helpsimple searchclear selectionselect all
Dictionary assisted search
All words
Any word/input
Exact phrase
in these
lexical topics:
  • Physics, Techniques and Procedures
  • Normal Anatomy
  • Musculoskeletal Imaging
  • Breast Imaging
  • Gastrointestinal Imaging
  • Urogenital Imaging
  • Chest Imaging
  • Cardiovascular Imaging
  • Neuroradiology
  • Head and Neck Imaging
  • Paediatric Imaging
 
 
 
Physics, Techniques and Procedures

Cathode

the negative electrode of a vacuum tube, such as a diode, triode or pentode. This electrode acts as electron source for production of the current that flows through the tube when certain conditions are met. In an X ray tube, which basically functions as a diode, the cathode is generally made of a helically wound tungsten wire (filament), 0.2-0.3 mm in diameter, and a second electrode, the Wehnelt electrode (also named focusing cup), which is used to focus the electrons as a narrow beam which hits the anode (Fig.1). In order to produce the high tube currents necessary at exposure, between 100 mA up to 2 A, the filament needs to be heated to temperatures around 2 700 K. In order to reduce evaporation of the filament, it is heated to this high temperature for as short time as possible at the exposure. It is therefore preheated to around 1 500 K, at which temperature the evaporation of cathode material is negligible. Often the cathode has two or even three filaments in order to produce different focal spot sizes on the anode (Fig. 1).

MN

To view high resolution images,
please register first.

Click  here to register.

Already registered? Enter your e-mail in the window below.
Re-register

Fig.1

Single and double focus cathode of an X-ray tube, cross-sectional view (left), and seen from the filament side (right).
Cathode, Fig.1