We report the first observations of beam losses due to bound-free pair
production at the interaction point of a heavy-ion collider. This process is
expected to be a major luminosity limit for the Large Hadron Collider (LHC)
when it operates with 208Pb82+ ions because the localized energy deposition by
the lost ions may quench superconducting magnet coils. Measurements were
performed at the Relativistic Heavy Ion Collider (RHIC) during operation with
100 GeV/nucleon 63Cu29+ ions. At RHIC, the rate, energy and magnetic field are
low enough so that magnet quenching is not an issue. The hadronic showers
produced when the single-electron ions struck the RHIC beampipe were observed
using an array of photodiodes. The measurement confirms the order of magnitude
of the theoretical cross section previously calculated by others.Comment: 4 pages, 5 figures. Added journal ref. Corrected typos. Fixed fig 1.
  Minor improvements to fig. 1,3,4. Rephrased a small number of sentences
  (p1,3,4). Added numerical values of the aperture and the displacement for Au
  (p 2). Changed reference 5, added name in acknowledgments (p 4

A. Drees

A. Fasso

F. Pilat

J. M. Jowett

J. M. Jowett

P. Grafström

R. Bruce

S. Gilardoni

S. R. Klein

S. Tepikian

W. Fischer

English

arXiv

We report the first observations of beam losses due to bound-free pair production at the interaction point of a heavy-ion collider. This process is expected to be a major luminosity limit for the Large Hadron Collider (LHC) when it operates with 208Pb82+ ions because the localized energy deposition by the lost ions may quench superconducting magnet coils. Measurements were performed at the Relativistic Heavy Ion Collider (RHIC) during operation with 100 GeV/nucleon 63Cu29+ ions. At RHIC, the rate, energy and magnetic field are low enough so that magnet quenching is not an issue. The hadronic showers produced when the single-electron ions struck the RHIC beampipe were observed using an array of photodiodes. The measurement confirms the order of magnitude of the theoretical cross section previously calculated by others.We report the first observations of beam losses due to bound-free pair production at the interaction point of a heavy-ion collider. This process is expected to be a major luminosity limit for the Large Hadron Collider (LHC) when it operates with 208Pb82+ ions because the localized energy deposition by the lost ions may quench superconducting magnet coils. Measurements were performed at the Relativistic Heavy Ion Collider (RHIC) during operation with 100 GeV/nucleon 63Cu29+ ions. At RHIC, the rate, energy and magnetic field are low enough so that magnet quenching is not an issue. The hadronic showers produced when the single-electron ions struck the RHIC beampipe were observed using an array of photodiodes. The measurement confirms the order of magnitude of the theoretical cross section previously calculated by others

Bruce, R.

Jowett, J.M.

Gilardoni, S.

Drees, A.

Fischer, W.

Tepikian, S.

Klein, S.R.

