Radiation Quantities And Units Exposure

Published Date: 02 Nov 2017

The quantity which is most common used to express the radiation at a specific point is known as exposure. The SI unit used to calculate exposure is coulomb per kilogram of air i.e. (C/kg) and conventional unit us roentgen (R).

Their relationship can be written as

1R = 2.58 * 10^-4 per kg of air

Or

1 C/kg of air = 3876 R.

Air Kerma

Another quantity which is used to express radiation quantity delivered to a point, such as entrance surface of a patient’s body which is known as Air Kerma. It is the originated from acronym, Kerma is known as Kinetic Energy Released per unit mass (of air).As it is the measure of radiation energy, its unit joules (J) which is actually absorbed or deposited in unit mass (kg) of air. Therefore the unit of the quantity kerma is J/kg which is equal to gray (G).

Radioactivity

Absorbed Dose

 The quantity that express the concentration of radiation energy absorbed at a specific point in the tissue of the body is known as Absorbed dose.

The SI unit is Gray (Gy) which is equivalent to the absorption of 1 J of radiation energy per kg of tissue. And its conventional unit is rad,which is equivalent to 100 ergs of energy absorbed per g of tissue.

The relationship between these two units is:

1 Gy = 100 rad

1 rad = 100 erg/g = 0.01 J/kg =0.01 Gy

Dose Equivalent

The product of absorbed dose Gy and the quality factor (for radiation) is known as the dose equivalent and it is denoted by H.

Mathematically it can be written as:

Dose equivalent = Dose equivalent * Quality factor

H= Gy*QF

Where QF is quality factor.

The radiation we use for medical imaging (x-rays, gamma, beta and positron) all have quality factor QF is equal to 1.

Dose equivalent (H) = Dose equivalent (Gy)

The SI unit of dose equivalent H is Sievert (Sv), which is equal to J/kg.

Its conventional unit is Roentgen equivalent man (rem).

1 Sv = 100 rem

Or 1 rem = 1/100 * J/kg

Effective Dose equivalent

2. Interaction of Radiation with matter

In this part of the chapter our task is study the behavior of radiation when it interacts with matter. This study will help us in finding the answer of our task and helps to understand that how the radiation interacts with living tissues of our bodies.

We will study all types of radiation which interacts with matter but we will give our most of the attention to the gamma rays because this type of radiation is used in medicine for treatment purpose. In each type of radiation interaction ions produce, that is why it is called ionizing radiation.

http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Interaction_of_Radiation_with_Matter

2.1 Ionizing radiation

Ionizing radiations are produced from the nuclear reactions and these nuclear reactions can be artificial or natural. There are different sources of ionizing radiation they can be produced by a very high temperature, particle accelerators, electromagnetic fields, supernova explosions.

These radiations are composed of the particles which have sufficient kinetic energy to free or release an electron from an atom or molecule. Alpha particles, Beta particles, Neutrons, Gamma rays, X-rays and cosmic rays are the general examples of ionizing radiation.

http://en.wikipedia.org/wiki/Ionizing_radiation

Now we will discuss the different types of ionizing radiations but we will focus more on gamma rays.

2.1.1 Alpha Particles or Radiation:

Alpha particles are generally He nucleus which have 2 protons and 2 neutrons in it. They have positive 2 electric charge on them. When they pass through the atom, interact with the electrons of the outer most orbital shell and exert electrostatic force of attraction on them. Relative to the other radiations, these are the most massive. Alpha particles can be divided into two paths; electric and magnetic fields. Most of the energy loss is for the electronic ionization and excitation. Their interaction with matter is very strong as they have positive 2 charge on them. Due to their strong interaction, they have a very small range e.g. a paper sheet. Alpha particles can interact with living tissues strongly and inside the body, all of the energy is absorbed which can destroy the cells.

Figure

2.1.2 Beta Radiation

Beta particles have negative 2 charge on them. These are charged particles which are comparatively light mass and they intermediately interact with matter. When they interact with matter, they are attracting by nuclei of the atom and force back (repelled) by the electrons and cause ionization. Their interaction can be categorized by two important mechanisms form radiation protection points of view, which are given below.

2.1.2.1 Excitation and Ionization

The interaction among the electric field of the beta particles and the electrons of the orbit of the absorbing medium results the inelastic collision which produce the electric excitation and ionization. Due to interrupted spectrum of the beta particles, the particular ionization reduce from nearly 200 ion pairs with the gain of energy of beta, and attaining a least value of nearly 60 ion pairs at energy of beta of 1Mev.

2.1.2.2 Bremsstrahlung

Second mechanism of cutting the energy of beta particle is Bremsstrahlung which is most important mechanism indeed. If a high velocity charged particle passes by a medium, it sometimes experience nuclear scattering which gives the discharge of uninterrupted electromagnetic energy which is called as Bremstrahlung. This energy of x-rays is electromagnetic energy of a low range and becomes gumptious (energetic) if heavy metals are used for the stopping purpose. Light materials are not use for this purpose because these reduce the bremstrahlung and they are used to absorb the beta rays.

