Radiobiology2

Prof.Dr.Tarek Elnimr

L 2Presented to the

Biology Departments in Faculty of

Scienceson February 15 ,

2009

2

It is found that a few naturally occurring substances consist of atoms which are unstable.-that is they undergo spontaneous transformation Into more stable product-Such substances are said to be radioactive- and the transformation process is known as radioactive decay.- Radioactive decay is usually accompanied by the emission of charged particles and gamma rays. As a result of that transformation process, these unstable nuclei emit radiations of three main types, called alpha, beta and gamma radiation.

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FissionThe nucleus is divided into two parts, fission fragments. and3-4 neutrons. Examples: Cf-252 (spontaneous), U-235 (induced)

-decayThe nucleus emits an -particle (He-4). Examples: Ra-226, Rn-222

-decayToo many neutrons results in -decay. n=>p++e-+. Example:H-3, C-14, I-131.Too many protons results in -decayp+=>n+ e++ Examples: O-16, F-18 or electron capture (EC). p+ + e-=>n+ Examples: I-125, Tl-201

86226

84222

24Ra Rn+

Part 2: Radiation Physics 4

If there are initially N atoms, the initial rate of decay will be N/t

This change in the decay rate can be expressed as - dN/dt , being negative as it is decreasing

With time, whereas,- dN/dt = N ........................(1)This equation can be rearranged- dN/Ndt = N/N , - dN. dt / N. dt = N . dt/N - - dN / N = dt , N = No exp(- t) , where- No is the number of nuclei present initially- N is the number of nuclei present at time t

- is the radioactive decay constant

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It is impossible to know at what time a certain radioactive nucleuswill decay. It is, however possible to determine the probability l of decay in a certain time. In a sample of N nuclei the number of decays per unit time is then:

2lnT

eN=N(t)

Ndt

dN

2/1

t-0

Nuclear Activity

Radioactive decay is described by N(t), N0: number of radionuclide at

time t = 0 and t, resp. : decay constant [1/t]

Activity A = average decay rate [decays per second]

Nuclear activity is measured in curie: 1 [Ci] = 3.7 1010 decays/sec(orig.: activity of 1 g of 226Ra)

Practical: 1 mCi, Ci. SI unit is becquerel [Bq] = 1 decay/second

0( ) tN t N e

1/ 2

0.693T

99mTc

0tdN t

A t N t A t A edt


A CBλ1

λ2

)ee(A

B(t)

eA=A(t)

tt

12

20

t1-0

21


Secular equilibriumTB<<TA ≈ ∞

Transient equilibriumTA ≈ 10 TB

No equilibriumTA ≈ 1/10 TB

99Mo-99mTc


99Mo87.6% 99mTc

140 keVT½ = 6.02 h

99Tc

ß- 292 keVT½ = 2*105 y

99Ru stable

12.4%

ß- 442 keV 739 keVT½ = 2.75 d

Alpha Alpha Decay Decay

44He Nucleus Ejected He Nucleus Ejected from from 222222Rn NucleusRn Nucleus

+2+2

4He + He + 218PoPo

+

+

+

+

+

+

+

+

+

Radon - 222Radon - 222

Alpha Decay...Alpha decay is a common radioactive process

encountered with heavier isotopes. The alpha particle is a helium nucleus having a mass of 4 and a charge of +2. Isotopes with mass numbers less than about 150 (Z 60) seldom yield alpha particles. Alpha particles progressively lose their energy as a result of collisions as they pass through matter and are ultimately converted into helium atoms through capture of two electrons from their surroundings.

Radiation

Radiation

• Occurs spontaneously

• Due to change in # of protons, atom becomes -2 charge

• Radiation released

Alpha Radiation ()• Particle released when the nucleus kicks out 2 neutrons and 2 protons

• Relatively massive

• Relatively slow

• Total charge of +2

Mass number changes by 4 and atomic number changes by 2

Beta Radiation ()• Particle released when the nucleus changes a neutron into a proton and a beta particle

• Relatively small mass

• Relatively fast moving

• Total charge of -1

Atomic Mass Number remains constant

PN

Gamma Radiation ()• Pure energy. Released from the nucleus when an alpha or a beta is emitted

• No mass

• Speed of light

• No charge

NO CHANGE

Beta Decay...Beta decay is a radioactive process in which, the atomic number changes but the mass number stays the sameThere are types of decay are encountered: negatron formationpositron formationelectron capture

Beta Radioactivity Beta particles are just electrons from the

nucleus, the term "beta particle" being an historical term used in the early description of radioactivity. The high-energy electrons have greater range of penetration than alpha particles, but still much less than gamma rays. The radiation hazard from betas is greatest if they are ingested

Beta RadioactivityThe emission of the

electron's antiparticle, the positron, is also called beta decay. Beta decay can be seen as the decay of one of the neutrons to a proton via the weak interaction. The use of a weak interaction Feynman diagram can clarify the process

Gamma Decay...Gamma rays are produced by nuclear

relaxations. Gamma-ray emission is the result of a nucleus in an excited state returning to the ground state in one or more quantized steps with the release of monoenergetic gamma rays. Gamma rays, except for their source, are indistinguishable from X-rays of the same energy.

X-Ray Emission...X-Ray emission are formed from electronic

transitions in which outer electrons fill the vacancies created by the nuclear process. One of the processes is electron capture. A second process which may lead to X-rays is internal conversion, a type of nuclear process that is an alternative to gamma-ray emission.

Radioactive Half Life...It is time taken for the radioactive substance to reduce to half its activity. Mathematically it is given by:

t1/2 = ln 2/Decay Constant

Radioactive Half LifeRadioactivity is measured by means of a

detector that produces a pulse of electricity for each atom undergoing decay

Quantitative information about decay rates is obtained by counting these pulses for a specific period

table with decay data obtained by successive one-minute counts is shown on next slide…

Minutes

Counts

Minutes

Counts

1 180 7 168

2 187 8 170

3 166 9 173

4 173 10 132

5 170 11 154

6 164 12 167Total counts = 2004Average counts/min = 167


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Radiobiology2