CERN Document Server

Observations of Beam Losses Due to Bound-Free Pair Production in a Heavy-Ion ColliderR. Bruce,* J. M. Jowett, and S. GilardoniCERN, Geneva, SwitzerlandA. Drees, W. Fischer, and S. TepikianBNL, Upton, New York, USAS. R. KleinLBNL, Berkeley, California, USA(Received 13 June 2007; published 3 October 2007)We report the first observations of beam losses due to bound-free pair production at the interaction pointof a heavy-ion collider. This process is expected to be a major luminosity limit for the CERN LargeHadron Collider when it operates with 208Pb82 ions because the localized energy deposition by the lostions may quench superconducting magnet coils. Measurements were performed at the BNL RelativisticHeavy Ion Collider (RHIC) during operation with 100 GeV=nucleon 63Cu29 ions. At RHIC, the rate,energy and magnetic field are low enough so that magnet quenching is not an issue. The hadronic showersproduced when the single-electron ions struck the RHIC beam pipe were observed using an array ofphotodiodes. The measurement confirms the order of magnitude of the theoretical cross section previouslycalculated by others.DOI: 10.1103/PhysRevLett.99.144801 PACS numbers: 29.20.Dh, 25.75.qWhen fully-stripped heavy ions of atomic numbers Z1,Z2 are brought into collision at the interaction point (IP) ofa collider, a number of electromagnetic interactions areinduced by the intense fields generated by the coherentaction of all the Z1;2 charges in either nucleus (for a review,see Ref. [1]). Some of these ‘‘ultraperipheral’’ interactionshave much higher cross sections than the hadronic nuclearinteractions that are the main object of study. Among them,the bound-free pair production (BFPP), sometimes knownas electron capture from pair production, occurs when thevirtual photon exchanged by the ions converts into a pair,and the electron is created in an atomic shell of one of theions: Z1  Z2! Z1  e1s1=2;:::  Z2  e: (1)The resulting one-electron atoms have a slightly largermagnetic rigidity than the original bare nucleus.(Magnetic rigidity is defined as p=Qe  B for a parti-cle with momentum p and charge Qe that would havebending radius  in a magnetic field B). Since the trans-verse recoil is very small, this ‘‘secondary beam’’ willemerge at a very small angle to the main beam. However,it will be bent and focused less by the guiding magneticelements and may be lost somewhere in the collider ring.It has long been known that this process, together withelectromagnetic dissociation of the nuclei, could be amajor contribution to the intensity and luminosity decayof ion colliders [2,3]. It was realized more recently [4,5]that, in certain conditions, the BFPP beam will be lost in awell-defined spot, initiating hadronic showers in the vac-uum envelope of the beam. The resulting localized heatdeposition could induce quenches of superconductingmagnets. Detailed calculations have been given elsewherefor 2:76 TeV=nucleon 208Pb82 operation of the LHC atCERN [5–8]. The consequent luminosity limit is expectedto occur at a level close to the design performance. It istherefore vital to test our quantitative understanding of thefeatures of the BFPP process in order to ensure safe op-eration of the LHC, uninterrupted by lengthy quench-recovery procedures. BFPP has been measured in fixedtarget experiments [9–11] at lower energy but not, untilnow, in a collider. An opportunity arose during 63Cu29operation of RHIC at Brookhaven National Laboratory.The flux of ions in the secondary BFPP beam emergingfrom the interaction point is the product of the colliderluminosity and the BFPP cross section. The partial crosssection for electron capture for a given low-lying atomicbound state i on nucleus 1 has the approximate form [12] i  Z51Z22Ai logcm  Bi (2)where cm is the Lorentz boost of the ions in the center-of-mass frame and Ai, Bi depend only weakly on Z. The totalcross section for BFPP to one of the colliding ions is then BFPP Xii (3)where the sum is over all atomic shells. Some numericalvalues from [12] are given in Table I. The cross section forcollisions of 100 GeV=nucleon 63Cu29 was not calculatedin Ref. [12] but we have estimated it as 0.2 b by interpo-lation of the data given in Fig. 7 of Ref. [12], scaled with Zand logcm according to Eq. (2). Applied to other tabulatedvalues in Ref. [12], this method produces an agreementwithin 10%.PRL 99, 144801 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending5 OCTOBER 20070031-9007=07=99(14)=144801(4) 144801-1 © 2007 The American Physical SocietyThe impact point of the lost ions is predicted by trackingthe orbit of the BFPP beam in the collider optics until itintersects the physical aperture (vacuum pipe) of the beamline. To lowest order in the uncorrelated betatron ampli-tudes, a, b, and relative magnetic rigidity deviation fromthe central value, , the horizontal displacement from thecentral orbit, at a distance s from the IP is xsa2sqcossb2sqsinsdxs:(4)Averaging over all ions gives the transverse emittance ofthe beam: ha2  b2i  ; s is the usual ‘‘Twiss func-tion’’ determined by the focusing properties of the accel-erator, s  Rs0 u1du is the betatron phase and dxsis the locally generated dispersion (dx0d0x00) thatencodes the spectrometer effect of the bending magnets.An ion following the central trajectory (i.e., in themiddle of the bunch) in the BFPP beam enters the IPwith a  b    0 and emerges, still with a  b  0,but   1=Z 1. This ion will hit the beam pipe at thefirst location where the horizontal aperture Ax satisfiesAxs  dxs=Z 1. In the arc, Ax  3:455 cm.During 197Au79 operation at RHIC,  is not largeenough for this to occur (xmax ’ 2:5 cm). However, during63Cu29 runs [13], the larger  (Table I) meant that the ionsshould be lost in a well-defined location where they can bedetected directly. Thanks to the small cross section, therewas no risk of quenching any superconducting materials.Since the BFPP beam was not visible to the existingdetectors, beam loss monitors in the form of p-i-n diodes(PDs) of the type Hamamatsu S3590 were mounted on theoutside of the magnet cryostat downstream of the IP of thePHENIX experiment. The PDs have a time resolution of25 ns and the counts were read out every second. Theywere initially positioned in a wide configuration (WPD),3 m apart (see Fig. 1), and later moved closer together(CPD), 0.5 m apart, around an observed count-rate maxi-mum at 141.6 m.A linear interpolation of dxs between magnet ends,based on the design model of the collider, identified theelements within which the BFPP beam was lost: a 9.7 mlong dipole magnet starting at s  129:6 m, followed by a2.12 m drift space and a quadrupole magnet, shown inFig. 1. More precise tracking, including chromatic effects( dependences), and a final step of analytic orbit calcula-tion inside the dipole, predicted an impact at si ’ 135:5 mwith an angle of incidence of hx0sii ’ 2:7 mrad. Trackingan assumed Gaussian distribution of the amplitudes a andb in Eq. (4) gives the distribution of impact momenta in thespot around this point. Figure 2 shows the main 63Cu29beam together with the 63Cu28 BFPP beam propagatingfrom the IP.To the extent that the RHIC hardware differs from thedesign model, the impact point may be shifted. The globaldistortion of the central orbit by misalignments of themagnets is partially corrected by localized corrector mag-nets. A least-squares fit was made to orbit data from beamposition monitors (BPMs) and measured misalignments (oforder 1–2 mm) of the quadrupole magnets in the form xsxsXjssj	ssjqsinssjj: (5)Here s is the unit step function, x the first two terms inEq. (4) and j are the angular kicks from misalignmentsand correctors at locations sj. Unfortunately not enoughdata were available to make a satisfactory fit. Instead, thepossible spread of the impact point was estimated bycalculating the propagated error using Eq. (5). A possibleerror on the horizontal quadrupole misalignments of 1 mmwas assumed, corresponding to the 95% confidence inter-CPDWPD132140144148s (m)0 20-20 x (cm)QuadrupoleDriftDipole 136FIG. 1 (color online). Left: The energy deposition from atypical shower shown in a thin slice in the x-s plane throughthe geometry. The arrow (red online) near the center of the figureindicates the impact point and the arrows along the right side(green and orange online) show the positions of the PDs in theCPD and WPD. Right: The 3D model of the geometry asimplemented in FLUKA around the impact point. The squares(orange online) show the PDs in the WPD.TABLE I. BFPP cross sections, typical peak luminosity, BFPP rates and relative changes in magnetic rigidity at RHIC and LHC.Values are taken directly where possible, or estimated by fitting sums of contributions of the form (2), to the information in Ref. [12].  1=Z 1 is the fractional deviation of the magnetic rigidity.BFPP (barn) L=1027 (cm2 s1) BFPP rate (kHz)  (%)LHC Pb-Pb 2759 GeV=nucleon 281 1 281 1.2RHIC Au-Au 100 GeV=nucleon 114 3 342 1.3RHIC Cu-Cu 100 GeV=nucleon 0.2 20 4 3.6RHIC Cu-Cu 31 GeV=nucleon 0.08 1 0.08 3.6PRL 99, 144801 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending5 OCTOBER 2007144801-2val of the measurement and the fact that the magnets mayhave moved between early 2005 when the BFPP measure-ment was done and late 2006 when the misalignments weremeasured. The absolute error on the BPM data was taken tobe 0.5 mm, reflecting the limited data and possible non-reproducibility. Combining these errors gives an impactpoint of 135:5 2 m. Keeping the nominal optics, butdisplacing the beam pipe by 1.4 mm at the impact pointaccording to the measured misalignment of the dipole,moves the impact point to 136.0 m.Several distributions of BFPP ions at the PHENIX IP,centered on different initial offsets and angles, weretracked to obtain their impact coordinates on the insideof the vacuum pipe. These were fed into a 3D model (seeFig. 