Why isn't there Bremsstrahlung Radiation for Energy less than 20 keV for Tungsten?

Question

https://www.researchgate.net/figure/Bremsstrahlung-and-characteristic-radiation-spectra-are-shown-for-a-tungsten-anode-with_fig4_8365056

Fast electrons produce X-rays in the anode of an X-ray tube through two different methods. The first one is the interaction of electrons with the nuclei of the target atom. As an electron passes near a nucleus, the positive charge of the nucleus affects the negative charge of the electron, attracting it towards itself and changing its direction.

This causes the electron to undergo decelerated motion and, consequently, lose energy. The kinetic energy lost by the electron is emitted as a photon. The radiation produced in this way is called "general radiation" or "bremsstrahlung."

Bremsstruhlung Radiation's Energy Magnitude depends on,

1. the voltage of X-Ray Tube that accelerates and gives kinetic energy to electron

2. how close does an electron passes near a nucleus

3. atomic number (element type, Z) of that nucleus

So,

• why isn't there Bremsstrahlung Radiation for Energy less than 20 keV for Tungsten?

• Is it about the source of thermionic emission of the electron source, which bombards the cathode? So no start of bombardment(emission of the electron) before 20 keV from the anode to the cathode??

Answer 1

This is a result of the filtering material (usually aluminium).

The theoretical unfiltered bremsstrahlung spectrum is linearly increasing with decreasing energy.

Since lower energy x-rays are more easily attenuated (i.e. filtered), the measured continuous bremsstrahlung spectrum has a maximum at intermediate energy and goes back to zero at low energy.