- Email: [email protected]
The PEP-II collider and commissioning detectors
~ l I I I I I1'__,1"i "J-'i*&'~[Ikl PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 55A (1997) 310-314
The PEP-II Collider and Commissioning Detectors Thomas S. Mattison ~* ~Stanford Linear Accelerator Center Stanford University, Stanford CA 94309 PEP-II will collide 9 GeV e- with 3.1 GeV e + at high luminosity on the T(4S) resonance to produce B-meson pairs for study of CP-violation. Progress on its design and construction at SLAC is reported. Since PEP-II has a complicated interaction region and high beam currents, and will be completed before the new BABAR detector is available, a set of smaller detectors will be used to measure the beam-induced backgrounds during PEP-II commissioning.
1. T h e P E P - I I C o l l i d e r The near-future of the SLAC e+e - program focuses on study of CP-violation in the B-meson system, using the PEP-II collider and the BABAa detector. The measurement requires events with one neutral B-meson decaying to a CP-eigenstate, and the other decay tagging it as B or B. In decays of the T(4S) resonance, the B and B are produced in a coherent state such that the CPasymmetry cancels between events where the tagging B decays first and those where it decays second. The T(4S) decays produce B's with such low center-of-mass velocity that the decay time cannot be determined by flight distance. The solution is to collide beams of unequal energies to produce the T(4S) with a significant velocity, which will be shared by the decay products, /3 and B. The separation between the decay vertices along the boost direction then determines the difference between the decay times. The measurement requires about 10 9 B-decays, which requires 10 times higher luminosity than any existing e+e - eollider. Luminosity depends on bunch charge, bunch size, and collision frequency. For stable operation, the focusing effect of one beam on the other during collision must be much less than the beam divergence, which limits bunch charge increase and bunch size reduction. The divergence is limited by the physical and dynamical aperture of focusing magnets, *Work supported by U.S. Department of Energy contract DE-AC03-76FS00098 0920-5632/97/$17.00 c~:, 1997 Elsevier Science B,V All rights reserved. PIl: S0920-5632(97)00192-8
their distance from the interaction region (IR), depth of focus, and bunch length. Once these factors have been exhausted, further luminosity improvements require increasing the collision frequency, which requires higher beam current. The engineering implications are the need for more radio frequency (RF) power, thermal problems from synchrotron radiation and beam-induced fields, and instability from beam-induced fields. Energy asymmetry requires two separate rings of magnets (which is also useful for avoiding additional beam-beam collisions), and a complicated IR to focus the unequal energy beams and to combine and separate them. Crossing the beams at an angle couples transverse and longitudinal beam motions, which caused serious problems in the original DORIS ring at DESY. The KEKB collider being built in Japan to measure CPviolation has a small crossing angle, which simulations predict will be stable due to a machine lattice with unusual longitudinal beam modes. PEP-II has zero crossing angle like all existing e+e - machines, and separates the beams with DC magnets. The magnets produce copious synchrotron radiation, but most of it is aimed away from the detector, and the rest can be blocked by careful design. The PEP-II collider is being built at the SLAC site by a collaboration of SLAC, Lawrence Berkeley National Laboratory (LBNL), and Lawrence Livermore National Laboratory (LLNL) [1]. It consists of rings for 3.1 e + and 9.0 GeV e - in the existing PEP-I tunnel, and uses the SLAC
TS. Mattison/Nuclear Physics B (Proc. SuppL) 55A (1997) 310 314
Figure 1. PEP-II Collider at SLAC.
