Re: A simple way to MAKE x-rays????

["Cary Renquist" <crenquis@goldengate.net>]

I have attached a file that  I came across a
couple of years ago -- perhaps it will be of
some interest to you.

Cary


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   Cary Renquist                       crenquis@goldengate.net
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htt v. 1997-05-08

 

Extracted from TV.tube.to.Xray.tube.html, 4 Aug 1997, 21:57

 



Generating X-Rays with Receiving Tubes



------------------------------------------------------------------------

This article, which describes the experiments of Bob Templeman of 

Chicago, IL, is from  the Bell Jar  (electronic version) No. 2 (October 

1994), which was condensed from material originally presented in Volume 

3, Numbers 1 ? 2 (Winter ? Spring 1994) of  the Bell Jar . Full  

circuit schematics as well as sample radiographs are presented in the  

original articles. Devoted to the vacuum experimenter, the intent of  

the Bell Jar  is to broaden interest in vacuum technology through 

useful discussions of theory and technique, and to present ways in 

which a variety of apparatus may be assembled using common and 

inexpensive materials. Information on  the Bell Jar  may be obtained by 

sending email to the editor, Steve Hansen, at hansen35@delphi.com or by 

writing to 35 Windsor Dr., Amherst, NH 03031. New numbers will be 

mailed at approx. quarterly intervals. Email subscriptions are free and 

may be obtained by contacting the editor. Comments, contributions and 

criticisms are always welcome. Copyright 1994, Stephen P. Hansen.  ;

------------------------------------------------------------------------



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 (i.e. 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.



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' 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 mF 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.



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. First, 

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 is 

provided as a kit from Electronic Goldmine (P.O. Box 5408, Scottsdale, 

AZ 85261). This kit, #C6430, which has an audio output, currently lists 

for $59.95.

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.

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.) Maintaining a dosimetry program is advisable and 

some suggestions are provided later in the article. Finally, the safest 

practice is to operate the tube from a remote location.

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. These showed the internal circuitry of several potted 

modules as well as the internal structure of a 308 Winchester rifle 

shell.

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's 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 a sealed-in molybdenum cathode with a 

magnesium target. 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.