For the first time, the deflection of an ultra-relativistic, fully stripped Pb82+ ion beam in a bent silicon crystal has been observed. The ions were provided by the CERN-SPS in the H4 beam at a momentum of 400 GeV/c per unit of charge. A 60 mm long silicon crystal, bent over 50 mm to give a 4 mrad deflection angle, was used in this experiment. The measured Pb ion deflection efficiency is comparable to the one obtained with protons at an equivalent ratio of momentum per charge, and is found to be about 15\% for a beam with a divergence of 35 microradians (FWHM). The interaction rate observed in a background counter is found to drop when the crystal is well aligned with the beam. This corroborates further the channeling model, which predicts that channeled ions are steered away from regions of high electron densities as well as the nuclei in the crystal

Biino, C

Clément, M

Doble, Niels T

Elsener, K

Gatignon, L

Grafström, P

Keppler, P

Major, J V

Mikkelsen, U

Møller, S P

Taratin, A M

Uggerhøj, Erik

CERN Document Server

First Observation of the Deflection of a 33 TeV Pb Ion Beamin a Bent Silicon CrystalC. Biino, M. Cle´ment, N. Doble, K. Elsener, L. Gatignon, P. Grafstro¨m,U. Mikkelsen (CERN, Geneva, Switzerland), A. Taratin (JINR, Dubna, Russia)S.P. Møller, E. Uggerhøj (ISA, Aarhus University, Denmark)P. Keppler, J. Major (MPI fu¨r Metallforschung, Stuttgart, Germany)AbstractFor the first time, the deflection of an ultra-relativistic, fullystripped Pb82+ ion beam in a bent silicon crystal has beenobserved. The ions were provided by the CERN-SPS in theH4 beam at a momentum of 400 GeV/c per unit of charge.A 60 mm long silicon crystal, bent over 50 mm to give a4 mrad deflection angle, was used in this experiment. Themeasured Pb ion deflection efficiency is comparable to theone obtained with protons at an equivalent ratio of momen-tum per charge, and is found to be about 15% for a beamwith a divergence of 35 microradians (FWHM). The inter-action rate observed in a background counter is found todrop when the crystal is well aligned with the beam. Thiscorroborates further the channeling model, which predictsthat channeled ions are steered away from regions of highelectron densities as well as the nuclei in the crystal.1 INTRODUCTIONDuring the past ten years, the deflection of high energy pro-ton beams by planar channeling in bent silicon crystals hasbeen intensively studied and this technique is now being ap-plied routinely at accelerator laboratories around the world[1], [2], [3], [4]. Planar channeling, the steering of particlesby the collective fields of crystalline planes, takes placewhen particles enter the crystal within a small angle (theso-called critical angle) to these planes. For small bend-ing angles, channeling persists throughout the full crystallength.Apart from the attraction of a bent crystal as a cheap andeasy way to extract and split particle beams, it has been as-tonishing to see how well the channeling models, originallydeveloped for MeV particles, still apply at the highest en-ergies available at proton accelerators today, i.e. up to 900GeV. Recently, the validity of the models for planar deflec-tion were also tested in a different material, i.e. a germa-nium crystal, and very good agreement was found [5]. Ions,on the other hand, have hardly been used in connection withbent crystals at accelerators, the only exception being anearly test with 53 GeV/c C6+ nuclei at Dubna [6], wherea deflection efficiency could not be measured. Predictionsfor bent crystal assisted beam extraction from the heavy ionaccelerators in Dubna and Brookhaven are available sincea few years [7]. With the availability of the ultrarelativisticPb ion beam at CERN, it was thus of particular interest totry and deflect such a very high energy beam with a bentcrystal.The fully stripped 33 TeV Pb ions challenge the exper-imenter further due to the high background (-electrons,pions, etc.) observed wherever such ions pass materialor, worse, are stopped in a beam-line aperture limitation.Special precaution has therefore to be taken to identify Pbions in the detectors and to distinguish them from lighterparticles with lower charge leading to background events.The high charge state, however, provides one with a verystrong signal from Pb ions passing the detectors based onenergy loss or scintillation. While many experiments turnto Cerenkov counters for clean Pb ion identification, in thepresent case standard scintillation counters were used withphotomultipliers run at a strongly reduced gain comparedto that used for singly charged relativistic particles.On the other hand, the large number of interaction prod-ucts from Pb ions passing through material allows to testone of the hypothesis’ of channeling theory: Positivelycharged channeled particles are steered through the crystalat large impact parameters, i.e. away from the lattice nu-clei, and channeled ions should thus experience a reducedinteraction rate compared to non-channeled ions.2 EXPERIMENTThe experiment was carried out in the H4 beam line in theNorth Area of the CERN-SPS. The beam was calculatedto be parallel in the vertical plane, in which the deflectionby the bent crystal should take place. The experiment wasperformed in three phases: (a) In order to verify the experi-mental arrangement and test the detectors, a proton deflec-tion experiment with a 450 GeV/c beam from the SPS wasperformed prior to the Pb ion run. This test gave confidencethat the bent crystal deflected protons as efficiently as ex-pected from the experience acquired over the past years.