k. oishi, k. kosako and t. nakamura institute of technology, shimizu corporation, japan
DESCRIPTION
id=17 SATIF-10. OPTIMUM SHIELDING DESIGN FOR ELECTRON LINAC ONCOLOGY FACILITIES INCORPORATING SEISMIC BASE-ISOLATION STRUCTURE. K. Oishi, K. Kosako and T. Nakamura Institute of Technology, Shimizu Corporation, Japan. 1. Background. - PowerPoint PPT PresentationTRANSCRIPT
K. Oishi, K. Kosako and T. NakamuraInstitute of Technology, Shimizu Corporation, Japan
id=17 SATIF-10
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Recently electric linac oncology facilities incorporating seismic base-isolation structure have become very popular in Japan to prevent strong impact to the equipments by the earthquakes.
When the boundary of the controlled area is set below the base-isolated foundations, additional shielding of iron plate is needed and the cost for shielding increases up to $100M.
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3
Since no one usually accesses underneath the base-isolated foundation, except short maintenance period for base-isolation structure, it is reasonable to set the boundary around the downstairs room by fence.
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concrete
concrete
air
air
room
pit
floor
side
X
Z
IC
target andcollimator
0°18.4°33.7°45°53.1°59.0°63.4°
iron
However, it is very difficult to estimate the dose rate of new boundary by using the simple shielding calculation method, because of rotating gantry head and effect of scattering radiation in the base-isolation floor.
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1.To investigate the complicated behavior of photons in the “pit” by the analysis,
(1) Which angle of the gantry will influence the most to the dose rate at the new boundary ?
(2) How much can we reduce the thickness of additional iron plate ?
(3) Which component of the radiation is dominant at the new boundary ?
2. To verify the calculated results by the measurement.
3. Assess the feasibility of simple calculation method from this study.
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2. Objectives
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Code: MCNP5 Library: EL3 for electron, MCPLIB04 for photon Source: 10 MeV electrons (pencil beam) Target: Cu (2 cm-thick) Tally: # cell tallies for photon and electron flux
: lower layers in pit (1 cm-thick, 10x20cm2)# point detectors for photon at positions
estimated.
•The criteria of the boundary dose rate is 8.3 micro Sv/h. ( 1,300 micro Sv/3 month) /156 h = 8.3 micro Sv/h )
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63.4°
air
concrete
concrete
air
air
room
pit
floor
side
X
Z
IC
target andcollimator
0°33.7°45°53.1°59.0°
boundary ofcontrolled area
cell tallies
100 cm
450 cm
iron
(1) Which angle of the gantry will influence the most to the dose rate at the new boundary ?
10-3
10-2
10-1
100
101
102
103
104
0 100 200 300 400 500 600
Photon Dose-rate distribution at upper position in pitspace for floor direction shielding (beam angle = 0 to 73.9 deg.,
current = 64.8 A).
0 deg.33.7 deg.45.0 deg.53.1 deg.59.0 deg.63.4 deg.69.6 deg.73.9 deg.
Distance on X-direction from IC [cm]
The highest dose rate was obtained at the direction 0 degree.
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(2) How much can we reduce the thickness of additional iron plate ?
X
air
concrete
concrete
iron
air
air
room
pit
floor
sideZ
IC
target andcollimator
0°
boundary ofcontrolld area
cell tallies
100 cm
450 cm
10-5
10-3
10-1
101
103
0 100 200 300 400 500 600
Photon Dose-rate distribution at upper position in pitspace for floor direction shielding (beam angle = 0 deg.,
current = 64.8 A).
iron 55cmiron 50cmiron 45cmiron 40cmiron 35cmiron 30cm
iron 25cmiron 20cmiron 15cmiron 10cmiron 5cmiron 0cm
Distance on X-direction from IC [cm]
No additional iron shield is necessary.
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(3) Which component of the radiation is dominant at the new boundary ?
10-2
10-1
100
101
102
103
104
0 100 200 300 400 500 600
Photon Dose-rate distribution at upper position in pitspace for floor direction shielding by with or without pit floor
(beam angle = 18.4 deg., current = 64.8 A, iron thickness = 5 cm).
with concrete floor in pitwithout concrete floor in pit
Distance on X-direction from IC [cm]
attenuation
scattering
concrete
pit
IC
boundary ofcontrol area
iron
To estimate the component of dominant radiation to the new boundary, we have performed calculation with and without base floor.
The influence of scattered radiation from the floor became dominant to the new boundary at the position greater than 3m.It decreased constant inclination.
