Clay Minerals (1981) 16, 69-80.
FURTHER STUDIES OF A FERROUS IRON DOPED
S Y N T H E T I C K A O L I N : D O S I M E T R Y OF
X-RAY INDUCED DEFECTS
A. H. CUTTLER
Department of Environmental Sciences, Plymouth Polytechnic, Drake Circus, Plymouth, Devon PL4 8AA
(Received l7 December 1979; revised26 June 1980)
A B ST R A C T: The structure of kaolin has been examined together with aspects of dosimetry and
energy loss mechanisms of radiation to explain the formation ofg = 2 EPR centres. The analysis
points to the formation of a trapped hole on the 'inner layer' oxygen atoms of kaolin located at the
boundaries between divalent ion and trivalent ion 'cells', in particular at the boundaries with
excess negative charge. Direct interaction of X-rays with atoms and the possibility of proton
recoil are eliminated. The means of production appears to be by transfer of charge following
ionization of atoms by secondary electrons, with transfer of vacancies ultimately to the oxygen
ions. Mechanisms which result in a decrease in signal strength with increase in concentration are
examined. It is concluded that the cell mechanisms discussed are consistent with the rates of
production and that at 20 Mrad (air) the number of centres should be reaching saturation.
Studies b y A n g e l & Hall (1972), M e a d s & M a l d e n (1975) a n d H e r b i l l o n et al. (1976) o f
n a t u r a l k a o l i n s have revealed a stable defect centre which gives an electron p a r a m a g n e t i c
r e s o n a n c e ( E P R ) signal at g = 2. Studies b y A n g e l et al. (1974), Jones et al. (1974) a n d
A n g e l et al. (1977) o f synthetic k a o l i n s d o p e d with d i v a l e n t ions have s h o w n t h a t it is
possible to p r o d u c e a similar centre b y X - i r r a d i a t i o n . This c a n be o b s e r v e d after a n n e a l i n g
at 473 K. It was f o u n d t h a t similar results c o u l d n o t be o b t a i n e d using ultra-violet
r a d i a t i o n b u t n o e x p l a n a t i o n was given. F u r t h e r studies b y M 6 s s b a u e r s p e c t r o s c o p y have
s h o w n t h a t the i r o n substitutes t r i o c t a h e d r a l l y in place o f a l u m i n i u m (Cuttler, 1980). In
the p r o p o s e d m o d e l C u t t l e r suggested t h a t the g = 2 centres are linked with the b o u n d a r i e s
between t r i o c t a h e d r a l F e z + 'cells' a n d d i o c t a h e d r a l A P + 'cells' at which charge i m b a l a n c e
occurred. The detailed n a t u r e o f the sites a n d the p r o d u c t i o n m e c h a n i s m s have n o t been
elucidated. A n g e l et al. (1974) a n d Jones et al. (1974) f a v o u r e d the e x p l a n a t i o n o f a
t r a p p e d hole on an oxygen a t o m b r i d g i n g a single d i v a l e n t ion a n d a t e t r a v a l e n t silicon
ion. In the case o f s u b s t i t u t i o n o f single F e z+ for A P + ions it is difficult to e x p l a i n w h y the
F e 2+ does n o t c o n v e r t to F e 3+. A n a l t e r n a t i v e suggestion o f an O2- molecule was also p u t
f o r w a r d . C u t t l e r (1980) p r o p o s e d similar sites b u t with the restriction t h a t the divalent
ions were situated on the b o u n d a r i e s between t r i o c t a h e d r a l d i v a l e n t a n d d i o c t a h e d r a l
cells. F o r this t r i o c t a h e d r a l F e 2+ model, the ferrous ions are necessary for charge b a l a n c e
in each cell. In the case o f O2- the site is f o r m e d to reduce excess negative charge. M e a d s &
M a l d e n (1975) p r o p o s e d three types o f site. O n e o f these, involving s u b s t i t u t i o n o f A P +
for Si 4+, is n o t p e r t i n e n t to this discussion. T h e o t h e r two sites were n o t identified b u t it
was suggested t h a t they were c o n n e c t e d with the inner a n d o u t e r h y d r o x y l groups. F r o m
the m o d e l p r o p o s e d by C u t t l e r (1980) it is possible to p o i n t to specific sites at which the
0009-8558/81/0300-0069502.00
9 1981 The Mineralogical Society
70
A. H. Cuttler
defect must be located. From a knowledge of the dosimetry and particle energy loss
mechanisms it might be possible to identify which centres are formed and how they are
produced. Snell (1965) examined briefly some consequences of the atomic rearrangement
following ionization in an insulating solid. He stated that it is possible for ions to be
moved to regions of lower potential. Such mechanisms will be assessed with regard to the
irradiated kaolins together with the probability of formation at the levels of doping (1 ~o or
less of divalent ions).
