Radiometric methods involve detecting gamma rays from geological environments through various approaches, relying on natural radioactivity first discovered by Henri Becquerel. Key elements examined include uranium, thorium, and potassium, which are used for dating rocks and mineral prospecting due to their natural occurrence and properties. The document also outlines the types of instruments used for detection and the importance of calibration for accurate radiometric surveys.

1. Radiometric Methods

2. Radiometric Method
• Radiometric method is based on the detection
of gamma rays from geological environments
using airborne, car borne and ground
methods.
• The method is based on natural radioactivity
that was discovered in 1896 by Henri
Becquerel when studying uranium sulphate.

3. Natural radioactivity
• Radioactivity is the process of decay or change in
atomic nucleus that results in change of charge or
its state of mass or both.
• There are three products of radioactive decay:
alpha rays, beta rays and gamma rays. The first
two are not really rays but are particles as they
have both mass and electrical charge.
• Gamma rays are electromagnetic radiations that
are produced as a product during loss of alpha or
beta particle.

4. Radioactivity
• Radioactivity is a spontaneous disintegration
of some naturally occurring elements into
stable elements. The nuclei of an element X
with atomic number z are made up of protons
and neutrons. The total number of protons
and neutrons is the mass number A.
• Atoms with atomic number greater than 82
are unstable and tend to disintegrate into
small atoms.

5. isotopes
• Atoms having the same atomic number but
different mass numbers are called isotopes.
Elements having atomic number (Z >83) are
very unstable.
• Such atoms will disintegrate producing -
particles, -particles -ray radiation.
• During the process heat is also generated.

6. Alpha and Beta Particles
• The particles are ordinary electrons with
negative charge.
• Alpha particles are helium nuclei consisting of
two protons and two neutrons.
• Once a helium particle is ejected from the
parent nucleus it acquire orbital electrons
from the environment and becomes a neutral
helium atom.

7. Radiometric Decay

8. Uranium 238

9. Gamma Rays
• Gamma radiations have their origin in a
nuclear decay process. These are EM waves of
extremely short wavelength.
• Gamma rays have no electrical charge neither
mass and therefore are capable of penetrating
through earth materials.

10. Characteristics of gamma rays
• Affect photographic emulsions.
• Ionise gas making it electrically conducting.
• Produce scintillation or phosphorescence in
certain minerals and chemical compounds.
• These properties are used in their detections.

11. Radioactive equilibrium
• Parent atoms decay into
daughter atoms in which
also decay into other
atoms until equilibrium is
reached. At equilibrium
the number of daughter
atoms decaying per unit
time is the same as the
number being created by
disintegration of the
parent atoms.
• 1N1 =2N2=3N3
• Where  is the decay
constant and N is number
of atoms.

12. Occurrence of Radioactive elements
• Over 20 natural occurring elements have been
reported, however uranium, thorium,
potassium and rubidium are the only
important elements in geology.
• These elements are used in dating of rocks
and others have applications in prospecting
for minerals, groundwater and hydrocarbon
resources.

13. Importance of others
• The reason why other elements are not useful
is that they occur in minute quantities in the
earth’s crust.

14. Potassium Minerals
• Mineral
• (i) Orthoclase and microcline feldspars [KAlSi3O8]
• (ii) Muscovite [H2KAl(SiO4)3]
• (iii) Alunite [K2Al6(OH)12SiO4]
• (iv) Sylvite, carnallite [KCl, MgCl2.6H2O]
• Occurrence
• (i) Main constituents in acid igneous rocks and pegmatites
• (ii) Main constituents in acid igneous rocks and pegmatites
• (iii) Alteration in acid volcanics
• (iv) Saline deposits in sediments

15. Thorium Minerals
(i) Monazite [ThO2 + rare earth phosphate]
(ii) Thorianite [(Th,U)O2]
(iii) Thorite, uranothorite [ThSiO4 + U]
• Occurrence
• (i) Granites, pegmatites, gneiss
• (ii), (iii) Granites, pegmatites, placers

16. Uranium Minerals
• (i) Uraninite [oxide of U, Pb, Ra +Th, rare earths]
• (ii) Carnotite [K2O.2UO3.V2O5.2H2O]
• (iii) Gummite [uraninite alteration]
• Occurrence
• (i) Granites, pegmatites and with vein deposits of
Ag, Pb, Cu, etc.
• (ii) Sandstones
• (iii) Associated with uraninite

17. summary

18. Process of Radioactive decay
• The rate of radioactive
decay is given by:
• Where  is the decay
constant and N is
number of atoms
present.
• Integrating both sides
we arrive at
• For half life this can be
written in the form of
N
N e T
0
1
2
2
  
N N e T
 
0

dN
t
N


 
dN
N
t
N
N T
 
 
 
0 0
Ln N T
( )  

19. Radioactive decay
• This reduces to by:
• The time T can be
determined.
• If you know the decay
constant then one may
find the time taken for
the number of atoms to
decay to a value in the
rocks and hence you
can determine the age.
  
