VITESS Tool Compute Chopper Phases

# VITESS Tool Chopper Phases

This application determines the optimal apertures and phases of disc choppers of a time of flight (TOF) instrument. It is designed for instruments installed on a spallation source, but it can also be used for choppers in TOF instrument at a reactor source. Then the first chopper must be regarded as the source. It is meant to deliver the input data for MC simulation with the VITESS software package, especially those of the disk chopper module.

There are 2 options:
A) ComputeChopperPhases
The phase of a chopper is calculated (cf. chapter 'direction of rotation...'). The length of the pulse is taken into account. The phase is chosen in a way that the center of the pulse reaches the center of the chopper (at the given length). The aperture of the chopper is used to give additional information (about the wavelength range that can pass the chopper).
B) DesignChopperSystem
For a set of choppers, the apertures must fit to each other as well as the phasing (cf. chapter 'pulse length....'). This application helps to design the chopper system and to get information about the usable wavelength range at the detector (cf. chapter 'evaluation time.....').

In both cases you can choose the average (option 'a') or the minimal (option 'm') wavelength that can pass. Some of the input or output data may need an explanation:

### 'pulse length' and 'max. pulse length', 'frame overlap chopper' and 'wavelength band chopper'

The 'pulse length' and the 'wavelength band chopper' (WB chopper) determine that part of the pulse that can pass the chopper system and can be evaluated at the detector. There are 2 ways of determining the apertures of the frame overlap choppers:
a) The idea is, not to stop any neutron of the right velocity by a frame overlap chopper that starts in that time interval. The frame overlap
choppers shall only prevent frame overlap.
b) The frame overlap choppers shall not increase the half shadow time, i.e its half shadow is at most as large as that of the wavelength band chopper. With this condition the apertures are larger than by condition a).
The maximal pulse length is necessary to determine the first arrival of neutrons of the pulse.

### 'evaluation time', 'beam diameter'

The evaluation time is the time of a pulse that can be used for data collection and evaluation. The wavelength band chopper and the actual frame overlap chopper are considered in the calculation.
- the 'TOF for desired wavelengths' is the time interval from the time of arrival of the fastest neutrons starting at the beginning of the pulse until the time of arrival of the slowest starting at t=t_p (pulse length).
- Neutrons starting at the end of the pulse (t=t_max, max. pulse length) can have a higher velocity than planned and arrive before the fastest from the beginning of the pulse. Analogously, neutrons from the beginning of the pulse can arrive after the slowest starting at t=t_p. Therefore, the 'TOF range at the detector' is larger than the 'TOF for desired wavelengths'.
- The 'maximal evaluation time' is between the arrival of the last neutrons of the previous pulse and the first of the following pulse.
- The 'optimal evaluation time' is even shorter: The evaluation time should further be reduced by the time, in which the intensity is reduced by the chopper moving into the beam. (No evaluation in the time of "half shadow"). The time that the chopper needs to pass the beam depends on the beam diameter and the distance: center of beam - center of chopper. This time must then be multiplied by the ratio: distance source - WB chopper to distance WB chopper - detector.
Warning: The calculated times are not valid any more, if the frame overlap chopper is positioned behind the wavelength band chopper.

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The following input data need to be given:

Parameter Unit | Phase calculation | Chopper system | Description | Range or Values | Command Option
repetition rate of pulses [1/s] |  | X | frequency of pulses generated by the source | > 0 | -R

rounds / min.
[1/min]

X

X

rotational speed of the choppers; (different speeds in one chopper system cannot be treated by this tool)
> 0: chopper phase approaches + infinity with time; => initial phase < 0
< 0: chopper phase approaches - infinity with time; => initial phase > 0

> 0 or
< 0

-s

distance source - detector [m] |  | X |  | > 0 | -D
pulse length [ms] | X | X | length of the neutron pulse | >=0 | -p
center of pulse / delay [ms] | X | X | average starting time of the neutrons pulsed source: half of pulse length continuous source: zero or half of pulse or anything else, depending on zero time | no restriction | -d
max. pulse length [ms] |  | X | pulsed source: time at which intensity of pulse is practically zero continuous source: usually = pulse length | >= pulse length | -m
aperture (WB) chopper [deg] | X | X | for phase calculation, this is the aperture of the only chopper under consideration for the chopper-system design, this is the aperture of the wavelength band chopper | > 0 | -a
distance source - (WB) chopper [m] | X | X | distance to the chopper defined in the previous row | >0 and < dist. detec. | -C
beam diameter at WB chopper [cm] |  | X | the beam diameter influences the half shadow time and is therefore necessary to calculate the usable wavelength range | > 0 | -c
distance: beam - chopper axle [cm] |  | X | (as the beam diameter), the distance beam - chopper-axle influences the half shadow time and is therefore necessary to calculate the usable wavelength range | > beam diameter | -A
Distance source - FO chopper [m] |  | X | distance to the frame overlap chopper, whose aperture and phase shall be determined | > 0 and < dist. detec. | -F
beam diameter at FO chopper [cm] |  | X | the beam diameter determines the opening and closing and is therefore necessary to calculate the aperture of the frame overlap chopper | > 0 | -f
wavelength [Ang] | X | X | minimal or average wavelength that shall pass the chopper (system) - s.a. next row | > 0 | -W
wavelength range determined by | X | X | the wavelength is: a: the average wavelength m: the minimal wavelength of the wavelength band that passes the chopper (system) | > 0 | -w
FO chopper |  | X | the frame overlap chopper apertures are chosen in a way that they h: do not increase the half shadow time n: do not stop any neutron of the right wavelength range that is able to pass the wavelength band chopper | h or n | -k

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