1. 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 450 mwe
Neutron production and propagation with unadorned GEANT4
Calculations with toy model (Noel Stanton)
Excel spreadsheet calculations
Muon flux from Hurwitz & 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

2. 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

3. Comparison of toy model and GEANT4 results for line and sheet illumination by muons
GEANT4 simulation:
H&P muon 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

4. 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 Hz (16 Hz from top panel)
Neutron rates per day in 3.5 m vessel:
Active tagging ignored E4 d-1
Active tagging ignored and <10 MeV in vessel 7.9 E3 d-1
(=> muon not thru vessel)
No tag seen d-1
Recoil proton rates per day in 3.5 m vessel:
Active tagging ignored E4 d-1
Active tagging ignored and <10 MeV in vessel 2.6 E3 d-1
No tag seen d-1
For now, multiply these preliminary rates by a safety factor of 5-10 !!
N. Stanton 3 Apr 06

5. 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 E4 d-1
From CHESS shield E3 d-1 Muon not thru vessel 7.86 E3 d-1
From rock d-1
TOTAL E4 d-1 TOTAL E4 d-1
With perfect tagging d-1 With perfect tagging 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

6. 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

7. 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

8. 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