Dr. Kamal K. AliUniversity of Baghdad

College of science Geology 2nd year

Radioactivity & Isotopes geology

Lecture 1 • Atom stracture.• Nuclides• Elements , isobars, isotones, isotopes

Matter is made up of very small particles called atoms • Each atom has a very small and very dense core called nucleus. • Most of the mass of atom is contained in the nucleus

• A proton has a unit positive charge. • A neutron is an uncharged particle.• An electron has a negative charge.

Atom

Nucleus structure

Z electrons

Z protons

N neutrons

A mass number (nucleons) = Z + N

Nucleus

XA

Z

• The electrons move in orbits around the nucleus. • There are a lot of empty spaces within atom • A nucleus consists of a number of protons and neutrons. • Protons and neutrons also known as mass number (nucleons).

100m500m

1cm

Dimentions and Distance

Diameter of Nucleus is range of 1.75 fm- 15 fmfm=10-15m

Masses

…then the mass of the nucleus is

108 Tonnes

Suppose the mass of the electron1 g…

Neutron slightly heavier than protonMass n=1.67495x10-27 KgMass p= 1.67265x10-27 kgMass p> mass e about 1836 times

What is nuclide?

• A nuclide is an atom of a particular structure. Each element has nucleus with a specific number of protons.

• Nuclide notation • Example 6C ,

• Proton 1p , Neutron n , electron e

XA

Z12

1 1

0 -10

Elements- Isotopes-IsobarNumber of p (Z) = atomic number. It

determines the chemical element.Ex. Z=92 is Uranium, Z=90 is Thorium Z= 86 Radium Z=27 CobaltNumber of Neutron (N)Mass number OR nucleon number(A)=Z+NNumber of electron=number of proton(It

impacts chemical reaction)

ISOTOPES• Thommson,1914, Aston, 1919:(There are different mass number(A) for a

given (Z) for an element. That is mean they have different (N)……..THEY ARE ISOTOPES.

Isotopes: Greek phrase : isos topos meaning :“The same place” isotopes of the same

elements occupy the same position in periodic table.

I s o t o p e s

Isotopes • Isotopes have the same mass

number(Protons)(Z) and different number of neutrons(N).• So # of neutrons determine the

isotope of the element • Ex: Z=92 : N=146 238U ; N=143 235U Z=27 : N=32 59Co ; N=33 60Co

Oxygen (3 isotopes)

Nucleon number = 16Atomic number = 8

Nucleon number = 16Atomic number = 8

Nucleon number = 17Atomic number = 8

Nucleon number = 17Atomic number = 8

Nucleon number = 18Atomic number = 8

Nucleon number = 18Atomic number = 8

17

Hydrogen (3 isotopes)

Nucleon number = 2Atomic number = 1

Nucleon number = 2Atomic number = 1

Nucleon number = 3Atomic number = 1

Nucleon number = 3Atomic number = 1

Nucleon number =1Atomic number = 1

Nucleon number =1Atomic number = 1

Deuterium

Tritium

Isotopes• Isotopes either stable or

unstable(radioactive)• 59Co : stable isotope;• 60Co: radioactive(radioisotope)

IsobarNuclides with same mass(nucleon)

numbers(A)but with slightly different # of protons(Z).• Ex: Rubidium 87Rb&Strontium87Sr.

• Carbon -12 6C and Boron-12 5B

383712 12

• It is important to know of isobars because unless they are separated chemically beforehand they interfere with one another when isotope abundance are measured with spectrometer.

Isobar

Isotones• Atoms have the same number of neutrons(N)

but different proton number(Z). For example:• Boron-12 and carbon-13…….HOW?• S-36, Cl-37, Ar-38, K-39 and Ca-40 nuclei all

contain 20 neutrons …….• What is the difference between isobars and

isotones C-12, B-12…..C-13 , B-12

Exercises

comments charge Location 3 basic particles

proton

neutron

Electron

ExercisesNo. of

(N)No. of

(A)Number of

(Z) Term

isotopes

isobars

isotones

Questions 1. A carbon-12 atom and a carbon-13 atom are

called…………………

2. Isotopes are atoms of the same element with

different numbers of ……………………

3. Isotopes has similar …………… properties and

different ……………. properties.

4. Oxygen-18 has …… electrons, 8 protons and

………….. neutrons.

5. An element P has a nucleon number of 31 and

proton number of 15. Therefore, the number of

electrons is ………… and the number of neutrons is

………

6. Element Q has two isotopes. If Q-127 has 53

protons, how many protons does Q-131 have ?

