Why did I write this instructable? Well, to have a shot at winning that objet 3D printer!
And why would I want that?
To set up a hackerspace of course! There aren't any within 50 miles of my town, and that leaves those who wish to build things; people like my friends and I, sh*t out of luck. This is especially a pain when I'm trying to design a better, manufacturable, portable x-ray machine and have no access to a 3D printer!
I've spent the better part of two months writing this guide, just for the chance to give some geeks (me included) the tools they deserve. If you could vote for my instructable, that'd be just awesome :-)
X-rays can kill. At the very least, they can give you cancer, which also kills. If you do not fully understand the dangers of ionizing radiation, and are not competent enough to handle voltages exceeding 50,000eV do not, under any circumstances replicate what I have done here.
Step 1: Be safe: Radiation Sickness
Acute radiation sickness occurs when your body has absorbed a large amount of ionizing radiation, usually on the order of several sieverts. What makes radiation lethal is the effect it has on DNA. When a high energy particle, be it a photon or some other particle collides with DNA it breaks bonds and rearranges the bases. Normally your cells can repair this damage, but if a cell fails at that task it often commits suicide before it divides. For long living cells such as muscle this isn’t too much of a problem, since the other cells have time to replace the dead ones. For short-lived cells though, this apoptosis becomes a major issue as cells are dying too fast to be replaced.
Such short lived cells include the mucus-making cells that line the intestinal wall. When exposed to enough radiation, these mucus cells start to die off en masse, and so are not replaced. No mucus cells means there will be no mucus, and no mucus means there is no protection from stomach acid. The intestine stops absorbing food particles, acid burns the tissue, and eventually you die of sepsis. If somehow you survive this ordeal, you will now need a bone marrow transplant since the short-lived bone marrow cells have died off. Radiation sickness symptoms include nausea, stomach pain and a lack of energy, and a detailed chart of symptoms can be found here.
And that’s why we shield ourselves from ionizing radiation! Keep in mind that it takes a very large amount of radiation to cause radiation sickness, not something a fiestaware plate or even a radium painted clock could ever produce. However, a Coolidge tube is certainly capable of generating very intense radiation!
Step 2: Be Safe: Know thy Energy!
There are multiple different types of radiation and each type must be treated differently when it comes to radiological protection. Some types require more shielding than others and since this is a guide I will now do some explaining. First with particle radiation, then with electromagnetic radiation. But before we do that let’s discuss energy.
Radiation can have different energy levels, energies which are measured in electron-volts (eV). One electron-volt is defined as the amount of energy gained by one electron as it moves through an electric field of one volt. For example, green light photons usually have an energy of about 2.3eV, while blue light has an energy of 3eV. More energetic radiation is able to cause more damage when it hits something, and this is why microwaves such as those emitted from cell phones (0.00001eV) cause no chemical damage while gamma rays which may have an energy of 5 million eV can cause major damage.
Generally higher energy radiation is harder to shield than lower energy radiation, but when it comes to particle radiation the type tends to play more of a roll when determining penetration. Usually particle radiation tends to be the least penetrating...
Step 3: Be Safe: Pesky Particles
Although alpha particles are very high energy, often having energies in the MeV range, they are very large. Because of that they are stopped very easily. In fact an alpha particle cannot even make it past a piece of paper, or even skin for that matter. Alpha particles usually have a hard time making it through more than 3cm of air, so therefore no special shielding is necessary for alpha radiation. Just don’t eat an alpha emitter and you will be fine.
The next type of radioactive decay is beta decay, a process in which a neutron is converted into a proton and in exchange an electron and a neutrino is ejected. The neutrinos are of no concern since they are small, light and neutral, and thus pass through any matter they encounter and fly off into space like a ghost. The speedy electron known as a beta particle has a negative charge though, so it can interact with matter and thus pose a hazard. Fortunately beta particles are not very penetrative; all that is needed to shield them is a piece of aluminum.
