A high voltage full wave rectifier

Earlier I posted about building a full wave rectifier from an LED diode bridge to use with our foxhole radio.  The transformer we've been playing with is alternating current, which means the electrons in our partial vacuum are first accelerated in one direction, then reversed and accelerated in the other direction many times a second.  To live up to their full potential what we really need is direct current.  However, normal diodes won't work, the voltage will easily spark across them.  So I got a hold of four microwave diodes (W10163432) designed for handling high voltage.  Also, I didn't want circuitry with this much power laying around bare on a table top, so we planned to embed them in paraffin wax (pure paraffin wax is a better insulator than almost anything except some types of plastic) in an old brie container (M likes to snack on brie).  Perhaps this is overkill, but it also has the benefit of making the bridge rectifier a single portable unit. 

Above, gathering materials and diagramming our strategy. 

First we needed to set up a double boiler to melt the paraffin.  I was planning to use two of V's pots but she forbade me from getting melted wax all over her new soup pot, so I picked up a stack of recycled aluminum trays for a couple dollars and used them as our crucible to melt the wax. 

We punched holes and ran some copper wire through for handles to lift the crucibles out of the boiling water.  If you try this, do not use a single wire across the middle for a handle (like the one on the left).  We tested this with water first and it tilts to one side and dumps whatever is in it.  A much better arrangement is to have three connections like the one in the middle, this gives much more control and allows the contents to be tilted and poured out. 

Above, a block of paraffin is completely melted. 

M pouring wax into the base of the brie container. 

Then we soldered the diodes together with wires to access each corner of the bridge.  It was a horrible soldering job, the thick (AWG 12) wire and large diodes sunk all the heat away from our cheap soldering iron repeatedly giving cold joints and having to retry; it was ugly in the end with solder blobs but the circuit worked.  (Pay attention to the cathode stripes on the diodes.  They have to run from one corner to the other.) 

We mashed the circuit down part way into the first layer of wax while it was still warm and started melting the second block of paraffin to pour over it. 

Above, the now transparent upper layer of wax is setting over the diode circuit.  Below is with the lid added and holes punched for the wires.  The circuit takes AC at two of its corners and converts it to DC at the other two corners.  Later I marked the + and - ends so I wouldn't forget. 

It was a hot day, while we waited for the wax to set the kids went outside to play and cool off for a while. 

Then we hooked it up and tried it out.  The most obvious difference was the DC plasma beam was much more responsive to a magnetic field (on the end of the screwdriver). 

The path near the magnet is bent toward a curve perpendicular to the magnetic field from the magnet. 

I was curious if we could detect any bremsstrahlung (braking radiation) from the setup.  This is seen with high voltage particles that undergo changes in acceleration (like running into the wire or glass on the sides).  The change causes the electron (or ion) to shed energy in the form of a photon, like the light emitted when electrons are captured by positive ions in the tube, but the photon can be even higher in energy than visible light, in the ionizing X-ray range. Long story short, we got some very spurious results. 

I placed our homemade geiger counter on a remaining piece of the shoepad (that we used to insulate the quad gyros from vibrations) because the vacuum pump was vibrating the table top.  In this position you can see a reading of over 20,000 counts per minute, or 165 µSv/hr!  30 minutes of this is about the dose received by people living within 16 km of Three Mile Island during its nuclear accident, but it would take about 3 hours to equal the exposure of a mammogram.  Fortunately however, this is a completely false reading.  The electric field caused the counter to go haywire, some times it would read zero for long periods of time, once symbols went all over the screen and I had to re-upload the program, and at other time it gave transiently high spike readings like this. 

Above is a plot of the recorded counts per minute on a log scale.  Counts around 10 are normal background levels around here.  Counts above 100 and long stretches of zero are not normal and presumably complete artifacts.

Real Uranium Glass!

I found some large uranium glass marbles for sale on ebay for a few dollars!  Above I am holding them under a blacklight for my first test of authenticity.  You can see them fluorescing chartreuse!  See my earlier blog post for this test with fake uranium glass. 

Next I hooked up our geiger counter.  In the picture above it recorded 414 counts per minute which is elevated well above the background rate. 