Base floor
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concrete
concrete
concrete
air
air
room
pit
floor
side
X
Z
IC
target andcollimator
boundary ofcontrolled area
cell tallies
40 cm
5 cm
125 cm
130 cm
100 cm
450 cm
600 cm
100 cm
100 cm
10 cm
iron150 cm
airiron
100 cm
space for turn table
170 cm
0 cm
5. Comparison between calculated and measured 5. Comparison between calculated and measured resultsresults
Final shielding design
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10-2
10-1
100
101
102
103
104
0 100 200 300 400 500 600
Gamma-ray dose rate distributions in the underground pit for0 degree irradiation by 10 MV linac at the Tama-PFI hospital.
lower pit: Experimentlower pit: Calculation
Distance from center [cm]
0 degree irradiation(Cu target 2.0 cm-thick)
Very good agreement was obtained within 20%, except beneath iso-center and boundary.
10-2
10-1
100
101
102
103
104
0 100 200 300 400 500 600
Gamma-ray dose rate distributions in the underground pit for18.4 degree irradiation by 10 MV linac at the Tama-PFI hospital.
lower pit: Experimentlower pit: Calculation
Distance from center [cm]
18.4 degree irradiation(Cu target 2.0 cm-thick)
We have measured dose rate along the pit floorWe have measured dose rate along the pit floor by ionization by ionization chamber.chamber.
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Reasons of disagreement between calculated and measured Reasons of disagreement between calculated and measured resultsresults
The structure of turn table in the treatment The structure of turn table in the treatment roomroom
concrete
pit
X
Z
IC
boundary ofcontrol area
iron space for turn table
0 cm
# Anisotropy of turn table base was not considered in the calculation model.
# Pipes and some equipments near the boundary were not considered in the calculation model.
Pipes near the boundaryPipes near the boundary
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10-5
10-4
10-3
10-2
10-1
100
101
250 300 350 400 450 500 550 600
Photon Dose-rate distribution at upper position in pitspace for floor direction shielding (beam angle = 0 deg.,
current = 64.8 A).
iron 55cm
iron 50cm
iron 45cm
iron 40cm
iron 35cm
iron 30cm
iron 25cm
iron 20cm
iron 15cm
iron 10cm
iron 5cm
iron 0cm
Distance on X-direction from IC [cm]
Thickness ofiron (cm)
TVL (cm)
55 406.6450 391.0645 370.3240 376.0135 363.2230 389.4625 362.3720 400.9915 390.2410 366.885 358.980 372.41
6. Possibility of simple calculation method6. Possibility of simple calculation method
TVL in the pit varied from360 to 400 cm
Although the structure of the shielding wall is different, the slope of attenuation coefficient is almost the same. The energy spectra of attenuated photons are also very similar. We may define Tenth Value Layer (TVL)in the pit for simple calculation method.
10-15
10-14
10-13
10-12
10-11
10-10
10-9
10-8
10-2 10-1 100 101
Gamma-ray energy spectra at upper position in pit space for floor direction shielding (beam angle = 0 deg.) of Tama-PFI Hospital.
iron=55cmiron=50cmiron=45cmiron=40cmiron=35cmiron=30cmiron=25cmiron=20cmiron=15cmiron=10cmiron=5cmiron=0cm
Gamma-ray energy [MeV]
concrete
pit
IC
boundary ofcontrol area
iron
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ii xt
conironXU
Dt
UTDtDtL
IE
/
2
60
10
0.11060
: effective dose [μSv/(3 month)] at an evaluation position I0 : X-ray dose rate [Gy/min] at 1 [m] of distance from target 60 : conversion coefficient from minute to hour unit 106 : conversion coefficient from Gy to μGy unit L : distance [m] from target to an evaluation position Dt : transmittance of shielding material with thickness t [cm] where ti: effective thickness [cm] of shielding material
xi: Tenth value layer [cm] of shielding material T : operated time [hours/(3 month)] U : directional useful rate 1.0 : conversion coefficient from Gy to Sv unit. f : correction factor
XU E
fDt pit
Dose rate outside of the primary barrier
concrete
pit
IC
boundary ofcontrol area
iron
Conventional calculation method for primary shield for linacsConventional calculation method for primary shield for linacs
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ii xt
pitconironXU
Dt
fDtUTDtDtL
IE
/
2
60
10
0.11060
10-5
10-3
10-1
101
103
0 100 200 300 400 500 600
Photon Dose-rate distribution at upper position in pitspace for floor direction shielding (beam angle = 0 deg.,
current = 64.8 A).
iron 55cmiron 50cmiron 45cmiron 40cmiron 35cmiron 30cm
iron 25cmiron 20cmiron 15cmiron 10cmiron 5cmiron 0cm
Distance on X-direction from IC [cm]
f
Result of simple calculation method = 0.57 micro Sv/h Result of MCNP5 calculation method = 0.36 micro Sv/h
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7. Conclusion7. Conclusion
1. We have performed the analysis of the behavior of photons in the pit and found out no additional iron shielding was necessary, which carried out an extreme reduction of costs.
2. Good agreement between calculated and measured results were obtained, and passed the examination by the authority.
3. It has been suggested there is a possibility of simple calculation method for the estimation of dose rate at the new boundary.
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