The last type of particle radiation is known as neutron radiation; something that is created when atoms are either fused together or fissioned apart. Unlike all other forms of radiation, neutrons can actually turn things radioactive! This is because when a neutron smacks an atom it may stick to it, turning that atom into another stable isotope or possibly a radionuclide. Unless you are either playing with Farnsworth Fusors or uranium reactors neutron radiation is not much of a concern, but nonetheless it is best shielded with light materials of all things, materials such as water and aluminum. Large amounts of water make an excellent neutron moderator, but because of this the human body does too. Therefore neutron radiation is especially dangerous to living things so do everything in your power to avoid it.
While you're not going to encounter any particle radiation when playing with x-rays, it's always best to be informed!
Step 4: Be Safe: Use Shielding!
First let’s start with gamma rays. In certain radionuclides the atom’s nucleus is left in an excited state after beta or alpha decay. This energy is then released via a very high energy photon. By high energy I mean several MeV, and due to that gamma rays are very penetrative. It takes quite a lot of material to stop them, so lead is often the material of choice for gamma shielding. If for some reason you have a very active gamma source use plenty of lead to shield it. Something like 5cm or more of that grey metal should be sufficient.
The other type of electromagnetic radiation I have to discuss is x-rays. X-Rays are produced when electrons dump a large amount of energy into a single photon, thus creating a very high energy light particle. X-Rays are a lot like regular light: they travel in straight lines, can be reflected somewhat, and scatter in the air much like a green laser beam. When experimenting with x-rays, always make sure your lab is of light construction. While cinderblock walls are great for stopping x-rays from escaping your lab, they are also great for reflecting them back at you! It’s better to have them escape rather than to have them bounce around.
When possible, be sure to either point your x-ray beams down to the earth or up in the air: anywhere where it is unlikely to be intercepted by an animal or human. NEVER power up an x-ray tube in a shared residence or an apartment without full knowledge that the radiation will be contained, and NEVER intentionally expose yourself to x-radiation.
It is important to shield yourself from x-rays to prevent overexposure! The amount of shielding required is entirely dependent on the energy and quantity of x-rays being stopped. Lead is the ideal shield for x-rays because it is cheap, easily workable and has a high nuclear charge; something that lets it absorb electromagnetic radiation very well. For convenience I have prepared this chart of energy vs. attenuation vs. amount of lead needed using the standards set by the International Atomic Energy Agency. [Image 1]
Step 5: What is a Coolidge tube?
A Coolidge tube’s method of operation deviates not too far from that of a diode. The heater is given a bit of current to warm it to incandescence, where the now hot tungsten cathode boils off a cloud of electrons while simultaneously focusing them into a beam. These electrons are then attracted to the positively biased anode and move towards it at a very high speed. Upon arrival at the anode, the high energy electrons lose energy through collisions with the metal atoms. Most of these electrons will do little more than heat the anode, but about 2% will generate x-rays in a process called bremsstrahlung.
Step 6: How do they produce X-Radiation?
Literally translating to ‘braking radiation’, bremsstrahlung is the process where a high speed electron ‘brakes and sling-shots’ around an atom's nucleus, dumping its kinetic energy on one photon. An imparting electron might have an energy in excess of 60keV so some very energetic photons are be made; photons which fling off into space and become the x-rays we all know and love.
What determines the energy of the x-rays produced is the amount of voltage present on the anode. It’s quite simple actually; more voltage means more electron attraction, and more attraction results in a faster electron beam. Faster electrons are able to make higher energy photons, and thus “harder”, higher energy x-rays would be generated.
Bremsstrahlung is a continuous spectrum of radiation akin to a “white light” source. Since most electrons graze a few atoms before they have the chance to sling-shot, they often lose some energy before they make any x-radiation. A whole range of x-ray energies is thus produced.
The maximum energy that an x-ray can have is limited to the energy of electron producing it, itself directly proportional to the voltage applied on the tube’s anode. Often this energy is measured as kilovolts peak, or kVp. In reality the majority of the x-rays produced are low energy, ‘soft’ x-rays, but these are greatly attenuated by the tube’s glass wall.
