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
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klaus.ott@helmholtz-berlin.de
RADSYNCH 2017, NSRRC, Hsinchu, Taiwan
Helmholtz-Zentrum Berlin, BESSYII, Albert-Einstein-Str.15, 12489 Berlin, Germany
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Thin target (1 rad. length) Gamma radiation At t fe fence ce: : 3.2-10 mSv/year Absorb rber: r:1.0-3.2 mSv/year Thin target (1 rad. length) Neutron radiation At t fe fence ce: : 4.6-10 mSv/year Absorb rber: r:10 -22 mSv/year
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)
Absorber is now shielded 1TVL PE Neuron dose/10
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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 Gamma radiation At t fe fence ce: : 0.32-1.0 mSv/year Absorb rber: r: 0.032-0.1mSv/year Undulator chamber Neutron radiation At t fe fence ce: : 1.0-2.2 mSv/year Absorb rber: r:4.6-10 mSv/year
Absorber is now shielded 1TVL PE Neuron dose/10
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Average annual dose: 5.46 mSv/ a for 6000 h/ a 1.82 mSv/ a for 2000 h/ a Scenario Gamma dose
(average of range)
Neutron dose
(average of range)
Sum Thin target 6.6 mSv/ a 7.3 mSv/ a 13.9 mSv/ a Undulator chamber 0.66 mSv/ a 1.6 mSv/ a 2.26 mSv/ a Down- stream dipoles 0.066 mSv/ a 0.16 mSv/ a 0.226 mSv/ 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
elect rons unchanged)
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Injection in experimental hall At t fe fence ce: : 1.0-3.2 µSv/shot
Loss at thin target in tunnel At t fe fence ce: : 0.3-1.0 µSv/shot Loss at ID chamber in tunnel At t fe fence ce: : 0.03-0.1 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
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Loss at ID chamber in tunnel At t fe fence ce: : 0.03-0.1 µSv/shot Absorb rber: r: 1.0-3.2 µSv/shot Loss at thin target in tunnel At t fe fence ce: : 0.2-0.46 µSv/shot Absorb rber: r:4.6-10 µSv/shot Injection in experimental hall At t fe fence ce: : 3.2-10 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
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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
High energy part of the spectrum (E> 7 MeV) 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 Cross section for ionisation in fillgas much higher for lower energies
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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
High energy part of the spectrum (E> 7 MeV) 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 Calculation of measurement errors and correction factors by FLUKA calculations of spectra
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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
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m m t
C H B C B A p
m
− + ⋅ = 1
− + − ⋅ = q p p A Ht 4 2
2
C B H q
m
⋅ =
Biorem B MLS Biorem A MLS LB6419 MLS Measured dose rate(10Hz)/µSv/h True dose rate(10Hz)/µSv/h
m m t
Alpha = 2.59E-2 cps/(µSv/h) Biorem A 1.94E-3 cps/(µSv/h) Biorem B 1.71E-2 cps/(µSv/h) LB6419 Only detector dependent parameter included, result is usable for other acclerators: Measured burst dose/nSv
14 28 56 83 111 139 56 222 278 333
True burst dose/nSv
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s I
t t t t m < − = / lg
=
⋅ − − =
N i tp i tp H
c H W c H C
1
exp 1 / 1
s
t b t t m ≥ + − = / lg
Time distribution of neutrons inside moderator after prompt pulse (Dinter, Tesch) 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
= dt t m t m t W ) ( / ) ( ) (
⋅ − − = − = c H W m P m P
tp i i i
exp 1 ) , ( 1 ) 1 , (
Htp/c true events /accelerator pulse Wi probability of event in ith channel Widths of time channel = tau Only 1 or 0 events possible Ch = measured events/ true events = measured dose/true dose N = number of time channels between accelerator pulses Biorem type A MLS: tau = 17 µs Dead-Time Effects of Neutron Detectors due to Pulsed Radiation
HZB, PTB, Charité
Conclusion: increase in far saturation range results from exponential time distribution in moderator
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Intercomparison of Radiation Protection Instrumentation in a Pulsed Neutron Field
. Trompier, M. Volnhals, M. Wielunski U Milano, HZB, IFNF , CNAO, INBE, PSI, DESY , PTB, CERN, U Liverpool, IRPNS, HZM
