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The microchannel plate as vertex detector in high energy physics experiments
Nuclear Instruments and Methods 184 (1981) 405-407 North-Holland Publishing Company
405
THE MICROCHANNEL PLATE AS VERTEX DETECTOR IN HIGH ENERGY PHYSICS EXPERIMENTS Douglas M. POTTER Dept. of Physics and Astronomy, Rutgers State University, Piscataway, New Jersey 08854, U.S.A. Received 15 September 1980
The properties of a microchannel plate exposed to a high energy hadron beam have been studied. When the beam was passed through the plate transverse to the microchannel axes, about 2 microchannels per millimeter were excited and an rms error of less than 10 tzm was obtained. Prospects for improvement are discussed.
1. Introduction In the past few years there have been both great interest and significant progress in the development of vertex detectors capable of high temporal and spatial resolving power. The interest has been kindled by the prospect of measuring in high energy interactions the production cross-sections, lifetimes, and branching ratios of particles carrying the new flavor quantum numbers beauty arid charm. Several features of the microchannel plate [1] (MCP) make it an attractive candidate for use as such a track-sensitive target. First, the intrinsic resolving time of the MCP is less than a nanosecond, so extremely high interaction rates should be possible. Second, the small diameter of the channels (typically, 12 /am) implies that the rms error per excited channel could be less than 5/am. Third, the MCP is "live" so that information from it can be used in the event trigger. Fourth, the MCP would provide a true projection of an event topology, and thus solve in a simple fashion the depth of field problems associated with other visual techniques. Finally, by combining the MCP with a phosphor output screen and an optical switch, one could easily construct a triggerable device for recording data on photographic trim. Note that triggerability distinguishes the MCP qualitatively from nuclear emulsions and bubble chambers and that "liveness" similarly distinguishes it from these detectors as well as from spark and streamer chambers. The attractive features of the MCP must be weighed against a disadvantage. The theory of secondary emission [2] predicts that the probability for a particle to excite a microchannel in the MCP is pro0029-554X/81/0000-0000/$02.50 © North-Holland
portional to the local specific ionization. That probability can be estimated using data on MCP and channeltron efficiency for incident low energy electrons, [3] and lies between 1% and 10% for a minimum ionizing particle. Thus, in a conventional MCP, the number of points per millimeter of minimum ionizing track is expected to be between about 1 and 10. The consequence of this low point density is that, while the MCP should be useful for reconstructing secondary vertices resulting from charm and beauty decay, its utility for observing them directly will be limited [4]. A simple demonstration of the feasibility of using the MCP as a track chamber-target has been carried out and is discussed below.
2. Test setup In logical sequence the scheme of the test setup consisted of the following: an MCP and output phosphor (both contained in a channel plate image intensifter); a lens; an optical switch and optical amplifier (comprised by a gated four-stage magnetically focussed image intensifier); and a camera to "record the data. A hadron beam was passed through the MCP transverse to the axes of the microchannels. The proximity focussed channel plate image intensifter tube (PFCIT)was an ITT type F4111; it contained a 0.020" thick 18 mm diameter MCP with 11 tam diameter channels spaced by 12.5/am. The output phosphor was P20, which has a decay time of about 200/as. In order to avoid potential problems with spatial
406
D.M. Potter / Microchannel plate as vertex detector
resolution in the remainder of the apparatus, the image of the PFCIT output phosphor was projected onto the input of the four-stage tube with a magnification of about 6. The lens system used consisted of a 50 mm f l . 4 Canon TV16-5014, followed by a 300 mm achromat. This system has an MTF of 0.5 at about 40 lp/mm. Experience with the remaining apparatus (which will be described in detail elsewhere) has shown it adequate to record a 20 /am diameter dot at the PFCIT phosphor. Gating of the system was accomplished by pulsing on the second stage of the four-stage tube from its quiescently off state for about 100/as. The system was tested at BNL in the A2 test beam, which was set for 7 GeV/c positive particles. The trigger to the gate pulse generator required a coincidence between a signal from a beam particle and a signal from the MCP. The beam signal was defined by a coincidence of pulses from two scintillation counters in the beam and an anticoincidence from a hole veto scintillation counter. The MCP signal was generated by integrating for about 1/as the output from a PMT viewing the PFCIT output phosphor.
