Alpha Spectrometry is a fascinating technique because it allows you to have accurate information about the radioactive decay of heavy nuclei and about the physics of the interaction of charged particles with matter. But this is a rather difficult technique, even more difficult than gamma spectrometry. The difficulties of this technique lie in the type of detector, usually a solid state silicon detector (rather expensive) that produces a very weak signal which requires, to be analyzed, very low noise amplifiers.
The measurement has to be made in vacuum conditions (however not high vacuum) so that the alpha particles are not shielded from the air. The sources that are measured have to be carefully prepared so as to have a layer as thin and uniform as possible so that alpha particles are not diffused and absorbed within the source itself.
Despite these difficulties it is possible, with a fair amount of work and patience, prepare a DIY instrument that can give a lot of satisfaction.
The alpha radioactive nuclei (typically heavy nuclei) can decay by emitting alpha particles (helium nuclei) with energies of the order of a few MeV, with spectra with lines, corresponding to the energy levels of involved nuclei .
In the figure aside it is an example of energy spectrum of alpha emissions of U-238.
The alpha-active nuclei are heavy nuclei with atomic number greater than 82 (lead). Examples are Polonium, Radium, Thorium, Uranium, etc …
The alpha decay has been explained theoretically by G. Gamow in the first half of the previous century making use of the tunnel effect in quantum mechanics. In the figure is a graph which shows the wave function of the alpha particle inside the nucleus and outside, beyond the Coulomb barrier. Although the alpha particle does not have enough energy to overcome the barrier it is seen as outside the nucleus the wave function is not zero and thus there is a non-zero probability that the alpha particle is ejected from the nucleus. Using this model it is possible to explain with good accuracy the characteristics of alpha decay.
Solid State Detector
In a semiconductor, the equivalent of the ionization energy is the band-gap energy to promote an electron from the valence to the conduction band. In Si at room temperature, Eg = 1.1 eV, compared to ~15 eV to ionize a gas. A charged particle moving through Si therefore creates more ionization and a larger signal.
When n-type and p-type silicon are put in contact, creating a p-n junction, the flow of the two different free charges across the boundary creates a depletion zone, an electrically neutral area near the junction where an internal electric field sweeps out any free charge. By reverse biasing the junction, the depletion zone can be made large, ~hundreds of microns. If an energetic charged particle ranges out in the depletion zone, an amount of ionization proportional to the particle’s initial energy will be created there, and swept out. By plating metallic ohmic contacts on the outer surfaces of the crystal, it is possible to both apply the bias and collect the free charge from the depletion zone, so that the whole assembly is a high gain, solid state version of the capacitive ionization chamber.
In our project we have used the detector shown in the image aside (thanks to Professor John Bland).
It features the following technical data :
– Canberra PIPS SPD-100-12 (partially depleted)
– Active area = 100mm2
– FWHM 12KeV at 5MeV
– Bias Voltage = 40V
– Thickness = 100μm
The signal produced by the detector has very low amplitude and therefore it requires an appropriate amplification. Given the very low level of the signal you must use very low noise amplifiers, also the bias voltage must be free of ripple, which is why we have adopted a power-based batteries. The preamplifier of the signal is based on a charge sensitive preamplifier type (CSP): the current pulse generated by the detector is converted into a voltage pulse by means of the charge of a capacitor. In the scheme below it is presented ta basic diagram of a charge preamplifier :
Response of a CSP
At time domains lasting up to a few microseconds, the CSP output is the time integral of the current pulse from the PIPS/Surface barrier detector. The output rise time is approximately equal to the duration of the current pulse, although the speed of the CSP sets a lower limit to this rise time.
Because the CSP Produces an output voltage step that is proportional to the time integral of the current input and remembering that :
the CSP output is proportional to the total charge (Q) from the PIPS detector. At much longer time domains the response of a CSP to a fast current pulse from a PIPS detectors is in the form of a tail pulse. A tail pulse has a fast initial rise time followed by a very long exponential decay back to the baseline. A tail pulse response from a CSP module is shown below.
The reason for the exponential decay is the resistance that is placed in parallel to the feedback capacity. This solution is necessary so that the CSP can respond to subsequent pulses.
