By

Amr A. Abd-Elghany, Ph.D.

Radiation Biophysics

Atomic Structure•An atom is composed of electrons (with a negative charge), protons (with a positive charge) and neutrons (no charge).

•The number of electrons equals the number of protons in an atom so that the atom has no net charge (electrically neutral).

• Different materials (for example, gold and lead) will have different numbers of protons/electrons in their atoms.

• However, all the atoms in a given material will have the same number of electrons and protons. •Electron mass (9.1x10-31kg) is smaller than proton mass (1.67x10-27kg).

protons

neutrons

electrons

Atom This atom has 7 protons and 7 neutrons in the nucleus. There are 7 electrons orbiting around the nucleus.

The electrons are maintained in their orbits around the nucleus by two opposing forces. The first of these, known as electrostatic force, is the attraction between the negative electrons and the positive protons. This attraction causes the electrons to be pulled toward the protons in the nucleus. In order to keep the electrons from dropping into the nucleus, the other force, known as centrifugal force, pulls the electrons away. The balance between these two forces keeps the electrons in orbit.

(See next three slides)

• Electrostatic force is the attraction between the positive protons and negative electrons. Electrons in the orbit closest to the nucleus (the K-shell) will have a greater electrostatic force than will electrons in orbits further from the nucleus. • Another term often used is binding energy; this basically represents the amount of energy required to overcome the electrostatic force to remove an electron from its orbit. For our purposes, electrostatic force and binding energy are the same. • The higher the atomic number of an atom (more protons), the higher the electrostatic force will be for all electrons in that atom.

Centrifugal force pulls the electrons away from the nucleus.

EF CF

The balance between electrostatic force and centrifugal force keeps the electrons in orbit around the nucleus

Electron Binding Energy and Energy Levels

-The binding energy of an electron (Eb ): is defined as the energy required to completely separate the electron from the atom.

-When energy is measured in the macroscopic world of everyday experience, units such as joules and kilowatt-hours are used.-In the microscopic world, the electron volt is a more convenient unit of energy.-The kinetic energy (the “energy of motion”) of the electron depends on the potential difference between the electrodes.

-One electron volt: is the kinetic energy imparted to an electron accelerated across a potential difference (i.e., voltage) of 1 V.

How much energy does an electron gain when it is accelerated across a voltage of 20,000 V?

E= 20,000 eV

-The electron volt can be converted to other units of energy:1 eV = 1.6 × 10−19 J = 1.6 × 10−12 ergNote: 103 eV = 1 keV106 eV = 1 MeV

Kinetic energy of electrons specified in electron volts. A: The electron has a kinetic energy of 1 eV.B: Each electron has a kinetic energy of 10 eV.

Electron Transitions, Characteristic and Auger Emission

-On the other hand, an outer-shell electron may drop spontaneously to fill a vacancy in an inner shell. This spontaneous transition results in the release of energy.

-To move an inner-shell electron to an outer shell, some external source of energy is required.

-When an electron is removed from a shell, a vacancy or “hole” is left in the shell.

-An electron may move from one shell to another to fill the vacancy. This movement, termed an electron transitionelectron transition.

equals the difference in binding energy between the two shells involved in the transition. ΔE = E2-E1 = h ν

-The transition energy is released as a photon.

-The energy released during an electron transition is transferred to another electron.

This energy is sufficient to eject the electron from its shell. The ejected electron is referred to as an Auger electron.

A: Electron transition from an outer shell to an inner shell. B: Electron transition accompanied by the release of a characteristic photon. C: Electron transition accompanied by the emission of an Auger electron.

Types of Radiation• Ionizing Radiation.

• Non Ionizing Radiation.

Two major forms of Radiation• Particulate Radiation.

• Electromagnetic Radiation.

Both forms can interact with matter, and transfer their energy to the matter.

Types of Ionizing Radiation

• Nature: Helium atoms (2 neutrons and 2 protons).

• Charge: +2.

• Mass: equal to the mass of helium atoms.

• Penetration: it can be stopped by a sheet of paper.

