Radiation BiophysicsRadiation biophysics is concerned with the interaction of ionizing radiations with matter and the effects on biological systems. Radiation in lower energy interacts with matter by excitation processes but high energy radiation can ionize atoms and the liberated ‘primary electrons’ can further ionize or excite neighboring atoms.The biological effects of radiation depend on the energy and the penetration power of the incident radiations and the nature of the target. α rays have less penetrating power comparing to - rays and X-rays. But, α rays have very high ionization power. Hence the biological damage caused by α rays are more compared to - rays and X-rays.

Ionizing RadiationAll radiations of the electromagnetic spectrum have similar characteristics, but differ widely in their energies- from low energy radio and microwaves to infrared, visible and ultraviolet light to high energy X-rays and - rays. Radiation biophysics, in general, is concerned with the interaction of ionizing radiations (above 500eV), such as X-rays, - rays, β- rays, α rays, protons and neutrons, with matter and the resulting damage to the systems.

Excitation and IonizationInteraction of radiation with matter can lead to either (i) excitation or (ii) ionization, depending on the energy of the incident radiation. If the energy of the incident radiation is below the threshold ionization potential, it can lead to excitation from one state to another, as incident energy is not sufficient to knock out the electrons from the attractive force of the nucleus of the atom. Excitation energies are in the UV region.High energy radiations such as X-rays and - rays have sufficient and even excess kinetic energy above the ionization threshold potential of atoms so that they can completely remove electrons from the atoms, leading to ionization.

A A + e-

Radiation SourcesSources of ionizing radiations are natural as well as manmade. Cosmic rays, radiations emitted by radioactive decay, such as α- particles, protons, neutrons and photon emission are natural sources. The interaction of any radiation with matter depends on its energy, penetration power and other characteristics specific to the radiation and to the target.

α- raysα rays are high energy Helium nuclei released in the α decay, that occurs in heavy nuclei(generally Z> 83) with excess of protons and neutrons, such as thorium, uranium, and radium.

The penetrating power of α rays is very low- they can be stopped by a thin sheet of paper. However, they have very high ionization power. The α particles can cause considerable radiation damage to skin tissues in organism, since all their energy is deposited in a very high small tissue volume and is therefore radiotoxic. β- rays

β- rays are high energy electrons produced by the transformation of neutrons into protons with emission of electrons. Transformation of protons into neutrons induces the emission of positrons.

Neutron Proton + Electron + AntineutrinoProton Neutron + Positron + Neutrino

Interaction of high energy β- rays with matter results in excitation, ionization and Bremsstrahlung processes. The penetration of β- rays, which is about ~1cm in biological tissues, is higher than that of α rays.

Protons and Neutrons

Protons are hydrogen nuclei. Neutrons transfer their kinetic energy through elastic, inelastic and particle capture processes. In the inelastic collision process with an atomic nucleus, part of the kinetic energy is utilized to excite the nucleus and the excited nucleus emits radiation. Neutrons do not produce ionization directly, but indirectly by ejecting protons of the atomic nuclei of the target material. In effect, the biological effects of neutrons are due to the protons they produce.

X-rays and - rays

Both X-rays and - rays are high energy photons, irrespective of their mode of production, and their effects on biological systems are quite similar. While X-rays are produced due to atomic transitions of core electrons on bombardment of any heavy element target by high speed electrons, - rays are produced in nuclear reactions, whenever there is a transition of a nucleus to the ground state or by annihilation of an electron-positron pair.

Interaction of Radiation with Matter

There are 3 processes by which high energy radiations interacts with matter.1) Photo-electric effect2) Compton effect3) Electron- positron pair production

Photoelectric effectAt lower energies (<30KeV) the photoelectric effect comes into operation. In this process all the energy of the incident radiation is used in ejecting a bound electron resulting in ionization. The electrons thus produced are called photoelectrons. Photoelectrons are absorbed locally, causing ionization and excitation. The photoelectric effect is predominant at lower energies (<0.1 MeV) and for elements of high atomic numbers.

