Uzunian, A V

Vasilev, A

Yarba, J V

CERN Document Server

 STATE RESEARCH CENTER OF RUSSIAINSTITUTE FOR HIGH ENERGY PHYSICSIHEP 2001–24A. Uzunian1, A. VasilievInstitute for High Energy Physics, Protvino, RussiaJ. YarbaFermi National Accelerator Laboratory, USAFIRST RESULTSON SIMULATION OF RADIATION ENVIRONMENTAT BTeV ELECTROMAGNETIC CALORIMETER1 E-mail : uzunian@mx.ihep.suProtvino 2001UDK 539.1.01 m–24AbstractUzunian A., Vasiliev A., Yarba J. First results on simulation of radiation environment at BTeV electro-magnetic calorimeter: IHEP Preprint 2001–24. – Protvino, 2001. – p. 9, ﬁgs. 6, refs.: 11.We present the preliminary results of the radiation environment calculations for BTeV electromag-netic calorimeter. Two problems associated with this background are considered – the absorbed doseaccumulation in the lead tungstate crystals (PbWO4) and hadron ﬂuence formation in electronic zone ofthe electromagnetic calorimeter.aNNOTACIQuZUNQN a., wASILXEW a., qRBA ‘. pERWYE REZULXTATY MODELIROWANIQ RADIACIONNOJ OBSTANOWKI NA“LEKTROMAGNITNOM KALORIMETRE “KSPERIMENTA BTeV: pREPRINT ifw— 2001–24. – pROTWINO, 2001. –9 S., 6 RIS., BIBLIOGR.: 11.w RABOTE PREDSTAWLENY PREDWARITELXNYE REZULXTATY RASˆETOW RADIACIONNOJ OBSTANOWKI NA“LEKTROMAGNITNOM KALORIMETRE “KSPERIMENTA BTeV. w SWQZI S “TIM RASSMOTRENY DWE PROBLEMY –NAKOPLENIE POGLO]ENNOJ DOZY W KRISTALLAH WOLXFROMATA SWINCA ((PbWO4) I FORMIROWANIE FL@-ENSA ADRONOW W ZONE “LEKTROMAGNITNOGO KALORIMETRA.c© State Research Center of RussiaInstitute for High Energy Physics, 20011. IntroductionBTeV is a dedicated B-physics experiment in the C interaction region at the Fermilabcollider to study direct CP-violation, mixing and rare decays in B-mesons. The BTeV Proposalwas approved in June 2000 [1]. BTeV is an international collaboration to which IHEP joinedin 1999. The IHEP group’s interest, together with FNAL, Syracuse University and Universityof Minnesota, is an Electromagnetic Calorimeter (EMCAL) of the BTeV setup. EMCAL isplanned to consist more than 20,000 of PbWO4 crystals.The study of the radiation environment formation is the essential part for the BTeV upcomingTechnical Design Report preparation. The following subjects could be under the consideration:• absorbed dose and radiation hardness of detectors;• background particle (charged hadrons, neutrons, gammas, electrons and highly ionizingparticle) rates and its tolerable levels for detecting elements and electronics;• residual levels of induced radioactivity, safety issue and access procedures;• gamma/electron background due to induced radioactivity;• possible ways for background reduction and shielding requirements.In this paper we present the ﬁrst results of radiation background calculations in BTeVEMCAL. To deﬁne conditions for normal operation of the detector, it appears to be an importanttask to estimate the expected levels of the absorbed dose and the ﬂuences of neutrons and chargedparticles in the EMCAL elements (crystals, phototubes, read-out electronics).To calculate the radiation levels, Monte Carlo simulation using MARS(IHEP) code wasperformed[2,3]. MARS is the standard IHEP package used (together with FLUKA code [4]) forradiation environment calculations for LHC project at CERN [5,6].2. Conditions of simulationsThe nominal luminosity of Tevatron, 2 · 1032cm−2s−1, together with 1 TeV beam energy,assuming an inelastic cross-section of 60 mb, will generate 1.2 · 107 inelastic pp events persecond. Together with particles produced in interactions with BTeV material, this create ahostile radiation environment. Each year of BTeV operation was assumed to be 