Radiogenic isotopes and dating methods k-Ar methods decay process ..

Radiogenic isotopes and dating methods k-Ar methods

1. RADIOGENIC ISOTOPES Submitted by – Niharika naidu     M.Sc. 1st Sem geochemistry

2. CONTENTS Introduction of isotopes Types of isotopes Radiogenic isotopes        1)alpha decay         2) beta decay         3) gamma decay Law of radioactive decay Half-life concept Decay system of K-Ar

3. WHAT IS ISOTOPES ? Isotopes are defined as atoms of same element that have the same atomic number (the same number of protons) but different mass number ( different number of neutrons)

4. TYPES OF ISOTOPES     Stable isotopes Stable isotopes are Atoms having stable nuclei They do not show radioactivity.  Half-life is very long or doesn’t have a half-life at all.  used for applications  Where radioactivity should not be present.     Unstable isotopes  Unstable isotopes are atom having unstable nuclei. They show radioactivity.  Half -life is short and can be calculated easily.  used for application where radioactivity is importance (Eg-in DNA analysis)

5. Unstable isotopes (Radiogenic isotopes)  Radiogenic isotopes that are produce by radioactive decay of unstable parent nuclides. Generally they have high number of Neutron compare to proton(N>>p) therefore neutron will breakdown in form of particles α β γ will emitted then in order to archive stability. A radiogenic isotope itself may be a radioactive.

6. Radioactive decay Radioactive decay is the Spontaneous process by which unstable atomic nuclei lose energy by emitting radiation. There are three common type of decay: Alpha decay Beta decay Gamma decay

7. Alpha decay  A large number of radio nuclides undergo decay by emission alpha particles from the nucleus  An alpha particles is composed of two neutrons and two protons (nucleus of He).  In alpha decay atomic number decrease by two and mass number decrease by 4

8.                                 Beta decay Beta decay is a radioactive decay in which beta particle is emitted from an atomic nucleus. There are two types of beta decay  BETA MINUS DECAY Radionuclides with an excess of neutron (eg.withN:Z>1) decay spontaneously by emitting from the nucleus a negatively charge beta particle or negatron . The negatron decay may be regarded as the transformation of a Neutron into a proton, an electron which is expelled from the nucleus as a B particle, and an antineutrino by reaction. In this atomic mass is decrease by 1 and atomic number is increase by 1.       BETA PLUS DECAY Radionuclides that are deficient in neutron (e.g. with N:Z<1) decay occur by emission from the nucleus of a positively charged beta particle or positron. The positron decay may be regarded as the transformation of a proton into a neutron that Is accompanied by the emission of a positron and neutrino according to the reaction. In this atomic mass is increase by 1and atomic number is decrease by 1.

9. Gamma decay Gamma decay is a type of radioactive decay in which an unstable atomic nucleus emits a gamma rays . Gamma rays a High energy Photon. Unlike alpha and beta decay, Gamma decay does not change at atomic number or mass number of the nucleus.it only affects the energy level of the nucleus.

10. Electron capture  Electron capture is a mode of beta decay in which an electron – commonly from an inner (low energy) orbital is capture by the atomic nucleus. The capture electron combines with a proton, converting the proton into a neutron. Also called k-capture because the capture usually comes from the atom k-shall.

11. Law of radioactive  According to the theory of radioactive decay ,the rate of decay of an unstable parent atom in a sample at any time is proportional to the number of parent atom existing at that time mathematically ,this can be expressed as. When there is large number of nuclei,                                         Rate of decay   ∝ number of nuclei in sample Rate of decay =number of nuclei decay/time N nuclei present DN nuclei decay  Let in time 'dt'

12. Dating equation

13. Concept of Half-life (t1/2 ) A time after which the concentration of parent nuclei became half of its original concentration. Rate of decay ∝   the number of parent atoms remaining in the sample at any time, Therefore, constant proportional of the remaining parent atoms decay in a fixed time period, Half of the unstable atoms in a sample thus decay during a specific time interval that is some functions of  λ   Half-life : the time required for half of the unstable atoms to decay, Initial number of parent atoms:N After one half life:1/2N After two  half-lives :1/4N So on......

14. Potassium –Argon system   The K-Ar dating method is especially useful for determining the age of lava. Potassium is a common element found in many materials ,such as Feldspar, mica or k is a major element and mostly confined to the upper continental crust. Calcium is a common element in continental crust so it becomes a sense of ambiquity in order to differentiate radiogenic and non-radiogenic calcium.

15. Therefore argon40 is selected for radiometric method because it is inert,so it does not chemically react with other elements, since argon is usually a gas it can escape rocks when they are extremely hot. so all the argon originally in the rock is thought to have escaped before the rock hardens. the argon in the rock today, is that which comes from K40. Ar can only trapped by many mineral phase he blocking temperature  helps to reset the clock esily  . Blocking temperature –temperature at which  40Ar will trap into solid phase is know as blocking temperature.

16. Decay - unlikely to other isotropic system it involve two phase of decay. 1)electron capture  2)beta decay

17. Advantages:- k-Ar method is suitable to date the minerals for example biotite, hornblende , nepheline ,muscovite and no suitable for orthoclase and microcline. It is use to date magmatic events and age of the magma . this method can be utilised to determine the age of rocks of Mesozoic, Paleozoic ,Archean . References:- Kula c. Mishra" introduction to geochemistry principles and applications". e-PGPathshala google