Linac as upgraded for the SLC as its injector, as shown in Figure 1. The design luminosity is 3 x 1033 cm-2s -1, which is achieved by colliding 1658 bunches, with a current of 2 Amperes of e + and 1 Ampere of e - . All of the PEP-I magnets have been removed, refurbished for use in the High Energy Ring (HER), and the dipoles have already been reinstalled in the tunnel. The HER vacuum chamber is copper to minimize outgassing due to the high synchrotron radiation power levels, and is smooth to minimize higher order mode (HOM) heating and beam impedance. The distributed ion pumps for the HER arcs were produced at LLNL, and have been electron-beam welded into copper vacuum chambers at SLAC, which are now being installed in the tunnel. Quadrupolesextupole-vacuum-chamber units for the HER are also being installed. The first full sector of the HER is to be pumped down in September 1996. The Low Energy Ring (LER) uses short, strong bend magnets to maximize the synchrotron damping rate. The LER magnets have been designed by LBNL, and are under construction in collaboration with the Beijing Institute for High Energy Physics. The LER vacuum chamber has discrete copper photon stops with local pumping between the bend magnets, similar to the
311
LBNL Advanced Light Source (ALS). The aluminum extrusions between the photon stops have antechambers along the path of the synchrotron radiation fans. The extrusions are being delivered to LBNL and machined, with welding to be done at Argonne National Laboratory. The PEP-II RF system is based on copper cavities (since most of the power goes into the beam rather than the walls, there is no economic advantage to superconducting cavities). The cavities are single-cell and nearly spherical to minimize the number of HOMs, and the remaining modes are damped by lossy tiles at the ends of special wavegnides that propagate HOMs but not the accelerating mode. A prototype has been tested to full power, and the cavities are under construction at LLNL. The first production 1.2 MW 486 Mttz klystron from Phillips has been delivered to SLAC and tested to full power. To further stabilize the beam, PEP-II has bunch-by-bunch feedback systems for longitudinal and transverse modes. The phase and transverse position errors of each bunch are measured and corrected continuously. The prototypes for the PEP-II digitalsignal-processors and correction kickers are being used already as the production feedback system for the Advanced Light Source (ALS) at LBNL. PEP-II will benefit from the low-emittance damped beams and high e + currents of the SLC. The beams are extracted from the Linac at the 3.1 and 9.0 GeV points, and are transported to the beam switchyard in new bypass lines on the ceiling of the Linac tunnel. These lines are already installed, and the e - beam has been transported to the end of its bypass line. There is a single Interaction Region for a detector. The beams collide with zero crossing angle, and are separated by the B1 dipole magnets starting 20 cm from the focal point. The primary SR masks are built into the B1 apertures. The Bls have conical tips to allow the detector acceptance to extend to within 350 mrad of the beam axis. The Q1 final quadrupole starts at 90 cm and focuses both beams. Since B1 and Q1 are inside the 15 kG detector solenoid field, they are Samarium-Cobalt permanent magnets. The central beampipe is 1.2 m m thick Beryllium with water cooling. The beampipe, masks, Bls and Qls
312
??S. Mattison/Nuclear Physics B (Ptvc. Suppl.) 55A (1997) 310 314
are housed in a 21.5 cm radius Support Tube. The central part of the Support Tube structure is 1% X0 thick carbon fiber reinforced epoxy. Outside the detector on each side is one quadrupole that focuses the LER beam with a slot for the HER beam, and a doublet that focuses the HER beam with slots for the LER beam. The IR masking and orbit has been carefully designed to minimize detector backgrounds from the synchrotron radiation produced by magnetic separation [2]. All large procurements for PEP-II have already been placed. Over the first two years of the project, $80 million has been spent, and the full $52 million for Fiscal Year 1996 and $45 million for FY1997 are expected. The HER will be commissioned starting in April 1997, with LER commissioning starting in July 1998. The BABAR Detector [3] at the single IR features a silicon vertex detector, a lowmass drift-chamber in a 15-kG superconducting solenoid field, a Cesium-Iodide crystal calorimeter, a DIRC (Detector of Internally Reflected Cherenkov radiation) for particle identification, and muon and neutral hadron identification in the fine-grain instrumented iron flux return. The BABAR Collaboration has nearly 500 physicists, roughly half non-US, from 10 nations and 76 institutions. The US funding for BABAR is $64 million (including salaries), and foreign funding is $22 million (excluding salaries). BABAR construction will be completed in December 1998, followd by a cosmic-ray test in early 1999, and physics with PEP-1I in April 1999.