Due to uninterrupted energy dispersion, the absorption of beta particles in any material is also uninterrupted. This energy range can have maximum value for different types of materials which is linked to the beta particles having maximum energy.

2.1.3 Gamma Radiation

Gamma rays have no electric charge or in the other words these rays are neutral and have zero rest mass. That is why the interaction of these rays with matter is very weak. It has three mechanism which are important from radiation protection point’s of view.

http://www.ehs.utoronto.ca/services/radiation/radtraining/module3.htm

These mechanisms are given below:

2.1.3.1 Photoelectric absorption/ effect

The photoelectric effect is an effect in which a photon is absorbed by the electron of an atom with later ejection of an electron from the atom. The energy of the emitted or outgoing electron would be

E= h

Where B.E represents the binding energy of the electron

In this way a free electron cannot absorb a photon and also the momentum is conserved. We get photoelectric effect on bound electrons with the nucleus absorbing the bouncing or recoil momentum.

Theoretically, the photoelectric effect is difficult to treat strictly because the Dirac wave functions for the atomic electron are very complex.

In photoelectric absorbtion process, a photon interacts with an absorber atom in which the photon completely disappears and at its place an energetic photon-electron is emitted by the atom from one of its bound shells. The interaction takes place with the whole atom and not with the single free electrons. If we use gamma ray of sufficient energy the photoelectric effect will occur at the most tightly bound or k-shell of the atom. The photoelectron will come out with an energy given by:

Here is the binding energy of photoelectron in its original shell. If the gamma-ray energies are more than a few hundred keV then the photoelectron takes off the majority of the original photon energy.

Inadequacy to the photoelectron, the interaction also forms an ionized absorber atom which has a vacancy in one of its bound states. The vacancy is rapidly filled through the capture of a free electron from the medium and/or by the rearrangement of electrons from the other shells. Due to this one or more characteristic X-ray photons may also be generated. While mostly these X-rays are reabsorbed close to the original site through photoelectric absorbtion involving loosely bound shells, their migration and their possible escape from radiation detectors can determine their response. In some fraction of the cases, an Auger electron is emitted that may substitute for the characteristic X-ray in taking away the excitation energy.

For the gamma-rays of relatively low energy photoelectric effect is predominant mode of interaction. If the absorber material is of high atomic number Z then the process is enhanced. No analytic expression is valid for the probability of photoelectron absorbtion per atom over all ranges of and Z, but a rough approximation is given as:

Where the value of exponent n varies between 4 and 5 over the gamma ray energy area. This relationship shows that photoelectric absorption probability depends greatly on atomic number of the absorber. That is why materials with high Z are preferred.

2.1.3.2 Compton scattering

In Compton scattering the process of interaction takes place between the incident gamma ray photon and an electron in the absorbing material. It is most often the predominant mechanism for the gamma rays that have energies typical of radioisotope sources.

In Compton scattering, the incident gamma ray photon is deflected at an angle from its original direction. A portion of energy is transferred from the photon to an electron that is assumed to be at rest, which is then called as recoil electron. In this process all angles of scattering are possible. So, the value of energy transferred can range from zero to a large fraction of the gamma-ray energy.

For any given interaction we can write the relation between the energy transfer and the scattering angle by simply writing the simultaneous equations for the preservation of energy and momentum.

Figure:

From the above figure we can show that:

Where the rest mass energy of electron is represented by . If the scattering angle is very small the a very little amount of energy is transferred. Some amount of energy from the original energy is always kept back by the incident photon even when the angle is extreme at .

The probability per atom of the absorber of Compton scattering depends on the number of electrons that are used as scattering targets. That’s why they increase linearly with Z.

Klein-Nishina formula formula predicts the angular distribution of scattered gamma rays for the differential scattering corss section :

Where and is the classical electron radius.

2.1.3.3 Pair Production

The process of pair production is energetically possible when the energy of gamma ray exceeds twice rest mass of energy of an electron (1.02 MeV). Practically the probability of this process to occur is very low until the gamma ray energy approaches several Mev and due to this reason pair production is predominantly limited to high energy gamma rays. The interaction takes place in coulomb field of a nucleus, the gamma ray photon anishes and is interchanged by an electron-positron pair. In the process all the extra energy taken by photon above the 1.02 MeV needed to create the pair goes into kinetic energy that is divided by the positron and the electron. Because the positron will later annihilate after decelerating in the absorbing material, two annihilation photons are normally formed as secondary products of the interaction.

There is no simple expression that explain the probability of pair production per nucleus, but its magnitude changes approximately as the square of the atomic number of the absorber.

2.1.4 Neutrons

Neutrons are neutral (electrically) and have some rest mass they also interact with matter weakly just like gamma rays. They interact with matter by collision. As they have the mass same like protons, their largest interactions happen with Hydrogen atom. After some collisions their energy reduce, at last completely absorbed. As human tissues have a large amount of water in them, neutrons are very risky. Protection from neutrons radiations can get from materials have H or any other light nuclei.