Simulations of Chopper Jitter at the LET Neutron Spectrometer at the ...

Simulations of Chopper Jitter at the LET Neutron Spectrometer at the ISIS TS2 53. Chopper JitterPhysical beam choppers are never perfectly at the desired phase, but rather deviate bya random error at any given time. Whereas most neutron simulations are performedwith mathematically perfect beam choppers, here a random phase error (jitter) is addedto give more physically realistic choppers. For the purposes of this article we choose thejitter to be Gaussian, parametrized by the width of the Gaussian error[6]The size of the jitter is then defined as the width of the Gaussian error, which is usedas the jitter parameter.For every event where a neutron ray reaches a chopper, the phase θ is calculated thus:θ = θ 0 + j ∗ rand norm ∗ ω (1)Where θ 0 is the chopper position without jitter, j is the jitter parameter for the chopperwith units of time, rand norm is a normalized Gaussian random number (with σ 2 = 1)and ω is the angular velocity of the chopper.To illustrate the effect of chopper jitter, we consider the basic equation for flight time:t = α ∗ λ ∗ L (2)where λ is the neutron wavelength, L is the flight length, and the constant α isα = m n /h ≈ 252 µs/m/Å.[10]Hence, if the jitter is the only source of uncertainty, we have that the uncertainty in thewavelength is:dλ = dt(3)αLSo for a realistic jitter value, (dt), of 0.4 µs in the second resolution choppers, located at15.7 m from Res1, this contribute to the uncertainty in the wavelength (dλ) of 6*10 −5 Å,and thus a dE of 0.1 µeV for 5 meV neutrons. In comparison the resolution is at about20 µeV at 250 Hz.Since the energy of neutrons arriving in the physical detector bank is calculated bythe ToF-method, a small discrepancy in the arrival time at the sample, can, over the 3.5m from the sample to detector, translate into a much larger difference in the neutronenergy perceived by the detector. Considering again the jitter dt = 0.4 µs in the 2ndresolution choppers, this corresponds to an uncertainty in the perceived wavelength (dλ)of 510 −4 Å, and thus a dE of 1 µeV for 5 meV neutrons.Some neutron instruments compensates for jitter with a ’veto’ system, where anevent is discarded if an unacceptable discrepancy of the chopper phase is detected. Theveto scheme is not included in the present simulations.4. Simulation resultsThe instrument was built in the McStas neutron simulation package[2, 3], by addingthe physical components (moderator, guide sections, beam choppers) sequentially, with