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Background radiation sources and average doses
Source Dose (mSv/y) Multiply by 100 to obtain dose in mrem/y.
Source Australia Germany United States World
Natural Radiation - external
Cosmic Rays 0.30 0.28 0.30 0.39
Soil, building materials 0.40 0.40 0.30 0.48
Radon gas 0.90 1.1 2.0 1.2
Natural Radiation - internal
40 K, 14 C, 226 Ra 0.24 0.28 0.40 0.29
Medical&Dental 0.80 0.90 0.53 0.40
TOTAL 2.6 3.0 3.5 2.8

To physically limit radiation doses, we use shielding    , increase the distance from a source, and limit the time of exposure .

[link] illustrates how these are used to protect both the patient and the dental technician when an x-ray is taken. Shielding absorbs radiation and can be provided by any material, including sufficient air. The greater the distance from the source, the more the radiation spreads out. The less time a person is exposed to a given source, the smaller is the dose received by the person. Doses from most medical diagnostics have decreased in recent years due to faster films that require less exposure time.

The image shows a dental patient wearing a lead apron sitting in a chair. X-rays emitting from an x-ray tube that is placed on the side of the patient’s jaw are passing through only the affected area of his teeth.
A lead apron is placed over the dental patient and shielding surrounds the x-ray tube to limit exposure to tissue other than the tissue that is being imaged. Fast films limit the time needed to obtain images, reducing exposure to the imaged tissue. The technician stands a few meters away behind a lead-lined door with a lead glass window, reducing her occupational exposure.
Typical doses received during diagnostic x-ray exams
Procedure Effective dose (mSv)
Chest 0.02
Dental 0.01
Skull 0.07
Leg 0.02
Mammogram 0.40
Barium enema 7.0
Upper GI 3.0
CT head 2.0
CT abdomen 10.0

Problem-solving strategy

You need to follow certain steps for dose calculations, which are

Step 1. Examine the situation to determine that a person is exposed to ionizing radiation.

Step 2. Identify exactly what needs to be determined in the problem (identify the unknowns). The most straightforward problems ask for a dose calculation.

Step 3. Make a list of what is given or can be inferred from the problem as stated (identify the knowns). Look for information on the type of radiation, the energy per event, the activity, and the mass of tissue affected.

Step 4. For dose calculations, you need to determine the energy deposited. This may take one or more steps, depending on the given information.

Step 5. Divide the deposited energy by the mass of the affected tissue. Use units of joules for energy and kilograms for mass. If a dose in Sv is involved, use the definition that 1 Sv = 1 J/kg .

Step 6. If a dose in mSv is involved, determine the RBE (QF) of the radiation. Recall that 1 mSv = 1 mGy × RBE ( or 1 rem = 1 rad × RBE ) .

Step 7. Check the answer to see if it is reasonable: Does it make sense? The dose should be consistent with the numbers given in the text for diagnostic, occupational, and therapeutic exposures.

Dose from inhaled plutonium

Calculate the dose in rem/y for the lungs of a weapons plant employee who inhales and retains an activity of 1.00 μCi of 239 Pu size 12{"" lSup { size 8{"239"} } "Pu"} {} in an accident. The mass of affected lung tissue is 2.00 kg, the plutonium decays by emission of a 5.23-MeV α particle, and you may assume the higher value of the RBE for α s from [link] .

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Source:  OpenStax, Basic physics for medical imaging. OpenStax CNX. Feb 17, 2014 Download for free at http://legacy.cnx.org/content/col11630/1.1
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