Ln
T T
( ) .
2 0 693
2 2

20. Instrumentation
• The earliest instruments of
detecting natural radiations
was the Geiger Mueller
Counter. The GM works on
principle of ionization that a
particle enters the tube
with argon gas it pulls an
electron from the argon
atom that is then attracted
to central wire put +400V .
On its way it knocks more
electrons that are detected
at once.
• GM is efficient for detecting
alpha particles although
versions for detecting
gamma and beta particles
also exist.

21. Geiger Mueller Counter

22. Scintillation Meters
• Some substances emit light when hit by
radioactive particles such as gamma rays. ( ZnS,
anthracene, stilbene, scheelite). NaI crystals
when treated with thallium is a activated in
causing scintillation.
• Most of the gamma ray detectors are made from
NaI crystals. The light is emitted as photons and
the intensity of light emitted is proportional to
the energy of the gamma ray absorbed.

23. Gamma Ray Spectrometers
• Gamma ray spectrometers are scintillometers that are
designed to distinguish the gamma rays according to
their energy levels.
• Gamma rays from the three natural radioactive
elements have different energy levels and hence can
be characterized.
• The intensity of light emitted induces a voltage whose
amplitude is proportional to the original -ray energy.
This can occur when the -ray loses all its energy at
once by photo conversion.
• Light is received by a photo multiplier Tube, converting
light into a voltage

24. GRS

25. GRS
• GRS consists of the detector and Console. The
detectors are ranked according to crystal size.
The larger the crystal size the better is the
detector.

26. Energy spectrum of natural radiation
• Considering the energy
spectrum it can be
shown that the
spectrum has energy
peaks at 1.46 MeV, 1.76
MeV, and 2.62 MeV.
• Using these peaks one
can distinguish gamma
rays from K, U and Th.

27. Energy Windows
• Channel 1: Total Count 0.4 - 3.00 MeV
• Channel 2: Potassium 1.36 – 1.56 MeV
• Channel 3: Uranium 1.66 – 1.86 MeV
• Channel 4: Thorium 2.42 - 2.82 MeV

28. Converting counts into element
Content
• Gamma Ray Spectrometers may measure
elements in form of counts per unit time or
counts may be converted into element
content if the GRS is calibrated.
• If GRS is calibrated the gamma counts are
converted in element content ( U and Th in
ppm whereas K is given in %).
• Any GRS can be calibrated if you have samples
with known concentrations of U, Th and K.

29. element
• Thorium (all the gamma
rays from Th channel are
originating only from Th)
• Th(pp)= k1Tc
• Uranium Channel records
gamma rays from Th and
• U(ppm)= k2(Uc-S1Tc)
where k1 is constant for
Thorium Channel, and k2
is constant for Uranium
channel and S1 is stripping
ratio.

30. element
• otassium channel records
gamma rays from both Th and
Uranium Channel
• K% = k3(Kc-S2(Uc-S1Tc)-S3Tc)
where k3 is constant for
Potassium Channel, and S2
and S3 are stripping ratio for U
and Th gamma rays recorded
in the Potassium channel. Thus
if the six constants are known
then one can convert gamma
ray counts into element
content.

31. Radiometric Surveys
• Radiometric surveys
may be conducted by
airborne sampling rate
is 60-70 m.
• Background radiations
are obtained by flying
over a body of water or
orienting crystals
upward.
• Radiations are affected
by weather and in
particular rain. A wet
soil traps some of the
radiations in the soil.
Radiometric surveys
should be conducted
during the dry season
to have uniform soil
conditions.

32. continued
• Other factors affecting
readings are
topography. Over the
hills less readings are
counted as compared in
valleys because of the
circle of influence.
• Most spectrometers are
calibrated assuming a
flat surface.
• Any departure from
that geometry affects
the reading positively or
negatively.

33. Data Presentation
• Results of radiometric surveys may be
represented as maps for each element, or
profiles, or composite map in which response
of all 3 elements are represented using the
three primary colours (RGB), Red for K, Green
for Thorium and Blue for Uranium.
• Enhancement of anomalies by calculating
element ratios ie U/Th, U:K.

38. Applications
• The radiometric methods are applied in
mineral exploration of radioactive minerals:
• Uranium and Thorium deposits.
• Deposits associated with uranium minerals
such as phosphates and carbonatites.
• Gold deposits associated with hydrothermal
alterations.
• Placer deposits ( monazite).