Since 22nd October 2012 the synchrotron light source  BESSY is operated in top up mode. Losses of electrons  during injection cause an electromagnetic cascade, that consists  of high energetic photons of the bremsstrahlung, and  secondary electrons and positrons from the pair creations.  The bremsstrahlung spectrum has a maximum at 1.022 MeV  owing to pair creations. The spectrum has a high energetic  tail, that reaches up to the electron energy of 1.7 GeV at  BESSY. The low energy part of the electromagnetic cascade  is produced by compton scattering or the photo   effect. Due  to the opened beamshutters during top up injections, the low  energetic part of the bremsstrahlung spectrum can reach the  experimental hall. We used the particle interaction and transport  code FLUKA for the calculations of both the fluence  and the dose distribution. We calculated the gamma spectra  of the radiation through the shielding walls and through the  front ends. We discussed the question whether additional  safety measures are necessary for top up operation due to  the low energy part of the spectrum. From our calculations  we determined the correction factors for our ionisation chambers  of the ambient dose measurement syste

Bergmann, Y.

Ott, K.

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FLUKA CALCULATIONS OF GAMMA SPECTRA AT BESSY ∗K. Ott †, Y. Bergmann, Helmholtz-Zentrum Berlin, BESSYII, Berlin, GermanyAbstractSince 22nd October 2012 the synchrotron light sourceBESSY is operated in top-up mode. Losses of electronsduring injection cause an electromagnetic cascade, that con-sists of high energetic photons of the bremsstrahlung, andsecondary electrons and positrons from the pair creations.The bremsstrahlung spectrum has a maximum at 1.022 MeVowing to pair creations. The spectrum has a high energetictail, that reaches up to the electron energy of 1.7 GeV atBESSY. The low energy part of the electromagnetic cascadeis produced by compton scattering or the photo - effect. Dueto the opened beamshutters during top-up injections, the lowenergetic part of the bremsstrahlung spectrum can reach theexperimental hall. We used the particle interaction and trans-port code FLUKA for the calculations of both the fluenceand the dose distribution. We calculated the gamma spectraof the radiation through the shielding walls and through thefront-ends. We discussed the question whether additionalsafety measures are necessary for top-up operation due tothe low energy part of the spectrum. From our calculationswe determined the correction factors for our ionisation cham-bers of the ambient dose measurement system.INTRODUCTIONThe storage ring BESSY is in operation since 1998 andsince 1999 used for a regular scientific program with syn-chrotron radiation. It has an extended double bend achromatlattice with a 16-fold symmetry. 14 straight sections havebeen used for the installation of wigglers, undulators andwave length shifters (WLS). Two sections are used for therf system and the injection septum. In 2011 the microtronhas been replaced by a 50 MeV linac which is operated aspreinjector for the synchrotron. This measure makes muchhigher charges in a single bunch possible and it was one ofthe prerequisites for the top-up operation [1]. The installa-tion of a cryogenic in-vacuum undulator is in preparation(EMIL project). Hutches are only necessary at the WLS andthe cryogenic in-vacuum undulator. At all other beamlinesthe synchrotron radiation is absorbed in the vacuum systembecause of the low critical energy which is about 2.5 keV atdipole beamlines and about 1 keV at W/U beamlines. Upto now there are 51 beamlines in operation. An overviewof BESSY is given in Figure 1 the most important machineparameters are shown in table 1.At every ratchet at the closest transversal distance to themachine a stationary γ und neutron measurement system isinstalled outside the shielding wall in the experimental hall.The detectors are a ionisation chamber and a BF3 counter.Measurement errors of neutron monitors are possible due∗ funded by the Bundesministerium fuer Bildung, Wissenschaft, Forschungund Technologie and by the Land Berlin† Email: klaus.ott@helmholtz-berlin.deFigure 1: BESSY II.Table 1: Machine ParametersLinac energy 50 MeVCharge per pulse 2 nCBooster energy 0.2 - 1.9 GeVStored beam energy 1.7 GeVBooster circumference 96 mStorage ring circumference 240 mMax. injection frequency 10 HzMax. top-up inj. frequency 