There are three main sources of the radiation background in MeRHIC: forward synchrotron radiation generated upstream of the detector, the direct backward radiation caused by the photons hitting beampipe downstream of the detector, and the indirect secondary radiation caused by hard photons hitting vacuum systems, masks, collimators, absorbers or any other elements in the interaction region. In this paper, we first calculate the primary radiation distribution by employing electromagnetic theory. Then we obtain the direct backward scattering rate by applying the kinematic Born approximation deduced from scattering dynamics. The diffuse scattering cross section is calculated as a function of the surface properties of the MeRHIC vacuum system. Finally, the dominating physical processes and minimization of indirect secondary radiation is presented and discussed

Beebe-Wang, J.

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NLABORATORYBNL..81796..2009..CPInvestigation ofthe radiation background in theinteraction region ofthe medium-energy electronrelativistic heavy ion collider (MeRHIC)J. Beebe-WangPresented at the Particle Accelerator Conference (PAC09)Vancouver, B.C., CanadaMay 4-8, 2009Collider-Accelerator DepartmentBrookhaven National LaboratoryP.O. Box 5000Upton, NY 11973-5000www.bnl.govNotice: This manuscript has been authored by employees of Brookhaven Science Associates. LLC underContract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting themanuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up,irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow othersto do so, for United States Government purposes.This preprint is intended for publication in a journal or proceedings. Since changes may be made beforepUblication, it may not be cited or reproduced without the author's permission.DISCLAIMERThis report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor anyagency thereof, nor any of their employees, nor any of their contractors,subcontractors, or their employees, Inakes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, or anythird party's use or the results of such use of any information, apparatus, product,or process disclosed, or represents that its use would not infringe privately ownedrights. Reference herein to any specific commercial product, process, or serviceby trade name, trademark, manufacturer, or otherwise, does not necessarilyconstitute or imply its endorsement, recommendation, or favoring by the UnitedStates Government or any agency thereof or its contractors or subcontractors.The views and opinions of authors expressed herein do not necessarily state orreflect those of the United States Government or any agency thereof.INVESTIGATION OF THE RADIATION BACKGROUND IN THEINTERACTION REGION OF THE MEDIUM-ENERGY ELECTRONRELATIVISTIC HEAVY ION COLLIDER (MeRHIC)*Jo Beebe-Wangt , Brookhaven National Laboratory, Upton, NY 11973, USAAbstractThere are three main sources ofthe radiation backgroundin MeRRIC: forward synchrotron radiation generated up-stream ofthe detector, the direct backward radiation causedby the photons hitting beampipe downstream of the detec-tor, and the indirect secondary radiation caused by hardphotons hitting vacuum systems, masks, collimators, ab-sorbers or any other elements in the interaction region. Inthis paper, we first calculate the primary radiation distribu-tion by employing electromagnetic theory. Then we obtainthe direct backward scattering rate by applying the kine-matic Born approximation deduced from scattering dynam-ics. The diffuse scattering cross section is calculated as afunction of the surface properties of the MeRRIC vacuumsystem. Finally, the dominating physical processes andminimization of indirect secondary radiation is presentedand discussed.INTRODUCTIONA staged approach towards the development ofa high en-ergy RRIC-based electron-ion collider has been proposedat BNL [1]. In the first stage, a medium-energy electronRelativistic Heavy Ion Collider (MeRRIC) would be con-structed. It would utilize a high energy ion beam, accel-erated in one of the two existing rings of the RHIC facil-ity, colliding with a medium energy (4 GeV) electron beam,generated by a proposed energy-recovery linac (ERL). Thecentral detector is equipped with a solenoid of up to 5 Tto facilitate particle identication. A pair of strong dipoleswith a magnetic field of 3 T are added at both end of thesolenoid, forming forward spectrometers. These dipolemagnets also provide the necessary separation of the twobeams [2]. Synchrotron radiation is generated close to theinteraction point and requires attention as part of the inter-action region design. The detector needs to be shieldedfrom synchrotron radiation photons with an appropriatemasking scheme. It is also necessary to investigate thebackground radiation in the physics experiment detector.FORWARD SYNCHROTRON RADIATIONIN THE INTERACTION REGIONThe hard photons generated by the electron beam bentby the strong horizontal dipole field upstream of the detec-'" Work perfonned under the United States Department ofEnergy Con-tract No. DE-AC02-98CHl-886.t