Radiation depth dose

BARTLETT, D.T., FRANCIS, T.M. and DIMBYLOW, P.J. (1989). Methodol-ogy for the cahbration of photon personal dosemeters Calculation of phantom backscatter and depth dose distributions, Radiat. Prot. Dosim. 27, 231-244. 

GROSSWENDT, B. and HOHLFELD, K. (1982). Angular dependence of specified depth dose equivalent quantities in the ICRU sphere for photon irradiation, Radiat. FYot. Dosim. 3, 169-174. 

Another exposure tool is available in gamma radiation. While the correlation is not always perfect, there is a high degree of similarity in the response of polymers to the radiation from 60Co and from 50 keV electrons. Because of the penetrating nature of gamma rays, the exposure is not restricted to thin films or small amounts of polymer. Also, the absorbed dose is not complicated by the depth-dose function which must be used when electron-... 

ZAGORSKI, Z.P. Dependence of depth-dose curves on the energy spectrum of 5 to 13 MeV electron beams , Radiat.Phys.Chem 22 (1983) 409-418. 

FIG. 7.9. Depth-dose curves for electromagnetic radiation in water. Distance between source and water surface is 0.8 m area of beam at water surface 0.01 m. (From Spinks and Woods.)... 

Electron beam irradiation has been carried out with an electrocurtain accelerator manufactured by Energy Sciences, lnc.(model CB/150/15/180). The samples were placed on steel plates in aluminum trays and passed through the conveyor system of the electron beam apparatus. The maximum available dose per pass was 20 Mrad, hence, for the highest dose used in this study(40 Mrad), two passes were utilized. In light of the depth-dose profile at 175 kilovolts electron energy level of the EB system, the radiation dose will be nearly unifiorm throughout the sample thickness(3 mil). 

The radiation therapy in practice requires from the neutron source a high dose rate, good depth dose, well-defined horizontal and vertical beams movable around the patient, and a variable beam size up to about 20 x 20 cm. In addition, an average neutron yield of 4 x 10 /s, as well as 125 cm specified source-screen distance (SSD), and 2 cm maximum source diameter are needed for cancer therapy. A typical treatment schedule for malignant diseases involves two fractions per week, modal tumor doses of 1.2-1.3 Gy per firaction, and total doses in the range of 14-21 Gy. 

Assistance from the NEC s radiology lab under Adair Morrison saved Eldorado time and money on the BTU project. Its help with the shutter system and the collimator were particularly valuable. Morrison also experimented with small ionization chambers and water phantoms - targets prepared to mimic the human body - to measure the amount of radiation that the cobalt-60 therapy machine would deliver to different depths within a patient (a measurement at a particular depth was known as a depth-dose ). Johns conducted similar work in Saskatoon. ... 

About one third of all localized tumors, which contribute to about 60% of all cancers, are cured by radiotherapy alone or by radiotherapy in combination with surgery [2]. For another one-third of the localized tumors, however, a failure of local control is observed, and this group is expected to benefit from the development of techniques allowing a better conformation of dose to the tumor. The compromise between tumor cure and minimal side effects is largely determined by the physical properties of the radiation type under consideration, namely the depth dose profile. The depth dose profile defines the ratio of the dose delivered to the tumor and the dose delivered to the healthy tissue. 

Besides these distinct microscopic features, particle radiation also differs considerably from photon radiation with respect to the macroscopic distribution, i.e., the depth dose distribution. The typical shape is caused by the velocity-dependent stopping power, as described by the Bethe-Bloch-formula [63, 64] ... 

Within the framework of therapeutical applications of ion beams, this depth dose profile is often called inverted , because it shows low energy deposition at the entrance to tissue and high dose deposition in depth, in contrast to photon radiation, where in general the dose deposited in depth is lower than the dose deposited in the entrance region. The only exception to this general rule is the build-up effect for very high energetic photon beams, which can be explained by the forward... 

Warman, J. M. de Haas, M. R Luthjens, L. H. Horn, M. L. High-energy radiation monitoring based on radio-fluorogenic co-polymerization III Fluorescent images of the cross-section and depth-dose profile of a 3 MV electronbeam. Radial Phys. Chem. 2013,84, 129-135. 

DEPTH PROFILE. The secondary electrons produced by ionization processes from an incident beam of high-energy electrons are randomly directed in space. Spatial "equilibrium" is achieved only after a minimum distance from the surface of a polymer in contact with a vacuum or gaseous environment (of much lower density). Consequently, the absorbed radiation dose increases to a maximum at a distance from the surface (2 mm for 1 MeV electrons) which depends on the energy of the electrons. The energy deposition then decreases towards zero at a limiting penetration depth. 

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

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. 

Calibration of the intensities of the radiation flelds is traceable to the NIST. The ionization chambers and electrometers used by the service laboratories to quantify the intensity of the radiation fields must be calibrated by the NIST or an accredited secondary standards laboratory. The intensity of the field is assessed in terms of air kerma or exposure (free-in-air), with the field collimated to minimize unwanted scatter. Conversion coefficients relate the air kerma or exposure (free-in-air) to the dose equivalent at a specified depth in a material of specified geometry and composition when the material is placed in the radiation field. The conversion coefficients vary as a function of photon energy, angle of incidence, and size and shape of backscatter mediiun. 

The U.S. Environmental Protection Agency has proposed that "The annual alpha radiation dose rate to members of the critical segment of the exposed population as the result of exposure to transuranium elements in the general environment should not exceed either 1 millirad per year to the plumonary lung, or 3 millirad per year to the bone" (54). The USEPA also derived a soil contamination level of 0.2 pCi/m2 Tl cm depth, soil particles less than 2 mm) as a reasonable "screening" level for which the resultant dose rates to the critical segment of the exposed population could be reasonably predicted to be less than the guidance recommendations. 

---