Page 52 - 00. Complete Version - Progress Report IPEN 2014-2016

52 Application of Ionizing Radiations | Progress Report

Development of Semiconductor Dosimeters ski (Cz) silicon substrate. As dosimeter, each
unbiased diode was housed in a PMMA probe
Medical applications: Silicon diodes have been (Fig 6) and directly coupled to a digital elec-
used for photon beam dosimetry in the field trometer to measure the current produced
of medical imaging and clinical diagnostic across its p-n junction during the irradiation.
owing to their higher sensitivity per unit of In all measurements, the current was linear-
volume in comparison with ionization cham- ly dependent on the dose rate and negligible
ber. However, in medical dosimetry, where a sensitivity decay was observed for accumulat-
precise measurement of the absorbed dose is ed doses up to 10 Gy. Other application that
required, the routinely use of Si devices de- deserves be reported was the dosimetry re-
mands periodic recalibration of the dosime- sponse evaluation and beam data acquisition
ter due to the sensitivity decay with the ac- performance of a miniature Float Zone (FZ) sil-
cumulated dose. This drop in sensitivity is icon diode for clinical photon beams of 6 MV
the most important radiation damage effect and 15 MV. Results obtained with this diode
that imposes constraint on the widespread evidenced linear and stable dose response
use of ordinary silicon diodes as dosimeters. even for field size of 1 x 1 cm². Furthermore,
This scenario has been changed with the de- the output factor values gathered in several
velopment of silicon devices with enhanced field sizes point out the promising applica-
tolerance of radiation in the framework of tion of this specific FZ diode for the commis-
High Energy Physics research projects. Sam- sioning of small megavoltage fields (Fig 7).
ples of these radiation-hard diodes, free sup-
plied by Helsinki Institute of Physics (HIP) and Radiation Processing: The rising demand for
University of Hamburg, have been used as de- applications in the field of radiation process-
tectors in real-time dosimetry systems devel- ing with very high doses has pushed research-
oped for diagnostic radiology, mammography es to develop dosimetry systems to with-
and computed tomography. The best results stand harsh radiation environment. In this
were obtained with Epitaxial (EPI) silicon di- context, to improve quality assurance in elec-
odes processed on n-type thin epitaxial sili- tron beam processing of many products, ac-
con layer grown on a highly doped Czochral- tive semiconductor dosimeters were designed
based on radiation-hard diodes
processed on n-type Magnetic
Czochralski (MCz) silicon sub-
strate. The main application en-
visaged was to monitor 1.5 MeV
electron beam processing cover-
ing the dose range from 10 kGy
to 2 MGy and dose rates up to 8
kGy/s. During the electron irradi-
ation, the diodes were unbiased
and directly coupled with a digi-
tal electrometer in a short circuit
mode to record real-time currents
and, consequently, dose rates. The
Figure 7.Miniature FZ diode in a waterproof protec-
tion and its connection to a coaxial cable. dose delivered to the product can

Instituto de Pesquisas Energéticas e Nucleares