(b) The 208Pb82+ ions with a momentum of 400 GeV/c percharge, thus 32.8 TeV/c, were used in a first option withmore generous collimator settings, resulting in a wider ver-tical angular distribution with a FWHM of 50 rad, verysimilar to the beam divergence in the proton run. With thisbeam, the first Pb ion deflection experiment was performed.(c) Finally, the same Pb ion beam, but with more restrictivecollimator settings, was found to have a vertical angulardistribution of 35 rad (FWHM), leading to a higher de-deflected   beamH1H2H3    incident33 TeV Pb ionsSC1goniometer with(110) Si crystal   4.66 mSCbgFigure 1: Schematic view of the experimental arrangement.Pb ions are deflected vertically by the bent silicon crystal.Scintillators Sc1 and H1, H2, H3 are tuned to detect Pb82+ions, while SCbg is tuned for background particles.flection efficiency (see Table 1 below).The experimental arrangement is schematically shown inFigure 1. The location in the H4 beam, equipped with a go-niometer on an x-y translational table, was originally pre-pared for tests of proton beam splitting in a bent crystal inview of the NA48 CP violation experiment [1]. The crucialmovement of the goniometer for the present experiment,i.e. the vertical rotation used to align the (110) planes ofthe crystal with the beam, has a step-size of 35 rad, some-what large compared to the critical angle for channeling at400 GeV/c/Z momentum, which is 8 rad, but similar tothe Pb ion beam divergence.The Pb82+ ions were steered onto the entrance face ofthe 1.5 mm thick crystal by aiming at a small scintillator(SC1, 2 mm high, 5 mm thick and 7 mm wide) installedupstream of it. The crystal, 60 mm long in beam direc-tion and 18 mm wide, was bent over 50 mm of its lengthin a new version of the now ’classical’ three-point bendingdevice, which is described in [5]. The resulting bend an-gle for this experiment was 4 mrad. The deflected Pb ions-1.0 -0.9 -0.8 -0.7 -0.6 -0.501angle [mrad]deflected Pb82+ [arb. units]0.5Figure 2: Measurement of the Pb ion vertical beam diver-gence. Good agreement is found between an angular scanwith the crystal (dots) and a scan of the beam angle usingsteering magnets (+).were detected in a set of three scintillators (’hodoscope’:H1, H2, H3), each 10*10 mm2 in size and 5 mm thick inbeam direction, installed on a motorised support. The cen-tral scintillator, H2, overlaps with H1 and H3 by about 2mm in the vertical (deflection) plane. The high voltage onthe photomultipliers of these scintillation counters was re-duced and discriminator thresholds adjusted well above thedetection level for minimum ionising particles (typical H.Tvalues were -1650 V for protons, -1250 V for Pb ions). Anadditional scintillation counter (SCbg), 100*100 mm2 insize and 10 mm thick, was installed outside the beam in or-der to detect the interactions produced by lead ions passingthrough the crystal. Thus, its high-voltage was adjusted toa level just below the one used for minimum ionising par-ticles. The beam divergence of the incident Pb ions wasdetermined in two ways: (a) by measuring the intensity ofthe deflected Pb ion beam for varying goniometer angles,(b) by varying the incident beam angle with a combina-tion of two vertical steering elements (TRIMs), located 42and 29 meters upstream of the crystal. These magnets al-lowed to change the incident angle without changing thebeam position at the crystal. Typical results for both typesof angular scans are shown overlayed in figure 2 for the firstexperiments with a larger beam divergence. The agreementbetween the two methods is excellent, and the divergencein this case is found to be 50 rad (FWHM).    