2. Machine-Induced Backgrounds Synchrotron radiation (SR) is generated when the beam passes through bend magnets or by the tail of the beam in focusing magnets. Lost particles are produced by beam collisions with residual gas. Bremsstrahlung produces energetic photons, and off-energy beam particles. Coulomb scattering produces on-energy particles with large angles. The scattered particles can travel long distances in the machine, then generate electromagnetic (EM) showers in the beampipe, and debris from these showers can enter the detector. A few scattered particles hit the inner layer of the
BABAR silicon detector directly. The design of PEP-II employed computer programs to calculate synchrotron radiation from the beam core and tails in the IR magnets and SR absorption in masks, and to calculate beam-gas scattering, trace scattered rays, and production of shower debris [4]. The IR masks, magnets, and orbits were designed to eliminate direct SR on the beam pipe, and to minimize scattered SR. The machine apertures and collimators were designed to minimize beam-gas particles hitting the beampipe near the IR. The mass of the B1 and Q1 permanent magnets absorbs most of the shower debris of those particles that do hit. Initially the EGS4 Monte Carlo was used to calculate synchrotron radiation masking, scattering, and EM showers from lost particles. The PEP-II beamline components have been incorporated into the GEANT Monte Carlo simulation of BABAR, and electroproduction of hadrons has been added to GEANT for calculating trigger rates [5]. Recently, GEANT has been further extended to calculate synchrotron radiation production and masking in the full BABAR geometry. The accelerator ray-tracing program TURTLE was extended to include beam-gas scattering processes. A detailed calculation of outgassing due to SR-induced heating, beampipe vacuum conductance, and distributed pumping is used to calculate the pressure profile.
3. Background Detectors New accelerators often have beam-induced background problems, and PEP-II has high beam currents and an exotic IR. Parts of PEP-II are to be commissioned in Spring 1997, and the rest in Summer 1998, but BABArt is not scheduled to he ready for beam until Spring 1999. The competing BELLE detector in Japan will be ready at the same time as BABAR, but the KEK-B collider will not be available for early commissioning. BABAR and PEP-II physicists have formed a BackgroundCommissioning group to operate simple detectors for measuring backgrounds at the IR as soon as there is any beam in PEP-II. The emphasis is on finding and fixing accelerator problems using real-time information sent to
TS. Mattison/Nuclear Physics B (Proc. Suppl.) 55A (1997) 310-314
the PEP-II control room by continuously operating detectors. The background measurements will be compared quantitatively to Monte Carlo simulations, which will be recalculated for each IR configuration. This allows useful measurements to begin before the final IR is installed. Some detectors are optimized for the expected backgrounds (keV X-rays from synchrotron radiation and MeV g a m m a rays from lost-particle shower debris) while others are prototypes or analogs of BABAR components. There are two phases of PEP-II commissioning. The high-energy ring will be operated in 1997 with a simplified IR. In 1998, the low-energy ring will be completed, and the final Support Tube with the B1 and Q1 permanent magnets and small Beryllium beampipe will be used. Background detectors must then either fit completely inside the Support Tube in the space between the conical B1 magnets, or else be completely outside the Support Tube. One background detector to be located outside the Support Tube is a water Cherenkov detector. It consists of a water tank roughly 30 cm on a side with a quartz window and 36 phototubes on one face, and will be accompanied by a scintillator hodoscope. It will check for beaminduced backgrounds that could generate light inside the DIRC Standoff-Box, a tank containing several tonnes of water and 10000 phototubes that is outside the BABAR detector magnet. This mini-Standoff-Box, and a scintillator hodoscope, is being constructed by the University of Cincinnati group. Also outside the Support Tube will be two prototype modules of the BABAR CsI barrel calorimeter. Each module (about 75 kg) has 21 crystals with photodiode readout. In addition to measuring the total number of MeV photons, which dominate the radiation dose, the prototype modules are large enough to fully contain showers and thus to measure the spectrum of energetic photons (and tracks) that will determine the trigger rate of BABAIt. The spectrum of synchrotron radiation X-rays will be measured using Cadmium-Zinc-Telluride crystal detectors. Two crystals of 3 x 3 × 2 m m have been purchased from eV Products [6] and