107 seconds.We used the PYTHIA generator to generate 2000 minimum bias events as input particlesﬁle for MARS.12.1. Radiation transport codeMARS as Monte-Carlo code was designed for the simulation of hadron cascades, electron-photon showers and muon transport in heterogeneous matter of arbitrary complexity in energyrange from several TeV down to thermal energy(for neutrons).By its original philosophy MARS is a fully biased code, and as one of its features uses aninclusive approach for the Monte Carlo simulation of particle production. Within the inclusiveMonte Carlo approach only a limited number of particles of ﬁxed types are born in interaction actbased on phenomenological formulae. The kinimatic characteristics of the secondary particlesfrom the corresponding double diﬀerential cross-section are simulated. The real multiplicityof the generated particles is taken into account via the introduction of particle weights. Suchan approach gives a possibility to use ﬂexible tuning of a cascade tree construction algorithmproviding a considerable gain in calculational eﬃciency depending on the kind of problem. Theinclusive Monte Carlo version, although not useful for precise physics simulations, allows fastestimations of mean values, such as particle ﬂuxes, spectra and absorbed energy, and is thereforewell suited for background calculations. MARS uses a library of constants, consisting of 28groups and covering neutron energy range up to 14.5 MeV (including one thermal group withEn < 0.215 ·10−6 MeV) [8]. The energy thresholds in our simulation were 2 ·10−9 MeV (thermalenergy region) for neutrons, 10 MeV for charged hadrons and 100 keV for gammas and electrons.MARS(IHEP), BTEV-b130501001502002503003500 200 400 600 800 1000 1200Z, CMR, CMFig. 1. BTeV geometry used for radiation background calculations.22.2. Geometry descriptionThe ﬁrst approximation of BTeV geometry implicated in MARS is shown in Fig.1. Thefollowing elements and subsystems were taken into account:• beam pipe: R= 6 cm, 1.5 mm of thickness, Al, for Z < 96 cm,and R= 4.85 cm, 1.5 mm of thickness, Al, for Z ≥ 96 cm;• silicon pixel detector, forward silicon tracker, forward tracking system, RICH;• dipole magnet, magnetic ﬁeld components Bx, By, Bz and its dependence from Z-axis only(we have not the azimuthal structure of magnetic ﬁeld in calculations);• EMCAL (PbW04, density=8.28 g/cm3), location: Z=(710-732) cm, R= (15-150) cm;• phototube windows (quartz, density =2.64 g/cm3), location: Z=(732-732.2) cm,R = (15 - 150) cm;• hadron absorber, muon toroids;• experimental cavern was approximated as cylindrical volume with walls made by the or-dinary concrete (density =2.35 g/cm3)The same rough geometry description is quite enough for EMCAL absorption energy esti-mation. Here, it should be noted that the conﬁguration is not ﬁnal and is likely be revisitedlater.3. Result of calculationsIn the EMCAL (and in all BTeV environment) energy deposition eﬀect will be inducedby charged hadrons, electrons, gammas and neutrons ( though the last two are not directlyionizing, they can induce ionizing energy deposition). Total absorbed dose eﬀect is a typicalcase of cumulative eﬀects. This happens during the whole time the device is exposed to radiation.The results of calculations in terms of absorbed dose and particle ﬂuence are presented.Absorbed dose unit in the International System is the Gray (Gy). (Note that 1 Gy =1 J/kg = 100 rad = 6.24 · 106 GeV/g). The mathematical deﬁnition of the absorbed dose (D)is:D = lim(E/m); m→ 0,where E is the absorbed energy in the volume V , within of which we have the matter ofmass