0.000.100.200.300.400.50   -55 -45 -35 -250.000.100.200.300.40 hodoscope position [mm]normalized count rate H1*H2*SC1Figure 3: Vertical scan of the hodoscope (stepsize 1 mm)for the more divergent (’wide’) Pb ion beam. For a non-aligned crystal (top), the straight beam peak is visible. Itsintensity is reduced for the well-aligned crystal (bottom),and an additional peak of upwards deflected Pb ions is visi-ble. (Note: The integrated number of counts is very similarin the two plots.)3 RESULTSThe undeflected and deflected Pb ions downstream of thecrystal can be detected in a given hodoscope scintillatorby scanning the hodoscope vertically through the beams.The best position resolution (about 2 mm) in such a scanis obtained when the coincidence between two overlappingscintillation counters, e.g. H1*H2, is used. The result ofsuch a scan, for the case of the larger Pb beam divergence,is shown in Figure 3. The top of this figure shows the scanfor a non-aligned crystal, while the bottom shows the re-sult for a well-aligned crystal. In Figure 4, the same result-55 -50 -45 -40 -35 -30 -250.000.100.200.300.400.50H1*H2*Sc1/Sc1normalized count ratehodoscope position [mm]Figure 4: Vertical scan of the hodoscope (stepsize 0.5 mm)for the less divergent (’narrow’) Pb ion beam. The coin-cidence rate in the overlapping counters H1*H2, for ionswhich passed through SC1, is measured and normalised tothe incoming beam rate. The undeflected beam is shownat -49 mm, 19 mm lower than the Pb ions deflected by thecrystal. Some ions, visible between the two peaks, were in-tially channeled and later lost in the bent part of the crystal.is shown with smaller step-size for the narrow angular dis-tribution of the incident Pb ion beam. Here, the straightand deflected Pb ions are clearly visible, and a region ofpartially deflected (i.e. initially channeled and then lost)ions can also be seen. Not visible in Figure 3 and 4 is theexcellent background rejection of the counters: below thestraight as well as above the bent beam, zero counts havebeen recorded.Analysing the integrated count rates in such a scan, thedeflection efficiency can be deduced as the ratio betweenthe deflected and the incident Pb ions. Results are sum-marized in Table 1. For the undeflected beam peak, a cor-rection concerning the interactions in the crystal has to beapplied to the integral found in the hodoscope scan. Thesize of this correction in the present case can be estimatedfrom the observation that only 65% of the ions counted inSC1 are detected in SC1*H2, downstream of the crystal, forthe non-aligned case (cf. upper part of Fig.3). The remain-ing systematic errors in the efficiency analysis stem fromtwo sources: (1) The efficiency may be underestimated dueto the fact that SC1 is thicker than the crystal (2 vs. 1.5mm), and the beam is wider than both, FWHM about 3.5mm. The results presented here have been corrected forthis effect. The uncertainty introduced in the efficienciesis estimated to be 1%. (2) The efficiency may be wronglyestimated depending on the cut that is chosen around thebent beam peak in Fig. 3 and 4. (A report comparing themeasured deflection efficiencies for Pb ions with those cal-culated by A. Taratin’s simulations is in preparation.)The observed interaction rate in the ’background’counter SCbg (cf. Fig. 1) is found to drop by about 15%for a well-aligned crystal (at the peak in Fig. 2), i.e. wherethe largest fraction of Pb ions is channeled and deflected bythe crystal. This observation is in agreement with the ex-pectation based on channeling theory: well channeled ionsare steered through the crystal far away from the nuclei,and the probability for interactions is thus reduced. Theresults obtained with the ’background’ counter during an-gular scans will be reported in more detail elsewhere.Pb beam divergence deflection efficiency’wide’ beam 50 rad 82 %’narrow’ beam 35 rad 142 %Table 1: Measured deflection efficiencies for the 33 TeVPb ions. Statistical errors are negligible; systematic errorsare given. For more details, see text.4 CONCLUSIONThe present experiment has, for the first time, shown thefeasibility of deflecting a very high energy, fully strippedPb ion beam by means of a bent crystal. The experimentalresults are corroborating the channeling model widely usedfor protons at energies from MeV to TeV. In addition, thedeflection efficiency for these ions is comparable to the onefor protons, assuming an equivalent beam emittance andcrystal geometry. This is all the more positive as one mighthave feared a strong loss of ions due to interactions with theatoms in the crystal lattice. The present results can be seenas a first, encouraging step towards extracted ion beamsat the Nuclotron (Dubna) and at RHIC (Brookhaven), asproposed by [7].5 REFERENCES[1] N. Doble, P. Grafstro¨m and L. Gatignon, Nucl Instr. Meth.B119, 181 (1996)[2] R.A. Carrigan, Jr et al., Nucl. Phys. B163, 1 (1980)[3] V.M. Biryukov et al., Physics-Uspekhi 37, 937 (1994)[4] K. Elsener et al., Nucl Instr. Meth. B119, 215 (1996)[5] C. Biino et al., CERN-SL/97-05(EA), Phys. Lett. B, in print.[6] L.I. Belzer et al., JETP Lett., Vol. 46, 381 (1988)[7] A.D. Kovalenko, A.M. Taratin, E.N. Tsyganov, Preprint JINRE1-92-8, and A.D. Kovalenko et al., JINR Rap. Comm.No.4(72)-1995

First observation of the deflection of a 33 TeV Pb ion beam in a bent silicon crystal

First observation of the deflection of a 33 TeV Pb ion beam in a bent silicon crystal

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