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Cristina Biino INFN Torino

Mini-Workshop on Crystal Collimation CARE CERN – 8 March 2005. Interaction of particles with strong crystalline fields – Some application in high energy beam lines at CERN. Cristina Biino INFN Torino. Introduction. The application of coherent phenomena of high energy

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Cristina Biino INFN Torino

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  1. Mini-Workshop on Crystal Collimation CARE CERN – 8 March 2005 Interaction of particles with strong crystalline fields – Some application in high energy beam lines at CERN Cristina Biino INFN Torino

  2. Introduction The application of coherent phenomena of high energy particles in strong crystalline fields, to NA48, an experiment to measure the direct CP violation in the decay of neutral kaons, lead to a series of detailed studies at CERN beamlines. (NA43 collaboration - Aarhus, CERN, Firenze, Grenoble, MPI, Johannesburg, Strasbourg, Torino) In particular we studied : • channeling of high energy protons in bent crystals • pair production enhancement in aligned crystals. I present some of the results obtained. NA48 used successfully channeling in a Si crystal to split and deflect the proton beam and an Ir crystal to efficiently convert photons (minimizing multiple scattering). Cristina Biino

  3. Talk Summary • Channeling efficiency in Si crystals • Channeling measurements in Ge crystals • Radiation damage • Pair Production in W and Ir crystals • 33 TeV/c Pb ions deflection • NA48 applications: channeling and pair production Cristina Biino

  4. The experiment configuration for Si Schematic view of the experimental arrangement in the H8 microbeam at the CERN SPS. DC1 and DC2 are drift chambers to detect incoming and outgoing particle positions, SC1 and SC2 are trigger scintillators, SC3 is an anticounter installed to reduce background in DC2. The bent crystal is mounted on a two stage goniometer with 1.7 mrad stepsize in turn (horizontal) and tilt (vertical). Primary 450 GeV/c proton; Secondary mixed pion/proton of 200 GeV/c Cristina Biino

  5. SPS - H8 microbeam The SPS-H8 microbeam is a unique beam for channeling studies. It has an extremely small emittance, obtained by successive collimation of the primary beam. At the position of the crystal the beam size is around 2x1 mm2FWHM to be compared to the typical crystal size of 1x10x50 mm3. Furthermore the horizontal divergence is around 3mrad rms to be compared to p = 7 mrad for (110) Si at 450 GeV. The beam intensity is up to a few 106protons per burst. Cristina Biino

  6. A classical 3-point bender four solid state detectors on the crystal measured dE/dx for channeled particles Channeled positive particles experience less energy loss than non channeled ones. Cristina Biino

  7. The crystal alignment In order to align the crystalline planes with the proton beam, an angular scan using the goniometer turntable is performed. The count- rate in a window around the low dE/dx region is used as a sensitive signal for channeling. A typical scan is shown in this figure.The width of the curve is an indication of the critical angle for channeling (in this scan +/- 9.4 mrad) while the slopes are indicating that the beam divergence is small. Cristina Biino

  8. dE/dx spectra • Channeled positive particles experience less energy loss than non channeled ones. • The figure shows the dE/dx spectra as measured by a surface barrier detector on the straight upstream end of the crystal: • for a non aligned crystal the well-known Landau distribution is observed. • for the aligned crystal we observe two peaks : the higher dE/dx corresponds to particles which are not within the critical angle for channeling, while the lower dE/dx peak represents particles channeled. • (The ratio of particles in the peaks indicates that the 450 GeV beam was more parallel.) random 200 GeV/c aligned 450 GeV/c aligned Cristina Biino

  9. Results for Si (111) at 450 GeV The measured deflection efficiencies for450 GeV protonsare shown fordeflection angles ranging from 1.4 to 11.5 mrad. The experimental values are compared to calculations for a crystal with uniform curvature.Deflection efficiencies of up to 50%were observed for (111) planar channeling in agreement with theoretical calculations. Cristina Biino

  10. High-Z crystals: Ge(110) The equivalent magnetic field of a bent crystal increases linearly with Z of the crystal and in principle this should lead to: • a larger critical angle • smaller dechanneling losses An experiment with 450 GeV/c and with 200 GeV/c protons was performed in 1995 at CERN to investi- gate the deflection efficiency obtainable with a germanium crystal. Cristina Biino

