- Email: [email protected]
PHENIX RPC R&D for the fast RPC muon trigger upgrade
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
PHENIX RPC R&D for the fast RPC muon trigger upgrade Beau Meredith University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
For the PHENIX RPC upgrade group a r t i c l e in f o
a b s t r a c t
Available online 19 January 2009
The PHENIX collaboration at relativistic heavy ion collider (RHIC) will measure the maximal parity violating W asymmetries in polarized proton–proton collisions at high transverse momentum in order to make the most precise measurement of the quark and the anti-quark polarizations of the proton. For this measurement, the collaboration is installing resistive plate chambers (RPCs) similar to the CMS Endcap RPCs in the two existing muon spectrometers. In these proceedings, we shall discuss the research and development (R&D) that has been done by the collaboration in order to both understand and develop RPC technology. The R&D has been carried out at Georgia State University, the University of Colorado at Boulder, and the University of Illinois at Urbana-Champaign with many other institutions contributing as well; each university has focused on the different aspects of the RPCs. Georgia State has focused on detector design, performance, and stability; Colorado has tested the front end electronics and termination schemes; Illinois has performed position resolution studies as well as two-dimensional efficiency scans of RPCs. Additionally, a cosmic ray teststand for final gap QA is currently operational at Brookhaven National Laboratory (BNL). & 2009 Elsevier B.V. All rights reserved.
Keywords: Resistive plate chamber Muon trigger Proton spin PHENIX BNL RHIC
1. Introduction The relativistic heavy ion collider (RHIC) at Brookhaven National Laboratory (BNL) is both for heavy ion and for polarized pp collisions. The PHENIX experiment at RHIC plans to precisely measure the quark and anti-quark polarizations by measuring pffiffi muon yields from W-boson decay in longitudinally polarized s ¼ 500 GeV pp collisions (see Ref. [1] for more details). Presently, PHENIX has two forward muon spectrometers (see Fig. 1) each of which consist of three wire chamber stations with cathode strip readout (Muon Trackers or MuTr), a lampshade magnet that produces a radial magnetic field, and five layers of Iarocci streamer tubes interlaced with steel absorbers for muon identification. The leptonic decay of the W-boson results in high momentum muons; Ws can be identified in the PHENIX muon spectrometers at high pT 420 GeV, well above the pT of muons from decay backgrounds [2]. The present muon trigger has a rejection factor (RF) of approximately 500; however, an RF of 10,000 is needed to select high pT W-decay muons and reject low pT backgrounds in the trigger (see Ref. [1]). In order to satisfy the requirement of RF ¼ 10; 000, three resistive plate chamber (RPC) stations will be installed in each spectrometer. The PHENIX RPCs are based on the CMS Endcap design [4]. A double-gap design is used wherein the readout plane
E-mail address: [email protected] 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.12.236
is sandwiched between the two gaps. A gas mixture of 95:4.5:0.5 R134A:Isobutane:SF6 is used. Both the gap widths and the plate thicknesses are 2 mm (see Refs. [1,3] for more details). The detector requirements displayed in Table 1 have been shown to be sufficient to obtain the required RF. In order to measure PHENIX-specific performance parameters and to acquire experience with the RPC technology, a multiinstitutional effort of RPC research and development (R&D) has been carried out within the PHENIX collaboration. Four cosmic ray teststands have been built. Each teststand was created to study the different aspects of the R&D: Georgia State University (GSU) has focused on detector design, performance, and stability; the University of Colorado at Boulder (CU) has tested the front end electronics and termination schemes; the University of Illinois at Urbana-Champaign (UIUC) has performed position resolution studies as well as two-dimensional efficiency scans of RPCs. Additionally, the teststand at BNL is for the final QA of the gas gaps; this teststand can simultaneously test up to nine RPC chambers at once. In this contribution, we shall discuss the RPC R&D efforts in PHENIX and results to date.
2. Studying the CMS design In order to understand the CMS design, it was decided to build generations of prototypes and add features of the CMS design as the generations progressed. GSU was largely responsible for
ARTICLE IN PRESS B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
767
Fig. 2. Diagram of baseline trigger for forward upgrade.
Fig. 1. Cross-sectional view of PHENIX with the RPC upgrades.
