Page 7 - PR 2014 2016 03 Applications of Ionizing Radiations

Applications of Ionizing Radiations | Progress Report 49

Dosimetry Systems of medical imaging and clinical diagnostic

owing to their higher sensitivity per unit of
Industrial Dosimetry in Radiation Processing volume in comparison with ionization cham-
ber. However, in medical dosimetry, where a
Reliable absorbed-dose measurements trace- precise measurement of the absorbed dose is
able to recognized national and international required, the routinely use of Si devices de-
dosimetry standards play an important role in mands periodic recalibration of the dosimeter
radiation processing. In all electron and gamma due to the sensitivity decay with the accu-
radiation processes dosimetry, it is required as mulated dose. This drop in sensitivity is the
a necessary quality control, to establish the most important radiation damage effect that
process and for research and development imposes constraint on the widespread use of
studies. The Industrial Dosimetry Laboratory ordinary silicon diodes as dosimeters. This sce-
(IDL) is in charge of a wide range of dosimetry nario has been changed with the development
activities covering the conventional scope of of silicon devices with enhanced tolerance of
radiation processing applications and the de- radiation in the framework of High Energy
velopment of new products and services. The Physics research projects. Samples of these
dosimetry procedures are carried out in agree- radiation-hard diodes, free supplied by Hel-
ment with the International Organization for sinki Institute of Physics (HIP) and University
Standardization (ISO) and American Society of Hamburg, have been used as detectors
Testing and Materials (ASTM) standard guides in real-time dosimetry systems developed
and practices. To establish reliable dosimetry for diagnostic radiology, mammography and
systems, the IDL has participated of the inter- computed tomography. The best results were
comparisons of gamma dose measurements, obtained with Epitaxial (EPI) silicon diodes
organized by International Dose Assurance Ser- processed on n-type thin epitaxial silicon
vice (IDAS) offered by the International Atomic layer grown on a highly doped Czochralski (Cz)
Energy Agency (IAEA). Fricke solution has been silicon substrate. As dosimeter, each unbiased
used as reference standard and alanine as diode was housed in a PMMA probe (Fig 6) and
transfer standard dosimeters. Absorbed-dose directly coupled to a digital electrometer to
measurements in the products are carried measure the current produced across its p-n
out using polymethylmetacrylate (PMMA) junction during the irradiation. In all measure-
as a routine dosimetry system provided by ments, the current was linearly dependent on
Harwell Dosimeters Ltd. - UK, certified by the the dose rate and negligible sensitivity decay
International Dose Assurance Service (IDAS) was observed for accumulated doses up to 10
of the International Atomic Energy Agency Gy. Other application that deserves be report-
(IAEA) and the alanine dosimetry system to ed was the dosimetry response evaluation
increase the functional dose range. Additional- and beam data acquisition performance of
ly, the radiochromic thin film FWT-60 and the a miniature Float Zone (FZ) silicon diode for
Cellulose Triacetate Film – CTA dosimeters are clinical photon beams of 6 MV and 15 MV.
available for specific applications. Results obtained with this diode evidenced
linear and stable dose response even for field
Development of Semiconductor Dosimeters size of 1 x 1 cm². Furthermore, the output
factor values gathered in several field sizes
Medical applications: Silicon diodes have been point out the promising application of this
used for photon beam dosimetry in the field specific FZ diode for the commissioning of