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Radiation Protection Information.
 
RADIATION PROTECTION INFORMATION.

Introduction

Radiation protection, sometimes know as radiological protection, is the science of protecting people and the environment from the harmful effects of ionizing radiation, which includes both particle radiation and high energy electromagnetic radiation.

Ionizing radiation is widely used in industry and medicine, but presents a significant health hazard, It causes microscopic damage to living tissue, resulting in skin burns and radiation sickness at high exposures and cancer, tumors and genetic damage at low exposures.


Types of Radiation

Different types of ionizing radiation behave in different ways, so different shielding techniques are used.

*   Particle radiation consists of a stream of charged or neutral particles , both charged ions and subatomic elementary particles. This includes solar wind, cosmic radiation, and neutron flux in nuclear reactors.
  • Alpha particles (helium nuclei) are the least penetrating. Even very energetic alpha particles can be stopped by a single sheet of paper.
  • Beta particles (electrons) are more penetrating, but still can be absorbed by a few millimeters of aluminum. However, in cases high energy beta particles are emitted shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite) [1]. In case of beta + radiation (positrons), the gamma radiation from the electron-positron annihilation reaction poses additional concern.
  • Neutron radiation is not as readily absorbed as charged particle radiation. Neutrons are absorbed by nuclei of atom in a nuclear reaction.
  • Cosmic radiation is not a common concern, as the Earth's atmosphere absorbs it and the magnetosphere acts as a shield, but it poses a problem for satellites and astronauts and frequent fliers are also at a slight risk. Cosmic radiation is extremely high energy, and is very penetrating.
*   Electromagnetic radiation consists of emissions of electromagnetic waves, the properties of which depend on the wavelength.
  • X-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. In some special applications, depleted uranium is used, but lead is much more common, several centimeters are often required. Barium sulfate is used in some applications too. However, when cost is important, almost any material can be used, but is must be far thicker. Most nuclear reactors use thick concrete shields to create a bioshield with a thin water cooled layer of lead on the inside to protect the porous concrete from the coolant inside.
  • Ultraviolet (UV) radiation is ionizing but it is not penetrating, so it can be shielded by thin opaque layers such as sunscreen, clothing and protective eyewear. Protection from UV is simpler than for the other forms of radiation above, so it is often considered separately.
In some cases, improper shielding can actually make the situation worse, when the radiation interacts with the shielding materials and creates secondary radiation that absorbs in the organisms more readily.


Principles of Radiation Protection

Radiation protection can be divided into occupational radiation protection, which is the protection of workers; medical radiation protection, which is the protection of patients; and public radiation protection, which is protection of individual members of the public, and of the population as a whole. The types of exposures, as well as government regulations and legal exposure limits are different for each of these groups, so they must be considered separately.

There are three factors that control the amount, or dose, of radiation received from a source. Radiation exposure can be managed by a combination of these factors :

1.  Time        : Reducing the time of exposure reduces the effective dose proportionally. An Example of reducing radiation doses by reducing time of exposures might be Improving Operator training to reduce the time they take to handle as source.
               *    This can be mathematically expressed as :
                                   Dose = Dose Rate x Time.
2.  Distance  : Increasing distance reduces dose due to the inverse square law. Distance can be as Simple as handling a source with forceps rather than fingers.   
              *      Mathematically it can be expressed as :
                                       I1     = d22
                                      I2     = d12
          Where, I1  and I2 are the radiation dose rate at distance d1 and d2  respectively.
3. Shielding : Adding shielding can be also reduce radiation doses. The effectiveness of a material as a Radiation shield is related to its cross-section for scattering and absorption, and to a First approximation is proportional to the total mass of material per unit area Interposed along the line of sight between the radiation source and the region to be Protected. Hence, shielding strength or "thickness" is conventionally measured in Units of gm/cm2. The radiation that manages to get through falls exponentially with The thickness of the shield. In x-ray facilities, the plaster on the rooms with the x-ray Generator contains barium sulfate and the operators stay behind a leaded glass Screen and wear lead aprons. Almost any material can act as a shield from gamma or X-rays if used in sufficient amounts.
Practical radiation protection tends to be a job of juggling the three factors to identify the most cost effective solution.

In most countries a national regulatory authority works towards ensuring a secure radiation environment in society by setting requirements that are also based on the international recommendations for ionizing radiation (ICRP - International Commission on Radiological Protection):

- Justification : No unnecessary use of radiation is permitted, which means that the advantages must outweigh the disadvantages.
- Limitation : Each individual must be protected against risks that are far too large through individual radiation dose limits.
- Optimization : Radiation doses should all be kept as low as reasonably achievable. This means that it is not enough to remain under the radiation dose limits. As permit holder, you are responsible for ensuring that radiation doses are as low as reasonably achievable, which means that the actual radiation doses are often much lower than the permitted limit.


Shielding Design

Radiation shielding shall be designed by the specialize / medical physicist during the early planning stages, since the shielding requirements may affect the choice of location and the types of room/building construction, and it it necessary to submit the final shielding drawing and the specification to the pertinent regulatory agencies for review and approved.

Shielding reduces the intensity of radiation exponentially depending on the thickness. This means when added thickness are used, the shielding multiples. For example, a practical shield in a fallout shelter is ten halving-thicknesses of packed dirt, which is 90cm (3 ft) of dirt. This reduces gamma rays by a factor of 1/1, 024, which is 1/2 multiplied by itself ten times. Halving thicknesses of some materials that reduce gamma ray intensity by 50% (1/2) include [1].