EXPERIMENTAL
PROCEDURE
Although not essential to this paper, the experimental procedures used by Angel & Hall
(1972), Angel et al. (1974), Jones et al. (1974) Angel et al. (1977) and Cuttler (1980) will
be summarized for completeness. A thin layer of synthetic kaolin (Angel et al., 1977)
weighing 5 mg was placed in the exit port of a crystallographic X-ray generator and
irradiated with X-rays at either 40 or 20 kV. The X-ray output was monitored with a
Baldwin-Farmer 0.6 cc air equivalent ionisation chamber. The irradiation time for the
samples was chosen to give an exposure dose of 20 MR, equivalent to an absorbed dose in
air of ~ 20 Mrad. This dose was sufficient to enable a g = 2 EPR signal to be observed,
part of which could be removed by annealing at 473 K for 2 h, leaving a g = 2 signal which
was not removed until 723 K (Angel & Hall, 1972). X-rays with different peak energy did
not markedly alter the signal strength.
If it is accepted that the signal is associated with a trapped hole it follows that the
electron must have been removed from the structure to be trapped elsewhere.
Alternatively, it is possible that one hydrogen atom was removed. Formation of an O2molecule requires the removal of H2 +, i.e. two hydrogen atoms, and an electron. It is,
therefore, necessary to examine the means of production. To accept any mechanism for
production, it is necessary to show that (a) it is highly probable and (b) it is consistent with
observed effects.
STRUCTURE
AND DOSIMETRY
In order to examine the probability of production of specific defects it is necessary to
know the constitution of kaolin and the number of defect centres. To consider energy loss
mechanisms it is necessary to calculate the total energy absorbed in the given dose and to
assess, in detail, how this may be transferred to the kaolin. It is also instructive to estimate
the flux of X-rays for comparison with the number of defect centres.
Constitution o f kaolin
Table 1 gives the masses and numbers o f atoms of each element in kaolin and the
fractional masses based on the chemical composition A14Si4018H8. The cross-sections for
X-rays given by McGinnies (1959) and Grodstein (1957) are included and the weighted
cross-section calculated. The mass of iron as FeO is assumed to be either 1~o or 0.07~ by
weight. A change to Fe203 would make only a 10~o difference and will not affect
subsequent discussion to any significant extent. At the mean energies involved, the
dominant mechanism is photo-electric absorption; hence the absorbed dose will be
proportional to the cross-section to a first-order approximation. Comparison of the
Fe (II)-doped synthetic kaolinite
71
e.
o
~
6
b>
oo
I
O~
0
7?
X
77
X
obob
6
6
6
oo~
6
X
~
.
xx
e~
o
.~,
.~=..
P
8
<
Z
~o
~o=
X
.
tr
A. H. Cuttler
72
weighted mean cross-sections for kaolin and air gives a ratio of 3.5 which has been used to
scale the measured air dose to an absorbed dose for kaolin. F r o m the trioctahedral model
proposed for the Fe 2+ ions (Cuttler, 1980), the m a x i m u m occurrence of defect centres is
one in three with a probability that it is even lower. The ratio of iron to aluminium ions is
1 : 56 at the 1~o level and 1 : 802 at the 0.07~ level, which is expected to produce most defect
centres (Angel et al., 1974). The figure 0"07~o iron as FeO is equivalent to the 0"04~o MgO
used in the magnesium doped kaolin. In view of these small concentrations it is of interest
to know if the centres are formed as a result of general ionisation or whether the divalent
ions selectively enhance the formation of centres. These alternatives will be examined
later.