7. Chlorine-35 and chlorine-37 are isotopes. There

are 20 neutrons in Chlorine-37. How many protons

are there in Chlorine-35 ?

8. A neutral atom of element G has 26 protons and

28 neutrons. The nucleon number for element G is

………………….

Exercises

Isotopes Types• Isotopes either Stable or

Unstable(Radioactive)• 59Co,1H, 2H, 16O,18O : Stable

Isotopes;• 60Co, 3H,: Radioactive

(Radioisotope)

Radioactivity• Radioactivity was discovered and studied by Henri

Becquerel and then Pierre and Marie Curie from 1896 to 1902.

• Radioactivity is the phenomenon by which certain nuclei transform (transmute) spontaneously into other nuclei and give off particles or radiation to satisfy the laws of conservation of energy and mass.

• Proton is made of pieces called quarks(2up, 1down).• When proton breaks apart it produces high energitic

pieces.• So radioactivity is explosion of the nucleus of an

atom.

Radioactivity and Radiation• When we have a radioactive nucleus exploding,

the pieces is going flying out with energy a typically have 1 MeV. And some with 2MeV., WHILE IN CHEMICAL REACTION of hydrogen and oxygen the energy product about 1 eV.

• This the dangerous of radioactivity and this is the advantage of radioactivity.

• The pieces come out when a nucleus explode we called them radiation.(radioactivity=explosion, Radiation=pieces come out).

Lecture 3: Mechanism of radioactive decay and types of radiation

• Radioactive decay is a nuclear process and is independent of chemical and physical states of nuclide. It depends on the neutron/proton ratio and on the mass-energy relationship of the parent, daughter and emitted particles.

• For the nuclides of low atomic mass, the greatest stability is achieved when the number of neutrons and protons are approximately equal(N=Z), but as atomic mass increase, the neutron/proton ratio increase until N/Z=1.5

Lecture 2: Radioactivity and StabilityElements for which Z is an even number have far more isotopes than elements for which Z is an odd number.Fluorine (Z=9), sodium (Z=11), phosphorus (Z=15), and scandium(Z=21) have just a single isotope.

The distribution of natural stable isotopes in the neutron–proton diagram. After N=20, the zone of stable nuclei moves away from the diagonal for which the number of neutrons equals the number of protons. For N>20, the number of neutrons then exceeds the number of protons. This zone is called the valley of stability as it corresponds to a minimum energy level of the nuclides.

Mechanism of radioactive ,Type of Radiation1-Alpha particle (a helium nucleus 4

2He)

• For the nuclides of high atomic mass such as U-238 are unstable. U-238 have 146 neutrons and 92 protons. So its nucleus emits Alpha particles (α) to reach the stability. Alpha particle is (two protons and two neutrons) positive charge(+2). In this case number of protons and neutrons in the nucleus reduces by two for each. So it becomes 90 proton and 144 neutron and the new nuclide is Thorium-234. we can express it as follow:

U α + Th 2:146 2:92

23892

42

23490

Alpha particles

• The particles have large kinetic energies but are rapidly slowed down by collisions with other atomic nuclei. At thermal energies they soon gain two orbital electrons and become indistinguishable from other helium atoms.

• The average distance travelled in solid rock before this occurs is measured in fractions of a millimeter.

Beta-minus particle• Alpha emission produces nucleus rich in neutrons and

deficient in protons and still unstable. So a neutron disintegrates into a proton and an electron. The electron expelled from the nucleus as a negative particles called Beta particle(β-), They differ from other electrons only in having higher kinetic energies. To satisfy the law of conservation of energy and mass, it is assumed that the nucleus emits energy particles has no mass nor charge called an antineutrino along with the electron. The decay equation is written :

•1n 1

1 p + β- + v

• neutron -------------- proton + electron + antineutrino.• So in Th-234 it decays to Pa-234 by emitting β- as follow:

234 Th 234 Pa + β- + v

0

9190

Gamma ray

• In many cases the nuclide produced by beta minus decay is left in an exited state, which subsequently decay to the ground state nuclide by release of energy. This may be lost as gamma (γ) ray(electromagnetic ray) of discrete energy.

A

zP* A

zP + e- (mission of photon)

• The metastable state or “ Isomers” of the product nuclide are denoted by super fix ‘m’

Beta-plus and electron capture

• Nuclides deficient in neutrons e.g. K-38 may decay by two different processes:

positron emissionElectron capture. Both processes yield a product nuclide that

isobar of the parent, by transformation of a proton into neutron

1

1 p 1

0n + β+ + v

proton neutron + β+ + neutrino

Electron capture

• A nuclear proton is transform into a neutron by capture of an orbital electron, usually from the inner shells.

