CERN EP-TA1-SD Seminar 14.2.2001 Radiation Damage in Silicon Detectors - An introduction for non-specialists - Michael Moll CERN EP - Geneva ROSE Collaboration (CERN RD48) ROSE - Research and Development on Silicon Detectors for Future Experiments http://www.cern.ch/rd48
Outline Motivation - The challenge of LHC-experiments Radiation induced changes in silicon detector properties Increase of leakage current Change of effective doping concentration (depletion voltage) Decrease of charge collection efficiency Radiation hard silicon detectors Oxygen enriched silicon Damage projections for LHC operation Microscopic defects in silicon Relation between microscopic and macroscopic properties of the damage Summary and Outlook Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-2 -
Motivation Promising new physical results are related to some very rarely produced particles High event rate (10 9 /s at LHC), very good spatial resolution and fast signal read out required, can be fulfilled with silicon detectors, however: Detectors and electronics will be harshly irradiated! ATLAS - Inner Detector: Φ eq up to 3 10 14 cm -2 per operational year 10 years of operation have to be guaranteed 3x10 14 cm -2 Φeq [cm -2 ] 10 15 10 14 10 13 10 12 ATLAS - Inner Detector Pixel Pixel SCT - barrel SCT - barrel total Φ eq neutrons Φ eq pions Φ eq other charged hadrons Φ eq What is the impact on silicon detectors? 0 10 20 30 40 50 60 R [cm] How can we make silicon radiation harder? Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-3 -
PKA - Primary Knock on Atom Primary Damage Simulation (Fig.: van Lint 1980) 50 KeV PKA (average recoil energy for PKA produced by 1 MeV neutrons) Displacement threshold in Silicon: Single lattice atom (Frenkel pair): E d» 25 ev Defect cluster E C» 5 kev Neutrons (elastic scattering) E n > 185 ev for single displacement E n > 35 kev for cluster Electrons E e > 255 kev for single displacement E e > 8 MeV for cluster 60Co-gammas Compton Electrons with max. E g»1 MeV (no cluster production) Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-4 -
D(E) / (95 MeV mb) 10 4 10 3 10 2 10 1 10 0 10-1 10-2 10-3 10-4 Displacement damage functions 4 2 0.8 1 0.6 0.4 neutrons pions protons 10 0 10 1 10 2 10 3 10 4 neutrons protons electrons pions For Silicon: 100 MeVmb = 2.14 kevcm 2 /g 10-5 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 particle energy [MeV] NIEL - Non Ionizing Energy Loss NIEL - Hypothesis: Damage parameters scale with the NIEL (Be careful, does not hold for all particles / damage parameters, see later) Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-5 -
Testing Structures - Simple Diodes Example: Test structure from ITE Very simple structures in order to concentrate on the bulk features Typical thickness: 300µm Typical active area: 0.5 0.5 cm 2 Openings in front and back contact optical experiments with lasers or LED Different producers used (by the ROSE Collaboration) ITE (Warsaw), Sintef(Oslo), MPI-Halbleiterlabor Munich, ELMA (Moscow), Canberra (Olen,Belgium), Diotec (Radosina, Slovakia), Schottky-diodes (Hamburg), ST Microelectronics (Catania, Italia), CIS (Erfurt) Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-6 -