Table 1 PHENIX RPC requirements. Parameter
Allowable range
Efficiency Time resolution Average cluster size Rate capability Number of streamers
495% p3 ns p2 strips 0:5 kHz=cm2 o10%
building and testing these prototypes. Design characteristics such as wrapping the gaps with copper foil to reduce crosstalk and noise, using polycarbonate spacers protruding beyond the bakelite edges and high resistivity EVA glue on the edges to keep the dark currents low, and using linseed oil coated gaps to reduce the noise rate were added to incrementally improve the prototype design and to study the impact of each design feature on the detector performance.
simulations is whether or not a clustering algorithm would be needed in the level-1 trigger. If the cluster size distributions are too large and no clustering algorithm is present, we cannot determine the change in azimuthal angle (we trigger on high pT muons, which have very small angular changes) between RPC stations with good precision for high momentum tracks; hence, the RF would be insufficient. To resolve this issue, a realistic cluster size (number of contiguous strips that fire in a given event) distribution was needed as an input for the simulation. A cluster size distribution of a double-gap RPC with 0.5 cm strips was experimentally measured for cosmic ray muon tracks (see Fig. 3(a)). The voltage was chosen to be 9:3 kV, which lies on the efficiency plateau (see Fig. 3(b)). This cluster size distribution was then integrated into the simulations. The RFs obtained from these simulations are shown in Table 2. Note that this is the combined RF for both North and South arms. In this table, Df1;2 is a cut on the change in azimuthal angle between RPC1 and RPC2. N and S refer to the north and south muon spectrometer arms. One can see that a 21 cut produces a RF of 15,000, well above the 10,000 that is desired without a significant loss in efficiency. From this study, it was concluded that no cluster algorithm is needed for the level-1 trigger if a cut of 21 is used.
3. Forward RPC trigger 5. Timing resolution studies The RPC is an ideal detector for the PHENIX forward upgrade because of its timing resolution, rate capability, and efficiency [5,6]. As stated in the introduction, we aim to increase the RF of the muon spectrometers from approximately 500 to 10,000. In addition to the RPC upgrade, an upgrade of the MuTr FEE is taking place which allows the MuTr stations to be used in the trigger. The baseline upgrade trigger will consist of four detectors: RPC1 (or MuTr1), MuTr2, RPC2, and RPC3 (see Fig. 1). The trigger logic effectively proceeds as follows (also see Fig. 2): (i) Identify a candidate track (note: muon tracks will curve owing to a radial magnetic field between the three MuTr stations, see Fig. 1). (ii) Draw a line connecting RPC1, two hit positions. (iii) Project this line onto MuTr2. (iv) Require the hit position in MuTr2 to be within three strips of the projection.
In order to reject beam related backgrounds and cosmic muon backgrounds, a safe requirement is to have a detector with a timing resolution below 3 ns. Bakelite RPCs have a timing resolution that is well below this limit, and this is a key reason RPCs were chosen for the muon trigger. Timing resolution studies were performed on a setup that mimics the final installation—a protoype RPC manufactured at Korea University (the CMS forward RPC production site, see Refs. [1,3]) with 35 cm strips connected to a CMS AD board readout by a TDC module. One outstanding question is whether or not termination is required in order to cancel reflected pulses that degrade the timing resolution. Table 3 summarizes the results of this study. The timing resolution was measured at the center of the strip and at the end opposite the termination. The resolution is well under 3 ns for all cases. However, the actual RPCs used in PHENIX will have longer strips (see Refs. [1,3]), and so additional timing resolution studies are taking place at the BNL teststand on RPCs with strip lengths that are used in the RPC stations.
4. Trigger simulations In order to measure the RF of the trigger, Monte Carlo studies have been performed. A simulation was carried out in which 106 pffiffi Pythia s ¼ 500 GeV pp events were run through a GEANT model of PHENIX. One issue that needed to be resolved by the
6. Basic R&D In order to better understand some fundamental aspects of the RPC (e.g. position resolution limits, behavior upon variation of SF6 ,
ARTICLE IN PRESS 768
B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
Fig. 3. (a) Cluster size distribution and (b) efficiency for a Georgia State University prototype with 0.5 cm strips.
Table 2 Rejection factor as a function of the azimuthal angular cut for a track between RPC1 and RPC2.
Df1;2 cut
Trigger efficiency (25 GeV muons)
Rejection factor
21 cut 31 cut
95% N/92% S 98% N/96% S
15,600 9317
Table 3 FEE efficiencies and timing resolutions for a RPC strip.
