Step 3: Additional Info

Here's a small comic which demonstrates how the detector was originally supposed to work. I have no illusions of being good, or even acceptable, at drawing.

What actually happened was not a sudden change in the conductivity of the photodiode silicon lattice, but a short-lived buildup of charge as demonstrated by the oscilloscope photographs, akin to the photoelectric effect.

Reverse-bias is *supposed* to improve event capture by expanding the depletion layer, and cause the charge buildup to be shorter lived (but still visible), improving performance... but I could not conspire to get such a thing to work.

If the correct components find me, I will try to repeat this experiment with them in an attempt to cheaply increase sensor area, as well as render the experiment in its entirety more easily reproducible... the current photodiode is a Hamamatsu S1223-01. I have some left over from a failed experiment involving single-photon detection. A very nice product, however rather expensive for a hobbyist.
I once had to build a low-pass filter and pre-amplifier for an instrumentation project. I hung my device onto the A/D card and got a mess of high-frequency noise. I thought that the coax cable from instrument to pre-amp would be noise free, but I was wrong. When I hung the pre-amp/filter off the instrument and sent the amplified signal over the coax, the noise was negligible. The take-home, I guess, was to pay attention to signal paths. Besides shielding the pulse-generating circuitry in a cage, it sounds like paying attention to all signal paths and sources of noise may help. Are you using bypass capacitors on the V+ pins as well as being generous with ground planes? I'm not an EE, but I know there are guides on the websites of providers of instrumentation ICs, like Texas Inst., Analog Devs., and so forth. Good luck.
I remember using 220uF and 0.1uF power supply bypass capacitors. The ground plane for the open-loop amplifier stage is quite generous, and miniaturized as far as the components would allow. In later revisions, the preamplifier, diode, and particle source were made into a module about the size of a US dime and encased in grounded copper foil. This gave the best performance, and looked doubleplus cool.<br><br>I return to work on this project from time to time, the most frustrating part is the cost of the diodes... 25$ each, and you get a scary phone call at home asking why you need milspec parts (basically, just answer honestly). I've tried different exposed die semiconductors as replacements (MOSFETs, power transistors, diodes) without success. I still have a few left to try based on older &quot;budget&quot; particle detector designs documented by some universities (updated with modern tech of course).<br><br>Really I'm surprised that an exposed power transistor didn't work when I tried it. It should work, must be doing something wrong. <br><br>-S<br><br>
A couple of suggestions for reducing noise: Use an active filter to remove the 60Hz band from your signal. It looks like the pulses of interest are much shorter than the 16.67 ms period of 60Hz. So a high-pass or notch analog active filter could be included in the amplifier chain. Also, does the avalanche diode detector become quieter at lower temperatures? If so, could it be cooled by, eg., a peltier device?
Well, you're right! In other designs I used a twin-T notch filter for this task (active or passive). Due to the high amplification though, other sources of noise can be problematic too (serial comms, nearby switching power supplies)... so when I designed this some time ago, I decided that a Faraday cage was the best option. Another (smaller) problem is that signal frequency and pulse duration can vary by quite a bit (multiple hits at the same time, maybe also Bremsstrahlung), though it's still not near 60hz. I think pulse duration is a bimodal normal distribution (2 normal curves), and frequency is a poisson distribution... so it involves some interesting math.<br><br>Avalanche photodiodes are expensive, so I didn't use one... but yes, you can cool them to decrease noise, and (if you cool them enough) even use them as single photon detectors. This is actually a normal (but large surface area, small capacitance, and milspec) PIN photodiode.
I think I speak for most of us when I say this. "HUH?" :)
That's where I was three weeks ago, my friend. Then, a colleague told me that "no one can really conceptualize quantum physics". I firmly believe that with due diligence, any aspect of reality can be understood by just about any human being. This project was ultimately inevitable. Mind you, this is still all rather confusing to me too. I have not proven my colleague wrong just yet. Wish me luck ;)
