A Modular Low Frequency EPR Spectrometer for Studying Objects

A Modular Low Frequency EPR Spectrometer for Studying Objects

A Modular Low Frequency EPR Spectrometer for Studying Objects with
Cultural Heritage Significance
W.J. Ryan, L.E. Switala, M. Hoffman, W. Brown, N. Zumbulyadis, J.P. Hornak
Magnetic Resonance Laboratory, Rochester Institute of Technology, Rochester, NY 14623
Overview
Sample Probes
Unlike X band EPR, Low frequency EPR (LFEPR) spectroscopy can
be used to examine large objects with cultural heritage significance
non-invasively and non-destructively.1 This capability is due to their
large volume magnets and lower operating frequency (ν < 500 MHz).
The former permits the placement of large intact objects in the
spectrometer while the latter makes it easier to construct surface coil
(SC) probes. In this poster we describe the features of our
The
laboratory-built, continuous-wave, LFEPR spectrometer.1,2
spectrometer’s modular design allows us to easily change its
frequency and capabilities.
cm
30 cm diameter, 45 cm long solenoidal
electromagnet with STS probe.
15 cm ID Helmholtz style modulation coil
with 2 cm surface coil on a 15 cm
diameter Ming Dynasty bowl.
Surface Coil Probe
2 cm, 7 turn, spiral, inductively coupled
Surface Coil Probe
2.9 mm, 5 turn, solenoid, inductively coupled
Single Turn Solenoid
Probe with surrounding
modulation coils and
internal slotted,
inductively coupled,
STS resonator.
LFEPR Program
Three Spectrometer Control Programs
1. Spectrum: Conventional Bo sweep and record lock-in signal
2. Field Cycle: Record lock-in signal while sweeping Bo up and down
3. Time Domain: Fixed Bo and record signal as a function of time
Specifications
Magnet:
Type: Electromagnet
Size: 15 cm Diameter, 45 cm Length, 800 turn #10 Cu wire
Field Sweep Range: 0-34 mT
Frequency Range: 100-500 MHz
Modulation Frequency: 10 kHz
Modulation Coils:
15 cm ID Helmholtz, 1.8 Ω
10 cm ID Solenoid, 2.3 Ω
4.3 cm Saddle, 13 Ω
Sample Probes:
Single Turn Solenoid: 150, 205, 251, 299, 354, 400, 440 MHz
Surface Coil: 2.9 mm 247 MHz, 2 cm 271 MHz
Largest Sample Diameter: 15 cm
LabView object oriented programming code and user
interface for the spectral acquisition program. Code sweeps
the magnetic field and collects data from the lock-inamplifier.
A1
RF
RF Sweeper
PD1
-20 dB
Marker
X
Y
O-Scope
P1
PD2
φ
÷2
A2
Control
A3
LabView
Computer
A4
IEEE-488
50Ω
F1
SW1
Lock-In
Amp
DBM
T1
SW2
AF
F2
PS2
Boost
Magnet
10A DC
Power
Supply
Amp 1
Probe
Modulation
Coils
PS1
DC
Power
Supply
Magnet
Primary Magnet
0-1V
Adder
D/A 1
D/A 2
Source
A1
A2
A3
A4
DBM
Amp 1
Amp 2
F1
F2
PD 1
PD 2
Sweeper
SW1
SW2
T1
Programmable Test Sources PTS-500
0-10/1 dB Variable Attenuator, Wavetek
0-10/1 dB Variable Attenuator, Wavetek
0-50/10 dB Variable Attenuator, Wavetek
0-10/1 dB Variable Attenuator, Wavetek
Doubly Balanced Mixer, Merrimac DM-2A-200
AF Amp Carver TFM-15, Techron 7570
Lock-In Amplifier, SRS 830 DSP
Low Pass Filter
High Pass Filter
-20 dB Directional Coupler, Merrimac CP-20-215
Power Divider (÷2), Pasternack PE2000
Wavetek 7062
DPDT Switch
DPDT Switch
Merrimac HJ-300 Magic Tee with 50 Ω Load
Size
6.3 cm3
SC
2.9 mm
2 cm
a
Example of spectral output from
the field cycling program (#2). The
sample is ferri/ferro magnetic
electrophotographic toner fused
on paper. For comparison, a 299
MHz, field cycled x0.01 spectrum
of
paramagnetic
DPPH
is
inserted.
ν (MHz)
150
205
251
299
354
400
440
245
271
Q
127
196
212
186
180
444
445
75
109
Relative Sensitivity
0.01
0.02
0.02
0.03
0.01
0.03
0.02
1.00
0.09
b
Sensitivity plots for the planes perpendicular to the surface of the a) 2 cm, and b) 2.9
mm diameter surface coils.
Summary
PTS-500
Tuning
Type
STS
R
I
T
Example of spectral output from the time
domain program (#3). Sample is the
barconde-39 of the letters RIT printed
with ferrimagnetic electrophotographic
toner fused on paper. Barcode was
scanned under the 2.9 mm SC.
A modular LFEPR spectrometer design for operating at frequencies
between 100 and 500 MHz is presented. The spectrometer can be
easily reconfigured to utilize different volume and surface coil sample
probes. Three different control programs allow the instrument to
record field swept, field cycled, and time domain spectra. For
applications, see the other posters done by our lab.
References
1. W.J. Ryan, N. Zumbulyadis, J.P. Hornak, MRS Proceedings, mrsf13-1656-pp03-03
doi:10.1557/opl.2014.708 (2015).
2. J.P. Hornak, M. Spacher, R.G. Bryant, Meas. Sci. Technol. 2 (1991) 520-522.