FAQ - X-ray radiation!!

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George Dowell
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Re: FAQ - X-ray radiation!!

Post by George Dowell » Wed Feb 06, 2013 12:37 am

RadPro Calculator- Fooling around with dose at 10 cm from a 30 kVp 10 Ma
X-Ray source, shielded by iron or lead.

Dose at 10 cm with 2mm Iron shielding (fusor chamber? = 5 R/H

Dose at 10 cm with 2mm Pb shielding = 1 10E-21

It takes little effort to shield 30 kV rays.

George Dowell
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Richard Hull
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Re: FAQ - X-ray radiation!!

Post by Richard Hull » Wed Feb 06, 2013 5:37 pm

The assumption here in these calcs is that you are operating a formal, tungsten target "x-ray tube". All the energy is in a focused beam blasting "naked" into air. The fusor is never this kind of device, of course, so all the unshielded values given by this program are super wrong to start with, when referencing a fusor, as the levels will be 2-3 orders of magnitude lower due to the isotropic emission and SS lower Z target-shell shield.

A better test is to let the x-ray tube filter and target be 2mm of iron which is closer to the natural fusor. Unfortunately, we aren't allowed to choose the target, but can choose Cu for our filter. The program, though, forces a minimum of 85kv for that filter and will not accept lower KV numbers. Thus, it is valueless, on many levels, for use with a fusor.

We have actually, physically measured at the SS shell of a fusor working around 35kv @10ma. We record about 100 mr/hr at the shell ~3 - 5cm distant and at 1 meter, less than one mr/hr. This is not worth consideration as a rad hazard if your operating station is over a meter distant from such a fusor and operated for infrequent, short periods.

For the paranoid, hyper-ventilator's, a millimeter or two of lead will leave you in, hopefully, a "feel good" operational environment.

For me, 50-60 kv applied is when I might put up a thin Pb shield, assuming I am 1.5 meters from the fusor.... as it is in my case.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

George Dowell
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Re: FAQ - X-ray radiation!!

Post by George Dowell » Thu Feb 07, 2013 6:34 am

Richard, may I ask what you measured the 100 mR/H with? A low energy intergrating ion chamber I assume? George Dowell

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Re: FAQ - X-ray radiation!!

Post by George Dowell » Thu Feb 07, 2013 6:43 am

By the way a tungsten target does not turn on until above 59 keV (tungsten K edge) . A real X-Ray tube designed to work at lower HV would have an MO or copper target. Or in the case of a fusor, a very large iron target. A peak of about 6 keV would result, with a background of Bremsstrahlung from 0-35 keV (average of around 1/3 that).

Geo

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Chris Bradley
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Re: FAQ - X-ray radiation!!

Post by Chris Bradley » Thu Feb 07, 2013 6:57 am

George Dowell wrote:
> By the way a tungsten target does not turn on until above 59 keV (tungsten K edge) .
There is an X-ray continuum too, related directly to the electrons' energies. It is not just the atomic signature from which one gets X-rays.

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Re: FAQ - X-ray radiation!!

Post by George Dowell » Thu Feb 07, 2013 7:36 am

Correct Chris, that's caused by electron Bremsstrahlung.

It is not exactly an X-Ray by definition, it is Bremsstrahlung radiation, but ionizing photon radiation nonetheless. Photon's are often called by the manner of their origin, i.e. X-Rays, Gamma Rays, Annihilation Radiation,Bremsstrahlung etc.

At the receiving end, they are all the same thing and indistiguishable from one another.

George Dowell

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Carl Willis
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Re: FAQ - X-ray radiation!!

Post by Carl Willis » Thu Feb 07, 2013 4:58 pm

Hi George,

Richard has mentioned at least three times toward the top of the thread that an ion chamber is preferred for dosimetric measurement of x-rays. The model I've seen him use is a Victoreen 471, and he has photos of it here: viewtopic.php?f=13&t=5972#p34416

I'd argue that the commonly-understood meaning of "x-ray" implicitly includes bremsstrahlung. There does not appear to be any confusion about what physical mechanism produces radiation in the amateur fusion environment, and no need for affecting a hyper-distinction in the nomenclature.

-Carl
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Re: FAQ - X-ray radiation!!

Post by JohnCuthbert » Thu Feb 07, 2013 5:21 pm

I'm fairly sure that most of what Dr Roentgen named X rays were what would later be classified as bremstralung, but that didn't stop them being X rays any more than light isn't light if it didn't come from the sun.

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Richard Hull
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Re: FAQ - X-ray radiation!!

Post by Richard Hull » Thu Feb 07, 2013 6:02 pm

Yes, I use a large chambered victoreen ion chamber. Two other similar ion chamber instruments brought by others over the years have agreed to within 20%. 200mr pen dosimeters placed at varying ranges have also agreed to within a similar error range.

I have used as many as 20 pens in a single test before. My pocket dosimeter has never registered a readable amount in all the various runs where I have worn one.

The fusor, as such, is a pretty low emission x-ray device up to about 35kv due to the SS shell shielding. I rely heavily on distance and the inverse square law to operate with zero shielding up to about 40kv. at 1.5 meters there is no reading seen on the 3mr/hr lowest range with two Victoreens and one Eberline ion chamber detectors that I have.

Also about 1 run session of max output for a total of 30 minutes every three months or so (average) keeps the time averaged absorbed exposure to effectively nothing.

In short, no significant x-radiation from naked fusor IV operating between 20kv and 40kv, provided you are positioned a meter or more from it over much of its full power, operational time.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

George Dowell
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Re: FAQ - X-ray radiation!!

Post by George Dowell » Thu Feb 07, 2013 7:05 pm

John Cuthbert wrote:
> I'm fairly sure that most of what Dr Roentgen named X rays were what would later be classified as bremstralung, but that didn't stop them being X rays any more than light isn't light if it didn't come from the sun.

Yes actually it does John if we are to be anything like scientifically correct. We use terms like Radio, Laser Light, Arc Light, Lime Light, X-Rays etc for a reason even though they are all electromagnetic radiation.
For example in the case of a fusor or *any* vacuum chamber with two electrodes, electron flow and HV, there will be a number of sources of ionizing photon radiation. If the "target" i.e. anode is iron, the peak radiation will be at 6 keV, from the characteristic XRF of iron. Each anode element would have it's own energy of characteristic X-Ray emission, but the electron beam must be of sufficient energy to excite this emission. That's why X-Ray tubes have different target material for different usage. Ironically those very low energy X-Rays are specifically filtered in radiology because they don't penetrate tissue,rather absorbed, therefore don't add to an image, but cause the burns.

Understanding this allows the observer to select the correct instrument to monitor the dose. For example, many if not most instruments are blind to 6 keV due to self shielding.

The ion chamber I use for that is a Victoreen 440 RF/D which has a very thin magnesium window, specifically designed to allow low energy X-Rays in.

PS, to put it in perspective, my experiments with X-Rays use sources of 20 to 30 kV at up to 100 MICROAMPS only. Yes these are isotropic sources. A normal all metal fusor is a diffuse source. No one can say what Bell Jar fusors are.

George Dowell

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