X Ray Spectroscopy with PIN Photodiode

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 Intrinsically 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 include at least one 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.

The photodiode was placed inside a light-tight metal box. Both the box chassis and the lid were connected to the signal ground reference.

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.

Signal Processing

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 :

Building the Charge Sensitive Preamplifier (CSP)

The charging preamplifier and the photodiode bias circuit are shown in the following diagram. The operational amplifier is the model OPA656, characterized by JFET input stage with low noise, low bias current and wide bandwidth. This component is particularly suitable for the production of high speed and low noise integrating stages. The photodiode is inversely biased through a 100MΩ bias resistor, to reduce the Johnson noise. To reduce the capacity of the photodiode junction, the maximum reverse voltage of 30V is used. This allows you to reduce overall capacity and reduce noise.

To achieve low noise noise preamplifier (CSP), we used a DEM-OPA-SO-1A demo board for SMD operational amplifiers. The PCB shown in the side figure allows the creation of classic circuits with OP AMP using SMD components.
The PCB wiring diagram, with the indication of the components is shown in the picture below.
of course, not all components are to be used, some will be replaced by jumpers, others simply will not be mounted and will therefore be an open circuit.
The SMD components that we have used are the following:

L1 = L2 = EMI-Suppression Ferrite Chip 600R 0,5Ω
C1 = C2 = Tantalum Chip Capacitor, SMD EIA Size 3528, 20V, 2,2 μF
C4 = C5 = Multilayer Ceramic Chip Capacitor, SMD 1206, 50V, 0,1 μF
R6 = parallel of Rf (100 MΩ) and Cf (0,5 pF) feedback CSP
R4 = coupling capacitor Cc 0,1 μF
R7 = jumper
R3 = jumper

The following image shows the general scheme of the demo board.

Building the Shaper Amplifier

As explained in the previous paragraph on signal processing, after the charge-sensitive preamplifier, a shaper amplifier is required to amplify the signal produced by the CSP and to make it similar to a gaussian pulse. The shaper also greatly improves the S/N ratio.
The shaper consists of a first differentiator stage followed by an integrator, stage as in the following scheme.

The signal from the integrator stage is then amplified with an adjustable gain stage and buffered before being sent to the output, as in the following scheme:

To realize the shaper, we used an operational amplifier demo board: PDIP-EVM of Texas Instruments:

The following image shows the general scheme of the demo board. Naturally, not all components are to be used, some will be replaced by jumpers, others simply will not be mounted and will therefore be an open circuit.

Pulses

Here are some examples of pulses that are obtained from the shaper with maximum amplification. The first two images show the impulses produced by an Am-241 source, as we see the amplitude is about 100 mV for the 60 KeV energy line.
The third image shows the pulses that are obtained with a Sr-90 source, with average amplification, as the pulses amplitude is noticeably larger.

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.

Gamma Spectrum Americium 241 (²⁴¹Am)

Americium-241 was directly obtained from plutonium upon absorption of one neutron. It decays by emission of a α-particle to ²³⁷Np; the half-life of this decay was first determined as 510 ± 20 years but then corrected to 432.2 years. Americium is the only synthetic element to have found its way into the household, where one common type of smoke detector uses ²⁴¹Am in the form of americium dioxide as its source of ionizing radiation. This isotope is preferred over ²²⁶Ra because it emits 5 times more alpha particles and relatively little harmful gamma radiation. The amount of americium in a typical new smoke detector is 1 microcurie (1 µCi, 37 kBq) or 0.28 microgram. This amount declines slowly as the americium decays into neptunium-237, a different transuranic element with a much longer half-life (about 2.14 million years). With its half-life of 432.2 years, the americium in a smoke detector includes about 3% neptunium after 19 years, and about 5% after 32 years.

Americium decays by means of the 4n + 1 chain of Np-237 is commonly called the “neptunium series” or “neptunium cascade”. In this series, only two of the isotopes involved are found naturally, namely the final two: bismuth-209 and thallium-205. A smoke detector containing an americium-241 ionization chamber accumulates a significant amount of neptunium-237 as its americium decays. The following elements are also present in it, at least transiently, as decay products of the neptunium: actinium, astatine, bismuth, francium, lead, polonium, protactinium, radium, thallium, thorium, and uranium. Since this series was only studied more recently, its nuclides do not have historic names. One unique trait of this decay chain is that it does not include the noble-gas radon, and thus does not migrate through rock nearly as much as the other three decay chains. The total energy released from californium-249 to thallium-205, including the energy lost to neutrinos, is 66.8 MeV.

The graph below shows the gamma spectrum of americium 241 acquired by the photodiode with the apparatus described above. The main peak at 60 KeV is evident and the secondary peak around 20-25 KeV is composed by the 26 KeV emission of the americium and the X-lines of the neptunium between 15 and 25 KeVs.

Conclusions

The use of a Si-PIN photodiode, reverse polarized at maximum voltage to increase the depletion zone, and used with a low noise CSP amplifier, has been proved to be suitable as a beta and gamma ray detector. With an MCA you can also obtain qualitative information on the energy spectrum. Of course, the resolution is rather coarse, but can be improved by acting mainly on the electronic front-end in order to reduce the noise and thereby increase the signal/noise ratio of the pulses.

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