Muons as a Tool for Probing
Supercritical Water Chemistry
Paul W. Percival
Simon Fraser
University
and
TRIUMF
2
Supercritical Water Oxidation
There are drastic changes in the physical properties of water close
to and above the critical point (Tc = 374°C, Pc = 221 bar).
This leads to unusual chemistry:
 organic compounds are miscible in SCW
 combustion of organic materials is possible
A Flame in Water!
30% methane in water
2000 bar, 450ºC
W. Schilling and E.U. Franck, Ber.
Bunsenges. Phys. Chem. 92 (1988) 631.
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Paul Percival, May 2009
3
Supercritical Water Oxidation
SCWO facilities are being developed for the destruction of
hazardous waste such as PCBs, sewage sludge, chemical weapons.
It has even been proposed for water recycling on long space flights.
There is ~500 tons of blister and nerve agents stored at the
Blue Grass Army Depot in Kentucky. The plan is to neutralise
these materials and then destroy the hydrolysate by means of
SCWO.
Many of these reactions involve free radical intermediates, but
there is very little direct knowledge about these transient species
under such extreme conditions.
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4
Studying Reactions in High Temperature Water is Difficult
… but muon spin spectroscopy has a number of advantageous
features for the study of “hostile” environments:
 Muons can be injected into most materials. The momentum can be
adjusted for optimum penetration of the sample. Optical windows
are unnecessary.
 Spin polarized muons give high sensitivity and do not need radiofrequency or other radiation.
 The detectors are simple devices (plastic scintillators) outside
the controlled sample environment.
 the short lifetime of the muon, µSR is inherently suited to the
study of transients.
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Paul Percival, May 2009
5
Muonium — a light isotope of hydrogen
Mu e
e
The properties of a single
electron atom are determined
by the reduced mass

1
1
1
1
=
+
»
mr mN me me
m = 19 mp
 = 2.2´ 10- 6 s
reduced mass of Mu = 0.995 mr(H)
ionization potential = 13.539 eV
Bohr radius = 0.532 Å
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6
Centres for Muon Spin Spectroscopy
NOW
J-PARC
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7
The TRIUMF Joint Venture
TRIUMF is owned and operated by a consortium of Canadian universities:
Member Universities
Associate Members
University of Alberta
University of Guelph
University of British Columbia
McMaster University
Carleton University
Queen's University
University of Manitoba
University of Regina
Simon Fraser University
Saint Mary's University
Université de Montréal
York University
University of Toronto
University of Victoria
TRIUMF's operating costs are funded by a contribution from the federal
government through the National Research Council of Canada (NRC).
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8
TRIUMF
Meson
Hall
Cyclotron
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The TRIUMF Meson Hall
Cyclotron
muon beam line
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Looking down on the TRIUMF M9 Beam Area
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Apparatus for Hydrothermal Studies in HELIOS
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High pressure and temperature cell
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The
Phase Diagram
The Phase
Diagram of
of Water
Water
FLUID
Pressure / bar
50—450°C
250—350 bar
221
LIQUID
ICE
1.0
Critical
point
Triple
point
VAPOUR
STEAM
0 100
374
Temperature /°C
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14
µSR: Muon Spin Spectroscopy
Muon Spin Rotation
• spin polarized muon beam
• muons are implanted in the sample
• muons decay:
+  e+ +  + e
 = 2.2 s
• angular distribution of e+ has maximum in muon spin direction
• precession of muon spins in transverse magnetic field
• equivalent to free induction decay in pulsed NMR or ESR
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15
The Muon Lifetime Experiment
 + ® e+ +   +  e
M
µ+
beam
 = 2.2 s
F
S
e+
B
P
lifetime histogram
start
stop
N
CLOCK
0
time
N = N0 e- t /  + background
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Paul Percival, May 2009
16
Muon Spin Rotation, µSR
M
µ+
beam
F
S
Apply a magnetic field
perpendicular to the initial
spin direction.
e+
B
P
µSR histogram
start
stop
N
CLOCK
time
N = N0 e- t /  [1 + aP (t )cos (t + )]+ background
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The µSR Histogram
800
N0 e- t /  [1 + aP (t )cos (t + )]+ B
count
600
400
Subtract the constant background and
divide out the exponential decay …
200
0
0
1
2
3
4
5
6
time / s
aP (t )cos (t + )
muon asymmetry
… to get the muon asymmetry,
which represents the time
dependence of the muon spin
polarization.
0.2
0.1
0
-0.1
-0.2
0
1
2
3
4
5
6
time / s
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18
Muonium in Supercritical Water
400°C, 245 bar
0.2
Asymmetry
0.1
D
signal
0.0
-0.1
-0.2
0.0
0.5
1.0
1.5
time / s
2.0
2.5
3.0
Percival, Brodovitch, Ghandi et al., Phys. Chem. Chem. Phys. 1 (1999) 4999
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19
Muon Asymmetry
Muonium in supercritical water at 196 G
0.00
0.05
0.10
0.15
0.20
Fourier Power
time / µs
400°C
245 bar
200
220
240
260
280
300
320
340
Frequency / MHz
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20
Muonium Kinetics
Muonium reactions are pseudo-first order because…
only a few million Mu atoms are needed for an experiment.
Mu + A ¾ ¾
® products
Muon Asymmetry
d [Mu]
= kM [A][Mu] = [Mu]
dt
The Mu signal decays
exponentially
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
time / µs
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Muonium Kinetics
 =  0 + kM [A]
0 is the first-order rate
constant for pure solvent
second-order rate constant
Mu decay rate / s
d [Mu]
= kM [A][Mu] = [Mu]
dt
-1
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.00
0.05
0.10
0.15
0.20
[MeOH]
 - 0
kM =
[A]
In addition to chemical decay, the muon decays radioactively, with a lifetime
which is independent of chemical environment.
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22
A fall-off of rate is common for reactions in high T water

