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Underappreciated X-rays

Posted: Wed Aug 29, 2018 6:58 pm
by Roberto Ferrari
Fellows
Found this article about undesired x-ray production in a lab during high voltage and vacuum experiments.
Is an interesting exercise about characterizing, measuring and controlling stray radiation.

Re: Underappreciated X-rays

Posted: Mon Nov 19, 2018 10:36 pm
by Chris Giles
However, it appears typical that unconditioned electrodes can have can have vacuum emission currents
of 1 µA or more [15–18] and indeed, this is typical of the levels observed in the apparatus described here
Interesting paper. If I understand correctly they are describing all current flowing in vacuum between Anode & Cathode as "leakage" current so in practice that is all/most of the current from the HV line in a Fusor. They are looking at uA current levels. Admittedly they are measuring very close to the device and I presume the intensity drops off with an inverse-square law. I guess the radiation would be proportional to the leakage current (?) so almost 1,000x these levels? Any views or measurements anyone?
... we observed a dose rate as large as 2.5 mSv hr−1 immediately outside the apparatus when leakage currents of ∼1–10 µA were present at applied voltage in the ±20–±30 kV range. ...This level of radiation represents a substantial hazard under conditions frequently used in university physics laboratories.

Re: Underappreciated X-rays

Posted: Tue Nov 20, 2018 1:16 am
by Richard Hull
100mrem = 1 mSv. All of this is known relative to fusor operation. At the glass view port window, pointing down at the absorbing concrete floor in my lab, at 35kv, 10ma I read about 300 mrem/h rate. This equals 3.5mSv. This is why I suggest never pointing a view window at anything other than an absorber and away from any nearby human activity. However, at my operating station, 7 feet from the fusor, I read no more than 1mrem/hr.

This is from a normal .0625-inch thick walled, 304 SS fusor operating at near 1,200,000 fusions per second. This is good but not great fusion display.
A 1 mrem per hour rate is nothing to fear. Plus the operational time at this level of fusion is normally well under 5 minutes.

This tends to indicate that with a fusor, you have to expend 350 watts of energy into a running fusor to receive a 1mrem/hr rate at 7 feet, providing one is not stupid enough to have a view window pointing at them.

A fusor loses in a dangerous radiation situation at voltages under 40kv due to all the radiation prevention issues. This is mostly due to a steel chamber absorbing all but the most potent x-rays which are out on the tail of the maxwellian, and the short exposure time, coupled with the inverse square law.

Richard Hull