a. False. In a self-rectified tube, electrons travel towards the anode only during the positive half of the waveform. During the negative half of the cycle, the filament has positive potential and does not emit electrons; at high tube currents, the anode may reach temperatures sufficient for thermionic emission and electrons may flow towards the filament, destroying the tube.
b. False. In a half-rectified circuit, the negative half of the voltage waveform is eliminated. As potential across the tube during this period of the cycle is zero, there is no electron flow and no X-ray production.
c. False. In a fully rectified circuit, the negative half of the cycle is ‘flipped’ into positive. Therefore, in both halves of the cycle, the filament and the target have the correct polarity to produce X-rays. However, the voltage across the tube fluctuates from zero to maximum, and the mean energy of the photons produced is relatively low.
d. True. With three overlapping fully rectified wave forms, the fluctuations in the tube voltage (ripple) are greatly diminished. The voltage across the tube never falls to zero and the efficiency of X-ray production is higher, as is the mean photon energy. High-frequency generators provide yet further improvements with a practically constant potential.
e. True. More filtration is needed to eliminate photons with lower energies produced by the latter.