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2. Safety and other Practical Considerations

An X-ray tube operating at about 40,000 V and 25 mA, is converting 1 kW of power, and the X-radiation coming from that tube is extremely dangerous. If you put your hand within 1 or 2 cm of that tube, and allow the full blast of radiation from the window to strike your hand, even for a short time you will get 3rd degree burns and these burns will not heal. The penetrating power of the X-rays, of course, depends very much on the wavelength. Copper K alpha radiation, wavelength about 1.5 $\times$10-10 m, will penetrate up to 2 m in air and the penetration is still so great at that range, that you can observe it on a zinc sulphide screen. If you use a sensitive Geiger counter, you will find that the radiation can still be detected at a much greater distance. The penetration of molybdenum radiation is even greater.

Anyone planning to work with X-rays at all, should get in contact with the appropriate local safety service. They will give details about what standards should be applied, and where one can get the radiation stickers and brightly coloured emblems to warn passers-by that there are X-rays around. They will also supply film badges which all workers should wear if they are constantly near the X-ray source. In my laboratory, I keep one film badge on the generator constantly rather than wear it on a laboratory coat, because most of the time there is no person standing next to the generator. The idea of keeping the film badge on the generator is to pick up an average background scatter. In our case, the scatter from the Philips generator is very small indeed.

(The only way to get any readable results on the film badge is to deliberately expose the film badge directly to the beam! Under these conditions the film badge is burnt so black that it is not readable on the densitometers at the radiation safety offices. This action will normally result in a frantic telegram asking whether you are still alive!)

Nowadays most generators and cameras are fitted with mechanical and electrical interlocks but regulations differ considerably from one country to another so we shall not go into detail. The importance of good safety practice however cannot be overemphasized.

An X-ray generator contains an extremely large capacitor, so when the generator is switched off, it is still not completely dead, electrically speaking, and in fact very large charges may still be held on the capacitor within the generator for anything up to 15 min. Anyone planning to open up the generator to do any adjustments inside, should never do it immediately the machine is turned off, otherwise there is danger of a 40,000 V shock.

When a generator is started up for the first time after quite a long lay-off, you must remember that you are dealing with very sensitive electronic apparatus and an X-ray tube which will not appreciate a very large thermal or electrical shock. The correct sequence is to switch on the generator but not to apply the high voltage to the tube for 5 min at least. Once the generator has warmed up, then the high voltage may be turned on. The high voltage should be left at the minimum value for 5 or even 10 min and then the voltage and current settings may be slowly increased, usually step by step, to the required values. If you are using the generator every day, this process need not be followed, but if the generator has been shut down for about a month, then the applied voltage should never be increased suddenly, otherwise it is quite possible to detonate the X-ray tube.

The converse applies equally well when the generator is to be switched off. The high voltage and the tube current should be reduced gradually from their maximum operating values down to the minimum values. The high voltage should then be switched off while still leaving the generator electronic circuits and the tube filament switched on. After a further 5 min one can switch off the generator completely. This process allows the target of the tube to cool down because the cooling water is still flowing through the generator even when the high voltage is off. Once you switch off all the power to the generator, the cooling water normally does not flow. If you switch off a generator running at 40,000 V by just punching `OFF', the applied voltage is removed from the tube, the water flow immediately stops, with the results that the target becomes badly over-heated. This sort of practice will ruin a tube very quickly.

It is important to remember that probably more trouble is caused by a poor supply of cooling water than by anything else. Generators are sensitive not only to the volume flowing through them, but also to the pressure of the water. (A generator will not start unless there is an adequate flow of water at the correct pressure.) It is, therefore, very necessary to maintain a constant flow at an adequate pressure. The best way to do this is to have a closed circuit water supply with a small pump sending the water through the generator and then back to the ballast tank. Normally some kind of a cooling device is also needed; (a small water tower is suitable) and then this will maintain the temperature of the water. There are safety devices built into every generator and these devices fairly obviously will be sensitive to dirt in the water and so it is necessary to put at least one and perhaps two filters (one coarse, one fine) into the water line. With such an arrangement, one can operate continually for 4 or 5 years with the same 100 gal of water running through the generator. The alternative is to try to use tapwater from the mains and simply pour this water through the generator at the rate of 1,000 gal/day straight down the drain. This may sound cheap and easy but it is not, because there is no guarantee whatsoever that the water in the main will be coming through at 70 lb/in2 constantly day and night. If you want to operate your generator day and night this is what you need. In practice, the mains pressure can fluctuate between 20 and 100 lb/in2. A perfectly adequate water pump and cooling system can be installed quite cheaply if you are prepared to build some of it yourself. Unfortunately, the smallest commercially available cooling towers are far too big for this particular job. A half horsepower motor and a small Mono pump are perfectly adequate to supply water at 70 lb/in2, enough to cool about 3 kW of power. In our laboratory we cool 3 generators from one small pump.


next up previous
Next: Structure of Crystals Up: An Introduction to the Scope, Potential Previous: Generation and Properties of X-rays

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