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alara

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alara

مُساهمة من طرف adam في الخميس يونيو 24, 2010 4:32 pm

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 exposure, 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 an exposure reduces the effective dose proportionally. An example of reducing radiation doses by reducing the time of exposures might be improving operator training to reduce the time they take to handle a source.
  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.
  3. Shielding: Adding shielding can 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.
[edit] Types of radiation


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

In some cases, improper shielding can actually make the situation worse, when the radiation interacts with the shielding material and creates secondary radiation that absorbs in the organisms more readily.
[edit] Shielding design


Shielding reduces the intensity of radiation exponentially depending on the thickness.
This means when added thicknesses are used, the shielding multiplies. For example, a practical shield in a fallout shelter is ten halving-thicknesses of packed dirt, which is 90 cm (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] (see also Kearney, ref):
MaterialHalving Thickness, inchesHalving Thickness, cmDensity, g/cm³Halving Mass, g/cm²
lead0.41.011.312
concrete2.46.13.3320
steel0.992.57.8620
packed soil3.69.11.9918
water7.2181.0018
lumber or other wood11290.5616
depleted uranium0.080.219.13.9
air6000150000.001218
Column Halving Mass in the chart above indicates mass of material, required to cut radiation by 50%, in grams per square centimetre of protected area.
The effectiveness of a shielding material in general increases with its density.
[edit] ALARP

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 or 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.

adam

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مُساهمة من طرف د.عوض محمد الخضر في الجمعة يونيو 25, 2010 2:13 am

معلومات قيمة أخي ادم ذكرتنا أيام كنا طلاب وعملت لينا رفرشمنت للذاكرة مشكووووور وربنا يوفقك

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مُساهمة من طرف أميرة عثمان خوجلي في السبت يونيو 26, 2010 8:06 pm

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