X-rays with a dental Chirana

After receiving a donation of two nice X-ray intensifying screens from Mr. Máca, a Czech radiologist (many thanks!) in 2011, it wasn’t long before I got a hand on a real X-ray tube, obtained through barter trade, with some luck, too. Only then were my previous failed X-ray attempts with DY86‘s and 6VS-1‘s marked with success, at least!

First success: an MP3 player radiograph

Well of course I’m not going to teach you how it’s possible for you to X-ray your hand at home, step-by-step (that’d be one hell of a long article); only to disregard safety precautions and develop yourself burns afterwards; so rather than that I am going to mention the experience how my first working X-rays went through the building process. 🙂

X-ray tube “Chirana” DOK-70, Czechoslovakia
Markings lost, Umax is 70 kV nevertheless.
-We made our own x-ray tubes back then!

To produce X-rays, one can go in cold- or hot-cathode tubes. Most cold cathode tubes as of nowadays are rare items which belong to your local museum, however, some were manufactured for schools as a physics teaching material. Combined with a Ruhmkorff inductor, these tubes generated copious amounts of X-rays enough to make a realtime radiograph on a zinc-sulfide-based fluorescent screen.
Hot-cathode X-ray tubes, also called “Coolidge tubes”, are used to generate X-rays to this date in hospitals, security and lab equipment, etc.  These need to be heated up before operation and a more powerful supply than an induction coil is needed. They are also more sturdy, have stronger vacuum and can operate for much longer than the prehistoric cold-cathode ones.

Intensifier screens DuPont Cronex RAPID (800)
Blue emitting and rare-earth based.

 As I did not want to mess with a photosensitive film and developing it afterwards; I have chosen to photograph the intensifier screen – in dark – with a remotely-controlled camera, something like a flouroscope. The photo can be then converted to grayscale and made a negative on a computer.
An absolute minimum of 40 kilovolts, under load, is needed to make any more-or-less reasonable radiographs in real-time, or with short exposure time (up to 5 seconds). Many low powered drivers might give e.g. a voltage of 50 kilovolts when the arc strikes, however it falls down sharply at load /an arc, ionised air, is also a load/. So if you have a similar X-ray setup and you’re not getting any fluoroscopic pictures whatsoever, make a stabilized high voltage supply, decrease filament current (to decrease overall current draw) or increase input power (output kilovoltage directly, if you can).
40 kilovolts is enough to X-ray small plastic objects, 50-60kV for biological specimen and over 70-80 kilovolts for metallic things. Lesser energy X-rays are easily shielded out, but they also absorb themselves better (in tissues, e.g.) !

The “digitalization” process

Now for the setup itself. Hadn’t been lucky to obtain a real, oil-submerged 120 kilovolt transformer, so I had to use some of the high voltage drivers I’ve made formerly. So: the main driver is my DST-driven ZVS; fed with 50 volts and the resonant capacitor tuned to 220 nF for approximately 50 kilovolts out (arc striked at 7.5cm). This was hooked directly to the anode. My cathode had been directly connected to ground and heated through a classic mains transformer through my triac phase regulator. Having a regulated filament supply is very crucial, since with this you affect the overall current draw from your HV supply. Be it too big and the output kilovoltage won’t be enough for radiographs, so you need to fiddle with this.
Most of these filaments need a few volts (not more than 4 or 5), AC or DC, at a few amps. Or, it is possible to heat it up as I did. Trial and error 🙂

X-ray of a measuring tape

Then, the object to be X-rayed is placed between the anode window/main beam and the opened intensifying screen. A camera is then set with an appropriate exposure value and self-timer, then focused on the other side of the screen. For this setup I’ve used an exposure of 3 seconds and a self-timer of 10. After setting that up, I turn on the heater and immediately escape to a safe place, in my case I usually hide behind a wardrobe, or a concrete wall – with a proper radiometer on my neck, sensitive to soft X-rays (e.g. the IT-65), to determine whether its safe or not. And just before the camera strikes, high voltage is remotely turned on during the exposure process, and shut down immediately, along with heater supply, after the photo is done.

Microchip

Now, I haven’t had a kilovoltmeter during the process, but the voltage on the tube (under load) had been, judging from the pictures’ contrast, no less than ca. 32 kV and no more than 40 kV, so if using a flyback supply, make some voltage headroom for your output, you won’t destroy an X-ray tube with excessive voltage easily. Or get a real x-ray transformer and rectifier. In such a case, invest also in shielding: 40-50keV is a lot easier to shield out than 150, let’s say. 🙂

A relay, out of focus…

As for radiation protection, a classic Geiger counter with a metallic probe/Geiger tube is not going to tell anything about the energy of your rays. In addition, due to the design, it is going to show readings 10 to 10 000 times lower than they really are!
I suggest having a ionisation chamber-based meter handy (like the IT-65.. 🙂 ), plus you might need to remove any metallic/lead shielding, if present, in your meter.
For empirical values of general X-ray tubes, be sure to check out RadPro calculator, to give you a basic idea of the main beam’s radiation intensity. 🙂

Flyback supply test (7.5 cm) with a modern
“draw-an-arc-and-see” approach

And that’s about size of it folks; enough said, I think. As you can see this article was not a guide, but rather a description of the setup itself. I really don’t suggest playing with X-rays if you aren’t informed how they work, how are they shielded and what advantages, disadvantages and dangers they pose.
Finally, if you got yourself lost in all that text, here’s a video of the whole setup. Watch and enjoy 🙂