Radiation

Radiation: The process of emitting energy in the form of waves or particles.

Where does radiation come from?Radiation is generally produced when particles interact or decay.

A large contribution of the radiationon earth is from the sun (solar) or from radioactive isotopes of the elements .

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Radioactivity

By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:

1) Alpha particles (α)2) Beta particles (β)3) Gamma-rays (γ)

By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:

1) Alpha particles (α)2) Beta particles (β)3) Gamma-rays (γ)

Alpha Particles (a)

Radium

R226

88 protons138 neutrons

Radon

Rn222

Note: This is themass number, whichis the number ofprotons plus neutrons

86 protons136 neutrons

+ nnp

p

a (4He)2 protons2 neutrons

alpha-particle ( )a is a Helium nucleus. It’s the same as the element Helium, with the electrons stripped off !

Beta Particles (b)

CarbonC14

6 protons8 neutrons

NitrogenN14

7 protons7 neutrons

+e-

electron(beta-particle)

We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. In symbolic notation, the following process occurred:

n p + e ( + )n

Yes, the same neutrino we saw

previously

Gamma particles (g)In much the same way that electrons in atoms can be in an excited state, so can a nucleus.

NeonNe20

10 protons10 neutrons

(in excited state)

10 protons10 neutrons

(lowest energy state)

+

gamma

NeonNe20

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

How do these particles differ?

Particle Mass*(MeV/c2)

Charge

Gamma (g) 0 0

Beta (b) ~0.5 -1

Alpha (a) ~3752 +2

* / m = Ec2 *m = E / c2

INTERACTION OF RADIATION WITH MATTER

The interaction of radiation with matter depends on:i) The type and energy of the incident radiation.ii) The chemical and physical properties of the target material.iii) The manner in which the incident radiation interacts with the

material. This section contains the mechanisms by which ionizing radiation

interacts and loses energy as it moves through matter. This subject is extremely important for radiation measurements because the detection of radiation is based on its interactions and the energy deposited in the material of which the detector is made. Therefore, to be able to build detectors and interpret the results of the measurement, we need to know how radiation interacts and what the consequences are of the various interactions.

• For the discussion that follows, ionizing radiation is divided into three groups:

• Charged particles: electrons (e-), positrons (e+), protons (p), deuterons (d) , alpha particles (α), heavy ions (A > 4).

• Photons: gammas (γ) or X-rays.• Non-charged particles: Neutrons (n).• This classification is convenient because each group

has its own characteristic properties and can be studied separately.

INTERACTIONS OF CHARGED PARTICLE WITH MATTER

• Charged particles traveling through matter lose energy in the following ways:

• In Coulomb interactions with electrons and nuclei.

• By emission of electromagnetic radiation.• By emission of Cerenkov radiation. Cerenkov

radiation is visible electromagnetic radiation emitted by particles traveling in a medium, with speed greater than the speed of light in that medium. It constitutes a very small fraction of energy loss.

INTERACTIONS OF PHOTONS WITH MATTER

• Photons, also called X-rays or g rays are electromagnetic radiation, are considered as particles that travel with the speed of light c and they have zero rest mass and charge.

• There is no clear destination between X-rays and γ–rays. The term X-rays is applied generally to photons with E < 1 MeV. Gammas are the photons with E > 1 MeV. X-rays are generally produced by atomic transitions such as excitation and ionizations. Gamma rays are emitted in nuclear transitions.

• There are several possible interactions of photons, but the three most important ones are: the photoelectric effect, Compton scattering and pair production.

A) The Photoelectric Effect• The energy of the gamm a-ray

photon is completely transferred to an orbital electron which is ejected from its atom .

• The gamma-ray no longer exists after the collision. The ejected electron then causes ionization until it loses its energy, and is captured by an atom. The photoelectric effect is more likely to occur when the photon energy is low, i.e. below 0.5 MeV and the absorber is a heavy material

B) Compton Scattering• Higher energy photons may lose

only part of their energy to the atomic electron which is again ejected from its atom .This electron goes on to create ionization. The remaining energy is taken up by another photon of reduced energy which is scattered in a new direction. The new photon will either be absorbed by a photoelectric effect, or if the energy is still high by further Compton scattering.