Irradiation Standard Measurements MeV neutrons Reactor, Be(d,n) or T(d,n) 10MeV, 23 GeV protons 192 MeV pions 60 60 Co-gammas Measurements Extracted Parameters C/V at room temperature depletion voltage U dep N eff I/V at room temperature current at depletion voltage I(U dep ) DLTS (Deep Level Transient Spectroscopy) microscopic defect parameters TSC (Thermally Stimulated Currents) microscopic defect parameters Annealing Experiments Isothermal annealing studies at 20 C 120 C Isochronal annealing studies 20 C 400 C Temperature dependent modeling Damage Scenarios for HEP-Experiments Understanding of microscopic defect kinetics Defect Engineering Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-7 -
Radiation induced changes in detector properties Change of depletion voltage Due to defect levels that are charged in the depleted region most problematic! Increase of leakage current Bulk current due to generation/recombination levels Decrease of charge collection efficiency Due to damage induced trapping centers Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-8 -
ST Microelectronics - standard diodes Different orientations (<111> and <100>) and resistivities CV measurements before irradiation Capacitance [pf] Standard silicon : ST - Microelectronics test structures before irradiation 10 2 5 10 1 W333-A3 (GR3477) : <111> 1K W330-A3 (GR3476) : <100> 1K W336-A3 (GR3474) : <111> 2K W335-A3 (GR3473) : <100> 2K W339-A4 (GR3475) : <111> >5K M.M oll 01-S GS -S TD -CV -BEF OR E-IRR 1 5 10 50 100 500 bias voltage[v] Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-9 -
N eff N eff - effective doping concentration U dep [V] (d = 300µm) V dep d 2 5000 1000 500 100 50 10 5 N eff positive - n-type silicon (e.g. Phosphorus doped - Donor) N eff negative - p-type silicon (e.g. Boron doped - Acceptor) N eff proportional to depletion voltage and 1/(device thickness) 2 n - type type inversion "p - type" 10 14 cm -2 1 10-1 10 0 10 1 10 2 10 3 Φ eq [ 10 12 cm -2 ] 600 V 10 3 10 2 10 1 10 0 10-1 N eff [ 10 11 cm -3 ] [Data from R. Wunstorf 92] Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-10 -
Systematic analysis of annealing data Example: oxygen enriched silicon after 24 GeV/c proton irradiation Instead of depletion voltage plot the change in the effective space charge DN eff N ( Φ, t) = N N ( t ) 0 Φ eff eq eff eff eq, 2.0. 10 13 24 GeV/c proton irradiation 2.0. 10 13 N eff [1/cm 3 ] 1.5. 10 13 1.0. 10 13 10.3. 10 14 p/cm 2 6.2. 10 14 p/cm 2 4.2. 10 14 p/cm 2 3.4. 10 14 p/cm 2 2.0. 10 14 p/cm 2 1.2. 10 14 p/cm 2 5.9. 10 13 p/cm 2 2.9. 10 13 p/cm 2 1.6. 10 13 p/cm 2 1.5. 10 13 1.0. 10 13 5.0. 10 12 5.0. 10 12 0.0 5 10 1 5 10 2 5 10 3 5 10 4 5 10 5 annealing time at 60 o C [min] 0.0 Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-11 -
N eff [10 11 cm -3 ] N 10 8 6 4 2 Annealing behavior of N eff - Hamburg model - ( Φ, t) = N 0 N ( Φ t ) eff eq eff eff eq, N a = g a Φ eq N eff = Change of N eff with respect to N eff 0 (value before irradiation) g C Φ eq N Y, = g Y Φ eq N C N C0 long term reverse annealing: 1 N Y = NY, 1 1+ t /τ y second order parameterization (with N y, =g y Φ eq ).gives best fit But: τ y independent of Φ eq underlying defect reaction based on first order process! 0 1 10 100 1000 10000 annealing time at 60 o C [min] N ( Φ, t) = N ( Φ, t) + N ( Φ ) + N ( Φ t) eff eq a eq C eq Y eq, short term annealing: N a = Φ eq gai exp t τ i i first order decay of acceptors introduced proportional to Φ eq during irradiation stable damage: N C CO ( 1 ( c Φeq )) + gc Φeq = N exp incomplete donor removal + introduction of stable acceptors Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-12 -
