Measuring dose equivalent in an aviation environment using PIN diodes

Transcription

1 Measuring dose equivalent in an aviation environment using PIN diodes Alex Hands Neutron Users Club meeting, NPL Wednesday 4 th November 29 1

2 Background Cosmic Rays consist primarily of protons and alpha particles with energies extending up to and beyond 1 2 ev (c.f. LHC ~1 13 ev) Interactions with the atmosphere produce various secondary particles including neutrons across a wide energy range Neutron flux builds up to a maximum at 6, feet but is reduced by two or three orders of magnitude at sea level At aviation altitudes dose levels can be significant for air crew or frequent flyers Solar Particle Events can result in doses above the legal annual maximum for pregnant aircrew in a single flight 2

3 The Current Deep Solar Minimum Is Giving Record High Cosmic Ray Fluxes In this environment, at 4 kft long haul doses could exceed 1 µsv (1 return trip = 2% of legal annual limit to general public or pregnant aircrew) 3

4 Aviation dosimetry Response to mixed field radiation environment essential Dose equivalent commonly measured on microdosimetric scale Gas-filled proportional counters with equivalent micron scale Silicon microdosimetric detectors with tissue-equivalent converters (suffer from small sensitive volume) This enables ICRP quality factors applied based on linear energy transfer (LET) to be applied directly Converts absorbed dose to dose equivalent * TEPC e.g. (n,α) Unit density wall Low density gas (< 1-4 g cm -3 ) 4

5 Original solid-state monitor - CREAM Cosmic Radiation Effects and Activation Monitor (CREAM) designed in 198s to actively monitor high energy cosmic ray particles in space Two Shuttle Activation Monitor (SAM) units built PIN diode array measures charge depositions from directly and indirectly ionising particles in 9 channels Shuttle flight delayed due to Challenger disaster New CREAM unit developed to fly on Concorde Dose equivalent calculated via semiempirical comparison to tissue equivalent measurements CREAM SAM-1 5

6 CREAM in Space On MIR Space Station ( ) On Space Shuttle ( ) 6

7 QDOS latest version: Internal batteries and data storage Single wide area PIN diode 15 energy deposition channels (.1 MeV 1 MeV) Live accumulated dose & dose rate readings (plus alert indicators) 7

8 Calibration flights Routes flown by Capt Ian Getley of Qantas On Boeing-747 Flights cover wide range of latitudes Other instruments flown for crosscomparison (including a TEPC) 8

9 Simple dose conversion Currently used for LCD screen display Low LET counts (which dominate count rate) scaled with single scale factor derived from broad range of flights Start Time Ch Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8 Ch 9 Ch 1 Ch 11 Ch 12 Ch 13 Ch 14 3/1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 22: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: /1/29 23: Dose Rate (µsv/hour) Rayhound Dose /11/28 14:52 3/11/282:52 Date Dose Rate (µsv/hour) Total Dose (µsv) Total Dose (µsv) 9

10 Effect of Geomagnetic Rigidity Divergence from true TEPC dose near equator (due to changing neutron intensity) QDOS / TEPC dose ratio Rigidity (GV) 1

11 Calibration for dose equivalence Most high LET energy deposition in diode is due to secondaries following nuclear reactions between atmospheric neutrons or protons and silicon Light ion secondary distributions established via Monte Carlo simulations Secondary particles: Nucleon flux.5 mm Si Flux per unit lethargy (arb.) Alpha Proton E (MeV)

12 Calibration for dose equivalence (2) Particle track broken up into microdosimetric scale sections: ~1 MeV proton (range 2µm) KeV per segment Deposited energy Q Quality factor 2 µm segments 12

13 Calibration for dose equivalence (3) Any given energy pulse could originate from many particle/track length combinations Different (maximum) path lengths through sensitive volume (RHS particles escape the diode) To calculate a mean quality factor for each channel we need: Energy distribution of secondary ions (as previously plotted) Path length distribution in sensitive volume (dependent only on diode geometry) E.g. a 6 MeV pulse could be 6 MeV proton depositing all it s energy or 9 MeV proton traversing 5 um 17.5 MeV proton traversing 1 um etc.(nb alphas too) 13

14 Calibration for dose equivalence (4) Considering all combinations leads to preliminary quality factor (Q*) for application to all measured energy channels to calculate dose equivalent Q* factor Ch Ch 1 Point at which alpha particles begin to dominate 5 Ch 5 Ch E dep (MeV) 14

