IN5 resolution in a nutshell

Double entry plot extracted from the analytical formulae [1]. The green lines are the IN5B resolutions vs the chopper velocity (X-axis) and the wavelength (Y-axis). The white lines define regions with different frame overlap ratio (a region where N=3 means that we need to spin the frame overlap chopper 3 times slower - 2 neutron pulses over 3 lost - than the other choppers to avoid frame overlaping) according to the rule _max = 1.5 _o where _o is the incident wavelength. The numbers stands for the frame overlap ratio

As an example, a wavelength of 5 Å and a chopper velocity of 8500rpm give a resolution of 100 µeV with a frame overlap ratio of 3 (as we are close from the boundary a ratio of 2 might be also correct).

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Figure 1

The color scale superimposes the measured absolute flux with the above calculated figure. It combines with the measured flux (see below).

Absolute flux measurements

The figure below shows the absolute flux as measured by gold foil activation for one velocity, 8500 rpm, i.e. half the maximal velocity, for a frame overlap ratio of 1 (same velocity for all the choppers). Doubling this velocity leads to a loss of flux by a factor 4.

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Figure 2

Compute your own optimal resolution/flux parameters

The neutron intensity (here the neutron current I[n/cm²/s]) depends on the incident flux, the chopper velocities and the frame overlap ratio:

 

_o( ) [n/cm²/s] is the absolute flux per wavelength (unknown), N is the frame overlap ratio (see Fig. 1 above) and  is the master chopper velocity.
With I_o( ) coming from Fig. 2 above, we have then:

Examples of possible combination

[*] Measured by gold-foil activation.

1. R.E. Lechner, Proceedings of the ICANS-XI, KEK reports 90-25 (Eds: M. Misawa, M. Furusaka, H. Ideka, N. Watanabe, 1991).