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Dental X-ray Machine

Dental X-ray Machine

This project is something I have had on my mind for several years now. It all started when I saw reports of members on the 4HV forum doing their own X-ray experiments. What's more it didn't appear to require much skill in the electronics department, which I lacked at the time. Some of the inspiring experiments can be seen here and here. Of course I thought it was absolutely amazing, but at the same time the idea of exposing myself to radiation didn't appeal to me at all. So I decided to NOT to conduct X-ray experiments. Well, obviously curiosity got the better of me, and I ordered an X-ray tube from ebay a year ago. Mind you this was three years later, so my will power isn't terrible, and above all I had picked up some knowledge on safety. At the same time I had purchased the core for the big-Mofo transformer, and had also started working on a CW multiplier. That the two would one day be united wasn't planned. Construction continued over the following months, and it wasn't until 6 months later I finally had the CW multiplier and a high voltage AC transformer ready. I've written this article to reflect an ideal progression of events, what actually happened was lot's of trial and error, some earlier x-ray experiments which never resulted in anything, and so on.

SAFETY: Before you read any further you should be aware of the dangers associated with conducting x-ray experiments. If your common sense suggests that this is utter madness then you're predisposed for safety, which is good. Otherwise I'll need to scare you with some quick facts. X-rays are ionizing radiation just like gamma rays, which means exposure WILL cause damage to living tissue, which in effect increases your chances of CANCER. Yes, the terrible disease you've heard so much about. The measured radiation intensity from my x-ray tube when shielded by 1mm aluminum is 110 Sv/hr. A lethal dose of radiation is between 1 and 10 Sv, or a what you would receive after a casual 5 minute exposure. In comparison the dose at 1 meter from a 1kg block of depleted uranium is only 0.14 µSv/hr, so even hot minerals are in a completely different league than an X-ray tube. So if the idea of exposing yourself to plutonium seems stupid, imagine direct x-ray exposure. Ionizing radiation can pass through low density materials with the ease of light through glass, so the only real protection is distance and thick, dense shields. As though X-rays aren't scary enough, they can also reflect and scatter, sending X-rays in completely new directions so a directional shield isn't enough. Think of an X-ray tube as a lightbulb, if you can see the light it emits, you're not entirely safe.

dental x-ray machine setup

X-ray tubes are a primitive form of particle accelerator which accelerate electrons. The Coolidge X-ray tube works by heating a filament which "boils" off electrons, just like in a standard vacuum tube/valve. When the filament becomes sufficiently warm, the electrons gain enough kinetic energy to leave the filament, and create a small cloud of electrons around the filament. When a high voltage is applied to the tube it creates a strong electric field between the anode and cathode, which attract the electrons emitted from the cathode toward the anode. The flow of electrons from the cathode to the anode can be measured as an electrical current. The electrons pick up speed as they are accelerated by the electric field, and eventually crash into the anode. In about one in every hundred collisions an electron will interact with a tungsten atom, and the atom will emit an x-ray photon to rid itself of the extra energy. The other 99 collisions result in increased temperature at the anode. The intensity of x-rays is proportional to the anode current, and the ability to penetrate matter is roughly proportional to the square of anode voltage. The spectrum of x-rays emitted from a tube will depend on the anode material and voltage. Generally the peak X-ray energy will be the energy the voltage source can impart on a single electron, which is often expressed in keV, which is the kinetic energy gained by an electron accelerated by a 1kV electric field. The bulk of the X-rays created are in the lower end of the spectrum, however the glass walls of the tube will filter out any x-rays below 20kV. It's explained pretty well on this site.

X-ray Machine divider resistors

I had worked on and off at the actual X-ray machine, which was little more than an X-ray tube holder with a shield. After some testing, Harry from the 4HV forums kindly donated a pile of goodies to keep me going (thanks again Harry)! Among the goodies was a moving coil µA ammeter, and high voltage resistors which I desperately needed. Attempts at measuring the anode current and voltage with my DMM had failed, and only old vintage equipment has worked so far. The anode current, or current passing through the tube, is measured using the moving coil ammeter with different sets of resistors in parallel to get 1mA, 2.5mA and 5mA ranges. It a simple matter of applying Ohm's Law to determine the resistor values when the internal impedance of the meter is known. A three state ON-OFF-ON switch is used, with OFF being the default 1mA range. For measuring the high voltage on the anode I built a voltage divider using high resistance resistors (courtesy of Harry :-)) which were sealed in an oil filled PVC pipe. The resistance of the resistors is 200M each, giving 1G ohm + 1.1M for the entire string. The total power dissipation in the divider is kept reasonable this way. The X-ray machine can be seen above and is simply a tube holder as mentioned before. I've mounted the ammeter on the machine along with the voltage source for the filament. The shield was constructed from a MOT (microwave oven transformer) which had been cut up previously. The laminations were arranged to provide an average of 30mm thickness, and the total weight is 3.6kg of steel. I've calculated the ratio of passed radiation through the shield to be 1 to 1300000 for 80keV x-rays, according to data here. The level of directional x-rays is minute at the very least.

