C. Sbarra

F. Palmonari

Giuseppe Levi

R. Martelli

Undetermined

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November 5, 2003 12:19 WSPC/Trim Size: 9in x 6in for Proceedings finemesh-simSIMULATION OF THE FINE MESH PHOTOMULTIPLERSIN THE TOF OF AMS-02G.LEVI, R.MARTELLI, F.PALMONARI, C.SBARRA∗Dep. of Physics and INFN, Bologna, ItalyThe photomultiplers Hamamatsu R5946 “fine mesh” have been simulated in orderto understand their behavior in magnetic field, in view of their use in the TOFsystem of AMS-02 experiment, a cosmic ray spectrometer to be operated in spaceafter 2005. The simulation, compared with the data taken, has been a usefull tool infinding a proper calibration procedure in the laboratory, calibration whose resultsare crucial for the final phototubes displacement on the whole TOF structure.1. IntroductionThe Alpha Magnetic Spectrometer (AMS) is the first large particle detectorthat will be installed on the International Space Station and will measurecosmic ray fluxes for at least three years in a low orbit (about 400 km)around the earth1. It will detect rigidities up to 2.5 TV, by using a super-conducting magnet which will provide a maximum field of 0.87 Ta. Thephotomultiplier tubes of the plastic scintillator Time Of Flight System ofAMS-02 will operate in the strong (2-3 kG) and badly shaped fringing fieldof the dipole magnet2. That led to the choice of the R5946 Hamamatsu“fine mesh” tubes, for their intrinsic capability of tolerating the magneticfield3. Since the year 2000, a simple model of the fine mesh photomultipli-ers was used to understand their time behavior in magnetic field. Later themodel has been refined to reproduce the gain characteristics. Finally, themodel–data permitted to identify the main parameters to look for duringcalibration.∗rapporteur e-mail: sbarra@bo.infn.itaThe detector is nicknamed AMS-02 (TOF-02) in the following.1November 5, 2003 12:19 WSPC/Trim Size: 9in x 6in for Proceedings finemesh-sim22. Fine mesh simulationSince the year 2000 a proper simulation of the Hamamatsu fine mesh hasbeen developed with the aim of understanding the time response of thePMs just tested in magnetic field. The data taken and the result of thisfirst simulation (properly tuned for the purpose) have been shown at theICRC 20014. Later a complete simulation, of both the single photoelectronresponse and the gain in magnetic field, has been implemented10.In the following the model on which is based the complete simulationof fine mesh PM is given and some features of it are described.2.1. The modelThe basic model of the fine mesh simulation starts with a single photo-electron (phe) extracted from a random point at the surface of the photo-cathode. Then, the phe is followed, solving the equations of motion withelectricb and magnetic field, up to the first dynode: during its way, it canmiss the first dynode surface and is lost. The cathod–first dynode distanceis 3.4 mm and the voltage drop twice that for the following 16 dynodeswhose spacing is 0.9 mmc. The electron impinging the first dynode sur-face extracts secondaries, up to a maximum number, when all the avaiableenergy is over. Therefore the simulation implements multiplication on anenergy basisd: in the energy computing, it’s followed the phenomenologicaldistribution of the secondary electrons shown in Fig.1, taking into accountthe energy given to the whole lattice and the minimun energy thresholdfor extraction. This threshold energy depends on the angle of incidenceof electron on dynode surfacee and consequentely is related to the angleθ between B and the PM axis, because at high θ there are more inclinedelectrons that seem to extract more easily (see Ref.8).The dynodes are a planar mesh of orthogonal wires (Fig.2 shows amesh). The photoelectron can thus pass through a hole of the first dyn-ode, without extracting secondaries12, and go to the following stagef . Ithas been implemented this possibility as an overall probability (namely thebAssumed uniform to semplify the model.cThe partition ratios beeing 2:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1dNot