Page 167 - 00. Complete Version - Progress Report IPEN 2014-2016

Nuclear Science and Technology | Progress Report 167

which accepts up to four different steps in the up to 1% of their detection efficiency per year.
counting procedure, moving the source closer
to the detector at pre-programmed intervals, In collaboration with the Advanced Studies In-
and handling the data acquisition accordingly. stitute of the Aeronautic Technological Center
The system, comprised of a step motor and an (IEAv/CTA), a project aiming to the monitor-
ABS plastic source holder and positioned, cou- ing of ground neutron doses originated from
pled to an Arduino Uno programmable board cosmic rays is under development. The proj-
and to a PC, has an easy-to-use graphical inter- ect also includes the development of a specif-
face (GUI) and interacts with Canberra’s Ge- ically-designed neutron monitor.
nie-2000 data acquisition software. The pro-
totype was thoroughly tested and the source In collaboration with the Relativistic Heavy Ion
positions were found to be accurately repro- Group of the Physics Institute of the Universi-
duced every time, showing that the system can ty of São Paulo, we are developing a neutron
be safely used in comparative measurements. detector based on the Gas Electron Multipli-
er concept (GEM). In this detector, the radi-
A low-noise charge-sensitive preamplifier built ation ionizes gas molecules generating free
from low-cost commercially available com- electrons that are accelerated by strong elec-
ponents was developed, and its performance tric 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 cou- collected in the anode. Since neutrons do not
pled to boron-deposited glass. The deposition produce ionizations, we use enriched 10B or
is 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 convert-
ers by the 10B + n δ 7Li + 4He reaction. The
A very extensive analysis of the long-term ef- 7Li and 4He ions are the particles that produce
ficiency 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 detec-
detectors from the 2 major brands (Ortec and tor 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, albe- The Neutron Activation Analysis technique is
it suffering from a small, manageable, reso- a very well-established and largely used ana-
lution decrease over time. The analysis of the lytical technique in IPEN at the Neutron Acti-
detection efficiency, though, showed similar vation Laboratory (LAN). Nevertheless, several
results for both brands, with detectors losing tasks have been undertaken in the last years