ISBN 978-3-95450-132-8 - http://accelconf.web.cern.ch/AccelConf/IPAC2014/papers/thpme090.pdfInternational audienceSuper luminous flavor factories, as SuperKEKB in Japan, aim to achieve very high luminosity thanks to a newly employed concept, the nano-beam scheme, where ultra-low emittance beams collide at very large crossing angle. Luminosity optimisation and dynamic imperfections require fast luminosity measurements. The aimed precision, 10−3 in 10 ms, can be achieved thanks to the very large cross-section of the radiative Bhabha process at zero-photon scattering angle. As a result of huge particle fluxes, diamond sensors are chosen to be placed just outside the beam-pipe, downstream of the interaction point, at locations with event rates consistent with the aimed precision and small enough contamination by backgrounds from single-beam particle losses. We will present the results concerning the investigation of the optimal positioning of our diamond sensors, taking into account the rate of Bhabha particles as well as their interactions with the beam pipe material

Bambade, P.

El Khechen, D.

Jehanno, D.

Rimbault, C.

HAL-IN2P3

FAST LUMINOSITY MONITORING USING DIAMOND SENSORS FORTHE SUPER FLAVOUR FACTORY SUPERKEKBD. El Khechen∗, C. Rimbault, P. Bambade, D. Jehanno, LAL, Orsay, FranceAbstractSuper luminous flavor factories, as SuperKEKB inJapan, aim to achieve very high luminosity thanks to anewly employed concept, the nano-beam scheme, whereultra-low emittance beams collide at very large crossingangle. Luminosity optimisation and dynamic imperfec-tions require fast luminosity measurements. The aimedprecision, 10−3 in 10 ms, can be achieved thanks to thevery large cross-section of the radiative Bhabha process atzero-photon scattering angle. As a result of huge particlefluxes, diamond sensors are chosen to be placed just out-side the beam-pipe, downstream of the interaction point, atlocations with event rates consistent with the aimed pre-cision and small enough contamination by backgroundsfrom single-beam particle losses. We will present the re-sults concerning the investigation of the optimal position-ing of our diamond sensors, taking into account the rate ofBhabha particles as well as their interactions with the beampipe material.INTRODUCTIONSuperKEKB [1], the upgrade of KEKB machine at KEKin Japan, is a very high luminosity e+e− circular collider,dedicated to B meson physics study experiment Belle II [2].It consists of a 4 GeV positron ring and a 7 GeV electronring. The very high luminosity (8 1035cm−2s−1) can beachieved using the nano-beam scheme [3], whereby theultra-low emittance beams collide at a large crossing an-gle (83 mrad), thus reducing the effective interaction regionlongitudinally and β∗ parameters without a severe beamblow-up. In this type of rings, beams have relatively shortlifetimes, of the order of 10 minutes, mainly due to Tou-schek large angle intra-beam Coulomb scattering, as wellas to the zero angle radiative Bhabha scattering at the in-teraction point (IP) and should be thus continuously re-injected. Fast luminosity monitoring is needed as inputfor luminosity optimisation and feedback in presence ofdynamic instabilities. To carry on, the radiative Bhabhaprocess at zero photon scattering angle will be considered,because of its large cross-section. Because of its radiationhardness and fast signal collection, diamond sensor (Fig. 1)is chosen for our measurements.BHABHA RATE REQUIREMENTSThe radiative Bhabha at zero photon scattering angleprocess (Fig. 2) corresponds to an electron-positron beamparticles scattering through the exchange of a quasi-realphoton at almost zero angle. It is one of the main sources∗ elkheche@lal.in2p3.frFigure 1: Diamond sensor.of backgrounds via particle losses in such colliders, as wellas a limitation for beam life time. However this process canalso be useful for luminosity monitoring since the amountof scattering is large and proportional to the luminosity.The cross-section calculated