lecture 29. radiation protection

Ahmed GroupAhmed GroupLecture 29Lecture 29

Lecture 29.

Radiation protection


• General philosophy• Stochastic and deterministic effects• Effective dose – relative weighting factors• Equivalent dose – tissue weighting factor• Committed dose• Collective exposure dose• Dose limits for occupational and public exposure• ICRP and NCRP


General philosophy

Second International Congress of Members prepare X-rayRadiology in Stockholm, 1928 protection recommendations

The congress set up the International X-Ray and Radium ProtectionCommittee; after World War II it morphed into two commissions:

1. The International Commission on Radiological Protection (ICRP)

2. The International Commission on Radiation Units and Measurements (ICRU)


Stochastic Effects and Deterministic Effects

Stochastic Effects.

1. No occupational exposure should be permitted untilthe age of 18 years;2. The effective dose in any year should not exceed 50 mSv(5 rem);3. The individual worker’s lifetime effective dose should notexceed age in years x 10 mSv (1 rem)

These limits apply to the sum of the effective dose from externalradiation and the committed effective dose from internalexposures.


Deterministic Effects

1. 150 mSv (15 rem) per year for the lens of the eye.

2. 500 mSv (50 rem) per year for localized areas of the skin and the hands and feet

The additional limits for deterministic effects are requiredbecause the weighting factors


Quantities and Units

Dose

The quantity used to measure the “amount” of ionizing radiationis the absorbed dose, or simply dose. This is defined as the energyabsorbed per unit mass, and its unit is joules per kilogram, thegray (Gy), named after the British physicist who contributed tothe development of ionization chamber theory.In the past the unit was the rad (radiation absorbed dose), definedas an energy absorption of 100 erg/g.Consequently, 1 Gy equals 100 rad.


Radiation Weighting Factor (WR)

Some radiations are biologically more effective, for a given dose,than others. This is taken into account by weighting the absorbeddose by a factor related to a quality of radiation.A radiation weighting factor (WR) is a dimensionless multiplier used to place biologic effects (risks) from exposure to differenttypes of radiation on a common scale.Radiation weighting factors are chosen by the ICRP as representative of RBE, applicable to low doses and low doserates, and for biologic end points relevant to stochastic lateeffects.


Radiation Weighting Factor (WR)Weighting factors recommended by the ICRP for different types of radiation


Equivalent dose – relative weighting factorsIn radiation protection, the equivalent dose is the product of the absorbed dose averaged over the tissue or organ and the radiationweighting factor selected for the type and energy of radiationinvolved.

Equivalent dose = absorbed dose x radiation weighting factor

If the absorbed dose is measured in gray, the equivalent dose isin sievert (Sv), named after the Swedish physicist who designedearly ionization chambers.1 Sv of either neutrons or X-rays does result in equal biologic effect(1 Gy of these radiations does not produce the same effect). TheICRP has recommended a new name for this quantity: Radiation weighted dose


Equivalent dose – relative weighting factors

If a radiation field is made up of a mixture of radiations, theequivalent dose is the sum of the individual doses of the various types of radiations, each multiplied by theappropriate radiation weighting factor.Thus, if a tissue or organ were exposed to 0.15 Gy (15 rad) of cobalt-60 gamma-rays plus 0.02 of 1-MeV neutrons,the equivalent dose would be:

(0.15 x 1) + (0.02 x 20) = 0.55 Sv, or 55 rem


Effective dose – tissue weighting factors

The sum of all of the weighted equivalent doses in all the tissues ororgans irradiated is called the effective dose, which is expressed bythe formula:

Effective dose = Σ absorbed dose x WR x WT

for all tissues or organs exposed.

The table on the next slide lists the tissue weighting factors recommended by the ICRP.


Effective dose – tissue weighting factors


Committed Equivalent Dose

In the case of external radiation, the absorbed dose is deliveredat the time of exposure, but for radionuclides, the total absorbeddose is distributed over time, as well as to different tissuesin the body.To take into account the varying time distributions of dose delivery,the ICRP defined the committed equivalent dose as the integral over 50 years of the equivalent dose in a given tissue afterintake of a radionuclide.This time was chosen to correspond to the working life of a person.


