Instrumentation & Measurement Magazine 23-9 - 40

successive block is mandated to shape the received voltage
into semi-Gaussian pulses and grant that the output baseline
is no longer dependent on pulses counting rate. To this aim, an
ad-hoc filtering section, based on the cascade of a high-pass filtering stage (with pole-zero cancellation), low pass filtering
stage, amplification, a further low filtering stage and a final
baseline restorer, has been realized. The obtained signal is then
given as input to a peak detector (to generate a trigger signal
for the ADC when the semi-Gaussian pulse reaches its peak
value) and an attenuator (to adapt the pulse amplitude to the
input ADC range).
A microcontroller (STM32F303VCT6) with built-in ADC
module is mandated to carry out the energy spectrum estimation along with its transmission to the embedded system.
Regarding the measurement algorithm, voltage provided by
the conditioning section is digitized with nominal sampling
rate and vertical resolution equal respectively to 5.1 MHz and
12 bits. To reduce latency and computational burden of the microcontroller, acquired samples are transferred by means of a
direct memory access (DMA) channel from ADC data register into 30 memory locations managed according to a circular
buffer strategy. When a trigger signal is generated from the
conditioning section, the maximum of the last 30 values provides the code associated with energy of the detected event
(particle or ray); the code is exploited as index to update the histogram of absolute occurrence. Since the resolution of the ADC
is 12 bits, the node is nominally capable of resolving 4096 levels of event energy, and the number of times that a certain ADC
code is digitized directly corresponds to the occurrences of an
event with that specific energy level. The whole histogram is
periodically transferred via serial universal synchronous/
asynchronous receiver transmitter to the embedded system.
To prevent artifacts due to the harmful interference with sampling and updating operations, the memory size dedicated to
histogram values is twice its dimension, in such a way as to allow a double buffer configuration; when one half of the buffer

is involved in the histogram update, the other one is sent to the
embedded system, and vice-versa. The embedded system accumulates the received histograms, thus providing the desired
energy spectrum that is finally sent to the IoT platform for storage and visualization.
With regards to power supply issues, actual consumptions
have not been a key issue, due to the specific considered measurand. In fact, nodes based on Geiger sensors to continuously
measure counts associated with the environmental radiation;
this way, they are designed to be main powered or equipped
with battery packs locally recharged by photovoltaic panels.
On the contrary, spectrometer nodes are in sleep condition for
most of their operating life, being activated only when Geiger
nodes detect an anomalous counts value; this way, spectrometers are battery powered and battery size (22000 mAh,
typically adopted for quadcopter power source) was chosen in
such a way as to grant an active interval sufficient to identify
the radioactive material.

Results
The performance of the IoT platform was assessed by means
of a number of tests. Specifically, sensor nodes were first verified in the laboratory through known radioactive sources. In
[6], measurement results collected during preliminary tests on
a hybrid platform showed the reliability of the Geiger nodes.
Further tests were carried out on the spectrometer nodes; in
Fig. 5a, the measured energy spectrum of the gamma detector
is shown. Energy peaks related to a cobalt isotope (60Co, located at 1100 and 1400 keV) can be straightforwardly detected
and recognized. With regards to the most relevant metrological specifications, the full-scale range of the spectrometer is
equal to 2 MeV with an energy resolution (evaluated as full
width at half maximum) of 100 keV.
In regards to the alpha detector, its performance was preliminarily assessed by putting a radioactive source (made of
three isotopes Plutonium-239, Americium-241, Curium-244)

Fig. 5. Energy spectra obtained by the realized wireless sensors for reference (a) gamma and (b) alpha ray sources.
40	

IEEE Instrumentation & Measurement Magazine	

December 2020



Instrumentation & Measurement Magazine 23-9

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