Increase of Leakage Current I / V [A/cm 3 ] 10-1 10-2 10-3 10-4 10-5 10-6 n-type FZ - 7 to 25 KΩcm n-type FZ - 7 KΩcm n-type FZ - 4 KΩcm n-type FZ - 3 KΩcm p-type EPI - 2 and 4 KΩcm n-type FZ - 780 Ωcm n-type FZ - 410 Ωcm n-type FZ - 130 Ωcm n-type FZ - 110 Ωcm n-type CZ - 140 Ωcm p-type E PI - 380 Ωcm 10 11 10 12 10 13 10 14 10 15 Φ eq [cm -2 ] Increase of leakage current independent of: Conduction type (p or n), resistivity oxygen and carbon content crystal orientation <111>, <100> Temperature dependence: E g I exp 2 k BT cooling strongly reduces current α [10-17 A/cm] 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 n-type FZ - 10 to 20 KΩcm n-type FZ - 7 KΩcm n-type FZ - 4 KΩcm n-type FZ - 3 KΩcm p-type EPI - 2 and 4 KΩcm n-type FZ - 780 Ωcm n-type FZ - 420 Ωcm n-type FZ - 130 Ωcm n-type FZ - 110 Ωcm n-type CZ - 140 Ωcm p-type EPI - 380 Ωcm 10 11 10 12 10 13 10 14 10 15 Φ eq [cm -2 ] Damage parameter a definition: I α = V Φ measured after 80min at 60C: a 80/60 = (3.99 ± 0.03)10-17 A/cm used for fluence (NIEL) calibration Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-13 - eq
Annealing of leakage current 10 1 hour 1day 1 month 1 year α [ 10-17 A/cm ] 8 6 4 2 0 W unstorf (92) α =2.9x10-17 A/cm 106 o C 21 o C 49 o C 60 o C 80 o C 10 1 10 2 10 3 10 4 10 5 10 6 time [ min ] Annealing at RT (21 C): good agreement with previous parameterization (Wunstorf 92, parameters for non inverted detectors) Annealing at higher temperature (long term at RT): new parameterization: (α ) ( t / τ ( T) ) + ( α β ln( ( T) t / )) α( T, t) = α1 exp 1 0 θ t0 exponential term: activation energy: E A = 1,11eV, ν = 1.2 10 13 s -1 correlated with defect at E C -0.46eV (DLTS) logarithmic term: acceleration factor θ(t) exp(1.3 ev / k B T) no saturation value! (no a ) Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-14 -
The ROSE Collaboration CERN-RD48 (R&D On Silicon for future Experiments) 1995: formed by the Co-Spokespersons: F.Lemeilleur (CERN, now retired), G.Lindström (Hamburg, now retired), S.Watts (Brunel,London) 2000: Final report to LEB - Oxygenated silicon will be used for the pixel detectors of ATLAS/CMS 38 HEP- and Semiconductor groups (125 persons), 7 associated companies and many observers For detailed information: http://cern.ch/rd48 Objectives: develop radiation hard detectors which ensure operation for the whole lifetime of the Large Hadron Collider (LHC) experimental program interact with industry both for growing and defect engineering the appropriate detector grade material as for processing diodes in the most optimal way exchange know how with the LHC experiments both as to the experimental demands as to recommendations from results of the ROSE-studies Standard and new material Macroscopic parameters Defect engineering strategy : Material characterization (PL, IR, DLTS, SIMS,..) Defect kinetics (Need [O],[C],[P],[B],[Sn],..) Defect characterization (DLTS, TSC, PL, TCT,..) Device model Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-15 -
Oxygen and Carbon - the key ingredients - 10 18 Polovodice CZ OFZ/JET ITME Oxygen Concentration - [O] (cm -3 ) 10 17 10 16 MACOM Epitaxial OFZ/Gas DOFZ Polovodice CFZ Polovodice ITME Epitaxial Two growth rates Standard FZ 10 15 10 15 10 16 10 17 Carbon concentration - [C] (cm -3 ) DOFZ - Diffusion Oxygenated Float Zone Silicon Controlled introduction of oxygen, easily included in manufacturing process, low cost Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-16 -
Oxygen diffusion 5 [O] [atoms/cm 3 ] 10 17 5 9d 1200 o C 12h 1100 o C 6h 1100 o C 10 16 0 30 60 90 120 150 depth [µm] DOFZ - Diffusion Oxygenated Float Zone Profiles measured by SIMS (Secondary Ion Emission Spectroscopy) Open question: Which is the minimum diffusion time/ oxygen concentration needed to get the beneficial effect? Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-17 -
