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

2. 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.

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

4. 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

5. 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

6. Detector geometry (2)

7. 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

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

9. 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

10. 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

11. 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)

12. Backup slides

13. 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.