313
are being tested by the LBNL and Colorado State University groups. They have about 5% energy resolution and useful efficiency for photons from 10 to 100 keV, and will be mounted in steerable lead collimators to provide directionality. The French groups at LAPP (Annecy) and Saclay are constructing a ring of 12 CsI crystals of 60 x 60 × 150 mm with photomultiplier readout to measure the rate of MeV photons and tracks. While the detector will be available for 1997 commissioning, it will be most interesting in 1998 when it will close around the Support Tube and be mounted on a remotely moving trolley. This will give the definitive measurement of background as a function of ¢ and Z, and will measure the effectiveness of the self-shielding from the magnets inside the Support Tube. The straw-tube chamber from the Crystal Ball detector is being revived for PEP-II background measurements. The chamber has 800 straws in 4 double layers, with pulse height readout at both ends for Z measurement by charge division. The chamber is built as two half-cylinders that are assembled around the beam pipe, and layers 1-2 separate from layers 3-4. All four layers will be used in 1997 commissioning, but only the outer two layers in 1998, because the inner two layers are too long to fit between the B1 magnets inside the Support Tube. The French group at Orsay, along with LBNL and Cincinnati, are constructing a small Time Projection Chamber to fit inside the straw-tube chamber for 1998 commissioning. It will have 6 sectors with 16 rows of cathode pads divided into 192 total pad channels, and instrumented with fiash-ADCs. The chamber covers from 40 to 100 mm in radius and 180 m m in Z. About 90 o of ¢ is missing to allow the chamber to be mounted on the beampipe. The many institutions of the BABAR silicon vertex tracker group are providing a complete silicon strip module with final on-detector electronics and a full readout chain for PEP-II commissioning. The module will be installed in the slot of the mini-TPC, as close to the beampipe as possible. In addition to measuring the radiation levels, it will check for beam-induced RF-pickup problems with the silicon detector readout, which
314
E£ Mattison/Nuclear Physics B (Proc. Suppl.) 55A (1997) 310 314
4. S u m m a r y Beampipe
Straw Chamber
Mini-TPC with slot for
The PEP-II collider and BABAR detector are being built at SLAC to study CP-violation in the B-meson system. The measurement requires high luminosity and colliding e + and e - beams with unequal energy, which requires high currents and a complicated interaction region. PEP-II construction is proceeding well, with commissioning of parts of the machine to begin in 1997, and commissioning of the rest of the machine in 1998. There have been extensive calculations of machine-induced backgrounds. A variety of small detectors will be used to measure the backgrounds before BABAP, rolls online for physics in early 1999. REFERENCES
Figure 2. Commissioning Detectors near Interaction Point. Half of the straw chamber has been removed to make the other detectors visible. The Mini-TPC is 18 cm long and 20 cm in diameter. Not visible are PIN diodes next to the beampipe, or other detectors far from the beampipe.
is completely inaccessible in the final BABAR detector configuration. Figure 2 is a rendering of the silicon strip, mini-TPC, and outer straw-tube detectors next to the PEP-II beampipe and B1 magnets. The Stanford University group will operate silicon photodiodes as radiation detectors at many positions next to the beampipe in both the 1997 and 1998 configurations. The Hamamatsu diodes are 10 mm square or 3.4 × 30 m m and give a current proportional to radiation level. Pairs of diodes will be used, with one bare diode and one shielded by 1 mm of lead to discriminate synchrotron radiation from lost-particle shower debris. Similar diodes will be used in the BABArt detector as the trigger to dump the PEP-II beam a n d / o r inhibit injection if the radiation level becomes high.
1. J. Seeman, PEP-II Status and Plans, in: L. Genari and R. Siemann (eds.), Proceedings of the 1995 Particle Accelerator Conference, IEEE, Piscataway, NJ, 1996. 2. M. Sullivan, et aL, Interaction Region Design at the PEP-II B-Factory, SLAC-PUB7206 (1996). Also in: Ch. Petit-Jean-Genaz (ed.), 5th European Particle Accelerator Conference (EPAC 96), to be published, 1996. 3. D. MacFarlane, The BABAR Detector, these Proceedings, 1996. 4. PEP-II: An Asymmetric B-Factory (Conceptual Design Report), SLAC, Stanford, 1993. 5. BABAR Collaboration (D. Boutigny, et al., BABAR Technical Design Report, SLAC, Stanford, 1995. 6. eV Products Inc., 375 Saxonburg Blvd., Saxonburg, PA 16056.
