Radiation measurement of my X-ray tube

Wanting to know the actual level of radiation produced by my X-ray tube, I borrowed a professional radiation dosemeter from the university (many thanks to Frau Feuser!). This device measures the equivalent dose (= energy dose here) of gamma-rays from 30keV to 1.3MeV. Measuring range is 0.1uSv/h (micro-Sievert per hour, 1Sv = 100rem) to 10mSv/h. However, the readings given are only approximate (due to the statistical character of X-radiation and the fact that I had to read the meter from a distance of about 3m).

I measured the intensity of the unshielded tube (DY 802 at 30uA current, about 48kV accelaration voltage) at 50cm and 1m distance and different angles. The first picture below illustrates the geometry. The second picture is a polar graph of the result, i.e. the length of the arrows corresponds to the intensity in the resp. direction.

The legal limit of radiation intensity for people who are not professionally exposed (i.e you and me) is 15mSv per year or, at 40h exposure per week, 7.5uSv/h. This can be considered a safe limit. As you see from the graph, Intensity even at 1m from the tube well exceeds this. An additional measurement gave the following readings:

distance in m 1 2 3
Intensity in uSv/h 50 15 7

The safe limit is reached only at 3m, about the largest distance I can get between me and the tube, living in a 14m^2 room. The table also shows that the inverse-square-distance law is (approximately) valid at these distances.

The tube also shows a distinct azimuthal dependence of the intensity:

The reason for this are the two metal wires leading from the tube socket to the filament (see diagram). They "cast a shadow" of several 10 degrees.

As it's not quite convenient to operate the tube from 3m distance, and even there the radiation is still high (normal background is below 0.5uSv/h), I usually have a lead shield (1mm lead as used by roofers) around my tube that lets only a relatively narrow beam through (small hole). This is enough to reduce radiation to normal background (except for the beam direction, of course). Be warned that a 6mm diameter hole at a distance of 10mm from the center of the tube (i.e about on the surface) still allows radiation into a cone of 30 degrees opening angle, that can still accidently hit you. To make a really narrow beam, I use a 40mm long, 5mm diameter lead tube. The following graph shows the effect of the simple shield with 6mm hole:

The lead cylinder had been adjusted in such a way that the hole matched the gap between anode cylinder and cathode shield (see diagram) and was not in the shade of one of the wires mentioned above. This way, the maximal intensity at 50cm is not much lower than without shield. Intensity decreases with angle just as without shield then, but is cut off rather abruptly at about 15 degrees. The radiation is so limited to a cone of 30 degree opening angle, all other directions are safe.

One more note on constructing a lead shield: Don't forget about the directions near the tube axis. For practical reasons, I haven't measured intensity there, but I did notice in earlier experiments that it's not quite zero! A simple lead cylinder will probably not be sufficient.

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Jochen Kronjaeger