Materials
Halving Thickness,
inches
Halving Thickness,
 cm
Density,
g/cm3
Halving Mass,
g/cm3
Lead
0.4
1.0
11.3
12
Concrete
2.4
6.1
3.33
20
Steel
0.99
2.5
7.86
20
Packed soil
3.6
9.1
1.99
18
Water
7.2
18
1.00
18
Lumber or
other wood
11
29
0.56
16
Depleted
uranium
0.08
0.2
19.1
3.9
Air
6000
15000
0.0012
18

Column Halving Mass in the chart above indicates mass of material, required to cut radiation by 50%, in grams per square centimeter of protected area.

The effectiveness of a shielding material in general increases with its density.


Shielding Design for an Exposure Room
  • An exposure room is a shielded enclosure specially designed to provide adequate protection against ionizing radiation to persons in its vicinity.
  • The use of an exposure room allows other work in the vicinity to continue without any due consideration given to the potential risk caused by high radiation.
  • It is important to plan the design of the exposure room for immediate and foreseeable future needs before commencing its construction.
  • Detailed drawings or sketches are prepared of the installation and its surroundings, including dimensions of each enclosed area, thickness, density and type of shielding materials on all sides including those above and below the exposure area.
  • Entrances are identified and distances to potentially occupied areas adjacent to, above and below the exposure area are indicated.
  • The basic design principles for all exposure rooms are similar although there is a slight different in term of shielding characteristics required in the shielded enclosure for X-rays or gamma radiation.
  • The shielding design should also consider both the primary and scattered radiation. The amount of shielding required should be calculated with reference to the dose rate, use factor and occupancy factor.
  • Documentation which shows results of calculations, measured radiation levels and maximum expected radiation levels inside the shielded enclosure and in all areas adjacent to it should be established and properly kept.
  • When the design of the exposure room has been established, no subsequent changes which affect radiation safety should be made unless they are proved to be more effective and are authorized or approved by the AELB.


Common Types of X-Ray or Radiation Rooms

  • X-Ray / Radiology / Radiographic Imaging Rooms
  • Fluoroscopy Rooms
  • Computerized Tomography / C.T. Scanner Rooms
  • Positron Emission Tomography / C.T. Scanner Rooms
  • Emergency Rooms (E.R.), Special Procedures and Operating Rooms (O.R.)
  • Chiropractic X-Ray
  • Dental X-Ray
  • Mammography
  • Angiography Room / Cardiac Catherization Labs
  • MRI (Magnetic Resonance Imaging) or Radio Frequency Rooms
  • Ultrasonography / Ultra Sound Rooms
  • Gamma Knife Rooms
  • Linear Accelerators / Radiation Therapy
  • Proton Beam Room
  • NDT Rooms


Main Article : ALARP

ALARP is an acronym for an important principle in exposure to radiation and other occupational health risks and stands for "As Low As Reasonably Practicable". The aim is to minimize the risk of radioactive exposure or other hazard while keeping in mind that some exposure may be acceptable in order to further the task at hand. The equivalent term ALARA, " As Low As Reasonably Achievable", is also commonly used.

This compromise is well illustrated in radiology. The application of radiation can aid the patient by providing doctors and other health care professionals with a medical diagnosis, but the exposure should be reasonably low enough to keep the statistical probability of cancers or sarcomas (stochastic effects) below an acceptable level, and to eliminate deterministic effects (e.g. skin reddening and cataracts). An acceptable level of incidence of stochastic effects is considered to be equal for a worker to the risk in another work generally considered to be safe.

This policy is based on the principle that any amount of radiation exposure, no matter how small, can increase the chance of negative biological effects such as cancer, though perhaps by a negligible amount. It is also based on the principle that the probability of the occurrence of negative effects of radiation exposure increases with cumulative lifetime dose. These ideas are combined to form the linear no-threshold model. At the same time, radiology and other practices that involve use of radiations bring benefits to population, so reducing radiation exposure can reduce the efficacy of a medical practice. The economic cost, for example of adding a barrier against radiation, must also be considered when applying the ALARP principle.

There are four major ways to reduce radiation exposure to workers or to population:

  • Shielding. Use proper barriers to block or reduce ionizing radiation.
  • Time. Spend less time in radiation fields.
  • Distance. Increase distance between radioactive sources and workers or population
  • Amount. Reduce the quantity of radioactive material for a practice.


References

  • IRPA, International Radiation Protection Association as worldwide association of individuals engaged in radiation protection.
  • Radiation protection of patients International Atomic Energy Agency information on the safe use of radiation in medicine.
  • ICRP, International Commission on Radiation Protection.
  • ICRU, International Commission on Radiation Units.
  • IAEA, International Atomic Energy Agency.
  • UNSCEAR, United Nations Scientific Committee on the effects of Ionizing Radiations.
  • HPA (ex NRPB), Health Protection Agency, UK.
  • NRCP, National Council on Radiation Protection and Measurement, USA.
  • IRSN, Institute for Radioprotection an Nuclear Safety, France.


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All calculations for lead shielding requirements should be determined by a qualified health radiation physicist or other radiation "expert" as defined by NCRP or ICRP, and currently recognized by the country/state/province/region/area in which the project occurs.

CUSTOMER OR END USER SHOULD HAVE ALL RADIATION SHIELDING PROJECTS COMPLETELY TESTED AND SURVEYED BY A THEIR ORIGINAL PROJECT RADIATION PHYSICIST OF RECORD,

WHO MUST BE A QUALIFIED RADIATION HEALTH PHYSICIST OR OTHER RADIATION "EXPERT" AS DEFINED BY EITHER NCRP OR ICRP, AFTER INSTALLATION AND PRIOR TO OCCUPANCY AND USE.

Please see DIAGNOSTIC RADIATION SHIELDING CONSIDERATIONS for more information.

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