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Changing Samples

**** DO NOT APPROACH THE BEAM PATH WHILE THE SHUTTER IS OPEN ****

The sample position (driven by motor #2) and the detector bank (driven by motor #9) must be coupled together during data collection. However, it is difficult to load and unload the sample while the sample table and the detector bank are coupled. Therefore, one should uncouple the instrument in order to load a sample and couple the instrument to begin collecting data. The coupling and uncoupling can be done with the commands:

tr- Uncouples motors #2 and #9.
tr+ Couples motors #2 and #9.


When a run (or sequence of runs) is finished, the priority is to first close the neutron beam shutter by turning the shutter switch to the BEAM OFF position. The shutter switch is located to the left of the computer terminal. Once the beam shutter is closed, uncouple the sample table (motor #2) from the detectors (motor #9) and separate them by executing a sequence of commands such as:

tr- Uncouples the sample table from the detectors.
d9h=80 Sends motor #9 to the hardware angle of 80.
d2h=50 Sends motor #2 to the hardware angle of 50.


While this sequence is only an example, it creates enough space to easily change samples. **** DO NOT APPROACH THE BEAM PATH WHILE THE SHUTTER IS OPEN ****



FAST NEUTRON BACKGROUND

The "fast background" is the result of neutrons with incident energies higher than the cut-off limit reaching the detectors and should be subtracted from all sample run. In order to measure the fast background for a sample, it is necessary to block the 'good' neutrons from the detectors. This is achieved by placing a cadmium shield between the sample position and the detector bank. In order to place the Cd shield properly, first ensure that the beam shutter is closed, uncouple the sample table (motor #2) from the detectors (motor #9) and then separate them by executing a sequence of commands such as:

tr- Uncouples the sample table from the detectors.
d9h=80 Sends motor #9 to the hardware angle of 80.
d2h=50 Sends motor #2 to the hardware angle of 50.


Once the Cd shield is in position, execute the appropriate TIME sequence run set up. Typically one can set-up a fast background run to count from 60 to 300 seconds per energy point and choose a coarse energy step size. Additionally, a "blank" (sample can without the sample) can be measured which can be then subtracted from the sample run.


Instrument scientists
Craig M. Brown (301) 975 5134
Terry Udovic (301) 975 6241
Juscelino Lećo (301) 975 8867


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Go to the NCNR Thermal Neutron Instruments page.


Last modified 04-May-2004 by website owner: NCNR (attn: Craig Brown)