Above is a plot of the radiation level over time with the uranium glass, flanked by periods of background radiation.  It is plotted in units of micro-sievert per hour. 

Also, out of curiosity, I picked up a gas mantle that contains radioactive thorium and tested it. 

There was no noticeable increase in hits, which is not unexpected.  Thorium emits alpha particles which are stopped by the packaging (which I left on) and the glass of the GM detector tube.  However, there may be tiny amounts of beta emission from trace beta emitting thorium isotopes and elements in the decay series (thorium 232 decays to radium 228 with is a beta emitter).  Interestingly, thorium has a lot of properties that make it useful as a potential nuclear fuel source and research into thorium-based reactors is taking place (link). 

So, I had to try combining the uranium glass with the uranium ore to see how high I could get the CPM.

Here it is at 786 CPM or 6.4 µSv/h!  This was the highest spike, mostly it was around 4-5 µSv/h.  A CT scan is about 20 mSv, which corresponds to about 4000 hours (or 167 days, half a year) of exposure at this level.

Geiger Counter, § III

I wanted a reliable radiation source to test out the geiger counter.  As I have discussed in earlier posts, I haven't had much luck with vaseline glass.  Either it turned out to be fake or I can't get it shipped here from the EU.  There has essentially been no vaseline glass production in the last 50 years (since the beginning of the cold war); everything out there is an antique.  Vaseline/uranium glass is easy to find for sale online but can cost several $100's because they are sought out by depression glass collectors. 

So I switched strategies and went straight for the source, uranium ore.  There are mines all over the western US from Texas to Washington and it is easy to order small pieces through the mail for testing; you've got to love the US!  Otherwise we would be forced to get our radioactive material the old fashioned way, by stealing it from « nationalists » and getting chased around mall parking lots⸮ 

Just for the record, I am keeping it doubly sealed and stored out of reach.  It is a harmless amount of radiation when kept this way.  I can help illustrate this below.

In this picture the ore is right against the tube, which just registered 306 counts per minute.  The reported activity for this fragment is 600 CPM but this tube does not detect alpha particles.  I am leaving the ore in the plastic bag to minimize possible dust contamination. 

In the graph above I've converted CPM to μSv/h (micro-Sieverts per hour).  The highest point, 4 μSv/h, corresponded to just over 500 CPM--for this particular GM tube.  It was singing away at almost 10 chirps per second!  But first let me explain, the initial low level in the plot, below 0.5 μSv/h was to establish a background level of radiation.  Then the sharp rise to 3 μSv/h came when I put the uranium ore directly against the tube.  Then it jumped up and down as I rotated the ore to find the hottest spot which seems to be somewhere near 3 - 3.5 μSv/h.  I left it there for a few minutes then moved the ore 1 cM away from the tube, and the signal dropped to 1.5 to 2 μSv/h, then I moved it 2 cM away and it dropped to 0.5 to 1 μSv/h.  I kept doing this until it was back down to background levels with the ore >10 cM away.  (Because of the sensitivity of orientation what I really should have done is fix the ore's position and moved the tube's distance, but the important point remains.)  If the ore is not immediately next to you, there is essentially no increase in radiation.  Just being in the same room, for example, has no effect.  The rapid drop off comes from the radiation spreading out in the a sphere, which grows rapidly and dilutes the strength, as distance increases by small amounts. 

To put this in perspective, the doses from a chest x-ray, mammogram, or GI x-ray range from 2,000 - 18,000 μSv.  To get a dose of 10,000 μSv you would have to carry the ore in your pocket for 119 days.  In fact carrying it around for an entire year, still keeps you under the maximum yearly limit of exposure (50,000 μSv) at maximum exposure orientation.  The yearly limit is off the scale above at 5.70 μSv/h when converted to a per hour unit. 

The only real potential danger is if fragments were ingested or inhaled.  Then almost all of the emissions are absorbed over an extended time-period.  So it is my job to ensure that that will not happen. 