Leakage Current Annealing - Oxygenated Silicon I / V [A/cm 3 ] 10-1 10-2 10-3 10-4 10-5 10-6 n-type FZ - 7 to 25 KΩcm n-type FZ - 7 KΩcm n-type FZ - 4 KΩcm n-type FZ - 3 KΩcm p-type EPI - 2 and 4 KΩcm 80 min 60 C n-type FZ - 780 Ωcm n-type FZ - 410 Ωcm n-type FZ - 130 Ωcm n-type FZ - 110 Ωcm n-type CZ - 140 Ωcm p-type EPI - 380 Ωcm 10 11 10 12 10 13 10 14 10 15 Φ eq [cm -2 ] α(t) [10-17 A/cm] 6 5 4 3 2 1 80 min 60 C oxygen enriched silicon [O] = 2. 10 17 cm -3 parameterisation for standard silicon 0 10 0 5 10 1 5 10 2 5 10 3 5 10 4 5 10 5 annealing time at 60 o C [minutes] 6 5 4 3 2 1 Damage parameter a: α = independent of F eq, used for fluence (NIEL) calibration Oxygenated and Standard Silicon show same annealing V I Φ eq Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-18 -
Influence of Carbon and Oxygen concentration 24 GeV/c proton irradiation N eff [cm -3 ] 1E+13 9E+12 8E+12 7E+12 6E+12 5E+12 4E+12 3E+12 2E+12 1E+12 0 Carbonated Standard (P51) O-diffusion 24 hours (P52) O-diffusion 48 hours (P54) O-diffusion 72 hours (P56) Carbon-enriched (P503) Compared to standard silicon: b [C] = 0.0437 b St = 0.0154 b [O] = 0.0044 0.0053 0 0 1E+14 2E+14 3E+14 4E+14 5E+14 Proton fluence (24 GeV/c ) [cm -2 ] Standard High Carbon less radiation tolerant High Oxygen more radiation tolerant Oxygenated 500 400 300 200 100 V FD for 300 mm thick detector [V] Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-19 -
Oxygen and standard silicon - Particle dependence - 23 GeV protons - 192 MeV pions - reactor neutrons N eff [10 12 cm -3 ] 7 6 5 4 3 2 1 standard FZ neutrons pions protons oxygen rich FZ neutrons pions protons 0 0.5 1 1.5 2 2.5 3 3.5 Φ eq [10 14 cm -2 ] 400 300 200 100 Strong improvement for pions and protons Almost no improvement for neutrons V dep [V] (300µm) Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-20 -
N eff [10 11 cm -3 ] N 10 8 6 4 2 Annealing behavior of N eff - Hamburg model - ( Φ, t) = N 0 N ( Φ t ) eff eq eff eff eq, N a = g a Φ eq N eff = Change of N eff with respect to N eff 0 (value before irradiation) g C Φ eq N Y, = g Y Φ eq N C N C0 long term reverse annealing: 1 N Y = NY, 1 1+ t /τ y second order parameterization (with N y, =g y Φ eq ).gives best fit But: τ y independent of Φ eq underlying defect reaction based on first order process! 0 1 10 100 1000 10000 annealing time at 60 o C [min] N ( Φ, t) = N ( Φ, t) + N ( Φ ) + N ( Φ t) eff eq a eq C eq Y eq, short term annealing: N a = Φ eq gai exp t τ i i first order decay of acceptors introduced proportional to Φ eq during irradiation stable damage: N C CO ( 1 ( c Φeq )) + gc Φeq = N exp incomplete donor removal + introduction of stable acceptors Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-21 -
Systematic analysis of annealing data Example: oxygen enriched silicon after 24 GeV/c proton irradiation Instead of depletion voltage plot the change in the effective space charge DN eff N ( Φ, t) = N N ( t ) 0 Φ eff eq eff eff eq, 2.0. 10 13 24 GeV/c proton irradiation 2.0. 10 13 N eff [1/cm 3 ] 1.5. 10 13 1.0. 10 13 10.3. 10 14 p/cm 2 6.2. 10 14 p/cm 2 4.2. 10 14 p/cm 2 3.4. 10 14 p/cm 2 2.0. 10 14 p/cm 2 1.2. 10 14 p/cm 2 5.9. 10 13 p/cm 2 2.9. 10 13 p/cm 2 1.6. 10 13 p/cm 2 1.5. 10 13 1.0. 10 13 5.0. 10 12 5.0. 10 12 0.0 5 10 1 5 10 2 5 10 3 5 10 4 5 10 5 annealing time at 60 o C [min] 0.0 Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-22 -