Compton effectAt medium energies (<10MeV) the Compton effect is dominant. The Compton effect is inelastic scattering, where the incident photon transfers part of its energy to a weakly bound electron. This electron is called recoil electron. As a result of the energy transfer during collision the photon will have a reduced energy. The Compton effect is prominent at moderately high energies and for materials of low atomic number.

Pair ProductionRadiations (photons) with energies > 1.02Mev in the presence of a nucleus of an atom will split up into an electron and positron (a particle and its anti particle). The presence of nucleus is essential for the conservation of angular momentum. This process is called pair production.

Beer- Lambert Law

The radiation passing through matter is attenuated due to absorption and scattering. Attenuation due to absorption follows the Beer-Lambert law

I = I0 exp (-μ x) I transmitted intensity I0 incident intensity μ linear attenuation coefficient x thickness of the material (target)

The linear attenuation coefficient of the material is related to its atomic number and, therefore, the mass attenuation coefficient μm is dependent on density, ρ, given by,

μm = μ / ρThe total attenuation is the sum of 3 components due to 3 processes- photoelectric (μ p), Compton (μc), and pair production (μπ).

i.e. μtotal = μp + μc + μπ

Dosimetry- Measurement of radiation

Dosimetry deals with the various methods of radiation measurements, both physical and chemical.

Absorbed Dose (D)Absorbed dose, D, is the energy absorbed by unit mass of the material.

D = Energy imparted / MassThe unit of absorbed dose is Grey.

Gy = 1 J/kg

Dose equivalentRadiation induced biological effects also depends on the type of radiation and the nature of the target etc.Dose equivalent, is the absorbed dose, weighted according to the damage potential of the incident radiation. The unit of Dose equivalent is Sievert, Sv.

Effective Dose EquivalentNot all parts of the organisms are equally sensitive to a given radiation. So, different parts of the body should be given appropriate weightage.

Radiation dose is measured by various methods- by physical Dosimetry that includes ionization chambers, GM counters, luminescence and photographic methods and by Chemical Dosimetry.

Physical DosimetryHigh energy radiations can be measured with gas filled scintillation and semiconductor detectors. The measurement of the number of ionizations is the basic principle on which physical Dosimetry is based. Dosimetry of photon radiation is carried out by methods based on ionization of air. A closed chamber is filled with an inert gas and is subjected to steady ionizing radiations which will produce a fixed number of ions per second. Photon detectors are good for measuring lower energy range radiations and electron detectors are more useful in higher energy radiation regions, where secondary electrons play a prominent role in causing biological effects.In all these detectors, the amplitude of the pulse is non-linearly related to the applied voltage and detecting devices perform with maximum efficiency in their appropriate voltage regions (ex. GM counters have maximum efficiency in their operating region).The ionization chambers one of the most commonly used radiation detectors. In any ionization chamber device there is a closed chamber containing air or inert gas and the electric field is applied between the two electrodes. There are 3 distinct regions of the operation of the counter1) Ionization region

2) Proportional counter region3) Geiger region

In the ionization region (<300V), ionization of the gas in the chamber caused by high energy radiation is proportional to the incident radiation.In the proportional counter region, the applied voltage is above the ionization level but below the Geiger voltage. In this region the number of the electrons reaching the anode increases rapidly with the applied voltage, due to the formation of secondary ion pairs. But, the pulse amplitude is still proportional to the initial ionization and the total number of the secondary electrons is proportional to the number of primary ion-pairs produced by the original ionizing radiation. Detectors operating under these conditions are called proportional counters. They are ideal for counting high counting rates and are essential for the detection of particles of high ionizing power, such as α particles.In the Geiger region the applied voltage is higher than the proportional counter voltage. In a GM counter, a high potential is applied to the central anode ire which is surrounded by a cylindrical cathode. The tube is filled with a gas mixture (ethane + argon) and is sealed. A thin mica or glass window acts as an entry area for incident radiation. An incident ionizing particle initiates ion-pair production in the GM tube. These ion-pairs collide with neighboring gas molecules and ionize them. Such cascade collisions of ions produce an avalanche of ionization, and the electrons upon reaching the anode produce photons which in turn produce ionization. Geiger counters are simple, rugged and easy to operate but sluggish. They are used for the detection and monitoring of high energy radiation and β- particles.Semiconductor detectors are also used in physical Dosimetry.