Energy deposition and photon flux
Energy deposition. The experimental dose of 20 Mrad (air) is equivalent to 70 M r a d
(kaolin). To compare this figure with the binding energies of atoms it is necessary to
convert to electron volts. One rad (radiation absorbed dose) is defined as an energy
deposition of 10 -5 J g - i irrespective of the nature of the radiation. One electron volt =
1.6 x 10 -19 J, hence
70 Mrad - 70 • 106• I0 -s = 4.4 x 1021 eV.
1.6 • 10 -19
Since there are 3.9 x 1022 atoms per gram of kaolin the average energy deposited per atom
would be of the order of 0.1 eV, too small to expect any observable defects. It would
appear, therefore, that selective enhancement must take place to increase the local dose
around some atoms and particularly the Fe z+ or Mg 2+ ions responsible for the g = 2 sites.
Photon flux. The average photon flux has been estimated by rule of thumb methods
used in radiation dosimetry. The average photon energy is taken as 1/3 of the peak energy.
This gives 13 keV for 40 kV peak X-rays and 7 keV for 20 kV peak X-rays, The numbers of
interacting photons become 3.4 • 1017 and 6-8 x 1017, respectively. The photon flux is not
known but, using the weighted cross section (for 10 keV) of cr = 15 cm 2 g-1 and the mass
thickness m = 5 mg cm -2, the decrease in photon flux e -~rm can be calculated.
The product am= 15 x 5 x 10-3=0.075.
The number of photons absorbed
=(1-e-'~m)=am=O'075 or 7.5~ to a first approximation. The photon fluxes will
therefore be 1/0.075= 13, i.e. an order of magnitude greater. (Since the cross-section
decreases with increasing energy, the factor will be less than 13 for 40 kV peak X-rays and
greater for 20 kV peak X-rays.) This figure for the flux, ~ 5 x 1018 X-rays cm -2, is barely
comparable with the number of Fe 2+ ions and possible defect sites at the 0"07~o level. A
highly selective mechanism would be required to produce the number of defects. It is
concluded that direct interaction o f X-rays with the atoms adjacent to a g = 2 site is not
sufficient to explain the origin of these sites. It is necessary to consider energy loss
mechanisms in detail to understand how the defects may be produced.
Energy loss mechanisms
The X-rays may impart their energies in two ways. One would be to interact with the
hydrogen and dislocate it by direct energy transfer. The second would be to create
energetic electrons mainly by the photo-electric effect. These would lose their energy by
Fe ( II)-doped synthetic kaolinite
73
interaction with other electrons, i.e. by ionization. The atoms affected would rearrange
themselves to restore neutrality; in most cases further energy changes might be involved.
Hydrogen recoil
The maximum recoil energy of hydrogen atoms may be calculated from the Compton
scattering formula in the low energy limit. The energy imparted to protons by X-rays
scattered through an angle 0 is
Ep=Ex-Exl= Ex2/mpC2r;(1 - c o s 0)
(1)
1 + ~-~x2 ( 1 - c o s 0)
mpC
Ex is the initial and Ex I the final photon energy, me the proton mass and c the velocity of
light. (See e.g. Davisson, 1965.) The maximum transfer occurs for
0 = 180 ~ when Ep = 2Ex2/mpC2 - 2 Ex2/mpC2
E~
l+--
(2)
mpC2
since Ex ~ mpC2. The energy required to release a proton is calculated from data by Fripiat
et al. (1967),where the binding energy of the OH bond in kaolin is taken to be 115 kcal
m o l - ~. This, when converted to electron volts, becomes 5 ev per (OH). It is assumed that
the proton must recoil with more than this energy. Inserting this figure in expression (2)
gives a minimum photon energy of
Ex
= ~Ep
X/ 2
-
X/9.38 x 108 x 5
2
- 48keV.
(rnpc2= 938 MeV).
It is possible that recoil could take place following collision by a fast electron. In this case
the maximum recoil energy of the proton again occurs for back scattering of electrons
Ep = m4m-~-em~ 2 Ee
(eWmp)
(3)
where the subscripts e and p apply to electron and proton. This is just the case for classical
particle momentum transfer. Since
mp~me,
Ep=4 me Ee
mp
(4)
for Ep = 5 eV, Ee = 2.3 keV (this is the minimum energy required). Although from energy
considerations this mechanism could occur, the probability is too small to consider
further. This conclusion is supported by the independence of the signal strength on the
peak X-ray energy; this would also affect the electron energy.
