Measurement Errors and Upgrades of the Ambient Dose Measurement - PowerPoint PPT Presentation

Measurement Errors and Upgrades of the Ambient Dose Measurement System at BESSY Y. Bergmann, M. Martin, K. Ott, L. Pichl Helmholtz-Zentrum Berlin, BESSYII, Albert-Einstein-Str.15, 12489 Berlin, Germany 1 RADSYNCH 2017, NSRRC, Hsinchu, Taiwan klaus.ott@helmholtz-berlin.de

2 Introduction • Calculated annual doses and dose rates • Measurement problems • Solution of measurement problems • Measured annual doses BESSY • Top-up operation experiences • Summary

3 Annual dose through open BS - sc1 Top-Up mode: 6000 h/a user operation (250 days) Annual dose close to front ends: < 11 mSv/a (fence) (calculated with Fluka 90 % injection efficiency 2E15/16 e-/a) Thin target (1 rad. length) Thin target (1 rad. length) Absorber is now Gamma radiation Neutron radiation shielded 1TVL PE At t fe fence ce: : 3.2-10 mSv/year At t fe fence ce: : 4.6-10 mSv/year Neuron dose/10 Absorb rber: r:1.0-3.2 mSv/year Absorb rber: r:10 -22 mSv/year

4 Annual dose through open BS - sc2 Top-Up mode: 6000 h/a user operation (250 days) Annual dose close to front ends: < 3.2 mSv/a (fence) (calculated with Fluka 90 % injection efficiency 2E15/16 e-/a) Undulator chamber Undulator chamber Absorber is now Gamma radiation Neutron radiation shielded 1TVL PE At t fe fence ce: : 0.32-1.0 mSv/year At t fe fence ce: : 1.0-2.2 mSv/year Neuron dose/10 Absorb rber: r: 0.032-0.1mSv/year Absorb rber: r:4.6-10 mSv/year

5 Annual dose through open BS Scenario Gamma dose Neutron dose Sum (average of range) (average of range) Thin target 6.6 mSv/ a 7.3 mSv/ a 13.9 mSv/ a Undulator 0.66 mSv/ a 1.6 mSv/ a 2.26 chamber mSv/ a Down- stream 0.066 mSv/ a 0.16 mSv/ a 0.226 dipoles mSv/ a Average annual dose: 5.46 mSv/ a for 6000 h/ a 1.82 mSv/ a for 2000 h/ a Conclu lusio ions: E Experim im ent al h l hall ll radio iolo logic ically lly cont rolle lled a area Em ployees ees a and user ers a are e radiat ion w orker er c cat eg egory B B Radia iat io ion t t hrough s shie ield ldin ing le less 1 1 m Sv/ a (num ber o of inj ect ed e elect rons unchanged)

6 Radiation through open BS Top-Up mode crash conditions: 1 nC/ shot (100 % losses) Gamma radiation Loss at ID chamber in tunnel Injection in experimental hall Loss at thin target in tunnel At t fe fence ce: : 0.03-0.1 µSv/shot At t fe fence ce: : 1.0-3.2 µSv/shot At t fe fence ce: : 0.3-1.0 µSv/shot Conclusions: Doses by losses in E-hall or tunnel differ only by a factor of 3 Interlock safed exclusion area + absorber avoids dangerous doses under crash conditions

7 Radiation through open BS Top-Up mode crash conditions: 1 nC/ shot (100 % losses) Neutron radiation Loss at thin target in tunnel Loss at ID chamber in tunnel Injection in experimental hall At t fe fence ce: : 0.2-0.46 µSv/shot At t fe fence ce: : 0.03-0.1 µSv/shot At t fe fence ce: : 3.2-10 µSv/shot Absorb rber: r:4.6-10 µSv/shot Absorb rber: r: 1.0-3.2 µSv/shot Absorb rber: r:10-32 µSv/shot Conclusions: Absorber is now shielded with 1 TVL PE (neutron dose /10) Doses by losses in E-hall or tunnel differ by a factor of 20 (fence) Interlock safed exclusion area + absorber avoids dangerous doses under crash conditions

8 Measurement problems Errors of neutron measurements (AB and Leake counters) High energy part of the spectrum (E> 10 MeV) not detectable For detection neutron energy has to be moderated down to thermal energy (25 meV) 10B(n, α )7Li fillgas BF3 Burst doses (> 100 nSv/ burst) not detectable Dead time effect of proportional counter (several µsec), short injection pulses convolution time synchrotron (e.g. 320 nsec ), low repetition rate (e.g. 10 Hz) short beam flashes by stochastic beam dumps Errors of gamma measurements (Ionisation chambers) High energy part of the spectrum (E> 7 MeV) not detectable Cross section for ionisation in fillgas much higher for lower energies

9 Solution of measurement problems Errors of neutron measurements (AB and Leake counters) High energy part of the spectrum (E> 10 MeV) not detectable 1.) Calculation of measurement errors and correction factors by FLUKA calculations of spectra 2.) Developed upgrade set to expand measurement range from 10 MeV to 1 GeV Burst doses (> 100 nSv/burst) not detectable 1.) Experiments to determine dead time losses, calculation of correction functions 2.) Usage of faster preamplifiers to reduce deadtime 3.) Usage of passive dosimeters close to front -ends Errors of gamma measurements (Ionisation chambers) High energy part of the spectrum (E> 7 MeV) not detectable Calculation of measurement errors and correction factors by FLUKA calculations of spectra 9