Scintillation countersThe fluorescent effect, that is, the conversion of high energy radiation into a visible spectrum by chemicals, is measured by scintillation counters. Almost all nuclear detectors employed in nuclear medicine utilize scintillation technique. Silver activated zinc sulphide is a good scintillator for α particles and for counting - rays sodium iodide doped with thallium iodide is generally employed.There are 2 types of scintillation counters- solid scintillation counter and liquid scintillation counter.In liquid scintillation, the scintillation solution consists of solvents such as benzene, toluene, etc and solutes such as naphthalene, Carbazole etc.The solvent acts as a medium for absorbing the energy from the ionizing radiations. This results in the formation of excited solvent molecules, which will eventually transfer energy to the solute molecules, which acts as a source of photons.High energy charged particles (such as electrons) traveling with a velocity, v, through a medium of refractive index, n, radiate a part of their energy if their velocity, v, is above the threshold.

V > C / n.Where C velocity of lightThis radiation is called Cerenkov radiation.

Chemiluminescence and BioluminescenceChemiluminescence and bioluminescence result from internal reactions in a solution leading to products in excited states.

A + B C* + DC* C + hυ

The degree of darkening of a photographic emulsion due to radiation is the basis of the photochemical measuring devices such as photographic plates, film etc.

Chemical DosimetryOxidation of ferrous to ferric ions (Fe2+ Fe3+ ) by the products of radiolysis of water is a convenient method of chemical Dosimetry. This is the basic principle of the Fricke’s dosimeter. The Fricke’s dosimeter has a dilute solution of a mixture of ferrous ammoniumsulphate and NaCl and H2SO4. The reaction is carried out at pH = 1.5.Radioactive IsotopesAnother common source of ionizing radiation is radioactive isotopes. Radioactive isotopes (radio nuclides) are of considerable importance in the diagnostics of radiation effects on biological systems as well as therapeutic purposes in nuclear medicine.

RadioactivityCertain atomic nuclei (radio nuclides) which are unstable achieve stability by radioactive decay, giving rise to other types of isotopes and atoms. The β- decay processes transform neutrons to protons and protons to neutrons with the emission of electrons and positrons respectively. The α decay process emits α particles and transition of the excited nuclide to ground state causes the emission of photon. The rate of radio active decay can be expressed by Poisson’s distribution and is characteristic of that nuclide and is independent of physical and chemical environments. The rate of decay is given by

N = N0 exp (-λt)Where N0 number of radioactive atoms at the initial time (t=0) N number at time t λ decay constantThe period during which the radioactive decay of a radionuclide is half of its initial value is called half-life (τ).

τ = 0.693 / λAverage life time, T= 1/ λThe amount of radioactivity, A, ( A=λN), is measured in terms of disintegration rate.The unit is Becquerrel, Bq.

1 Bq = 1 disintegration/ second1 Bq = (1/3.7) x 10-10 Curie.

Production of Radio nuclidesRadio nuclides are isotopes (elements with same atomic number but different mass numbers) that reach stability by emitting radiation or particles. A radio nuclide is characterized by its half-life, type of decay and the type of particle emission and its energy. The most important method of producing artificial radio nuclides is by neutron bombardment of a required material in an atomic reactor. The transformation is denoted by

A (δ, X) BWhere A initial nucleus B final nucleus

δ bombarding particleX emitted particle (radiation)

Application of Radioactive TracersRadioactive labeling has extensive applications in biological & clinical fields in many assay methods and in monitoring of chemical reactions.