15 Contribution from direct ionisation Low LET counts are dominated by direct ionisation depositions (high energy protons, electrons & photons) with quality factors of unity Balance of direct/indirect counts established by simulation: Relative contriubtions to total count rate (arb. scale) Shape in good agreement with real flight data (direct & indirect proton contributions plotted together) E dep (MeV) total neutron gamma proton electron Flight data Direct ionisation fraction Simple function derived: (average over latitude & altitude) 1% at high LET 1% at low LET E dep (MeV) 15

16 Quality factors Direct ionisation fraction applied to quality factors Q* factor Predominantly affects low LET Q* values ( 1) Unconstrained Q* Constrained Q* Constraining function E dep (MeV) Count fraction (F) due to direct ionisation 16

17 Effect on channels contribution to dose Applying Q*(E) factors to absorbed energy depositions results in a heightened contribution to dose from high LET channels 18% 16% 14% Simple method SYD-EZE (inc. Q*) SYD-EZE (w/o Q*) EZE-SYD (inc. Q*) EZE-SYD (w/o Q*) Dose fraction 12% 1% 8% 6% 4% 2% New method % Ch Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8 Ch 9 Ch 1 Ch 11 Ch 12 Ch 13 Ch 14 Channel # 17

18 Tissue Equivalence Absorbed dose is in silicon, needs to be converted to absorbed dose in tissue before Q* factors are applied for dose equivalent CERN-EU Reference Facility (CERF) used for concurrent experiments with TEPC Comparison of absorbed dose in simulated atmospheric environment yields factor of 1.4 for tissue equivalence (agrees well with theoretical values for protons and alphas) 18

19 Effect on TEPC comparison Binned doses from Qantas flights recalculated using new method (both tissue equivalence factor and Q*(E) factors applied) Dose ratio New dose / TEPC Screen dose / TEPC Rigidity (GV) Agreement across a range of rigidities implies that the varying neutron contribution is now properly accounted for 19

20 Flight example Good agreement with TEPC measurements over a long haul route covering a wide rigidity range Dose rate (Sv hr -1 ) LHR-SIN 31 Jan 9 21:36 : 2:24 4:48 7:12 9:36 12: Rigidity (GV) Altitude (km) QDOS Screen TEPC Altitude Rigidity 2

21 Flight example 2 High Southern Latitude Route from Buenos Aires to Sydney on 7 Dec. 28 cf. QARM (no TEPC flown) 14 EZE-SYD 7 Dec 8 12 Dose rate ( µsv hr -1 ) Rigidity (GV) Altitude (km) :24 16:48 19:12 21:36 : 2:24 4:48 7:12 9:36 Altitude Rigidity QARM QDOS Screen dose Route Dose: RH 75.6 µsv; QARM 78.7 µsv 21

22 Recent measurements Trans Polar Monitor flown on trans-polar routes in recent months (without TEPC) Very few measurements of this type but route becoming more important Dose rate ( µsv hr -1 ) Altitude (km) Rigidity (GV) Aug 9 9:36 8 Aug 9 12: 8 Aug 9 14:24 Toronto-HKG 8 Aug 9 8 Aug 9 16:48 8 Aug 9 19:12 8 Aug 9 21:36 9 Aug 9 : 9 Aug 9 2:24 9 Aug 9 4:48 9 Aug 9 7:12 Screen dose QDOS Altitude Rigidity QARM Total Dose 62 µsv 22

23 Recent measurements Trans Polar Route not always trans-polar Dose rate ( Sv hr -1 ) Altitude (km) Rigidity (GV) Aug 9 14:24 1 Aug 9 16:48 1 Aug 9 19:12 HKG-Toronto 11 Aug 9 1 Aug 9 21:36 11 Aug 9 : 11 Aug 9 2:24 11 Aug 9 4:48 11 Aug 9 7:12 11 Aug 9 9:36 Screen dose QDOS Altitude Rigidity QARM Total Dose again ~6 µsv (NB cruising alt only ~33-35 kft) + greatly exposed to SPEs 23

24 Conclusions New calibration approach considers energy depositions to derive a millimetre scale quality function (Q*) from first principles Excellent agreement with dosimetry standard TEPCs Use of such monitors could reduce dose uncertainties to around 5% and allow costeffective crew rostering. They could also enable avoidance of major solar particle events which have the potential to give annual dose limits in a single flight. Extensive flight programme planned through next solar maximum together with TEPCs and other monitors, hopefully including transpolar routes. Thanks to: Clive Dyer, Fan Lei, Peter Truscott, Keith Ryden, Paul Morris (QinetiQ) Capt Ian Getley (Dept of Aviation, University of New South Wales & Qantas), Les Bennett, Bryce Bennett, Brent Lewis (Royal Military College of Canada), Graeme Taylor (NPL), Markus Fuerstner (CERN) 24