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Fast track-finding processor based on RAM look-up table for the VENUS detector at KEK
522
Nuclear Instruments and Methods in Physics Research A269 (1988) 522-526 North-Holland, Amsterdam
FAST TRACK-FINDING PROCESSOR BASED ON RAM LOOK-UP TABLE FOR THE VENUS DETECTOR AT KEK T. OHSUGI, Y. CHIBA, I . HAYASHIBARA, A. TAKETANI and S. YASUISHI Hiroshima University, Hiroshima 730, Japan
Y. ARAI, H. SAKAMOTO and S. UEHARA KEK, National Laboratory for High Energy Physics, Oho-mache, Tsukuba, Ibaraki 305, Japan
Received 18 August 1987 and in revised form 28 December 1987
We have developed a fast track-fending processor using signals from the central tracking chamber of the VENUS detector in the TRISTAN experiments . Particle tracks are recognized by a look-up table made with a high-speed static RAM. This method enables us to implement the track finder m the first level triggering . The track finder has been working excellently under heavy background due to synchrotron radiation. A processing time of 110 ns is attained . 1 . Introduction In e + e - colliding experiments, a track signal from the interaction points is a clear indication of beam-beam interaction. For example, in an event like e +e- -> p +p.-, track information is only available for event triggering. In such a case, a fast track finder must be one of the indispensable triggering devices. Many ideas for fast track-finding methods have been proposed and utilized [1] in the experiments at SPEAR, PETRA, and PEP, but only few have been implemented in the first level triggering [2], because most of the devices need a rather long processing time to find tracks. The tracking chambers had also rather long electron drift times. We have developed a fast track finder using a highspeed static RAM (random access memory) in which a look-up table method is taken to find tracks from data of a tracking chamber. This type of track finder matches well with tracking chambers having a small cell . This device has been implemented in the VENUS detector in the TRISTAN experiment . Since the look-up table method is of totally parallel-processing type, we expect the highest processing speed, which enables us to make a dead-timeless triggering at the first level in the VENUS detector. The look-up table for triggering used to be made with a FPLA (field programmable logic array) mainly because of its high speed capability . In the present case, however, we need a more flexible device, because track-finding conditions should be easily changeable according to various experimental requirements. A rewritable look-up table which is constructed 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
of high-speed static RAMs is, therefore, more suitable for the present track finder. 2. book-up table method The principle of the look-up table is very simple as seen from fig. 1. One has to prepare a size of memory modules large enough to cover the number of signal inputs . The input signals are supplied to the address lines of these memories with timing signal, and then the track flag written in advance comes out as a memory readout action . If one wants to make up such a naive look-up table, one needs an enormous number of memories. For example, processing of only 64 signal inputs charged track
4KX 1
bit
or not
interaction point Fig. 1. Principle of a look-up table for the track finder.
52 3
T. Ohsugi et al. /Fast track finding processor
requires a 264-bit table which is obviously impossible to be implemented. However, if we look at the table contents carefully, most of the area of the naive look-up table is found to be empty space. Only some special combinations of the input signals are realized and other combinations never come up in practical use. Such specific combinations could be selected out analytically or by a Monte Carlo simulation. We classify the selected combinations to specific clusters so that the number of input signals forming the combinations is equal to or less than the number of address lines of an available memory chip. The required look-up table can be made up with this memory chip . Thus, how to make a grouping of input signals is the most critical point in designing the look-up table. 3. Signal preparation for the track finder The central drift chamber of the VENUS detector has 10 groups (2 layers in one group) of axial sense wires and 9 layers of stereo-wires [3]. To locally solve the left-right ambiguity, two axial layers in the same group are staggered by a half cell as shown in fig. 2. For our triggering purpose, only signals from inside 7 groups of axial wires are used in track finding. The reason why we have selected signals from the innermost 7 groups is as follows: A track originating from the interaction point is a good signature for beam-beam interactions . For the identification of a track coming from the interaction point, a distance of closest approach to the beam line on the R-0 plane is a good parameter. Obviously the shorter lever arm gives the better resolution of distance in the closest approach determination. In addition, inner groups can cover a wider angular region, resulting in a larger acceptance which is always favorable for interesting events .