Energy transfer by electrons
As with other radiations, the energy transfer by electrons is energy dependent,
increasing logarithmically as the energy decreases (see e.g. Knop & Paul, 1965). For silicon
74
A. H. Cuttler
TABLE 2. Extrapolated rangeenergy relationship for aluminium
Energy
keV
21.5
10
3.4
1.0
0.34
Range in
mg cm -2
Aluminium
/tm*
1
0.26
0.046
0.0058
0.001
4.3
1-1
0-2
0.025
0.0043
* Assuming density = 2.3 g cm -3.
the rate o f deposition (or loss) with distance dE/dx is 4.21 eV per micrometre at 10 keV.
Possibly o f greater significance is the range. By extrapolating curves given by Marshall &
W a r d (1937), the range for low energy electrons is calculated and given in Table 2.
A l t h o u g h the extrapolation to lower energies is difficult to justify over such a large range
(10 keV-0.3 keV), due to straggling and other effects, it appears that at energies less than
0-3 keV the range is comparable with the dimensions o f the unit cell o f kaolin. The
probability o f 'ionizing' adjacent ions as well as causing double ionization must be high.
To understand the significance o f these energy losses, it is also helpful to examine the
binding energies o f the electrons in the various shells associated with the elements
constituting kaolin. The binding energies are quoted with reference to the Fermi level in
the solid state. Shifts o f the order o f 1 eV might occur with changes in chemical
composition. Table 3 lists these binding energies as given by Hfigstrom et al. (1965). It is
noted that in magnesium, aluminium, and silicon, no information is given with regard to
the M shell in which electrons will be very loosely bound. In addition, in iron the 3d (Miv,
My) subshells are only partially filled and the NI subshell is filled but weakly bound. The
binding energy o f the oxygen L shell has been estimated by extrapolation. As electrons
TABLE 3. Electronic binding energies of the constituents of
kaolin
Element/shell
H
O
Mg
AI
Si
Fe
K
L~
Lll,in
14
532
6t*
1305 63
52
1560 87
73
1839 118 100 99
7112 842 721 709
MI MII,m MIV,V N
*
*
*
94
54
3
*
Note: All energies in electron volts (eV) relative to the Fermi
level.
* Denotes subshells containing electrons in normal atoms
but data not given. Expected values are of order leV.
t Extrapolated value.
75
Fe ( II)-doped synthetic kaolinite
|169 @@
,
(a)
(b)
(c)
(d)
Key
9 silicon
@ aluminium
9 iron (Fe2+)
o
0 oxygen
9 hydroxyl groups
FIG. 1. Proposed sites for ag=2 EPR signal: (a) loss of H +, (b) loss of H and H + to form 02(dashed), (c) trapped hole near Si +, (d) near (OH)- group.
lose energy it will become possible to release further electrons only from the lower energy
shells, e.g. the oxygen L shell or the iron M~v, Mv and N1 subshells. The consequences of
the release of these electrons will be examined in relation to models of the defect structure.
A second feature of the ionization is the return to the stable state of the atom or ion, Snell
(1965) has shown that in the emission of/~ rays the daughter atom has a positive charge. In
more than 10% of the decays the charge is greater than two units of positive charge. At low
energies one would expect more highly charged atoms by the following arguments. At low
atomic numbers a vacancy in the K or L shell is filled mainly by Auger transitions for
example K L L , K L M , LLM, L M M , M M N etc. The nomenclature signifies the transfer of
an L electron to the K shell with the emission of another L electron from the atom, etc.,
(for a detailed treatment the reader is referred to Bergstr6m et al., 1965). The consequence
of such events is that the a t o m (or ion) will be left more strongly ionized with two or more
electrons missing. A second consequence of the transition is that the electron leaving the
a t o m will have low energy and hence will create more local ionization.
76
A. H. Cuttler
DISCUSSION
To explain the nature of the centre giving rise to the g = 2 EPR signal it is necessary to find
a mechanism which can lead to such centres and also to show that the rate of production is
sufficient to account for such centres. In view of the concentrations of magnesium or iron,
0.04 to 1~o, or roughly 1/1200 to 1/80 'divalent cells' to normal cells, it is necessary to
explain how the centre is linked to the divalent iron cells. In proposing this model of
trioctahedral divalent ion 'cells' to replace dioctahedral trivalent ion 'cells', Cuttler (1980)
examined four possibilities: the release o f H + at the boundary with excess positive charge
(Fig. la), the removal ofH2 + to form 02 molecules (Fig. lb) and trapped holes attached
either to oxygen ions bridging Fe z+ and Si 4+ ions or attached to the outer hydroxyl
radicals (Figs lc and ld, respectively). In the latter three cases the sites are located at
boundaries with excess negative charge.
