Parameter

Assumption

Explanation

Workload

1200 mA-min/wk

This is based on data from our inspection database.  Some facilities average between 500-600 patients per month.  Assume 600 patients per month, 4 weeks/month, 4 films/patient, 120 mAs/film

kVp

25-35 kVp

Assume kVp spectrum estimated by D.J. Simpkin.  14 % @ 20-25 kVp, 70 % @ 26-30 kVp, 16% @ 30-35 kVp

Occupancy Factor

1

All occupiable areas are assumed for design purposes to be fully occupied.  This is to allow for future changes in areas adjacent to the room

Primary use factor

0.25

For machines that are capable of permitting primary beams to miss the film holder, it is assumed that they are directed to any particular barrier 1/4 of the time

Secondary use factor

1

Secondary radiation always strikes the barrier

Leakage technique factor

35 kVp, 5 mA

Assume the machine will meet 100 mR/hr at 1 meter leakage limit when operated continuously at 35 kVp and 5 mA

Primary Distance

1.5 meters

Based on observation during inspections.  Many machines are positioned close to the vertical barrier. 

Secondary Distance

1.5 meters

Same

Leakage Distance

1.5 meters

Same

SID

65 cm

Most machines have SIDs near 65 cm

Beam size at SID

720 cm²

Area of a 24 by 30 cm cassette

Scattering Angle

135 degrees

A conservative scattering angle

Shielding design goal

0.02 mGy per week
(1 mGy per year)

Based on NCRP No. 147

mm

inches

Lead

0.139

5.5E-3

Concrete

16.6

0.65

Gypsum

45.8

1.8

Steel

0.731

0.029

Plate Glass

17.4

0.687

Wood

238

9.36

mm

inches

Lead

0.193

6.6E-3

Concrete

23.1

0.9

Gypsum

60.3

2.37

Steel

1.03

0.0405

Plate Glass

24.5

0.964

Wood

389

15.3