lecture 11

# lecture 11 - 1/23/2012 Health Physics Principles EEE 498,...

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1/23/2012 Lecture 1 1 Health Physics Principles EEE 498, EEE 591 Lecture 11 Radiation Dosimetry: Units, External Exposure Spring 2012, ASU Robert Metzger, [email protected] , BYEN335 Objective Find a unit that is descriptive of radiation risk. The unit should be measurable and easy to understand. We don’t have it. The units for radiation dosimetry evolved over time and are complex. In addition to being complex, there are two sets of them: Traditional and SI. Let’s look at them in the order they appeared.

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1/23/2012 Lecture 1 2 Exposure The first unit is that of radiation exposure. The traditional unit is the Roentgen which is defined as 2.58 x 10 -4 Coulombs per kg air. It is simply a measure of charge liberated in air. It is defined for gamma and x-rays only . There is no such thing as a Roentgen of alpha or beta radiation. The SI unit for exposure is just C/kg. Dose The traditional unit for r adiation a bsorbed d ose is the rad and is defined as 0.01 Joules per kilogram. It is defined for all radiations and all media and is just the average energy absorbed per gram of the material. The SI unit for dose is the Gray. The Gray is equal to 1 J/kg or 100 rads.
1/23/2012 Lecture 1 3 Dose Equivalent The long term risks from exposure to ionizing radiation (discussed in detail in Bio Effects) is dependant on both the energy absorbed and the density of the ionization path in the tissue. Densely ionizing radiations are more dangerous. To account for this, a new unit was developed that factors the increased risk associated with the densely ionizing radiations. Dose equivalent is a risk based unit and all radiation dose standards use this unit. Quality Factor The dose equivalent is the dose in rads or Grays multiplied by an arbitrary unitless number called the Quality Factor. The QF is related to the Relative Biological Effectiveness (RBE). RBE Energy quired to oduceaGiven Effect Energy quired to producetheSame Effect with keV x rays Re Pr Re 200

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1/23/2012 Lecture 1 4 Quality Factors Type of Radiation Traditional SI Gamma and X- ray 1 1 Beta 1 1 Alpha 20 20 Neutron 2 - 11 5 - 20 Dose Equivalent Traditional Unit is the R oentgen E quivalent
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## This note was uploaded on 02/11/2012 for the course EEE 498 taught by Professor Barnaby during the Spring '08 term at ASU.

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lecture 11 - 1/23/2012 Health Physics Principles EEE 498,...

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