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Stopping X-rays

Posted: Fri Jun 22, 2001 6:46 pm
by Richard Hull
I recently spoke to this question on the old intranets site, but it was not posted in the radiation forum. So here goes another pass in the proper forum.

X-rays are the first major radiation to issue from the working fusor and continues to get worse as the applied voltage rises.

Most amateur fusor demo models never produce a single x-ray and are therefore pretty benign in this area.

A deuterium filled fusor operated above 10kv will not only start to produce neutrons, but x-rays as well.

It is rather difficult to get a serious neutron flux from any amateur fusor operated under 30kv. Thus, the neutron hazard from short period runs in fusors up to 30kv is not a serious or immediate hazard.

X-rays, are another matter. They begin to spill out of viewports at about 12kv and the intensity grows to a dangerous level by 18kv. A geiger counter placed near a viewport at 20kv will usually saturate the entire counter and stop it from working, temporarily!!!

Ionization counters are best to sniff out x-rays and obtain a quantitative exposure. However we don't care about quantitative. We only care about detection. So most any low cost geiger counter will serve to alert us as to where x-rays are leaking out.

X-rays in the 15-30kv range are, for the most part, "soft" and are, therefore, considered "burning rays". They will be stopped totally in the first few millimeters of flesh and do maximum damage. (burn) by the same token they are easily stopped dead or shielded totally out of the picture. If you are not using your view port, cover it with a 1/16 inch thick sheet lead cover or a 1/4" thick aluminum disk. continuously sniff around this area with a geiger counter when the fusor is running to check for x-ray leaks at the edges.

Most of you will want to stuff a camera into the viewport and remote view the opertion on a video monitor. Make sure the area around the camera's lens and viewport is shielded to avoid scattered radiation.

Most metal fusors will have a lethal x-ray problem emerge when the entire shell becomes transparent above 70kv, but that is another issue and no one is likely to be there any time soon.

Remember, the eye is one of the most sensitive organs in the body to radiation. Never stare into a view port with high voltage in excess of 6kv applied.

Richard Hull

Re: Stopping X-rays

Posted: Tue Jul 03, 2001 6:22 pm
by mhecht
Richard, could you share any calculations or formulas for calculating the dosimetry ? Thanks!

Re: Stopping X-rays

Posted: Tue Jul 03, 2001 9:31 pm
by Richard Hull
X-rays are not like nuclear particlate radiation. It consists of a spectrum of photon energies. Photon energies determine their individual contribution to the absorbed dose. ( a summation of a continuum of energies in a specific target) The roentgen is a pretty worthless measure as it determines the number of ions created in a given amount of air.

We are hunks of bipedal meat. For doseage or ionizations in flesh, there is the REM or roentgen equivalent man. What complicates doseage from a fusor is that each one is different in its own right. Many use different structures, materials, etc. In other words, these are not carefully crafted x-ray tubes of uniform design which can be codified and charts given on dosage. Add to this the fact that we operate at widely varying voltages and currents and the whole calculating part of this business is a crap shoot.

X-rays are created when high energy electrons get close to or slam into atoms. The type of x-ray is based on its energy, how it is produced and what type of atom the electron encountered.

In our fusor the best we can hope for is to actually just detect the x-rays and stop them via simple detection schemes and lead shielding. As I mentioned, it doesn't take much to shield well, and at below 40kv, the only way out of the device is the view port. So the threat is not great and easily dealt with.

For those seeking a strong quantitative reading of dose, only a calibrated ionization chamber will work.

The x-ray energies will vary over the full range of applied voltage with most being well below 3/4 of the applied potential.

The penetrating power of x-rays is strictly a function of their energy and the absorbed dose is based on the quantity of x-rays (current) and the integrated energy combined.

Richard Hull