Fig. 1. A beam track recorded by the MCP test beam.
3. Results The results of the test are best illustrated by the raw data. Fig. 1 shows a beam track; about 2 dots per millimeter of minimum ionizing track are recorded. Measurements made using an image plane digitizer show that the dot sizes range between 100 and 200 /am and that the rms displacement of the dots from a line least-squares fit to them is less than 10/am. More careful adjustment of exposure during subsequent tests without beam reduced the median size of the dots to about 75/am. Fig. 2 shows two interactions in the MCP; these data were obtained by requiring in the trigger a large pulse from the PMT viewing the PFCIT phosphor. Nuclear breakup is presumably responsible for the heavily ionizing tracks seen in these interactions. The blob shape at regions of very high ionization is probably the result of blooming in the fourstage tube or reflections in the optical systems; it is not intrinsic to the MCP. The photographic density of a dot depends on the electron amplification in the microchannel, and therefore on the local depth in the MCP at which the rnicrochannel is excited by a particle. This effect is illustrated in fig. 3, which evidently records the passage of a particle on a trajectory not parallel to the
fll
Fig. 2. Two hadronic interactions recorded by the MCP device. The tiny dots correspond to single photons, which can arise from reflections in the optical train between the PFCIT and the four-stagetube.
Fig. 3. A track recorded for a particle probably traveling skew to the plane at the MCP.
D.M. Potter / Microchannel plate as vertex detector
plane of the MCP. In principle, the MCP can thus provide simultaneously information on all three coordinates of points comprising a charged particle trajectory. Because the decay constant of the PFCIT output phosphor was inappropriately long, no useful information could be obtained on time resolution during this test.
4. Discussion The results discussed above demonstrate only the feasibility of using the MCP as a track chamber-target; they should not be interpreted as representing the properties of a practical detector. On the contrary, several improvements can be anticipated and are discussed in the following paragraphs. The dot size, which in the present test device is almost certainly limited by the velocity of the electrons transverse to the microchannel axes, can be reduced either by further collimation in the MCP or by magnetic focussing. The latter technique as used in magnetically focussed image intensifiers routinely yields 10-20/am diameter dots. Another improvement would be to use a thicker MCP. In addition to increasing the fiducial volume of the target, the thicker MCP would result in saturated electron multiplication, which has two useful properties. First, an electrical signal from the MCP would be used in the trigger logic; thus the extremely good time resolution of the MCP could be exploited. Second, dots of uniform and therefore, easily photographed intensity would be produced. An additional consequence is that information on the depth of particle trajectories in the MCP would be lost.
407
The spatial resolution of the test devices was probably limited by the dot size. Dots of 10-20/am diameter will not obscure the expected inherent resolution of less than 5/am rms. A new development which would enhance the general usefulness of the MCP would be to increase the secondary emission coefficient of the microchannel walls; such an improvement would proportionately increase the probability of channel excitation by minimum ionizing particles. Work along these lines is currently in progress [5]. I would like to thank J. Cuny of ITT for the loan of the PFCIT, M. Deutsch for the loan of the camera, H. Brown and the staff of B.N.L. for their help, and D. Johnson and B. Nordholt for their cheerful assistance in this work.
References [1] For a review see, e.g., J. Wiza, Nucl. Instr. and Meth. 162 (1979) 587. [2] See, e.g., R.E. Simon and B.F. Williams, IEEE Trans. Nucl. Sci. NS-15 (1968) 167. [3] J.P. Macau, J. Jamar and S. Gardier, IEEE Trans. Nucl. Sci. NS-23 (1976) 2049. [4] This disadvantage m a y be more of a technical inconvenience than a fundamental limitation, tn order that a secondary (decay) vertex be observed directly, it m u s t n o t be obliterated by tracks from the interaction vertex. This requirement translates into a limit on track width. However, the condition that a secondary vertex be reconstructable from daughter tracks is related to the resolution in impact parameter of the daughter trajectories at the interaction vertex, and is generally easier to satisfy than the requirement on track width. The resolution in impact parameter depends only on the square root of the n u m b e r of points comprising a track. [5] P. Rehak, private communication.