1) of the impact region geometry, the PDs, and thedipole magnetic field, implemented in the FLUKA 2006.3Monte Carlo code [14–16], in order to simulate the showerin the magnet and particles emerging from the cryostat.This code has been benchmarked by others in the relevantenergy range [16].Within 2–2.5 m from the average impact point most ofthe shower inside the magnet has died out, although parti-cles outside propagate farther. This can be understoodthrough a rough estimate: The typical average nuclearinteraction length for the superconducting coil and theiron cold bore is about 20 cm. Convoluted with the impactpoint spot size of about 1.8 m, this gives a total showerlength of about 2.8 m, assuming that the shower lasts fiveinteraction lengths. The maximum PD signal was expectedabout 2.5 m downstream of the impact point. When theshower is contained within the dipole, moving the impactpoint along s only translates the shower. If the impact pointis moved closer toward the end of the magnet or the angleof incidence decreased, more of the shower emerges intothe void before the quadrupole and the profile changesqualitatively. A second peak in energy deposition appearsat the entrance of the quadrupole and eventually exceedsthe first one.The PDs count the number of minimum-ionizing parti-cles (MIPs) that pass their active area of 1 cm2. In thesimulation, the number of MIPs entering each PD wasrecorded, assuming a detection efficiency of 30%. Sincethe PDs are much more sensitive to particles enteringthrough the side orthogonal to the cryostat, only theseparticles were counted. At the expected BFPP ion produc-tion rate of 4 kHz, and the 0.01 MIPs expected per PD perion, pileup from multiple interactions was not an issue.Both WPD and CPD configurations were simulated forvarious impact points and a typical result is shown togetherwith measured data in Fig. 3.Data were collected from the PDs during 14 fills ofRHIC with 63Cu29 ions. An example of the recordedsignal is shown in Fig. 4, together with the zero degreecalorimeter (ZDC) count rate (which is directly propor-tional to the luminosity). The PD signals show a goodtemporal correlation with luminosity and are very localizedalong s, close to the predicted impact position. With a 1 sbinning, the correlation coefficient, r, between the PDsignal and ZDC is 0.7 at 141.6 m and decreases graduallyto 0.1 at PDs outside the predicted impact area. With a 30 sbinning the highest r rises to 0.98. This makes it veryunlikely that some other source than the collisions couldbe responsible for the signals.This becomes even clearer from van der Meer scans inwhich the beam orbits are scanned transversely across eachother and the luminosity variation is recorded as inRef. [17]. Figure 5 shows the ZDC rate and the signal ofone of the PDs as a function of the orbit bump amplitude.Since the data were taken parasitically during normalcolliding-beam operation, the PD signals were often pol-luted by other losses, e.g., from collimation. Therefore thecleanest data sets with least interference were picked out(near the beginning of each fill) and the averaged count ratewith background subtracted, normalized to luminosity, wascalculated for each PD (see Fig. 3). The background noiselevel was calculated for each PD as the average signalwithout luminosity.In the WPD, the maximum signal was recorded by thePD at 141.6 m but moved to 140.5 m in the CPD. The countrates ranged from 1 to 20 Hz depending on PD position and120 125 130 135 140 145 150s m02.557.51012.515PDcountrateHzClose sim.Wide sim.Close meas.Wide meas.FIG. 3 (color online). The PD signals from both configurationstogether with simulation results, averaged over the fills and nor-malized to a typical average luminosity of 9:1	 1027 cm2 s1.The simulation was produced from the central BFPP orbit in thenominal lattice but with the beam pipe misaligned by 1.4 mm atthe impact location according to measurements. Because of largestatistical error bars, the simulation gives only the order ofmagnitude to expect in the measurement.25 50 75 100 125 150s m0.060.040.020.020.040.06x mFIG. 2 (color online). The horizontal projection of the 1 envelope of the BFPP beam and the nominal beam emergingfrom the PHENIX IP. The BFPP beam hits the inside of thevacuum chamber at 135.5 m.PRL 99, 144801 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending5 OCTOBER 2007144801-3luminosity. This agrees well with the simulation. However,the maximum in the simulation came from the PD at138.6 m, which does not exactly agree with the measure-ments. This means that, in reality, the second peak in theenergy distribution outside the cryostat is actually higherthan the first, while in the simulation the first peak ishigher. As can be seen in Fig. 1, the peak in the showercould also escape between the PDs in the WPD. However,if the impact point is translated within the 2 m error bar, thesimulated maximum moves further downstream and a fairagreement can be found. Moreover, the PD signals them-selves have error bars—apart from the statistical error asmall relative change in the counting efficiency betweenthe PDs could change the result. This has not been mea-sured so we give no numerical estimate.One might hope to extract a value for the cross sec-tion for BFPP between colliding copper ions at100 GeV=nucleon. However, because of the uncertaintiesin both the number of MIPs entering a specific diode in thesimulation and the recorded count rates, we can onlyconclude from the good agreement that the theoreticalestimate of the BFPP cross section has the right order ofmagnitude.No signal was detectable at 31 GeV=nucleon, consistentwith the much lower (L	 BFPP) (see Table I).In conclusion, we made the first measurements of thelocalized loss of a BFPP generated secondary beam in anion collider. These measurements were done with copperbeams at RHIC. We found the location of the maximumloss monitor signal at 140.5 m from the IP, within 2 m of itscalculated location, and the measured event rates in our PDdetectors within a factor of 2 of calculated ones. Thedeviations between calculated and measured values areconsistent with estimated errors. This is a valuable test ofour ability to make quantitative predictions of this effectfor the LHC, where it is expected to be one of the mostrestrictive luminosity limits.We thank A. J. Baltz, R. Gupta, K. Hencken, J. B.Jeanneret and T. Roser for helpful discussions.*Also at MAXlab, Lund University, Sweden.Electronic address: roderik.bruce@cern.ch[1] C. Bertulani, S. R. Klein, and J. Nystrand, Annu. Rev.Nucl. Part. Sci. 55, 271 (2005).[2] H. Gould, Lawrence Berkeley Laboratory ReportNo. LBL-18593, 1984.[3] A. J. Baltz, M. J. Rhoades-Brown, and J. Weneser, Phys.Rev. E 54, 4233 (1996).[4] S. R. Klein, Nucl. Instrum. Methods Phys. Res., Sect. A459, 51 (2001).[5] J. M. Jowett, J.-B. Jeanneret, and K. Schindl, Proceedingsof the Particle Accelerator Conference 2003, Portland,Oregon, edited by Joe Chew, Peter Lucas, and SaraWebber (IEEE, Piscataway, New Jersey, 2003), p. 1682.[6] O. S. Bru¨ning et al., CERN Report No. CERN-2004-003-V1, 2004.[7] J. M. Jowett, R. Bruce, and S. Gilardoni, Proceedings ofthe Particle Accelerator Conference 2005, Knoxville,Tennessee (IEEE, Piscataway, New Jersey, 2005), p. 1306.[8] R. Bruce, S. Gilardoni, and J. M. Jowett, CERN LHCProject Note 379, 2006.[9] A. Belkacem et al., Phys. Rev. Lett. 71, 1514 (1993).[10] H. F. Krause et al., Phys. Rev. Lett. 80, 1190 (1998).[11] P. Grafstro¨m et al., Proceedings of the ParticleAccelerator Conference 1999, New York (IEEE,Piscataway, New Jersey, 1999), p. 1671.[12] H. Meier et al., Phys. Rev. A 63, 032713 (2001).[13] F. Pilat et al., Proceedings of the Particle AcceleratorConference 2005, Knoxville, Tennessee (IEEE,Piscataway, New Jersey, 2005), p. 4281.[14] A. Fasso et al., CERN Report No. CERN-2005-10, 2005.[15] A. Fasso et al., Proceedings of the Computing in HighEnergy and Nuclear Physics 2003 Conference, La Jolla,California (SLAC, Stanford, California, 2003).[16] S. Roesler, R. Engel, and J. Ranft, SLAC ReportNo. SLAC-PUB-8740, 2000.[17] A. Drees et al., Proceedings of the Particle AcceleratorConference 2003, Portland, Oregon, edited by Joe Chew,Peter Lucas, and Sara Webber (IEEE, Piscataway, NewJersey, 2003), p. 1688.1.5 1 0.5 0 0.5 1 1.5bump amplitude mm0246810ZDCcountratekHz1.5 1 0.5 0 0.5 1 1.5bump amplitude mm00.250.50.7511.251.51.75PDcountrateHzFIG. 5. The ZDC count rate (proportional to luminosity, left)and background-subtracted PD count rate (right) vs relative orbitdisplacement between beams in a van der Meer scan. A Gaussianfit of the form A expxx022x  is shown together with the data.The two fits had x  0:49 mm and 0.44 mm and 	2=DOF 0:23 and 0.20, assuming a measurement error of 500 Hz on theZDC signal and 0.3 Hz on the PD.0 2000 4000 6000 8000 10000 12000Time s2468101214PDcountrateHz5000.10000.15000.20000.25000.ZDCcountrateHzPD 135.6 mPD 138.6 mPD 141.6 mZDCFIG. 4 (color online). Count rates measured on the ZDCluminosity monitors (black, right scale) and the three PDs withthe highest signal [shades of gray, left scale (colors online)]during a store with the WPD. The data was binned in 30 secintervals. A clear correlation between the luminosity and the PDcount rates can be seen.PRL 99, 144801 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending5 OCTOBER 2007144801-4