Frequency o f occurrence
The number of unit cells per gram of kaolin is 1.2 x 102~ since there are four aluminium
(or silicon) ions per unit cell. The number of divalent ion 'cells' as defined by Cuttler
(1980) is 2.4 x 1021 g - i since there are effectively two divalent ion 'cells' per unit cell. The
number of divalent ion 'cells' at a concentration of 0"04~o magnesium or 0.07~ iron is
2 x 1019 g-1. This number is of the same order as the photon flux and greater than the
number of interacting photons. It is, therefore, assumed that the centres are not formed by
direct interaction of X-rays but result from the secondary and subsequent ionization
resulting from the photo-electrons (and occasional C o m p t o n electrons) released by the
X-rays. For an electron of 10 keV, the range is estimated to be 1.1/~m in aluminium and
would be similar in kaolin. The dimensions of the unit cell of kaolin are 0.53 x 0.89 • 0.73
nm, hence on average a secondary electron would traverse approximately 1600 unit cells
or 3200 of the smaller cells. With an estimated 6.8 • 1017 interacting photons the number
of cells through which the secondary electrons passed would be about 2 x 102% i.e. the
same as the actual number per gram. Allowing for statistical fluctuations, it would appear
reasonable to infer that the production rate is sufficient and that the number of centres
produced would be reaching saturation at the doses received (20 Mrad air). The fact that
the centres were formed by secondary electrons and subsequent ionisation would also
explain the apparent independence of signal strength for X-rays of different peak
energy.
The probability that individual 'cells' will be affected by radiation appears sufficiently
large to consider other aspects. The number of atoms in each 'cell' is 17 or 18 depending on
whether it contains trivalent or divalent ions. It is, therefore, necessary to consider how
the energy transfer processes result in a centre associated with a particular site. Two
points arise here: first, some centres, such as a trapped hole or an 0 2 - molecule, may be
located on more than one site--there are at least two bridging oxygen atoms and several
possibilities for 0 2 - ; secondly, if one also considers adjacent cells and the possibility of
charge transfer, then one may reduce the transfer processes from possibly 17 atoms to one
to perhaps four to one or less. It would, therefore, appear that on grounds of probability
the secondary electrons are capable of generating a sufficient number of defects but that
transfer processes might be required to bring the defect to the appropriate site.
Fe ( II)-doped synthetic kaol&ite
77
Transfer processes
Two types of process must be considered; one which gives rise to liberation of H + or
H2 + and another which gives rise to a trapped hole. The direct removal of H + has been
shown to be impossible both on the grounds of the X-ray flux and the energies of the
X-rays and most unlikely for secondary electrons. To release hydrogen following
ionization it would be necessary to 'ionize' the associated oxygen so that it had a positive
charge. To remove H § from O H would leave the oxygen as 02 . Thus to remove H § it
would be necessary to remove three electrons from the oxygen atom. In the case of H2 +
one would need to remove five electrons from two neighbouring (OH) groups. Since
oxygen has only two shells, it will be doubly ionized only if(i) the L shell is doubly ionized
by Auger transition, (ii) the L shell is doubly ionized by two different processes or (iii) the
L shell electrons of oxygen fall into the deeper shells of aluminium, magnesium or iron. To
remove three electrons would necessitate a high local deposition of energy of the order of
20 eV, far in excess of the average value of 0-1 eV per atom. Such processes, though
possible, would occur throughout the lattice. In this case one would not expect only the
specific divalent 'cells' to be affected. Since the g = 2 signal is not observed in a kaolin with
no divalent ions, it must be assumed that if a hydrogen bond is broken the hydrogen is
sufficiently mobile to return to its original site. One might expect the same to be true for
the centre associated with the divalent 'cells'. It is inferred, therefore, that the removal of
H § from a hydroxyl (with possible trapping of a hole) or H2 + leading to an 0 2 - are
unlikely mechanisms.