A disadvantage of this choice is larger noise. However, we can sufficiently reduce the X-ray noise by taking coincidences between two signals from adjacent layers staggered in the same group as shown in fig. 2. This is because a charged particle passing through the chamber generates correlated signals in adjacent cells staggered, whereas random noise by synchrotron radiation does not show such a correlation . By taking the coincidence, we form 7 layers of signals for the trigger. Then, these signals are grouped to form a basic trigger cell as shown in fig. 2. A track element would, thus, be found within a spatial resolution defined by this cell size. The cell size is selected to reduce accidental triggers due to soft X-ray backgrounds and to minimize the number of signals feeding the track finder. Eventually, 64 azimuthal sectors were formed and a total of 64 X 7 = 448 signals were supplied to the track finder in our case. 4. Track recognition on R-0 projection To find a track element, a fan-shape sector is formed to cover 1/64 of the R-di projection, which is shown
2
OR ed
Coincidence Cell for trigge Sense wire Trock Pt =300 MeV
Fig. 2. Signal preparation scheme for the track finder. The coincidence between signals from two layers staggered is taken, and ORed to form 64 cells per trigger layer.
Fig. 3. The cell structure of a 12-bit subsector. Each subsector is covered by one RAM chip, resulting in four chips covering one fan-shape sector which is the unit of track counting for the trigger.
52 4
T. Ohsugi et al. / Fast trackfinding processor
by the thick solid line in fig. 2. This sector can recognize a track having more than 0.2 GeV/c transverse momentum. The fan-shape sector is decomposed into four 12-bit subsectors, being matched to a 4k-bit, highspeed static RAM. Cell structures of four subsectors are shown in fig. 3. The whole of the R-0 projection is covered by 256 chips of 4k-bit RAM, Any combinations of these 12 bits are programmable. Let us give a typical example of how to make a trigger pattern. Basic hit patterns are generated on the subsector by simulated tracks originating from the interaction point. By taking account of the inefficiency of chamber cells, a pattern in which any one or two cells are missing in the 7 layers should be counted as a basic pattern. Since there are noise hits which overlap with track signals, all patterns including any of the basic patterns must be adopted as trigger patterns . If one allows a missing of the inner-most cell in the track identification, two adjacent sectors may generate track flags by one real track because of overlapping of the fan-shape sectors. This kind of double-counting problem can be solved by the so-called cluster counting technique. A track separation in a jet is not required for trigger purposes. Each memory chip generates on/off signals depending on whether a track has been recognized or not. A total of 256 bit information is generated from 256 chips in 8 modules, which can be read out via FASTBUS, and is very helpful to the second level trigger as well as to track finding in off-line analysis . Four signals which correspond to one basic fan-shape sector are ORed, resulting in 64 track flags which are supplied to the next-step logic.
5. Track finder module One FASTBUS module covers 1/8 of the R-0 projection, in other words, a look-up table for eight sectors is installed on one FASTBUS slave module which is realized with a 6-layer printed-circuit board. FASTBUS specification is well matched to a large-size table, because of its large addressing capability and large board size. An auxiliary connector having a large number of pins is open to the user, and this is important for a large size look-up table needed for our track finder. Functions of FASTBUS standard implemented in this module satisfy the minimum requirements for a slave module, i.e., geographical addressing, secondary addressing and the control-status register no . 0. Fig. 4 shows a schematic diagram of the track finder logic. Input signals should be stable for 20 ns before and after the latch timing of input signals. The latchtiming signal produced by beam crossing latches the 76 input signals in the input register, and starts looking for a track pattern. They are rearranged in 32 sets of 12-bit addresses and are fed to address lines of 4k-bit RAM chips in the look-up table. Then, a readout pulse produced by beam crossing triggers the readout action. We used a CMOS static RAM, HM6147HLP-35 (Hitachi), for which 35 ns access time was guaranteed . In practice, signals come out within 20 ns for most of the RAM chips. RAM outputs are summed over 4 RAMS corresponding to one basic, fan-shape sector. The resulting 8 outputs per module are supplied to the next-step logic.