FEE efficiency (%) Time resolution (ns)
Center terminated
End terminated
End not terminated
98.6 1.34
96.1 1.32
99.6 1.49
Center (end) means the resolution was measured in the center (end opposite termination) of the strip.
efficiency as a function of particle rate), a teststand was built at the UIUC possessing tracking via drift chambers (resolution 1.0–1.5 mm) in x- and y-planes and timing information from three paddle scintillators as seen in Fig. 4. The RPCs used have an open gap design (see Fig. 5(a)) and are immersed in the gas flowing through closed aluminum cylinders. The double-gap RPCs have 2 mm gaps and 2 mm bakelite-plate thicknesses. The gap is maintained by Teflon spacers (washers) at the corners which fit around Teflon screws holding the RPC together. The RPC’s active area measures approximately 6 19 cm. Because of this design, one can easily change the parameters of the double gap to allow rapid R&D. For instance, readout strip layouts are routinely modified by swapping PCBs, and gap widths can be easily modified by using spacers of different thicknesses. Both TDC and ADC information are read out. A position resolution study was performed to see how a finer granularity of strip sizes affects the achievable position resolution. Strip widths of 1, 0.6, and 0.3 cm were used in this study. An example printed circuit board is shown in Fig. 5(b). The
Fig. 4. The cosmic ray teststand at UIUC contains drift chambers in the x- and y-planes above and below the RPCs to allow precise tracking. The RPCs are placed inside the aluminum cylinders. Both ADC and TDC information are read out.
Fig. 5. (a) The open gap design of the UIUC RPC prototype uses Teflon spacers to keep the gap width fixed and Teflon screws and nuts to hold the gap together. This allows for rapid prototyping. (b) A PCB with 1.0 cm wide strips used in the position resolution study.
muon position in the RPC was calculated in three different ways: (i) Finding the geometrical center of the cluster (cluster center method).
ARTICLE IN PRESS B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
769
Fig. 6. Example of position resolution plot from the UIUC teststand for gaps with 1.0 and 0.3 cm strips.
Fig. 7. Position resolution versus applied high voltage for three different strip widths (1.0, 0.6, and 0.3 cm).
(ii) Finding the position of the strip with the maximum charge (ADC Max method). (iii) Finding the mean of Gaussian fit to the charge distribution on the strips (Gaussian method).
The RPC position is then compared to the drift chamber position to calculate the position resolution. Fig. 6 shows an example position resolution plot for 0.3 and 1.0 cm strip widths. Fig. 7 shows the Gaussian position resolution as a function of applied high voltage. Note that for the 0.3 cm strips the RPC position resolution approaches that of the drift chambers. Additionally, the resolution may be limited by slight geometrical alignment issues. In addition to the position resolution studies, an SF6 scan and a gap-nonuniformity test were performed; however, these results will not be shown here.
Fig. 8. Strip layout used for rate capability study. The cylindrical sources are placed at one end of the RPC; the long side of the source runs parallel with the strips.
Finally, a rate capability study is underway in which two 6 cm cylindrical 0.5 mCi 55Fe sources are placed at the edges of the gas gaps. The strip layout shown in Fig. 8 is used. The background photon rate drops off (linearly near the source) in intensity as the strip distance from the source increases; in this way we are capable of measuring the efficiency as a function of multiple rates without varying the source intensity. Fig. 9 shows an efficiency radiograph of the irradiated gap. The rate drops from approximately 10 kHz=cm2 at the strip closest to the source (at the bottom of the efficiency radiograph) to 100 Hz=cm2 near the center of the chamber and then to 10 Hz=cm2 at the last strip (approximately 20 cm away from the source). The efficiency shows a significant dependence on the rate, as established in many previous results.
ARTICLE IN PRESS 770
B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
R&D has been performed in the PHENIX collaboration to both acquire experience with the technology and studying PHENIX specific performance parameters. References [1] B. Hong, et al., in these proceedings. [2] G. Bunce, N. Saito, J. Soffer, W. Vogelsang, Ann Rev. Nucl. Part. Sci. 50 (2000) 525. [3] R. Towell, et al., in these proceedings. [4] S. Park, et al., Nucl. Instr. and Meth. A 550 (2005) 551; S. Park, et al., Nucl Phys. B Proc. Suppl. 158 (2006) 16. [5] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. A 187 (1981) 377. [6] G. Carboni, et al., Nucl. Instr. and Meth. A 533 (2004) 107.
Fig. 9. An example of radiograph of the RPC with the source in place.