1

1 p + e- 1

0n + β+ + v

Outer orbital electron falls into the vacancy produced by electron capture, emitting a characteristic X-ray. The product nucleus may be left in exited state, in which case it decays to the ground state by gamma emission (γ).

Gamma rays and X- rays are both electromagnetic rays , they are differ where they are produced.

Isotopes measurementThe Mass Spectrometer (MS)

• “There would be no isotope geology without MS”.• The principle of MS. Atoms of the chemical element whose isotopic

composition is to be measured are ionized in a vacuum chamber.

The ions produced are accelerated by using a potential difference of 3-20 kV.

This produces a stream of ions , and so an electric current.

This electric current is passed through a magnetic field.

The magnetic field exert a force perpendicular to the electric current and so bends the beam of ions.

The lighter ions are deflected more than the heavier ones and so the ions can be sorted according to their mass.

So, the relative abundance of each isotope can be measured from the relative values of the electron currents produced by each stream of ions separated out by this way.

Isotopes measurementThe Mass Spectrometer (MS)

The Mass Spectrometer (MS)

1. The source: To generate ions from atoms.To accelerate the ion by potential

differences.To shape the beam, through

calibrated slits in the high voltage plates.

2. Magnet: It deviates the ions and this deflection separates them by mass.

3. Collectors: collect and integrate the ion charges so generating an electric current. all of which are maintained under vacuum.

Short Quiz

• Decide which of these nuclei are stable or unstable:

1. 235U 2. Ra 3. C 4. Mg

5. I 6. Ne

92

223

8812

6

24

12

10

2212853

Isotopes measurementThe Mass Spectrometer (MS)

• Suppose atoms of the element in question have been ionized. The ion acceleration is:

eV =1/2mν 2 eV : electric energy

e:ion charge 1/2mν 2 : kinetic energym: mass of the ionν: its speed…………….then

ν = (2eV/m)1/2

The Mass Spectrometer (MS)• Magnetic deflection is given by equating the

magnetic force Beν to centripetal acceleration (ν2/R)multiplied by mass m, where B is the magnetic field and R the radius of curvature of the deflected path:

Beν=(ν2/R)m……where B=magnetic fieldNote: when a charge particle moves through a

magnetic field: Magnetic force F= magetic field(B) X electical charge(e) X its

velocity(ν)………THEN: νBeR= ν2m

The Mass Spectrometer (MS)ν= BeR/m ….. ThenBeR/m= (2eV/m)1/2

B2 e2 R2/m2= 2eV/m …. B2 e2 R2m= m22eVm/e= B2R2/2V In which B in tesla, R in meters, m in atomic

mass, and V in Volts.Atomic mass unit m= 1.6605402 X10-27 kgElectron charge e = 1.60219 X10-19 coulombs

Exercises

• A mass spectrometer has a radius of 0.3m and an acceleration voltage of 10 000 V. The magnetic field is adjusted to the various masses to be measured. Calculate the atomic mass corresponding to a field of 0.5 T.

• Answer: Just apply the formula with suitable units:

• m= (B2R2/20721V) X 1012 = (0.5)2x(0.3)2x1012/20721x10000=108.58

Exercises

• If hydrogen ions (mass number=1) are accelerated with a voltage of 10 kV, at what speed are they emitted from the source?

• Answer Just apply the formula : ν = (2eV/m)1/2

ν= (1.9272x1012)1/2 = 1388 km/s

Atomic mass unit m= 1.6605402 X10-27 kgElectron charge e = 1.60219 X10-19 coulombs

Ionization techniques

1. Thermal-ionization mass spectrometry.

2. Electronic bombardment.3. Inductively coupled plasma mass

spectrometry(ICP-MS).4. Ionic bombardment in secondary

ion mass spectrometry(SIMS)

Short Quiz

• Two of the isotopes of the element whose atomic number is 10 have mass numbers of 20 and 22.

• Write the symbol of each isotope.– How many protons does each of the isotopes

have?– How many neutrons does the isotope with mass

number 20 have? How about for the other one?

R = Abundance of Heavy isotope (rare)/ abundance of light isotope

standardsample

standard

Tritium 3H• Tritium 3H values reported as

absolute concentration called Tritium Unit (TU), where on TU corresponded to I tritium atom per 108 hydrogen atoms. Tritium values may also be expressed in terms of activity (pCi/l), where

1TU= 3.2 pCi= 7.2 dpm/l