Back plane
Front panel BCS timing signal CS
76
4Kx32
to majority logic
in RAM output register
RAM WE
tr m ouxi?iar connectyor
Pattern register FASTBUS protocol circuits
Control lines AD lines
Fig. 4. Block diagram of the track finder logic.
525
T. Ohsugi et al / Fast trackfinding processor
A track-status flag (found or not found for each track pattern) is ready to be read out within 110 ns after the arrival of the latch-timing signal . A processing time of 75 ns has been attained by fine adjustment of the timing . If one designs this type of look-up table on the most advanced CMOS gate array, one can probably achieve less than 15 ns processing time . Patterns in the look-up table are generated in a host computer and written via FASTBUS. The input-signal pattern and the output-bit pattern can be readout via FASTBUS. Power consumption of this module has been measured to be about 25 W under the static condition. 6. Performance of the track finder Our track finder was found to work excellently in spite of enormous X-ray noise in the initial stage of TRISTAN operation. Fig. 5a shows a typical Bhabha event under the heavy X-ray backgrounds. Fig. 5b indicates trigger cells fired in the same event. Comparison of these two figures clearly shows that most of the X-ray noise is rejected from the trigger signal fed to the track finder. The requirement of a coincidence between two adjacent cells staggered to form the trigger signal was confirmed to be extremely effective for rejecting these X-ray backgrounds . By requiring that at least 5 out of 7 layers in a pattern must be fired for the track recognition, two real tracks were uniquely identified as shown in fig. 5c. The probability of finding X-ray noise per unit cell was found-to be inversely proportional to the distance from the beam axis, and the average number of wires hit by X-rays was about 400 (- 6% of all sense wires) in the beginning of TRISTAN operation. Under this condition, the probability of the track finder giving one false track from these random noises was evaluated to be 10 -5 . A charged-track trigger was issued by requiring two collinear tracks or more than three tracks. For more than one week running with this trigger condition, no false trigger was recorded by the track finder . Thus it turns out that there is no mistake for more than 6 x 10 10 beam crossings, showing an excellent discrimination capability for accidental tracks due to random noises of X-rays . A charged-track trigger must be given by only one track coming from the interaction point. Fig. 6 shows how the track-detection efficiency decreases as the distance of closest approach of the track to the beam line increases. One can see that most of the cosmic rays distributed uniformly were discriminated by the track finder, but some events due to beam-beam pipe interaction could survive because some of the tracks produced at the beam pipe had a small value of closest approach . The transverse momentum (Pt) dependence of the expected acceptance of the track finder is shown
Fig. 5. A typical Jhabha event observed by the VENUS central drift chamber at the first stage. (a) Each dot corresponds to a hit wire. (b) Trigger cell fired for the same event. (c) The fan-shape sectors which could recognize tracks.
1000
500
00 1 00
50
100
-150
distance of minimum approach (cm)
200
Fig. 6. The distribution of closest approach to the beam line for the tracks detected by the track finder.
52 6
T Ohsugi et al. /Fast trackfinding processor
ÛC a â
Fig. 7 . Transverse momentum dependence of track detection acceptance. The solid, dashed and dotted line indicate accep tances expected for cases of the chamber efficiency of 100%, 99% and 98%, respectively . in fig . 7 for a track pattern having Pt = 0 .7 GeV/c . The solid, dashed and dotted lines indicate the acceptances for cases of chamber efficiency of 100%, 99% and 98%, respectively . We started the experiment with a track trigger of the widest acceptance pattern, in other words, the lowest P, pattern in the look-up table . Then we suffered from too large rates of trigger by low Pt and curling tracks produced at the beam pipe . As soon as this was discovered, a fast change of the track pattern to one of higher P, was made. This new pattern could cut off tracks having Pt less than 0 .7 GeV/c, and enabled us to make measurement without any significant dead time. This confirms that the present programmable look-up table is very flexible and powerful . 7. Conclusion We have developed a track-finding processor with a look-up table of parallel processing type for the firstlevel trigger of the VENUS experiment . The track finding for trigger is made with a cell granularity of 1/64 azimuthally and 7 layers radially. The accessing time is
less than 110 ns, irrespective of the number of tracks because we employ totally parallel processing . The requirement of adjacent two-layer coincidence to form the trigger signal is essential to discriminate synchrotron radiation noise . The present programmable look-up table has enough flexibility in changing track-finding conditions and this flexibility is indispensable for efficient and versatile triggering . Our track finder is now working excellently as one of main devices in the VENUS triggering system under rather heavy backgrounds .
