Question

Your quality control/quality assurance manager has requested your assistance in the testing and repair facility. One of the test procedures utilizes a radiation source that is emitting gamma radiation at a rate of 50 mrem/hour at a distance of 1 foot. This testing is done for approximately 2 hours per day, 2 days per week. The Occupational Safety and Health Administration (OSHA) has a PEL of 1.25 rem per quarter and 5 rem per year.

1. Determine the employee's exposure for 1 year.
2. Calculate the exposure rate if the employee was moved to a distance of 3 feet from the radiation source.
3. Calculate the exposure rate of the employee if a 5 cm lead shield was installed between the source and the detector. The employee is working at a distance of 1 foot from the source. [µ for lead, (662 keV gamma ray) = 1.23cm^-1]

Answer 1

Exposure time for a year = 2x2x52 = 208 hours

exposure per year = 208 x50mrem = 10400mrem / year = 10.4 rem /year, which his higher than the prescribed limits.

Exposure rate is inversely proprotional to the ( distance)²

exposure = 10.4 /9 = 1.155

Intensity of the shielded radiation (I) = I₀ x exp ( - mu x t)

where t is the thickness of the shield and mu is linear attenuation coefficienct of lead

I = I₀ x exp( -1.23 x5)

= I₀ x 0.002133

= 10.4x0.002133 =0.0222