7. Summary The RPC forward trigger upgrade will enable PHENIX to measure the quark polarization distributions of the proton. RPC
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
PHENIX RPC R&D for the fast RPC muon trigger upgrade Beau Meredith University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
For the PHENIX RPC upgrade group a r t i c l e in f o
a b s t r a c t
Available online 19 January 2009
The PHENIX collaboration at relativistic heavy ion collider (RHIC) will measure the maximal parity violating W asymmetries in polarized proton–proton collisions at high transverse momentum in order to make the most precise measurement of the quark and the anti-quark polarizations of the proton. For this measurement, the collaboration is installing resistive plate chambers (RPCs) similar to the CMS Endcap RPCs in the two existing muon spectrometers. In these proceedings, we shall discuss the research and development (R&D) that has been done by the collaboration in order to both understand and develop RPC technology. The R&D has been carried out at Georgia State University, the University of Colorado at Boulder, and the University of Illinois at Urbana-Champaign with many other institutions contributing as well; each university has focused on the different aspects of the RPCs. Georgia State has focused on detector design, performance, and stability; Colorado has tested the front end electronics and termination schemes; Illinois has performed position resolution studies as well as two-dimensional efficiency scans of RPCs. Additionally, a cosmic ray teststand for final gap QA is currently operational at Brookhaven National Laboratory (BNL). & 2009 Elsevier B.V. All rights reserved.
Keywords: Resistive plate chamber Muon trigger Proton spin PHENIX BNL RHIC
1. Introduction The relativistic heavy ion collider (RHIC) at Brookhaven National Laboratory (BNL) is both for heavy ion and for polarized pp collisions. The PHENIX experiment at RHIC plans to precisely measure the quark and anti-quark polarizations by measuring pffiffi muon yields from W-boson decay in longitudinally polarized s ¼ 500 GeV pp collisions (see Ref. [1] for more details). Presently, PHENIX has two forward muon spectrometers (see Fig. 1) each of which consist of three wire chamber stations with cathode strip readout (Muon Trackers or MuTr), a lampshade magnet that produces a radial magnetic field, and five layers of Iarocci streamer tubes interlaced with steel absorbers for muon identification. The leptonic decay of the W-boson results in high momentum muons; Ws can be identified in the PHENIX muon spectrometers at high pT 420 GeV, well above the pT of muons from decay backgrounds [2]. The present muon trigger has a rejection factor (RF) of approximately 500; however, an RF of 10,000 is needed to select high pT W-decay muons and reject low pT backgrounds in the trigger (see Ref. [1]). In order to satisfy the requirement of RF ¼ 10; 000, three resistive plate chamber (RPC) stations will be installed in each spectrometer. The PHENIX RPCs are based on the CMS Endcap design [4]. A double-gap design is used wherein the readout plane
E-mail address: [email protected] 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.12.236
is sandwiched between the two gaps. A gas mixture of 95:4.5:0.5 R134A:Isobutane:SF6 is used. Both the gap widths and the plate thicknesses are 2 mm (see Refs. [1,3] for more details). The detector requirements displayed in Table 1 have been shown to be sufficient to obtain the required RF. In order to measure PHENIX-specific performance parameters and to acquire experience with the RPC technology, a multiinstitutional effort of RPC research and development (R&D) has been carried out within the PHENIX collaboration. Four cosmic ray teststands have been built. Each teststand was created to study the different aspects of the R&D: Georgia State University (GSU) has focused on detector design, performance, and stability; the University of Colorado at Boulder (CU) has tested the front end electronics and termination schemes; the University of Illinois at Urbana-Champaign (UIUC) has performed position resolution studies as well as two-dimensional efficiency scans of RPCs. Additionally, the teststand at BNL is for the final QA of the gas gaps; this teststand can simultaneously test up to nine RPC chambers at once. In this contribution, we shall discuss the RPC R&D efforts in PHENIX and results to date.
2. Studying the CMS design In order to understand the CMS design, it was decided to build generations of prototypes and add features of the CMS design as the generations progressed. GSU was largely responsible for
ARTICLE IN PRESS B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
767
Fig. 2. Diagram of baseline trigger for forward upgrade.
Fig. 1. Cross-sectional view of PHENIX with the RPC upgrades.
