Scan/Reflection

ONLINE provides an automatic procedure that maximizes the intensity of a reflection by varying the diffractometer angles. The corresponding menu is invoked by:

ONLINE> SCAN/REF*LECTION

The following qualifiers may be supplied:

/COA*RSE
The scan range will not be adjusted.

/EXEC*UTE
The centering starts immediately. Control is not passed to the menu driver. If the centering was successful and the /store qualifier is supplied, the reflection is copied to the UB matrix queue element (UB_MATRIX). The user has to move the diffractometer near the reflection before the command is executed. This is typically done in command files. Example:

move/qx=1/qy=1/qz=2
scan/refl/exec
if !gra_status() then end
fc_qz() = 3
scan/refl/exec
if !gra_status() then end
...

The call to GRA_STATUS is used to test the completion status of the centering function. The status is 0, if if the centering algorithm failed or if the user interrupted the routine.

If the procedure does not find a peak, it doubles the scan range and retries the search. If the second search fails also, the function terminates with an error condition. In the non-automatic mode the user is asked for confirmation before the scan range is doubled.

SCAN/REFL may also be used to executed single angular scans. Use these qualifiers: /_two_theta, /_omega, /_chi and /_phi.

/FLOAT When reflection are added to the UB_MATRIX, their hkl values are rounded to integer numbers unless /FLOAT is supplied. In this case hkl is assigned the q-vector which was in effect when the centering procedure started (was stored when PF1 is pressed).

/NOBA*CKGROUND_SUBTRACTION
Suppresses the subtraction of the background during the determination of the peak.

/NODIS*PLAY Suppresses any graphical output. Most likely this is used in the /EXEC mode.

/_AUTO_FILTER=1_or_0

/_CENTERING_M*ODE=int_expr

/_CENTERING_ORDER=text_expr

/_NP_SCAN=int_expr The number of scan points.

/_RANGE_CHI=val

/_RANGE_OMEGA=val

/_RANGE_PHI=val

/_RANGE_TWO_THETA=val The initial values for the scan parameters.

The menu is displayed below (table 7.8).

**Table 7.7:** The **scan/reflection** Menu
--- REFLECTIONS --- SPECTRA> 2_Theta Omega Chi Phi A-Move [Deg.] Position 54.2565 27.03447 56.34136 -82.74974 --------------------------------------------------------------------------- The q-Vector Qx Qy Qz PSI A-Move Position 4.000172 0.00340366 -0.00808777 -50.0104 --------------------------------------------------------------------------- UB-Matrix: Mode: Bisecting Psi-Par.: 0 ASN: 2 Signal: 554 Cent. Order OTCP C. Mode: CMS Auto Filter NO --------------------------------------------------------------------------- Scan Range [Deg] 0.45 0.3 1.5 1 No. of points 21 Conv. Ratio 0.01 Iterations 1 --------------------------------------------------------------------------- PF1 - Center Reflection \| PF2 - Store Reflection \| PF3 - Ed Reflections PF4 - Exit \| KP7 - TWO_THETA Scan \| KP9 - OMEGA Scan KP3 - CHI Scan \| KP5 - PHI Scan \| KP_PERIOD - Dis. angles KP0 - Print \| KP1 - Save to disk \| KP_PERIOD - Dis. angles

Table 7.7: The scan/reflection Menu

                         --- REFLECTIONS ---
   SPECTRA>    
                    2_Theta      Omega          Chi           Phi
  A-Move [Deg.]
  Position          54.2565       27.03447      56.34136      -82.74974

---------------------------------------------------------------------------
  The q-Vector      Qx           Qy             Qz            PSI
  A-Move
  Position          4.000172      0.00340366    -0.00808777   -50.0104
---------------------------------------------------------------------------
  UB-Matrix:
  Mode:             Bisecting     Psi-Par.: 0            ASN: 2
  Signal:           554           Cent. Order  OTCP
  C. Mode:          CMS           Auto Filter  NO
---------------------------------------------------------------------------
  Scan Range [Deg]  0.45          0.3           1.5           1
  No. of points     21
  Conv. Ratio       0.01          Iterations    1
---------------------------------------------------------------------------
 PF1 - Center Reflection  | PF2 - Store Reflection  | PF3 - Ed Reflections
 PF4 - Exit               | KP7 - TWO_THETA Scan    | KP9 - OMEGA Scan
 KP3 - CHI Scan           | KP5 - PHI Scan          | KP_PERIOD - Dis. angles
 KP0 - Print              | KP1 - Save to disk      | KP_PERIOD - Dis. angles

When ONLINE enters the menu routine, it checks whether an UB_MATRIX exists.