NIM A 737 (2014) 203 -213
Biorem 79 nSv/Burst @ 10 Hz = 2.84 mSv/h true dose rate 5.68 mSv/h
Activation monitors Commercially available monitors True doses are twice burst doses for values from table Prototypes 13
High Energy Neutron Fields at Accelerators
. Bergmann, M. Martin, A. Weber HZB, Charité
Response function Biorem Fluence to dose conversion ICRP & Pelliccioni data CERN Neutron spectrum BESSY 1 m concrete Correction factor 2.98
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FLUKA Calculations of Neutron Spectra at BESSY K. Ott Proceedings EPAC 2006
High Energy Neutron Fields at Accelerators
. Bergmann, M. Martin, A. Weber HZB, Charité
Correction factor 2.22 Response function Biorem Fluence to dose conversion ICRP & Pelliccioni data CERN Neutron spectrum BESSY 0.8 m heavy concrete
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1 cm Pb with 4 % Sb to improve mechanical stability 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 fl fluence 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
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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)
Maximum shifts from 100 to 1 MeV
E/GeV Neutron Fluence in Lethargy Units Spectrum on Biorem PE Surface Spectrum on Biorem lead surface Spectra calculated for Biorem on roof of CERF bunker
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Instrument Intercomparison in the High Energy Mixed Field at the CERN-EU Reference Field (CERF) Facility
. Manessi, K. Ott, R. Scherpelz, M. Silari CERN, HZB, U Milano, NPI/ASCR, U Liverpool, PNNL
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Lead moderator developed at HZB with FLUKA calculations Accuracy demonstrated at CERF at CERN Cheap and reliable possibility to expand measurement range of standard neutron monitors from 10 MeV to 1 GeV Since Jan. 2015 in usage at BESSY – in future at bERLinPro
Patentamt: 17h Nov. 2014 Deutsches Gebrauchsmuster DE 20 2013 011 938 U1 Upgr grade de pr progr gram : 1.) l ) lea ead m oder erat ors 2.) fa fast st er pream plifi fiers s
τ = 1.
1.9µ 9µs f form er 10µ 10µs
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FLUKA Calculations of Gamma Spectra at BESSY
. Bergmann HZB
Proceedings of IPAC (2014) MOPRO059 H< 7/HΣ =0.374 (through wall) 0.967 (inside) H< 7/HΣ =0.058 (through aperture) Correction factor = 17.2 ! Correction factors = 2.67 and 1.03 Transversal direction Forward direction I- Chamber Emax = 7 MeV
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2014 2014 2015 2015 Experimental hall sections 1 – 16 (1 injection) odd high beta even low beta Green een: gamma red ed: : neutrons Cosmic gamma radiation 0.6 mSv/a in Berlin not subtracted Sum red 6.49 mSv Sum red 2.64 mSv red2015/ 2015/ red2014 2014 = 2. 2.46 46 in in agreem eem en ent w it it h high en ener ergy co corr. . fa fact ct or Lea ead m oderat ors in in usa sage si since ce
2015
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Act ivat ions at at sep sept um an and at at st st art an and en end of
t raight s t s: locat ion of
elec ect ron losses sses
Passive dosimetry close to front-ends: no top-up crash conditions occured
Example activations at tapers < 2µSv/h
Considerable measurement errors corrected: Dead time: correction formulas and faster preamplifier High energy: lead moderator and correction factors Control of top-up with injection efficiency much better Limitation of number of injected electrons (same as during decay mode) successful True annual dose outside exclusion areas < 1 mSv/ a To consider: work inside hutches (BS closed): shielding effect of BS to
neutrons, multiple scattered synchrotron radiation around BS
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Ahead: Cryogenic In-Vacuum undulator (2017), BESSY – VSR (2020)
Neutron monitors due to dead time effects and high energy neutrons Ionisation chambers due to high energy photons
neutron radiation showed dead time effects are common at commercially available neutron monitors from several nSv/burst on
response functions
usage at BESSY since start of 2015
undulator and BESSY VSR ahead
radiation workers, prerequisite DIS personal dosimeter certified for pulsed radiation)
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