Coolidge Dental X-ray Tube

The tube I purchased was from an old Ritter X-ray machine, and that's just about all that was known about it besides a bunch of serial numbers. There was minor browning of the glass, and the anode looked smooth, suggestingSchematic little use. However I had no way to tell if the tube still held a vacuum, so my 75USD might simply have paid for an expensive, but awesome paper weight. So I asked a technician at for some specs on the tube, which were given. What I got suggested the tube works in the range of 40 - 70kV, at anode currents from 0 - 15mA. Still nothing about the filament voltage/current though, so I had to tread cautiously to avoid burning the filament. Before doing anything I connected the tube to a "low" high voltage supply (old AC flyback at 30kV) which would reveal the presence of gas. If the tube lights up or shows a visible arc it has leaked and cannot be used for x-rays. I checked the tube's vacuum before driving the filament, as driving a filament without a vacuum would have burned the it. To drive the filament I eventually settled on a simple linear regulator, the LM338T and empirically determined the correct voltage range for my filament. This was done by slowly increasing the filament voltage with high voltage on the anode, until the tube started to conduct. After several measurements of the filament voltage and anode current I determined that the filament needed to operate between 3 and 4V, which is roughly the norm for Coolidge X-ray tubes. The exact filament voltage is crucial, as once the tube starts conducting more than 1mA even tiny changes in the filament temperature greatly increase the anode current. For all practical purposes constant voltage will work when driving the filament, either AC or DC. To the left is the basic circuit I used for providing test voltages for the filament. The 1k potentiometer was later changed for a smaller value and a fixed resistor to decrease the adjustment range and provide greater control over the voltage.
With the X-ray tube passing current without breaking down, I assumed it was radiating x-rays. Once again, my friend Harry provided by gifting me with two quartz fiber dosimeters in the range of 200 and 500cGy. These dosimeters have a small quartz fiber which is charged by the charging unit. The charge leaks away incredibly slowly, and causes deflection of the fiber much like in an electroscope. I noticed no difference in the reading level even after a week had passed, in fact both dosimeters are still at zero as I write this, and it has been weeks since I charged them last. Ionizing radiation will ionize gas molecules in the chamber however, and accelerate the charge leakage rate. The amount of leaked charge can be read directly in the dosimeter, and is calibrated to correspond to a set amount of ionizing radiation. I proceeded to irradiate the dosimeters at different positions relative to the tube, which would tell me where the x-ray intensity is greatest and where points of low x-ray intensity exist. Irradiating the dosimeters in the main beam at various distances allowed me to determine the average dose based on the inverse square law. At 2mA, 65kV and just 7cm from the anode spot the dose was measured to be 108 Gy/hr. With this is defined as Io, the corresponding radiation measurements matched up pretty well. You can see for yourself how much distance helps reduce dose by using the inverse square law. At a right angle to the tube, facing the user, the dose was measured to be roughly half under the same conditions. Directly above the tube the dose was measured to be ~0. Whether the x-rays are actually focused somehow or the anode simply shields them I'm unsure, though I'd bet it's the anode that's shielding.

darkroom processing image

With x-rays confirmed the next step is to attempt taking radiograms. X-rays themselves can expose photographic paper, but at a very slow rate. To help reduce the dose to patients, intensifying screens are used, which consist of a phosphor coated screen. The phosphors will fluoresce when exposed to x-rays, and create enough visible light to greatly speed up the exposure rate. An intensifying screen will fluoresce enough under a Coolidge tube to appear visible to the naked eye in low light conditions. Intensifying screens can be purchased as x-ray cassettes from hospitals upgrading their equipment. The benefit of using a complete x-ray cassette is that it simplifies exposing photopaper, as the cassettes are light-proof when closed, allowing x-ray exposures regardless of the ambient light conditions. For taking x-rays I had to learn basic photographic processing, which wasn't as hard as I thought. A bathroom was converted into a darkroom by taping cardboard over the one window in the room. I used some old ice-cream containers as chemical trays, and opted to simply use developer and fixer chemicals for processing of the images. I used Ilford Developer, Ilford Rapid fixer and Fotokjemika EMAKS resin-coated fiber paper for all of the photographies. The photographic paper was soaked in the developer for 2-7 minutes, rinsed, soaked in the fixer for 2 minutes and rinsed again. After they had dried I scanned them into my PC. In the above picture the developer appears amber, when fresh it's clear.

