INVERSE SQUARE PROBLEMS
If a radiographer receives 50mR of exposure at 3 feet from a source for one hour, how much will the radiographer receive if he moves four feet from the source.
Hints to solve this problem:
A) Identify the formula
B) Place the factors in the problem into the formula
C) Ask yourself : With the change proposed in the problem, would the radiographer receive more or less exposure
D) Calculate the problem
E) Did the answer agree with the change you predicted?
A) I1 ( D2) squared
I2 (D1) squared
B) What is I1? 50mR
What is I2 ? X
What is D2? 4 feet
What is D1? 3 feet
C) less
D) 50 4 feet squared
X 3 feet squared
50 16 feet
X 9 feet
16X = 450
X =28 mR
E) yes, the exposure rate when down.
TRY THIS!!
If a radiographer receives 50mR of exposure at 3 feet from a source for one hour, how much will the radiographer receive if he moves two feet from the source and stays in for only 20 minutes.
A) Identify the formula
B) Place the factors in the problem into the formula
C) Calculate the problem and decide, without factoring in the time at this point , if the exposure rate will increase or decrease
D) Take you answer to the inverse square problem and now factor in the time difference
A) I1 ( D2) squared
I2 (D1) squared
B) What is I1? 50mR
What is I2 ? X
What is D2? 2 feet
What is D1? 3 feet
C) Exposure should increase!
50 2 feet squared
X 3 feet squared
50 4 feet
X 9 feet
4X = 450
X = 112.5mR PER HOUR
d) The tech was only in the room for 20 minutes with this new exposure versus one hour.
20 minutes/60 minutes = 0.33
112.5mr x 0.33 = 37.12 mR