1) Radioimmunoassay (RIA) One of the important applications of radioactive tracers is the Radioimmunoassay (RIA) method. This method can be employed to assay substance present in small quantities with a high degree of specificity and can be used in the diagnostic monitoring of abnormalities of body functions, enzyme catalysis and other biochemical reactions.RIA is based on the high degree of affinity of an antibody for specific antigens. The general principle of RIA is the quantitative measurement of the ratio of labeled antigen (Ag*) molecules to an unknown quantity of assay antigen, Ag (that is present in the sample) competing for the binding sites of a known quantity of an antibody (Ab).In RIA a known quantity of labeled antigen (Ag*) is added to a known quantity of antibody (Ab)(in addition to the unlabelled antigen, Ag).When there is an excess of antigen molecule (Ag and Ag*) for a known quantity of antibody, Ab, there arises a competition of Ag with Ag* for the same sites of the antibody, Ab. At equilibrium,

Ag + Ab Ag . Ab

Ag* + Ab Ag*. Ab

In this reaction, increase of Ag leads to decrease in Ag*. Ab complex (therefore decrease in its radioactive counts). The assay is based on the quantitative evaluation of the total

labeled antigen, Ag*, bound to the antibody (Ag*.Ab complex) from the radioactive counts of the free and bound ligands from which the amount of unlabelled (Ag) can be calculated.At equilibrium, the ratio of the antigen in the bound state (F) and free (F) phase is

B/F= [Ag.Ab]/ [Ag]A dose-response plot of F/B versus the antigen concentration is a hyperbolic curve and the reciprocal of the receptor concentration can be calculated from the slope of the curve.In RIA, the selection of the radiolabel is determined by the structural and sensitivity requirements of the immuno response system. 131I or 125I is the generally employed radio label. Once the equilibrium state is reached the components are separated. This can be carried out by several methods such as- Chromatophoresis, gel filtration, salt precipitation etc. The choice of the counting system depends on the type of isotope used for labeling.

2) AutoradiographyRadioactive tracers are employed in autoradiography. It is a record of the traces of ionizing radiations emitted by the radioactive compound on a photographic emulsion. This technique is very similar to the detection of the paths of high energy particles photographically. In biological studies, alpha particles and gamma rays are not employed. Beta emitters such as 3H, 32P and 35S isotopes are generally preferred. The degree of darkening of exposed photographic emulsion is a measure of the amount of radio active substance present. Various variations of the general technique, such as micro-, macro-, and electron microscope autoradiography are in use to cater to different requirements. In macro- as well as in micro-autoradiography, the organisms or their parts are placed in contact with a photographic emulsion to produce an autoradiograph. Electron microscope autoradiography is employed to achieve higher resolution than possible with light microscopy.

Biological Effects of Radiation

Dose Response RelationshipsBiological radiation data are analyzed by dose response relationship graphs. The end result of the dose-response is the result of many intermediary reactions. For chemical reactions where the radiation process is indirect, the relationship between dose and the observed chemical change depends upon the reaction product. If the product does not

interact with free radicals, then the number of molecules damaged is directly proportional to the dose applied.

Target TheoryThe dose-response relationship is explained by the target theory, which is based on the assumption that within a cell a target has to be hit directly to induce cell damage. The radiation damage can either be due to single-hit dose or multi-hit dose response. The dose-response relationship for a single hit dose is semi-logarithmic and the surviving fraction, S, is

S = Ns/No = e-D

where No is the no. of targets exposed and Ns is the number surviving after irradiation and is the inactivation coefficient, which depends on the volume of the target, dose-rate etc.The inactivation coefficient of the single hit dose-response relationship represents irreversible radiation damage and does not take into account other factors such as below-lethal damage and cellular repair processes. The DNA is taken as the principal target for radiation damage. Dose-response curve which are sigmoid in shape are due to multi-hit processes combined with molecular repair processes. An end-effect can be due to a combination of single hit and multi hit. In multi-hit processes, the damage is dose-rate dependent at higher doses.

Radiolysis of WaterIn biological systems, water is abundant. Its high affinity for electrons makes the radiolysis of water a primary event in the initiation of primary damage in the living cells. Absorption of radiation by water leads to ionized species which further react with neighboring water molecules or other polar molecules, to form a variety of new reactive species. Free radicals are not the primary products of the radiolysis of water, but secondary products from subsequent decomposition of ions and excited species produced initially.