Observations of beam losses due to bound-free pair production in a heavy-ion collider

We report the first observations of beam losses due tobound-free pair production at the interaction point of a heavy-ioncollider. This process is expected to be a major luminosity limit for theLarge Hadron Collider (LHC) when it operates with 208Pb82+ ions becausethe localized energy deposition by the lost ions may quenchsuperconducting magnet coils. Measurements were performed at theRelativistic Heavy Ion Collider (RHIC) during operation with 100GeV/nucleon 63Cu29+ ions. At RHIC, the rate, energy and magnetic fieldare low enough so that magnet quenching is not an issue. The hadronicshowers produced when the single-electron ions struck the RHIC beampipewere observed using an array of photodiodes. The measurement confirms theorder of magnitude of the theoretical cross section previously calculatedby others

UNT (University of North Texas) Digital Library

arXiv:0706.2292v1  [physics.acc-ph]  15 Jun 2007First observations of beam losses due tobound-free pair production in a heavy-ion colliderR. Bruce,∗ J.M. Jowett, and S. GilardoniCERN, Geneva, SwitzerlandA. Drees, W. Fischer, and S. TepikianBNL, Upton, NY, USAS.R. KleinLBNL, Berkeley, CA, USA(Dated: June 15, 2007)We report the first observations of beam losses due to bound-free pair production at the interactionpoint of a heavy-ion collider. This process is expected to be a major luminosity limit for the LargeHadron Collider (LHC) when it operates with 208Pb82+ ions because the localized energy depositionby the lost ions may quench superconducting magnet coils. Measurements were performed at theRelativistic Heavy Ion Collider (RHIC) during operation with 100 GeV/nucleon 63Cu29+ ions. AtRHIC, the rate, energy and magnetic field are low enough so that magnet quenching is not anissue. The hadronic showers produced when the single-electron ions struck the RHIC beampipewere observed using an array of photodiodes. The measurement confirms the order of magnitude ofthe theoretical cross section previously calculated by others.PACS numbers: 29.20.Dh, 25.75.-qWhen fully-stripped heavy ions of atomic numbers Z1,Z2 are brought into collision at the interaction point (IP)of a collider, a number of electromagnetic interactions areinduced by the intense fields generated by the coherentaction of all the Z1,2 charges in either nucleus (for a re-view, see [1]). Some of these “ultra-peripheral” interac-tions have much higher cross sections than the hadronicnuclear interactions that are the main object of study.Among them, the bound-free pair production (BFPP),sometimes known as electron capture from pair produc-tion (ECPP), occurs when the virtual photon exchangedby the ions converts into a pair, and the electron is cre-ated in an atomic shell of one of the ions:Z1 + Z2γ−→ (Z1 + e−)1s1/2,... + Z2 + e+ (1)The resulting one-electron atoms have a slightly largermagnetic rigidity than the original bare nucleus. (Mag-netic rigidity is defined as p/(Qe) = Bρ for a particlewith momentum p and charge Qe that would have bend-ing radius ρ in a magnetic field B). Since the transverserecoil is very small, this “secondary beam” will emerge ata very small angle to the main beam. However it will bebent and focused less by the guiding magnetic elementsand may be lost somewhere in the collider ring.It has long been known that this process, together withelectromagnetic dissociation of the nuclei, could be a ma-jor contribution to the intensity and luminosity decay ofion colliders [2, 3]. It was realized more recently [4, 5]that, in certain conditions, the BFPP beam will be lost∗Also at MAXlab, Lund University, Sweden.; Electronic address:roderik.bruce@cern.chin a well-defined spot, initiating hadronic showers inthe vacuum envelope of the beam. The resulting local-ized heat deposition could induce quenches of supercon-ducting magnets. This has been extensively discussedelsewhere, particularly for the case of 2.76 TeV/nucleon208Pb82+ operation of the LHC at CERN [4, 5, 6, 7, 8].The consequent luminosity limit is expected to occur ata level close to the design performance. It is therefore vi-tal to test our quantitative understanding of the featuresof the BFPP process in order to ensure safe operationof the LHC, uninterrupted by lengthy quench-recoveryprocedures. BFPP has been measured in fixed target ex-periments [9, 10, 11] at lower energy but not, until now,in a collider. An opportunity arose during 63Cu29+ op-eration of RHIC at Brookhaven National Laboratory.The flux of ions in the secondary BFPP beam emergingfrom the interaction point is the product of the colliderluminosity and the BFPP cross section. The partial crosssection for electron capture for a given low-lying atomicbound state i on nucleus 1 has the approximate form [12]σi ≈ Z51Z22 (Ai log γcm +Bi) (2)where γcm is the Lorentz boost of the ions, in the centre-of-mass frame and Ai, Bi depend only weakly on Z. Thetotal cross section for BFPP to one of the colliding ionsis thenσBFPP =∑iσi (3)where the sum is over all atomic shells. Some numericalvalues from [12] are given in Table I. The cross sectionfor collisions of 100 GeV/nucleon 63Cu29+ was not cal-culated in [12] but we have estimated it as 0.2 barn by2TABLE I: BFPP cross sections, typical peak luminosity,BFPP rates and relative changes in magnetic rigidity at RHICand LHC. Values are taken directly where possible, or esti-mated by fitting sums of contributions of the form (2), to theinformation in [12]. δ = 1/(Z − 1) is the fractional deviationof the magnetic rigidity.σBFPP L/1027 BFPP δ(%)(barn) (cm−2s−1) rate (kHz)LHC Pb-Pb 281 1 281 1.22759 GeV/nucleonRHIC Au-Au 114 3 342 1.3100 GeV/nucleonRHIC Cu-Cu 0.2 20 2.9 3.6100 GeV/nucleonRHIC Cu-Cu 0.08 1 0.08 3.631 GeV/nucleoninterpolation of the data given in Fig. 7 of [12], scaledwith Z and log γcm according to Eq. (2). Applied toother tabulated values in [12], this method produces anagreement within 10%.The impact point of the lost ions is predicted by track-ing the orbit of the BFPP beam in the collider optics untilit intersects the physical aperture (vacuum pipe) of thebeam line. To lowest order in the uncorrelated betatronamplitudes, a, b, and relative magnetic rigidity deviationfrom the central value, δ, the horizontal displacementfrom the central orbit, at a distance s from the IP isx(s) = a√2β(s) cos (µ(s)) + b√2β(s) sin (µ(s)) + dx(s)δ.(4)Averaging over all ions gives the transverse emittanceof the beam: 〈a2 + b2〉 = ǫ; β(s) is the usual “Twissfunction” determined by the focusing properties of theaccelerator, µ(s) =∫ s0β(u)−1du is the betatron phaseand dx(s) is the locally generated dispersion (dx(0) =d′x(0) = 0) that encodes the spectrometer effect of thebending magnets.An ion following the central trajectory (i.e. in the mid-dle of the bunch) in the BFPP beam enters the IP witha = b = δ = 0 and emerges, still with a = b = 0, butδ = 1/(Z − 1). This ion will hit the beam pipe at thefirst location where the horizontal aperture Ax satisfiesAx(s) = dx(s)/(Z − 1).During 197Au79+ operation at RHIC, δ is not largeenough for this to occur. However, during 63Cu29+runs [13], the larger δ (Table I) meant that the ionsshould be lost in a well-defined location where they canbe detected directly. Thanks to the small cross section,there was no risk of quenching any superconducting ma-terials. Since the BFPP beam was not visible to theexisting detectors, beam loss monitors in the form ofPIN diodes (PDs) of the type Hamamatsu S3590 weremounted on the outside of the magnet cryostat down-stream of the IP of the PHENIX experiment. The PDshave a time resolution of 25 ns and the counts were readout every second. They were initially positioned in aCPDWPD132140144148s (m)0 20-20 x (cm)QuadrupoleDriftDipole 136FIG. 1: (color online) Left: The energy deposition from atypical shower shown in a thin slice in the x−z−plane throughthe geometry. The red arrow indicates the impact point andthe orange and purple arrows show the positions of the PDsin the CPD and WPD. Right: The 3D model of the geometryas implemented in FLUKA around the impact point.25 50 75 100 125 150s HmL-0.06-0.04-0.020.020.040.06x HmLFIG. 2: (color online) The horizontal projection of the 1 σenvelope of the BFPP beam and the nominal beam emergingfrom the PHENIX IP. The BFPP beam hits the inside of thevacuum chamber at 135.5 m.wide configuration (WPD), 3 m apart (see Fig. 1), andlater moved closer together (CPD), 0.5 m apart, aroundan observed count-rate maximum at 141.6 m.A linear interpolation of dx(s) between magnet ends,based on the design model of the collider, identifiedthe elements within which the BFPP beam was lost: a9.7 m long dipole magnet starting at s = 129.6m, fol-lowed by a 2.12 m drift space and a quadrupole mag-net, shown in Fig 1. More precise tracking, includingchromatic effects (δ-dependences), and a final step of an-alytic orbit calculation inside the dipole, predicted animpact at si ≃ 135.5 m with an angle of incidence of〈x′(si)〉 ≃ 2.7 mrad. Tracking an assumed Gaussian dis-tribution of the amplitudes a and b in Eq. (4) gives thedistribution of impact momenta in the spot around thispoint. Fig. 2 shows the main 63Cu29+ beam togetherwith the 63Cu28+ BFPP beam propagating from the IP.To the extent that the RHIC hardware differs fromthe design model, the impact point may be shifted. Theglobal distortion of the central orbit by misalignments ofthe magnets is partially corrected by localized correctormagnets. A least-squares fit was made to orbit data frombeam position monitors (BPMs) and measured misalign-ments (of order 1–2 mm) of the quadrupole magnets in3the formx(s) = xβ(s) +∑jΘ(s− sj)××√β(s)β(sj) sin (µ(s)− µ(sj)) θj . (5)Here Θ(s) is the unit step function, xβ the first two termsin Eq. (4) and θj are the angular kicks from misalign-ments and correctors at locations sj . Unfortunately notenough data were available to make a satisfactory fit.Instead, the possible spread of the impact point was esti-mated by calculating the propagated error using Eq. (5).A possible error on the horizontal quadrupole misalign-ments of 1 mm was assumed, corresponding to the 95 %confidence interval of the measurement and the fact thatthe magnets may have moved between early 2005 whenthe BFPP measurement was done and late 2006 when themisalignments were measured. The absolute error on theBPM data was taken to be 0.5 mm, reflecting the limiteddata and possible non-reproducibility. Combining theseerrors gives an impact point of 135.5± 2 m. Keeping thenominal optics, but displacing the beam pipe by 1.4 mmat the impact point according to the measured misalign-ment of the dipole, moves the impact point to 136.0 m.Several distributions of BFPP ions at the PHENIXIP, centered on different initial offsets and angles, weretracked to obtain their impact coordinates on the insideof the vacuum pipe. These were fed into a 3D model (seeFig. 