from the dedicated code BB-Brem [4] at the Υ(4S) center of mass energy, with a pho-ton energy cut set to 1%.Ebeam, is about 150 mbarn, in-cluding the correction for very small transverse beam sizes(beam-size effect) [6]. To achieve the aimed precisions,from initial luminosity phase to the optimal one, around7 to 8 ×10−4 of the total Bhabha cross-section should bedetected in the sensors as displayed in Table 1.Figure 2: Radiative Bhabha at zero photon scattering angle.MONITOR LOCATIONThe dynamics of the Bhabha events has been generatedusing GUINEA-PIG++ [5], a beam-beam interaction simu-lation tool mainly used for high energy e+e− collider stud-ies, in which the radiative Bhabha process at very small an-gle is treated as a Compton scattering process convolutedwith a spectrum function for the quasi-real photon. Energyspectrum of the Bhabhas is represented in Fig. 3 for theLow Energy Ring (LER). Low energy Bhabhas will escapethe beam thus exiting the beam-pipe at successive locationsdownstream of the IP at different rates.The scattered electrons/positrons particles have been thentracked by SAD code [7] developed in KEK, up to 25 me-ters downstream of the IP (Fig. 4), in order to estimatethe rates of the Bhabha particles deflected enough to hitthe beam pipe and potentially generating signals in a 5 x5 mm2 diamond sensor placed just outside of the 40 mmradius and 6 mm thickness copper beam-pipe. Several lo-cations have been considered, after the first bending mag-nets, quadrupoles and in the drifts. As a result, the opti-THPME090 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-83442Copyright©2014CC-BY-3.0andbytherespectiveauthors06 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and InstrumentationTable 1: Least Required Fractions of Bhabha Cross-section in the SensorsLuminosity (cm−2 s−1) Number of Bhabha Aimed precision Required fraction1034 1.5× 107 in 10 ms 10−2 in 10 ms 6.7 x 10−48 x 1035 1.2× 109 in 10 ms 10−3 in 10 ms 8.3 x 10−4mal position for our sensor is to be at S=13.9 m from theIP where a fraction of 1.65 % of the total Bhabha cross-section should be, in a first approximation, intercepted inthe diamond. While the LER presents a clear proportional-ity between horizontal deflections of the scattered positronsand their energy (l.h.s of Fig 5), enabling ”monochromatic”measurements of 3.4 GeV, the High Energy Ring (HER)showed non-linear distributions (r.h.s Fig. 5), potentiallydue to chromaticity corrections, in addition to a very lowand fluctuating amount of intercepted Bhabha fractions.Figure 3: Energy distribution of Bhabha positrons whichare already lost (green), detectable (blue) and still in thebeampipe (red) at 13.9 m.Figure 4: Beta functions and dispersion functions over first25 m downstream the IP for the LER.VACUUM CHAMBER GEOMETRYBhabha positrons hit the copper beam-pipe at a verysmall angle of 5 mrad, crossing 1.2 m of the material whichcorresponds to ≈ 80 radiation lengths. In this case, mostof the particles produced in the showers induced by theinteraction of the scattered positrons with the beam-pipeare absorbed, and the probability for having charged sec-ondaries exiting the beam-pipe is very small. Nevertheless,Figure 5: Horizontal extension (in m) of scattered Bhabhaparticles Vs their energy (in GeV), respectively for thepositrons in LER on l.h.s and for the electrons in HER onr.h.s.the extension of the electromagnetic showers contained inthe Molie`re radius enables few of them to exit. An ideaof creating a window (Fig. 6) at 45 degrees in the driftbetween the two quadrupoles QKBLP and QLC1LP wasconsidered, to enable the exit of the core of the showersand then to increase the signal in the sensors. GEANT4simulations were performed, considering the material andthe vacuum chamber geometry, to estimate the actual num-ber of particles which will hit the diamond sensor and todetermine if a window is required. Different geometriesof the beam pipe were considered, such the case of a nor-mal cylindrical beam