4.2 KiB
Alpha Particle Spectra
Radioactive elements can be identified based on their individual and characteristic alpha particle energies.
Since alpha particles are very heavy (they are in fact Helium nuclei or positively charged Helium ions: 2 neutrons + 2 protons) in comparison with other sources of ionizing radiation, they interact strongly with matter.
That means they may have lost considerable amounts of energy before reaching the detector depending on the properties of the source material and the surrounding air.
Alpha-spectra can be difficult to interpret because of this. A Simulation of the measurement helps to identify the radioactive elements and can compensate for specific effects - a very common method in nuclear and particle physics. A good, free simulation program is described here.
It's important to approach the specimen as close as possible with the diode. If the BPX61 is in touch with the surface, the remaining air gap is about 2 mm. The density of air - depending on temperature, atmospheric pressure and humidity - should be estimated as precisely as possible for best results. In professional settings, vacuum pumps are used in order to reduce the air density which shrinks the width of the peaks considerable and reduces overlaps in a spectrum (especially on the left-hand sides of peaks).
Energy Calibration
The following linear equation is used to map the arbitrary pulse amplitudes, as measured by a CM108 USB Soundcard, to the corresponding particle energy given in keV:
y [in keV] = m*x + t (simple linear equation)
m = 0.4958436 (slope)
t = -116.40845593 (offset)
x = maximum pulse amplitude (absolute value of the downward facing portion)
x is usually called 'channel number' in MCA software (Multi Channel Analyzer)
In combination with this calibration, the minimum trigger level or threshold for recorded pulses must be set at -300 (c.f. [[pulse_recorder.py line 33 | 53a7770a66/data_recording_software/pulse_recorder.py (L33)]]; the same level must be used in the web browser application or any other pulse recorder). Pulses with an amplitude of -300 represent the minimum energy threshold of 33 keV. Everything smaller than that is considered spurious electronic noise an not a real pulse from a particle.
Note: this formula can be quite different for other soundcards and headset inputs as input sensitivity varies a lot. It applies to the low-cost CM108 USB Soundcards only if set at 100% input volume, recording at 16-bit/48.000 Hz and with gain boost enabled - please read these operating system specific notes carefully.
For further context, please refer to the discussions of figures 4, 5, 9, and 10 in the paper.
Electron Energies
Beta decays produce electrons with continuous energy distributions.
The total decay energy is isotope specific but always shared between an anti-neutrino and an electron in each disintegration. As a consequence, characteristic electron energy lines do not occur like in measurements of alpha decays.
Furthermore, with thin silicon diodes, most electrons from natural sources of radioactivity are only partially absorbed within the sensitive layer and are even capable to leave the relatively thin diode.
Gamma/X-ray Photon Spectra
Thin layers of silicon, like the approximately 50 micrometers deep depleted region of a BPX61 diode when biased with about 8 V, absorb only a small fraction of gamma or X-ray photons. Above an energy of 100 keV, the detection probability is already below 0.5% (C.f. figure 1 in the paper) Therefore, characteristic gamma and x-ray energy lines may be only identified in the spectrum, if the radioactive object does not emit alpha and/or beta radiation else which is rarely the case. Characteristic energy lines from X-ray photons may be observed if high enough. The lower/minimum detection threshold of the alpha-spectrometer was verified at about 33 keV using an X-ray machine.