I present an overview of radiation monitoring for vertex detectors and the abort system for the Fermilab Tevatron. Details on the detectors, inputs, and measurements for the Run 1 time period are provided. Plans for the monitoring during Run 2 are discussed. The measurements imply an approximately even mix of radiation from beam-beam collisions and beam losses

Derwent, P.F.

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F Fermi National Accelerator LaboratoryFERMILAB-Conf-99/194-ECDF and D0Radiation Monitoring for Vertex Detectors at the TevatronP.F. DerwentFor the CDF and D0 CollaborationsFermi National Accelerator LaboratoryP.O. Box 500, Batavia, Illinois 60510July 1999Published Proceedings of Vertex ’99, the 8th International Workshop on Vertex Detectors,Texel, Netherlands, June 20-25, 1999Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of EnergyDisclaimerThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any oftheir employees, makes any warranty, expressed or implied, or assumes any legal liability orresponsibility for the accuracy, completeness, or usefulness of any information, apparatus,product, or process disclosed, or represents that its use would not infringe privately ownedrights. Reference herein to any specic commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the United States Government or any agencythereof. The views and opinions of authors expressed herein do not necessarily state or reectthose of the United States Government or any agency thereof.DistributionApproved for public release; further dissemination unlimited.Copyright NoticationThis manuscript has been authored by Universities Research Association, Inc. under con-tract No. DE-AC02-76CHO3000 with the U.S. Department of Energy. The United StatesGovernment and the publisher, by accepting the article for publication, acknowledges thatthe United States Government retains a nonexclusive, paid-up, irrevocable, worldwide licenseto publish or reproduce the published form of this manuscript, or allow others to do so, forUnited States Government Purposes.Radiation Monitoring for Vertex Detectors atthe TevatronP. F. DerwentRepresenting the CDF and D0 collaborationsFermi National Accelerator Laboratory, Batavia, IL 60510, USAAbstractI present an overview of radiation monitoring for vertex detectors and the abortsystem for the Fermilab Tevatron. Details on the detectors, inputs, and measurementsfor the Run 1 time period are provided. Plans for the monitoring during Run 2 arediscussed. The measurements imply an approximately even mix of radiation frombeam-beam collisions and beam losses.1 IntroductionThe Tevatron has run as a 900 GeV proton-antiproton collider in variousperiods since 1987. With a circumference of 2 km, the revolution periodis 21 sec. During Run 1A, which covered the time period 1992-1993, anaverage store had 1e12 particles and a total of  30 pb 1 delivered luminosity.The CDF Collaboration had a vertex detector (CDF SVX) [1] with singlesided DC coupled sensors connected to radiation soft readout electronics (theSVXD chip). During Run 1B, which covered the time period 1994-1996, theaverage particle intensity doubled to 2e12 and there was a total of  120 pb 1delivered luminosity. The CDF Collaboration had a vertex detector (CDFSVX') [2] with single sided AC coupled sensors connected to radiation hardelectronics (the SVXH chip). In Run 2, scheduled to start in spring 2000, thebeam energy is expected to increase to 1 Tev per beam and the beam intensityto go up to 1e13 total particles. The stated goal is delivery of 2 fb 1. Both theCDF (CDF SVXII) [3] and D0 (D0 SVX) [4] collaborations plan on havingvertex detectors using double sided AC coupled sensors with radiation hardelectronics (the SVX3D chip for CDF, the SVX2 chip for D0).Preprint submitted to Elsevier Preprint 30 June 19992 Beam Loss and Abort at the TevatronThere are several time scales for beam loss at the Tevatron, ranging from100's of hours to 10's of sec. The single beam lifetime (without particle lossfrom collisions) is on the scale of 100's of hours and is not a signicant eectin radiation to vertex detectors. The luminosity lifetime, dominated by theparticle loss through collisions and emittance growth, is on the time scale of10's of hours. Though a slow time scale, the particle ux from collisions is asignicant source of radiation for vertex detectors (of course, it is good in thatthe detectors see particle collisions and detect vertices!). Device failure, suchas problems with power supplies, diagnostic, cryogenic, or vacuum equipment,is a major source of beam loss and has time scales ranging from millisecondsto seconds. Standard manipulations of the beam (e.g., the transition from theinjection lattice to the low  collision lattice) are also a major source of beamloss with time scales in the seconds to 10's of seconds range. Finally, the fastestloss mechanism is a pre-re (unintentional ring) of the abort kickers, whichhas a time scale of 1 revolution (21 sec). Most of the loss mechanisms havegrowth rates long compared to the revolution period.To protect the superconducting magnets in the Tevatron, the abort systemwas designed to do a single turn abort with a fast kicker rise time during adesigned abort gap [5]. The abort system has concentrator modules in 24 lo-cations around the ring, with standard inputs from vacuum equipment, powersupplies, quench protection monitors, and beam loss monitors (described be-low). Each