• Compton scattering occurs in all materials and predominantly with photons of medium energy, i.e. about 0.5 to 3.5 MeV.

C) Pair Production • Gamma-ray photons with energy

greater than 1.02 MeV may interact with a nucleus to form an electron-positron pair. This amount of energy is just sufficient to provide the rest masses of the electron and positron (0.51 MeV each). Excess energy will be carried away equally by these two particles which produce ionization as they travel in the material. The positron is eventually captured by an electron and annihilation of the two particles occurs. This results in the release of two photons each of 0.51 MeV known as annihilation radiation. These two photons then lose energy by Compton scattering or the photoelectric effect.

D) Coherent scattering • In addition to Compton scattering, another type of

scattering can occur in which the gamma-ray photon interacts coherently with all the electrons of an absorber atom. This coherent scattering or Raleigh scattering process neither excites nor ionizes the atom, and the gamma-ray photon retains its original energy after the scattering event. Because virtually no energy is transferred, this process is often neglected in basic discussions of gamma-ray interactions. However, the direction of the photon is changed in coherent scattering. The probability of coherent scattering is significant only for low photon energies (typically below a few hundred keV for common materials) and is most prominent high-Z absorbers.

NEUTRONS

( 1 )Slow Neutrons • Slow neutrons are that with energies from

zero to about 1000 eV

(2 )Intermediate Neutrons

• Intermediate neutrons having energies in the range 1000 eV-0.5 MeV. We haven't enough information about intermediate neutrons than about slow neutrons due to the difficulty of finding efficient detectors.

(3 )Fast Neutrons

• Neutrons with energies having range between 0.5 and 10 MeV are called fast neutrons. This energy region range gives the possibility of many nuclear reactions which are energetically impossible at lower ranges of which the inelastic scattering is dominant.

(4 )Very Fast Neutrons

• These are neutrons having energies in the range 10-50 MeV and is distinguished from the proceeding by the appearance of nuclear reactions involving the emission of more than one product such as the (n, 2n) reaction.

(5 )Ultra Fast Neutrons

• Neutrons with energies beyond 50 MeV are called ultra high neutrons. They are produced by p-n interactions induced in nuclei by high energy protons. The cosmic radiation is also a source of neutrons with energies well above those which are likely to be produced by accelerations.

Interactions of Neutrons• In common with gamma rays, neutrons carry no charge and

therefore cannot interact in matter by means of the Coulomb force. Neutrons can also travel through many centimeters of matter without any type of interaction and thus can be totally invisible to a detector of common size. As a result of the interaction of the neutron with the nucleus of the absorbing material, it may either totally disappear and be replaced by one or more secondary radiations, or else the energy or direction of the neutron is changed significantly.

• In contrast to gamma rays, the secondary radiations resulting from neutron interaction are almost heavy charged particles. These particles may be produced either as a result of neutron-induced nuclear reactions or they may be the nuclei of the absorbing material itself, which have gained energy as a result of neutron collisions.

RADIATION UNITS

• Shortly after the discovery of x-rays in 1885 by Professor Roentgen scientists began to notice the harmful effects of exposure to this radiation. There were many years before people realized how dangerous x-rays and other radiation could be. Quite number of pioneer radiologists suffered severe injuries, and some even died, as a result of prolonged exposure to dangerously high intensities of X-rays. These early workers in the field had no means of measuring the harm caused by radiation accurately and depended on unreliable effects such as the degree of skin reddening caused by the exposure, or on timing the exposure from a certain type of X-ray machine to establish quantity.