Acknowledgements We are grateful to prof. S . Ozaki, Prof . K . Takahashi and Prof. Y . Sumi for their continuous encouragement and support to this study . We thank all members of the VENUS group for their support and many useful discussions . We wish to express our thanks to Prof. 1 . Endo for his invaluable discussions and continuous support in the course of this work. We thank Mr . S. Takubo for his kind help in figure preparation . The design and simulation work was done with the computer of the Data Analysis Laboratory for High Energy Physics, Hiroshima University .
References See, for examples, H. Brafmann, H. Breidenbach, R. Hettel, T . Himel and D . Horelick, IEEE Trans. Nucl . Sci . NS-25 (1978) 692 ; P . Scluldt and H.J . Stuckenberg, Nucl . Instr . and Meth . 178 (1980) 571 ; G. Franke, DESY 80/109 (November 1980) ; P. Waloschek, DESY 80/114 (November 1980) ; M . Ronan, J . Millaud and T . McGathen, IEEE Trans . Nucl. Sci . NS-19 . (1982) 427 ; J.J. Thaler, V .J. Simatis, j .J. Becker and W.J . Wisniewski, IEEE Trans. Nucl. Sci . NS-30 (1983) 236 . [21 CELLO Collaboration, H.J . Behrend et al., Phys . Scripta 23 (1981) 610. [31 R . Arai, H. Boemer, N . Ishihara, T. Kohriki, T. Kondo, S . Nakamura, Y . Watase and F . Suekane, Nucl . Instr. and Meth . 217 (1983) 181 .
Nuclear Instruments and Methods in Physics Research A269 (1988) 522-526 North-Holland, Amsterdam
FAST TRACK-FINDING PROCESSOR BASED ON RAM LOOK-UP TABLE FOR THE VENUS DETECTOR AT KEK T. OHSUGI, Y. CHIBA, I . HAYASHIBARA, A. TAKETANI and S. YASUISHI Hiroshima University, Hiroshima 730, Japan
Y. ARAI, H. SAKAMOTO and S. UEHARA KEK, National Laboratory for High Energy Physics, Oho-mache, Tsukuba, Ibaraki 305, Japan
Received 18 August 1987 and in revised form 28 December 1987
We have developed a fast track-fending processor using signals from the central tracking chamber of the VENUS detector in the TRISTAN experiments . Particle tracks are recognized by a look-up table made with a high-speed static RAM. This method enables us to implement the track finder m the first level triggering . The track finder has been working excellently under heavy background due to synchrotron radiation. A processing time of 110 ns is attained . 1 . Introduction In e + e - colliding experiments, a track signal from the interaction points is a clear indication of beam-beam interaction. For example, in an event like e +e- -> p +p.-, track information is only available for event triggering. In such a case, a fast track finder must be one of the indispensable triggering devices. Many ideas for fast track-finding methods have been proposed and utilized [1] in the experiments at SPEAR, PETRA, and PEP, but only few have been implemented in the first level triggering [2], because most of the devices need a rather long processing time to find tracks. The tracking chambers had also rather long electron drift times. We have developed a fast track finder using a highspeed static RAM (random access memory) in which a look-up table method is taken to find tracks from data of a tracking chamber. This type of track finder matches well with tracking chambers having a small cell . This device has been implemented in the VENUS detector in the TRISTAN experiment . Since the look-up table method is of totally parallel-processing type, we expect the highest processing speed, which enables us to make a dead-timeless triggering at the first level in the VENUS detector. The look-up table for triggering used to be made with a FPLA (field programmable logic array) mainly because of its high speed capability . In the present case, however, we need a more flexible device, because track-finding conditions should be easily changeable according to various experimental requirements. A rewritable look-up table which is constructed 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
of high-speed static RAMs is, therefore, more suitable for the present track finder. 2. book-up table method The principle of the look-up table is very simple as seen from fig. 1. One has to prepare a size of memory modules large enough to cover the number of signal inputs . The input signals are supplied to the address lines of these memories with timing signal, and then the track flag written in advance comes out as a memory readout action . If one wants to make up such a naive look-up table, one needs an enormous number of memories. For example, processing of only 64 signal inputs charged track
4KX 1
bit
or not
interaction point Fig. 1. Principle of a look-up table for the track finder.