The alternative possibility is a trapped hole. Here it may be argued that as a result of
ionization, and the short range of tertiary electrons, the possibility of ionization of
neighbouring atoms is high. Also it is possible for an aluminium atom to capture an
electron from a neighbouring oxygen atom and release either another electron from
aluminium, silicon or oxygen. Therefore it would appear that double 'ionization' o f A 13+,
Si 4+ o r 0 2 - is quite feasible. In the case of Fe 2+ even higher states of ionization would be
possible. Such processes would tend to transfer charge from the A13+ and Si 4+ and
possibly Fe 2+ to oxygen. A second feature of the binding energies that would favour
ionization of oxygen is the energy of the tertiary electrons. For energies less than 50 eV
ionization could occur only in oxygen, hydrogen and the Miv, v and N subshells of iron.
This argument assumes that for Mg 2+, A13+, and Si 4+ the M subsheils are empty. These
considerations point to a high probability of removal of electrons from oxygen atoms. In
the regions where there is an excess of negative charge it is proposed that one of the ejected
electrons is trapped elsewhere - - probably at the boundary with a positive charge excess.
Thus on reorganization a 'trapped hole' remains. In view of the short range of the
electrons it is possible that at the higher concentrations, the groups of 'cells' proposed by
Cuttler (1980) are too large in extent to allow the trapping of such an electron. Hence one
would expect the signal strength to decrease after an optimum concentration is reached.
One may explain the inability of ultraviolet light of wavelength 250 nm (,-~ 5 eV) to
produce defects similar to those induced by X-rays on the grounds that the electrons do
not have sufficient energy to travel far from the oxygen or that they cannot produce
double ionization. At present it is not possible to analyse in greater detail the mechanism
of energy transfer. In view of the possibility of transferring charge from aluminium,
silicon, magnesium or iron ions to the 02 , the last two electrons of which will be weakly
bound, it is believed that a hole is more likely to be trapped on an inner oxygen atom than
78
A. H. Cuttler
on outer (OH) groups since the probability is twice as high and one might expect the
trapped hole to be more strongly bound. It is suggested that studies with X-rays and
ultra-violet radiation of monochromatic energies, such as are now available using
synchrotron radiation, might enable the mechanism of production of the g = 2 E P R
centres to be understood completely. The deduction that the centre is formed as part of
the general ionization implies that the rates of formation in iron-doped and magnesium
doped kaolins will be similar. This has still to be confirmed.
CONCLUSIONS
From a consideration of the dosimetry and energy loss mechanisms at the atomic level it
has been possible to draw some conclusions about the formation of g = 2 paramagnetic
centres (A-centres) in kaolin. The conclusions are:
1. That proton recoil induced either by X-rays or electrons is unlikely to produce the
defects for X-rays of the energies used, i.e. < 40 kV at an integrated dose of 20 Mrad (air).
This is concluded both on the grounds of available energy and photon fluxes.
2. The defects are created as a result of the 'ionization' caused by photo- and C o m p t o n
electrons released by the X-rays. (The occurrence of divalent ion 'cells' at levels of 1~ or
less is too low to permit creation directly by interactions of the X-rays with the atoms in
the divalent ion 'cells' or their neighbours.)
3. To explain the link between the g = 2 centre and the divalent ion 'cells', transfer
mechanisms must occur.
4. The removal of protons, H +, following multiple ionization due to vacancy cascades
is improbable, hence the formation of O2- is unlikely.
5. The binding energies of the constituents of kaolin, namely oxygen, aluminium,
silicon and magnesium or iron favour multiple ionization of oxygen atoms due to the low
binding energy of the oxygen L-electrons. Two processes may contribute to this
ionization: (i) the low binding energy which favours energy transfer from low energy
electrons and (ii) cascade vacancies in the heavier elements which m a y be filled by a
transfer of electrons from the oxygen ions.
6. Due to the statistical nature of the energy losses and the uncertainties associated with
the collision processes at low energies, it is proposed that:
(i) The range of low energy electrons of energy < 50 eV is of the order of the
dimensions of the unit cell. Electrons from the boundary with excess negative charge
may travel far enough to become trapped at the boundaries with excess positive charge.
The result will be a trapped hole.
