Page 203 - 00. Complete Version - Progress Report IPEN 2014-2016

Nuclear Science and Technology | Progress Report 203

system for low energy X-rays using
NASC and values of the core basis – BEF. It was a free air ionization chamber
possible to conclude that the best methodolo-
gy to evaluate the elements enrichment in the A primary standard system was established
Pantanal, for the present study, was the BEF. for low energy X-rays (10 kV to 50 kV), using
a free air ionization chamber with concentric
Ionizing Radiation Metrology cylinders, Victoreen (Model 481-5), at the Cali-

bration Laboratory of Instruments (LCI). A new
Development and characterization ionization chamber alignment protocol was de-
of special ionization chambers for veloped for the radiation system and a mod-
computed tomography beams ification on the micrometer housing used for

the movement of the internal cylinders was
The use of computed tomography (CT) for im- made. The results obtained for the stability
aging procedures is growing due to advanc- and characterization tests showed to be with-
es in the CT equipment technology, because in the limits established by the standards IEC
they allow the acquisition of images with bet- 61674 and IEC 60731. The correction factors
ter resolution than through other techniques. for photon attenuation in the air, transmis-
Therefore, they are responsible for increasing sion and scattering in the diaphragm, scatter-
the dose radiation of patients during the pro- ing and fluorescence and ion recombination
cedure. This fact led to a greater concern about were also determined. These values were com-
the doses received by patients who undergo pared with those obtained by the German pri-
this type of examination. To perform the do- mary standard laboratory, Physikalisch-Tech-
simetry in CT beams, the most widely used in- nische Bundesanstalt (PTB), showing good
strument is the pencil type ionization chamber, agreement. Finally, the absolute values of the
because this dosimeter has a uniform response quantity air kerma rate for the standard qual-
to the incident radiation beam for all angles. ities direct beams MWV28 and WMV35 and
The conventional ionization chamber, which the attenuated beams WMH28 and WMH35
is available on the market, has a sensitive vol- were determined; the results are in agreement,
ume length of 10 cm; however, some studies with a maximum difference of 3.8%, with the
have shown that this dosimeter has under- values obtained using the secondary standard
estimated the dose values. Therefore, in this system of LCI.
study two ionization chambers with sensitive
volume lengths of 10 cm and 30 cm, making Application of special ionization chambers
use of low cost national materials, were devel- for quality control in mammography
oped at the Calibration Laboratory of Instru-
ments. The characterization of these chambers Mammography is an examination which uses
was performed, and the results were obtained X-rays to obtain images of the internal anat-
within the international recommended limits. omy of the breast. To provide a correct diag-
As an application, the developed ionization nosis, it is necessary that the equipment con-
chambers and a commercial chamber were stantly undergoes quality control programs
tested in a clinical tomographer. The devel- to ensure that patients submitted to this ex-
oped ionization chambers were analyzed in a amination do not receive more than the nec-
complete way for their possible uses. essary dose, thus avoiding unwanted possible
biological effects. The ionization chambers are
Establishment of a primary standard widely used as detectors to perform the qual-