2. Characteristic Production
Characteristic or k-line production is the second mode in which an electron might produce an x-ray. In this method, electrons knock others out of an atom’s lowermost shell and leave a hole which must be filled. This unstable arrangement is then promptly made stable by electrons from higher shells who jump down to fill the hole, emitting an x-ray photon during the journey.
Tungsten k-shell electrons have a binding energy of 69.5keV, so to kick these out your impacting electrons must have energies greater than 69.5keV. Typically one would need to give the anode a bit more than 72kV to accomplish this, hence the standard 75kV x-ray tube.
After a k-shell electron gets the boot, its hole will immediately be filled by an electron from tungsten's l-shell; binding energy 10.2 keV. The difference between these two energy states; 69.5 keV and 10.2 keV gives us the characteristic tungsten x-ray energy of 59.3 keV. A molybdenum anode would produce two peaks, one at 19.7keV and the other at 17.6keV.
Interestingly, this process can be used to identify elements based on their k-lines. By bombarding a sample with electrons and measuring the output spectra, an x-ray florescence analyzer can determine what elements a compound contains.
Step 7: Anode Current
Conveniently, these tubes usually come with some graphs to help one set parameters for the design of a machine; [image 1] displays the relation between heater voltage and anode current for the tube which I've chosen. From what the curve tells, one must apply 2.6V to the heater in order to allow a decent 3mA to flow through the tube’s anode. While 3mA might not sound like a lot of current, at 75,000V that is a respectable 225 watts of power! Assuming a typical 3% efficiency, this would equate to 6.7W of x-ray energy out.
When one thinks about how a 100W light-bulb emits on average 4 watts of visible light, it becomes quite clear that the tube will emit quite a hefty amount of radiation; certainly enough to expose a film.
Step 8: Thermal Limitations
Fortunately, manufacturers of both x-ray machines and x-ray tubes are required by federal law [CFR Title 21] to provide anode heating and cooling curves for their devices [image 1]. It's evident that operating this tube at a power of 225W would limit the maximum exposure time to a bit less than 1 minute, with a 5 minute cool-down period. Of course x-ray exposures are never actually 1 minute long; usually they are only a few seconds at most.
Provided the tube is not abused, 225W would not be an unreasonable power to operate it at.
Step 9: Generating an extra-high tension
While all of these methods have their advantages and pitfalls the Cockroft-Walton voltage multiplier would be the circuit of choice for this project. Properly designed cascades are capable of transforming large powers with comparatively little loss, and their lightweight and small stature make them well suited for a small x-ray generator.
Refer to the first schematic. By feeding an alternating current into this circuit, one can sacrifice cycles and current in return for a doubled, tripled or even quadrupled DC voltage. All and all the circuit's mode of operation is rather simple, as it is nothing more than a cascade of Greinacher voltage doublers [second image]
On the negative alternation, the bottommost plate of C1 is charged to -10kV via D1. Afterward, the positive alternation puts C1 in series with another 10kV creating a total potential difference of 20kV, which is shared with C2 via D2. This 20kV can then be discharged, or another cascade may be added to create 30kV, or another to make 40 kilovolts. In reality though, it takes several more cycles for the stack to reach its full potential due to parasitic resistances limiting what would otherwise be very high currents.
Step 10: Designing a CW
There is nothing overtly complex about this math; it's simply the peak DC input value multiplied by the number of stages in the multiplier stack. Of course, this is only the theoretical output voltage. Large high voltage capacitors are expensive and bulky, so in most cases we are stuck using small capacitors –and their high XC.
Now take a gander at equation #2.
The large impact of n in the latter half of the equation tells that using as few stages as possible in a multiplier will help minimize voltage drop. Fewer stages would mean fewer series-strung capacitors, and thus a lower XC. Likewise, I ÷ ( f * C ) tells us that both higher frequencies and larger capacitances will reduce the voltage drop under load. In both scenarios XC would be reduced. A practical multiplier would thus contain no more than 5 stages, operate at a very high frequency and have capacitors whose values are not less than 500pF.