Reverse annealing amplitude N Y Reverse annealing time constant τ Y Extraction of damage parameters N Y [10 13 cm -3 ] 2.5 2 1.5 1 0.5 0 standard FZ oxygenated FZ 24 GeV/c proton irradiation 0 1 2 3 4 5 6 7 Φ eq [10 14 cm -2 ] N C [10 13 cm -3 ] 1.2 1 0.8 0.6 0.4 0.2 24 GeV/c proton irradiation standard FZ g c =1.93 10-2 cm -1 oxygenated FZ g c =5.61 10-3 cm -1 0 0 1 2 3 4 5 6 7 Φ eq [10 14 cm -2 ] Stable damage parameter N C N Y /N Y 1.0 0.8 0.6 0.4 0.2 24 GeV/c proton irradiation normalised reverse annealing standard FZ τ Y = 50 min oxygenated FZ τ Y = 220 min 24 GeV/c proton irradiated O-enriched diodes g c improved by a factor of 3 saturation of reverse annealing delayed reverse annealing 0.0 10 0 5 10 1 5 10 2 5 10 3 5 10 4 annealing time at 80 o C [min] Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-23 -
Damage Projection - ATLAS Pixel Detector - B-Layer (4cm) Radiation level: F eq Three scenario: 1 year = eq (year) = 3.5 10 14 cm -2 (full luminosity) > 85% charged hadrons 1 year = 100 days beam (-7 C) (1) 3 days 20 C and 14 days 17 C (2) 30 days 20 C (3) 60 days 20 C Rest of the year: no beam (-7 C) 2000 standard silicon 2000 V dep (200µm) [V] 1500 1000 500 operation voltage: 600V 1500 1000 500 oxygenated silicon 0 1 2 3 4 5 6 7 8 9 10 time [years] Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-24 -
Damage Projection - Strip Detector - 30 cm Radiation level: 400 F eq Scenario: 1 year = eq (10years) = 2 10 14 cm -2» 50% charged hadrons 1 year = 100 days beam (-7 C) 14 days 20 C 251 days no beam (-7 C) 400 V dep (300µm) [V] 300 200 100 standard silicon oxygenated silicon 300 200 100 0 1 2 3 4 5 6 7 8 9 10 time [years] Dashed line - 2 days 20 C and 14 days 14 C warm-up Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-25 -
Warning: Variation of standard material N eff [10 12 cm -3 ] 12 10 8 6 4 2 standard FZ silicon oxygenated FZ 0 0 1 2 3 4 5 6 7 8 9 10 Φ proton [10 14 cm -2 ] 800 700 600 500 400 300 200 100 V dep [V] (300 µm) Strong variation of standard silicon Reproducible results for oxygenated silicon Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-26 -
Summary (1/4) - Leakage Current NIEL - Non Ionizing Energy Loss NIEL - Hypothesis (damage parameters scale with NIEL) has to be used very carefully! (e.g. not valid for changes in V dep of oxygenated and standard silicon detectors) Devices used for investigations Very simple diodes in order to concentrate on the bulk damage effects Macroscopic Damage Effects - Leakage Current Strong increase of leakage current after irradiation Cooling of detectors necessary Leakage current damage parameter a - material independent (no impurity, resistivity or conduction type dependence) - scaling with NIEL for fast neutrons, pions and protons Leakage current annealing after irradiation same for all type of materials 60 Co-gamma irradiation: No leakage current (bulk current!) annealing observed at room temperature! Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-27 -
Summary (2/4) - Depletion Voltage Macroscopic Damage Effects - Depletion Voltage - (N eff ) In general the space charge becomes more negative after irradiation (type inversion). The initial n-type silicon inverts to a quasi "p-type" silicon The main depletion zone evolves from the back electrode (n + ) after type inversion Changes in N eff consists of three basic components (neutrons, pions, protons): - Short term or beneficial annealing (Decrease of depletion voltage for inverted detectors) - Stable damage (No change in time after irradiation) - Reverse