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Hahn Echo Program

for RadioProcessor and iSpin-NMR


Overview

The Hahn-Echo experiment applies a 90 degree pulse, waits a period of Tau, and then applies a 180 degree pulse. This causes an echo in the FID response of the sample, which peaks at a duration of Tau after the end of the 180 degree pulse. The Hahn-Echo experiment program is available for download at the bottom of this page, as well as with the other NMR programs in the RadioProcessor section of the SpinAPI Example Downloads.


Hahn Echo Example Results
Figure1. Hahn-Echo Experiment Results (a sample of household cooking oil using a 10.8MHz permanent magnet)


Figure 1 shows the resultant FID response when capturing data continuously during a Hahn-Echo experiment. The large negative spike is caused by the 180 degree pulse being applied. Figure 2, below, shows the pulse sequence used by the RadioProcessor to perform this experiment. Each vertical line is a new instruction, and the names of the durations used are shown above each time range.


Hahn Echo Pulse Sequence
Figure2. Hahn Echo Pulse Sequence



Running the Hahn Echo Program

The Hahn-Echo program accepts command line parameters to specify the values for each of the pulse program's parameters. It takes the following arguments as inputs:


Argument
Parameter
Units/Values
1
File Name

2
Board Number
#
3
De-blank Bit
0, 1, 2, or 3
4
Debug 0 or 1
5
ADC Frequency
MHz
6
Spectrometer Frequency
MHz
7
Spectral Width
kHz
8
Amplitude
0.0 - 1.0
9
90 Degree Pulse Time
us
10
90 Degree Pulse Phase
degrees
11
180 Degree Pulse Time
us
12
Include 90
0 or 1
13
Bypass FIR
0 or 1
14
Number of Scans
#
15
Tau
us
16
De-blanking Delay
ms
17
Transient Delay
us
18
Repetition Delay
s
Table1. Hahn Echo Input Arguments


These arguments must all be passed as input parameters when launching the hahn_echo.exe file. Omitting or passing arguments out of order will cause the program to return an error. To simplify this process, a hahn_echo.bat file is provided. This file allows for easy manipulation of argument values, and ensures that they will passed in the correct order. To use the hahn_echo.bat file simply:
  • Open the hahn_echo.bat file in a text editor
  • Set the parameters to the desired values
  • Save any changes made
  • Ensure that the hahn_echo.bat and hahn_echo.exe files are in the same directory
  • Double-click the hahn_echo.bat file
After following these procedures, the hahn_echo.exe will run with the specified parameters and print those parameters as well as the progress of the experiment to a terminal window. Once experimentation is finished, it will save the results to ASCII, JCAMP, and Felix files.



Viewing Hahn Echo Results

After execution of the Hahn Echo program, the output files can be used with external programs to visualize the data.

The LabVIEW NMR Interface can process .txt files. To do this:
  • Navigate to the 'Process Data' tab
  • Point the 'Import File Name' path to the Hahn Echo's .txt output.
  • Press 'Import Data From ASCII File' to view the data.

The Felix NMR program can process .fid files. To do this simply ensure that the Felix NMR program is installed properly and double-click on the .fid file.



Hahn Echo Acquisition Tips

Setting the input parameters correctly is critical in obtaining good Hahn Echo data. Below are suggestions for determining these parameters:
  • Start with a simple Single-Pulse NMR experiment:
    • Find the values of spectral width and number of points which optimize the signal-to-noise ratio without cutting off the FID.
    • Find the 90 and 180 degree pulse widths. These are the amounts of time it takes for a transmitted pulse to cause a 90 and 180 degree FID response. The 180 degree pulse width should be roughly double that of the 90 degree pulse width.
    • Find the resonance frequency and output phase which achieve a maximum real amplitude at the start of acquisition. The value of the first real data point should be a maximum, and the value of the first imaginary point should be zero.
    • These procedures are described in more detail here (using the LabVIEW NMR Interface).
  • Use the established Single-Pulse NMR parameters with the Hahn Echo program. The relevant parameters are:
    • BOARD_NUMBER
    • ADC_FREQUENCY
    • SPECTROMETER_FREQUENCY
    • SPECTRAL_WIDTH
    • AMPLITUDE
    • P90_TIME
    • P90_PHASE
    • P180_TIME
    • TRANSIENT_DELAY
    • REPETITION_DELAY
  • Estimate the TAU time. This should be approximately equal to the time it takes for your Single-Pulse NMR FID to decay to zero.
  • Do an initial run of the Hahn Echo program:
    • The FID from the initial 90-degree pulse should match the Single-Pulse NMR results.
  • Adjust the TAU parameter as necessary to produce the desired echo. A larger TAU will result in a more diminished echo. You can also increase the NUMBER_OF_SCANS parameter to average results over multiple runs. This will improve signal-to-noise ratio.


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