Nuclear Physics B (Proc. Suppl.) 55A (1997) 310-314
The PEP-II Collider and Commissioning Detectors Thomas S. Mattison ~* ~Stanford Linear Accelerator Center Stanford University, Stanford CA 94309 PEP-II will collide 9 GeV e- with 3.1 GeV e + at high luminosity on the T(4S) resonance to produce B-meson pairs for study of CP-violation. Progress on its design and construction at SLAC is reported. Since PEP-II has a complicated interaction region and high beam currents, and will be completed before the new BABAR detector is available, a set of smaller detectors will be used to measure the beam-induced backgrounds during PEP-II commissioning.
1. T h e P E P - I I C o l l i d e r The near-future of the SLAC e+e - program focuses on study of CP-violation in the B-meson system, using the PEP-II collider and the BABAa detector. The measurement requires events with one neutral B-meson decaying to a CP-eigenstate, and the other decay tagging it as B or B. In decays of the T(4S) resonance, the B and B are produced in a coherent state such that the CPasymmetry cancels between events where the tagging B decays first and those where it decays second. The T(4S) decays produce B's with such low center-of-mass velocity that the decay time cannot be determined by flight distance. The solution is to collide beams of unequal energies to produce the T(4S) with a significant velocity, which will be shared by the decay products, /3 and B. The separation between the decay vertices along the boost direction then determines the difference between the decay times. The measurement requires about 10 9 B-decays, which requires 10 times higher luminosity than any existing e+e - eollider. Luminosity depends on bunch charge, bunch size, and collision frequency. For stable operation, the focusing effect of one beam on the other during collision must be much less than the beam divergence, which limits bunch charge increase and bunch size reduction. The divergence is limited by the physical and dynamical aperture of focusing magnets, *Work supported by U.S. Department of Energy contract DE-AC03-76FS00098 0920-5632/97/$17.00 c~:, 1997 Elsevier Science B,V All rights reserved. PIl: S0920-5632(97)00192-8
their distance from the interaction region (IR), depth of focus, and bunch length. Once these factors have been exhausted, further luminosity improvements require increasing the collision frequency, which requires higher beam current. The engineering implications are the need for more radio frequency (RF) power, thermal problems from synchrotron radiation and beam-induced fields, and instability from beam-induced fields. Energy asymmetry requires two separate rings of magnets (which is also useful for avoiding additional beam-beam collisions), and a complicated IR to focus the unequal energy beams and to combine and separate them. Crossing the beams at an angle couples transverse and longitudinal beam motions, which caused serious problems in the original DORIS ring at DESY. The KEKB collider being built in Japan to measure CPviolation has a small crossing angle, which simulations predict will be stable due to a machine lattice with unusual longitudinal beam modes. PEP-II has zero crossing angle like all existing e+e - machines, and separates the beams with DC magnets. The magnets produce copious synchrotron radiation, but most of it is aimed away from the detector, and the rest can be blocked by careful design. The PEP-II collider is being built at the SLAC site by a collaboration of SLAC, Lawrence Berkeley National Laboratory (LBNL), and Lawrence Livermore National Laboratory (LLNL) [1]. It consists of rings for 3.1 e + and 9.0 GeV e - in the existing PEP-I tunnel, and uses the SLAC
TS. Mattison/Nuclear Physics B (Proc. SuppL) 55A (1997) 310 314
Figure 1. PEP-II Collider at SLAC.