concentrator module serves as a repeater for a 5 MHz permit signal,taking away any of the inputs drops the repeater and the permit signal. Whenthe gap in the permit signal has propagated to the kicker location, there is atmost 1 revolution period to time in the abort gap and re the kicker.The Tev beam loss monitor is a 110 cc argon ionization chamber, operatingclose to standard temperature and pressure. It is connected to a log-integratingpreamplier and calibrated to a direct scaling of rads/second into the monitor.The abort threshold is set at 10 rads/sec during low eld operation, droppingto 1.5 rads/sec at high eld (as the beam energy has gone up by a factor ofroughly 7). The standard loss monitor has an integration time constant of 60msec.Simulations of beam loss around the Tevatron interaction regions predict ap-proximately 5  10 8 rads deposited in the vertex detectors per proton lost.As the original CDF SVX, with rad soft electronics, had an expected lifetimeof around 30 krads (when the noise would double) and there was 1e12 par-ticles circulating in the Tevatron, a total beam loss would lead to 50 krads.Therefore, a monitoring and abort system was designed specically to protectthe CDF SVX.23 CDF Run 1 SystemThe CDF Run 1 radiation monitor and abort system used two dierent mea-surement methods. First, a set of thermoluminescent dosimeters (TLDs), sen-sitive to ionizing charged particles, were used to measure a total radiationdose. Located in an array at  2.5 m from the nominal interaction point, theywere at 3 dierent radii and 6 azimuthal positions. The arrays were changedat quasi-monthly intervals. The second method made use of the beam lossmonitors discussed in the previous section. Located just downstream of theTLDs on the inside and outside of the beam pipe in the plane of the Tevatron,the BLMs gave both an integral measurement and an instantaneous rate.The BLMs were connected to a log-integrating preamplier with an integrationtime constant of 960 msec. The output of the preamplier was fed to a specialpurpose CAMAC module. On this module there was an 8 bit ADC, sampledevery 10 revolution periods (210 sec), digital comparators for both alarm andabort thresholds looking at the ADC output, a 2k deep cyclical buer, and anmicroprocessor to integrate the waveform. The abort thresholds were set at10 rads/sec during injection and machine study periods and 2 rads/sec whenstable beams reached collision.Of the 265 proton-antiproton stores in Run 1A, 6 were aborted by the CDFBLMs only. 2 of these were hardware failures (power to the CAMAC cratefailed, taking away the beam permit), 1 during tuning, 1 followed a correctormagnet power supply trip, 1 from local losses, and 1 by hand. In Run 1B, of the520 stores only 2 were aborted by CDF only (vacuum problems). Aborts weremore frequent during studies periods, when the Tevatron was not at stableoperating points. There were many other stores where the abort thresholdwas reached at CDF but they were consistent with losses elsewhere in thering. As the interaction region low  systems are a high loss point, the BLMsin the interaction region would often be the rst to reach the abort thresholdsthough other regions were approaching the threshold. There were multipleinstances where the `early' abort from the CDF BLMs saved the Tevatronfrom quenching and made re-establishment of beam collisions quicker andeasier.4 CDF Run 1 ExperienceIn gure 1, an hour of recorded radiation and luminosity information is dis-played. The rst 25 minutes cover the time period of beam injection (little orno recorded radiation), the low  transition (from injection lattice to collisionlattice after the energy ramp) as demonstrated by the change in the low 3Typical Run 1A Store0.001.002.003.004.005.006.007.008.000 1 0 2 0 3 0 4 0 5 0 6 0Time (minutes)Luminosity (10^30/cm^2/sec)Low Beta Quad Current (kAmps)Integrated Dose (Rads)Fig. 1. A time history of a typical Run 1A store. The integrated dose (in rads),the instantaneous luminosity (1030/cm2/sec) and low  quad current (kAmps) aredisplayed for a 1 hour time period.quad current, the preparation of beams for collision, and the start of physicsoperation. During this period, there is a sharp step in the integrated radia-tion during the low  transition (about 1.5 rads in 30 seconds), followed bya medium rate as the beams are prepared for collision. The nal 35 minutesshow a steady growth in the integrated radiation associated with the particleux from beam collisions. In gure 2, the cyclical buer (covering 400 msec)is shown for one particular abort. Note the slow growth in the radiation (10'sof msec time scale) and then a faster growth (100's of sec) near the end. Thislevel was the highest instantaneous rate seen during collider Run 1, approxi-mately 16 rads/sec. It occurred during the low  transition with the thresholdset at 10 rads/sec.It was expected that there would be some mix of radiation from beam collisions(expected to fall as r 2, where r is the radial distance from the beam) and beamlosses (expected to fall as r 1). With measurements from the TLD arrays(3 radial points), the best t in Run 1A was r 1:5. Since the CDF SVXDchip performance changed signicantly during the course of Run 1A due toradiation damage, one can also t the change in noise and electronic gainversus radial position. In gure 3, the noise and gain normalized to the initial(0 radiation) values are plotted versus radius. Overlayed are the expected noiseand