Electrons and secondary electrons

Electrons in their interactions can release secondary electrons of finite range.

These are known as delta rays and are partly responsible for the buildup exhibited by electron beam depth dose curves.

Beam Quality of kilovoltage therapy x-ray beams

Beam quality of a kilovoltage beam must be specified with both kVp and HVL.

Two different beams with different kVp may have the same HVL. For example a heavily filtered kV xray beam and a medium filtered kV beam of somewhat higher kV x-rays can have the same HVL, so specifying both kVp and HVL will specify beam quality with much less ambiguity.

Photon interactions in a medium

Photons either interact of they don't. They are not like electrons which lose energy continously.

Primary photons are the ones transmitted by and have not had any interaction with the media traversed.

Megavoltage photon interactions can release electrons of significant energy.

Output of a kV therapy unit

The output of a kilovoltage therapy unit is roughly proportional to

Output proportional to (tube voltage in kv) squared and the Z of the target

Ref Johns and Cunningham 1983.

kV x-ray unit and target requirements

The requirements of an x-ray unit target are the following:

High Z and High melting point

X-ray spectrum and thickness of the target

The X-Ray spectrum is influenced by the thickness of the target.

Electron absorption in the target cause heating up of the target.

Electrons and the target

Electrons impinge on a small circular area of about 3 mm diameter in the target in a clinical accelerator.

Mechanism of x-ray production

The mechanism of x-ray production is NOT the same in a kV x-ray unit and an accelerator unit!

In a kV machine, the potential difference is V between the cathode and the anode accelerates the filament electrons.

In an accelerator, however the electrons are accelerated to near the velocity of light. They are bunched together and injected into the microwave carrying cavities to be accelerated by the waves in the phase stable position.

Half life of a free neutron

The half life of a free neutron is 12 minutes

This is why a tray is activated.

Si Unit of Radioactivity

The SI Unit of Activity is the Becquerel

One Becquerel corresponds to the following number of nuclear transformations per second: 1

1 Bq= 1dps

1 Curie= 37 GBq

Mean life of a radioactive source

The mean life of a radioactive source is given by 1.44 x T 1/2

Table of Radionuclide properties

1.17 and 1.33 Mev Gammas- Co 60
0.662 MeV Gamma- Cs 137
Several gammas of mean energy around 400 KeV- Ir-192
Several gammas of mean erergy around 0.8 MeV Rn 222
Mean energy of 28 KeV0 125 I

Half lifes

Co-60- 5.26 years
Cs-137- 30 years
Ir-192- 74 days
Ra-226- 1626 tears
I-125- 59.6 days

Electron Capture

Electron capture usually occurs in High Z radioactive elements.

Electron capture and positron emission are competing modes of decay ( and they lead to the same daughter nuclide by converting a proton into a neutron)

Gamma decay

Gamma usually follows beta particle emission in radioactive decay.

Beta decay

Beta decay is usually associated with NEUTRON RICH radionuclides

In a beta decay, a neutron becomes a proton so there is a tendency to increase proton number. So nuclides decaying by beta decay are neutron rich (having too many neutrons compared to the number of protons)