52 3
T. Ohsugi et al. /Fast track finding processor
requires a 264-bit table which is obviously impossible to be implemented. However, if we look at the table contents carefully, most of the area of the naive look-up table is found to be empty space. Only some special combinations of the input signals are realized and other combinations never come up in practical use. Such specific combinations could be selected out analytically or by a Monte Carlo simulation. We classify the selected combinations to specific clusters so that the number of input signals forming the combinations is equal to or less than the number of address lines of an available memory chip. The required look-up table can be made up with this memory chip . Thus, how to make a grouping of input signals is the most critical point in designing the look-up table. 3. Signal preparation for the track finder The central drift chamber of the VENUS detector has 10 groups (2 layers in one group) of axial sense wires and 9 layers of stereo-wires [3]. To locally solve the left-right ambiguity, two axial layers in the same group are staggered by a half cell as shown in fig. 2. For our triggering purpose, only signals from inside 7 groups of axial wires are used in track finding. The reason why we have selected signals from the innermost 7 groups is as follows: A track originating from the interaction point is a good signature for beam-beam interactions . For the identification of a track coming from the interaction point, a distance of closest approach to the beam line on the R-0 plane is a good parameter. Obviously the shorter lever arm gives the better resolution of distance in the closest approach determination. In addition, inner groups can cover a wider angular region, resulting in a larger acceptance which is always favorable for interesting events .
A disadvantage of this choice is larger noise. However, we can sufficiently reduce the X-ray noise by taking coincidences between two signals from adjacent layers staggered in the same group as shown in fig. 2. This is because a charged particle passing through the chamber generates correlated signals in adjacent cells staggered, whereas random noise by synchrotron radiation does not show such a correlation . By taking the coincidence, we form 7 layers of signals for the trigger. Then, these signals are grouped to form a basic trigger cell as shown in fig. 2. A track element would, thus, be found within a spatial resolution defined by this cell size. The cell size is selected to reduce accidental triggers due to soft X-ray backgrounds and to minimize the number of signals feeding the track finder. Eventually, 64 azimuthal sectors were formed and a total of 64 X 7 = 448 signals were supplied to the track finder in our case. 4. Track recognition on R-0 projection To find a track element, a fan-shape sector is formed to cover 1/64 of the R-di projection, which is shown
2
OR ed
Coincidence Cell for trigge Sense wire Trock Pt =300 MeV
Fig. 2. Signal preparation scheme for the track finder. The coincidence between signals from two layers staggered is taken, and ORed to form 64 cells per trigger layer.
Fig. 3. The cell structure of a 12-bit subsector. Each subsector is covered by one RAM chip, resulting in four chips covering one fan-shape sector which is the unit of track counting for the trigger.