This problem is compounded by the fact that many capacitors are placed in series in a multiplier. The high impedance nature of this circuit allows any sort of load to pull down the voltage substantially. This pulldown can be so profound that time was spent developing a formula to estimate the voltage drop under various conditions.
Step 11: Designing *the* CW
The operating frequency should be set to 70kHz to keep capacitive losses in the transformer’s windings to a minimum, yet that still should be high enough to push 3mA through without too much voltage drop.
Please refer to image #2.
This 2.6kV drop is certainly reasonable and may be compensated for by increasing the input voltage by an additional 650 volts.
However there is one minor problem; this formula is for the most part, useless. While the theoretical voltage drop should be 2.6kV, it will most likely be on the order of 6 to 12kV. Although this is significantly higher than predicted is not impossible to compensate for by upping the frequency and input voltage a bit.
Step 12: Oscillators!
In order to obtain 225W at 12kV we need 54mA, and assuming a 100% efficiency we’d likewise need to draw 6.25A from a 36V source. Of course 100% efficiency is unobtainable, but 10A at 36V is still not unreasonable for a hobbyist to supply.
A choice oscillator for this task is the current-fed ZVS Oscillator; an LC resonant zero-voltage switching circuit. Although a Hartley or a Colpitts oscillator could be used, both are not switched under no-load conditions, and would thus burn significant amount of power when the MOSFET travels through its linear region.
Step 13: How the ZVS works
When power is applied to the circuit, current begins to flow through L1 and into the MOSFETs’ drains via the center tapped load coil. Simultaneously, this voltage appears on both gates and begins to turn the MOSFETs on. Since no two FETs are alike, one turns on faster than the other and drags down the voltage on the opposite MOSFET’s gate. One FET is now latched on while the other is off. The tank capacitor prevents the circuit from staying in this state as the LC resonance causes a sinusoidal reactance in the circuit; reactance which will ‘flip’ the MOSFETs’ states and feed more current into the tank. This oscillation keeps going until power is removed, or some instability such as a saturated load inductor latches one MOSFET on and explodes the circuit.
While the circuit would work in theory, such an oscillator proves to be very unreliable without some modifications. Instead of connecting the gates directly to the LC tank, it’d be wiser to instead leave them normally pulled up via a pair of resistors, where the LC tank would alternately ground the gates via ultra-fast feedback diodes. This method ensures that no stray currents lock up the gates and kill the circuit. [image 2]
Of course MOSFET gates will not tolerate a VGS in excess of 30V, so it is a wise idea to use 18V zener diodes to protect the gate from such excess voltages. 10K discharge resistors ensure that no stray charges are left on the gate while it is being pulled down by a feedback diode.
If that confused the hell outt'a ya, this simulation might help!
Step 14: Designing a ZVS
Parallel resonant LC oscillators often have a poor tank power factor, with this one being no exception. When drawing 8 amps the circulating tank current may exceed 50A! This can become a problem if the DC resistance of the load coil is high. To combat this one may wish to use a larger capacitor than inductor, since a metalized polyester capacitor’s parasitic resistance tends to be smaller than a large coil’s.
[look at image 2]
Since we designed the CW to run on 70kHz we'd need that same frequency out of this ZVS. To achieve that with 680nF of tank capacitance, we’d need roughly 7.5μH of tank inductance or about 5 turns of wire on an average ferrite core. Since 5 turns of copper wire has for the most part an insignificant series resistance I^2 R losses in the coil will be minimized.Trouble may arise if the coil voltage raises high enough to saturate the transformer's core, but an air gap should solve any issues.
Step 15: The transformer
Special attention must paid to prevent saturation that could otherwise occur at 15 volts per turn; pick a core with a low permeability, a large cross sectional area, a small magnetic path and then set a 0.5mm air gap.