Here's where I can't buy it. &lt;a rel=&quot;nofollow&quot; href=&quot;http://en.wikipedia.org/wiki/Schr%C3%B6dinger's_cat&quot;&gt;Schr&Atilde;&para;dinger's cat&lt;/a&gt;. Put a cat in a box with a quantum particle (which you can't be certain about) that is a trigger to a device that will kill the cat if it decays, and close them up in such a way as the cat is unable to affect the quantum particle.&lt;br/&gt;&lt;br/&gt;Copenhagen interpretation says that since you don't know the state of the particle until you observe it, it exists in both states, decayed and not decayed. Since that particle is the trigger to the cat's death, the cat is both dead and alive at the same time. It exists in both states, until we observe it. The act of observing the cat, and therefore the particle, forces it/them into a single state, and the cat will only remember that state (if it's alive to remember it).&lt;br/&gt;&lt;br/&gt;I don't buy it. The cat is either dead, or alive. Not both. Can't be that way. That Wiki link contains an exceprt from a letter from Einstein. He agrees. Reality gets in the way of the experimental situation.&lt;br/&gt;<br/>
When this experiment was tried, the cat said, &quot;Stuff this!&quot; and hoofed it through the nearest window.
Schrodinger's thought experiment is meant to point out the paradoxes in using the Copenhagen Interpretation to explain macroscopic phenomena. <br/><br/>Here's a non-quantum example: Assume coin flips are random. You have foolishly bet your friend that if a particular coin flip lands heads, you will give him 2$, and if it lands tails, you gain 1$ from him. As soon as the coin is tossed, you have on average lost fifty cents: (0.5)*(-2)+(0.5)*(1)=0.5, even though it is impossible for you to actually have lost this amount, because when the coin lands it is either tails or heads.<br/><br/>My assertion is that Schrodinger's cat is not *actually* both alive and dead, just as the bottle of cyanide is not actually both broken and unbroken. However, because we are dealing with probability theory, and both states of the cat are equally likely, (0.5)*(living)+(0.5)*(dead)=equally living and dead. Both of these are examples of how probabilistic models of reality do not ever correspond to the result of any given real incident. <br/><br/>A real life example: I study trees. I have measured the diameter of many trees, and discovered that the arithmetic mean value is some number, say 23 centimeters. This is meaningless on its own until I also tell you that the 95% confidence interval for that estimate is from say... 22.5 to 23.5 centimeters. We &quot;know&quot; that there is in fact a &quot;real&quot; mean value... but not only is it not necessary for any actual tree to have that diameter... but the real mean value is (sometimes equally) likely to be either greater or less than our estimated value.<br/><br/>Similarly, if we were (in rather poor taste I think) run Schrodinger's thought experiment in real life ten thousand times... we would have approximately 5000 live and 5000 dead cats. At no point in reality is any cat really alive and dead except on paper... <br/><br/>Probabilistic models do not describe what the status of any actual event will be while it is happening, only the average final outcome of the event if it is run many times. The Copenhagen Interpretation is interesting because it is a demonstration of a system in which the *only* models that predict behavior are probabilistic ones. Furthermore, it may suggest that the only models that predict anything are probabilistic, and that the &quot;reliability&quot; of classical physics is only a result of the many combined probabilistic events in macroscopic systems. As a side note, you then have Chaos Theory which asserts that extremely large, complex systems are also probabilistic... a beautiful and inconvenient symmetry, don't you think?<br/><br/>...And there you have my Copenhagen Interpretation Interpretation.<br/>
Sweet flaming bagels, is there no way to edit comments? The first sentence should read: Schrodinger's thought experiment is meant to point out the paradoxes in using Quantum Mechanics to explain macroscopic phenomena. The Copenhagen Interpretation attempts to resolve this and other paradoxes.
Well do you ever worry that your cat has died while you where out? It's confusing but it makes sense in terms of us, but whatabout things we observe wihtout understand/interpreting... It makes more sense in a mental way than physical...
OK, well I'm too lazy to post the newer developments here, just go read the talk @ defcon 17.<br />
Thanks for this. I'm a lapsed physicist, and I really enjoy things like this that don't require me to spend weeks relearning calculus. Looking forward to your statistical analysis.
Wait, you don't really need calculus in the real world?? Cool! (maybe its just one of those GPA things)
Calculus+statistics=winning at computer games.&lt;br/&gt;&lt;br/&gt;Or at investing, if you live in the past and don't consider it just another computer game.&lt;br/&gt;<br/>
lol. I'm not that much of a math fan to go so far as to calculate the odds of me beating others(or the computer). I'm a fan of strategy games(command & conquer) & FPS games. I'm just glad that It's not imperative to doing engineering.
I finished the stats analysis, I produced 100 megabits of data by sampling this system with a microcontroller running a special case of a von neuman whitening algorithm. This took a rather long time, as you might imagine.<br/><br/>I tested the output in Linux with the NIST test suite: <a rel="nofollow" href="http://csrc.nist.gov/groups/ST/toolkit/rng/index.html">http://csrc.nist.gov/groups/ST/toolkit/rng/index.html</a><br/><br/>The results were pretty good. Random data has a predictable failure rate for tests for non-randomness (as strange as that sounds). The observed failure rate was not significantly different than the expected rate. <br/>