Linac as upgraded for the SLC as its injector, as shown in Figure 1. The design luminosity is 3 x 1033 cm-2s -1, which is achieved by colliding 1658 bunches, with a current of 2 Amperes of e + and 1 Ampere of e - . All of the PEP-I magnets have been removed, refurbished for use in the High Energy Ring (HER), and the dipoles have already been reinstalled in the tunnel. The HER vacuum chamber is copper to minimize outgassing due to the high synchrotron radiation power levels, and is smooth to minimize higher order mode (HOM) heating and beam impedance. The distributed ion pumps for the HER arcs were produced at LLNL, and have been electron-beam welded into copper vacuum chambers at SLAC, which are now being installed in the tunnel. Quadrupolesextupole-vacuum-chamber units for the HER are also being installed. The first full sector of the HER is to be pumped down in September 1996. The Low Energy Ring (LER) uses short, strong bend magnets to maximize the synchrotron damping rate. The LER magnets have been designed by LBNL, and are under construction in collaboration with the Beijing Institute for High Energy Physics. The LER vacuum chamber has discrete copper photon stops with local pumping between the bend magnets, similar to the
311
LBNL Advanced Light Source (ALS). The aluminum extrusions between the photon stops have antechambers along the path of the synchrotron radiation fans. The extrusions are being delivered to LBNL and machined, with welding to be done at Argonne National Laboratory. The PEP-II RF system is based on copper cavities (since most of the power goes into the beam rather than the walls, there is no economic advantage to superconducting cavities). The cavities are single-cell and nearly spherical to minimize the number of HOMs, and the remaining modes are damped by lossy tiles at the ends of special wavegnides that propagate HOMs but not the accelerating mode. A prototype has been tested to full power, and the cavities are under construction at LLNL. The first production 1.2 MW 486 Mttz klystron from Phillips has been delivered to SLAC and tested to full power. To further stabilize the beam, PEP-II has bunch-by-bunch feedback systems for longitudinal and transverse modes. The phase and transverse position errors of each bunch are measured and corrected continuously. The prototypes for the PEP-II digitalsignal-processors and correction kickers are being used already as the production feedback system for the Advanced Light Source (ALS) at LBNL. PEP-II will benefit from the low-emittance damped beams and high e + currents of the SLC. The beams are extracted from the Linac at the 3.1 and 9.0 GeV points, and are transported to the beam switchyard in new bypass lines on the ceiling of the Linac tunnel. These lines are already installed, and the e - beam has been transported to the end of its bypass line. There is a single Interaction Region for a detector. The beams collide with zero crossing angle, and are separated by the B1 dipole magnets starting 20 cm from the focal point. The primary SR masks are built into the B1 apertures. The Bls have conical tips to allow the detector acceptance to extend to within 350 mrad of the beam axis. The Q1 final quadrupole starts at 90 cm and focuses both beams. Since B1 and Q1 are inside the 15 kG detector solenoid field, they are Samarium-Cobalt permanent magnets. The central beampipe is 1.2 m m thick Beryllium with water cooling. The beampipe, masks, Bls and Qls
312
??S. Mattison/Nuclear Physics B (Ptvc. Suppl.) 55A (1997) 310 314
are housed in a 21.5 cm radius Support Tube. The central part of the Support Tube structure is 1% X0 thick carbon fiber reinforced epoxy. Outside the detector on each side is one quadrupole that focuses the LER beam with a slot for the HER beam, and a doublet that focuses the HER beam with slots for the LER beam. The IR masking and orbit has been carefully designed to minimize detector backgrounds from the synchrotron radiation produced by magnetic separation [2]. All large procurements for PEP-II have already been placed. Over the first two years of the project, $80 million has been spent, and the full $52 million for Fiscal Year 1996 and $45 million for FY1997 are expected. The HER will be commissioned starting in April 1997, with LER commissioning starting in July 1998. The BABAR Detector [3] at the single IR features a silicon vertex detector, a lowmass drift-chamber in a 15-kG superconducting solenoid field, a Cesium-Iodide crystal calorimeter, a DIRC (Detector of Internally Reflected Cherenkov radiation) for particle identification, and muon and neutral hadron identification in the fine-grain instrumented iron flux return. The BABAR Collaboration has nearly 500 physicists, roughly half non-US, from 10 nations and 76 institutions. The US funding for BABAR is $64 million (including salaries), and foreign funding is $22 million (excluding salaries). BABAR construction will be completed in December 1998, followd by a cosmic-ray test in early 1999, and physics with PEP-1I in April 1999.