bbwang@bnl.govtor would produce unacceptable background conditions inthe detector if they were allowed to hit any surfaces within.The width of this hard synchrotron radiation fan is reducedas much as possible by a primary synchrotron radiationcollimator. In order to facilitate complete removal of theremaining hard synchrotron radiation fan, a weak verticaldipole magnet is added bet\veen the central solenoid andthe strong dipole magnet. The bending angle of this dipoleis chosen such that at the location ofthe secondary collima-tor, the hard synchrotron radiation fan is completely sepa-rated from the electron and ion beams, and can therefore beremoved completely by the collimator [2]. The physical pa-rameters and synchrotron radiation properties of both hardand soft bending magnets are listed in Table 1.Table 1: Physical parameters and synchrotron radiationproperties of bending magnets located upstream and down-stream of the MeRRIe detector.I Magnet Magnet Bending I Critical RadiationL(UP/dO\Vn Length Angle I Energy Powerstreams) [m] [mrad] [keY] [W]up hard I 3.0 200 9.5 2403up soft 1.0 5 0.7 4.5down hard 3.0 250 11.8I3755down hard 1.0 4 0.6 2.9 iThe photon spectrum of forward synchrotron radiationcan be calculated with [3]:d2 n POI S(w/wc)dtdE Eb (w/wc)where Po is the synchrotron radiation power, J is the elec-tron relativistic factor E[~taz/E~est' Ee is the critical pho-ton energy, and the S-function is defined as:9!3 1°0s(~) = <8" w K 5/3(Z) dz.we 1rWc w/'--'cHere, K S / 3 (z) is the modified Bessel function of the sec-ond kind. Based on the physical parameters and the ra-diation properties listed in Table 1 the calculated photonspectrum of fonvard synchrotron radiation in the interac-tion region (without masks) is shown in Figure 1.BACKWARD SYNCHROTRONRADIATION IN INTERACTION REGIONOn the downstream side, the strong spectrometer dipoleproduces a very wide, hard synchrotron radiation fan thatFigure 2: Cross-section of diffuse scattering for h =0.5.MULTIPLE SCATTERING ISSUES.......... A=-G.5, E""2?k0V.1\=0.6, E=2.7keV-. A:::."O.e, E=2.7keVk=1, E::27keV=-- A=5, E=:2.7keV-- A::o:100, E=2.7keV.,..., A=O,5, E=10keVA-OO. E=10keV"" '" A.=C.8, E=10keVA:1, E=10keV"" .... A:;:5, E=10keV- - A:100. E=10keV10' ,------.--,-------.--,-------,----,------;:r====O;=~=;l--- A=IJ.5, E=-2.7keVA=0.6. E=2.7keV- A-08. E=2.7keVA=1, E=2,?k'2lV- A=5, E=2.7keV- A=100, E=2.7keV_ ... A=iJ.5, E=10keV~ ~ ~:g~: ~:~g~:~A=1, E=10keV- ~ A=5. E:1Dk.V- - A=100, E=10keV1C~14L---'--_.L____'__-"-------L_--'----___'_ _'__~J"..:...!'._'"Jo G 6 10 12 14 15Diffuse angle [degree]Figure 3: Cross-section of diffuse scattering for h = 1.Some hard photons produced in the strong dipole mag-nets could have energies up to 70 keY, as can be observedfrom Figure 1. Even though these photons are maskedfrom the detector opening, they could undergo multiplescattering through photon-matter interactions and cause in-creased background in the detector. It is therefore neces-sary to minimize the secondary scattering by careful de-sign and choice of material for vacuum systems, masks,collimators, absorbers and any other elements in the in-teraction region. There are many physical processes in-volving photon-matter interactions. These include the pho-toelectric effect, Rayleigh (coherent) scattering, Comptonculated cross-section of diffuse scattering over the rangeof MeRRIe radiation energy and surface parameters A,with h=O.5 which represents a somewhat 'jagged' surfacestructure. The unit of the cross-sections are normalized toLxLy . It can be observed from these results that on typi-cally rough surfaces, with 11, 0( 0.5, the diffuse scatteringcross-section is very small (0( 10-12). The worst case sce-nario corresponds to the extremely smooth surface, h=l. Inthis case, as shown in Figure 3, the diffuse scattering cross-section could be as high as 1O~2 in the normalized unit.c: l ~ upstream hard bend-§ _,! ~. upstream soft bend5: 101c~: downstream hard bend'-rfLi =-=d~o~w~nsQtr=ea=m~,=so==ft=b=en,;od~LL~~~~JLJ10'"~101102103104105Forward Radiation Photon Energy E leV]Figure 1: Photon spectrum of forward synchrotron radia-tion in the interaction region without masks.needs to be absorbed properly. To prevent back-scatteredphotons from entering the central detector, any direct line-of-sight into the detector has to be avoided. This is againaccomplished by adding a weak, vertically bending dipolebetween the solenoid and the strong spectrometer dipole(see Table 1). Two synchrotron radiation masks on thedownstream side of the detector minimize the opening an-gle of the back-scattered photon distribution entering thecentral detector beampipe [2]. However, the soft syn-chrotron radiation fan generated in the upstream verticalbending dipole magnet would still pass through the openingof the detector and the first downstream mask, then hit thesecond downstream mask or beam pipe, and cause back-ward scattering of soft X-rays.In order to estimate the backward scattering rate, we ap-ply the kinematic Born approximation deduced from scat-tering dynamics. The diffuse scattering cross-section ofsoft X-rays off rough surfaces can be