Mike's notes Flashcards
radiation safety accessories
linacs:
- door interlocks
-LPO button - 20 s after pressed, beam can turn on
-Beam on and off lights
-beam status indicator
-emergency stop switches
-radiation warning signs: Indicates high radiation area (if>25 μSv/h) at Vault door
-2 TV cameras
-2-way intercom
-enable/disable switch
-beam off button
brachy only:
-radiation area detector
-door alarm
CNSC annual dose limits
NEW : 20 mSv/yr averaged over 5 years, < 50 mSv/yr in any one given year
public: 1 mSv/yr
preganant woman:4 mSv from time she announces it
shielding max permissible dose (P)
NEW: 1 mSv/yr
public: 0.05 mSv/year
NCRP suggests 5 mSv/yr in controlled area, 1 mSv/yr in uncontrolled area
what is CNSC
canadian nuclear safety commission
what is nrc
nuclear regulatory commission (US)
pregnant NEW dose
4 mSv from declaration to end of pregnancy
PER CNSC FINAL ANSWER
diagnostic dose
3 mSv/yr
50% from CT and 25 % from nuc med
background radiation
1mSv year from cosmic (0.3 mSv), terrrestrial (0.3 mSv), and internal (0.4 mSv)
2 mSv from Radon
ALARA limits vs ICRP60 limits
ALARA limits are 1/20 those of ICRP60 or CNSC
what do you do if radiation level is > 25 uSv/h?
post sign
for a NEW working 40 h per week and 50 wk/yr, the 25 uSv/hr corresponds to 50 mSv/yr if beam is always on
properties of leakage radiation
depends on design
limitied to 0.1% of primary beam
originates from target
assumed to be isotropic
properties of scatter radiation
assumed to come mostly from patient
use largest field size fr measurement (40x40)
Assumed to be isotropic
when is neutron shielded needed?
E >/= 10 MV
barrier types
primary
-directly in path of radiation beam
-must shield for primary, scattered, and leakage radiation
secondary
-not in direct beam path
-accounts for scattered and leakage radiation
typical barrier thickness
Primary 2.1 - 2.4 m
Secondary 0.9 - 1.2 m
Thickness depends on energy, workload, occupancy, and distance.
why do we use 35x35 instead of 40x40 for max field size?
max field size not perfectly square (clipped corners)
-35x35 with collimator rotated 45 degrees
DONT USE THIS QUESTION
why is hydrogen content of shielding material important?
for neutron shielding
different shielding materials
concrete
heavy concrete
steel
lead
earth, dry packed
TVL
TVL = ln(10)/u
u is broad beam linear attenuation coefficient
more shielding needed with broad beam vs narrow beam to stop additional scatter
relate TVL to HVL
TVL =HVL * ln(10)/ln(2)
for broad beam, why are subsequent TVLe < TVL1?
TVLe is subsequent (equilibrium) TVL
Beam hardening counteracted by scattering to lower energies in broad beam geometry
leakage TVL < primary TVL also because leakage spectrum (after passing through the linac head) is softer than primary spectrum.
concrete primary TVL1, TVLe
TVL1 = 37 cm, TVLe = 33 cm for 6 MV
TVL1 = 45 cm, TVLe = 43 cm for 18 MV
concrete leakage and scatter TVL for 6 MV
leakage TVL1 = 34 cm, TVLe = 29 cm
scatter TVL = 17 cm
concrete leakage and scatter TVL for 18 MV
leakage TVL1 = 36 cm, TVLe = 34 cm
scatter TVL = 19 cm