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Gamma PIN - Semiconductor Radiation Detector

An attempt to design and deliver universal and accessible radiaton analyser to be used in nuclear spectroscopy

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All the time we are exposed to various kinds of radiation. The one that we are least aware
of is the ionizing radiation and elementary particles associated with it. They pass through objects and our body leaving a trail of ions in their path and inducing changes.
Energies of those corpuscles change as they travel through distinct environments.
Having the ability to measure energy of each particle enables us to identify it's source whether
it is terrestrial - coming from some mineral or extraterrestrial like a cosmic ray hurtling through space.

A good radiation analyzer should be able to measure the energy of each quant,
it's trajectory, overall dose of radiation and energy spectrum of the source.
This research project is aimed at achieving and evaluating best results with accessible components. Based on the experience of previous designs and promising simulations
I hope to optimize the prototype version to use it for amateur nuclear spectroscopy.

The physical basis of capturing the ionizing radiation’s energy:

Ionizing radiation is a broad term for all the particles and quants that are capable of knocking away electrons from shells of atoms an molecules. There are a few of classified ionizing corpuscules and quants like:

  • α - alpha particle which is doubly ionized atom of helium
  • β - beta particle that may be an electron β or positron β+
  • γ - gamma radiation consisting of quants that are high energy photons.

There are more particles that are capable of ionizing matter by various interactions: muons, neutrons, protons but for the early stage of the research I would like to focus on the most common ones.

All those particles and quants carry energy, which is receding as they move through matter. This overall energy is lost due to various collisions and affections. As a result ions and electrons are liberated, which form tiny electric charge. The amount of the charge is directly proportional to energy of the particle to some extent.

Under special conditions this residue of charge left after passing of a particle may be collected and applied to estimate the energy and trajectory of it. The only thing required to do this is to measure the generated charge or current and analyse the results.

In this research project the medium to measure the radiation particles will be PIN diode. It is a flat slice of 3 semiconducting materials. Following the order P – positivly doped,  I- intrinsic (undoped) and N – negatively doped semiconductor. When the energetic particle or quant strikes the I – undoped region, it easily conveys the energy to the silicon atoms creating electrons and holes. Those two opposite kinds of charged particles then are attracted by the electric field coming from the biasing voltage applied to the diode.

As those opposing clouds of charge are separated a pulse of current occurs, which can be measured using ultra low noise and precise amplifier circuits.

Drawing illustrating pair generation in the semiconductor

The circuitry:

The whole instrument will be consisting of a few functional blocks, which will be tested and designed individually, but with consistency to provide high end results.                    

Graph describing the flow of information thorugh the device.

  • Sensing matrix – in the first stage it will consist of an array of common BPW34 PIN diodes. As the project develops I will test some larger ones I found on the Internet under the misterious name 2DU10 (10x10mm). If those common diodes fail to meet the expectations I may even move to professional detectors produced by First Sensor or photo multiplier diodes or replace the semiconducting area with a high voltage PMT and a chunk of scintillator.
  • Booststrapping circuit – while using larger diodes, their capacitance significantly reduces the charge generated by each event. There is a proposed solution that can be found on the website of Analog Devices. The simulations conducted for the detector in LTspice confirm the utility of this simple circuit, which will enable to use greater sensing areas....
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  • 10 × BPW34S Opto and Fiber Optic Semiconductors and ICs / Photodiodes
  • 1 × BF862 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs
  • 1 × LTC6268 Current FET Input Op Amp
  • 1 × LTC6232 Ultra Low Noise Rail-to-Rail Wideband Op Amp
  • 1 × MCP4561-502 Data Converters / Digital Potentiometers

View all 10 components

  • Inconclusive simulations

    Marcin Wachowiak08/01/2018 at 12:00 0 comments

    Recently I have tried to simulate the transimpedance amplifier once more to optimize the components for the best singal to noise ratio. This proportion is mostly affected by the feedback resistance, which determines how large the output signal will be. The additional feedback capacitor shuld be kept at lowest value preventing the op amp from disastrous oscilations.

    Simulated circuit

    The pulse coming from the detector had an estimated length of about 300ns and carried charge of one femto Coulomb. The feedback resistance was stepped in two scopes one ranging from 1M-100M. The other one was more focused on the peaking that ocurred in the scope 10k-350k. For resistances greater than 140M the simulations didn’t provide any reliable or sensible results. Results of this analysis also varied with the noise integration range. For this case I have set it to be from 1Hz-1MHz.

    Rf: 1M-100M scope
    Rf: 10k-350k scope


    Estimating results even in the simulations is really complicated as all the parameters are dependent on one another. This circuit needs to meet a few criteria:

    • Huge amplification for the ultra low current pulses
    • Highest possible SNR - Signal to noise ratio
    • Wide bandwidth to amplifiy even the shortest events and measure high doses

    This simulation does not include the thermal noise coming from the photodiode and the numerical analysis has its limitations. In the range of extremely large resistances or low currents it is unwise to relay only on them. Even though I ran lots of simulations still the results varied depeding on the scope. Finally, I decide to design the board and work on the prototype to pick the optimal values. One part after another I will assembly the detector and gather the data that earlier I could only estimate without certainty.

    The first examination will focus on the TIA - transimpedance amplifier . I intend to focus on the feedback resistor to see what results provide various resistors starting from 100k to 1G. I'm also curious about the noise from the photodiodes.

    If you have any suggestions about the simulations I am open to any discussion as it feels a little bit as if I was walking in the dark. I wish I could find one and only optimal value for the TIA, but the whole thing seems to be floating. Maybe PSpice has some better numerical analysis or tools, but unfortunalety the LTspice models/libs are coded and availible only for LTspice.

