Comparisons of neutron production by muons in GEANT4 and toy model Simulations with “old” pre-RAT GEANT4 (Peter Fisher) Muon flux and spectrum from Hurwitz and Pilcher @ 450 mwe Neutron production and propagation with unadorned GEANT4 Calculations with toy model (Noel Stanton) Excel spreadsheet calculations Muon flux from Hurwitz & Pilcher @ 450 mwe Neutrons / muon /(g/cm2) from Wang et al., <E> = 110 GeV Neutron production pointlike & isotropic, n’s go straight Shielding matls described by effective attenuation lengths Motivation Toy model helps intuition. Can be tuned from GEANT results? Comparisons reported here Line and sheet sources of muons Braidwood geometry (work in progress) N. Stanton 3 Apr 06

Comparison of toy model and GEANT4 results for line and sheet illumination by muons Vessel: 3.5 m radius CHESS concrete: 1.0 m thick, outside of cube at 4.5 m Muon line source: At x = 5.1m, 0.1 m inside rock Muon sheet source: Parallel to shielding face 12m wide, 20 m high Neutrons from sheet have more skewed paths thru shield than neutrons from line N. Stanton 3 Apr 06

Comparison of toy model and GEANT4 results for line and sheet illumination by muons GEANT4 simulation: H&P muon spectrum @ 450 mwe Toy model calculation: H&P total flux, Wang n/,  = 24.2 cm for CHESS Toy model (TM) predicts 3-5 times more n/ than GEANT  Too many n produced in TM? Attenuation length too long in TM? TM predicts smaller effect for more skew paths (S/L ratio)  Attenuation length too long in TM? Better average agreement with   19 cm in toy model. Seems very short! N. Stanton 3 Apr 06

Braidwood geometry GEANT4 simulation and preliminary results CHESS concrete box 1.0 m thick around vessel of radius 3.5 m Outside of box covered with “chambers”, essentially 100% efficient: Three 1 cm layers of Ar-CO2 (80-20), tag if > 10 keV deposit Rock starts 10 cm outside shield Note: 3.5 m rather than 2.6 m radius, no shielding from oil buffer yet Statistics-limited! Each generated event is propagated 20 times Preliminary results (4.5 M generated, 90 M propagated) Tagger rate 19.1 Hz (16 Hz from top panel) Neutron rates per day in 3.5 m vessel: Active tagging ignored 1.7 E4 d-1 Active tagging ignored and <10 MeV in vessel 7.9 E3 d-1 (=> muon not thru vessel) No tag seen 0.6 d-1 Recoil proton rates per day in 3.5 m vessel: Active tagging ignored 7.4 E4 d-1 Active tagging ignored and <10 MeV in vessel 2.6 E3 d-1 No tag seen 0.2 d-1 For now, multiply these preliminary rates by a safety factor of 5-10 !! N. Stanton 3 Apr 06

Braidwood geometry Compare toy model and GEANT4 TM: Spherical CHESS concrete shell 1.0 m thick around vessel of radius 3.5 m, CHESS  = 24.2 cm GEANT4: CHESS box, etc as on previous slide Neutron rates / day in 3.5 m vessel Toy model (tags ignored) GEANT 4 (tags ignored) From inside vessel 3.55 E4 d-1 From CHESS shield 7.92 E3 d-1 Muon not thru vessel 7.86 E3 d-1 From rock 100 d-1 TOTAL 4.35 E4 d-1 TOTAL 1.72 E4 d-1 With perfect tagging 30 d-1 With perfect tagging 0.6 d-1 (only rock events survive) Total is dominated by production in vessel (will be efficiently vetoed) TM total n rate into vessel is 2.5x GEANT4 total rate Non-vessel part agrees well. Note: shield contribution is insensitive to  Events in rock are tagged much more efficiently in GEANT4 We must understand this!! N. Stanton 3 Apr 06

Isolating contribution from vessel: compare DCA distribution DCA = Distance of closest approach to center of vessel of ’s that make n’s Look for distinctive shape of vessel contribution in TM:  r (R2-r2)1/2 To agree with GEANT4, TM vessel contribution must be renormalized down by factor 3.0 (consistent with result on previous slide) Remaining GEANT4 non-vessel contribution 7.9 E3 d-1, also consistent N. Stanton 3 Apr 06

Summary of GEANT4 & toy model comparisons of neutron production by muons Line and sheet muon illumination study Relative to GEANT4: Toy model over-produces neutrons in rock by  4x and/or under-attenuates neutrons thru CHESS shield Braidwood geometry study (3.5 m vessel, no buffer yet) TM over-produces neutrons in vessel by x 3 TM produces neutrons in shield OK (Primary vs equilibrium neutrons?) Note, neither vessel nor shield source sensitive to  of shield TM greatly overestimates untagged rock contribution This must be understood! Preliminary GEANT4 recoil proton rate looks promising but one should still apply a safety factor of 5-10 at this stage N. Stanton 3 Apr 06

Remaining work with “old” GEANT4 (before going over to RAT) Look at neutron & recoil proton energy spectra, make sensible cuts Look at fiducial (r < 2.6 m) region with oil buffer protection Vary tagging inefficiency (will admit shield events) What is tolerable? Examine event record to see how recoil protons are made Get better statistics N. Stanton 3 Apr 06