2. Machine-Induced Backgrounds Synchrotron radiation (SR) is generated when the beam passes through bend magnets or by the tail of the beam in focusing magnets. Lost particles are produced by beam collisions with residual gas. Bremsstrahlung produces energetic photons, and off-energy beam particles. Coulomb scattering produces on-energy particles with large angles. The scattered particles can travel long distances in the machine, then generate electromagnetic (EM) showers in the beampipe, and debris from these showers can enter the detector. A few scattered particles hit the inner layer of the
BABAR silicon detector directly. The design of PEP-II employed computer programs to calculate synchrotron radiation from the beam core and tails in the IR magnets and SR absorption in masks, and to calculate beam-gas scattering, trace scattered rays, and production of shower debris [4]. The IR masks, magnets, and orbits were designed to eliminate direct SR on the beam pipe, and to minimize scattered SR. The machine apertures and collimators were designed to minimize beam-gas particles hitting the beampipe near the IR. The mass of the B1 and Q1 permanent magnets absorbs most of the shower debris of those particles that do hit. Initially the EGS4 Monte Carlo was used to calculate synchrotron radiation masking, scattering, and EM showers from lost particles. The PEP-II beamline components have been incorporated into the GEANT Monte Carlo simulation of BABAR, and electroproduction of hadrons has been added to GEANT for calculating trigger rates [5]. Recently, GEANT has been further extended to calculate synchrotron radiation production and masking in the full BABAR geometry. The accelerator ray-tracing program TURTLE was extended to include beam-gas scattering processes. A detailed calculation of outgassing due to SR-induced heating, beampipe vacuum conductance, and distributed pumping is used to calculate the pressure profile.
3. Background Detectors New accelerators often have beam-induced background problems, and PEP-II has high beam currents and an exotic IR. Parts of PEP-II are to be commissioned in Spring 1997, and the rest in Summer 1998, but BABArt is not scheduled to he ready for beam until Spring 1999. The competing BELLE detector in Japan will be ready at the same time as BABAR, but the KEK-B collider will not be available for early commissioning. BABAR and PEP-II physicists have formed a BackgroundCommissioning group to operate simple detectors for measuring backgrounds at the IR as soon as there is any beam in PEP-II. The emphasis is on finding and fixing accelerator problems using real-time information sent to
TS. Mattison/Nuclear Physics B (Proc. Suppl.) 55A (1997) 310-314
the PEP-II control room by continuously operating detectors. The background measurements will be compared quantitatively to Monte Carlo simulations, which will be recalculated for each IR configuration. This allows useful measurements to begin before the final IR is installed. Some detectors are optimized for the expected backgrounds (keV X-rays from synchrotron radiation and MeV g a m m a rays from lost-particle shower debris) while others are prototypes or analogs of BABAR components. There are two phases of PEP-II commissioning. The high-energy ring will be operated in 1997 with a simplified IR. In 1998, the low-energy ring will be completed, and the final Support Tube with the B1 and Q1 permanent magnets and small Beryllium beampipe will be used. Background detectors must then either fit completely inside the Support Tube in the space between the conical B1 magnets, or else be completely outside the Support Tube. One background detector to be located outside the Support Tube is a water Cherenkov detector. It consists of a water tank roughly 30 cm on a side with a quartz window and 36 phototubes on one face, and will be accompanied by a scintillator hodoscope. It will check for beaminduced backgrounds that could generate light inside the DIRC Standoff-Box, a tank containing several tonnes of water and 10000 phototubes that is outside the BABAR detector magnet. This mini-Standoff-Box, and a scintillator hodoscope, is being constructed by the University of Cincinnati group. Also outside the Support Tube will be two prototype modules of the BABAR CsI barrel calorimeter. Each module (about 75 kg) has 21 crystals with photodiode readout. In addition to measuring the total number of MeV photons, which dominate the radiation dose, the prototype modules are large enough to fully contain showers and thus to measure the spectrum of energetic photons (and tracks) that will determine the trigger rate of BABAIt. The spectrum of synchrotron radiation X-rays will be measured using Cadmium-Zinc-Telluride crystal detectors. Two crystals of 3 x 3 × 2 m m have been purchased from eV Products [6] and