using a poissonian distribution of secondaries as in other models simulated previ-ously (see the one described in Ref.7).eBeing θinc the electron angle of incidence on mesh surface, the energy threshold iswritten as a function of θinc, around the work function of the mesh material.fAnd this is valid for each stage, even if is more critical for the cathod–first dynodestage10.November 5, 2003 12:19 WSPC/Trim Size: 9in x 6in for Proceedings finemesh-sim3Figure 1. Phenomenological distribution of Secondary Electrons Emitted (SEE) energyat the dynodes for an electron of 300 V (see Ref.7)Figure 2. Structure of a fine mesh dynode (see Ref.3)“transparency” T ) whose value has been tuned on real data and which isdescribed in the next paragraph. After the first dynode, each extractedelectron is followed up to the second dynode, and so on, solving equation ofmotion for each electron (getting lost those outside the dynode boundariesand considering the effect of the transparency) and following the multipli-cation avalancheg up to the anode.2.2. The “transparency” and the single photoelectronresponseThe transparency parameter T has been found very efficient to tune ourmodel on the experimental data. The experimental gain as function of theHigh Voltage is fitted assuming a linear dependence of T on the voltage dropgA proper indexing algorithm is used in order to minimize the program vectors dimen-sions.November 5, 2003 12:19 WSPC/Trim Size: 9in x 6in for Proceedings finemesh-sim4Simulated sphe spectra at different T11010 210 30 500 1000ADC ch.T=0B=0HV=2000 V 11010 210 30 50 100 150ADC ch.T=20%B=0HV=2000 V11010 210 30 200 400 600ADC ch.T=0B=0HV=1700 V 11010 210 30 20 40 60ADC ch.T=25%B=0HV=1700 VFigure 3. Simulated sphe response atdifferent T and B=0. The peak of thesingle phe signal is well separated at zerotransparency, but not at T∼ 20%: theelectrons who skipped the dynode do notmultiplicate, resulting in a lower gain.Simulated sphe spectra at different angles11010 210 30 50 100 150ADC ch.θ=0B=2000 GHV=1700 V 11010 210 30 50 100 150ADC ch.θ=20B=2000 GHV=1700 V1010 210 30 50 100 150ADC ch.θ=30B=2000 GHV=1700 V10 210 310 40 50 100 150ADC ch.θ=40B=2000 GHV=1700 VFigure 4. Simulated sphe response atdifferent angles θ, HV=1700 V andB=2000 G. As you can see, the sphe res-olution degrades notably as the angle in-creases (and this almost independentlyon the B-field magnitude).between dynodes: at higher voltage electrons are directed more efficientlyon the electrode wires and the transparency decreases. The overall meanvalue of T is tuned to reproduce the single photoelectron response. Infact, the model at T = 0 gives the typical “dimple” observed in non finemesh photomultiplierss, whereas for T ∼ 20% our model can reproducereasonably well the shape for R5946 tubes (see Fig.3).Finally the T parameter can be tuned to reproduce the behaviour ofthe “fine mesh” in magnetic field. This is shown in Fig.4 for the singlephotoelectron (sphe) spectrum. The effect of the magnetic field is double:on one side as θ increases more and more electrons are deflected outside thedynodes area and are lost, degrading the sphe resolutionh; on the other sidethe transparency T depends geometrically on the electron angle of incidence(θinc) on the planar meshi decreasing for high inclined electrons.hDefined as theσpeakpeakof the 1-started phe final gaussian distribution.iT ∝ cosθincNovember 5, 2003 12:19 WSPC/Trim Size: 9in x 6in for Proceedings finemesh-sim52.3. The PM time response in magnetic fieldThe time of each electron in the simulated avalanche can be recorded. Theanode distribution of the the transit timej simulated are compared with thedata taken of two PMs (low gain and high gain) in Fig.5, for a magnetic fieldof 3000 Gauss and at different θ angles. From these plots, the conclusionis that the transit time worsens with the angle, critically for PMs workingat higher voltage. Moreover, the transit time itself, at B = 0, is higher forthe high gain phototubesk.2.4. The PM Gain in magnetic fieldFrom the simulated single photoelectron distribution, in the case that just1 phe