Fwd: NEON- X-RAY bombarder

Hi All,

Seems as if the subject of X-ray production has come up simultaneously
on the Tesla list and the neon list. Neon John, who is temporarily off
of the Tesla list, wrote the following post to the neon list on the
subject of X-ray production that many may find interesting and
informative relating to the many posts here lately about hi-voltage and
X-rays. With John's gracious permission, he has agreed to let me forward
that post to the Tesla list. As both a retired nuclear health physicist
and a writer, there is much that can be learned from his post, and
although it was geared towards neonfolk, I think it is just as (or more)
relevant, interesting, and informative to T-coilers, since we work with
MUCH higher voltages. BTW, a bombarder is a non-current limited,
multi-kVA transformer, either dry, potted, or a pole pig, that is used
to heat neon tubing during processing. So, in the interest of
information and safety (and with Terry's indulgence :-) Here's John's

From: NeonGlow-at-webtv-dot-net (Tony Greer)
Date: Sun, 12 Sep 1999 01:01:05 -0500 (CDT)
To: TeslaTec-at-webtv-dot-net
Subject: Fwd: Re: NEON- X-RAY bombarder

To: neon-l-at-lists.io-dot-com
Subject: Re: NEON- X-RAY bombarder

> >  Did I hear right and do big bombarders make X-rays?  Are they strong
> enough
> >  to expose X-RAY film?
> Chris, I don't know about you, but I have always used a 15kva bombarder, one
> of which I still use in my basement, and if there was ever any X-rays, I
> never heard of such a thing. X-rays are usually generated from a radiated
> source, such as "radiation", and even though I am not a physisist, I don't
> think there is any radioactivivity being conducted from a power source. I
> will admit I have heard of people complaining about high power lines running
> over their home and maybe causing them cancer, but I am still skeptical about
> that too.
> As you said, you have to die from something, lets not make it worry that
> kills you...

OK, let's sort this out.

First some definitions.

The stuff that most people think of when they hear the word
"radiation" is ionizing radiation.  That is, radiation capable of
knocking electrons from atoms of substances with which it
interacts.  Light is non-ionizing radiation.  Radio waves are
non-ionizing radiation.  microwaves are non-ionizing radiation.  The
two types of ionizing radiation are gamma rays and X-rays.  The
difference is purely semantic - "gamma" is the term used when the
radiation emanates from the nucleus; "X-ray" is used when the
radiation originates from outside the nucleus.  Not knowing the
source, one cannot observe a beam of ionizing radiation and
determine whether it is gamma or X-ray.

The common source of X-rays is "bremsstrahlung" or "braking
radiation".  When an electron (for example) is rapidly decelerated,
one way it sheds energy by producing an X-ray.  The most common
method of slowing X-rays is to direct them to impact a dense
target.  This is how an X-ray tube works.  Electrons are emitted
from a hot filament and are accelerated across a vacuum by high
voltage applied between the filament and a target.  The electrons
impact the target and mostly heat it.  But a small fraction of the
electron energy is converted to X-rays.  For a given energy of
electron (measured in electron-volts or kilo-electron-volt (KeV)),
the production of X-rays varies as the square of the target's Z.  As
is intuitive, many more x-rays are produced from heavy, dense
materials than from light ones.  The typical target in an X-ray tube
is either tungsten or rhenium.

When a beam of electrons is accelerated toward a target by a voltage
of X KV, the X-rays produced vary in energy from X KeV toward zero
energy in a smooth continuum.  The intensity follows an inverse
curve with the highest intensity being toward the low energy end. 
Another mechanism for X-ray production is fluorescent X-rays.  With
this mechanism, an incident electron collides with an atom's shell
electron.  Instead of being knocked off the atom, the electron is
stimulated to a higher energy orbit.  When it decays back to its
rest energy orbit, it emits the excess energy as a photon.  This
photon is characteristic of the orbital position of the electron and
falls in the range of what is generally regarded as X-ray (some are
of such low energy as to straddle the boundary between X-ray and
light.)  This is completely analogous to how the fluorescent powder
in a neon tube works, where it absorbs UV light and emits the energy
as visible light.  Fluorescent X-rays are a significant portion of
the X-rays produced in a conventional tungsten-targeted X-ray tube.
(I have some nice photos of an X-ray tube that was just UPS'd
[broken] if anyone is interested in seeing the details.)

