Directional dose equivalent

Also ambient dose equivalent, directional dose equivalent, personal dose equivalent, organ equivalent dose. 

This quantity is defined by the ICRU, not by the ICRP, and is used mainly for operational aims in measurement of the radiation impact together with the quantities of directional dose equivalent and personal dose equivalent (see description of equivalent dose). The quantity ff (d) at a point in the radiation field is defined as the dose equivalent that would be produced by the corresponding aligned and expanded field in the ICRU-sphere at a depth d on the radius opposing the direction of the aligned field. The depth d = 10 mm is recommended for strongly penetrating radiation such as y- or neutron radiation. 

The normal or average risk from whole body gamma-ray exposure in the environment is only about 10% of that from average radon daughter exposure and much less in elevated indoor environments. Considering that the radon daughter lung cancer risk can be derived directly from exposure in most cases, effective dose equivalent is an unnecessary step. 

A particular set of values for tissue doses does not lead to the same numerical value for He and E. For example, assume a case in which the body is partially irradiated, the exposure is primarily to the chest, and the tissue doses are breast, 2 mSv lungs, 1 mSv active bone marrow, 0.2 mSv skin, 0.2 mSv other specific tissues and remainder tissues, negligible. The resulting values are He = 0.44 mSv and E = 0.25 mSv. For this reason, one cannot directly compare previous numerical values of He to current numerical values of E. Note also that a personal monitor located on the front at the chest would have indicated a dose equivalent in excess of 2 mSv, which is an overestimate of either or E. 

It is not practical in the work environment to measure the absorbed doses in the various organs and tissues necessary to compute He or E directly. Therefore, a number of quantitative relationships between He or E and various field or operational quantities have been developed and are available in the literature. The operational quantity named personal dose equivalent, HJ d), has been developed for the purpose of personal monitoring (ICRU, 1992), where d is the depth below a specified point on the body. For strongly-penetrating radiation, a depth of 10 mm is employed and the quantity is then specified as H iVS). The relationship between He or E and /fp(lO) is the most practical for use in determining He or E to workers for external exposure to low-LET radiation. 

In addition, multiple personal monitors are often used for situations in which a worker is exposed to a nonuniform radiation field, in an attempt to assess the region of the body receiving the highest deep dose equivalent. Approaches to the use of multiple personal monitors vary widely, and the number used and their locations depend on the particular work activity. For example, during work inside a steam generator, where the radiation fields are potentially isotropic, a total of 12 to 14 personal monitors may be placed at specific locations on both the front and the back of the body, and on top of the head. In other work situations, when the radiation field may be relatively directional but variable (e.g., during control-rod drive maintenance in a boiling-water reactor) the individual may wear all of the personal monitors at locations on the front of the body. 

H (d), at a point in a radiation field, is the dose equivalent that would be produced by the corresponding aligned and expanded field in the ICRU sphere at a depth d on the radius opposing the direction of the aligned field. A depth d= Q mm is recommended for strongly penetrating radiation. 

In practice, the instruments are properly calibrated to read directly Sv (or rem), or Gy (or rad). For some neutron detection instruments, the neutron flux is recorded. Then the dose equivalent is obtained after multiplying the flux by the conversion factor given in Table 16.4. Since different detectors do not have the same efficiency or sensitivity for all types of radiation and for all energies, there is no single instrument that can be used for all particles (a, y, n) and all energies. 

The assigned deep-dose equivalent and shallow-dose equivalent must be for the part of the body receiving the highest exposures. These data can be inferred from surveys or other measurements if direct data are not available. 

One of the limiting quantities in radiological protection against exposure of people is the effective dose (the others being equivalent doses to the lens of the eye and to the skin (e.g. see Section II-8 of Ref. [1]). As this is not a directly measurable quantity, operational quantities had to be created which are measurable. These quantities are ambient dose equivalent for strongly penetrating radiation and... 

It is assumed that within the range of exposure conditions usually encountered in radiation work, the risks of cancer and hereditary damage increase in direct proportion to the radiation dose. It is also assumed that there is no exposure level that is entirely without risk. Thus, for example, the risk factor for leukaemia is about 1 in 300 per sievert (see below for definition) of dose equivalent to the red bone marrow. In scientific notation, this is given as 3.4 x 10 per sievert. The mortality risk factor for all cancers from uniform irradiation of the whole body is about 3.4 x 10 per sievert for a UK working population, aged 20 to 64 years, averaged over both sexes. 

The new information on radiation risks prompted the National Radiological Protection Board (NRPB) to recommend that occupational workers exposure should be so controlled as not to exceed an average effective dose equivalent of 15 mSv per year. This recommendation has been considered by the Working Group on Ionising Radiations of the HSC ° and it can be expected that, pending revisions to the ICRP basic recommendations and the European Directive, this recommendation will be taken into account in an addition to the Approved Code of Practice during 1990. 

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