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162 Nuclear Science and Technology | Progress Report

two or three half-lives. In an attempt to over- tion decrease over time. The analysis of the
come these problems, a programmable sample detection efficiency, though, showed similar
positioning system was developed which ac- results for both brands, with detectors losing
cepts up to four different steps in the counting up to 1% of their detection efficiency per year.
procedure, moving the source closer to the
detector at pre-programmed intervals, and In collaboration with the Advanced Studies In-
handling the data acquisition accordingly. The stitute of the Aeronautic Technological Center
system, comprised of a step motor and an ABS (IEAv/CTA), a project aiming to the monitor-
plastic source holder and positioned, coupled ing of ground neutron doses originated from
to an Arduino Uno programmable board and cosmic rays is under development. The project
to a PC, has an easy-to-use graphical interface also includes the development of a specifical-
(GUI) and interacts with Canberra’s Genie-2000 ly-designed neutron monitor.
data acquisition software. The prototype was
thoroughly tested and the source positions In collaboration with the Relativistic Heavy Ion
were found to be accurately reproduced every Group of the Physics Institute of the Universi-
time, showing that the system can be safely ty of São Paulo, we are developing a neutron
used in comparative measurements. detector based on the Gas Electron Multiplier
concept (GEM). In this detector, the radiation
A low-noise charge-sensitive preamplifier built ionizes gas molecules generating free elec-
from low-cost commercially available com- trons that are accelerated by strong electric
ponents was developed, and its performance fields present in small holes bored on thin
proved to be acceptable when compared to the plates (typical 50 mm), subject to high volt-
reference ORTEC 142 preamplifier. age between their surfaces. Due to the high
field, these electrons produce secondary free
A neutron detector is under development electrons (multiplication process), which are
which uses a commercial photodiode coupled collected in the anode. Since neutrons do not
to boron-deposited glass. The deposition is produce ionizations, we use enriched B or
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made by laser ablation, in collaboration with natural B films deposited on the cathode and
the Laser Center at IPEN (CLA). on the GEM plates to work as neutron con-
verters by the B + n → Li + He reaction. The
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A very extensive analysis of the long-term 7 Li and He ions are the particles that produce
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efficiency stability of HPGe detectors was per- the primary free electrons. In this project, the
formed using the daily verification data gath- thin GEM plates were changed by thick plates
ered for the Neutron Activation Laboratory’s of FR4 (0.5 mm), which are common circuit
detectors over the last 25 years. Data from 11 boards used in electronics. As, for this detector
detectors from the 2 major brands (Ortec and called thick-GEM (TGEM) the materials are
Canberra) were corrected for source decay and found in the electronics market, it has a low
analyzed. The results showed that Ortec’s Pop- cost compared to the previous GEM plates.
Top detectors suffer from frequent vacuum
degradation, requiring periodic annealing to Enhancements in NAA
restore the detectors to the nominal energy procedures and analyses
resolution. The Canberra detectors didn’t re-
quire any annealing during these years, albeit The Neutron Activation Analysis technique
suffering from a small, manageable, resolu- is a very well-established and largely used

Instituto de Pesquisas Energéticas e Nucleares