(ii) The centre forming the g = 2 EPR signal is a hole on an oxygen ion bridging the
divalent ions and Si 4+ ion at the cell boundary with excess negative charge. There is a
possibility that an oxygen ion in the outer hydroxyl layer of kaolin could also trap a
hole. The rate of formation would be lower due to the fewer bonds with higher atomic
number elements. The effect of the hydrogen atom in the hydroxyl radical ( O H ) would tend to weaken the binding of the trapped hole hence the trapped hole on a
bridging oxygen in the inner oxygen/hydroxyl layer is favoured.
(iii) To explain the decrease in intensity of the signal as the concentration of divalent
ions increases, it is proposed that the divalent cells group together and that as the size of
the groups exceeds the range of the low energy electrons, the trapping mechanism for
the electron is no longer operative. The displaced electrons then return to the ions from
Fe (II)-doped synthetic kaolinite
79
w h i c h t h e y w e r e r e m o v e d . T h e a l t e r n a t i v e p r o p o s a l to e x p l a i n the d e c r e a s e in signal
i n t e n s i t y w i t h i n c r e a s e in c o n c e n t r a t i o n is t h a t f o r g r o u p s o f cells, the excess n e g a t i v e
c h a r g e is b a l a n c e d o v e r a r e g i o n e i t h e r by i n c l u s i o n o f e x t r a d i v a l e n t o r t r i v a l e n t ions.
F o r iron, o n e m i g h t find o c c a s i o n a l F e 3+ s u b s t i t u t i o n at the b o u n d a r i e s o f the F e z+
'cells'. F o r m a g n e s i u m n o s u c h s u b s t i t u t i o n is possible. T h e i n t r o d u c t i o n o f e x t r a i o n s is
n o t f a v o u r e d since t h e y w o u l d c r e a t e l a r g e d i s t o r t i o n s t h e m s e l v e s .
(iv) T h e failure o f u l t r a - v i o l e t r a d i a t i o n to i n d u c e s i m i l a r defects m i g h t be d u e e i t h e r
to (a) insufficient e n e r g y o r (b) the i m p o s s i b i l i t y , e x c e p t at v e r y h i g h fluxes, o f p r o d u c i n g
m u l t i p l e i o n i z a t i o n in a n a t o m .
It is s u g g e s t e d t h a t studies w i t h s y n c h r o t r o n r a d i a t i o n in the r a n g e 10 V to 2 k V m i g h t
assist in u n d e r s t a n d i n g t h e details o f the m e c h a n i s m s p r o d u c i n g the defect centres.
ACKNOWLEDGMENTS
The author wishes to thank Drs B.R. Angel, K.S. Richards and W.E.J. Vincent of Plymouth Polytechnic for
preparation of the synthetic kaolin and Dr S. Bowring, Department of Nuclear Medicine, Freedom Fields
Hospital for undertaking the dosimetry. The author also wishes to acknowledge receipt of a Science Research
Council Equipment Grant for M6ssbauer studies which enabled the model for the divalent ions to be elucidated.
REFERENCES
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ANGEL B.R., JONESLP.E. & HALLP.L. (1974) Studies of doped synthetic kaolinite I. Clay Miner. 10, 247-256.
ANGEL B.R., CUTTLERA.H., R1CHARDSK.S. & VINCENTW.E.J. 0977) Synthetic kaolinites doped with Fe 2§
and Fe 3+ ions. Clays Clay Miner. 25, 381-383.
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gamma-ray spectroscopy, Vol. 2 (K. Siegbahn, editor). North Holland Publishing Company, Amsterdam,
Holland.
CUTTLERA.H. (1980) The behaviour of a synthetic 57Fe-doped kaolin: M6ssbauer and electron paramagnetic
resonance studies. Clay Miner. 15, 429-444.
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gamma-ray spectroscopy, Vol 1. (K. Siegbahn, editor). North Holland Publishing Company, Amsterdam,
Holland.
FRIPIAT J.J., BOSMANSH. • ROUXHETP.G. (1967) Proton mobility in solids I; hydrogen vibration modes and
proton delocalisation in boehmite. J. Phys. Chem. 71, 1097-1111.
H~,GSTROM S., NORDUNG C. & SIEGBAHNK. (1965) Electron binding energies and kinetic energy against
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Siegbahn, editor). North Holland Publishing Company, Amsterdam, Holland.