m.Fluence (F ) is deﬁned as the total tracklength of particles per unit of volumeF =∑iliwiV, [1/cm2],were li is a particle tracklength within of the volume V and wi is a particle weight.Particle ﬂuence and absorbed dose, although correlated for a given particle type and energy ingiven medium, should never be treated as synonyms.For the real situation we need to take bounded volume and mass. In our case we have essen-tially diﬀerent radiation ﬁeld gradient along Z-axis of PbWO4 and along azimuthal ϕ-angle (theeﬀect of dipole magnet inﬂuence). Because of this we used the EMCAL segments (for which theabsorbed dose was calculated) which is contained byRi − (Ri + dR), Zi − (Zi + dZ):, dR = 3 cm, dZ = 1 cm,3abs(Y ) ≤ 5 cm for the vertical plane and abs(X) ≤ 5 cm) for the horizontal plane. We estimatethe accuracy of calculational results (because not a ”point-like” volume is used) as ∼ 20 % forthe worse case( the area with a big gradient of radiation ﬁeld). For these conditions, the EMCALabsorbed doses at η = 4.45(3., 2.27) versus Z-position are given in Fig.2. The maximum of theabsorbed dose value occurs between 3 and 8 cm deep depending on the η-range considered. Thedose proﬁles has a maximum on the PWO depth of ∼ 6 cm at η = 4.45 (vertical plane). In thiscase the absorbed dose is equal to ∼ 1.8 · 104 Gy/year for luminosity L = 2 · 1032cm−2s−1. Theminimal dose values of a few tens rad take place on EMCAL periphery. Fig.3 shows the azimuthstructure of absorbed dose proﬁles for the EMCAL ( in PbWO4, at (4-8) cm deep) and for thephototube windows. The azimuth ϕ-angle starts from vertical axis and was divided in 40 bins.Note that for absorbed dose calculation in front face of the phototubes we used quartz(SiO2,density=2.64 g/cm3) of 2 mm thickness. We can see that the maximum absorbed dose in thephototube window is 10 kGy/year.The maximum radiation level occurs close to the beam pipe.There is also a relatively narrowvertical band (were the absorbed dose higher than average dose) caused by the sweeping actionof BTeV dipole. Fig.4 gives the crystals number distribution as a function of the absorbeddose accumulated by crystals during the operation year (here dose means the maximum dosein any part of the crystals). The calculations indicate that ∼ 90% of the crystals have a yearlyaccumulated dose of less than 1000 Gy.MARS(IHEP), BTEV/Ecal (L=2∗1032 cm-2 s-1, 1Year=107s)10 210 310 410 50 5 10 15 20PWO-crystal              Z, cmabsorbed dose, Gy/year10-111010 210 310 410 50 5 10 15 20PWO-crystal               Z, cmFig. 2. PWO-crystals absorbed doses at η = 4.45(3., 2.27) versus of crystal depth for the vertical andhorizontal planes.4MARS(IHEP),BTEV/Ecal (L=2∗1032 cm-2 s-1, 1Year=107s)11010 210 310 410 50 1 2 3 4 5 6PWO-crystal                    φ, Radabsorbed dose, Gy/year10-111010 210 310 40 1 2 3 4 5 6Phtotubes(quartz)                        φ, RadFig. 3. Azimuth proﬁles of the PWO-crystals maximal absorbed doses (top ﬁgure), and absorbed dosesin the phototube windows (bottom). ϕ-angle starts at vertical axis.MARS(IHEP), EMcal PWO crystals (L=2∗1032 cm-2 s-1, 1Year=107s)1010 210 310 10 2 10 3 10 4    Absorbed dose, Gy/yearNumber of CrystalsFig. 