annealing (Increase of depletion voltage for inverted detectors) 60 Co-gamma irradiation: No beneficial and no reverse annealing! Determination of all damage parameters (Hamburg model + many isothermal annealing experiments) allows for the simulation of damage projections in the LHC experiments under various operational conditions. Annealing experiments (macroscopic parameters - V dep, α) isothermal and isochronal give already some very useful hints about the microscopic defects / defect reactions underlying the change of the macroscopic parameters! Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-28 -
Summary (3/4) - Microscopic Defects Damage at the Microscopic Level DLTS and TSC measurements reveal many electrical active defects Most of the defects are not charged in the space charge region (no influence on V dep!!) Comparison between 60 Co-gamma and neutron/proton irradiated silicon allows for determination between point defects and defects related to defect clusters. Annealing experiments (isochronal and isothermal) reveal correlation's between single defects and macroscopic parameters - reverse annealing and leakage current annealing Defect introduction rates and defect parameters can be used for the modeling of the overall defect kinetics in the silicon (including fluence ranges where DLTS technique is not applicable any more) Defect kinetics modeling Modeling of the defect kinetics suggests that oxygen rich silicon is radiation harder (V 2 O-model) Model can explain experimental data for gamma irradiated silicon (change of depletion voltage) Model gives qualitatively the macroscopic findings for hadron irradiated silicon detectors (depletion voltage). However, all models are so far incomplete. Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-29 -
Summary (4/4) - Oxygenated Silicon Oxygen enriched silicon - technological results Oxygen enrichment by diffusion is most effective / less cost intensive method Detectors produced on oxygen enriched silicon do not show any change in detector properties besides the improved radiation hardness (surface properties, performance before irradiation,etc.) Diffusion technology has been successfully transferred to several silicon detector manufacturers (SINTEF, Micron, ST, CiS) and full-scale microstrip detectors produced Oxygen enriched silicon - scientific results Effective doping changes can be improved by oxygenation of the material (factor 3 for stable damage parameter gc). Such improvement is only observed when the radiation environment contains a significant charged particle component. Reverse annealing saturates at high fluences (2 10 14 p/cm 2 ) for oxygen enriched silicon. Time constant for the process is larger by a factor of 2-4 allowing detectors to remain at room temperature for longer periods during maintenance periods: additional safety margin. Many open questions Further work on oxygenated/standard Silicon needed! The proton neutron puzzle: Violation of NIEL? Why does the reverse annealing saturate in oxygenated silicon? Which defect is responsible for the differences observed after proton and neutron irradiation? (V 2 O? ) more open questions and details see ROSE status reports. Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-30 -
Visit our web-site for The ROSE Collaboration more information status reports, preprints, publications, related conferences, etc. http://cern.ch/rd48 Remember, today is Valentines Day! A good day for ROSES? Michael Moll - CERN EP-TA1-SD Seminar - 14.2.2001-31 -