The menu displays the diffractometer angles and allows the user to move them. The same is true for the components of the scattering vector.

Suppose you are in the fixed psi mode: It is possible to move the diffractometer to any $\psi$ value. No matter which Psi-Par. has been selected. But once you specify a q-move, the program moves the diffractometer back to Psi-Par.

The user may specify the scan range for each angle separately. A scan range of 2 means current positions $\pm$ 1. No. of Points are the number of points for a scan. Conv. Ratio is a quantity that defines the convergence: During the centering of a reflection ONLINE determines the new position of the maximum for every iteration and compares it with the result of the previous iteration. If $abs(old-new)/range < Conv. Ratio$ , the centering is terminated.

ONLINE measures the intensity by evaluating the symbol SIGNAL, e.g.: SIGNAL = [vfc(c1, timer, 1.)]. SIGNAL is created in EXP_INI_LIST.

If AUTO_FILTER is set to 1, SPECTRA tries to keep SIGNAL between FILTER_MIN and FILTER_MAX by moving an attenuator (FC_FILTER()) in steps of FILTER_DELTA.

The description of the PF keys:

PF1 - Center Reflection:
ONLINE performs angular scans to find the maximum of a reflection. The order in which the angles are changed is determined by the symbol CENTERING_ORDER, e.g.: If CENTERING_ORDER is set to OTCP, centering starts with OMEGA, then TWO_THETA, CHI and finally PHI.

Every angle undergoes a coarse scan first. The coarse scan is used to determine the scan range (2 $\times$ FWHM). If the fine scan range is less than 50% of the old one, ONLINE repeats the angular scan before it changes the next angle.

If the signal does not go down to half of the maximum. ONLINE informs the user that it is not able to determine the width:" FIND_MAX failed to determine the new width, continue? ". In case the user responds with "Y", the range is doubled and the program continues. Otherwise an error is generated which causes the centering routine to terminate.

The maximum of a scan is found be calculating the center of gravity or by taking the highest y-value. That depends on the value of the flag centering_mode which is part of the UB matrix ( 0 - CMS, 1 - Max Y).

If the angular scans pass the convergence test, the program terminates. Otherwise the fine scans are repeated ITERATIONS times.

Figure 7.2 displays the graphical output which is produced by scan/refl.

PF2 - Store Reflection:
The current angular and q positions (hlk) are stored in the list of reflections.

Before hkl are stored they are rounded to integer numbers. If the reflection scan menu was called with the /FLOAT qualifier, hkl are stored as they were when the centering procedure started.

The stored angles are shifted by the offsets ( $angle_{store} = angle + angle_{offset}$ ). The offsets have been calculated in a previous RAFIN ^7.1 run.

The user has to edit the reflection list in order to assign the correct indices. If ONLINE finds that the current reflection has a negative $two\_theta$ value, it applies the above mentioned transformation.

If ONLINE finds a reflection with the indices hkl=0-0-0, it changes them to 0-0-100.

PF3 - Ed Reflections:
Edit the reflection list. This command can be used to change a reflection, add a new one or delete reflections.

PF4 (Ctrl-X)
Exit this menu.

KP0
The current diplay is sent to the default postscript printer.

KP1
The data are written to a disk file. The names is center_ $vers$

.fio.

KP7
A single $two\_theta$ scan.

KP9
A single $\omega$ scan.

KP_MINUS
A single $\chi$ scan.

KP5 $\phi$ Scan:
A single $\phi$ scan.

KP_KOMMA Dis. Angles: Display the angles that correspond to a specified q vector. Do not move any motor.

**Figure 7.2:** Monitoring **scan/refl**