La estética Kitsch como preámbulo de lo Chicha

X-ray equipment produces a continuous range of photon energies up to a threshold level dependent upon the tube voltage setting. The threshold photon energy level can be adjusted from 50 keV or less up to a maximum (for high energy equipment) of perhaps 30 MeV. The photon energy of gamma ray sources is fixed.

The output of radiation per unit time is variable for x-ray equipment up to the maximum mA rating of the tube. The output of radiation from a radioactive isotope is fixed by the source activity. The output of radiation produced by x-ray equipment is generally much greater than that produced by radioactive isotopes.

The penetrating power of ionising radiation is controlled by its maximum photon energy and the photon energy distribution. Table 4 gives an indication of the maximum steel thickness that can practically be radiographed using conventional x-ray equipment and the commonly encountered isotopes. The penetrating power of x-rays produce by self rectified equipment is less than that of x-rays produced by constant potential equipment operating at the same tube voltage. This is because the constant potential equipment produces a larger proportion of high energy radiation than does the self rectified.

Source of Radiation Useful Thickness Range/mm of Steel

Table 4. Useful thickness range for various sources of radiation.

Note: Steel sections of 500 or 600 mm can be radiographed using x-rays generated by linear accelerators or betatrons.

7.6.2 RADIOGRAPHIC CONTRAST

Low energy radiation is more easily absorbed by than high energy radiation.

A radiograph produced using small effective source size will usually be of higher quality than one produced with a larger effective source size. The average focal spot size an x-ray tube is similar to the average physical size of the gamma ray sources which are

commonly used. Most x-ray tubes have a fixed effective focal spot of between 1 and 4 mm.

With some x-ray equipment the focal spot size can be varied. Microfocus x-ray tubes may have an effective focal spot of less than 0.1 mm. The size of the focal spot in an x-ray tube tends to be larger for the higher maximum kilovoltage tubes. This is due to the need to dissipate the increased amount of heat generated at high kV. The practical source size for a radioactive isotope is determined by the maximum economically achievable specific activity.

Specific activity is usually expressed in curies (or becquerels) per gram. Table 5 below gives typical practical achievable maximum specific activity for 4 common isotopes.

Isotope

Note 1. Density is for compressed caesium chloride (CsCl) Note 2. Density is for thulium oxide (Tm₂O₃)

Table 5. Specific activity for common radioisotopes

Note that the maximum activity of a gamma ray source is limited by it’s physical size. The most useful isotopes are those which have a high value of practically achievable specific activity. In an iridium 192 source at the maximum achievable activity about 2.5 atoms per 100 million are radioactive. In a cobalt 60 source the figure is only about 1 atom in every 10,000 million.

The output of radiation from a typical x-ray machine is much greater than the output of radiation from a typical gamma source. This means that in x-radiography the use of long

focal to film distances is more economically feasible than is the case in gamma radiography.

Thus, even though the focus is similar in physical size when compared with the average gamma source, it is generally the case that geometric unsharpness is better for x-ray techniques than for gamma.

7.6.4 EXPOSURE TIME (FILM RADIOGRAPHY)

An exposure time of between 1 & 5 minutes is usual for x-ray radiography. A conventional self-rectified x-ray set operating at maximum kilovoltage and tube current will generally be capable of continuous use with an exposure time of up to 5 minutes followed by a rest period between successive exposures of around 1 or 2 minutes. If the exposure time is extended beyond 5 minutes then overheating will generally occur if the rest period is not considerably extended. Constant potential equipment intended for fixed installation usage will usually be capable of continuous operation at its maximum output rating. However, even with such equipment, exposure times in exceeding 10 minutes will generally be avoided.

The exposure time for gamma radiography tends to be longer. This is because the output of radiation (in photons per second) is generally much less. Gamma ray exposure times are usually in the range from about 30 seconds to 1 hour, but exposure times exceeding 24 hours are not unheard of. The required exposure time for a gamma ray source increases as the source activity reduces with time.

7.6.5 POWER SUPPLY

X-ray sets require power from a mains supply or mobile generator. Usually a 4.5 kW generator will provide sufficient power to operate a 300 kV self-rectified set.

Gamma radiography can in general be carried out without the need for a power supply.

7.6.6 PHYSICAL SIZE AND WEIGHT

An Iridium 192 isotope with a source activity of up to 100 curies can safely be stored in a container weighing approximately 15-20 kg which has outside dimensions of approximately 200 x 400 x 100 mm. Such isotopes are useful for the radiography of steel sections of up to 75 mm thick. Gamma ray sources can be used to make exposures in situations where access is extremely limited.

A typical self-rectified 300 kV rated x-ray set (which is useful for the radiography of steel sections of up to 60 mm thick) is on the other hand considerably less portable and less manoeuvrable. A typical 300 kV SR tube head could weigh 55 kg and measure 300 x 300 x 750 mm while the associated control panel might weigh as much as 30 kg and measure 450 x 350 x 250 mm. Low kilovoltage equipment offers improved portability and manoeuvrability but this has to be offset against the reduced penetrating power.

7.6.7 EQUIPMENT COST

The initial cost of x-ray equipment for site work is about 2-5 times that of a portable

8.0 METHODS OF PRODUCING A RADIOGRAPHIC IMAGE