The nELBE beam scanner
At nELBE a beam scanner is available to scan the spacial profile of the neutron beam. It consists of two plastic scintillator strips that are 1 m long/42 mm thick/11 mm wide. Each strip is equiped with two Hamamatsu R2059-01 photomultiplier tubes (PMT), one at each end. Both detectors can be moved by means of Berger-Lahr stepping motors. One detector moves in vertical the other one in horizontal direction.
How to move the detectors
The movement of the scanners is controled by means of a LabView program installed on PC FWK374.
The following steps have to be done to get the program started:
Scanner H (horizontal) | Scanner V (vertical) | ||
|
|
||
|
C:\Documents and Settings\ntofdaq\Desktop\Motorcontrol_nELBE |
||
|
double click Motorcontrol_nELBE.exe |
double click Motorcontrol_nELBE_2.exe |
|
|
COM4, Motor 1 | COM3, Motor 1 | |
|
click "Ausführen" button | ||
|
click "OK" button | ||
|
click "power on motor" button | ||
|
click "Ref" buttom wait until scanner has reached zero position |
* numbers in parenthesis corespond to the arrows shown in Figures 1 and 2 |
Figure 1: Illustration of the sequence to start the motor control
Figure 2: Illustration of the sequence to start the motor control - Confirm error message
Table 1: Assignment of the PMTs of the two scanner detectors
|
Table 2: Recommended voltage combinations
|
The different buttons and text fields have the following meanings:
Button | Meaning | Comment |
|
COM port of the USB to RS485 Converter | Should be changed only before starting the program |
|
Address of the motor | Can be changed by an internal rotary switch |
|
Velocity of the scanner used for the next movement |
Can be choosen in multiples of 12.5 mm/min |
|
Distance to go during the next movement | |
|
Final position of the next movement | |
|
Button to switch on the motor | Has to be pressed before any movement can be done |
|
Move motor over user defined distance into positiv direction | |
|
Move motor to user defined position | |
|
Move motor over user defined distance into negative direction | |
|
Load a list of position to be reached sequentially | The list is an ASCII file containing one comment line and then a list of number pairs separated by a space corresponing to the position in mm to be reached and the time in seconds to wait at this position. Examples can be found in the program directory C:\Documents and Settings\ntofdaq\Desktop\Motorcontrol_nELBE |
|
Find absolute zero position | The motor will move the detector until the limit switch is triggered and slowly move back about 3.5 mm. |
|
Move 5 mm in negative direction | This button has to be used, when the limit switch was triggered before the program was started. |
|
Move 5 mm in positive direction |
This button has to be used, when the limit switch was triggered before the program was started. |
|
Immediately stop current movement | If the List option is in use and the detector is waiting at a certain position this button will come into effect not before the waiting time has passed. |
|
Stop the program regularly | |
|
Stop the program irregularly | |
|
Current velocity and absolute position of the detector | |
|
Displays that lights up green if the motor is inialized and/or referenced |
How to run a beam scan
To perform a beam scan the transmission DAQ has to be used and the following preparations have to be done (if not yet done):
|
|
|
|
|
move cables in room 250 from patch field 9/10 to 19/20 / 29/30 |
|
see Pb loop control and Absorber |
|
perform step 1-9 from "DAQ manual" |
|
in setup.ini: "sis3820_LNEtime = 5" |
|
move scanner to beam position given in Tab.1 (velocity = 250 mm/min) and adjust voltage (cf. Tab. 2) to get count rate of about 10-20 kHz |
|
position = 0 mm, velocity = 250 mm/min |
Two different kinds of beam scans are usually performed. A wide range scan with moderate velocity over the the complete distance the detector can move or a narrow range scan with lowest velosity over a distance just covering the beam. The former takes 34 minutes the latter 28 minuntes.
The following steps have to be done to perform the scan:
wide range | narrow range | ||
|
set velocity to 250 mm/min | ||
set position | 0 mm | 335 mm | |
move to position (click "Pos") | |||
|
set scan velocity | 50 mm/min | 12,5 mm/min |
|
perform step 1-3 from "Running the DAQ" (DAQ manual) | ||
|
click "List" button | wide_scan_v=50_start=0.txt | narrow_scan_v=12.5_start=335.txt |
|
|||
|
perform step 5-6 from "Running the DAQ" (DAQ manual) | ||
|
position = 0 mm, velocity = 250 mm/min |
Using the position list forces the scanner to move to the final position and immediately back to start position. This helps to determine the absolute position of the beam center.
The content of the position lists is as follows:
wide_scan_v=50_start=0.txt
insert: [position in mm]spacebar[time of measurment in s] |
narrow_scan_v=12.5_start=335.txt
insert: [position in mm]spacebar[time of measurment in s] |