DIY阿尔法谱仪（二）
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 :
Amplifier Construction
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.
Pulses
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 exponentialdecay 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.
Multichannel Analyzer
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 amplificationparameter 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 .
Operating Procedure
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.