pipe and the case of a window. Inthe case without a window, simulations showed very lownumber of signals thus resulting in very low precisions onluminosity measurements even for the nominal luminosity.Simulations with a window showed much higher signals,especially when adding a radiator of iron at shower maxi-mum just in contact with the window.In SuperKEKB, the repetition frequency is ≈ 250 MHz sothe two beams collide every 4 ns. Fast signals are then re-quired for individual bunch crossing luminosity. This couldbe achieved using a 100µm thickness diamond, enabling ashort pulse of 2 ns [8]. The energy deposition per incidentparticles in such a diamond is displayed in fig. 7. The spec-trum corresponds to a Landau distribution with a peak en-ergy at 50 KeV (1 MIP). The figures 8 show the reachableprecisions on our measurements for the initial luminosityphase as a function of different energy thresholds for par-ticles detection, respectively for the normal beam pipe andthe window case. It appears that the aimed precision cannot be reached with a normal beam-pipe, contrary to thewindow case improving the results by a factor 10.Proceedings of IPAC2014, Dresden, Germany THPME09006 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-132-83443 Copyright©2014CC-BY-3.0andbytherespectiveauthorsFigure 6: The window designed by K. Kanazawa at KEK -Japan.Figure 7: Energy deposition per incident particles in100µm diamond.CONCLUSION AND NEXT PLANSFrom the simulation results presented in this paper,we were able to specify the best candidate position of oursensor in the LER at 13.9 meters downstream from the IP,where we should have enough Bhabha rates to achieve ouraimed precision. A window appears to be necessary to ob-tain reliable signals in the sensors. On the other hand, can-didate places in the HER are still under investigations tofind a position which suits our measurements. 150µm di-amond sensor characterisation study will start in the nextweeks by laboratory test in synergy with the ATF2 diamondsensor project [9]. Furthermore, first prototype sensor andthe fast readout electronics which is needed are also on-going to end up with a setup for beam test at SuperKEKBin 2015 for single beam run.ACKNOWLEDGMENTSWe would like to warmly thank Yoshihiro Funakoshi,Ken-ichi Kanazawa and Sadaharu Uehara for the fruitfuldiscussions and collaboration we are having and for theirencouragements. Part of this work has been funded by theP2IO LabEx (ANR-10-LABX-0038) in the framework “In-vestissements d’Avenir” (ANR-11-IDEX-0003-01) man-aged by the French National Research Agency (ANR)Figure 8: Precision in 10 ms on luminosity (1034 cm−2s−1) as a function of threshold applied on the energy de-posited per scattered particle in the normal case (upper) andfor a window (lower).REFERENCES[1] Accelerator design at SuperKEKB,YukiyoshiOhnishi,Prog. Theor. Exp. Phys. 2013, 03A0111, DOI10.1093/ptep/pts083.[2] T. Abe et al., Belle II Technical Design Report,arXiv:1011.0352 [physics.ins-det], Oct. 2010.[3] SuperB Collaboration, SuperB Progress Reports,arXiv:1009.6178v3 [physics.acc-ph], Dec. 2010.[4] R. Kleiss, H. Burkhardt, “BBBrem - Monte Carlo simula-tion of radiative Bhabha scattering in the very forward direc-tion”, NIKHEF-H/94-01, CERN SL/94-03(OP), Jan 1994.[5] D. Schulte, Ph. D. Thesis, University of Hamburg 1996.TESLA-97-08,https://trac.lal.in2p3.fr/GuineaPig.[6] G. L. Kotkin and V.G. Serbo,Phys. Rev. ST Accel. Beams 7,101001 (2004).[7] http://acc-physics.kek.jp/SAD/SADHelp.html.[8] http://www.cividec.at.[9] S. Liu et al., “Status of Diamond Detector Development forBeam Halo Investigation at ATF2” THPME092, these pro-ceedings.THPME090 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-83444Copyright©2014CC-BY-3.0andbytherespectiveauthors06 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and Instrumentation

Fast Luminosity Monitoring using Diamond Sensors for the Super Flavor Factory SuperKEKB

Abstract

Similar works

Full text

Available Versions

HAL-IN2P3