INVERSE SQUARE
HVL & SHIELDING
WORKED PROBLEMS
BADGES & REPORTS
SAFETY PRACTICE
100
What is the square of the distance (I proportional to 1/d squared)?
For a point source, intensity is proportional to the inverse of this function of distance.
100
What is the linear attenuation coefficient mu?
HVL equals ln(2) divided by this material property.
100
What is I = I0 e to the minus mu d?
Transmission through a shield is given by this exponential formula.
100
What is minimal — below the measurable threshold?
A reading of M on a dosimetry report, in place of a number, means this.
100
What is medical imaging (diagnostic x-ray and nuclear medicine)?
The primary source of artificial ionizing radiation to the general public.
200
What is one-fourth (25%)?
If you double your distance from a point source, intensity falls to this fraction of the original.
200
What is 3.6 mm?
The attenuation coefficient of aluminum at 100 kV is 0.1925 per mm. The HVL is this.
200
What is 26 mR per month?
Old apron 0.17 cm, new apron 0.23 cm, mu = 2.31 per cm, old under-apron dose 30 mR/month. New under-apron dose is about this.
200
What is no?
An x-ray technologist needs diagnostic medical x-rays after an injury. Should the occupational badge stay on?
200
What is radon?
This component of natural background typically contributes the largest average annual dose in the U.S.
300
What is 1.67 mR/hr?
A detector reads 15 mR/hr at 2 feet from the tube. By inverse square, the reading at 6 feet is this.
300
What is 3.6 mm?
Reducing a beam by 50% requires this thickness of aluminum if mu is 0.1925 per mm — same number as the HVL.
300
What is 0.2 mrem/hr?
A Yellow-II package with TI 0.2 must read no more than this at 1 meter before you accept it.
300
What is a low-background storage rack?
The control badge must be kept here, not in the x-ray room or on a car dashboard.
300
What are a syringe shield and an L-block?
In nuclear medicine, these two devices keep your hands and torso off unshielded syringes and vials.
400
What is 7.75 feet?
Starting at 15 mR/hr at 2 feet, this is about how far you must stand to get under 1 mR/hr.
400
What is 3.30 mm?
Five millimeters of a new shield drops exposure to 35% of the initial rate. The material HVL is this.
400
What is 50 mrem/hr?
Yellow-II surface limit is this; White-I surface limit is 0.5 mrem/hr.
400
What is to measure background or transit dose so it can be subtracted from staff badges?
Main purpose of the control badge.
400
What is the image-receptor / image-intensifier side?
During C-arm fluoro, stand on this side of the patient to cut scatter.
500
What is the square root of (I1 / I2)?
The formula for a new distance that produces a target intensity is d2 = d1 times this.
500
What is one-fourth, or 25%?
Two half-value layers leave this fraction of the original beam intensity.
500
What is DPM = CPM divided by detector efficiency?
Convert wipe-test CPM to DPM using this relationship.
500
What is quarterly?
For radiation workers receiving only minimal dose, this is the longest usual interval before exchanging dosimeters.
500
What is written (and voluntary)?
Declaration of pregnancy must be this in order to trigger the 500 mrem fetal limit — a verbal heads-up is not enough.
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