  • Evaulation of the old prototype based on TL072

    Marcin Wachowiak07/28/2018 at 15:16 0 comments

    The early stage prototype of the detector was designed to have an array of about 50 PIN diodes.
    To distribute them equally I divided the array into 7 rows with 7 diodes each. Every single row had
    it’s own sensing amplifier construced using TL072. This common and easily accesible solution was supposed to give me an insight to futher work with this kind of detectors. The design was preceded with simulations in LTspice to check what results may I expect.

    LTspice Gamma PIN prototype simulations
    Early stage TL072 design


    This prototype was created in a wafer fashion to make it smaller and stackable. The power supply was based on common step-up inverters using LM2577. By changing the feedback resistor they may work up to 65V. TL072 worked at 18V and diodes were biased by 60V. These power modules were shielded to reduce the noise emitted by the coils.

    Whole device
    Board footprint


    Unfortunatelly, in the final stage of the signal processing there have occurred an error stopping the grid detector from functioning properly. The pulse signal is negative and in the circuit there is diode added at the output of the TL072. It was supposed to prevent the signals from mixing if they were positive. This awful mistake is still present in the PCB files. If you wish to test your own detector make sure you have replaced the diodes with a proper solution that would stop the signals from interfering.

    Schematic of the early design


    I still managed to get use of the single row to measure some radiation. The first measurment was without any source. Even the background radiation managed to leave a trace, although the pulses were significantly smaller and happened less often.


    Low energy pulse coming from the background radiation

    Image of the collected pulses through short time with no source present

    Then I placed a thoron mantle at the sensing area. Simultaneously the pulses were higher and more frequent. For a single singal trigger I didn’t have to wait even a second.

    Detector with a thoron mantle placed upon it

    High energy event - particle coming from thoron mantle

    Pulses collected with the source present

     

    On the whole, this prototype has allowed me to focus on a few problems that will need to be solved:

    • Noise and parasitic capacitance should be kept as low as possible. The whole instrument for proper working conditions, will have to be enclosed in a grounded metal chasis.
    • The power supply circuits will need to be with reduced EMI and voltage ripple. This induces
      me to use charge pump voltage regulators, which should be suitable for the job.
    • For any future applications the single row detector should be made modular and portable to enable easy use. The sensor will be in a shape of a thin stripe with a connector at the edge.

    The early design was quite a crude one. The second one is going to be well thought over to provide reliable results.

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fabian wrote 07/27/2018 at 14:43 point

No dobra a do czego to?

Wiem, że obrazowanie taonowe było uzywane w piramidzie do określania gdzie są korytarze, nawet detektory materiałów rozszczepialnych sa na granicach i wykrywaja co ktoś przewozi. Chcesz zrobic coś co bedzie wykrywać przedmioty ?

Jak czułe jest to coś? Może jakas farba jest metaliczna? np. jak przeczytac tekst na kartce, której nie widzisz bo jest wewnątrz listu (druk gazetowy ma chyba ołów)

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Marcin Wachowiak wrote 07/27/2018 at 15:04 point

Na razie do mierzenia natężenia promieniowania, potem jak już uzyskam dobrą czułość po zbudowaniu drugiego prototypu do spektroskopii jądrowej i analizy jakie pierwiastki promieniotwórcze zawierają różne substancje. Np. suszone grzyby - cez, banany - potas itp. Ale to wszystko jest jeszcze w fazie rozwojowej. Pierwszy prototyp dawał średnie rezultaty bo był zrobiony na łatwo dostępnych częściach, ten powinien być znacznie lepszy. Można to potem rozbudowywać o trajektorie cząstek czy tworzenie obrazów prześwietleń. O tym pierwszym zaraz napiszę w logach.

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fabian wrote 07/28/2018 at 08:36 point

czyli cos jak w startreku do mierzenia wszystkiego. Kiedys byla kampania na kickstarterze do takiego urządzenia. Może i Tobie się uda. Czy możesz wycisnąć jeszcze kąt padania? Kiedyś widziałem detektor w formie stołu gdzie można bylo zobaczyć miony na całej powierzchni. Wyglądało niesamowicie. Ale mając też wygląd może jeszcze coś dało by się wyciągnać z badania. Może w zależności od układu promieniowania są jakieś właściwości.

banany akurat mają promieniotórczy potas 40 chyba. Ja bym się bardziej bał strontu 90 czy cezu 137 co swoja drogą wykryto ostatnio w winie w USA (mowi sie o przecieku z fukusima) no i nasze ulubione nawozy sztuczne.

czy promieniowanie oddziaływuje na siebie? np. czy promieniowanie jednego pierwiastka opóźnia dugi? dwa grzybki obok siebie.

Moim zdaniem obrazowanie jednak ma wieksza przyszłość niż spektroskopia

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Marcin Wachowiak wrote 07/28/2018 at 12:37 point

Zobaczę ile uda się osiągnąć podczas budowy tego detektora. Żeby obrazować najpierw muszę mieć sam spektroskop i nim mierzyć nim energie po przelocie przez jakiś materiał. Analizując jak duże straty wystąpiły przy przejściu może uda zrobić mały obraz gęstości ośrodka. W przypadku pomiarów obecne będzie pełno efektów rozmywających dokładność takich jak: cząstki/kwanty odbite od osłony próbki, różne energie dochodzące z różnych odległości dla niesymetrycznego źródła. Nie powinny one znacząco wpływać na pomiary ale będą wprowadzać błąd statystyczny.

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