A PIN diode (p-type, intrinsic, n-type diode) is a diode with a wide region of intrinsic semiconductor material (undoped) contained between a p-type semiconductor and an n-type semiconductor.
The advantage of a PIN diode is that the depletion region exists almost completely within the intrinsic region, which has a constant width (or almost constant) regardless of disturbances applied to the diode. The Intrinsic region can be made large as desired, by increasing the area in which the pairs of gaps can be generated.
The generation of charge carriers within the intrinsic region may be due to the radiation incident. For these reasons, many photosensors are based on a PIN diode, such as PIN photodiodes or phototransistors. In addition to the light radiation, the charge carriers can also be generated by particles, gamma radiation, X radiation: for this reason, a PIN diode can also be used as a solid state radiation detector.
In our tests a S1223 Hamamatsu photodiode was used: it is a “general purpose” photodiode, sensitive from 320 nm to 1100 nm. The photodiode has a dark current of 0.1 nA, a capacity of 10 pF and an inverse polarization bias voltage of up to 30 V.
Of course the photodiode was placed inside a light-tight metal box. Both the box chassis and the lid were connected to the signal ground reference.
The picture below shows the shielded box with the photodiode:
Interaction with Radiation
The ionizing particle enters into the sensitive area from the “window” of the photodiode and produces in its passage several hundred electron / hole pairs which are collected by the cathode / anode of the diode and produce the signal that is subsequently digitized.
We give some data from the literature on solid state sensors :
Silicon Bang Gap = 1,115 eV
Couple Production Energy e/h (300°K) = 3,62 eV
Electron ionization power = 80 e/μm
As you can see from the data shown above, an electron which runs 100 μm produces about 8000 charge carriers, and thus an easily detectable signal. The signal produced by the interaction of the electron in the sensitive area of the photodiode therefore depends mainly on the electron energy. There is, however, a maximum value of the energy that one electron can deposit on the detector and this value is related to the thickness of the active zone, beyond this value the electron can not deposit all its energy because it leaves the active zone.
The signal produced by the photodiode 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 photodiode 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 detector. At much longer time domains the response of a CSP to a fast current pulse from a 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 :
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, and the amplifier connected to the photodiode.
The images below show the output signals from the amplifier. You can see how the pulse produced has a Gaussian shape, with amplitude of about 50 mV and a duration of approximately 80 μs. These are the pulses produced by β particles that affect the photodiode.
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 / Beta 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.
Spectrum β Strontium 90 (90Sr)
Strontium-90 is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.8 years. It undergoes β−decay into yttrium-90, with a decay energy of 0.546 MeV. Strontium-90 has applications in medicine and industry and is an isotope of concern in fallout from nuclear weapons and nuclear accidents.
Naturally occurring strontium is nonradioactive and nontoxic at levels normally found in the environment, but 90Sr is a radiation hazard. 90Sr undergoes β− decay with a half-life of 28.79 years and a decay energy of 0.546 MeV distributed to an electron, an anti-neutrino, and the yttrium isotope 90Y, which in turn undergoes β− decay with half-life of 64 hours and decay energy 2.28 MeV distributed to an electron, an anti-neutrino, and 90Zr (zirconium), which is stable. Note that 90Sr/Y is almost a pure beta particle source; the gamma photon emission from the decay of 90Y is so infrequent that it can normally be ignored.
The photodiode can also be used for the measurements of beta particles, even if the electrons are more penetrating, and then the stopping power of the detector is lower; this means that the more energetic electrons can pass through the sensitive layer of the detector releasing only a part of their energy.
Below, the spectrum obtained from a sample source to 0,1μCi.
Spectrum β+ Sodium 22 (22Na)
The isotope Na-22 decays (in 99.95% of cases) with a half-life of 2.6 years, by positron emission or electron capture to the first excited state of 22Ne 1,274 MeV (which then relaxes by emitting gamma photon). The positrons emitted by the source annihilate inside the material that acts as a support to the source, producing two gamma photons of energy 0.511 MeV each.
In the picture on the left you see the scheme of sodium decay in which the maximum energy of the positron is shown which corresponds to about 544 KeV
As a sample for measurement was used a Sodium 22 sample source of 1uCi.
The use of a Si-PIN photodiode, inversely polarized to maximum voltage to increase the depletion area, and used with a low noise CSP amplifier, has been shown to be suitable as a beta detector (electrons, positrons). With an MCA it is also possible to obtain qualitative information on the energy spectrum. For this last use, however, there are limitations: at low energies the beta particles can not penetrate the protection window (which should then be removed), while high energy particles pass through the entire sensitive zone without being absorbed, there is therefore a maximum detectable energy. This limit can be estimated at about 1 MeV.
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