• Only hazard when inhaled.

Alpha particles

Beta particles

• Nature: Fast Electrons or positrons.

• Charge: -1.

• Mass: equal to the mass of electrons.

• Penetration: it can be stopped by a layer of clothing (few mm in tissue or meters in air).

• Only hazard when injected or inhaled.

Neutrons•Have the same mass as protons but are uncharged they behave like bowling balls.

Gamma rays• Nature: Photons (EM radiation).

• Charge: no charge.

• Mass: no mass.

• Penetration: very strong, so it can be used in diagnostic radiation.

• Hazards when injected or inhaled or due to external exposure.

Metastable:Elements that decay by emitting γ-ray only. The daughter nuclei differs from its parent only in energy.

Isotopes• Nuclei of a given element with different numbers of

neutrons (i.e. the same Z but differ in A).

(Z)6C12 (A) , 6C13 , 6C14 , 6C15

A = N + Z• Where A (mass no.), N (no. of neutrons) , Z (number

of protons or electrons). • There are 2 types of isotopes: • Stable isotopes: they are not radioactive and occur

naturally.• Radioisotopes: they are radioactive and emit

radiation. Some of them are natural and others can be artificial.

Laws and Definitions in Radioactivity

• The radioactive decay is described by:

A=Aoe- λt

Where A = activity present after time t

Ao = initial activity at time = zero

λ = decay constant in time-1

t = time taken to transform activity from Ao to A.

• Also activity

A = λ N• N is the number of radioactive atoms.

Half life time T1/2

• The time needed for all of the radioactive nuclei to decay to its half activity.

A=Aoe- λt -------------------- (1)

At t = T1/2 ----------- A=1/2 Ao

subst. in eqn. (1) ------ ½ Ao=Ao e- λT1/2

2= e- λT1/2

ln 2 = λ T1/2

T1/2 = 0.693/λ

Average (mean) life time Ƭ

• Average time required to all radioactive atoms to decay.

• Units of radioactivity.• Curie 1 Ci=3.7x1010 disintegration/sec.• Bequral Bq= 1 disintegration/sec.• 1 Ci = 3.7x1010 Bq.

Ƭ = 1/ λ

Ƭ = 1.44 T1/2

Calculate the time required to reduce the activity of a pure 40K from 7 µ Ci to 50 kBq? (T1/2=2days)?

• Given: T1/2=2 days Ao=7µCi=7x10-6x3.7x1010Bq

A=50kBq=50x103Bq• Required: t ?????

• Equations and calculations: T1/2=0.693/λ

λ = 0.693/2 = 0.3465 day-1

A=Ao e-λt

50x103 = 7x3.7x104 e-0.3465t

50=259 e-0.3465t

t= (ln 259/50)/0.3465 = 4.75 days.

Calculate the activity of 60Co after 72hr, if you know the activity at time 0 is 35MBq (decay constant=0.001 day-1)? Home work

Gamma Camera

• Is a device used to image gamma radiation radioisotopes this technique is called also scintillation camera.

• Gamma camera is used to view and analyze images of the human body or the distribution of the medically ingested, injected or inhaled radionuclides.

Nuclear Medicine imaging devices

Gamma Camera

Gamma Camera Components1-Collimators• The collimator provides an interface between

the patient and the scintillation crystal by allowing only those photons traveling in an appropriate direction.

Collimators

• Types of collimators

A) By the accepted energy.

B) By the geometric shape.

C) By the resolution.

Collimators

• By the accepted energy

Low Energy Collimator

High Energy Collimator

Medium Energy Collimator

Collimators

• By the geometric shape.

Parallel-Hole Diverging collimator

Converging collimatorPin-Hole Collimator

Collimators

Parallel-HolePin-Hole (more resolution & magnification) hip,thyroid

Converging لألعضاء أفضل وتحديد الصورة البدين Divergingلتكبير المريض حالة في للتصغير

Detector Scintillation Crystal

• The chosen material for the crystal is Na-I (Tl), 40-50 cmdiameter.