first x-ray color  first x-ray, x-ray

The photographic paper was cut into smaller sheets, and exposed between 20 and 60s at roughly 2mA. The first batch of radiograms I took were at reduced anode voltage, just 50kV. Due to most of the x-rays being in the 30-40kV range, the penetration was low enough to show details in plastic objects. I only took 4 x-rays at 50kV, and only one of them is interesting. It's evident I needed some practice placing the objects correctly, but otherwise it's a successful x-ray. The white spot in the one Lego figure is the shadow cast by a small button cell battery, you can also see a LED in the Lego figure's head. The exposure time was 35s, and the distance from the anode was 15cm. The x-rays were hardly penetrating, and the image is almost under-exposed. For this reason I upgraded the CW tower from four to six stages in order to increase the anode voltage. I then proceeded to take some more x-rays, which turned out considerably better. The white specs on the x-rays are from dust in my scanner.

color picture 2  x-ray 2

The 500cGy dosimeter, three sea shells, a rechargeable battery, an Ipod nano and a 0.5 mm steel transformer lamination were x-rayed. The exposure time was 30s at 15cm distance, 2mA of current and 70kV. The paper is much more exposed than in the previous x-ray, showing that the intensity of x-rays passing the glass wall of the tube has increased considerably. The x-rays pass the aluminum housing of the dosimeter with ease, while the steel lamination and Ipod backing are harder targets.

soft gun color  x-ray 3

X-rayed here is a cheap soft gun for shooting plastic BBs. At 70kV the plastic housing doesn't even show up. The exposure time was 51s at 15cm, I'm not sure why I left it for so long. Lead weights are used to make these soft guns seem more authentic, and are easily visible in the x-ray.

color, ipod and k750i  x-ray ipod and k750i
Here's the ipod again and a k750i cell phone. I used a thin, tin filter during this exposure to help increase contrast. I also exposed the image for 61s, the longest exposure yet.

color, multiplier, watch, dosimeter  x-ray, multiplier, watch, dosimeter
 This time it's the voltage multiplier unit from a color TV which had an old fashioned AC flyback inside, a digital watch and the 200cGy dosimeter. I also took a close up x-ray of the anode, to see how large the focal spot is. A small neodymium magnet was placed approximately under the center of the anode for reference. Due to the shield and cassette size I couldn't get close enough for a real good image, but it's close enough to give an outline of the anode shadowmain beam area. Above the magnet one can see the sharp shadow from the "anode heel effect". The tilt is probably due to the anode and cassette not being precisely parallel. As is visible from the exposure, the intensity diminishes rapidly behind the anode and to some extent behind the cathode. Out to the sides the intensity gradually decreases, but doesn't really become negligible until directly above the anode. I didn't use a filter for the image, and judging by checks with my dosimeter most of the higher energy x-rays are radiated in a wider angle than the image would suggest.

As of the time of writing this I've put the x-ray machine away, as it lacks proper shielding. I currently plan on upgrading it so the tube is completely encased in lead, and cooled by some means. Even a one minute exposure can overheat the tube, and it takes several hours for it to dissipate the heat as it can only radiate it as infrared. Almost all of the energy used to make x-rays is lost as heat, save the 1% mentioned earlier, so for continuous operation the anode would have to get rid off 100 - 300W of power depending on the anode current. That would require heatsinking of the anode, which is difficult considering the immense voltage present on it. As for the current steel shield, it may seem adequate, but many x-rays are reflected from nearby objects and can significantly increase one's radiation dose. Before I do any further experimentation I'll need proper shielding which reduces the risk of scattered x-rays.

Want more? See these Amateur X-ray projects as well:

c4r0's X-ray Pages
Henning Umland's High Voltage and X-Ray Experiments
Jochen Kronjaeger's X-ray Stuff
An Inexpensive X-ray Machine
Generating X-Rays with Receiving Tubes
Tera Lab
Leslie Wright's X-ray Page
X-rays with High Voltage Rectifiers
Danyk's Website (written in Czech)

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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.

Creative Commons License This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.