Understand the interaction of radiation with matter
Distinguish categories of radiation detector instruments
Recognize effects of shielding and distance from source
Interaction of radiation with matter
The interaction of radiation with matter occurs through the transfer of energy from a particle or a photon to the absorbing material.
The mechanisms of the interaction are the basis for radiation detection techniques and are needed for a better understanding of their biological effects.
When radiation interacts with matter, it can produce two effects: ionization and excitation.
Ionization involves the removal of a charged particle (an electron) from a molecule, leaving it with a net charge. Excitation is the transfer of energy without ejecting a particle. This energy will be dissipated as heat, light or chemical reaction.
Alpha and heavy charged particles
Their interaction is ruled by coulomb electrostatic forces:
F= q * q’ / d^2
So, the denser the material, the bigger the number of interactions per unit path length.
The mass of the particles is greater than the mass of the orbital electrons. This results in a straight path where the rate of energy loss is dependent on the charge and velocity of the particle.
This kind of interaction is the same as for heavy particles but the effect is different. As the mass of beta particles is equal to the mass of orbital electrons, the paths vary greatly since they suffer bigger angular deflection.
Beta particles can also loss energy by the production of bremsstrahlung (braking radiation). When a beta particle approaches a nucleus, the atomic nucleus attracts it due to the difference in mass and charge. When the kinetic energy exceeds its rest mass energy, there is a photon emission. The emission is directly proportional to the square of the atomic number and charge, and inversely proportional to square of the particle’s mass.
This phenomenon accounts for the production of x-rays.
In the shielding of beta particles, this becomes also an important issue and high atomic number elements cannot be used for this purpose.
These are pure energy carriers with no mass. They are not affected by electromagnetic fields. Interactions happen only by direct impact. There are three main mechanisms through wich photons are absorbed or attenuated:
All of the energy of the photon is absorbed by an orbital electron.
The incident photon energy is bigger than the binding energy for an orbital electron, so the electron is removed.
The removed electron, called photoelectron, will behave as a beta particle.
The atom remains ionized.
The incident photon is absorbed by an electron.
The electron emits a new photon in a new angle and gains kinetic energy.
The photon passes close to an atomic nucleus. It may annihilate and form an electron-positron pair.
The minimum energy required is 1.022MeV (2* electron mass * c^2); the remaining energy is imparted to the two particles as kinetic energy.
The positron will annihilate soon with an electron and two photons of 511 keV each are emitted at a 180-degree angle.
Linear attenuation coefficient:
Linear attenuation coefficient (µ) is a way to measure photon absorption. It expresses the probability that the intensity of a beam of photons, I, will be reduced by an absorber of thickness dx, and follows the equation:
dI / dx = - µ I
and integrating over a distance x: I = Io e exp (-µ x)
Radiation interaction with matter causes ionization and / or excitation of atoms and molecules. When atoms or molecules undergo recombination or excitation, energy is released. Usually this energy is thermal but for some materials a portion of this energy is released as visible or UV light. Materials having this property are called scintillators and radiation detectors made from them are called scintillation detectors.
The greater the energy the particle or photon carries, the greater the energy imparted to the scintillator and therefore the more light produced. So the scintillator is an energy sensitive device.
There are two kind of scintillation detectors:
Though both are based on the same property, organic and inorganic scintillation detectors are different. We will study inorganic detectors (from now on, whenever we refer to a scintilliator, we are talking about the inorganic type).
Individual atoms and molecules do not scintillate. Scintillator materials are active because of their crystal structure. In most cases, they are only activated when an impurity disturbs their normal structure.
The most common scintillator used in nuclear medicine is the NaI(Tl) (thallium-activated sodium iodide) because of its desirable properties. The crystal has a window on one side that allows the radiation to arrive, and an optical window on the other side to let the light photons go through. On the sides, it has an aluminium or stainless steel jacket which has reflector material (MgO or Al2O3) on the internal wall. The optical coupler is set to focus the light photons towards the photocathode in the first part of the photomultiplier (PM) tube. The PM tube is needed because the amount of energy produced by a single gamma ray or a beta particle, etc, is very small.
When a light photon impinges on the photocathode, an electron is ejected. There is a high voltage applied to the PM tube, typically between 900 and 1200V. Because of this high voltage, electrons are accelerated towards the dynode. Dynodes also have photoemittive properties and will eject several electrons for each incident one. This process will occur through the 9 to 12 dynodes of a PM tube, producing a multiplication of the accelerated electrons. Finally, a current will be collected on the anode.
This current is transformed into a voltage signal by an RC circuit. The voltage value will be proportional to the energy of the radiation event This current is transformed into a voltage signal by an RC circuit. The voltage value will be proportional to the energy deposited by the radiation event detected: the electrical charge deposited on the capacitor is produced by the electrons ( I = dQ/ dt), and the amount of electrons depends on the energy of the incident g-ray for a given PM and a given voltage supply.
The output voltage is Vo = Q/C, where C is the capacitance. Leakage goes through the resistor and then the voltage will decay exponentially with time: V=Vo exp (t/RC). The product RC is called time constant,t.
As the voltage level is a function of the energy of the incident photon, a window discriminator is used to select the energy range. This is usually called a "pulse height analizer" (PHA).
Pulses from a particular energy range can be accepted while all the others are rejected. Two parameters must be set: base level (E) and window width (DE). Each of these set a reference value for a comparator. If the meaured value is between E and E+ DE, then the pulse is accepted.
The following step depends on how the information will be displayed. Rate meter, digital display, markers or digital storage are some of the options.
Getting a spectrum
If the type and activity of a radioisotope are known, a calibration of the detector is possible. For this purpose, long mean life radioisotopes are used.
For a given high voltage, a relation between the window settings and the energy of the incident radiation is established. Once this curve is known, the scintillation detector can be used to establish the energy distribution of any radioisotope.