0.1 HzStored current 300 mAStored charge 1.5E12 e-Lifetime at 300 mA 7 hto high energetic neutrons [2], that are not detectable bystandard neutron monitors and by dead - time effects dueto pulsed radiation [3]. These papers include correctionfactors [2] and correction formulas [3] for these errors.We discuss in this paper the contribution of the high en-ergy photons that cannot be detected by our gammamonitors.This is the first time that such an investigation is conductedat a synchrotron light source.Proceedings of IPAC2014, Dresden, Germany MOPRO05902 Synchrotron Light Sources and FELsA05 Synchrotron Radiation FacilitiesISBN 978-3-95450-132-8219 Copyright©2014CC-BY-3.0andbytherespectiveauthorsTOP-UP OPERATIONSince 22nd October 2012 BESSY is operated in top-upmode. The storage ring current for this mode is 300mA. Thiswas also the maximum current of the former decay mode.The shorter life time at higher current made it necessary toincrease the injection efficiency from 30 % to 90 % to keepthe number of injected electrons/year constant in compari-son with the former decay mode. The annual dose throughthe shielding walls depends on the number of electrons in-jected per year. For that reason top-up is not a method tocompensate reduced lifetimes (e.g. because of in-vacuumundulators) by permanent injections. Top-up injections oc-cur at opened beamshutters so we installed interlock saveexclusion areas close to the front - end. These areas are onlyaccessible if every beamshutter of this section is closed andno injection is made. Located in these areas (Fig. 2) arethe mirror chambers that separates the synchrotron radia-tion from the bremsstrahlung, the lead absorber for the gasbremsstrahlung (30 cm) and for scattered bremsstrahlung inforward direction respectively.Figure 2: BESSY experimental hall front end restricted area.Besides the bremsstrahlung absorber we placed a whitePE cylinder for the usage of an albedo dosimeter (Fig. 2).The albedo dosimeters are not accurate because of the min-imum detectable dose of 100 µSv but they can detect theeffects of crash conditions. Because of the possible errors ofthe radiation monitoring we used the injection efficiency tocontrol the top - up injections. This is much more accurateand faster than the usage of radiation monitors. The injectionefficiency η is defined by:η =∆ISRISY · 2.4 (1)η is measured shot by shot and averaged over a 4 h period.If the 4 h - average is below 90 % the deviation is compen-sated by a penalty time (decay mode), if η < 60% the top- up operation is stopped immediately and the storage ringis operated in decay mode. Before top - up can be startedagain, at least one injection shot must be > 60% with closedbeamshutters.All measures are planned to hold the 1 mSv/a limit inthe accessible part of the experimental hall as it was beforeduring the decay - mode. But this has to be proven during thetest operation phase. A detailled description of the radiationprotection issues of the top-up operation is given in [4].Table 2: Top-up Safety MeasuresLinac charge limit 2 nC/shotBooster frequency limit 1 HzInjection frequency limit 0.1 HzISY < 0.3 mA SY beam not extractedISR < 200 mA no top - up injectionsISR > 300 mA injection stopsη < 60% top - up stopsη < 90% in 4 h penalty time decay modeLifetime at 300 mA < 5 h SY beam not extractedFLUKA CALCULATIONSWe calculated the photon fluence spectra as photons perprimary/(GeV · sr · cm2) with FLUKA [5]. To obtain theintegral binned results multiply these values have to be mul-tiplied by the energy difference of the respective bin and by2pi for one-way scoring. We used a logarithmic binning upto the gamma energy of 1.7 GeV and down to 10 keV. In ourspectra Figures the group fluences are divided by the lethargyinterval lg Ei+1 − lg Ei (i is the respective bin number). Weuse boundary crossing estimators with different detector ar-eas. To increase the statistics we used parallel computingand merged the resulting data. The Figures are made withFlair, and the calculations described so far are included inthe norm to process the data files. The calculation of the in-tegral binned results and from them the dose values / energybin is made with our program specgam which uses the sub-routine deq99c.f [6]. This subroutine contains the fluence todose conversion factors [7] and is included in FLUKA since2008. We integrated the doses up to the maximum energy ofour ionisation chambers (7 MeV) and