• In order to evaluate the hazards of radiation it is necessary to have a measure of the harm which is the radiation cause. The radiation effect is measured in terms of exposure or dose

A) Exposure

• One of the earliest observed properties of X-rays was their ability to ionize air. In 1928 the International Congress of Radiology specified this property as means of measuring the amount of X-radiation, and defined a unit which was named the roentgen, (R).

• 1 R = exposure to X-rays or gamma-rays of such intensity that the electrons produced by this radiation in 1cm3 of dry air, at standard temperature and pressure, generate along their tracks electron - ion pairs carrying a total charge of 1esu of either sign

• The SI unit of exposure is defined as 1 C / Kg air, without any new name proposed for it. Numerically

• 1R = 2.58 x 10-4 C/Kg air• The roentgen suffers from two limitations, first

it was defined in terms of electromagnetic radiation only, and secondly it was defined in terms of air only.

B) Absorbed Dose• Because of the limitation of roentgen (R), another unit called

the radiation absorbed does or rad was defined as energy absorbed per unit mass of material. So,

Where D is the absorbed dose and is the mean energy imparted by ionizing radiation to matter in a volume element and dm is the mass of matter in the volume element. The unit of the absorbed dose is rad defined as,

• 1 rad = 100 erg/g• The SI unit of absorbed dose is the Gray (Gy), defined as• 1 Gy = 1 J/Kg =100 rad

dm

dD

C) The Dose Equivalent• The units of absorbed dose defined in the pervious

section are quite adequate for the quantitative assessment of the effects of radiation to inanimate objects, like irradiated transistors or reactor fuel. For protection of people, however, the important thing is not the measurement of energy deposited (the absorbed dose), but the biological effects due to radiation exposure. Unfortunately, biological effects and absorbed dose do not always have one-to-one correspondence, and for this reason a new unit had to be defined, a unit that takes into accounts the biological effects of radiation.

• The first step toward that task was the introduction of a factor called the relative biological effectiveness (RBE), defined as

effect] biological same theproducing i typeradiation from dose [Absorbed

effect] biological certaina producing KeV)300 – (200 radiationgamma or ray -X from dose [AbsorbedRBEi

In the meaning of RBE, there are the following notes:

• RBE is defined in terms of photons; therefore, it follows that RBE=1 for electromagnetic radiation. Also, although the definition of RBE specifies the energy of the photons to be 200–300 KeV, RBE is taken as equal to 1 for photons of all energy.

• A given type of radiation does not have a single RBE, because RBE values depend on the energy of radiation, the cell, the biological effect being studied, the total does, dose rate and other factors.

• It is a well-known that the biological damage increase as the energy deposited per unit distance, the linear energy transfer (LET), increases. Thus, heavier particles (alphas, heavy ions, fission fragments) are, for the same absorbed dose, more biologically damaging than photons, electrons and positrons.

In 1963, the International Commission on Radiological Units and Measurements (ICRU) proposed the replacement of RBE by a new factor named the quality factor (QF). In 1973 the ICRU (82) recommended dropping "F" from QF, a suggestion that has now become practice. In 1977 the International Commission on Radiological Protection (ICRP) recommended that the dose equivalent (H) at a point in tissue be written as:

H = NQD Where Q = Quality factor its values are given in Table 1.1. D = absorbed dose N = product of all the modifying factors. The suggested value of N is 1 When the unit of absorbed dose is multiplied by the corresponding Q

value, the unit of dose equivalent (H) is obtained. The H units are 1 rem = Q x 1 rad and the SI unit, 1 Sievert (Sv) = Q x 1 Gy Thus, 1 Sv = 100 rem.

Table 1.1 Quality Factors for Several Types of Radiation

Radiation Type Q Radiation Type Q

Gamma-rays 1 Neutrons

X-rays 1 Thermal 2

Beta particles 1 0.02 MeV 5

Electrons 1 0.10 MeV 7.5

Positrons 1 0.50 MeV 11

Protons 10 1.00 MeV 11

Alpha Particles20

5.0 MeV8

Recoil nuclei 20 10 MeV 65