has been extracted from the cathode, it is possible to calculate thegain, which is by definition the number of final electrons at the anode.In absence of a correct function which can describe the fine mesh singlephotoelectron spectra, it can be assigned their mean value to the gain.In this way, various gains have been simulated at various B-fields and atvarious θ. The results of the simulation compared to the data taken areshown in Fig.6: the phototubes show a higher gain with the angle, up to alimit (almost 40 degrees) over which the gain has an abrupt fall; this almostindependently on the intensity of the B field, as both data than simulationresults show.3. ConclusionsA complete fine mesh simulation has been shown, of both timing than gaincharacteristics, in magnetic field and at various inclination θ of the PMaxis in the field. From the simulation results, together with the data taken,it is clear that a calibration procedure based on the usual fit of singlephotoelectron spectra, is no more applicable, due to the influence of the“Transparency” on the whole distribution. Those difficulties widen withincreasing B-field and angle θ, and they are particularly critical for PMsworking at higher HV, because one cannot increase freely their tension,which has shown to be the best treatment. The politics chosen by theTOF of AMS-02 is that of putting high gain PMs at the critical positions,after a calibration properly tuned for the fine mesh (not based on singlejTransit time is defined as arrival time at B = 0 minus the arrival time at B = 0kOf a factor of about 2 ps/Volt (from both data4 than simulation10)November 5, 2003 12:19 WSPC/Trim Size: 9in x 6in for Proceedings finemesh-sim6Fine mesh transit time vs angle-0.100.10.20.30.40.50.6x 10-80 10 20 30 40 50HV=1700VSimulated data B=3000 Gθ(degrees)Mean transit time (s)Measured data B=3000 G-0.100.10.20.30.40.50.6x 10-80 10 20 30 40 50HV=2200VSimulated data B=3000 Gθ(degrees)Mean transit time (s)Measured data B=3000 GFigure 5. Measured and simulatedtransit time of two different PMs in amagnetic field of 3000 G as a functionof the angle θ. High gain PM on toppicture, low gain PM on bottom.Fine mesh gain versus angle00.250.50.7511.251.51.7522.252.5-10 0 10 20 30 40 50 60 70Simulated data B=1500GSimulated data B=3000GMeasured data B=3000GMeasured data B=1500Gθ(degrees)Relative gain peakHV=2000 VFigure 6. Measured and simulated rel-ative gain of fine mesh response at vari-ous angles θ and B-fields. The data andthe simulation show an increasing gainwith the angle up to almost 40 degrees.photoelectron fitting functions at all), whose tension can be, during thethree years on ISS, increased as necessary to contrast the worsening oftheir time characteristics.References1. The AMS collaboration, Physics Reports, vol. 366/6 (Aug.2002), pp.331- 404.2. C.Sbarra et Al., “The Time of Flight System of the AMS-02 Space Experi-ment”, Proc. of the 7th ICATPP (2001). Printed by World Scientific.3. Hamamatsu Photonics: “Photomultipler tube R5946”, 1994.4. L.Brocco et Al..,“Behavior in strong magnetic field of the photomultipliers forthe TOF system of the AMS-02 space experiment”, Proc. 27th ICRC (2001).5. B. Bencheick et Al.., NIM A 315 (1992) 349.6. C.Sbarra, AMS-BO Int. Note (2002)7. G.Barbiellini et Al., “A simulation study of the behaviour of fine mesh photo-multipliers in magnetic field”, NIM A 362 (1995) 245-252.8. R. Chechik et Al..,WIS-93/63/Jul-PH (1993).9. C.Sbarra, “Comparison between fit functions of simulated Single Phe spectra”,AMS-BO Int. Note (2003).10. Amati L. et Al., “Simulazione dei fotomoltiplicatori “fine mesh” per il TOFdi AMS-02”, presented at the LXXXIX SIF (2003).11. Amati L. et Al., “Algoritmo genetico per l’ottimizzazione dei fotomoltiplica-tori del TOF di AMS-02”, presented at the LXXXIX SIF (2003).12. I Chirikov-Zorin et Al., NIM A 456 (2000) 310-324.

Simulation of the Fine Mesh Photomultiplers in the Tof of AMS-02

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Simulation of the Fine Mesh Photomultiplers in the Tof of AMS-02

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