beyond, to calculatethe part of the dose that cannot be detected by them. Theshielding wall in forward direction at the beamline anglesconsists of a 5 cm lead screen and 1 m haematite concrete,in the transversal direction the shielding wall consists of 1 mordinary concrete or close to the front - ends of 0.8 m heavyconcrete. As the first scenario we considered a beam loss atthe steel tube of the straight section downstream the plannedcryogenic in-vacuum undulator at an angle of 1 mrad. Thecalculations are conducted with magnetic fields in the dipolechambers. The gamma dose/primary e- for this scenario isshown in Fig. 3.As the second scenario we considered a beam loss down-stream the second dipole. The produced bremsstrahlungpenetrates the side wall of the tunnel and reaches the placewhere our ionisation chambers are located. Again the beamhits the vacuum tube at an angle of 1 mrad. The gammadose/primary e- is shown in Fig. 4.RESULTSIn the following H7− and H7+ means the integrated dosesfrom the spectra for energies < 7 MeV and > 7 MeV respec-tively and HΣ is the dose that results from the full integralof the spectrum. From the first scenario we calculated theMOPRO059 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-8220Copyright©2014CC-BY-3.0andbytherespectiveauthors02 Synchrotron Light Sources and FELsA05 Synchrotron Radiation FacilitiesFigure 3: Gamma dose rate from one point source in straightsection in pSv/primary.Figure 4: Gamma dose rate from one point source behinddipole in pSv/primary.energy spectra of the photons that pass the aperture of theratchet end wall (Fig. 5).Figure 5: Gamma spectrum in forward direction.From the first scenario we got H7−/HΣ = 0.058 or 17.2 ascorrection factor. From the second scenario we calculatedthe energy spectrum of photons inside the tunnel (red curvein Fig 6) and that who penetrated the wall (black curve inFig 6).Figure 6: Gamma spectra in transversal direction.From the first scenario get H7−/HΣ = 0.374 or 2.67 ascorrection factor for the spectrum outside the wall. Insidethe tunnel we have H7−/HΣ = 0.967 or 1.034 as correctionfactor in the transversal direction. The ratchet end wall atBESSY reduces the gamma radiation in forward directionby five orders of magnitude (as well as the beamshutters),so at the location of the ionisation chambers the transversalradiation is more important and the correction factor forthis direction should be used. In Berlin the natural gammabackground is 0.6 mSv/a. The highest gamma value in theexperimental hall at BESSY was in 2013 0.82 mSv/a, whichwas reached at the injection septum. When we subtract thenatural background we have 0.22 mSv/a which is corrected0.59 mSv/a but it is still low if we consider that this is thevalue for 6000 h/a of acclerator operation and the maximumstay is 2000 h/a.REFERENCES[1] P. Kuske et al., “Preparations of BESSY for Top-Up Op-eration”, EPAC’08, Genoa, June 2008, WEPC037 (2008)http://www.JACoW.org[2] K. Ott, “FLUKACalculations of Neutron Spectra at BESSY”,EPAC’06, Edinburgh, July 2006, THPLS018 (2006) http://www.JACoW.org[3] K. Ott, M. Helmecke, M. Luszik-Bhadra, M. Martin, A. We-ber, “Dead-time Effects of Neutron Detectors due to PulsedRadiation”, Rad. Prot. Dosi., vol. 155,2 (2013)[4] K. Ott,“Radiation Protection Issues of the Top-Up Opera-tion of BESSY”, Proc. 7th Intern. Workshop on RadiationSafety at Synchrotron Radiation Facilities (RADSYNCH13),BNL, Upton, New York, USA (2013) http://www.bnl.gov/radsynch13/papers.php[5] G. Battistoni, S. Muraro, P.R. Sala, F. Cerutti, A. Ferrari,S. Roesler, A. Fasso‘, J. Ranft, Proceedings of the HadronicShower SimulationWorkshop 2006, Fermilab 6–8 September2006, M.Albrow, R. Raja eds., AIP Conference Proceeding896, 31-49, (2007); A. Ferrari, P.R. Sala, A. Fassò, and J.Ranft, “FLUKA: a multi-particle transport code”, CERNYellow Report (2005), INFN/TC05/11, SLAC-R-773[6] S. Roesler, G. R. Stevenson, CERN-SC–2006–070–RP–TN(2006)[7] ICRP Publication 74, (New York: Pergamon Press 1996); A.Ferrari, M. Pelliccioni Rad. Prot. Dosi. 77, p159 (1998)Proceedings of IPAC2014, Dresden, Germany MOPRO05902 Synchrotron Light Sources and FELsA05 Synchrotron Radiation FacilitiesISBN 978-3-95450-132-8221 Copyright©2014CC-BY-3.0andbytherespectiveauthors

Fluka Calculations of Gamma Spectra at BESSY

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Fluka Calculations of Gamma Spectra at BESSY

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