1) of the impact region geometry, the PDs and thedipole magnetic field, implemented in the FLUKA 2006.3Monte Carlo code [14, 15, 16], in order to simulate theshower in the magnet and particles emerging from thecryostat. This code has been benchmarked by others inthe relevant energy range [16].Within 2–2.5 m from the average impact point most ofthe shower inside the magnet has died out, although par-ticles outside propagate farther. This can be understoodthrough a rough estimate: The typical average nuclear in-teraction length for the superconducting coil and the ironcold bore is about 20 cm. Convoluted with the impactpoint spot size of about 1.8 m, this gives a total showerlength of about 2.8 m, assuming that the shower lastsfive interaction lengths. The maximum PD signal wasexpected about 2.5 m downstream of the impact point.When the shower is contained within the dipole, mov-ing the impact point along s only translates the shower.As the impact point approaches the end of the magnet,and more of the shower emerges into the void before thequadrupole, the profile changes qualitatively. A secondpeak in energy deposition appears at the entrance of thequadrupole and eventually exceeds the first one. Smallerangles of incidence cause the shower to go farther andthe peak in the second magnet starts earlier.The PDs count the number of minimum-ionizing par-ticles (MIPs) that pass their active area of 1 cm2. In thesimulation, the number of MIPs entering each PD wasrecorded, assuming a detection efficiency of 30%. Sincethe PDs are much more sensitive to particles entering120 125 130 135 140 145 150s HmL02.557.51012.515PDcountrateHHzLClose sim.Wide sim.Close meas.Wide meas.FIG. 3: (color online) The PD signals from both configu-rations together with simulation results, averaged over thefills and normalized to a typical average luminosity of 9.1 ×1027cm−2s−1. The simulation was produced from the centralBFPP orbit in the nominal lattice but with the beam pipemisaligned by 1.4 mm at the impact location according tomeasurements. Due to large statistical error bars, the sim-ulation gives only the order of magnitude to expect in themeasurement.0 2000 4000 6000 8000 10000 12000Time HsL2468101214PDcountrateHHzL5000.10000.15000.20000.25000.ZDCcountrateHHzLZDCPD 135.6 mPD 138.6 mPD 141.6 mFIG. 4: (color online). Count rates measured on the Zerodegree calorimeter (ZDC) luminosity monitors (black, rightscale) and the three PDs with the highest signal (colors, leftscale) during a store with the WPD. The data was binned in30 second intervals. A clear correlation between the luminos-ity and the PD count rates can be seen.through the side orthogonal to the cryostat, only theseparticles were counted. At the expected BFPP ion pro-duction rate of 2.9 kHz, and the 0.01 MIPs expected perPD per ion, pileup from multiple interactions was notan issue. Both WPD and CPD configurations were sim-ulated for various impact points and a typical result isshown together with measured data in Fig. 3.Data were collected from the PDs during 14 fills ofRHIC with 63Cu29+ ions. An example of the recordedsignal is shown in Fig. 4, together with the Zero DegreeCalorimeter (ZDC) count rate (which is directly propor-tional to the luminosity). The PD signals show a goodtemporal correlation with luminosity and are very local-ized along s, close to the predicted impact position. Thecorrelation coefficient between the PD at 140.5 m and theZDC is 0.7, while the more distant PDs have correlationsaround 0.2 for the raw data. With a 30 s binning the4-1.5-1-0.5 0 0.5 1 1.5bump amplitude HmmL0246810ZDCcountrateHkHzL-1.5 -1 -0.5 0 0.5 1 1.5bump amplitude HmmL00.250.50.7511.251.51.75PDcountrateHHzLFIG. 5: The ZDC count rate (proportional to luminosity, left)and background-subtracted PD count rate (right) vs. relativeorbit displacement between beams in a van der Meer scan. AGaussian fit of the form A exp(−(x−x0)2σ2x) is shown togetherwith the data. The two fits had σx = 0.49 mm and 0.44 mmand χ2/DOF = 0.23 and 0.20, assuming a measurement errorof 500 Hz on the ZDC signal and 0.3 Hz on the PD.highest correlation coefficient rises to 0.98. This makesit very unlikely that some other source than the collisionscould be responsible for the signals.This becomes even clearer from Van der Meer scansin which the beam orbits are scanned transversely acrosseach other and the luminosity variation is recorded asin [17]. Figure 5 compares the ZDC rates with the sig-nal of one of the PDs as a function of the orbit bumpamplitude.Since the data were taken parasitically during normalcolliding-beam operation, the PD signals were often pol-luted by other losses, e.g., from collimation. Thereforethe cleanest data sets with least interference were pickedout (near the beginning of each fill) and the averagedcount rate with background subtracted, normalized toluminosity, was calculated for each PD (see Fig. 3). Thebackground noise level was calculated for each PD as theaverage signal without luminosity.In the WPD, the maximum signal was recorded bythe PD at 141.6 m but moved to 140.5 m in the CPD.The count rates ranged from 1 to 20 Hz depending onPD position and luminosity. This agrees well with thesimulation. However, the maximum in the simulationcame from the PD at 138.6 m, which does not exactlyagree with the measurements. This means that, in re-ality, the second peak in the energy distribution outsidethe cryostat is actually higher than the first, while in thesimulation the first peak is the higher. As can be seen inFig. 1, the peak in the shower could also escape betweenthe PDs in the WPD. However, if the impact point istranslated within the 2 m error bar, the simulated max-imum moves further downstream and a fair agreementcan be found. Moreover, the PD signals themselves haveerror bars—apart from the statistical error a small rela-tive change in the counting efficiency between the PDscould change the result. This has not been measured sowe give no numerical estimate.One might hope to extract a value for the crosssection for BFPP between colliding copper ions at100 GeV/nucleon. However, because of the uncertain-ties in both the number of MIPs entering a specific diodein the simulation and also the recorded count rates, wecan only conclude from the good agreement that the the-oretical estimate of the BFPP cross section has the rightorder of magnitude.No signal was detectable at 31 GeV/nucleon, consis-tent with the much lower (L× σBFPP) (see Table I).In conclusion, we made the first measurements of thelocalized loss of a BFPP generated secondary beam in anion collider. These measurements were done with copperbeams at RHIC. We found the location of the maximumloss monitor signal at 140.5 m from the IP, within 2 m ofits calculated location, and the measured event rates inour PD detectors within a factor of 2 of calculated ones.The deviations between calculated and measured valuesare consistent with estimated errors. This is a valuabletest of our ability to make quantitative predictions of thiseffect for the LHC, where it is expected to be one of themost restrictive luminosity limits.We thank A. J. Baltz, R. Gupta, K. Hencken andT. Roser for help and fruitful discussions.[1] C. Bertulani, S. Klein, J. Nystrand, Ann. Rev. Nucl.Part. Sci. 55, 271 (2005).[2] A.J. Baltz, M.J. Rhoades-Brown, J. Weneser, Phys. Rev.E 54, 4233 (1996).[3] H. Gould, Lawrence Berkeley Laboratory Report No.LBL-18593 (1984).[4] S. Klein, Nucl. Inst. & Methods A 459, 51 (2001).[5] J.M. Jowett et al., Proc. of the European Particle Accel-erator Conf. 2004, Lucerne p. 578 (2004).[6] O.S. Bru¨ning et al. (Eds.), CERN Report CERN-2004-003-V1 (2004).[7] J.M. Jowett, R. Bruce, S.Gilardoni, Proc. of the ParticleAccelerator Conf. 2005, Knoxville p. 1306 (2005).[8] R. Bruce, S. Gilardoni, J.M. Jowett, LHC Project Note379, CERN (2006).[9] A. Belkacem et al., Phys. Rev. Letters 71, 1514 (1993).[10] H.F. Krause et al., Phys. Rev. Letters 80, 1190 (1998).[11] P. Grafstro¨m et al., Proc. of the Particle AcceleratorConf. 1999, New York p. 1671 (1999).[12] H. Meier et al., Phys. Rev. A 63, 032713 (2001).[13] F. Pilat et al., Proc. of the Particle Accelerator Conf.2005, Knoxville p. 4281 (2005).[14] A. Fasso et al., CERN Report CERN-2005-10 (2005).[15] A. Fasso et al., Proc. of the Computing in High Energyand Nuclear Physics 2003 Conf., La Jolla (2003).[16] S. Roesler, R. Engel, J. Ranft, SLAC Report SLAC-PUB-8740 (2000).[17] A. Drees et al., Proc. of the Particle Accelerator Conf.2003, Portland p. 1688 (2003).

