An X-ray tube is a vacuum tube that converts electrical input power into X-rays. X-ray tubes evolved from experimental Crookes tubes with which X-rays were first discovered on November 8, 1895, by the German physicist Wilhelm Conrad Röntgen. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast to other sources of ionizing radiation, X-rays are only produced as long as the X-ray tube is energized. X-ray tubes are also used in CT scanners, airport luggage scanners, X-ray crystallography, material and structure analysis, and for industrial inspection.
The x-ray tube is a vacuum tube, containing a cathode and a high voltage anode. The cathode (or negative pole), as in the normal thermionic valves, is in turn composed of the heater filament (generally formed by copper alloy or other low-atomic metals, is powered by low voltage) and by the true cathode connected to the high voltage circuit. The anode (positive pole) instead, located at the opposite end of the tube, consists of a heavy (high-atomic atom) disk (plate), which can be either fixed or rotating. In the image below you can see an example of x-ray tube, 60 KV, used to make radiographs.
The cathode filament is heated by a current and begins to emit electrons by a thermionic effect; the electronic cloud around it is accelerated by the high voltage, which projects the electrons to the anode where they hit the metal disk: in the impact the kinetic energy they acquired became heat (99%) and radiation X (for 1%). The generation of X-rays is done by Bremsstrahlung (braking radiation) and by characteristic radiation.
Read the Disclaimer & Safety page carefully before trying to accomplish this project. It’s about working with high tensions and creating X-rays. Both of these things can be very dangerous. Carefully evaluate all risks, follow all safety regulations and never expose people or animals to X-rays.
BS7-W X-Ray tube
For our experiments we used the Soviet-Made BS7-W tube, which is easily available in the online market (eBay, sovtube). This tube is designed for X ray microscopy and microradiography applications and is characterized by a relatively low HV voltage, 15 KV, which corresponds to a “soft” X-ray emission, easily absorbed by organic materials. Another interesting feature is the emission of a collimated beam from the beryllium front window. Low voltage HV, low power, and collimated emission make it easier to use than other equipment.
|Filament Current||< 1.5 A|
|Filament Voltage||0.4 – 1.5 V|
|HV anode Voltage||4 – 15 KV|
|Anode Current||1.3 – 5.0 μA|
In the image below you see the BS7-W tube in all its “beauty” (the images are taken from the hardhack website in which they describe the tests done with the same apparatus).
In the pictures below you can see the beryllium front window from which the collimated beam comes out.
Powering the BS7-W tube
The tube is HV powered via a 6-stage CW (Cockcroft-Walton) multiplier. The multiplier is powered by a 12 V CCFL driver that provides approx. 2200 Vac.
Vin = 12Vcc –> HV CCFL = 2200V –> HV out = 13kV
The tube filament is powered with a DC – DC step-down converter voltage and current controlled. The voltage is taken from the power supply of the 12 V CCFL driver. The output voltage to the converter is limited to about 1.5 Vdc, and the current must not exceed 1 A.
The Cockcroft-Walton (or Villard) multiplier is a device that allows to convert an alternating low current to high DC voltage. The operation is based on rectifying diode and leveling capacitor. The use of multiple cascading modules, as in the diagram at the side, allows to obtain the multiplication of the input voltage.
Of course, note that the diodes and capacitors must be suitable to the operating voltage, in particular high-voltage components must be used. The picture below shows a 3-stage multiplier. To feed the X-ray tube we used two 3-stage modules for a total of 6 stages. The input alternating voltage is then multiplied by a factor of 6.
- Input voltage: max. 8,2 KVss
- Output voltage: max. 25 KV (20 KV Continuous operation)
- Output current: max. 5mA
- Operating frequency: max. 50 KHz
- Power: max. 125 Watt
The voltage multiplier is powered by a small CCFL (cold cathode fluorescent lamp) HV generator module, which operates at 12 Vdc and can deliver 2200 Vac. The module operates up to 16 Vdc at which it outputs about 3000 Vac The picture below shows the form used.
- Supply: 12 Vdc, 400 mA max.
- Power: 4.8 W max.
- Switching frequency: 40-50 KHz (load dependent)
- Output voltage (unloaded): @ 12 Vdc 2200 V, 2900 V @ 16 Vdc
A current-controlled power supply must be used for powering the filament. The maximum filament current is 1.5 A and should not be exceeded due to the danger of burning the filament. The tension at the filament varies during operation due to changes in filament temperature: the tension increases when the filament is heated. The module we used allows you to set the maximum output current.
Input voltage : 5 – 35 V
Output voltage : adjustable from 1,25 V to 30 V
Max Current : 3 A
The left trimmer adjusts the voltage
The right trimmer adjusts the current
The images below show the complete equipment. This is powered by 230 Vac line. The power supply voltage of the CCFL module can be varied continuously from 10 to 16 V so that the CCFL module produces a voltage ranging from 2000 V to 3000 V which corresponds to a HV voltage from 12 KV to 18 KV to the x ray tube.
HV insulation is very important in order to prevent electrical discharges. The tube is partially shielded with 1.2 mm lead sheet.
How to Run the Equipment
The power circuit is equipped with a main switch, HV driver power switch and terminals for remote switching.
The system is powered by a 230 Vac line voltage. As soon as the main switch is turned on, the 12 Vdc voltage must be set and the filament should be switched on and then the remote switch connected to the terminal board must be activated, of course far from X ray emissions.
X-Ray Emission Spectrum
The following chart shows the X emission spectrum of the 15 KV-powered tube. The spectrum was acquired with a 1 mm thick CsI(Tl) scintillator + PMT sensor. So thin crystal in order to be sensitive to low energy X-rays.
Making X-Ray Photos …
To obtain radiography images with our equipment we used quick developing films for dental use. These are plates contained in an opaque bag, which also contains separately the chemicals for the development of the plate. After exposing the plate to X-rays, the liquids must pushed on the plate and leave for about one minute. After this time the bag can be opened and the plate is washed out thoroughly.
To best visualize the x ray pictures we have created a small screen viewer with a transparent screen beneath which are placed high brightness LEDs, as shown in the picture below.