January 30, 2013 | In: CEDO preparation guide

Radiation Dose Rate Calculation

Dose Rate as a Function of Distance

The increase in dose rate as you move towards a radiographic source follows a fixed law called the Inverse Square Law.

If you know the dose rate reading at a given distance from a radiographic source, you can, by applying this law, determine the dose rate at any other distance.

 

The inverse square formula is as follows:

Dose = Dose Rate  X  (Distance where you know the dose rate)^2 / (Distance where you want to calculate the dose rate)^2

 

Dose as a Function of Shielding

Skillful use of shielding is an effective way to reduce the intensity of radiation to which you and your fellow workers are exposed.  In general, the more dense a material is, the more effective it will be as a shield for gamma rays.

Uranium metal is one of the most effective shielding materials for gamma rays. Tungsten is also very effective, as well as lead and steel.

Concrete is not as effective unless it is a really thick wall.

The most practical use of shielding can be achieved by the use of collimators. Collimators are small pieces of lead, uranium, but mostly tungsten. They partially surround the source to absorb radiation not directed towards the object being radiographed.

The small size of collimators makes them easily portable so they can be carried into the field.

The decrease in dose rate which you can achieve with the use of shielding techniques is expressed in Half-Value Layers (HVL) and Tenth-Value layers (TVL).

A half-value layer is the thickness of a specified absorbing material required to reduce the dose rate to one-half of it’s original value.

A tenth-value layer is the thickness required to reduce the dose rate to one-tenth of the original value.

Below are some HVLs and TVLs for different materials;

 

OBLIGATIONS OF OPERATORS

Every person who operates an exposure device shall…

(a)  Use a survey meter that is capable of measuring a dose rate of gamma radiation from the sealed source of between 20 µSv and 100 mSv per hour

(b)  Keep on hand, material that can be used to attenuate, by a factor of at least 100, radiation from the sealed source

(c)  Wear on trunk of his/her body a dosimeter that has direct reading display capability and emits an audible warning signal when the radiation dose reaches or exceeds 5mSv per hour or when total dose reaches or exceeds 2 mSv

(d)  Keep a record of the dose of radiation received by the person for each day the person operates the RED

(e)  Limit the radiation dose received by any person other than a nuclear energy worker to 0.1 mSv/week or 0.5 mSv/year

(f)  Place barriers to prevent entry to area where radiation dose is greater than 0.1 mSv per hour

(g)  Lock the exposure device when it is not being operated

 

MAXIMUM PERMISSIBLE DOSES (MPD)

Effective Dose Limit

A)  Nuclear Energy Worker (NEW) including pregnant NEWs

  1. One year  =  50 mSv    (5 REM)
  2. Five year =  100 mSv (10 REM)

B)  Pregnant NEW (balance of pregnancy)

4 mSv  (400 mREM)

C)  Non-NEWs

One calendar year =  1 mSv (100 mREM)

 

Equivalent Dose Limit

A)  Lens of an eye

  1. NEW = 150 mSv (15 REM) for One-year period
  2. Non-NEW = 15 mSv (1.5 REM) one-year period

B)  Skin

  1. NEW = 500 mSv (50 REM)  ~ 1 year
  2. Non-NEW = 50 mSv (5 REM) ~ 1 year

C)  Hands/feet

  1. NEW = 500 mSv (50 REM) ~ 1 year
  2. Non-NEW = 50 mSv (5 REM) ~ 1 year

 

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