Mu  OH  MuOH  eaq
Mu + Benzene
1.2
1011
350 bar
250 bar
> 310 bar
245 bar
1010
0.8
kMu / M-1s-1
kMu / 1010 M-1s-1
1.0
0.6
0.4
< 200 bar
1 bar (lit.)
109
108
0.2
0.0
0
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100
200
300
Temperature / °C
400
107
0
100
200
300
o
Temperature / C
400
Paul Percival, May 2009
23
Diffusion and collision-controlled kinetics
Reaction rates in liquids depend on diffusion of the reactants to form
the encounter pair, as well as the activated chemical reaction.
Mu  A
1
1
1


kobs kdiff kact
 products
MuA 
kdiff  4  RMu  RA  DMu  DA 
kact  f R A exp   Ea / RT 
The key factor in the fall-off
with temperature seems to
be the drop in the number of
collisions between a pair of
reactants over the duration
of their encounter.
where
fR 
PZ coll
-1enc  PZcoll
many collisions per encounter at low temperature
collision  encounter for gas-like behaviour at high T
Ghandi, Percival et al Phys. Chem. Chem. Phys. (2002)
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H + OH  H2O
current PWR reactors
Data limited to 200°C
next generation reactors
Buxton and Elliot,
JCS Far. Trans. 89 (1993) 485
8.0
AECL
k / 1010 M-1s-1
6.0
M1
M2
M3
4.0
2.0
0.0
0
100
200
300
400
500
Temperature /°C
Ghandi and Percival, J. Phys. Chem. A 107 (2003) 3006
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25
H abstraction from methanol by Mu (H)
H + CH3OH
Mezyk and Bartels, 1994
Percival et al., 2007
10.0
6
-1
k / 10 M s
-1
100.0
1.0
0.1
0
100
200
300
400
500
Temperature / °C
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Supercritical-Water-Cooled Reactor
Canada is one of ten countries (the Generation IV International Forum)
working together to lay the groundwork for fourth generation nuclear reactor
systems.
The priority R & D areas for Canada include
“improved understanding of radiolysis under supercritical water conditions and
the effect of radiolysis products on corrosion and stress corrosion cracking”.
The Supercritical-Water-Cooled Reactor
(SCWR) system is a high-temperature, highpressure water cooled reactor that operates
above the thermodynamic critical point of
water (374°C, 22 Mpa)
The SCWR system is primarily designed
for efficient electricity production.
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27
Muoniated Free Radicals
Mu
R2
R1
C
C
R4
R3
µSR:
precession frequencies  muon hyperfine coupling
µLCR:
resonance fields  other nuclear hyperfine couplings
hyperfine couplings  distribution of unpaired electron spin
e.g. cyclohexadienyl
H Mu
0.33
-0.10
0.33
•
-0.10
0.48
Temperature dependence of hyperfine couplings
 intramolecular motion
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Muon Avoided Level Crossing Resonance
Mixing of spin levels related by flip-flop transitions of the muon and another
nucleus results in loss of muon spin polarization at a characteristic field (Bres)
determined by the difference in muon and nuclear hyperfine constants.
BLCR
,  ,X
A+– A–
Energy
e
1   A  Ap   A  Ap  
 


