Page 56 - PR 2014 2016 07 Nuclear Science and Technology

198 Nuclear Science and Technology | Progress Report

reference values from the UCC, NASC and val- Establishment of a primary standard
ues of the core basis – BEF. It was possible to system for low energy X-rays using
conclude that the best methodology to evalu- a free air ionization chamber
ate the elements enrichment in the Pantanal,
for the present study, was the BEF. A primary standard system was established
for low energy X-rays (10 kV to 50 kV), using
Ionizing Radiation Metrology a free air ionization chamber with concen-

tric cylinders, Victoreen (Model 481-5), at the
Development and characterization Calibration Laboratory of Instruments (LCI). A
of special ionization chambers for new ionization chamber alignment protocol
computed tomography beams was developed for the radiation system and
a modification on the micrometer housing
The use of computed tomography (CT) for im- used for the movement of the internal cylin-
aging procedures is growing due to advances ders was made. The results obtained for the
in the CT equipment technology, because they stability and characterization tests showed
allow the acquisition of images with better res- to be within the limits established by the
olution than through other techniques. There- standards IEC 61674 and IEC 60731. The cor-
fore, they are responsible for increasing the rection factors for photon attenuation in the
dose radiation of patients during the procedure. air, transmission and scattering in the dia-
This fact led to a greater concern about the phragm, scattering and fluorescence and ion
doses received by patients who undergo this recombination were also determined. These
type of examination. To perform the dosimetry values were compared with those obtained
in CT beams, the most widely used instrument by the German primary standard laboratory,
is the pencil type ionization chamber, because Physikalisch-Technische Bundesanstalt (PTB),
this dosimeter has a uniform response to the showing good agreement. Finally, the absolute
incident radiation beam for all angles. The values of the quantity air kerma rate for the
conventional ionization chamber, which is standard qualities direct beams MWV28 and
available on the market, has a sensitive volume WMV35 and the attenuated beams WMH28
length of 10 cm; however, some studies have and WMH35 were determined; the results are
shown that this dosimeter has underestimated in agreement, with a maximum difference
the dose values. Therefore, in this study two of 3.8%, with the values obtained using the
ionization chambers with sensitive volume secondary standard system of LCI.
lengths of 10 cm and 30 cm, making use of
low cost national materials, were developed at Application of special ionization chambers
the Calibration Laboratory of Instruments. The for quality control in mammography
characterization of these chambers was per-
formed, and the results were obtained within Mammography is an examination which uses
the international recommended limits. As an X-rays to obtain images of the internal anato-
application, the developed ionization chambers my of the breast. To provide a correct diagnosis,
and a commercial chamber were tested in a it is necessary that the equipment constantly
clinical tomographer. The developed ionization undergoes quality control programs to ensure
chambers were analyzed in a complete way that patients submitted to this examination
for their possible uses. do not receive more than the necessary dose,
thus avoiding unwanted possible biological

Instituto de Pesquisas Energéticas e Nucleares