Modern X-ray tubes operate with a hard vacuum.  A hard vacuum is
necessary so that the electrons will have a high probability of
crossing the electrode gap without colliding with a gas molecule. 
Such collisions produce only heat and sometimes light.   The
original tubes that Crooks, Roentgen, etc worked with had soft
vacuums in them and typically did not use a filament.  That is,
there was no source of electrons and there was gas in the electron
path.  Yet they still made X-rays.  How?  With lots of very high
voltage and very low efficiency.  They used either a static machine
or an Oudin coil.  Either device was capable of generating hundreds
of thousands of volts.  This very high voltage could literally strip
electrons from the cold cathode in sufficient quantity that some
made it to the anode.

Now let's look at a neon tube.  If the electrodes cannot see each
other (that is, a clear optical path), then the discussion ends. 
Absent an external magnetic or electrostatic field, electrons don't
make turns.  As one can see in a poorly processed tube that
sputter-pumps its noble gas toward a high vacuum, electrons stream
out of the electrode, impact the first glass surface in the
line-of-sight - usually the turnback - and melt a neat little hole
in the glass.  Photos available if anyone is interested.

Assuming a straight tube with no turnbacks, let's look at the X-ray
situation.  The PEAK X-ray energy possible (which determines its
penetrating power) is the peak voltage available for accelerating
the X-rays. In the case of a 15KV bombarder, this would be 15,000
volts RMS * 1.414 = 21.21 kv peak (for a sine wave.)  A 21 KeV X-ray
has almost no penetrating power.  The half-value thickness (that
amount of absorber which reduces a beam intensity by 1/2) for air at
21 KeV is about 50cm.  A foil of most any metal will essentially
stop the X-rays.  The half-value thickness for lead is about
0.003cm. (all these numbers from the Radiological Health Handbook,
USGPO)  As you can easily calculate or simply see, the 1.25mm thick
leaded glass electrode casing would completely stop the X-rays.

This assumes X-rays are produced.  Under normal bombarding
conditions, they are not.  The pressure is too high, there are too
many gas molecules present and the mean free path is too short.  If
one evacuates the tube down to below about 250 micron (0.25 Torr),
one can sometimes see the results of some very soft X-ray
production.  This manifests itself as brightly glowing debris
particles in the tube.  These particles fluoresce in the X-ray
fluence.  This happens after the pressure gets low enough that any
glow discharge disappears.  The efficiency is very low because there
is still a relatively large amount of gas in the tube.  The
efficiency is further reduced because the target - iron, perhaps
with a nickel plating - is of fairly low Z.  To repeat, these X-rays
will NOT penetrate the electrode shell or the glass.  It will not
expose film.  There is no external radiation present and thus no

To summarize:

A tube:

*	At bombarding pressure - no x-rays
*	containing any sort of bend - no x-rays
*	that is straight and at relatively high vacuum - some X-rays
possible but are absorbed by the glass  envelope and the electrode

A home-made X-ray tube is easy to make.

Seal an automotive tail light bulb filament to a short hunk of 15mm
tubing, preferable flint or lead-free glass.  This is easily done by
cutting half the bulb off and sealing the resulting bulb stub to the
glass while purging the glass with a bit of argon to inert the
filament while it is hot.  Cut a piece of tungsten welding electrode
so that when it is stuck down an electrode shell and the electrode
sealed to the previous glass, the tungsten will reach to within
about 1/2" of the filament.  Wad some foil around the tungsten so
that it will interference fit into the electrode shell and will
remain in place when the electrode is held  upside-down. Seal the
electrode to the previously made glass. Evacuate the tube to the
highest vacuum possible (diffusion pump!) while baking the assembly
for several hours at at least 500 deg F.  Seal off and connect to
your source of 100KV.  An old X-ray power supply will do.  As will a
GM HEI ignition module and coil driven by a pulse generator.  If you
have DC, the tungsten must be positive.  AC is fine - the tube will
auto-rectify.  Light the filament to a yellow-white, turn on the
high voltage and viola!  X-rays.  The X-ray production increases
with age as the tungsten getters the residual gases.  The tungsten
is a point-source radiator and so the X-rays will emit in all
directions.  For any imaging purpose, most are wasted. But it'll
sure light up your neon phosphors!

I made one of these to try and get some use from the X-ray machine
that UPS broke the tube for while I get a replacement.  Worked fine
for a few seconds until the heat broke the glass.  Gotta retool with
Pyrex :-)

John De Armond
Neon John's Custom Neon
Cleveland, TN
"Bendin' Glass 'n Passin' Gas"