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McStas: ESS_moderator Component

[ Identification | Description | Input parameters | Output parameters | Links ]

The ESS_moderator Component

A parametrised pulsed source for modelling ESS long pulses.


  • Author: P Willendrup and E Klinkby, February 2014, derived from K Lefmann ESS_moderator_long
  • Origin: DTU
  • Date:(Unknown)
  • Version:(Unknown)
  • Modification history:
    • J. Saroun (


Produces a time-of-flight spectrum, from the ESS parameters
Chooses evenly in lambda, evenly/exponentially decaying in time
Adapted from ESS_moderator_long by: K Lefmann, 2001

Updates and simplified interface:
  1. The spectrum from the source(s) is defined via the sourcedef string input parameter which allows these values:
    • sourcedef="2001", legacy "Mezei moderators" from the original F. Mezei documents "ESS reference moderator characteristics for generic instrument performance evaluation", but rescaled to ESS TDR frequency, pulselength and power.
    • sourcedef="TDR", Mezei moderators, with a wavelength-dependent correction term to the cold flux, derived from 2012 MCNPX calculations by ESS neutronics group. Corrections calculated by K Lieutenant (Vitess) and implemented here by E Klinkby. NOTE: uses the 2001 brilliance for the thermal moderator!
    • sourcedef="2014", updated brilliance using formulation by Troels Schoenfeldt, including support for the "pancacke", i.e. flat geometry.
    • sourcedef="2015", updated brilliance using formulation by Troels Schoenfeldt, new butterfly baseline.
  2. The component can use target_index for focusing to a given beam port. Use an Arm() and ROTATED to position relatively to the moderator.
  3. The component relies on the new ess_source-lib which is expected to become further enriched during design-finaliziation and construciton of the ESS.

Note that this component does not implement "engineering reality" and currently uses a coordinate system centered on the moderator assembly. An updated moderator component which references the "Moderator focus coordinate system" will be released later during the spring of 2016.

Derived from ESS_moderator_long which was debugged intensively against Mezei note (4/12 2000) and VitESS @ Rencurel 2006. ----------------------------------------------- Correction by J. Saroun, NPI Rez: 1) version 2015: accepts negative port angles 2) version 2015: weight by cosine of the port angle Warning: The negative beamport angle is not taken into acccount by mcplot %VALIDATION Mezei-modererators validated against VitESS and Mezei note (4/12 2000) @ Rencurel 2006 Benchmarked against multiple versions of ESS moderator group simulation data 2013-2015

Input parameters

Parameters in boldface are required; the others are optional.
Name Unit Description Default
isleft 1 Fraction of thermal neutrons generated at the "left" moderator slab in case of "2013" or "2014" 0.9
Lmin AA Lower edge of wavelength distribution  
Lmax AA Upper edge of wavelength distribution  
cold_frac 1 Fraction of neutron statistics from cold source. It is implicitely assumed that supermirror allows each beamline to choose the desired fraction of cold and thermal neutrons (i.e. extreme idealization). 1.0
dist 0,0,dist [m] Distance from source to focusing rectangle; at 0
focus_xw m Width of focusing rectangle  
focus_yh m Height of focusing rectangle  
target_index 1 relative index of component to focus at, e.g. next is +1 this is used to compute 'dist' automatically. 0
tmax_multiplier 1 Defined maximum emission time at moderator, tmax= tmax_multiplier * ESS_PULSE_DURATION. Only in combination with sourcedef="2013", "2014" or "2015" 3
yheight_c m Height of the cold source 0.12
yheight_t m Height of the thermal source 0.12
n_pulses 1 Number of pulses simulated. 0 and 1 creates one pulse. The integrated intensity is constant 1
acc_power MW Accelerator power in MW 5
beamport_angle deg Direction within the beamport sector (0 < angle < extraction_opening for 2014, -extraction_opening/2 < angle < extraction_opening/2 for 2015) to direct neutrons. For sourcedef="2015", the only allowed values are 5,15,...,55 degrees measured from the central point. -1
sourcedef string ESS source "database", values: "TDR", "2001", "2013", "2014", "2015" "2015"
xwidth_c m Width / arc-length opening of the cold source. 0.1
xwidth_t m Edge of thermal source 0.18
extraction_opening deg Width of extraction-area in degrees (60 or 120 degrees). 120 deg only in combination with sourcedef="2014" and "2015". 120


[ Identification | Description | Input parameters | Output parameters | Links ]

Generated automatically by McDoc, Peter Willendrup <> / Wed Jan 12 15:13:20 2022

Last Modified: Wednesday, 12-Jan-2022 16:13:20 CET
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