  11. The experiment configuration for Ge Schematic view of the experimental arrangement in the H8 microbeam at the CERN SPS. DC1, DC2 and DC3 are drift chambers to observe beam profile. SC1 and SC2 (motorized), SC3 SC4 and SC5 used to define the beam and veto interactions with origin in the bending device; the scintillator hodoscope H1,H2 and H3 to detect the deflected beam, are scintillators installed about 5 m downstream of the crystal to measure the protons deflected by the crystal. Cristina Biino

  12. The crystal alignment In order to align the crystalline planes with the proton beam, an angular scan using the goniometer turntable is performed. The count- rate in the 3 H-counters is recor- ded and the bent beam is detec- ted as a function of the goniome- ter angle. A typical scan is shown in this figure.The width of the curve is an indication of the criti- cal angle for channeling (+/- 9.4 mrad in this scan) while the slopes are indicating that the beam divergence is small. Cristina Biino

  13. The horizontal beam profile in DC3 Straight and bent beam peaks as observed in the horizontal profile of drift-chamber DC3, 4.1 m downstream of the crystal. Results for the three different bending angles are shown. The region between the peaks contains protons which were initially channeled and later lost during the passage of the crystal (dechanneling). 8.1 mrad 4.6 mrad 2.4 mrad Cristina Biino

  14. Results for the bent Ge crystal 450 GeV/c The two data sets show different trends: the high energy result show a higher efficiency at low angles and a fast drop, while the lower energy data starts out at a not very high efficiency but drops slowly. 200 GeV/c The reason is probably related to the interplay of dechanneling lenght and crytical curvature. At low energy the dechanneling is comparatively low and the crytical curvature high; viceversa for the higher energy. Cristina Biino

  15. Radiation Damage • A crucial question for most applications is how sensitive is high energy proton channeling to proton irradiation. The irradiation must affect a significant fraction of the atoms encountered in one oscillation in the channel in order to reduce the deflection efficiency. • In 1996 an experiment was performed at CERN SPS where a (111) Si crystal (52x10x1mm3), used successfully as bent crystal to deflect 450 GeV protons in 1991,was irradiated in the T6 target station of the SPS during a full year (1992) Total dose received : 2.4 1020 protons/cm2 Cristina Biino

  16. The experiment configuration for irradiated Si Schematic view of the experimental arrangement in the H8 microbeam at the CERN SPS. DC1 and DC2 are drift chambers to detect incoming and outgoing particle positions, SC1 (hor. motorized) and SC2, SC3 are trigger scintillators, SC4 is an anti-counter installed to reduce background in DC2. The bent crystal is mounted on a goniometer with 1.7 mrad stepsize. H1,H2 and H3 are scintillators installed about 5 m downstream of the crystal to measure the protons deflected by the crystal. Cristina Biino

  17. Radiation Damage • In order to localize the beam impact on the irradiated crystal, it was exposed to a film about two years after irradiation in T6. Unfortunately, during the process the crystal broke into two pieces of 29 and 23 mm length, resulting in a strong anticlastic bending (a curvature perpendicular to the applied curvature) • Picture of contact radiography Cristina Biino

  18. Radiation Damage The reduction in deflec- tion efficiency, assuming linear dependence,corre- sponds to a deterioration coefficient of about : 6% / 1020 p/cm2 This means that NA48 could run up to 100 years in the intense proton beam before the crystal needs replacement. Cristina Biino

  19. Deflection of 33TeV Pb82+ Ions A 33 TeV beam of lead ions (beam div 50 mrad, 400 GeV per charge) is steered through 4 mrad using a bent crystal. About 15% of the beam particles are channeled and deflec- ted, as in the case of protons of an equivalent momentum. Cristina Biino

  20. NA48 – principle of simultaneous, nearly collinear KL and KS beams Cristina Biino

  21. NA48 – Schematic layout of KL and KS beams Cristina Biino

  22. NA48 – Layout of KL target and bent crystal zone Cristina Biino

  23. NA48 bending Silicon crystal • Advantages of using the crystal: • Deflects cleanly the proton beam in a very short length (equivalent to 14.4 TM) • Upstream muon sweeping action is not affected • Splits the desired beam fraction (about 5 10-5) • Garanties a sharply defined emittance of the outgoing beam in both hor. and vert. Planes. Cristina Biino