52 4
T. Ohsugi et al. / Fast trackfinding processor
by the thick solid line in fig. 2. This sector can recognize a track having more than 0.2 GeV/c transverse momentum. The fan-shape sector is decomposed into four 12-bit subsectors, being matched to a 4k-bit, highspeed static RAM. Cell structures of four subsectors are shown in fig. 3. The whole of the R-0 projection is covered by 256 chips of 4k-bit RAM, Any combinations of these 12 bits are programmable. Let us give a typical example of how to make a trigger pattern. Basic hit patterns are generated on the subsector by simulated tracks originating from the interaction point. By taking account of the inefficiency of chamber cells, a pattern in which any one or two cells are missing in the 7 layers should be counted as a basic pattern. Since there are noise hits which overlap with track signals, all patterns including any of the basic patterns must be adopted as trigger patterns . If one allows a missing of the inner-most cell in the track identification, two adjacent sectors may generate track flags by one real track because of overlapping of the fan-shape sectors. This kind of double-counting problem can be solved by the so-called cluster counting technique. A track separation in a jet is not required for trigger purposes. Each memory chip generates on/off signals depending on whether a track has been recognized or not. A total of 256 bit information is generated from 256 chips in 8 modules, which can be read out via FASTBUS, and is very helpful to the second level trigger as well as to track finding in off-line analysis . Four signals which correspond to one basic fan-shape sector are ORed, resulting in 64 track flags which are supplied to the next-step logic.
5. Track finder module One FASTBUS module covers 1/8 of the R-0 projection, in other words, a look-up table for eight sectors is installed on one FASTBUS slave module which is realized with a 6-layer printed-circuit board. FASTBUS specification is well matched to a large-size table, because of its large addressing capability and large board size. An auxiliary connector having a large number of pins is open to the user, and this is important for a large size look-up table needed for our track finder. Functions of FASTBUS standard implemented in this module satisfy the minimum requirements for a slave module, i.e., geographical addressing, secondary addressing and the control-status register no . 0. Fig. 4 shows a schematic diagram of the track finder logic. Input signals should be stable for 20 ns before and after the latch timing of input signals. The latchtiming signal produced by beam crossing latches the 76 input signals in the input register, and starts looking for a track pattern. They are rearranged in 32 sets of 12-bit addresses and are fed to address lines of 4k-bit RAM chips in the look-up table. Then, a readout pulse produced by beam crossing triggers the readout action. We used a CMOS static RAM, HM6147HLP-35 (Hitachi), for which 35 ns access time was guaranteed . In practice, signals come out within 20 ns for most of the RAM chips. RAM outputs are summed over 4 RAMS corresponding to one basic, fan-shape sector. The resulting 8 outputs per module are supplied to the next-step logic.
Back plane
Front panel BCS timing signal CS
76
4Kx32
to majority logic
in RAM output register
RAM WE
tr m ouxi?iar connectyor
Pattern register FASTBUS protocol circuits
Control lines AD lines
Fig. 4. Block diagram of the track finder logic.
525
T. Ohsugi et al / Fast trackfinding processor
A track-status flag (found or not found for each track pattern) is ready to be read out within 110 ns after the arrival of the latch-timing signal . A processing time of 75 ns has been attained by fine adjustment of the timing . If one designs this type of look-up table on the most advanced CMOS gate array, one can probably achieve less than 15 ns processing time . Patterns in the look-up table are generated in a host computer and written via FASTBUS. The input-signal pattern and the output-bit pattern can be readout via FASTBUS. Power consumption of this module has been measured to be about 25 W under the static condition. 6. Performance of the track finder Our track finder was found to work excellently in spite of enormous X-ray noise in the initial stage of TRISTAN operation. Fig. 5a shows a typical Bhabha event under the heavy X-ray backgrounds. Fig. 5b indicates trigger cells fired in the same event. Comparison of these two figures clearly shows that most of the X-ray noise is rejected from the trigger signal fed to the track finder. The requirement of a coincidence between two adjacent cells staggered to form the trigger signal was confirmed to be extremely effective for rejecting these X-ray backgrounds . By requiring that at least 5 out of 7 layers in a pattern must be fired for the track recognition, two real tracks were uniquely identified as shown in fig. 5c. The probability of finding X-ray noise per unit cell was found-to be inversely proportional to the distance from the beam axis, and the average number of wires hit by X-rays was about 400 (- 6% of all sense wires) in the beginning of TRISTAN operation. Under this condition, the probability of the track finder giving one false track from these random noises was evaluated to be 10 -5 . A charged-track trigger was issued by requiring two collinear tracks or more than three tracks. For more than one week running with this trigger condition, no false trigger was recorded by the track finder . Thus it turns out that there is no mistake for more than 6 x 10 10 beam crossings, showing an excellent discrimination capability for accidental tracks due to random noises of X-rays . A charged-track trigger must be given by only one track coming from the interaction point. Fig. 6 shows how the track-detection efficiency decreases as the distance of closest approach of the track to the beam line increases. One can see that most of the cosmic rays distributed uniformly were discriminated by the track finder, but some events due to beam-beam pipe interaction could survive because some of the tracks produced at the beam pipe had a small value of closest approach . The transverse momentum (Pt) dependence of the expected acceptance of the track finder is shown
Fig. 5. A typical Jhabha event observed by the VENUS central drift chamber at the first stage. (a) Each dot corresponds to a hit wire. (b) Trigger cell fired for the same event. (c) The fan-shape sectors which could recognize tracks.
