Radio Frequency µSR
The technique,
New Science,
Developments at ISIS.
This is Part I of a two part talk!On Friday, James will talk about the other pulsed environments that are
commonly used for muon experiments at ISIS.
Radio-Frequency µSRA technique akin to NMR/ESR.
0 2 4 6 8 10 12-0.16
-0.12
-0.08
-0.04
0.00b)
Differential Asymmetry
Time (µs)0.0 0.2 0.4 0.6 0.8 1.0
-0.2
-0.1
0.0
0.1
0.2
c)
Asymmetry
Time (µs)
20MHz precession about H0 (1476G)
0.2 MHz precession about H1 (15G)
e+ Detect
e+ Detect
µ+ P0 P0 rotates away from and about H0
H0H1
Envelope can tell us about muon charge state conversion reactions
Envelope can tell us about the local field distribution at muon site
ISIS is the best place to work!
Muon Pulse
High intensity R.F. Pulse Very low duty factor, eg 5×10-5
Data Acquisition
Variable ∆t (≈ 500ns)
δt (≈ 1µs)
Allows muon state kinetics to be studied
Determines rotation angle of muon polarisation eg π/2
80ns
Time structure removed
-1.0 -0.5 0.0 0.5 1.0
0.0
0.1
0.2
0.3
0.4
90∞ pulseRF ON
Free precessionRF OFF
Longitudinal Asymmetry
Time (µs)
-1.0 -0.5 0.0 0.5 1.0-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08 Free precessionRF OFF90∞ pulse
RF ON
Transverse Asymmetry
Time (µs)
Pulsed RF Techniques
e+ Detect
P0
H0
e+ Detect
P0
H0
F B
Measuring Dynamics
-1 0 1 2 3 4 5 6 7 8 9 10
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
Asymmetry
Time (µs)
Signal shown in 20MHz RRF
Boron diamagnetic resonance in 1672G
90o-τ-180o pulse sequence
180o
90o
Use a 90°- τ - 180° pulse seque nce to ref o cus the heteronuclear dipolar coupling between the muon an d neighbou ring nuclei.
In boron at room temperature, where muons are static, recovery of the full muon polarization (an echo) occurs at ~2τ or twice the pulse separation.
In general, the echo position and amplitude is sensitive to fluctuations in the local fields during the evolution periods.
This has been studied theoretically by Kreitzman (Hyp. Int. 65 (1990) 1055).
Beating the Pulse Width!
Pulsed RF techniques provide a method of measuring high precession frequencies at a pulsed muon source.
Time (µs)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Asymmetry
-0.04
-0.02
0.00
0.02
0.04
Time (µs)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Asymmetry
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06Time (µs)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Asymmetry
-0.04
-0.02
0.00
0.02
0.04
T=12K
T=17K
T=7.5K
• YNi2B2C• Type II• TC=15K• λ=103nm • ξ0=8nm
• Field 1034G• 13.6MHz
Normal state
Superconducting(mixed) state
(Hillier et al)
Muon State Kinetics (semiconductors)
Muons stopped in silicon form two paramagnetic states: MuT and Mu*
0 40 80 120 160 200 240 280 320
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
RF Data, zero delay Transverse Field Data
Diamagnetic Fraction
Temperature (K)
At low temperatures ~50% muons form MuT
and ~40% muons form Mu*.As the temperature is raised above ~120K conversion of the Mu* fraction into a diamagnetic state occurs.This increase is reflected in the RF results but not the transverse field data . . . why?
The question wa s ans we red by Kreitzman:
The muon spends a short time as Mu* before charge state conversion occurs, and the phase coherence of the TF signal is lost.
For RF measurements the muon polarization is locked along a large static field and is preserved regardless of muon state.
The RF signal measures the Final State
If the conversion rate is favourable, we can directly measure muon charge state conversion in Si (ionization of Mu*) by following the build-up of the diamagnetic state using delayed RF excitation.
0 2 4 6 8 10 12-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
Delay 2µs from muon pulse Delay 4µs from muon pulse Delay 6µs from muon pulse
Asymmetry
Time (µs)
Si, 132.5K
Muon Pulse
Data Acquisition
Variable Delay, 2µs, 4µs, 6µs
80ns
0 2 4 6 8 10 12
Time in microseconds
0
10
20
Asy
mm
etry
(%)
cis-PBD 137 K
LF 3512G (delay=3 s)LF 3512G (delay=1 s)
TF 40G
0 2 4 6 8 10
RF Delay ( s)
2
4
6
8
10
Dia
mag
netic
Am
plitu
de(%
)
237 K
212 K
137 K
87 K
Muon State Kinetics (polymers)
Similar behaviour is seen in a polymer system.
(Pratt and Watanabe, RIKEN-RAL)
RF Decoupling
Apply Continuous Wave decoupling methods to cancel the dipolar interaction between the muon and neighbouring nuclei.
Do two things at once:
Apply a long RF pulse (>20µs) at the resonance frequency of the nuclei we want to decouple. This pulse 'stirs' the nuclear spi ns and averages the dipolar coupling.
Apply a 90° pulse to the muons and measure the free precession signal.
Ca(OH)2 was chosen as a suitable test sample because, from other muon work, we knew:
1. The muon site and charge state.
2. That the muon couples strongly to three protons ea ch at distance 2.164Å , dipolar coupling between muon and protons, ωD, is ~133KHz.
3. That the muons are static at room temperature.
-2 0 2 4 6 8 10
-0.04
-0.02
0.00
0.02
0.04a)
Asymmetry
Time (µs)
λ = 0.12µs-1
With B1 of 40G for the decoupling pulse, at proton resonance frequency ~6.6MHz:
-2 0 2 4 6 8 10
-0.04
-0.02
0.00
0.02
0.04b)
Asymmetry
Time (µs)
λ = 0.02µs-1
Free precession signal of diamagnetic muons in Ca(OH)2 in a field of 1600G (~20.8MHz):
Note, for efficient decoupling ω1 > ωD⇒ ω1 > ~133KHz and B1 > ~26G (for protons)
The RF-µSR ProjectEPSRC grant (~£270k) awarded 1997 to commission an RF spectrometer on the DEVA beamline.
Follow-up EPSRC grant (~£330k) awarded jointly to RAL, UEA, Leicester and Manchester in December 2001 for scientific commissioning the new RF spectrometer, finishing 2005.
Current Activity
Four particular programme areas were highlighted in the application as being particularly suited to the application ofRF- µSR:
Gas phase studies - to investigate the reactivity of Mu with small molecules (such as NO and CO) to quantify quantum mass effects in reaction rates relative to H
RF d ecou p lin g tech niques - to study ion ic materials (P2O5-ZrO2 glasses and hydrogen oxide bronzes) to reveal proton mobility.
Muoniu m ch e mi s tr y - to study r adical reactions, investigate reaction rates and measure slowly formed species.
Proton st ates in semiconductors - to investigate the newly discovered shallow donor state in II-VI compounds.