Committed Effective Dose

If the committed equivalent doses to individual organs or tissues resulting from the intake of a radionuclide are multiplied by the appropriate tissue weighting factors and then summed,the result is the committed effective dose.


Collective Equivalent Dose

The quantities referred to previouslyall relate to the exposure of theindividual. They become appropriatefor application to the exposure of agroup or population by the additionof the term collective.Thus, the collective equivalent doseis the product of the average equvalentdose to a population and the numberof persons exposed. The unit is person-sievert


Collective Effective Dose

The collective effective dose isalso the product of the average effective dose to a populationand the number of personsexposed.

The unit is again the person-sievert


Collective Committed Effective Dose

In the case of a populationingesting or inhalingradionuclides that deposittheir dose over a prolongedperiod of time, the integralof the effective dose overthe entire population outto a period of 50 years is called

These collective quantities can be thought of as representing thetotal consequences of exposure of a population or a group

collective committed effective dose.


Dose limits for occupational and public exposure Aims and Objectives of Radiation Protection

ICRP: The objectives of radiation protection are:

1. to prevent clinically significant radiation-induced deterministiceffects by adhering to dose limits that are below the apparent orpractical threshold, and2. to limit the risk of stochastic effects (cancer and hereditaryeffects) to a reasonable level in relation to societal needs, values,and benefits gained.


Aims and Objectives of Radiation Protection

The difference in shape of the dose-response relationships for deterministic and stochastic effects.


Aims and Objectives of Radiation Protection

A L A R A

The objectives of radiation protection can be achieved byreducing all exposure to as low as reasonably achievable(ALARA) and by applying dose limits for controlling occupational and general public exposures.For radiation protection purposes, it is assumed that the risk of stochastic effects is strictly proportional todose without threshold throughout the range of doseand dose rates of importance in radiation protection.


Dose limits for occupational and public exposure

Tolerance dose – a dose to which workers could be exposedcontinuously without any evident deleterious acute effects.The maximum permissible dose was designed to ensure thatthe probability of the occurrence of injuries was so low thatthe risk would be readily acceptable to the average person.


Doselimits for occupationalexposure


Risks associated with current recommended limits

Risk estimates for radiation-induced cancer and hereditary effects


Risks associated with current recommended limits


1946 the NCRP (the National Council on Radiation Protection)received a charter from Congress as an independent body toprovide advice and recommendations on matters pertaining toradiation protection in the United States. It is still the basis ofradiation protection policy in the United States today, thoughlegal responsibility for radiation safety is variously in thehands of the:

- Nuclear Regulatory Commission- Department of Energy- State or city bureaus of radiation control


Organizations

1. Committees that summarize and analyze data and suggest risk estimates for radiation-induced cancer and hereditary effects:- International level: United Nations Scientific Committee on the Effects of Atomic Radiation (UNSEAR);- United States committee appointed by the National Academy of Sciences, is known as BEAR (Biological Effects of Ionizing Radiations) Committee.2. Committees that formulate the concepts for use in radiation protection and recommend maximum permissible levels:- International level: International Commission on Radiological Protection (ICRP);- United States: the NCRP. It normally follows the ICRP;- United States: the Environmental Protection Agency (EPA), the Nuclear Regulatory Commission, the U.S. Occupational Safety and Health Administration, the Department of Energy.


Limits for occupational exposure.

The NCRP recommends the limits for occupational exposuresummarized in the following slides.These limits do not include natural background radiation orradiation for medical purposes

In the United States the Environmental Protection Agency (EPA)provide the guidance to federal agencies. In agreement states, theNuclear Regulatory Commission formulates rules for by-productmaterials from reactors. Table on the next slide lists “agreementstates” as of 2003.


ICRP and NCRP

At the present time, there is a difference in the recommendations of the national and international bodies regarding the maximum permissible effective dose for occupational exposure (stochastic effects). The differences are highlighted in the Table on the next slide.


ICRP and NCRPcompared


Doses to which individuals are exposed vary enormously by severalorders of magnitude. Figure 15.2 compares the ranges of doses used inmedicine with doses received occupationally and from natural sources.

This chart uses the new SI units of gray andsievert


Comparison ofthe ranges of doses used inmedicine with doses received occupationally and from natural sources.

This chart uses the older unitsbased on the rad and the rem

lecture 29. radiation protection

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