10 Passive dosimeters (LiF) Albedo Dosimeters are used for ambient dosimetry on PE phantom Can measure pulsed radiation but doses must > 50 µSv 3 months period measurements on all 16 sections close to natural background

11 Determination of correction formula    − ⋅ 2 p p   A H   = ⋅ + −   = A 1 H H t A q = + − H m H m = p   m q ⋅  − ⋅ ⋅ B C B C t 2 4 B C   A H B m 333 278 Biorem A MLS 222 Biorem B MLS True dose rate(10Hz)/µSv/h True burst dose/nSv 56 LB6419 MLS 14 28 56 83 111 139 Measured dose rate(10Hz)/µSv/h Measured burst dose/nSv Only detector dependent parameter included, result is usable for other acclerators:  τ Alpha = 2.59E-2 cps/(µSv/h) Biorem A H  A = m α = = H 1.94E-3 cps/(µSv/h) Biorem B  − ⋅ α t ⋅ 1 H / f B c T 1.71E-2 cps/(µSv/h) LB6419 m

12 True dose rate by Poisson distribution ⋅   Time distribution of neutrons inside moderator N W H 1 ∑ = −  −  i tp C 1 exp   after prompt pulse (Dinter, Tesch) H   H / c c = i 1 tp = − < lg m t / t t t I s Ch = measured events/ true events = measured dose/true dose N = number of time channels between = − + ≥ lg m t / t b t t o s accelerator pulses ti = 180 µs to = 265 µs 1/10 - value times ts = 250.5 µs intersection time b = -0.44615 ti, to dep. on mod. volume ∫ = W ( t ) m ( t ) / m ( t ) dt Widths of time channel = tau Only 1 or 0 events possible ⋅ Biorem type A MLS: tau = 17 µs   W H = − = −  −  i tp P ( m , 1 ) 1 P ( m , 0 ) 1 exp   i i   c Conclusion: increase in far saturation range results from exponential time distribution in Htp/c true events /accelerator pulse moderator Wi probability of event in ith channel Dead-Time Effects of Neutron Detectors due to Pulsed Radiation K. Ott, M. Helmecke, M. Luszik-Bhadra, M. Martin, A. Weber HZB, PTB, Charité Rad. Prot. Dos. Doi:10.1093/rpd/ncs326 (2012) p1-16

13 EURADOS measurements at HZB cyclotron Activation monitors Prototypes Commercially available monitors True doses are twice burst doses for values from table Biorem 79 nSv/Burst @ 10 Hz = 2.84 mSv/h true dose rate 5.68 mSv/h Intercomparison of Radiation Protection Instrumentation in a Pulsed Neutron Field M. Caresana, A. Denker, A. Esposito, M. Ferrarini, N. Golnik, E. Hohmann, A. Leuschner, M. Luszik-Bhadra, G. Manessi, S. Mayer, K. Ott, J. Röhrich, M. Silari, F . Trompier, M. Volnhals, M. Wielunski NIM A 737 (2014) 203 -213 U Milano, HZB, IFNF , CNAO, INBE, PSI, DESY , PTB, CERN, U Liverpool, IRPNS, HZM

14 Folding spectra with response functions Response function Biorem Fluence to dose conversion ICRP & Pelliccioni data CERN Neutron spectrum BESSY 1 m concrete Correction factor 2.98 High Energy Neutron Fields at Accelerators Rad. Prot. Dos. to be published K. Ott, Y . Bergmann, M. Martin, A. Weber HZB, Charité FLUKA Calculations of Neutron Spectra at BESSY K. Ott Proceedings EPAC 2006

15 Folding spectra with response functions Response function Biorem Fluence to dose conversion ICRP & Pelliccioni data CERN Neutron spectrum BESSY 0.8 m heavy concrete Correction factor 2.22 High Energy Neutron Fields at Accelerators Rad. Prot. Dos. to be published K. Ott, Y . Bergmann, M. Martin, A. Weber HZB, Charité

16 Pb moderator In Pb nuclear reactions with high energy neutrons (n,2n) outgoing neutrons < 10 MeV Dev evel eloped ed by by FL FLUKA calc lcula lat io ions (H (HZB): ): lea ead m oderat or t o t o incr crease se t h t he fluence fl ce of t her erm alized ed neut rons in in t h t he de det e t ect or t u t ube be by by fa fact ct or 3 3 as upgr pgrade de se set fo for st andard m onit ors 1 cm Pb with 4 % Sb to improve mechanical stability

17 Effect of Pb moderator Spectra calculated for Biorem on roof of CERF bunker Spectrum on Biorem PE Surface Neutron Fluence in Lethargy Units Spectrum on Biorem lead surface Maximum shifts from 100 to 1 MeV E/GeV Red: Spectrum at Pb, Green: Spectrum at PE , neutron spectum Summarized from the three contributions (p, pi+ , K+ ) Beamtime at CERF in June 2012 (M. Helmecke, K. Ott)

18 Result Instrument Intercomparison in the High Energy Mixed Field at the CERN-EU Reference Field (CERF) Facility M. Caresana, M. Helmecke, J. Kubanak, G.P . Manessi, K. Ott, R. Scherpelz, M. Silari Rad. Prot. Dos. 161 (2014) 67-72 CERN, HZB, U Milano, NPI/ASCR, U Liverpool, PNNL