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I have used the
Theremino multichannel analyzer program with the regular sound card of
my PC which has a sampling frequency of 200 kHz. Gamma spectroscopy was
not the main goal of the radioactivity page so I did not add any
additional filter or sound card modification. The program has been
tested first with a pulses generator. The graph above shows on the right
the pulse seen by the sound card and the MCA program. The blue rectangle
at the left of the peak is the position of the baseline measurement.
When the pulses generator send constant intensity pulses to the MCA, one
narrow peak is obtained on the MCA graph. If the pulse amplitude are
changed quickly, a second peak at another position appears in the graph.
The left figure is the result of several step changes of the pulses
intensities. It illustrates the best resolution achievable with this
software in the present measurement conditions. The upper scale is the pulse voltage (mV).
For technical details, see: https://www.theremino.com/en/downloads/radioactivity |
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Gamma spectrum obtained with Monazite, NaI detector and Theremino MCA. Some of the
daughters of the thorium 232 are recognized. The monazite, a rare earth
phosphate, is known to contain some thorium. As I have no standard for
the high energy range of the spectrum there is an uncertainty of the
position of the peaks at high energy.
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Thorium nitrate spectrum with NaI detector. The same peaks are found in
this spectrum compared to the preceding monazite spectrum. Signal to
noise is better for the thorium nitrate due to its high activity.
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Spectrum of a blue apatite from India. The radioactive contamination of
this apatite seems to be thorium.
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Pechblende from Czechia gamma spectrum
with NaI detector. The Uraninite of the pechblende has a so high
activity that the MCA analyzer was saturated when a macroscopic sample
of around 10g was used. To reduce the count rate, a thin section (100 µ
thickness) has been used. The section gives the spectrum above, the 3
peaks of Pb214 are well resolved.
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Betafite - (Ca,U,Na)2(Ti,Nb,Ta)2O6(OH)7
- spectrum. The 3 Pb214 peaks confirm the presence of the uranium
238 series. The peak at 230 kev is higher for this rock compared to the
pechblende. The Pb212, the main peak of the thorium radioactive series
appears at about the same energy. This could indicate the presence of
thorium in addition of uranium in this rock. Due to the
low resolution of the NaI detector, the Th234 peak could be the sum of
the two Th234 gamma rays and some X rays.
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Kasolite-bearing rock - Pb(UO2)SiO4•(H2O) - has a lower activity
than the betafite sample so the spectrum above exhibits higher
noise. Nevertheless this spectrum indicates the presence of uranium and
his daughters Th234, Ra226,Pb214 and Bi214. The peak at high energy has
not been identified with certainty.
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Picture above is the gamma rays spectrum obtained with an old radium
watch. The Ra226 peak is higher compared to its daughter Pb214. If the
radium is pure, this spectrum could not exhibit the Th234 witch is not a
daughter of Ra226. Moreover, as it has been purified in the 20th
century, The Pb214 should not have time to be recreated. The same for
Bi214. One possible explanation of the spectrum could be the presence of
a mixture of Ra226 with some uranium and daughters remaining.
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Pure uranium oxide spectrum. The main peak is Th234. The Ra226 should be
absent in this powder, so the peak just below 200 Kev could be due to
U235.
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Natural lutetium is slightly radioactive because it contains the 176
isotope which is a beta emitter. The two peaks at 200 and 300 Kev are
characteristic of Lu176.
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The cadmium tungstate detector has also been used with the MCA analyzer.
Up to now no useful spectrum has been recorded with my detector. The
reason is still unknown. Probably the small size of the detector and the
geometry could be responsible for this lack of photo-peaks in the
spectrum.
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Monazite measured with the plastic detector. This broad spectrum does not
allow the identification of the Th232 daughters. It is not surprising
because this detector is built only with very low atomic elements so the
majority of the gamma rays lose only a part of their energy in the
detector so no photo-peak is detected. Note that the energy scale is not
calibrated.
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LYSO
spectra. Note the high background (blue curve). Again this spectrum has
very low resolution. |