Table 1 PHENIX RPC requirements. Parameter
Allowable range
Efficiency Time resolution Average cluster size Rate capability Number of streamers
495% p3 ns p2 strips 0:5 kHz=cm2 o10%
building and testing these prototypes. Design characteristics such as wrapping the gaps with copper foil to reduce crosstalk and noise, using polycarbonate spacers protruding beyond the bakelite edges and high resistivity EVA glue on the edges to keep the dark currents low, and using linseed oil coated gaps to reduce the noise rate were added to incrementally improve the prototype design and to study the impact of each design feature on the detector performance.
simulations is whether or not a clustering algorithm would be needed in the level-1 trigger. If the cluster size distributions are too large and no clustering algorithm is present, we cannot determine the change in azimuthal angle (we trigger on high pT muons, which have very small angular changes) between RPC stations with good precision for high momentum tracks; hence, the RF would be insufficient. To resolve this issue, a realistic cluster size (number of contiguous strips that fire in a given event) distribution was needed as an input for the simulation. A cluster size distribution of a double-gap RPC with 0.5 cm strips was experimentally measured for cosmic ray muon tracks (see Fig. 3(a)). The voltage was chosen to be 9:3 kV, which lies on the efficiency plateau (see Fig. 3(b)). This cluster size distribution was then integrated into the simulations. The RFs obtained from these simulations are shown in Table 2. Note that this is the combined RF for both North and South arms. In this table, Df1;2 is a cut on the change in azimuthal angle between RPC1 and RPC2. N and S refer to the north and south muon spectrometer arms. One can see that a 21 cut produces a RF of 15,000, well above the 10,000 that is desired without a significant loss in efficiency. From this study, it was concluded that no cluster algorithm is needed for the level-1 trigger if a cut of 21 is used.
3. Forward RPC trigger 5. Timing resolution studies The RPC is an ideal detector for the PHENIX forward upgrade because of its timing resolution, rate capability, and efficiency [5,6]. As stated in the introduction, we aim to increase the RF of the muon spectrometers from approximately 500 to 10,000. In addition to the RPC upgrade, an upgrade of the MuTr FEE is taking place which allows the MuTr stations to be used in the trigger. The baseline upgrade trigger will consist of four detectors: RPC1 (or MuTr1), MuTr2, RPC2, and RPC3 (see Fig. 1). The trigger logic effectively proceeds as follows (also see Fig. 2): (i) Identify a candidate track (note: muon tracks will curve owing to a radial magnetic field between the three MuTr stations, see Fig. 1). (ii) Draw a line connecting RPC1, two hit positions. (iii) Project this line onto MuTr2. (iv) Require the hit position in MuTr2 to be within three strips of the projection.
In order to reject beam related backgrounds and cosmic muon backgrounds, a safe requirement is to have a detector with a timing resolution below 3 ns. Bakelite RPCs have a timing resolution that is well below this limit, and this is a key reason RPCs were chosen for the muon trigger. Timing resolution studies were performed on a setup that mimics the final installation—a protoype RPC manufactured at Korea University (the CMS forward RPC production site, see Refs. [1,3]) with 35 cm strips connected to a CMS AD board readout by a TDC module. One outstanding question is whether or not termination is required in order to cancel reflected pulses that degrade the timing resolution. Table 3 summarizes the results of this study. The timing resolution was measured at the center of the strip and at the end opposite the termination. The resolution is well under 3 ns for all cases. However, the actual RPCs used in PHENIX will have longer strips (see Refs. [1,3]), and so additional timing resolution studies are taking place at the BNL teststand on RPCs with strip lengths that are used in the RPC stations.
4. Trigger simulations In order to measure the RF of the trigger, Monte Carlo studies have been performed. A simulation was carried out in which 106 pffiffi Pythia s ¼ 500 GeV pp events were run through a GEANT model of PHENIX. One issue that needed to be resolved by the
6. Basic R&D In order to better understand some fundamental aspects of the RPC (e.g. position resolution limits, behavior upon variation of SF6 ,
ARTICLE IN PRESS 768
B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
Fig. 3. (a) Cluster size distribution and (b) efficiency for a Georgia State University prototype with 0.5 cm strips.
Table 2 Rejection factor as a function of the azimuthal angular cut for a track between RPC1 and RPC2.
Df1;2 cut
Trigger efficiency (25 GeV muons)
Rejection factor
21 cut 31 cut
95% N/92% S 98% N/96% S
15,600 9317
Table 3 FEE efficiencies and timing resolutions for a RPC strip.