GRODSTEING.W. (1957) Attenuation coefficients from 10 keV to 100 MeV. U.S. National Bureau of Standards
Circular 583. Washington, USA.
HERBILLONJ.A., MESTDAGHM.M., VIELVOYEL. & DEROUANEE.G. (1976) Iron in kaolinite from tropical soils.
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Company, Amsterdam, Holland.
McGINNIES R.T. (1959) X-ray attenuation coefficients from l0 keV to 100 MeV U.S. National Bureau of
Standards Supplement to Circular 583. Washington, USA.
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80
A. H. Cuttler
RI~SUMI~: La structure du kaolin a 6t+ examin6e en m6me temps que divers aspects de la
dosim+trie et de m6canismes de perte d'6nergie des radiations en vue d'expliquer la formation de
centres RPE avec g = 2. Cette analyse indique la formation de trous pi6g6s dans le 'plan interne'
des atomes d'oxyg6ne du kaolin situ6s 5. la limite entre des 'mailles' contenant des ions bivalents et
trivalents. Ces trous existent en particulier 5. toutes les limites portant une charge exc+dentaire
n6gative. On a 61emin+ l'interaction directe entre rayons X et atomes, et la possibilit~ du recul du
proton. Les modes de formation semblent ~tre un transfert de charge, suivi de l'ionisation des
atomes par des 61ectrons secondaires, avec finalement un transfert des rides sur les ions oxyg6ne.
On examine les m+canismes qui entraTnent la diminution de l'intensit6 du signal lors de
l'accroissement des concentrations. On conclut que les m6canismes de maille qui ont ~t6 propos6s
sont en accord avec les vitesses de production et que sous 20 Mrad (air) le nombre de centres doit
atteindre la saturation.
K U R Z R E F E R A T : Die Struktur von Kaolin wurde zusammen mit Aspekten von Dosierung
und Mechanismen des Verlustes von Strahlungsenergie untersucht, um die Bildung von g = 2
EPR Zentren zu erkl/iren. Die Analyse weist aufeine Bildung von eingeschlossenen Hohlrfiumen
bei den 'Innenlagen' der Kaolin-Sauerstoffatome hin, die an den Grenzzonen zwischen di- und
trivalenten 'Ionenzellen', insbesonders an den Grenzen mit negativem Ladungs/iberschul3
lokalisiert sing. Eine direkte Wechselwirkung der R6ntgenstrahlen mit Atomen und die
M6glichkeit des Protonenr/ickstosses ist ausgeschlossen. Der Vorgang der Entstehung scheint
eine Ladungs/ibertragung zu sein, die der Atomionisierung durch sekund~ire Elektronen folgt und
wobei letztlich die Leerstellen auf die Sauerstoffionen iibertragen werden. Es werden die
Mechanismen untersucht, welche aufeinen Abfall der Signalst~irke bei gleichzeitigem Hfiufigkeitsanstieg hinauslaufen. Es wird der Schlul3 gezogen, dab die diskutierten Zell-Mechanismen mit
den Produktionstraten iibereinstimmen und ferner die Anzahl der Zentren bei 20 Mrad (Luft)
einen Sfittigungsbereich erreicht haben d/irften.
R E S U M E N : Se ha examinado la estructura del caolin junto con aspectos de dosimetria y
mecanismos de p~rdida de energia de radiaci6n, para explicar la formaci6n de centros
paramagn~ticos con g = 2. El anfilisis parece indicar la formaci6n de un hueco atrapado sobre la
'capa interna' de 5'tomos de oxigenos del caolln, localizado en los bordes entre 'octaedros' con i6n
trivalente y divalente, en particular, en los bordes con exceso de carga negativa. La interacci6n
directa de rayos X con los fitomos y la posibilidad de retroceso prot6nico han sido eliminadas. El
proceso de formaci6n parece ser la transferencia de carga que sigue a la ionizaci6n de 5'tomos por
electrones secundarios, con transferencia final de vacantes a los fitomos de oxigeno. Se discuten
mecanismos que se traducen en una disminuci6n de la fuerza de la serial con el aumento de
concentraci6n. Se concluye que los mechanismos discutidos son consistentes con las velocidades
de formaci6n y que a 20 Mrd (aire) el nt~mero de centros deberia llegar a la saturaci6n.