gain curves, using a radial dependence of r 1:5 and extrapolating from theTLD and BLM longitudinal position. The normalized noise follows the r 1:5curve very well, while the normalized gain actually falls faster. A r 1:5 behavior4Rate (Rads/sec)024681 01 21 41 61 80 5 0 100 150 200 250 300 350 400Time (msec)Rate (Rads/Sec)Fig. 2. A sample abort time history. This abort had the highest instantaneous rate(16 rads/sec) seen during collider Run 1 and occurred during the low  transition,at which time the threshold was 10 rads/sec. Note the jump from 8.5 rads/sec to 16in one sample time (210 sec).is consistent with a 50 - 50 mix of r 2 and r 1, meaning that the luminosityand loss dose are roughly equivalent.In gure 4, the total radiation at the innermost radii ( 3 cm) is shown for Run1A and 1B. Note that once past the startup period, there is a linear dependenceof radiation on the luminosity, again meaning the mix of luminosity and lossdose stays roughly equivalent even during stable running. For Run 1A, theslope was 300 rads/pb 1, for 1B, 550 rads/pb 1. The dierence between thetwo slopes is not understood. Even with the startup problems, it was found tobe very dicult (if not impossible) to dump signicant fractions of the beamin the interaction region.5 Run 2 PlansCDF plans to duplicate the same system used for Run 1: a combination ofbeam loss monitors and TLDs. Due to changes in the detector, the monitorpositions will change. They will move from   2.5 m upstream and down-stream of the interaction point to   4 m. The abort thresholds may beadjusted based on operating experience.5Noise and Gain Change vs Radius0.70.80.911.11.21.31.41.51.61.72 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8RadiusNormalized ScaleNoiseGainExpected NoiseExpected GainFig. 3. The normalized noise and gain change due to radiation damage as a functionof the radial distance from the beam. The lines are the expected change for a r 1:5behavior.CDF Run 1081 62 43 24 04 85 66 47 28 00 1 2 2 4 3 6 4 8 6 0 7 2 8 4 9 6 1 0 8 1 2 0Luminosity (pb-1)Radiation Dose (kRads)Run 1BRun 1AFig. 4. The total extrapolated dose to the inner most layer (at  3 cm) for theRun 1A and 1B periods. Once past startup, the dose is linearly dependent on thedelivered luminosity, meaning the 50 - 50 mix of luminosity dose and loss dose staysroughly equivalent even during stable running.6D0 plans to use both beam loss monitors and large surface Si diodes as ra-diation monitors. There will be 8 BLMs, located just outside the calorimetercryostat, connected to a log integrating preamplier and interfaced to theTevatron abort. As the D0 cryostat is very hermetic, it may be dicult to ex-trapolate from the BLMs (probably more sensitive to losses than luminosity)to the vertex detector. The second set of monitors, large surface Si diodes, willbe mounted directly on the the vertex detector.In the forward region, silicon disks with a wedge geometry will be used [4]. TheSi diodes will be mounted on the same bulkheads and will therefore samplethe same luminosity and loss dose as the detectors, giving information atthe exact location of interest (rather than extrapolating as the CDF monitorsystem requires). The current in the diodes will be integrated over various timescales and stored in cyclic buers, which are available for display. 1 minutelong averages will be archived for long term measurements. The electronicscan be gated to look at single beam crossings, allowing for a direct calibrationof single particle currents. This Si diode system will be very similar to onesuccessfully used previously by the OPAL collaboration [6].6 ConclusionsRadiation monitoring systems useful for both total dose measurements andas inputs to machine aborts have been successfully developed for the vertexdetectors at the Tevatron. They have been shown to be ecient and reliable,with most aborts correlating with recognizable machine problems. The radialdependence of radiation dose has been measured to be r 1:5, an even mixof that expected from beam collisions (r 2) and losses (r 1). It is dicultto dump all the beam in the detector locations, as most loss mechanismshave long time scales compared to the sensitivity of the loss monitors andthe Tevatron abort hardware. Signicant accidents only occurred when theTevatron lattice was perturbed (energy ramp, low  transition, device failure)and not at well understood operating points.7 AcknowledgmentsI wish to thank my colleagues on the CDF experiment who helped designand implement the radiation monitor system, as well as the members of theFermilab Accelerator Division who helped in putting it together and makingit work. I would also like to thank Sijbrand de Jong for information on the D0monitoring and abort system.7In conclusion, I would like to thank the organizers of the Vertex99 conferencefor the invitation. It was a splendid conference.References[1] D. Amidei et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 350, 73 (1994).[2] P. Azzi et al., Fermilab Report No. FERMILAB-CONF-94/205-E, 1994(unpublished).[3] See the contribution from Alan Sill in these proceedings.[4] See the contribution from Frank Lehner in these proceedings.[5] M. Harrison, Fermilab Report No. FERMILAB-UPC-153, 1981 (unpublished).[6] S. de Jong, Nucl. Instrum. Methods Phys. Res., Sect. A, 418, 22 (1998). Fromthe proceedings of Vertex97.8

Radiation monitoring for vertex detectors at the Tevatron

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