The three parts
of the Sodium Iodide detector:
- on top the NaI crystal surrounded by a humidity protection enclosure
with a front window. NaI is very sensitive to water vapor.
- on the left the SiPM component on a small PCB. It's front face is
glued to the crystal window. I have used oil for microscopy to make the
optical contact with the detector. The
SiPM is the
MicroFC−60035−SMT 6mm from ON semiconductor. It came on a
MicroFC−SMA evaluation board as seen on the image above.
- on the right the power supply filter. The SiPM must be polarized by a
30V supply. I have used three 9V batteries in series (this makes 28.5V
with new batteries) and a 1.5V small battery in series which can be
switched ON and OFF. A double filter (50 ohms resistors with 10 nF
capacitors) reduces the noise of the power supply. The SiPM is
equivalent to a reversed polarized diode as can be seen on the
schematics on next page, the output signal is taken across a 50 ohms
resistor via an SMA connector and cable. The signal is sent to a
preamplifier and shaper electronics described on next page. For the
details about SiPM see the brochure of ON semiconductor at the following
know the details of how radiation detectors work, the book :
Radiation Detection and Measurement by Glenn F. Knoll
is a must.
Plastic scintillator. When the material is illuminated with a
405 nm laser, a violet fluorescence appears. The scintillator is much
larger than the SiPM so a big amount of the luminescence is lost. The
SiPM is glued on the left side at the laser position.
The plastic scintillator must be covered with black tape to avoid
spurious light. The left picture shows the window cut in the black tape
where the SiPM must be positioned. The scintillator is first wrapped in
a first layer of white Teflon before being covered with the black tape.
The tape used has an inner face of white material. On the right, the
detector fully covered with the tape. SiPM is then glued on the small
Picture above shows the finished plastic detector equipped with the same
filter as NaI scintillator. For low level activity measurement, the
background of the detector must be kept as low as possible. Natural
radioactivity from the high atmosphere and the building materials is
also measured by the detector. To reach the low level background, a lead
shielding is used. The detector and material to be analyzed are placed
in an aluminum box surrounded by a few layers of lead. The box is closed
by a lead protected lid. The small box itself is introduced into another
metal box (not shown) again lined by another layer of lead. The total
thickness of the lead shield is around 2 cm. This is not enough to
really reduce the background to a very low level, it is only a
compromise between weight, cost and performance. The inner box must be
cleaned after each measurement to avoid contamination by an active
mineral in particular before a background measurement. The background
should be measured regularly to avoid fluctuations during the day or a
variation in the detector sensitivity in the long run.
The LYSO crystal used is very small (picture above), 2 cm long
and 4 mm thickness. Two crystals have been glued side by side and the
SiPM positioned on the side face. Again the crystals are covered by a
Teflon tape and a black tape. The detector is again equipped with the
same filter. The Raman spectrum of the lutetium-yttrium silicate has
been recorded, it is similar to the literature spectrum.
Cadmium tungstate from Russia. The crystal is equipped with a PIN diode
by the manufacturer. I did not use this diode for the measurements, I
rather fix an SiPM on the lateral face as usual and enclose the detector
in white Teflon and black tape. The recorded Raman spectrum of CdWO4 is
To compare the results of my scintillators to other detectors, I have
connected the gamma FTLAB on the left (network of PIN diodes) to an
Arduino micro-controller. The digital output and analog pulses of this
FTLAB board have been used. A low cost commercial Geiger counter on the
right has also been tested.