4. Crystals number distribution as a function of the maximal value of absorbed dose.5MARS(IHEP), Emcal/hadrons (L=2∗1032 cm-2 s-1)10 510 60 1 2 3 4 5 6Neutrons (total, including the thermal neutrons)                       φ, Radneutrons, Hz/cm210 310 410 50 1 2 3 4 5 6High-energy neutrons ,(E ≥ 14.5 MeV)                       φ, Radneutrons, Hz/cm210 310 410 50 1 2 3 4 5 6 High energy neutrons + ch.hadrons, (E ≥ 14.5 MeV)                       φ, Radhadrons, Hz/cm2Fig. 5. Azimuth structure of the hadron ﬂuence distributions just behind EMCAL.The radiation hardness studies of detectors and electronics are the important part of colliderexperiments activity[9,10]. The experimental data, available from LHC project activity, mainlyfrom photon and neutron (E < 20 MeV ) irradiation,indicate that only the light transmissionof the crystals is aﬀected by irradiation and not the scintillation mechanism itself. Usually, thehypothesis is that the light loss due to decreased transparency has no direct dependence on thetypes of incident particles, but only on the absorbed dose.However, in general case the radiation damage of PbWO4-crystals and electronics is not onlya function of absorbed dose, but also sensitive to the type of irradiation. So, the properties ofbulk silicon are signiﬁcantly degraded by displacement damage eﬀects, i.e. distortions of thecrystal structure.Thus, it is very useful to know both the hadron ﬂuence/spectra and absorbeddose to provide the radiation damage study.6We need to emphasize (together with [11]) although photon and neutron irradiation haveindicated that the damage in crystals is only a function of dose rate, this cannot be consideredas a proof that high-energy hadrons (with energy greater than a few tens MeV) could not causesome new type of damage. Compared to photons or reactor neutrons, high-energy hadrons willbe able to induce the inelastic nuclear reactions which will locally destroy the crystal lattice.In particular they can create nuclear fragments with very high linear energy transfer and leadto extended clusters of crystal lattice distortion. Thereto we will have the process of the newtypes of isotopes generation. This process can lead to deterioration or improvement of thecrystal radiation hardness. Therefore the radiation hardness studies of PbWO4-crystals shouldbe provided using a hadron environment which is similar to the BTeV/EMCAL expectations.The azimuth distributions of the hadron ﬂuence at diﬀerent η for region of the EMCALelectronics are presented in Fig.5. We have considered the neutrons ﬂuence and the chargedhadrons one separately to estimate its contribution in the total hadron radiation. One can ﬁndthat the maximal value of the neutron ﬂuence (including the thermal energy neutrons) is aboutof 106 n/(cm2s) and of 4 · 104 n/(cm2s) for the neutrons with energy greater than 14.5 MeV(luminosity = 2 · 1032cm−2s−1).The neutron energy spectra shape for the internal (R=(15-70) cm from beam pipe axis) andperipheral ((R=(100-150) cm) regions of the EMCAL electronics zone in Fig.6 are given. In thiscase the neutron spectra are averaged over the azimuth ϕ-angle. The spectrum above 100 keVis characterized by the evaporation and cascade peaks at neutron energies of 1 MeV and a fewtens of MeV, correspondingly.(L=2∗1032 cm-2 s-1) EMcal/neutron spectra, MARS(IHEP)11010 210 310 410 510 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 10 10 2 10 3 10 4 10 5           E, MeVneutrons, E∗dN/dE, 1/(cm2s)Fig. 6. Neutron energy spectra.4. ConclusionThe expected maximal annual EMCAL absorbed dose is about of 20 kGy (2 Mrad) at aradius of 15 cm from beam pipe axis (η = 4.45) for luminosity L = 2 · 1032cm−2s−1. Thisabsorbed dose value is at electromagnetic shower maximum, which is about 6 cm from the7front face of the EMCAL. This dose falls down three orders of magnitude to the boundary ofthe EMCAL, which is 150 cm from the center in the lateral direction. The calculations of theEMCAL absorbed dose map indicates that ∼ 90% of the crystals have a yearly