H2O H2O+ + e-

H2O H+ + OH-

H2O OH+ + H + e-

H2O+ + e- H2O*

H2O+ H+ + OH.

H.

+ OH+

h

ionisation

H2O* H. + OH

.

H2O + e- H. + OH-

H2O+ + H2O H3O+ + OH-

Hydrolysis of free radicals, OH., which are very reactive can form hydrogen peroxide,

which is a powerful oxidant.

OH.

+ OH. H2O2

Free radicals and H2O2 can react with the body system and can cause various kinds of damage including cancer.The role of oxygenOxygen is a highly reactive molecule. In the presence of oxygen the reactions produced by ionizing radiations are different and are more harmful biologically. In the absence of

oxygen, radicals R. and X

. can react with one another or dimerise, Rn.

R. + X

. RX

or R. + R

. + R..... Rn

Presence of oxygen blocks restoration processes and enhances radiation damage. In the

presence of oxygen, the formation of the peroxy-radical, RO2

., is the predominant

reaction

R. + O2 RO2

.

Peroxy radicals cannot dimerise or polymerise but give rise to hydroperoxide.

RO2

. + H RO2H

Irradiated water containing dissolved oxygen leads to the formation of relatively large

amounts of perhydroxy radicals (HO2

.). Therefore, all biological systems are more

radiosensitive in the presence of oxygen.

H2O + O2 HO2

. + OH2

.

Each perhydroxy radical can oxidize 3 molecules.

3RH + HO2

. 3 R

. + H2O

Therefore, a dose of radiation is more destructive to a biological system in the presence of oxygen than its absence.

Effects of Radiation on Living Systems

Ionising radiations can break the chemical bonds and radically alter the chemical structures of biological molecules. The principal reaction of the amino acids in aqueous media is deamination with the release of water. Free radicals, produced on radiolysis of water, can react with macromolecules and damage them. Presence of oxygen enhances the damage. Almost all proteins are denatured by ionisation which results in the lose of enzyme activity.Radiation damage to the genetic material can be hereditary or somatic. Radiations can directly damage the chromosomes, considered to be the root cause of the cell damage. A mutated cell may transmit flowed genetic message via its DNA to other cell generations. Radiation may cause cell division not to occur at a proper time. Therefore, acute damage is most likely to occur in those cells which are undergoing rapid cell divisions. Consequently, infants are more susceptible to radiation damage than adults. Radiation can also cause opaqueness and cataract in the eye.Radiation carcinogenesis is a stochastic process. In general carcinogenesis takes place in 2 steps- initiation (irreversible) and promotion. Ionising radiation acts as initiator as well as promoter. Carcinogenesis may occur mainly as a result of acute (short time span) doses from atomic weapons or therapy.All forms of ionising radiations are harmful above certain minimum dose levels. Exposure to ionising radiations cannot be completely avoided as human beings are exposed to these from natural as well as manmade sources of radiation. The mst important natural sources of radiation are the decay products of radon, 222Rn. 222Rn is the gaseous daughter product of radium, 226Ra. 222Rn is released into the atmosphere due to uranium, building construction etc.On inhalation of radon gas, the decay products are absorbed as aerosol particles by the bronchial tissues, leading to lung tissue damage and lung cancer.Other naturally occurring nuclides are 42K, 90Sr, and 131I being gaseous can spread over a large area and is absorbed by living organisms through the food chain. It causes minimal damage due to its very short life time. Of all machine produced sources, medical X-rays, gamma rays in nuclear medicine and in nuclear reactors constitute high health hazards.Accumulated low doses can start off chain reactions leading to genetic damage and cancer. A dose of 0.25Gy is considered as the limiting dose. Dosses in the range below 1Gy affect sensitive organs and tissues, such as bone marrow tissues. Doses≈ 1Gy damage eyes leading to blindness, cause deficiencies in the immune system and leads to anaemia. Doses in the range of 3-5Gy lead to death in few months due to bone marrow damage, haemorrhages, anaemia, infection and malnutrition. Doses of 10-40Gy lead to

death within weeks & very high doses, >100Gy cause death within hours or days due to the damage of central nervous system.