Thanks! My new oscilloscope does not seem to have high enough gain on the input amplifier to observe the effect as on my old scope... So it looks like I'll have to wait until I have time to build an external amplifier using some mosfets.
So perhaps a use for this would be to generate unpredictable random numbers to seed computer encryption algorithms?
Geez... you mathematicians are in a world all of your own. When I read this instructable, not only did my wave function collapse, but my BRAIN collapsed along with it!
Haha, actually I dislike math. I tolerate it when necessary. I'm "officially" a biologist. But yes, we (scientists) are often in a world of our own, and it is a little sad. It can be a cold, humbling, and beautiful world... and once you begin to see it, the desire to understand it never stops, and never goes away. I hope your brain gets better. Unless you don't want it to, that is.
I totally agree with your depiction of the world of Science being "cold, humble, and beautiful". I also have a life-long hunger to understand it, and my desire never stops. I am not a "scientist" but I do love science and technology. My math abilities are a little weak, but that doesn't impede my ability to seek out answers to questions about our physical world. I have a good grasp of many complex phenomenons and processes, and the more I learn, the more I seek. It's an addiction. *smile*
There exists no institution that can make you a scientist. Governments and universities would have you think otherwise, of course. All they can teach you are facts (if you're lucky), but science is a method, a process, a philosophy. It's not something anyone can really give you, or ever take away. I don't think it is possible to separate a love of science from being a scientist, or a love of technology from being an engineer. The people that hide behind their degrees and disagree are fateless cowards. I greet you, and everyone like you as my colleagues and equals.
Thanks for your kind words. I do not have a degree to hide behind, so that's a GOOD thing! Most of my education was acquired at the infamous School of Hard Knocks. I can not calculate the flight path of a rocket to the Moon, but I have a good understanding of the Physics of how it needs to get there. Without my intrinsic passion for Science, I would not be as smart as I am, and I would probably join the ranks of the average American by being obsessed with Football and Beer, neither of which I have any interest in.
I read this and my wave function collapsed. It'll take me weeks to get it all put back together again. Thanks. Thanks a lot.
Haha, that's some good infinitely recursive self-reference. ...The time to get it back together is known with a degree of certainty, so his/her current state is once again probabilistic if the analogy holds (which it probably doesn't)... well, I'm probably the only one that finds that funny.
"No cats were harmed in the making of this instructable". ;-)
you might find this interesting:<br/><br/><em>Some specially designed silicon diode detectors have been made with the active volume (the depleted region) very close to the surface of the detector and have an extremely thin window so that alpha particles can enter the active volume and deposit all their energy there. The charge collected is a measure of the energy of the alpha particle, and these detectors are common for alpha particle spectroscopy. One of the most common types of such detectors is called a surface barrier detector. The detectors are usually used with multichannel analyzers. Some silicon detectors have thicker active volumes and are used for beta particle energy analysis.</em> - <a rel="nofollow" href="http://www.hps.org/publicinformation/ate/q534.html">www.hps.org</a><br/>
Yeah, I'm using a detector with a large (13 square millimeters) depleted region near the surface. I removed the glass window on the device because it was thick enough to block alpha particles. Now the semiconductor is separated from the source by negligible air, and a few micrometers of glass. I didn't actually fall upon that page specifically while doing my initial research, I have access to scientific publications through my university, so I mainly used the primary literature. Interesting that there exist beta-detectors of similar design, I hadn't actually known that... of course beta decay isn't governed by quantum tunneling if I recall correctly! My setup is similar in character to the one (somewhat) described in: Deves et al, 2006. Characterization of Si p-i-n diode for scanning transmission ion microanalysis of biological samples. REVIEW OF SCIENTIFIC INSTRUMENTS 77, 2006.

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Bio: I publish my failures and my successes, as my teachers have done before me. I am a member of Foulab, an independent, nonprofit research and ... More »
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