First observations of beam losses due to bound-free pairproduction in a heavy-ion collider

We report the first observations of beam losses due to bound-free pair production at the  interaction point of a heavy-ion collider. This process is expected to be a major luminosity limit  for the Large Hadron Collider (LHC) when it operates with 208Pb82+ ions because the localized  energy deposition by the lost ions may quench superconducting magnet coils. Measurements were  performed at the Relativistic Heavy Ion Collider (RHIC) during operation with 100 GeV/nucleon  63Cu29+ ions. At RHIC, the rate, energy and magnetic field are low enough so that magnet quenching  is not an issue. The hadronic showers produced when the single-electron ions struck the RHIC  beampipe were observed using an array of photodiodes. The measurement confirms the order of  magnitude of the theoretical cross section previously calculated by others

eScholarship - University of California

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleFirst observations of beam losses due to bound-free pair production in a heavy-ion colliderPermalinkhttps://escholarship.org/uc/item/7x55698nAuthorsBruce, R.Jowett, J.M.Gilardoni, S.et al.Publication Date2008-06-20 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaarXiv:0706.2292v1  [physics.acc-ph]  15 Jun 2007First observations of beam losses due tobound-free pair production in a heavy-ion colliderR. Bruce,∗ J.M. Jowett, and S. GilardoniCERN, Geneva, SwitzerlandA. Drees, W. Fischer, and S. TepikianBNL, Upton, NY, USAS.R. KleinLBNL, Berkeley, CA, USA(Dated: June 15, 2007)We report the first observations of beam losses due to bound-free pair production at the interactionpoint of a heavy-ion collider. This process is expected to be a major luminosity limit for the LargeHadron Collider (LHC) when it operates with 208Pb82+ ions because the localized energy depositionby the lost ions may quench superconducting magnet coils. Measurements were performed at theRelativistic Heavy Ion Collider (RHIC) during operation with 100 GeV/nucleon 63Cu29+ ions. AtRHIC, the rate, energy and magnetic field are low enough so that magnet quenching is not anissue. The hadronic showers produced when the single-electron ions struck the RHIC beampipewere observed using an array of photodiodes. The measurement confirms the order of magnitude ofthe theoretical cross section previously calculated by others.PACS numbers: 29.20.Dh, 25.75.-qWhen fully-stripped heavy ions of atomic numbers Z1,Z2 are brought into collision at the interaction point (IP)of a collider, a number of electromagnetic interactions areinduced by the intense fields generated by the coherentaction of all the Z1,2 charges in either nucleus (for a re-view, see [1]). Some of these “ultra-peripheral” interac-tions have much higher cross sections than the hadronicnuclear interactions that are the main object of study.Among them, the bound-free pair production (BFPP),sometimes known as electron capture from pair produc-tion (ECPP), occurs when the virtual photon exchangedby the ions converts into a pair, and the electron is cre-ated in an atomic shell of one of the ions:Z1 + Z2γ−→ (Z1 + e−)1s1/2,... + Z2 + e+ (1)The resulting one-electron atoms have a slightly largermagnetic rigidity than the original bare nucleus. (Mag-netic rigidity is defined as p/(Qe) = Bρ for a particlewith momentum p and charge Qe that would have bend-ing radius ρ in a magnetic field B). Since the transverserecoil is very small, this “secondary beam” will emerge ata very small angle to the main beam. However it will bebent and focused less by the guiding magnetic elementsand may be lost somewhere in the collider ring.It has long been known that this process, together withelectromagnetic dissociation of the nuclei, could be a ma-jor contribution to the intensity and luminosity decay ofion colliders [2, 3]. It was realized more recently [4, 5]that, in certain conditions, the BFPP beam will be lost∗Also at MAXlab, Lund University, Sweden.; Electronic address:roderik.bruce@cern.chin a well-defined spot, initiating hadronic showers inthe vacuum envelope of the beam. The resulting local-ized heat deposition could induce quenches of supercon-ducting magnets. This has been extensively discussedelsewhere, particularly for the case of 2.76 TeV/nucleon208Pb82+ operation of the LHC at CERN [4, 5, 6, 7, 8].The consequent luminosity limit is expected to occur ata level close to the design performance. It is therefore vi-tal to test our quantitative understanding of the featuresof the BFPP process in order to ensure safe operationof the LHC, uninterrupted by lengthy quench-recoveryprocedures. BFPP has been measured in fixed target ex-periments [9, 10, 11] at lower energy but not, until now,in a collider. An opportunity arose during 63Cu29+ op-eration of RHIC at Brookhaven National Laboratory.The flux of ions in the secondary BFPP beam emergingfrom the interaction point is the product of the colliderluminosity and the BFPP cross section. The partial crosssection for electron capture for a given low-lying atomicbound state i on nucleus 1 has the approximate form [12]σi ≈ Z51Z22 (Ai log γcm + Bi) (2)where γcm is the Lorentz boost of the ions, in the centre-of-mass frame and Ai, Bi depend only weakly on Z. Thetotal cross section for BFPP to one of the colliding ionsis thenσBFPP =∑iσi (3)where the sum is over all atomic shells. Some numericalvalues from [12] are given in Table I. The cross sectionfor collisions of 100 GeV/nucleon 63Cu29+ was not cal-culated in [12] but we have estimated it as 0.2 barn by2TABLE I: BFPP cross sections, typical peak luminosity,BFPP rates and relative changes in magnetic rigidity at RHICand LHC. Values are taken directly where possible, or esti-mated by fitting sums of contributions of the form (2), to theinformation in [12]. δ = 1/(Z − 1) is the fractional deviationof the magnetic rigidity.σBFPP L/1027 BFPP δ(%)(barn) (cm−2s−1) rate (kHz)LHC Pb-Pb 281 1 281 1.22759 GeV/nucleonRHIC Au-Au 114 3 342 1.3100 GeV/nucleonRHIC Cu-Cu 0.2 20 2.9 3.6100 GeV/nucleonRHIC Cu-Cu 0.08 1 0.08 3.631 GeV/nucleoninterpolation of the data given in Fig. 7 of [12], scaledwith Z and log γcm according to Eq. (2). Applied toother tabulated values in [12], this method produces anagreement within 10%.The impact point of the lost ions is predicted by track-ing the orbit of the BFPP beam in the collider optics untilit intersects the physical aperture (vacuum pipe) of thebeam line. To lowest order in the uncorrelated betatronamplitudes, a, b, and relative magnetic rigidity deviationfrom the central value, δ, the horizontal displacementfrom the central orbit, at a distance s from the IP isx(s) = a√2β(s) cos (µ(s)) + b√2β(s) sin (µ(s)) + dx(s)δ.