Radiation Protection and Therapy

Radiation ProtectionAs radiolysis of water produces free radicals, addition of substances that complete for these free radicals could minimize the random effect. Some of the molecules rendering chemical protection against radiation are cysteine and molecules containing free sulphydryl (-SH) groups and chelating agents such as EDTA.Charge delocalization reduces the no. of radicals produced at the target, by diffusing them to wider areas. Therefore, aromatic agents are better protectors in charge-transfer mechanism. Macromolecules such as proteins can be protected by competitive removal of radicals or repair through a hydrogen-transfer reaction.

PH + OH* P* + H2OP* + XH PH + X*

Where P is a polymer, P* is the polymer radical and XH is the protective reagent. Colloidal sulphur is a protecting agent for enzymes.Protective of sensitive organs and sites from irradiation is another method of protection. As longer wavelength X-rays are absorbed by the skin surface leading to skin damage, filtering of such radiations can be achieved by shields (Al, Cu etc).

Radiation TherapyIn spite of the hazardous nature of ionizing radiations, living organisms have to live with them and they are very much part of nuclear medicine for diagnostic as well as therapeutic treatments. X-rays are used in Roentgenography and CAT-scan radiography. Radioactive isotopes are employed as tracers in many biological processes. Labeled radio nuclides can easily be monitored due to their radioactivity and high sensitivity. Various functions of organs and their abnormalities can be monitored by such methods. Some of these include diagnostics of the function of the thyroid gland by radio iodine, the function of kidneys by renography etc.Labeled proteins are used in clinical investigations. By such investigations any abnormalities due to the effects of radiation or chemicals can be evaluated.Tumors (rapidly dividing cells with undifferentiated structure) are more sensitive to radiation than strongly differentiated cells undergoing cell slow division. Use of ionizing radiation in cancer therapy is based on the sensitivity of different types of cells to these radiations. Present day radiation therapy relies on the administration of radiation dose not in a single shot but in fractions separated over time period. This procedure is to allow the recovery of normal calls from the effects of radiation.

Problems

1. The half-life of radioactive substance is 48 hr. How much time it will take to disintegrate to its 1/16th part.

Solution:

 2. Carbon-14 is one of the isotopes of carbon with a half life of 5,730 years. Find the

decay constant (λ) for this element.

Solution: We know the half life of C-14, from which we are expected to compute its decay constant. The equation that connects these two quantities is:

t1/2 = ln 2 / λ = 0.693 / λ

Therefore, rearranging terms, we get:

λ = ln 2 / t1/2 = 0.693 / t1/2 = 0.693 / (5730 x 365 x 24 x 60 x 60) sec = 3.836 x 10 -12 per second.

So the decay constant of Carbon-14 is 3.836 x 10-12 per second.

3. Cobalt-60 has a half life of 5.27 years. Find the average life time of each cobalt-60 atom.

Solution: The half life of Co-60 is given to be 5.27 years. To find the average lifetime (τ) of one cobalt-60 atom, the following formula must be used.

τ = 1 / λ

but we know that, λ = 0.693 / t1/2

Therefore the above equation becomes,

τ = t1/2 / 0.693 = (Half life of C0-60) / 0.693 = (5.72 x 365 x 24 x 60 x 60 sec)/ 0.693 = 8.25 years

Thus, the average life time of any cobalt-60 atom is 8.25 years.

4. The half-life of radium-226 is 1600 years. Suppose you have a 22mg sample. After how long will only 18mg of the sample remain?

Solution: λ = 0.693 / t1/2

1600=(0.693 x λ)λ=(0.693)/1600λ =~-0.000433216988

N=N0exp( -λt)18=22 x e-0.000433216988*t

t= ~463.210587512

t = 463.211 years