Generating X-Rays with Receiving Tubes

   
				

Old TV tubes are used as cold cathode x-ray emitters in a simple apparatus developed by Bob Templeman of Chicago, Illinois. With selected beam power tubes of the type used in the high voltage section of TV receivers, the intensity is adequate to make x-rays photographs of objects using standard films.

The full version of this article appeared in Volume 3, Number 1 of the Bell Jar with an update in Volume 3, Number 2.

Radiograph of potted electronic module (high voltage tripler) imaged on Agfapan 400 sheet film

Introduction: Cold and Hot Cathode X-Ray Tubes

The earliest x-ray tubes were of the cold cathode variety. These tubes, referred to as Crookes or Hittorf tubes, were of the general class of gas tubes since the pressure had to be in the ‘soft’ vacuum range (about 10-3 to 10-4 Torr) to permit the passage of electrons from the cathode to the x-ray producing target in a so-called ‘dark’ discharge. Higher pressures would result in a luminous discharge (as in a neon lamp) with only a small potential drop across the tube. Lower pressures (a 'hard' vacuum) would result in no current flow regardless of applied voltage.

The cold cathode tube went out of use shortly after 1910 when W. D. Coolidge introduced a tube with a hot cathode (thermionic) electron emitter. The Coolidge tube, which uses high vacuum (typically below 10-5 Torr), has a number of advantages over the gas tube.

With the gas tube, the electron current, at a given voltage, is dependent the voltage across the tube which, in turn, can vary depending upon the degree of vacuum. Furthermore, the degree of vacuum will change over time. This will affect the spectrum (hence the penetrating quality) of the x-ray output as well as the intensity. With a heated cathode in a high vacuum tube, the electron current may be controlled simply by varying the filament temperature. Then, by varying the voltage across the tube, the penetrating power of the x-rays (a function of the x-ray energy) may be varied. Thus, two important parameters may be controlled independently.

The operating regions of hot cathode rectifiers and x-ray tubes are shown in Figure 1.

Using Receiving Tubes in a Cold Cathode Mode

Bob Templeman has been able to use conventional vacuum tubes as cold cathode x-ray tubes. He has done most of his work with the 6BK4B, a beam triode used for voltage regulation of high voltage, low current dc power supplies in color and black-and-white television sets. The tube has an octal base and a plate cap. Bob has also tried several other tubes including the 6EN4 (which is very similar to the 6BK4B), 3AT2, 3CZ3A, and 3BW4. He has found that all will emit measurable amounts of x-radiation but only the beam tubes appear to provide sufficient radiation to expose standard films.

Since the tube is operated in a cold cathode mode, the tube’s degree of vacuum is quite important. Bob found that about one in eight tubes is able to produce enough radiation to expose his film. One might ask “why not just heat the filament to get an assured, controlled emission of x-rays?” The answer lies in the basic characteristics of a high vacuum diode. A ‘normal’ hot cathode vacuum diode, such as a rectifier tube, operates in a region where the tube current varies nearly linearly with the voltage drop. Thus, substantial increases in current would be required to produce a voltage drop across the tube significant enough to produce useful levels of x-rays. For normal tubes, the current would be well in excess of the tube's power rating. Normal operation for a rectifier tube is moderate to high current with a low voltage drop.

What is good for rectifiers is not good for x-ray tubes. In the case of the x-ray tube, the tube is operated in the upper part of the characteristic curve, the ‘saturation’ region. In this mode, the voltage can be increased with little increase of electron current. Getting the right balance between current and voltage is part of each tube's design. Also, as noted before, varying the filament temperature (e.g. by means of varying the filament voltage) allows the intensity of the tube’s output to be adjusted. For each filament temperature, there is a different current vs. voltage characteristic.

The High Voltage Power Supply

Bob uses a TV flyback driven voltage multiplier to power his tubes. This is a fairly common implementation using a pair of general purpose NPN transistors to drive a 10 turn primary which is added to a stock flyback.

The multiplier is of the cascade (Cockcroft-Walton) type. A modular tripler scavenged from a TV set can be used as these can usually be pushed to about 40 kV without failing. A better alternative is to make the multiplier from scratch using discrete diodes and ceramic disk capacitors. The diodes should be rated at 20 kV. A good value for the capacitors would be 0.001 uF at 15 kV.

As the flyback circuit will provide about 10 kV into the multiplier, six stages are required to boost the voltage to a maximum of 60 kV. The multiplier can be assembled on a piece of bare perf board with good separation between the components. To avoid excessive leakage or arcing, immerse the whole multiplier assembly in mineral oil. A rectangular plastic food storage dish makes a good container for this assembly.

A means of measuring the high voltage output is essential. A resistive divider is appropriate for this application. However, standard components are not suitable for high voltages, low current measurements. A good circuit for measuring the output voltage is a potted TV focus divider (RCA SK series or equivalent) which contains the necessary high voltage/high value resistors. The only additional components needed are one external resistor and a standard high impedance dc meter.

When testing the completed multiplier, avoid the temptation to draw sparks from the output. This will only stress the components and lead to premature failure.

Circuits are shown in Figure 2.