(4)Averaging over all ions gives the transverse emittanceof the beam: 〈a2 + b2〉 = ǫ; β(s) is the usual “Twissfunction” determined by the focusing properties of theaccelerator, µ(s) =∫ s0β(u)−1du is the betatron phaseand dx(s) is the locally generated dispersion (dx(0) =d′x(0) = 0) that encodes the spectrometer effect of thebending magnets.An ion following the central trajectory (i.e. in the mid-dle of the bunch) in the BFPP beam enters the IP witha = b = δ = 0 and emerges, still with a = b = 0, butδ = 1/(Z − 1). This ion will hit the beam pipe at thefirst location where the horizontal aperture Ax satisfiesAx(s) = dx(s)/(Z − 1).During 197Au79+ operation at RHIC, δ is not largeenough for this to occur. However, during 63Cu29+runs [13], the larger δ (Table I) meant that the ionsshould be lost in a well-defined location where they canbe detected directly. Thanks to the small cross section,there was no risk of quenching any superconducting ma-terials. Since the BFPP beam was not visible to theexisting detectors, beam loss monitors in the form ofPIN diodes (PDs) of the type Hamamatsu S3590 weremounted on the outside of the magnet cryostat down-stream of the IP of the PHENIX experiment. The PDshave a time resolution of 25 ns and the counts were readout every second. They were initially positioned in aCPDWPD132140144148s (m)0 20-20 x (cm)QuadrupoleDriftDipole 136FIG. 1: (color online) Left: The energy deposition from atypical shower shown in a thin slice in the x−z−plane throughthe geometry. The red arrow indicates the impact point andthe orange and purple arrows show the positions of the PDsin the CPD and WPD. Right: The 3D model of the geometryas implemented in FLUKA around the impact point.25 50 75 100 125 150s HmL-0.06-0.04-0.020.020.040.06x HmLFIG. 2: (color online) The horizontal projection of the 1 σenvelope of the BFPP beam and the nominal beam emergingfrom the PHENIX IP. The BFPP beam hits the inside of thevacuum chamber at 135.5 m.wide configuration (WPD), 3 m apart (see Fig. 1), andlater moved closer together (CPD), 0.5 m apart, aroundan observed count-rate maximum at 141.6 m.A linear interpolation of dx(s) between magnet ends,based on the design model of the collider, identifiedthe elements within which the BFPP beam was lost: a9.7 m long dipole magnet starting at s = 129.6 m, fol-lowed by a 2.12 m drift space and a quadrupole mag-net, shown in Fig 1. More precise tracking, includingchromatic effects (δ-dependences), and a final step of an-alytic orbit calculation inside the dipole, predicted animpact at si ≃ 135.5 m with an angle of incidence of〈x′(si)〉 ≃ 2.7 mrad. Tracking an assumed Gaussian dis-tribution of the amplitudes a and b in Eq. (4) gives thedistribution of impact momenta in the spot around thispoint. Fig. 2 shows the main 63Cu29+ beam togetherwith the 63Cu28+ BFPP beam propagating from the IP.To the extent that the RHIC hardware differs fromthe design model, the impact point may be shifted. Theglobal distortion of the central orbit by misalignments ofthe magnets is partially corrected by localized correctormagnets. A least-squares fit was made to orbit data frombeam position monitors (BPMs) and measured misalign-ments (of order 1–2 mm) of the quadrupole magnets in3the formx(s) = xβ(s) +∑jΘ (s − sj)××√β(s)β(sj) sin (µ(s) − µ(sj)) θj . (5)Here Θ(s) is the unit step function, xβ the first two termsin Eq. (4) and θj are the angular kicks from misalign-ments and correctors at locations sj . Unfortunately notenough data were available to make a satisfactory fit.Instead, the possible spread of the impact point was esti-mated by calculating the propagated error using Eq. (5).A possible error on the horizontal quadrupole misalign-ments of 1 mm was assumed, corresponding to the 95 %confidence interval of the measurement and the fact thatthe magnets may have moved between early 2005 whenthe BFPP measurement was done and late 2006 when themisalignments were measured. The absolute error on theBPM data was taken to be 0.5 mm, reflecting the limiteddata and possible non-reproducibility. Combining theseerrors gives an impact point of 135.5± 2 m. Keeping thenominal optics, but displacing the beam pipe by 1.4 mmat the impact point according to the measured misalign-ment of the dipole, moves the impact point to 136.0 m.Several distributions of BFPP ions at the PHENIXIP, centered on different initial offsets and angles, weretracked to obtain their impact coordinates on the insideof the vacuum pipe. These were fed into a 3D model (seeFig. 1) of the impact region geometry, the PDs and thedipole magnetic field, implemented in the FLUKA 2006.3Monte Carlo code [14, 15, 16], in order to simulate theshower in the magnet and particles emerging from thecryostat. This code has been benchmarked by others inthe relevant energy range [16].Within 2–2.5 m from the average impact point most ofthe shower inside the magnet has died out, although par-ticles outside propagate farther. This can be understoodthrough a rough estimate: The typical average nuclear in-teraction length for the superconducting coil and the ironcold bore is about 20 cm. Convoluted with the impactpoint spot size of about 1.8 m, this gives a total showerlength of about 2.8 m, assuming that the shower lastsfive interaction lengths. The maximum PD signal wasexpected about 2.5 m downstream of the impact point.When the shower is contained within the dipole, mov-ing the impact point along s only translates the shower.As the impact point approaches the end of the magnet,and more of the shower emerges into the void before thequadrupole, the profile changes qualitatively. A secondpeak in energy deposition appears at the entrance of thequadrupole and eventually exceeds the first one. Smallerangles of incidence cause the shower to go farther andthe peak in the second magnet starts earlier.The PDs count the number of minimum-ionizing par-ticles (MIPs) that pass their active area of 1 cm2. In thesimulation, the number of MIPs entering each PD wasrecorded, assuming a detection efficiency of 30%. Sincethe PDs are much more sensitive to particles entering120 125 130 135 140 145 150s HmL02.557.51012.515PDcountrateHHzLClose sim.Wide sim.Close meas.Wide meas.FIG. 3: (color online) The PD signals from both configu-rations together with simulation results, averaged over thefills and normalized to a typical average luminosity of 9.1 ×1027cm−2s−1. The simulation was produced from the centralBFPP orbit in the nominal lattice but with the beam pipemisaligned by 1.4 mm at the impact location according tomeasurements. Due to large statistical error bars, the sim-ulation gives only the order of magnitude to expect in themeasurement.0 2000 4000 6000 8000 10000 12000Time