405
THE MICROCHANNEL PLATE AS VERTEX DETECTOR IN HIGH ENERGY PHYSICS EXPERIMENTS Douglas M. POTTER Dept. of Physics and Astronomy, Rutgers State University, Piscataway, New Jersey 08854, U.S.A. Received 15 September 1980
The properties of a microchannel plate exposed to a high energy hadron beam have been studied. When the beam was passed through the plate transverse to the microchannel axes, about 2 microchannels per millimeter were excited and an rms error of less than 10 tzm was obtained. Prospects for improvement are discussed.
1. Introduction In the past few years there have been both great interest and significant progress in the development of vertex detectors capable of high temporal and spatial resolving power. The interest has been kindled by the prospect of measuring in high energy interactions the production cross-sections, lifetimes, and branching ratios of particles carrying the new flavor quantum numbers beauty arid charm. Several features of the microchannel plate [1] (MCP) make it an attractive candidate for use as such a track-sensitive target. First, the intrinsic resolving time of the MCP is less than a nanosecond, so extremely high interaction rates should be possible. Second, the small diameter of the channels (typically, 12 /am) implies that the rms error per excited channel could be less than 5/am. Third, the MCP is "live" so that information from it can be used in the event trigger. Fourth, the MCP would provide a true projection of an event topology, and thus solve in a simple fashion the depth of field problems associated with other visual techniques. Finally, by combining the MCP with a phosphor output screen and an optical switch, one could easily construct a triggerable device for recording data on photographic trim. Note that triggerability distinguishes the MCP qualitatively from nuclear emulsions and bubble chambers and that "liveness" similarly distinguishes it from these detectors as well as from spark and streamer chambers. The attractive features of the MCP must be weighed against a disadvantage. The theory of secondary emission [2] predicts that the probability for a particle to excite a microchannel in the MCP is pro0029-554X/81/0000-0000/$02.50 © North-Holland
portional to the local specific ionization. That probability can be estimated using data on MCP and channeltron efficiency for incident low energy electrons, [3] and lies between 1% and 10% for a minimum ionizing particle. Thus, in a conventional MCP, the number of points per millimeter of minimum ionizing track is expected to be between about 1 and 10. The consequence of this low point density is that, while the MCP should be useful for reconstructing secondary vertices resulting from charm and beauty decay, its utility for observing them directly will be limited [4]. A simple demonstration of the feasibility of using the MCP as a track chamber-target has been carried out and is discussed below.
2. Test setup In logical sequence the scheme of the test setup consisted of the following: an MCP and output phosphor (both contained in a channel plate image intensifter); a lens; an optical switch and optical amplifier (comprised by a gated four-stage magnetically focussed image intensifier); and a camera to "record the data. A hadron beam was passed through the MCP transverse to the axes of the microchannels. The proximity focussed channel plate image intensifter tube (PFCIT)was an ITT type F4111; it contained a 0.020" thick 18 mm diameter MCP with 11 tam diameter channels spaced by 12.5/am. The output phosphor was P20, which has a decay time of about 200/as. In order to avoid potential problems with spatial
406
D.M. Potter / Microchannel plate as vertex detector
resolution in the remainder of the apparatus, the image of the PFCIT output phosphor was projected onto the input of the four-stage tube with a magnification of about 6. The lens system used consisted of a 50 mm f l . 4 Canon TV16-5014, followed by a 300 mm achromat. This system has an MTF of 0.5 at about 40 lp/mm. Experience with the remaining apparatus (which will be described in detail elsewhere) has shown it adequate to record a 20 /am diameter dot at the PFCIT phosphor. Gating of the system was accomplished by pulsing on the second stage of the four-stage tube from its quiescently off state for about 100/as. The system was tested at BNL in the A2 test beam, which was set for 7 GeV/c positive particles. The trigger to the gate pulse generator required a coincidence between a signal from a beam particle and a signal from the MCP. The beam signal was defined by a coincidence of pulses from two scintillation counters in the beam and an anticoincidence from a hole veto scintillation counter. The MCP signal was generated by integrating for about 1/as the output from a PMT viewing the PFCIT output phosphor.
