Topic 8 - External Dosimetry
Types of X-ray Machines
Acceptable Leakage Limits
Schematic View of X-ray Room
Design of X-ray facilities
Uncontrolled vs Controlled Areas
Design of Primary Protective Barrier
Selecting Shielding - K values
T = 1
|Work areas such as offices, laboratories, shops, wards, nurses’ stations; living quarters; children’s play areas; and occupied space in nearby; buildings.|
T = 1/4
|Corridors, rest rooms, elevators using operators, and unattended parking lots|
T = 1/16
|Waiting rooms, toilets, stairways, unattended elevators, janitor’s closets, and outside areas used only for pedestrians or vehicular traffic.|
Diagnostic machine operated
220 mA for an average of 90 s wk-1
Calculate primary barrier thickness if lead or concrete were to be used to protect an uncontrolled hallway 15 ft from the tube target
Useful beam is directed horizontally toward the barrier 1/3 of the time and vertically into the ground the rest of the time
|Material||Range of Density
|Peak Voltage (kVp)
||HVL Lead (mm)
||HVL Concrete (cm)|
With the previous example, an existing 3 in. sand plaster wall separates the X-ray room and the hallway.
What thickness of lead must be added to the wall to provide the primary protective barrier?
Secondary barrier is designed to protect areas not in line of the useful beam
Protects from scattered and leakage radiation
“Quality” of these two components can be very different
Shielding requirements are computed separately, then summed
Calculation methods vary, this is one alternative to Cember
Use factor (U) = 1
Leakage limits previously given
Express as Y (R h-1 @ 1 m)
Given Y, secondary barrier can be computed as # half-value layers needed to restrict exposure to allowed levels.
t is tube operation time, min wk-1
B is the required reduction for the X-ray intensity
I is the average beam current in mA
The number of half-value layers that reduces the radiation to the factor B of its unshielded value is given by
N = -lnB/ln2 = -lnB/0.693
Same setup as previous example, but a therapy machine is installed.
Machine has a continuous tube current of 26mA at 300 kVp.
Average workload in the facility is 24,000 mA min wk-1.
How many HVLs of shielding would be needed to protect the laboratory from leakage?
Based on tube operating potential
If tube < 500 kVp, then scattered Xrays assumed to be same as primary beam
If tube potential > 500 kVp, then scattered photons are treated like primary ones at 500 kVp.
Value of K for scattered radiation:
Calculate the number of HVLs needed to protect the laboratory from scattered radiation from the therapy machine
What thickness of lead must be added to an existing 2.5 in plaster wall between the X-ray room and the laboratory to provide an adequate level of shielding?
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