HsL2468101214PDcountrateHHzL5000.10000.15000.20000.25000.ZDCcountrateHHzLZDCPD 135.6 mPD 138.6 mPD 141.6 mFIG. 4: (color online). Count rates measured on the Zerodegree calorimeter (ZDC) luminosity monitors (black, rightscale) and the three PDs with the highest signal (colors, leftscale) during a store with the WPD. The data was binned in30 second intervals. A clear correlation between the luminos-ity and the PD count rates can be seen.through the side orthogonal to the cryostat, only theseparticles were counted. At the expected BFPP ion pro-duction rate of 2.9 kHz, and the 0.01 MIPs expected perPD per ion, pileup from multiple interactions was notan issue. Both WPD and CPD configurations were sim-ulated for various impact points and a typical result isshown together with measured data in Fig. 3.Data were collected from the PDs during 14 fills ofRHIC with 63Cu29+ ions. An example of the recordedsignal is shown in Fig. 4, together with the Zero DegreeCalorimeter (ZDC) count rate (which is directly propor-tional to the luminosity). The PD signals show a goodtemporal correlation with luminosity and are very local-ized along s, close to the predicted impact position. Thecorrelation coefficient between the PD at 140.5 m and theZDC is 0.7, while the more distant PDs have correlationsaround 0.2 for the raw data. With a 30 s binning the4-1.5-1-0.5 0 0.5 1 1.5bump amplitude HmmL0246810ZDCcountrateHkHzL-1.5 -1 -0.5 0 0.5 1 1.5bump amplitude HmmL00.250.50.7511.251.51.75PDcountrateHHzLFIG. 5: The ZDC count rate (proportional to luminosity, left)and background-subtracted PD count rate (right) vs. relativeorbit displacement between beams in a van der Meer scan. AGaussian fit of the form A exp(−(x−x0)2σ2x) is shown togetherwith the data. The two fits had σx = 0.49 mm and 0.44 mmand χ2/DOF = 0.23 and 0.20, assuming a measurement errorof 500 Hz on the ZDC signal and 0.3 Hz on the PD.highest correlation coefficient rises to 0.98. This makesit very unlikely that some other source than the collisionscould be responsible for the signals.This becomes even clearer from Van der Meer scansin which the beam orbits are scanned transversely acrosseach other and the luminosity variation is recorded asin [17]. Figure 5 compares the ZDC rates with the sig-nal of one of the PDs as a function of the orbit bumpamplitude.Since the data were taken parasitically during normalcolliding-beam operation, the PD signals were often pol-luted by other losses, e.g., from collimation. Thereforethe cleanest data sets with least interference were pickedout (near the beginning of each fill) and the averagedcount rate with background subtracted, normalized toluminosity, was calculated for each PD (see Fig. 3). Thebackground noise level was calculated for each PD as theaverage signal without luminosity.In the WPD, the maximum signal was recorded bythe PD at 141.6 m but moved to 140.5 m in the CPD.The count rates ranged from 1 to 20 Hz depending onPD position and luminosity. This agrees well with thesimulation. However, the maximum in the simulationcame from the PD at 138.6 m, which does not exactlyagree with the measurements. This means that, in re-ality, the second peak in the energy distribution outsidethe cryostat is actually higher than the first, while in thesimulation the first peak is the higher. As can be seen inFig. 1, the peak in the shower could also escape betweenthe PDs in the WPD. However, if the impact point istranslated within the 2 m error bar, the simulated max-imum moves further downstream and a fair agreementcan be found. Moreover, the PD signals themselves haveerror bars—apart from the statistical error a small rela-tive change in the counting efficiency between the PDscould change the result. This has not been measured sowe give no numerical estimate.One might hope to extract a value for the crosssection for BFPP between colliding copper ions at100 GeV/nucleon. However, because of the uncertain-ties in both the number of MIPs entering a specific diodein the simulation and also the recorded count rates, wecan only conclude from the good agreement that the the-oretical estimate of the BFPP cross section has the rightorder of magnitude.No signal was detectable at 31 GeV/nucleon, consis-tent with the much lower (L × σBFPP) (see Table I).In conclusion, we made the first measurements of thelocalized loss of a BFPP generated secondary beam in anion collider. These measurements were done with copperbeams at RHIC. We found the location of the maximumloss monitor signal at 140.5 m from the IP, within 2 m ofits calculated location, and the measured event rates inour PD detectors within a factor of 2 of calculated ones.The deviations between calculated and measured valuesare consistent with estimated errors. This is a valuabletest of our ability to make quantitative predictions of thiseffect for the LHC, where it is expected to be one of themost restrictive luminosity limits.We thank A. J. Baltz, R. Gupta, K. Hencken andT. Roser for help and fruitful discussions.[1] C. Bertulani, S. Klein, J. Nystrand, Ann. Rev. Nucl.Part. Sci. 55, 271 (2005).[2] A.J. Baltz, M.J. Rhoades-Brown, J. Weneser, Phys. Rev.E 54, 4233 (1996).[3] H. Gould, Lawrence Berkeley Laboratory Report No.LBL-18593 (1984).[4] S. Klein, Nucl. Inst. & Methods A 459, 51 (2001).[5] J.M. Jowett et al., Proc. of the European Particle Accel-erator Conf. 2004, Lucerne p. 578 (2004).[6] O.S. Brüning et al. (Eds.), CERN Report CERN-2004-003-V1 (2004).[7] J.M. Jowett, R. Bruce, S.Gilardoni, Proc. of the ParticleAccelerator Conf. 2005, Knoxville p. 1306 (2005).[8] R. Bruce, S. Gilardoni, J.M. Jowett, LHC Project Note379, CERN (2006).[9] A. Belkacem et al., Phys. Rev. Letters 71, 1514 (1993).[10] H.F. Krause et al., Phys. Rev. Letters 80, 1190 (1998).[11] P. Grafström et al., Proc. of the Particle AcceleratorConf. 1999, New York p. 1671 (1999).[12] H. Meier et al., Phys. Rev. A 63, 032713 (2001).[13] F. Pilat et al., Proc. of the Particle Accelerator Conf.2005, Knoxville p. 4281 (2005).[14] A. Fasso et al., CERN Report CERN-2005-10 (2005).[15] A. Fasso et al., Proc. of the Computing in High Energyand Nuclear Physics 2003 Conf., La Jolla (2003).[16] S. Roesler, R. Engel, J. Ranft, SLAC Report SLAC-PUB-8740 (2000).[17] A. Drees et al., Proc. of the Particle Accelerator Conf.2003, Portland p. 1688 (2003).

First observations of beam losses due to bound-free pair production in a heavy-ion 
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Observations of Beam Losses Due to Bound-Free Pair Production in a Heavy-Ion Collider

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First observations of beam losses due to bound-free pair production in a heavy-ion collider

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