accumulateddose of less than 1000 Gy = 0.1 Mrad.The hadron ﬂuence just behind the EMCAL (EMCAL electronics zone) is essentially deﬁnedby the neutrons component. The low-energy neutrons (E ≤ 14.5MeV ) form approximately 90%of the hadronic ﬁeld in this region.It is necessary to provide the calculations of the particles (separately for charged hadrons,muons, gammas, neutrons, electrons+positrons) ﬂuence and its energy spectra in the regionwhere the sensitive elements are installed. It could help us to deﬁne the tolerable level of par-ticle rates for counting systems and electronics. So, for the next stages of calculations it isvery important to use the full scale BTeV geometry to take into account all possible sourcesof secondary particles including the BTEV-Tevatron interface(problem of machine generatedbackground).We would like to thank Joel Butler, Sheldon Stone and Yuichi Kubota for fruitful discussionsof the considered problems.References[1] BTeV Collaboration, Proposal BTeV, P918, Fermilab, May 2000.[2] I.Azhgirey, I.Kurochkin, V.Talanov,Development of MARS code package for radiation as-pects of electronuclear installations design, in: Proc. 15th Conf. on Charged particles Ac-celerators, Protvino, 22-24 October, 1996, p.74.[3] I.Azhgirey, I.Kurochkin, A.Sannikov, E.Savitskaya, Nucl. Insr. and Meth., A408, 535(1998).[4] P.A. Aarnio et al.,FLUKA86 user’s guide, CERN TIS-RP/168(1986);A. Fasso et al., FLUKA: performances and applications in the intermediate energy range,Meeting on Shielding Aspects of Accelerators, Targets and Irradiation Facilities, Arlington,Texas, April 28-29, 1994.[5] CMS Collaboration.The Muon Project Technical Design Report, CERN/LHCC 97-32, CMSTDR 3 (1997);A.Uzunian, CMS Radiation Studies Results ’99, Fourth Annual CMS/RDMS CollaborationMeeting, CMS Document 1999-113, CERN (1999).[6] CMS Collaboration. The Hadronic Calorimeter Project Technical Design Report,CERN/LHCC 97-31, June 20, 1997.[7] N.Mokhov et al,Particles .. , Atomizdat.[8] Voronkov A.V., Juravlev V.I. et al.USCONS: Code System to Compute Group Cross Sec-tions Using GNDL Data Library - Keldysh Institute of Applied Mathematics, Moscow,1983.8[9] CMS Collaboration. The Electromagnetic Calorimeter Project. Technical Design Report.CERN/LHCC 97-33, CMS TDR 4 (1997).[10] S.Baccaro et al.,Further understanding of PbWO4 scintillator characteristics and their op-timisation, CERN, CMS NOTE 2000/002 (2000);G.Davies et al., A study of the monitoring of radiation damage to CMS ECAL crystals,performed at X5-GIF,CERN, CMS NOTE 2000/020 (2000);H.F.Chen et al.,Measurements of PbWO4 crystals behaviour under irradiation, CERN CMSCR 1999/027 (1999);E.Auﬀray et al., Comparison of diﬀerent irradiation procedures for recently optimized rus-sian PbWO4 crystals, CERN, CMS NOTE 98/069 (1998).[11] M.Huhtinen,Dose proﬁles in ECAL crystals for various irradiation conditions, CERN, CMSNOTE 1998/055 (1998).Received May 14, 20019pREPRINT OTPEˆATAN S ORIGINALA-MAKETA, PODGOTOWLENNOGO AWTORAMI.a. uZUNQN, a. wASILXEW, ‘. qRBApERWYE REZULXTATY MODELIROWANIQ RADIACIONNOJ OBSTANOWKI NA “LEKTROMAGNITNOMKALORIMETRE “KSPERIMENTA BTeV.oRIGINAL-MAKET PODGOTOWLEN S POMO]X@ SISTEMY LaTEX.pODPISANO K PEˆATI 25.05.2001. fORMAT 60× 84/8.oFSETNAQ PEˆATX. pEˆ.L. 1.12. uˆ.-IZD.L. 0.9. tIRAV 160. zAKAZ 85.iNDEKS 3649. lr ß020498 17.04.97.gnc rf iNSTITUT FIZIKI WYSOKIH “NERGIJ142284, pROTWINO mOSKOWSKOJ OBL.iNDEKS 3649p r e p r i n t 2001–24, i f w —, 2001

First results on simulation of radiation environment at BTeV electromagnetic calorimeter

Abstract

Similar works

Full text

Available Versions

CERN Document Server