Step 16: The cathode PSU: Switch mode power conversion
Buck converters are the simplest of the switch-mode topologies. Essentially the circuit measures the voltage across a capacitor, and attempts to maintain a set voltage by varying the current supplied to that capacitor. Often this is done by varying the duty cycle on a rapidly switching MOSFET. While that might sound good in theory, in practice the very high currents experienced by the MOSFET would increase losses to intolerable levels. In the real world, an inductor is placed in series with the MOSFET to more or less ‘average’ this current flow.
This solution creates another problem though. Quickly interrupting an inductor’s current flow would create damaging high voltage spikes which could destroy the MOSFET. Typically, this is solved by putting a diode in antiseries with the inductor, but that would create a very lossy circuit. Instead, the diode is placed in antiseries with the load.
Peek at [image 1]
When the switch is closed, current flows through the load and filter capacitor via an inductor. When the specified voltage is reached on the capacitor, the switch is opened. Both the inductor and capacitor then deliver power to the load via a schottky flyback diode until the voltage falls enough that the control circuit once more turns on the MOSFET.
This happens thousands of times per second.
Step 17: Making an SMPS
Fortunately there is not much hair pulling to be done using this IC, apart from setting the feedback reference via a voltage divider connected to the output. This feedback pin feeds into an inbuilt comparator which the onboard oscillator uses to set the MOSFET’s duty cycle.
Be sure to put a big TVS diode across the IC's output! If this device fails shorted and there is nothing to clamp down the voltage, you'll burn out the cathode in your coolidge tube!
Step 18: Assemble the Radiating Head
making an x-ray device compact is near impossible.
That is of course, if one does not use insulating oil!
Most oils have a dielectric strength 4 times that of air, and eliminate the corona losses which would otherwise occur in an open air design. This reason, coupled with increased thermal conductivity is the reason why nearly all x-ray machines insulate all of their high voltage components with oil, and why both mine and yours should follow suit.
A junction box does a fine job of housing the EHT components. [Image 1] displays the junction box which houses my machine's Coolidge tube, its lead shield, the voltage multiplier and a 1.8 billion ohm resistor to measure the anode voltage. A 90kV this resistor will leak the 50uA needed to fully deflect a galvanometer.
The thickness of the box’s wall will attenuate the x-rays somewhat so there likely won’t be any low energy rays escaping. Depending on what you want to do this may or may not be a problem, but, x-rays with energies higher than 30keV should still be able to penetrate that thick plastic.
Step 19: Controlling the X-ray Head
Typically, an x-ray machine follows the following operating procedure:
1. The technician chooses an exposure time and kVp.
2. The tube’s heater warms up for a short period.
3. High voltage is turned on and x-ray photo is taken.
It’s a process that must be replicated in my machine. The safest, most logical method would be to base the circuit around a micro-controller. Not only would a microcontroller be reliable, but it would have the added benefit of allowing for easy modification later on.
Which is why I used an arduino.
Step 20: What should it all look like?
My 'something' has gone through several alliterations [images 1, 2], but in the end it was best to just put it in a wooden craft box. If I had access to a 3D printer I might have gone a different route, but alas, all I had at my disposal was a ratty old CNC machine.
It did do a good job routing the pinewood though, and in doing so it allowed my instructable to be eligible for this contest :-)
Step 21: Metering
You might remember the 1.8 billion ohm resistor that I placed inside the oil filled box. That resistor will allow 50uA through when 90kV is placed across it, so by placing a 50uA FSD meter in series with it we create a 90,000V meter. All that's left to do is make a scale!
Measuring the anode current takes a bit more math. No calculus though, just ohm's law. Before we calculate anything though, let's set some variables. The meter is 100uA full scale deflection, and we'd like to make it 3mA. Looks like we'll need a resistor!
Take a look at [image 2].
This is the scenario we must create. In order to do so though, we're going to have to measure the impedance of the galvanometer. An ohmmeter does a good job of that, and in this case the coil's impedance was 5 kiliohms.
See [image 3] for the maths!
Step 22: Nothing to see here...