1000
500
00 1 00
50
100
-150
distance of minimum approach (cm)
200
Fig. 6. The distribution of closest approach to the beam line for the tracks detected by the track finder.
52 6
T Ohsugi et al. /Fast trackfinding processor
ÛC a â
Fig. 7 . Transverse momentum dependence of track detection acceptance. The solid, dashed and dotted line indicate accep tances expected for cases of the chamber efficiency of 100%, 99% and 98%, respectively . in fig . 7 for a track pattern having Pt = 0 .7 GeV/c . The solid, dashed and dotted lines indicate the acceptances for cases of chamber efficiency of 100%, 99% and 98%, respectively . We started the experiment with a track trigger of the widest acceptance pattern, in other words, the lowest P, pattern in the look-up table . Then we suffered from too large rates of trigger by low Pt and curling tracks produced at the beam pipe . As soon as this was discovered, a fast change of the track pattern to one of higher P, was made. This new pattern could cut off tracks having Pt less than 0 .7 GeV/c, and enabled us to make measurement without any significant dead time. This confirms that the present programmable look-up table is very flexible and powerful . 7. Conclusion We have developed a track-finding processor with a look-up table of parallel processing type for the firstlevel trigger of the VENUS experiment . The track finding for trigger is made with a cell granularity of 1/64 azimuthally and 7 layers radially. The accessing time is
less than 110 ns, irrespective of the number of tracks because we employ totally parallel processing . The requirement of adjacent two-layer coincidence to form the trigger signal is essential to discriminate synchrotron radiation noise . The present programmable look-up table has enough flexibility in changing track-finding conditions and this flexibility is indispensable for efficient and versatile triggering . Our track finder is now working excellently as one of main devices in the VENUS triggering system under rather heavy backgrounds .
Acknowledgements We are grateful to prof. S . Ozaki, Prof . K . Takahashi and Prof. Y . Sumi for their continuous encouragement and support to this study . We thank all members of the VENUS group for their support and many useful discussions . We wish to express our thanks to Prof. 1 . Endo for his invaluable discussions and continuous support in the course of this work. We thank Mr . S. Takubo for his kind help in figure preparation . The design and simulation work was done with the computer of the Data Analysis Laboratory for High Energy Physics, Hiroshima University .
References See, for examples, H. Brafmann, H. Breidenbach, R. Hettel, T . Himel and D . Horelick, IEEE Trans. Nucl . Sci . NS-25 (1978) 692 ; P . Scluldt and H.J . Stuckenberg, Nucl . Instr . and Meth . 178 (1980) 571 ; G. Franke, DESY 80/109 (November 1980) ; P. Waloschek, DESY 80/114 (November 1980) ; M . Ronan, J . Millaud and T . McGathen, IEEE Trans . Nucl. Sci . NS-19 . (1982) 427 ; J.J. Thaler, V .J. Simatis, j .J. Becker and W.J . Wisniewski, IEEE Trans. Nucl. Sci . NS-30 (1983) 236 . [21 CELLO Collaboration, H.J . Behrend et al., Phys . Scripta 23 (1981) 610. [31 R . Arai, H. Boemer, N . Ishihara, T. Kohriki, T. Kondo, S . Nakamura, Y . Watase and F . Suekane, Nucl . Instr. and Meth . 217 (1983) 181 .