2      p 
e


e
, ,
X
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
Magnetic Field / kG
Magnetic Field
The differential line shape is from field modulation.
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29
Isopropyl from dehydration of 1-propanol
(a)
Ap /MHz
A+-A-
68.2
65.4
-57.7
Mu
10.4
10.6
10.8
11.0
11.2
H C
H
Field / kG
A+-A-
(b)
C
H H
C
H
H
LCR distinguishes the
signs of the hfcs
17.2
17.4
17.6
17.8
18.0
Field / kG
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30
The Mu adducts of acetone are isomeric radicals
(a)
A+-A-
92°C
136 bar
Mu
O
CH3
CH3
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Field / kG
(b)
H
A+-A-
350°C
250 bar
O
CH3
9.4
9.6
9.8
10.0
10.2
10.4
10.6
CH2Mu
10.8
Field / kG
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31
How to Study a keto-enol Equilibrium by Radical Trapping
H
High temperature:
OH
Mu
O
CH2Mu
Mu
Low temperature:
O
Mu
O
At 25°C the keto-enol equilibrium constant of acetone in water is
5  10-9
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Paul Percival, May 2009
32
Thanks…
…to my collaborators in the
Simon Fraser University Muonium Chemistry Group
in particular,
Dr. Jean-Claude Brodovitch
and my recent graduate students:
Brenda Addison-Jones (Ph.D. 2000)
Khashayar Ghandi (Ph.D. 2002)
Iain McKenzie (Ph.D. 2004)
Brett McCollum (Ph.D. 2008)
TRIUMF Centre for Molecular and Materials Science
Drs. Syd Kreitzman, Donald Arseneau, Bassam Hitti and staff
For funding:
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NSERC of Canada
Paul Percival, May 2009
33
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34
Pions Decay to Give Muons
At TRIUMF, pions are produced by bombardment of a Be or C production
target with 500 MeV protons. We take the positive pions, which decay to
positive muons:
+ ®  + +  
 = 26 ns


¬ ¾¾ e ¾ ¾®
uur
uur
p = - p
In the pion rest frame:
Momentum is conserved:
uur
uur
 = -  
Spin is conserved:
Spin and momentum are
related through helicity:
r r

×p
hˆ = r r
. p
hˆ = ± 1
For the neutrino, h = -1, i.e. the spin and momentum are anti-parallel.
Therefore, muons are produced
with  and p anti-parallel.
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
¾ ¾ ®
¬¾¾
p
e

¬ ¾¾
¾ ¾®
p
Paul Percival, May 2009
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Muon Beams are Spin Polarized
There are two commonly used types of muon beam:
Surface Muon Beams collect and transport muons which are created from the
decay of pions at or near the surface of the pion production target.
select these muons to get these spins:
beam line
Decay Muon Beams collect muons from pions which decay in flight.
In the  rest frame only backward {
} and forward muons are transported.
In the laboratory frame, both momentum bites travel in the forward direction.
The forward (momentum) muons (p~180 MeV/c) have backward spin;
the backward muons (p~80 MeV/c) have forward spin.
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36
Muon Decay
 + ® e+ +   +  e
The 3 particle decay results in a
spectrum of positron energies.
 = 2.2 s
N
The spatial distribution of positrons is
asymmetric and depends on the muon
spin polarization.
Consider the decay pattern which
results in the maximum positron energy:
The
is emitted in the direction of the
muon spin at the moment of decay.
More generally,
Emax = 12 m
= 52.8 MeV
Energy

e+
dN ( E , )
= C [1 + D( E )cos ]
dE d W
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µ
e+
e
e

Paul Percival, May 2009
37
Alcohols are dehydrated in superheated water
e.g. dehydration of tert-butanol forms isobutene
CH3
H3C
– H2O
H3C
OH
CH2
H3C
CH3
which can be spin-labelled by adding Mu
Mu
H3C
CH2Mu
H3C
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38
tert-butyl is also formed by dehydration of butanols
Power
350°C, 250 bar
tert-butanol
0
- H2O
H3C
sec-butanol
400
C H3
Mu•
OH
200
Frequency / MHz
C H2M u
- H2O
isobutene
A+-A-
OH
9.6
9.8
10.0
10.2
10.4
10.6
Field / kG
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39
Muon ALCR spectrum of tert-butyl radical in water
CH3
CH2Mu
1 Aμ  Ap
Bres 
2 γμ  γ p
A+-A-
CH3
200°C
250 bar
10.0
10.5
11.0
11.5
Field / kG
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Paul Percival, May 2009
40
1-propanol and 2-propanol both give isopropyl
Power
350°C, 250 bar
2-propanol
0
200
OH
400
Frequency / MHz
- H2O
H
Mu•
H3C
1-propanol
- H2O
propene
A+-A-
OH
C H2M u
10.4
10.6
10.8
11.0
11.2
Field / kG
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High pressure system
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