Producing X-Rays

When all is set with the high voltage circuitry and several candidate tubes are in hand, it is time to try generating some x-rays. X-rays are nothing to be treated casually. Bob surrounds his tubes with 2 to 4 inches of lead. (At 60 kV, 1/16 inch of lead is the absolute minimum.) Before turnng on the power, make sure that you have an operating x-ray monitor. This will be needed for checking tubes for output as well as for checking the effectiveness of the shielding. Bob uses a simple Geiger counter circuit which was provided as a kit from Electronic Goldmine (P.O. Box 5408, Scottsdale, AZ 85261). Finally, the safest practice is to operate the tube from a remote location.

Bob notes that the tubes tend to operate better when the normal cathode is positive, probably because of the slightly higher impedance in this configuration. As this tube element is smaller, the image tends to be a bit sharper. Still, receiving tubes are relatively diffuse emitters of x-rays and the images will be slightly fuzzy.

Arc-over is a problem at the voltages Bob has been using. Encasing the tubes in wax was tried but found to be only partially effective. Bob's prize 6EN4, the best emitter of x-rays, was destroyed in spite of this encapsulation. Immersing the tubes in mineral oil appears to be more effective. (Watch your druggist’s face when you purchase the several bottles of mineral oil which will be needed for insulation of the tube and multiplier!)

Even operating in a cold cathode mode, the current through these tubes at 40 to 60 kV is enough to cause heating. Furthermore, as the tube elements warm up, the cathode begins to emit electrons thermionically. This leads to increasing dissipation, lowered potential, and a shift of the x-ray emission toward the soft, less penetrating, region of the spectrum.

Bob supplied a number of radiographs which were printed with the original article and one of these is reproduced at the top of this page. The best radiographs taken to date have used Agfapan 400 sheet film. Typical exposures were 30 minutes with the tube about 8.5 inches from the film plane. With a negative bias of 40 kV applied to the plate cap, the tubes draw about 20 microamps.

Bob has been investigating the sensitivities of various phosphors to the x-rays emitted from his tubes. He had no luck with the phosphor salvaged from a fluorescent light tube but he did get a faint fluorescence from the phosphor scraped from the face of a broken color picture tube. The brightness was about comparable to that from a piece of standard medical rare earth phosphor x-ray intensifier screen.

Dosimetry

Any person who regularly works with any combination of high voltage and vacuum should maintain a dosimetry program.

Landauer (2 Science Road, Glenwood, IL 60425-1586, (708) 755-7000) provides film and thermoluminescent (TLD) dosimeters as part of their service. These are provided as either wearable badges or as room monitors. To sign up for the service you select a monitoring frequency (weekly, monthly, quarterly) and pay a small set-up fee plus a year's payment in advance. Before the end of each monitoring period, you receive a new badge. At the end of each period you send in the current badge and within 5 days you receive a report giving dosage for the period plus cumulative dosage. For a TLD dosimeter sensitive to x-ray, gamma, and beta radiation with a quarterly schedule, the cost is under $100 for a year.

Another approach to dosimetry and one which gives a continuous record is a Geiger counter provided by Aware Electronics (P.O. Box 4299, Wilmington, DE 19807, (302) 655-3800). Their RM-60 is a small monitor which interfaces directly to an IBM compatible computer via a phone type cable to the serial or printer port. A dedicated PC is not required as the software gathers the data and stores it to disk even while the computer is running other applications. The software displays the data in a scrolling bar chart format with date and time for each bar. Also provided is the cumulative average dosage. Cost for the RM-60 package is about $150. Aware's catalog also describes several other radiation monitoring items.

Further Reading

Using standard vacuum tubes to produce x-rays is nothing new. C. L. Stong’s “Scientific American Book of Projects for the Amateur Scientist” (Simon and Schuster, 1960) has a chapter describing Harry Simons’ impressive experiments using antique 01 tubes driven by a homebuilt Oudin coil. Mr. Simons also fabricated a variety of his own tubes, simple bulbs with sealed-in molybdenum cathodes with magnesium targets. The latter was deposited on the inside of the bulb, opposite the cathode, and was capacitively coupled to the Oudin coil by means of a layer of aluminum foil which was wrapped on the outside of the bulb. Simons evacuated his tubes to 0.1 mTorr before sealing.

A large number of books are available which deal with the physics of x-rays, x-ray production, radiographic techniques and safety. Before proceeding too far with experimentation with x-rays, please visit your local library to obtain further background information, particularly with regard to safety issues.

Addendum

James O'Neal of Alexandria, VA has provided some additional background on this tube that may explain why some tubes work well and others don’t. He writes:

“Those of us who are old enough may remember that there was something of a flap in the color TV industry during the late 60s when it was discovered that there was x-ray leakage from certain models of color TV. This was attributed primarily to the (then) quintessential 6BK4.

“The tube manufacturers solved the problem by redesigning the tube, going to a heavily leaded glass formulation. The “improved” 6BK4 is instantly distinguishable just by lifting it - it weighs substantially more than any “normal” tube of roughly the same dimensions. For experimenters, there may be some of the thin-skinned units either in old junked sets or in service shops that have been around for a while. A distributor of tubes such as Antique Electronic Supply (Tempe, AZ, 602-820-5411) might also be able to help.

“I did some experimentation with the original 6BK4 at the time of the flap and I found that the tube, when driven hard enough, could indeed produce x-rays. I still have a nice mental picture of the glass envelope of one of them brightly fluorescing a lime green color.”


More Information on X-Rays

The Department of Physics at the University of Uppsala maintains an excellent X-Ray Page.

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