Download Sugárkémiai áttekintés Schiller Róbert

.
Heavy ions, γ-photons, fast electrons – all of them produce
low-energy secondary electrons in the medium
(γ: photoeffect, Compton-effect, pair production).
The good part of the electron energy is imparted to the
atoms/molecules of the medium via Coulomb interaction
Er, t  
e
r  vt
3
r  vt 
~
Fourier Er, t   Er,  
Optical approximation– as if the medium were illuminated
with white light.
Hence: energy absorption is connected with optical
absorption spectra.
Types, sources of radiation
α
210Po (E=5,3 MeV)
β
90Sr
(Emax=2,18 MeV, <E>=0,765 MeV)
Accelerators: van de Graaff, LINAC, etc.
γ
60Co
(E=1,1; 1,3 MeV, <E>=1,25 MeV)
Energy loss of a charged fast particle:
4 Eme
dE
e4 z 2 M
LET  

n e ln
2
dx 8 0 E me
MI n
Mz 2 ne
dE


dx
E
Bragg-curve:
Number of ions as a
function of depth
Bragg-Gray : γ dose, measured in a thimble chamber
D fal
Dgáz
 dE

 dx

 dE

 dx
1

  fal
1

  gáz
Ideal case – poliethylene thimble filled with ethylene
gas
Absolute dosimetry:
Dose D coming from a volume element of the source,
reaching a volume element of the irradiated space
e  r
Delementary  A 2
r
One must know the activity
of the source, then Delementary
must be integrated over source and
irradiated space.
r
Radolysis of water
This was written in 1907. Ramsay thought it was a question of total
energy.
Others compared water radiolysis with electrolysis
Does this type of reaction scheme explain more than it was
devised to?
Effect of LET on the yields of radiaton chemical processes .
Diffusion controlled kinetics
Heavy ion
Fast electron
trajectories observed in a cloud chamber
Radiolysis products are not distributed homogeneously –
hence diffusion, recombination and chemical reaction proceed
simultaneously and in interaction.
Radical diffusion theory
ci
 D 2 ci  k r c i2   k jici c j
t
j
Neutral radicals, hence no Coulomb interaction.
Number of equations = number of radical types.
Coupled equations
LET effect due to different initial geometries.
Highland Park, USA, 1951
Some suspicion: two types of „H-atoms”?
Pulse radiolysis (a sister of flash photolysis)
Accelerators, with short and shorter pulse lengths are needed.
A not-too-modern equipment: Pune (India) 2006
The first spectrun of the hydrated electron (Hart and Boag, 1962)
As it was expected.
The first kinetic trace of the hydrated electron (Keene, 1963)
Chemistry of the hydrated electron
- The ideal of the reducing agent: no oxidised product left
- the perfect nucleophyilic partner
- very selective, in certain cases diffusion controlled rates
- previously unknown products, e.g.Ag0, Cu0
A naive model
(polaron in a dielectric
medium)
Hydrated Electrons in Water Clusters: Inside or Outside, Cavity
or Noncavity? László Turi *
Department of Physical Chemistry, Eötvös Loránd University, P.O. Box 32, Budapest
112, H-1518 Hungary
J. Chem. Theory Comput., 2015, 11 (4), pp 1745–1755
Detour – in parenthesis
(Simultaneous diffusion and migration:
 c ( x, t )
c
 2c
 ux
D
t
 x
 x2
u x  E x
Markov-process Master equation Fokker-Planck equation:
 P ( y, t )

1 

{a1 ( y) P} 
{a2 ( y) P}
2
t
y
2 y
c = concentration ; P = probability. But, as we know…
a1  
a2  2
)
Is electron formation a particular wonder?
Onsager problem: simultaneous ion migration and diffusion –
with the result: recombination.
Steady state approximation
R
+
I out
q 2
Wescape 
e
I in  I ou
Ibe
e
Iki
kTR
e
If so, complete ion-electron recombination
would proceed.
Solvated electrons discovered in a good number of
polar liquids: alcohols, amines, ethers
Electron spectra:
Electron yields in a series of polar liquids: the effect of energy
fluctuations
Hydrated electron yield in supercooled water
Excess electrons in a number of non-polar liquids
Electron mobilities
Hel, Xel, n-hexane, neo-pentane χ[10-3, 103] cm2/Vs
Energy of the localized state: Et ;
Bottom of the conductivity band: V0
Et < V0  lokalizáció (buborék)
Localization probability, P, defined by energy fluctuation:
1
P
2
x
e

t 2 / 2
dt
x
V0  Et

 2  kT 2Cv
   f 1  P 
Radiaton chemistry of organic molecules
R-CH2-CH2-R’
R-CH2· ·CH2-R’
R-CH2· + R-CH2-CH2-R’RCH3 + R-C·H-CH2-R’
H· + R-CH2-CH2-R’ H2+ R-C·H-CH2-R’
Generally speaking: bond cleavage and bond formation.
Main product usually H2
The failure of the organic moderated reactors.
But: chemistry of the nuclear reactors!!
β irradiation of oxidized iron surfaces, after that electrode
Impedance in aquoeus SO32- solution (hole capture).
Equivalent circuit:
Result:
Faraday process gets
faster due to irradiation
γ irradiation of carbon steels (Daub (2011)
Irradiation in aqueous solutions at different pH values
Ecorr= -0.65 V
Ecorr= 0 V
SCE; pH 10,6
γ-Fe2O3 is formed upon
the irradiated surface.
Raman spectra 
for comparison
Hydrogen economy
Hydrogen from water.
Catalytic cycle:
2AB + 2H2O  2AH + 2BOH
2BOH  2B + ½O2 + H2O
2AH  2A + H2
2A + 2B  2AB
Low-temperature extotherm, and high-temperature endotherm
stages are desired.
High temperature is always needed!
Gas-cooled nuclear reactors?
For example: UT-3 Process
CaBr2 + H2O  CaO + 2HBr
CaO + Br2  CaBr2 + ½O2
Fe3O4 + 8HBr  3FeBr2 + 4H2O + Br2
3FeBr2 + 4H2O  Fe3O4 + 6HBr + H2
[700 0C]
[550 0C]
[250 0C]
[600 0C]
Another possibility:
3
3H 2 O  3Cl 2  6HCl  O 2
2
8000 C
1000 C
4000 C
18HCl  3Fe 2 O 3   6FeCl 2  9H 2 O
6000 C
6FeCl 2  8H 2 O  2Fe 3 O 4  12HCl  2H 2
1
4000 C
2Fe 3 O 4  O 2   3Fe 2 O 3
2