FEE efficiency (%) Time resolution (ns)
Center terminated
End terminated
End not terminated
98.6 1.34
96.1 1.32
99.6 1.49
Center (end) means the resolution was measured in the center (end opposite termination) of the strip.
efficiency as a function of particle rate), a teststand was built at the UIUC possessing tracking via drift chambers (resolution 1.0–1.5 mm) in x- and y-planes and timing information from three paddle scintillators as seen in Fig. 4. The RPCs used have an open gap design (see Fig. 5(a)) and are immersed in the gas flowing through closed aluminum cylinders. The double-gap RPCs have 2 mm gaps and 2 mm bakelite-plate thicknesses. The gap is maintained by Teflon spacers (washers) at the corners which fit around Teflon screws holding the RPC together. The RPC’s active area measures approximately 6 19 cm. Because of this design, one can easily change the parameters of the double gap to allow rapid R&D. For instance, readout strip layouts are routinely modified by swapping PCBs, and gap widths can be easily modified by using spacers of different thicknesses. Both TDC and ADC information are read out. A position resolution study was performed to see how a finer granularity of strip sizes affects the achievable position resolution. Strip widths of 1, 0.6, and 0.3 cm were used in this study. An example printed circuit board is shown in Fig. 5(b). The
Fig. 4. The cosmic ray teststand at UIUC contains drift chambers in the x- and y-planes above and below the RPCs to allow precise tracking. The RPCs are placed inside the aluminum cylinders. Both ADC and TDC information are read out.
Fig. 5. (a) The open gap design of the UIUC RPC prototype uses Teflon spacers to keep the gap width fixed and Teflon screws and nuts to hold the gap together. This allows for rapid prototyping. (b) A PCB with 1.0 cm wide strips used in the position resolution study.
muon position in the RPC was calculated in three different ways: (i) Finding the geometrical center of the cluster (cluster center method).
ARTICLE IN PRESS B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
769
Fig. 6. Example of position resolution plot from the UIUC teststand for gaps with 1.0 and 0.3 cm strips.
Fig. 7. Position resolution versus applied high voltage for three different strip widths (1.0, 0.6, and 0.3 cm).
(ii) Finding the position of the strip with the maximum charge (ADC Max method). (iii) Finding the mean of Gaussian fit to the charge distribution on the strips (Gaussian method).
The RPC position is then compared to the drift chamber position to calculate the position resolution. Fig. 6 shows an example position resolution plot for 0.3 and 1.0 cm strip widths. Fig. 7 shows the Gaussian position resolution as a function of applied high voltage. Note that for the 0.3 cm strips the RPC position resolution approaches that of the drift chambers. Additionally, the resolution may be limited by slight geometrical alignment issues. In addition to the position resolution studies, an SF6 scan and a gap-nonuniformity test were performed; however, these results will not be shown here.
Fig. 8. Strip layout used for rate capability study. The cylindrical sources are placed at one end of the RPC; the long side of the source runs parallel with the strips.
Finally, a rate capability study is underway in which two 6 cm cylindrical 0.5 mCi 55Fe sources are placed at the edges of the gas gaps. The strip layout shown in Fig. 8 is used. The background photon rate drops off (linearly near the source) in intensity as the strip distance from the source increases; in this way we are capable of measuring the efficiency as a function of multiple rates without varying the source intensity. Fig. 9 shows an efficiency radiograph of the irradiated gap. The rate drops from approximately 10 kHz=cm2 at the strip closest to the source (at the bottom of the efficiency radiograph) to 100 Hz=cm2 near the center of the chamber and then to 10 Hz=cm2 at the last strip (approximately 20 cm away from the source). The efficiency shows a significant dependence on the rate, as established in many previous results.
ARTICLE IN PRESS 770
B. Meredith / Nuclear Instruments and Methods in Physics Research A 602 (2009) 766–770
R&D has been performed in the PHENIX collaboration to both acquire experience with the technology and studying PHENIX specific performance parameters. References [1] B. Hong, et al., in these proceedings. [2] G. Bunce, N. Saito, J. Soffer, W. Vogelsang, Ann Rev. Nucl. Part. Sci. 50 (2000) 525. [3] R. Towell, et al., in these proceedings. [4] S. Park, et al., Nucl. Instr. and Meth. A 550 (2005) 551; S. Park, et al., Nucl Phys. B Proc. Suppl. 158 (2006) 16. [5] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. A 187 (1981) 377. [6] G. Carboni, et al., Nucl. Instr. and Meth. A 533 (2004) 107.
Fig. 9. An example of radiograph of the RPC with the source in place.
7. Summary The RPC forward trigger upgrade will enable PHENIX to measure the quark polarization distributions of the proton. RPC