I'm reading a book entitled Nuclear Electronics by P.M. Nicholson. It's giving me quite a bit of trouble as I go through (an hour or two in, and I'm only on page 3) partly because I don't have a ton of background knowledge on the subject, though I do know a bit (please keep this in mind when answering).
In chapter 1, "An Outline of Detection Methods", section 1.1.3 "Electrons and positrons", the book says
For energies up to 10 MeV, electrons lose their energy to the detection medium mainly by excitation and ionization of the electrons of the medium, as in the case of heavily charged particles.
So far, so good. Then it says
For higher energy electrons the loss of energy as bremsstrahlung becomes increasingly important and the intensity of this varies as $Z^2$, where $Z$ is the atomic number of the medium. Thus, for example, 9 MeV electrons in lead lose as much energy due to bremsstrahlung as due to ionization.
After skimming the Wikipedia article on bremsstrahlung radiation, I have this understanding: generally, electrons decelerate as they come near a nucleus and emit a photon to uphold the law of conservation of energy, losing energy in the process. This in and of itself makes sense.
However, I didn't really find anything in the article (or understand anything, one of the two) that mentioned this was dependent on the particle energy. I mean, the electron slows down because of the electromagnetic field, loses some energy in the form of a photon, and goes on its merry way. I'm not sure how the electron's energy plays into this.
Secondly, I'm not sure I understand the book's example - firstly with how the energy plays into it (especially with what exactly it means with the 10 MeV threshold in the first part of the quote) and secondly what it means by "the intensity of this varies" and how $Z^2$ shows that.
If this is too broad, I can split it up into two questions, but I thought the questions were intertwined enough they could be asked together. Please provide an answer on the more intuitive side - I don't need to do tons of math with this, I just want to understand it. Any help would be much appreciated!