• The Na-I (Tl) crystal is stationary.

• The crystal transform the gamma-ray photon ------> Light photon

• Any damage to the crystal results in an inoperable scintillation camera and requires costly replacement of the crystal.

Photomultiplier tube

PhotocathodePhotomultiplier tube

DynodeConnected to High positive volt

Photomultiplier tube

• The Photocathode transform the light photon --- electron.

• The PMT multiplies the electron to be a significant detected signal.

Other circuits

• 1)Pre-Amplefier

• 2) Amplifier

Advantages of Gamma Camera

• The imaging time is only 1-2min.

• It can distinguish 2 sources about 5mm apart.

Radiation protection Units

ActivityActivity It describes the amount of radiation coming It describes the amount of radiation coming out of the sourceout of the source..--------------------------------1Ci (Curie) =3.7x101Ci (Curie) =3.7x1010 10 disintegration/secdisintegration/sec

))This is the activity of 1g of RadiumThis is the activity of 1g of Radium.(.(--------------------------------1Bq (Becquerel) =1 dis/sec1Bq (Becquerel) =1 dis/secThus, 1Ci= 3.7x10Thus, 1Ci= 3.7x1010 10 BqBq

ExposureExposure It describes the It describes the ionization of airionization of air due to the radiation coming out of due to the radiation coming out of the sourcethe source..

--------------------------------Roentgen (R) is the unit of exposure for x-rays and gamma raysRoentgen (R) is the unit of exposure for x-rays and gamma rays..1R = 2.58 x 101R = 2.58 x 104 4 Coulomb/kgCoulomb/kg

Absorbed Absorbed dosedose

It describes the amount of It describes the amount of energy absorbedenergy absorbed in the medium in the medium..----------------------------------------

11 Gray (Gy) = 1 Joule / kgGray (Gy) = 1 Joule / kg..11 Gy = 100 radGy = 100 rad

i.e., when one kg of tissue absorbs 1 joule, the tissue is said to have i.e., when one kg of tissue absorbs 1 joule, the tissue is said to have absorbed 1 Gyabsorbed 1 Gy..

Equivalent Equivalent dosedose

It accounts for the It accounts for the difference in the biological effectdifference in the biological effect caused by the caused by the absorbed dose from absorbed dose from different radiation sources compared to the different radiation sources compared to the effect caused by x-rays or gamma rayseffect caused by x-rays or gamma rays..

----------------------------------------------11 rem = 1 rad x Q.F. (The equivalent dose = absorbed dose x quality rem = 1 rad x Q.F. (The equivalent dose = absorbed dose x quality

factor)factor)Q.F. : is the quality factor of radiationQ.F. : is the quality factor of radiation

))Q.F. = 1 for x and gamma rays, electrons and positrons. Q.F. = 5-20 Q.F. = 1 for x and gamma rays, electrons and positrons. Q.F. = 5-20 for some neutrons, 20 for Alpha particles and 10 for protonsfor some neutrons, 20 for Alpha particles and 10 for protons.(.(

11 Sievert (Sv)= 100 remSievert (Sv)= 100 rem

Golden Rules for Radiation Protection

TimeTime (of exposure)(of exposure)

------------------------------The longer The longer the time of the time of exposure, the exposure, the higher the higher the radiation radiation dose. Do not dose. Do not spend spend unnecessary unnecessary time near time near radiation radiation sourcessources..

Distance Distance (from source)(from source)

--------------------------------The intensity of The intensity of radiation (I) is radiation (I) is inversely inversely proportional to proportional to the square of the square of the distance the distance from source from source (d(d22). So keep ). So keep away as much away as much as you can from as you can from sourcesource

Activity Activity (of source)(of source)

------------------------------------Choose the Choose the lowest activity of lowest activity of source that is source that is sufficient for the sufficient for the needed jobneeded job..

ShieldingShielding

----------------------------Proper Proper shielding of shielding of source and source and Personnel Personnel will always will always reduce reduce radiation radiation dosedose..

ALARA: As Low As Reasonably Achievable