[They are above]
Step 23: Radiography: X-Ray Cassettes
Photons! Lower energy photons, ones which we can see. We can use this property, 'X-Ray Fluorescence' to convert an x-ray beam into visible light, allowing us to witness the information contained within it. Often times, this is done with something called an 'intensifying screen'; a plastic sheet impregnated with an x-ray responsive phosphor.
Intensifying screens are contained in intensifying cassettes; little light tight folders which house x-ray film. [Image 1] ought to give you a good idea of what these things look like.
Types of Cassettes
Like many things in this world intensifying cassettes come in many flavors. Blue, Green, Rapid, Ultra Rapid, Normal...
'Ultra Rapid' screens will give you the brightest images, and thus shortest exposure times. This is not without its downfalls though. In order to appear bright, the crystals in these screens must be very large; large enough that the resulting image is a bit blurry.
'Regular' screens will likewise produce a sharper image, albeit a dimmer one. More x-rays and longer exposure times would then be needed, but this is the price we pay.
'Fine' cassettes will produce a very sharp image. Unfortunately it's also a very dim image...
Step 24: Radiography: Recording
+ The most traditional method would be to place a piece of paper film inside a cassette and develop it later. Although plastic-based film is certainly on death row, paper film is still readily made and bought by millions of people, and it's not going to go away any time soon. Usually one can pick up 100 sheets for about $90 on ebay; actually not too bad when you consider the price of printing high quality photos. [image 1]
+ The second, more modern method of imaging x-rays would be to use a flat panel detector. These however, cost $60,000 each. [image2]
+ A third method would be to tape the intensifying screen to a sheet of lead glass, then placing a digital camera behind it. Albiet a bit crude, a DSLR camera set to 10 second timer will do a fine job of imaging the screen. Lead glass is a must though, otherwise there will be a firestorm of noise in your image! [image 3]
If you have the money to spare, adding an image intensifying tube to your camera will greatly reduce the exposure time. Do not however buy a gen 1 or gen 2 tube; they're terrible, Go for generation 3 or nothing! [image 4]
Step 25: Radiography: Setting up a still life
It's not all that hard to set up for an x-ray image.
The first step, of course is to wait until it is dark outside. Now unless you happen to have a lead-lined room, I do not under any circumstances condone indoor radopgraphy. There are simply too many surfaces for the x-rays to reflect off of, especially if your house is of heavy construction.
The best place to take a radiograph is in your backyard with the beam pointed away from anything animate. A few acres of woodland for example, is a good target. Your neighbors house on the other hand, is not. That is, unless it is more than 80 yards away, in which case inverse square law reduces the radiation field to nil.
DO NOT EXPERIMENT WITH X-RAYS UNLESS YOU LIVE IN A SUBURBAN OR RURAL COMMUNITY. DO NOT, power up an x-ray tube in an apartment.
Now that we have some wanrings out of the way, let's get back to still lifes. It's not a terribly complex science; all you need to do is set up your x-ray source, your imaging device, and your object which is to be radiographed. The closer you place your object to the source the greater the magnification will be. Likewise, placing it directly in front of the film will produce a near life-size representation.
Step 26: Radiography: Kilovolts-Peak
[Image 1] is a radiograph of a steel gauge, set to a proper kVp for the job. Notice that all of the gauges are visible, albiet with the lighter ones a bit hard to see.
[Image 2] shows the same gauge, but at a higher kVp. The lighter gauges are now all but invisible...
[Image 3] shows the same gauge yet again, but this time at a lower kVp. Now everything is too dark.
[Image 4] is a flower imaged at about 28kVp. If we weren't able to adjust the kVp so low then the flower would be completely invisible! This is the benefit of building your own x-ray machine instead of buying one; you can adjust the kVp to whatever you want, not just from the usual 50 to 75kVp a dentistry machine will provide.
[Radiographs courtesy of Leslie Wright]
Step 27: That's about it!
All and all, it's an art and not a science. Like any art it takes practice to get good results, so don't be suprised if your first few x-rays look like crap. This is a dangerous art though, so please, be careful.
Also, please vote for me :-)
Some more information about radiography:
Henning Umland's site