Speaking of ingesting, how many bananas do you eat in a year⸮  Bananas are rich in potassium, which naturally includes the radioactive K-40 isotope.  The maximum exposure I measured above, if maintained for an hour, is in some ways similar to the total dose received from eating 35 bananas.  I am not trying to say that bananas or other foods rich in potassium are dangerous; I am just trying to put the exposure levels involved here in perspective. 

So now I have a reliable source to test with that is strong enough to give a clear signal but is still within safe levels of exposure. 

Geiger Counter, § II

❦ I needed a Geiger–Müller tube to add to the Geiger counter board.  The tube has low pressure gas in it between a high voltage anode and cathode.  When a particle of ionizing radiation passes through it it hits a gas atom and creates a + and - charged particle pair by knocking off an electron.  The pair race towards opposite ends of the charged tube.  Along the way they accelerate enough to knock other ions loose, which knock even more loose and create a cascade of charged particles.  This amplified signal registers in the circuit and is what allows single particles to be detected.  I found a J305 beta/gamma tube for sale for $31 by a company in China.  When the package arrived it had been checked by security.

❦ This tube can not detect alpha particles (helium nuclei), which are stopped by the glass.  But it can detect beta (high energy electron) and gamma (high energy photon) particles.  To try it out I temporarily connected it to the geiger counter shield which was plugged into the micro-controller underneath, which was connected to my computer.  I uploaded a program found here (link).  ☞ Note the conversion factor must be set correctly in the program for the model of tube you are using. 

❦ It clicked every few seconds and in the picture it registered 12 counts in the last minute (CPM, counts per minute), which, for this tube's sensitivity, corresponds to 0.0974 micro-Sieverts per hour of background radiation.

Geiger Counter, § I

❧ A few weeks ago I mentioned hooking a portable geiger counter to a drone as a way to measure radiation levels in dangerous areas.  Also, radiation from the 2011 Japanese Tsunami and Fukushima reactor damage is being monitored nearer to Japan and projected to be drifting across the Pacific nearer to Hawai'i.  There are some reports of small spikes in radiation levels at a monitoring station in Kauai.

❧ Not to sound alarmist, these are low levels of radiation here and probably harmless.  Aside from the Fukushima nuclear disaster, it would also just be fun to have a geiger counter to play around with.  Also, I was thinking of some higher voltage experiments with our
« new » vacuum pump and would like a way to monitor for x-rays.  Libelium electronics in Spain has been working on a cheap, portable geiger counter to help people in Japan monitor radiation levels.  The models with the Geiger–Müller tube included have been sold out, but the circuit boards are still available.  I ordered one and it came in the mail!  Basically the board is a shield that plugs into our Arduino microcontroller.  It has a transformer to convert the 500V required for a GM tube, and a speaker and LCD display to show the results. 

⁂

❧ Libelium also had an option of shipping a vaseline glass bead as a low level radiation source to test the geiger counter.  Since my vaseline glass find turned out to be fake I requested the glass bead.  However it was not included in the package with the circuit board.  Instead was a letter that said,

"La empresa que suscribe ante administracion de aduanas manifiesta:

Mediante presente escrito certifica que la mercancia amparada con nuestra factura ... que viaja bajo albaran ... no son productos de doble uso y no estan incluidos en el Anexo I del Reglamento (CE)№428/2009"

❧ Apparently the glass beads are considered « dual use » which implies they could have both a civilian and military use and are thus restricted from export out of the EU under regulation 428/2009.  I looked through the regulation document (link).  It mentions enriched uranium and depleted uranium, but I don't see anything about restricting trace amounts of natural uranium used to color glass beads.  Seriously, this is an absolutely harmless level of radiation.  What is Spain worried about, that the US will use glass beads to start a nuclear program⸮

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Update: a relative of mine helped me precisely translate this and it seems that I have confused a negative.  It thought it said no double use products could be included and that it was not included in the shipment because it was in 428/2009, but I am told that this actually says it is not in 428/2009 and thus not considered dual use.  This is even more confusing because they did not ship the glass beads.
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Another update: Now I get it; the current shipment, without the vaseline bead, is in compliance with 428/2009.  So the issue still stands, apparently vaseline glass can not be shipped out of the EU...?