مكملة الى الX-Ray

wavelength is found to depend on the tube voltage. F.gurc 1-4 shows the kind of curves obtained. The intensity is zero up to n certain wavelength, called the short-wmelength limit(Xswl), increases rapidly to a maximum and then decreases, with no sharp limit on the long wavelength side.. When the tube voltage is raised, the intensity of all wavelengths increases, and both the short-wavelength limit and the position of the max¬imum shift to shorter wavelengths. We are concerned now with the smooth curves in Fig. 1-4, those corresponding to applied voltages of 20 kv or less in the case of a molybdenum target. The radiation repre¬sented by such curves is called heterochromatic, continuous, or while radia¬tion, since it is made up, like white light, of rays of many wavelengths.
The continuous spectrum is due to the rapid deceleration of the electrons hitting the target since, as mentioned above, any decelerated charge emits energy. Not every electron is decelerated in the same way, however; some are stopped in one impact and give up all their energy at once, while others are deviated this way and that by the atoms of the target, successively losing fractions of their total kinetic energy until it is all spent. Those electrons which are stopped in one impact will give rise to photons of maximum energy, i.e., to x-rays of minimum wavelength. Such electrons transfer all their energy eV into photon energy and we may write


We now see why the curves of Fig. 1-4 become higher and shift to the left as the applied voltage is increased, since the number of photons pro¬duced per second and the average energy per photon are both increasing. The total x-ray energy emitted per second, which is proportional to the area under one of the curves of Fig. 1-4, also depends on the atomic num¬ber Z of the target and on the tube current i, the latter being a measure of the number of electrons per second striking the target. This total x-ray intensity is given by
I cent spectrum = AiZVm, (1~5)