Fig. 1. A beam track recorded by the MCP test beam.
3. Results The results of the test are best illustrated by the raw data. Fig. 1 shows a beam track; about 2 dots per millimeter of minimum ionizing track are recorded. Measurements made using an image plane digitizer show that the dot sizes range between 100 and 200 /am and that the rms displacement of the dots from a line least-squares fit to them is less than 10/am. More careful adjustment of exposure during subsequent tests without beam reduced the median size of the dots to about 75/am. Fig. 2 shows two interactions in the MCP; these data were obtained by requiring in the trigger a large pulse from the PMT viewing the PFCIT phosphor. Nuclear breakup is presumably responsible for the heavily ionizing tracks seen in these interactions. The blob shape at regions of very high ionization is probably the result of blooming in the fourstage tube or reflections in the optical systems; it is not intrinsic to the MCP. The photographic density of a dot depends on the electron amplification in the microchannel, and therefore on the local depth in the MCP at which the rnicrochannel is excited by a particle. This effect is illustrated in fig. 3, which evidently records the passage of a particle on a trajectory not parallel to the
fll
Fig. 2. Two hadronic interactions recorded by the MCP device. The tiny dots correspond to single photons, which can arise from reflections in the optical train between the PFCIT and the four-stagetube.
Fig. 3. A track recorded for a particle probably traveling skew to the plane at the MCP.
D.M. Potter / Microchannel plate as vertex detector
plane of the MCP. In principle, the MCP can thus provide simultaneously information on all three coordinates of points comprising a charged particle trajectory. Because the decay constant of the PFCIT output phosphor was inappropriately long, no useful information could be obtained on time resolution during this test.
4. Discussion The results discussed above demonstrate only the feasibility of using the MCP as a track chamber-target; they should not be interpreted as representing the properties of a practical detector. On the contrary, several improvements can be anticipated and are discussed in the following paragraphs. The dot size, which in the present test device is almost certainly limited by the velocity of the electrons transverse to the microchannel axes, can be reduced either by further collimation in the MCP or by magnetic focussing. The latter technique as used in magnetically focussed image intensifiers routinely yields 10-20/am diameter dots. Another improvement would be to use a thicker MCP. In addition to increasing the fiducial volume of the target, the thicker MCP would result in saturated electron multiplication, which has two useful properties. First, an electrical signal from the MCP would be used in the trigger logic; thus the extremely good time resolution of the MCP could be exploited. Second, dots of uniform and therefore, easily photographed intensity would be produced. An additional consequence is that information on the depth of particle trajectories in the MCP would be lost.
407
The spatial resolution of the test devices was probably limited by the dot size. Dots of 10-20/am diameter will not obscure the expected inherent resolution of less than 5/am rms. A new development which would enhance the general usefulness of the MCP would be to increase the secondary emission coefficient of the microchannel walls; such an improvement would proportionately increase the probability of channel excitation by minimum ionizing particles. Work along these lines is currently in progress [5]. I would like to thank J. Cuny of ITT for the loan of the PFCIT, M. Deutsch for the loan of the camera, H. Brown and the staff of B.N.L. for their help, and D. Johnson and B. Nordholt for their cheerful assistance in this work.
References [1] For a review see, e.g., J. Wiza, Nucl. Instr. and Meth. 162 (1979) 587. [2] See, e.g., R.E. Simon and B.F. Williams, IEEE Trans. Nucl. Sci. NS-15 (1968) 167. [3] J.P. Macau, J. Jamar and S. Gardier, IEEE Trans. Nucl. Sci. NS-23 (1976) 2049. [4] This disadvantage m a y be more of a technical inconvenience than a fundamental limitation, tn order that a secondary (decay) vertex be observed directly, it m u s t n o t be obliterated by tracks from the interaction vertex. This requirement translates into a limit on track width. However, the condition that a secondary vertex be reconstructable from daughter tracks is related to the resolution in impact parameter of the daughter trajectories at the interaction vertex, and is generally easier to satisfy than the requirement on track width. The resolution in impact parameter depends only on the square root of the n u m b e r of points comprising a track. [5] P. Rehak, private communication.