Background simulations: update and simulations of absorbed dose Ivan Logashenko Physics Department, Boston University December 12, 2008

Background simulation overview Hits in “sensitive detectors” energy deposition Cerenkov radiation Sensitive detectors: central cells central cell walls light guides muon veto counters human bodies Primary particle incoming neutrons stopped neutrons cosmic muons Simulation is done one particle at a time. Any hits in sensitive detectors are saved in the output file together with the full history of incoming particle.

Overall structure of simulation Beamline simulations Background simulations Sensitivity simulation B- and E-field calculations Storage simulations He3 simulations

Structure of background simulation code Geant4 engine version 4.9.0 patch 2 Analysis script (ROOT) Geometry description Output file Histograms XML-file materials size, shapes positions optical properties sensitive detectors Primary generators and tracking of the particles. G4NDL3.11 cross-section database for low-energy neutrons UCN production is included and used for normalization Hits in the sensitive detectors (energy deposition + Cerenkov photons) Full history of the particle, which caused the hit Loop through the hits and fill histograms Can be used in the sensitivity simulations Independent of the particular geometry

Ground electrodes and SQUIDs Detector geometry (1) Mu-metal shield and aluminum can Magnets and lead shield “Large” veto “Small” veto Ground electrodes and SQUIDs Concrete walls Collimator HV support and central enclosure

Detector geometry (2)

Incoming neutrons generator (1) Neutrons are generated at the collimator window with phase space according to output of beamline simulation (C.Crawford) Energy and angular distribution of incoming neutrons Histogram – beamline simulation, dots – primary generator of background simulation Interactions are switched off

Incoming neutrons generator (2) View from the side View from the top Interactions are switched off ~20 inch

Incoming neutron flux The background rates and absorbed dose depend on the incoming neutron flux – number of incoming neutrons per second The flux in calculated from the number of generated UCNs: from the background simulation, in average, we get 1 UCN (<200 neV) per 0.8 million of incoming neutrons. The UCN production cross-section in simulation agrees with the cross-section, quoted in the Design Report. from the design report, the expected UCN production rate is 0.3 UCN/cm3/sec common volume of measurement cells in simulation is 7.6 liters incoming flux is: 0.3*7600*0.8*106 = 1.8*109 n/sec If the incoming flux changes, the background rates and absorption doses should be adjusted proportionally

Absorbed dose 4 3 2 1 To calculate absorption dose, 4 “humans” are added to the picture: 1 – at the back 2 – right on top of the mu-metal shield 3 – 1.5 meters above the mu-metal shield 4 – in the control room Body elemental composition (by weight): 61% O, 23% C, 10% H, 2.6% N, 2% Ca, 1% P

Dose per 1 hour of the beam Dose per 1 year (107 sec of beam) Dose calculation 30 million incoming neutrons are simulated and the integrated energy deposition in each “human” is calculated. Almost all energy deposition is prompt – the integrated dose after 1 second after neutrons come is ~500 less that the dose during the first second. The dose [rad/sec] is calculated as follows: Energy_deposition[eV] * 1.8e9/3e7 * 1.6e-19[J/ev] / 90[kg] * 100[rad/Sv] Beam flux factor “Human” weight Dose per 1 sec of the beam Dose per 1 hour of the beam Dose per 1 year (107 sec of beam) At the back 0.35*10-6 rad/s 1.2 mrad/h 3.5 rad On top of the mu-metal shield 1.0*10-6 rad/s 3.6 mrad/h 10 rad 1.5 meters above mu-metal shield 0.30*10-6 rad/s 1.0 mrad/h 3 rad In the control room 0.10*10-6 rad/s 0.35 mrad/h 1 rad Statistical error is 10-20% (30-40% for the control room)

Backup slides

Materials Acrylic Aluminum6061 Mu-metal Lead G10 Concrete 60% Carbon + 32% Oxygen + 8% Hydrogen (Deuterium for the front window) Aluminum6061 97% Al + 1.2% Mn + 0.2% Cr + 0.3% Cu + 0.5% Fe + 0.5% Si + 0.2% Zn + 0.1% Ti Mu-metal 80% Nickel + 16% Iron + 4% Molybdenum Lead Pure lead G10 Si O2 C3 H3 Concrete 1% H + 0.1% C + 53% O + 1.6% Na + 0.2% Mg + 3% Al + 34% Si + 1.3% K + 4.4% Ca + 1.4% Fe Natural isotope composition is used for all elements.