy resolving capability
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of the new prototype• From fig. 8 it is seen that the energy resolution of the Lixiscope is only a factor of two worse than that of the same crystal coupled to a large 75 mm PMT. Considering the spatial resolution of about 2 lp m m -1 achieved by the Lixiscope with the crystal (fig. 4B), this is equivalent to having thousands o f NaI(T1)-PMT systems operating in tandem with only a factor o f two degradation in overall energy resolution• In the present phototype, the output is derived from a visible-light phosphor screen• In the pulsecounting and single-photon imaging mode, the output needs to be digitized. This can be accomplished by replacing the output phosphor screen with a position-sensing anode such as a grid or a resistive anode• In this fashion, for low input fluxes, the output of the device will be able to provide simultaneous information concerning the energy, counting rate, position and time variation of the incident gamma-rays. For visual examination, the image can then be integrated in a storage oscilloscope. The time response of the MCP is known to be of the order of a nanosecond. Therefore, the time resolution of such a device is primarily determined by the decay time of the front scintillator, e.g. 0.25/as for NaI(TI), and by counting statistics•
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6. Summary We have presented experimental data obtained from a new prototype Lixiscope which demonstrate
Lo I Yin et al. / A small rugged imaging X-ray spectrometer
its capabilities in simultaneous single gamma-ray p h o t o n counting, imaging, and energy resolution in the 2 0 - 2 0 0 keV range. Using a 0.3 m m thick NaI(T1)-fiber-optic-plate combination, the observed spatial resolution at 28 keV is 2 lp mm -1 ; the energy resolution at 23 keV is 72% fwhm as compared with 38% for the same crystal-fiber-optic-plate combination mounted on a 75 m m diameter PMT. Such operating characteristics make the Lixiscope ideally suited for low-flux imaging in the hard X-ray/soft gamma-ray regions where such an imaging spectrometer is greatly needed. The temporal resolution o f the device is essentially limited b y input statistics. The energy resolving capability can be utilized to advantage in selecting the operating energy ranges of a second-stage Lixiscope. We gratefully acknowledge the financial support of the Goddard Space Flight Center Director's Discretionary F u n d in performing this research. We thank U.J. Waiters and P.M. BaltzeU for obtaining the negative photograph o f single-photon scintillations shown in fig. 5. We also are indebted to M.R. Farukhi of Harshaw Chemical Co. for his efforts in fabricating the special NaI(T1)-fiber-optic-plate combination.
477
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