313
are being tested by the LBNL and Colorado State University groups. They have about 5% energy resolution and useful efficiency for photons from 10 to 100 keV, and will be mounted in steerable lead collimators to provide directionality. The French groups at LAPP (Annecy) and Saclay are constructing a ring of 12 CsI crystals of 60 x 60 × 150 mm with photomultiplier readout to measure the rate of MeV photons and tracks. While the detector will be available for 1997 commissioning, it will be most interesting in 1998 when it will close around the Support Tube and be mounted on a remotely moving trolley. This will give the definitive measurement of background as a function of ¢ and Z, and will measure the effectiveness of the self-shielding from the magnets inside the Support Tube. The straw-tube chamber from the Crystal Ball detector is being revived for PEP-II background measurements. The chamber has 800 straws in 4 double layers, with pulse height readout at both ends for Z measurement by charge division. The chamber is built as two half-cylinders that are assembled around the beam pipe, and layers 1-2 separate from layers 3-4. All four layers will be used in 1997 commissioning, but only the outer two layers in 1998, because the inner two layers are too long to fit between the B1 magnets inside the Support Tube. The French group at Orsay, along with LBNL and Cincinnati, are constructing a small Time Projection Chamber to fit inside the straw-tube chamber for 1998 commissioning. It will have 6 sectors with 16 rows of cathode pads divided into 192 total pad channels, and instrumented with fiash-ADCs. The chamber covers from 40 to 100 mm in radius and 180 m m in Z. About 90 o of ¢ is missing to allow the chamber to be mounted on the beampipe. The many institutions of the BABAR silicon vertex tracker group are providing a complete silicon strip module with final on-detector electronics and a full readout chain for PEP-II commissioning. The module will be installed in the slot of the mini-TPC, as close to the beampipe as possible. In addition to measuring the radiation levels, it will check for beam-induced RF-pickup problems with the silicon detector readout, which
314
E£ Mattison/Nuclear Physics B (Proc. Suppl.) 55A (1997) 310 314
4. S u m m a r y Beampipe
Straw Chamber
Mini-TPC with slot for
The PEP-II collider and BABAR detector are being built at SLAC to study CP-violation in the B-meson system. The measurement requires high luminosity and colliding e + and e - beams with unequal energy, which requires high currents and a complicated interaction region. PEP-II construction is proceeding well, with commissioning of parts of the machine to begin in 1997, and commissioning of the rest of the machine in 1998. There have been extensive calculations of machine-induced backgrounds. A variety of small detectors will be used to measure the backgrounds before BABAP, rolls online for physics in early 1999. REFERENCES
Figure 2. Commissioning Detectors near Interaction Point. Half of the straw chamber has been removed to make the other detectors visible. The Mini-TPC is 18 cm long and 20 cm in diameter. Not visible are PIN diodes next to the beampipe, or other detectors far from the beampipe.
is completely inaccessible in the final BABAR detector configuration. Figure 2 is a rendering of the silicon strip, mini-TPC, and outer straw-tube detectors next to the PEP-II beampipe and B1 magnets. The Stanford University group will operate silicon photodiodes as radiation detectors at many positions next to the beampipe in both the 1997 and 1998 configurations. The Hamamatsu diodes are 10 mm square or 3.4 × 30 m m and give a current proportional to radiation level. Pairs of diodes will be used, with one bare diode and one shielded by 1 mm of lead to discriminate synchrotron radiation from lost-particle shower debris. Similar diodes will be used in the BABArt detector as the trigger to dump the PEP-II beam a n d / o r inhibit injection if the radiation level becomes high.
1. J. Seeman, PEP-II Status and Plans, in: L. Genari and R. Siemann (eds.), Proceedings of the 1995 Particle Accelerator Conference, IEEE, Piscataway, NJ, 1996. 2. M. Sullivan, et aL, Interaction Region Design at the PEP-II B-Factory, SLAC-PUB7206 (1996). Also in: Ch. Petit-Jean-Genaz (ed.), 5th European Particle Accelerator Conference (EPAC 96), to be published, 1996. 3. D. MacFarlane, The BABAR Detector, these Proceedings, 1996. 4. PEP-II: An Asymmetric B-Factory (Conceptual Design Report), SLAC, Stanford, 1993. 5. BABAR Collaboration (D. Boutigny, et al., BABAR Technical Design Report, SLAC, Stanford, 1995. 6. eV Products Inc., 375 Saxonburg Blvd., Saxonburg, PA 16056.