chopcal¶

Chopper calculations that used to be locked inside McStas instruments, in Python.

A chopper train decides two things an instrument scientist cares about: where the wavelength band sits, and which neutrons a source need not bother emitting. chopcal answers both, using the same C the instrument uses — chopper-lib — so the numbers here and the numbers in a simulation cannot drift apart.

>>> import chopcal
>>> settings = chopcal.bifrost(wavelength_max=3.0)
>>> chopcal.lib.wavelength_windows(list(settings.values()))[0]
(1.1453582740163402, 3.0515432206219723)

Where to start¶

What it is for¶

Setting a chopper train. chopcal.bifrost gives the six BIFROST discs their speeds and delays from one number — the slow edge of the band you want.

Asking what a train passes. Given any set of discs, chopcal will tell you the wavelength bands that get through, on a grid, or exactly.

Not simulating what cannot arrive. A train that passes a percent of the plane makes a source spend ninety-nine percent of its rays on neutrons the discs stop. chopcal computes the region that does get through, and a sampler that draws from it — the same machinery McStas' own Polygon_ESS_butterfly uses.

Asking how much arrives. Give it a gridded source, such as a moderator's tabulated spectrum, and chopcal integrates it over the transmitted region exactly: the total, what reaches a monitor or the sample by arrival time and wavelength, or the mean and spread of wavelength in each arrival-time bin, the numbers a wavelength lookup table stores. With a finite beam it also weights each neutron by how likely it is to get through while a disc's edge is still sweeping the beam. See Weight the region by a source.

Setting the real choppers. chopcal.nicos gives BIFROST's speeds and experiment delays keyed by the NICOS devices that take them. See Set the choppers from NICOS.

What it is not¶

It is not a disc. chopcal asks when a chopper is open on the beam axis, widened by the beam's own width. Its weighted transmission models an edge crossing a uniformly lit beam as a linear ramp; it does not trace a neutron through a slit. That agrees with McStas for BIFROST's wedge-shaped openings to within counting error. For openings of another shape, or a beam that is not evenly lit, simulate the disc.

It is not a beamline either. Every neutron flies one path from source to sample; a guide's spread of path lengths smears arrival times, and only Region.transmit's path_spreads accounts for it, as a support rather than a distribution.