K. Oishi, K. Kosako and T. Nakamura Institute of Technology, Shimizu Corporation, Japan

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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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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44

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


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


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



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


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


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


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


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


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


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


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


current = 64.8 A).




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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Documents

K. Oishi, K. Kosako and T. Nakamura Institute of Technology, Shimizu Corporation, Japan