05 - Radioactive properties Flashcards
Z
A
Element
Z
number of protons
= atomic number
N
number of neutrons
A
= protons + neutrons
= mass number
isotope
same proton number
various neutron number
various number of mass
beta- decay
neutron to
–> Proton, Negraton and antineutrino
daughter: Z+1, N-1, Z+1+N-1=A
e. g. 87Rb -> 87Sr
beta+ decay
positron decay
Proton to
–> Neutron, Positron, Neutrino
daughter: Z-1, N+1, Z-1+N+1 =A
e. g. 18F -> 18O
e- capture decay
Electron -> split proton
–> Neutron & Neutrino
daughter: Z-1, N+1, Z-1+N+1
e. g. 40K -> 40Ar
alpha decay
2 Protons + 2 Neutrons
- -> charge 2+
- -> 4He-nucleus
daughter: Z-2, N-2, Z-2+N-2=A-4
e. g. 238U -> 234Th
Decay chains
U and Th isotops can’t reach stable via a single decay
!!
Dacay of unstable isotopes
!!
(formula)
N = N0 * e^( -lambda * t )
N = numer of remaining mother isotopes at time t
N0 = number of mother isotopes at time t=0
lambda = isotope specific decay constant
half life time
for daugther isotopes
D* = N0 * /1-e^(-lambda*t)
t = 1/lambda * ln( D* / N + 1 )
depth of penetration
of
radiation
extremely small
for apha & beta particles
thats why gamma ray is used
which radioactive decays produse measureable gamma rays?
uranium-radium series
(4.4 * 10^9 years half life)
Thorium series (1.4 * 10^9 years half life)
Potassium K40 (1.3 * 10^9 years half life)
characteristic peak
Potassium
1.46 MeV
characteristic peak
Thorium
2.62 MeV
characteristic peak
Uranium
1.76 MeV
major occurence
Potassium
micas
feldspars (K)
micaceous clays (illite)
radioactive evaporates
major occurence
Thorium
shales
heavy minerals
major occurence
Uranium
Phoshates
organic matter
Potassium
sources
- clay minerals: illite high, Kaolinite low
- rock-forming minerals: feldspars, mica
- evaporates: sylvite
Thorium
sources
- acid & intermediate rocks
- stable (ddon’t go in solution)
- with detrital sediments, never with pure chemical sediments (carbonates,aragonite)
- clay indicator
- in marine element
Uranium
sources
- acid & intermediate rocks
- result of weathering & alteration
- unstable salt
- in detrial & chemical sediments
- reducing,anoxic
K U Th in
Carbonates
all low
oxidizing env.
K U Th in
Stylolites, Phosphates
K, Th low
U high
reducing env.
K U Th in
Clay, marl
K, Th high
U low
K U Th in
Algal, glauconite
K (U) high
Th low
K U Th in
Igneous rocks
K, Th, U
increasing from mafic (basic) to felsic ( acid) rocks
K U Th in
sedimentary rocks
K, Th, U
increase with clay content
K U Th in
Carbonates
Th, K near 0, when pure
K,(U) algal origin or glauconite
U = 0 -> oxidizing env.
U variable reducing env or stylolithes
Th,K,U -> clay content - marl
Gammalog
lithological profiling
- > sediments: clean sand,carbonates and shaly and fining/coarening upwards
- > correlations
Gamma Ray Index
I_GR
determine clay content
I_GR = ( GR -GR(noshale) ) / ( GR(shale) - GR(noshale) )
I_GR = 0 =clean rocks I_GR = 1 = shale
Th/K ratio indicates?
various clay minerals
Th/U ratio indicates?
organic carbon content
> 7 continental oxidizing
<7 marine, grey-green shales
<2 black shale, phosphate
Radioactive Heat Generation
Mean heat flow at the surface: 65 mW/m²
Mean heat flow between mantle and continental crust: 20 mW/m²
Difference due to radioactive heat generation: 45 mW/m²
Total heat production
Heat generation A
A = 0.01 rho (9.52 U + 2.56 Th + 3.48 K)
