Read data from TOFTOF after convert:
Some words before the description of the procedure. Vanadium data are used to:
- Search for the elastic channel. This is done taking the maximum of Vanadium spectra, and assigning the origin to this channel
- Normalize of the intensity as a function of the angle (detector). That means that the program will integrate the peak of Vanadium (from the maximum, to 2*(half width at half maximum) in both directions of the peak) for every spectrum, will get a value, and afterwards it will divide each spectrum of your sample to this value. This has the problem that vanadium is NOT corrected for self absorption, and can give a slightly wrong angle dependence. You can then correct your vanadium using the self absorption correction procedure. It's also explained below.
- The detector efficiency correction, is energy dependent and has nothing to do with Vanadium. It simply takes in to account the efficiency of detector that would be less as the energy of the neutron increases, and is implemented as a formula.
| rtof | to read TOF data in a pseudo-IN6 format |
| 0 | read current cycle data |
| 0 | number offset |
| 13940-13943,13945 | sample numors |
| 13333-13334 | Background numors ( write - if you have no empty can measurement) or want to subtract it later on after self absorption correction |
| 1222-1223 | Vanadium numors ( - if you do not have it) used for normalization of the detected intensity. is not working correctly only in the TOFTOF ida version, therefore do not trust the intensities when not using that one. |
| - | Vanadium Background numors |
| y | to normalize to monitor (appears only if no vanadium numors were given). Works only correctly in the TOFTOF ida version (wavelength-dependence of monitor sensitivity was not accounted for) |
| y | to convert to energy |
| n | generate sig(2th,w): say y if you want to obtain d2σ/(dΩ dE) instead of S(2θ,E) – only difference is a factor kf/ki |
| 0 | elastic channel from vanadium. it will search the channel with highest intensity for every detector. do not take elastic channel from sample because this search is done in time space. the subsequent multiplication (when going to E space) with t4 shifts any broadened peak. |
| y | to correct detector efficiency |
| y | to take Debye Waller Factor from Vanadium. if measured with a non-standard temperature, frida will ask for confirmation of that temperature later on. check the DWF in that case. |
| 4 | to treat frame overlap at TOFTOF |
| 1000 | maximal energy gain (will scroll data) |
| 0.99 | maximal energy loss (will delete data) |
| n | do not sum spectra at equal angle so that bad detectors are not masked by taking the average with a good one |
| 1-3 | to exclude spectra which contain monitor informations (they are not all used but are in the converted file for consistency with IN6) |
| 3 | to delete spectra if vanadium data are bad |
| - | to go out from the read menu |
| df | the program will write the files it has (analog to “ls”) |
You can now perform the self absortion correction, explained here.
Self absorption correction of vanadium
We will perform the SAC for Vanadium, in order to divide by the right Q dependence our data… In file 1 is the vanadium, read WITHOUT vanadium numors.
| 1 | _sac |
| 0 | fully iluminated |
| 3 | hollow cylinder |
| 1.17 | outer radius of the sample, in this case vanadium without container from TOFTOF |
| 0.06 | thickness of vanadium (the TOFTOF one in this case) |
| 1 | The transmission are set to 1 because there is no sample holder |
| 1 | again |
| 0 | no backscattering |
| 72 | the number density for vanadium |
| 5.08 | scattering cross section of V |
| 5.08 | absorption xs |
| 80 | |
| 80 | seems reasonable |
And now you should get ONLY Assc, since there is no sample holder, there will be no Arel. And now:
| 1 | where our vanadium is |
| oy | |
| / | |
| y2 | |
| 2 | where our Assc is |
| 3 | our divided data |
| oi | |
| 4 | integrate |
| -0.1 0.1 | this should contain the elastic peak. extract the HWHM before (another oi option) and integrate from -2*HWHM to 2*HWHM. |
| 1 | to save as a file |
And now we have got the “numbers” to normalize your sample!!! Before going on, you should however check that the integral performed with and without SAC correction are not similar. If they are similar, maybe is not worth to do it for every sample… up to you!