expressed in polarcoordinates as [4]:p2 7'2 L L J 1 2 2hSUi) = el el2x y e-'1 qzAR Jo(qrR)RdRqzwhere Pel is the electron density of the surface material, Telis the classical electron radius, and q;; and qr = Jq~ + qBare the off-plane and in-plane scattering wave-vectors, re-spectively. .fa is the zero order Bessel function. h is asurface property parameter describing the roughness ofthesurface, ranging from 0 for extremely rough surfaces to 1for surfaces that are extremely smooth. A is a surface pa-rameter describing the correlation of the surface structure.In general, the above integral can be calculated numeri-cally. However, there are analytical solutions for h = 0.5and h = 1 given byI 2 2 / r 2 (A) 2 4] ~A7fPC..lTel.L:rLy. lqr + \"2 qz h = 0.5S(i1) = 21Tp2 r2 L L -bd el x Y e 2A'Iz h = 1.0Aq~ .In this investigation the surface material is stainless steeland the properties of iron are used. Figure 2 shows the cal-(incoherent) scattering, continuous energy loss, (e+, e-)pair production, positron annihilation, hadronic interaction,Bremsstrahlung, ionization and 8-ray production.A simulation study of secondary radiation with photonenergies up to 70 keVin the eRHIC interaction region wascarried out in BNL previously [5] using GEANT [6]. Thesimulation included an the physics processes listed aboveand implemented stainless steel vacuum chambers and cop-per radiation absorbers as in the current eRHIC design.It was noticed that the spectrum of backward radiationpeaked around 8 keY [5], which is close to the K-sheH en-ergies of iron (7.11 keY) and copper (8.98 keY). A carefulinvestigation indicates that in the photon energy range ofMeRHIC only the first three physical processes are impor-tant. Figures 4, 5 and 6 show the reaction coefficients ofphotons in the energy range of MeRHIC with 12 differentmaterials, for the photoelectric effect, Rayleigh (coherent)scattering and Compton (incoherent) scattering, respec-tively [7]. Comparing the coefficients of the three main re-actions, it can be seen that the secondary radiation is dom-inated by the values of the shell energies in the photoelec-tric coefficients (Fig. 4). The radiation background in theMeRHIC interaction region can be minimized with properknowledge of the photon energy distribution and selectionof suitable materials. In this regard, material propertieswith respect to the photoelectric effect are paramount.10° 10'Photon Energy [keV]Figure 4: Photoelectric absorption coefficientsN:§l1O' l :_.....~ ~~.~~LU$hwc:~ 100 IJ==~~....,..-~~ - 06C~ 1= nr:9 I2 ,- 24Cr1§10-f25Mnen -." 26 Fec D.' 27 Co~ 28 Ni~ -_. 29 Cu8 10-2 - _. 30 Zn10-1 10° 10' 102Photon Energy [keV]Figure 5: Rayleigh scattering coefficientsFigure 6: Compton scattering coefficientsCONCLUSION AND DISCUSSIONIn the current MeRHIe interaction design a collima-tion and mask system will be implemented to prevent syn-chrotron radiation from entering the central detector. A pairof weak vertically bending dipoles will be added betweenthe detector solenoid and the strong dipoles to provide thecomplete removal of "direct line-of-sight" into the detec-tor. The investigation of the radiation background in thedetector consists of evaluation of forward radiation, directbackward radiation, and the indirect secondary radiation.The photon spectrum of fOf\vard synchrotron radiationin the interaction region (without masks) provide good un-derstanding of the energy range and distribution of bothhard and soft X-rays in MeRRIe. The rate of direct back-ward scattering due to the soft X-rays is dominated by thesurface property. The rate of multiple scattering of hardX-rays is dominated by the photoelectric property of thematerial. These results offer a guideline for the choice ofthe material and surface in the design of vacuum system,masks, collimators and absorbers. Computer simulationswill be needed to obtain the final estimate of backgroundlevel in MeRRIe detector due to multiple scattering.REFERENCES[1] V. Ptitsyn et al., "MeRRIC - staging approach to eRHIC",Proceedings of this conference.[2] C. Montag, 1. Beebe-Wang, B. Parker, D. Trbojevic, "Inter-action Region Design for a RHIC-Based Medium-EnergyElectron-Ion Collider", Proceedings of this conference.[3] A. Hofmann, "Characteristics of Synchrotron Radiation",Proc. CERN Ace. School, CERN 98 A 04.[4] A. Gibaud and S. Hazra, "X-ray Reflectivity and DiffuseScattering", Current Science, Vol. 78, No. 12, 2000.[5] 1. Beebe-Wang et a1., "Synchrotron Radiation in eRHIC In-teraction Region" Proceedings of PAC 2005, p.2729, 2005[6] "GEANT, Detector Description and Simulation Too!",CERN Prog. Lib. Long Writeup W5013, 1994.[7] "Archive of Material Properties", University of Califomia atSan Diego, http://aries.ucsd.erlu'LIB/PROPS/PHOTON

Investigation of the radiation background in the interaction region of the medium-energy electron relativisitic heavy ion collider (MeRHIC)

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Investigation of the radiation background in the interaction region of the medium-energy electron relativisitic heavy ion collider (MeRHIC)

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