Shaping the tail pulse into a Gaussian pulse
The output of the CSP (with its tail pulse signal shape) should only be considered to be an intermediary step in producing a measurable output. The long tail makes digitizing the pulse heights impractical, because pulses will often ride on top of the long tail of one or perhaps several preceding pulses. To quicken the decay time of the pulses we recommend routing the CSP output into a shaping amplifier internship which produces a symmetrical bell-shaped pulse. Another important feature of the shaping amplifier is that much of the noise is filtered, improving the signal to noise ratio considerably. Signals that may be buried in the noise of the CSP output become clearly above the noise after the shaping internship.
In the scheme below it is presented the basic diagram of a shaping amplifier:
In following scheme the basic scheme of a signal processing chain is presented, it consists of bias, detector, CSP amplifier and pulse shaper :
For our alpha spectrometer we decided to use a commercial CSP, the model CR-110 made by Cremat. It is a hybrid CSP preamplifier with a feedback capacity of 1.4 pF and a feedback resistance of 100 MΩ, the time constant of the amplifier is 140 μs. We decided to adopt a commercial component (it is not expensive) because the CSP is a critical component and the functioning of the system depends greatly on the good performance of the CSP in terms of gain and low noise. The diagram below shows the connections of the component:
In particular the bias resistance and filter resistance were chosen of 10 MΩ. These values, taking into account a dark current of less than 100nA, guarantee a good compromise between the need to limit the drop in the bias voltage and that of having an adequate amplitude signal on the coupling condenser towards the CSP.
The shaping amplifier instead was “homebrew”, as already described in the post PMT Pulse Processing. The images below show the finished circuits inside a metal box that constitutes a shield against RF interference.
The images below show the output signals from the shaper (in yellow) and from the CSP output (in blue). You can see how the pulse produced by the shaper has a Gaussian shape, with amplitude of about 200mV and a duration of approximately 80μs. The pulse produced by CSP has instead an exponential decay with a much longer duration: 300μs.
Setup and Vacuum Chamber
The first solution that we tried in order to achieve the alpha spectrometer has been an old system no longer used (found on eBay ..): the famous “Nucleus”, image on side. It is an apparatus which includes a small vacuum chamber along with all the necessary electronics for the biasing the sensor and for processing the signal. But the evidence shows the old electronics proved not to be very reliable so we decided to remake both the vacuum chamber that all the electronics.
For the vacuum chamber we used a sealed die-cast aluminum container, drilled for the BNC connector and for through-connection for the pipe from the vacuum pump. In the picture below you can see the “new vacuum chamber” :
In the images below instead you see the overall setup of the apparatus and the vacuum pump, two-stage rotary pump.
After the hardware now we deal with software. The pulses generated by the shaper are acquired by MCA Theremino software through a USB sound card. Theremino MCA is the software that we have extensively used in gamma spectroscopy studies, widely reported on this blog. In Theremino web site there is a whole section on this application, with a rich set of documentation.
The version that we used is 7.2. This version has been specially modified to enable the usage also for alpha spectroscopy. In particular has been expanded the scale of energies up to 10 MeV, it has also been expanded adjustment range of MinEnergy and EnergyTrimmer parameters.
In the image below we show the alpha spectrum of Americium source Am241 (from smoke detector) which emits at about 5,5 MeV.
Because of the source shielding, alpha particles have an energy a little bit smaller than the real one and so the peak is shifted at about 4800 MeV. This source, easily available, can still be used for a first calibration of the software. The amplification parameter and the energy trimmer cursor must be adjusted so as to bring down the peak at about 4800 MeV. If acting on these adjustments it can not place the peak adequately, then it is necessary to act on the amplification shaper, up or down, so as to fall within the range of expected energies.
In order to have readily available the main alpha emitters isotopes it has been prepared a new file of the isotopes energies, comprising only alpha emitters with energies from 1.5 to 9 MeV. The new Isotopes_Energy.txt (isotopes_energy) files must be replaced to the existing file in Theremino_MCA/Extra folder (you should make a copy of the original file). The list of the isotopes will appear as shown in the side image .
To take a measurement it is necessary to insert the source inside the vacuum chamber, activate the vacuum pump and when the pressure has dropped to the minimum, give voltage to the detector. The detector should never be biased at atmospheric pressure and should never be exposed to light when powered otherwise be damaged.
The detector is very sensitive and its active area must never be touched, if you need to remove dust you could use a breath of air, also maximum working voltage must not be exceeded.
Another care that must be followed is that of not leaving alpha sources within the vacuum chamber for too long, this is because the sensor is slowly damaged by the particles themselves. It is also important to be careful to not “contaminate” the measuring chamber with residues of samples, this can be easily done using a disposable aluminum foil as a source of support.
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