  24. NA48 bending Si crystal The NA48 beam design aim is to obtain two simultaneous Kaons beams, KL and KS, as collinear as possible. At the same time the intensity of the proton beam creating the KS beam has to be reduced substantially, while maintaining a low emittance. For this a a bent crystal has been implemented to deflect a small fraction of the incident particles upwards. Before this application, bent crystal channeling had always been done by using the straight part at the end of the crystal as entrance. In order to be able to vary the deflection angle in a well defined way without changing the curvature of the crystal, a novel deflection scheme using a fraction of the curved crystal by aiming at the side was chosen. In fact, using the side of the crystal actually turns out to be a virtue since in that case there is a coupling between the horizontal position and the vertical angle, such that the channeled beam that exits is well defined in both planes. The measured background of muons in the experiment is about a factor 10 lower that what would be expected from deflection in a magnet. Cristina Biino

  25. NA48 – detailed layout of the KS target station Cristina Biino

  26. Pair Production by a Photon in an oriented single crystal In crystalline materials, the electric field experienced by an incident photon are strongly dependent on the direction of the photon with respect to the crystal axis. This leads to a change in the pair production cross section, and the effective radiation lenght, if the crystal is aligned with the incident beam direction. This effect, coherent pair production, find an experimental applica- tion in the NA48 AKS converter. Photons should be efficiently detected by inducing pair production in the converter, but neutral kaons passing through the converter should be disturbed as little as possible. The aligned crystal used as a converter could be made with reduced thickness and still maintain the same conversion efficiency, whilst reducing the average scattering angle of the kaons. Cristina Biino

  27. The NA48 AKS counter • Defines the beginning of the decay region for p+p- and p0p0 KS decays and determines the energy scale. • Is made of plastic scintilla- tion counters following a photon converter: - alignediridium crystal 3mm thick (0.98 X0 of amorphous iridium but 1.79 X0 for the aligned crystal and 30% less scattering if compared to a 1.33 X0 lead converter). Cristina Biino

  28. Measurements of Pair Production by a Photon in an oriented single crystal The pair production enhancement in W and Ir have been measured in dedicated experiments in the H2 secondary beam line at CERN SPS for photon energies between 5 and 150 GeV and a variety of incident angles up to 10 mrad from the crystal axis. Cristina Biino

  29. The tagged photon experiment The electron beam, whose energy can be varied by up to a maximum of 150 GeV/c is incident onto a converter, consisting of a 1 mm thick sheet of copper (7% X0). This radiator induces bremsstrahlung. The parent electron is deflected away from the axis by a bending magnet. Its position before and after the magnet is measured in both coordinates by a set of drift chambers (DC3 and DC4). These measurements allow to calculate the energy of the bremsstrahlung photon, for energies up to 100 GeV. On either sides of DC4, helium bags are used to reduce the probability of the photon interacting before reaching the crystal. The W (Ir) crystal is mounted on the AKS goniometer with a scintillator veto counter immediately upstream, to veto photons which produced showers before reaching the crystal. Downstream of the crystal there are the AKS scintillator counters to measure the shower multeplicity. Finally the photon shower energy is again measured in a leadglass block. Events were recorded with the photon beam with the crystal axis set to a variety of angles with respect to the beam axis and along different planes. Events with multeplicity greater or equal to one in the AKS counter were flagged as conversions. Since the photons are tagged you can compute the probability of pair production dividing by the number of incident photons Cristina Biino

  30. The crystal orientation Orientation of the (100) and (110) planes of the W crystal with respect to the crystal axis (100). The octant between the strong and the weak plane has been mapped by the photon conversion experiment. In order to align the crystal with the electron beam, an angular scan using the AKS goniometer is performed. The increase in the count rate in a scintillator, normalized by the number of incident electrons, is used as an indication of enhanced Bremsstrahlung and Pair Production and therefore of photon beam alignment. A typical angular scan is shown in the figure. Cristina Biino

  31. Pair Production Enhancement in the W crystal • enhancement definition: • = -9/7 X0(amorphous) /X ln(1-pcrystal) X0(amorphous) is the radiation length X is the crystal thickness pcrystal is the probability of conversion in the crystal Cristina Biino

  32. Pair Production Enhancement in the Iridium crystal Cristina Biino

  33. Conclusions NA48 used successfully channeling in a Si crystal to split and deflect the proton beam and an Ir crystal to efficiently convert photons minimizing multiple scattering. The crystal setting were very reliable and stable in time. NA48 achived the measurement of direct CP Violation with the precision of 2 10-4 Cristina Biino

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