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

The ILL_H521_NeXT Instrument

NeXT - Neutron and X-ray Tomography - cold neutron imaging at the end of H521

Identification

• Site: ILL
• Author: DRAFT, Peter Willendrup with Claude from public ILL web information
• Origin: ILL
• Date: September 2026

Description

DRAFT model of the NeXT cold-neutron imaging instrument at the ILL, located
at the end of the H521 guide, downstream of D16 and SuperADAM.

Source term and guide: the H5 horizontal cold source (HCS) and the H52 /
H521 guide geometry are copied from the ILL_H5_new example (proposed H5
layout from the 2011-2013 H5 upgrade design; the as-built geometry may differ).
The D16 and SuperADAM PG002 monochromators are kept IN TRANSMISSION: their
Bragg-reflected neutrons are removed from the beam (use upstream_monos=0 to
switch that off).

NeXT itself (public ILL data, 2026):
- pinhole diameters D = 5, 10, 15, 23, 30, 40 mm
- pinhole-sample distance L = 5.5 - 10.5 m (L/D up to ~2000)
- reference flux: 3e8 n/cm2/s at L/D = 333
- white beam, or wavelength selection by
mode=1: velocity selector, 2-20 AA, dl/l ~ 16 %
mode=2: double-crystal monochromator (DCM), 2-6 AA, dl/l ~ 5 %
- detector FoV up to 170x170 mm2 (scintillator + sCMOS, 2048x2048)
The sample is a simple cylinder (PowderN, e.g. Fe.laz) so that attenuation and
Bragg edges show up in the transmission spectra. Detector resolution/blur of
the scintillator/camera is NOT modelled (ideal position-sensitive monitor).

ASSUMPTIONS (to be checked against ILL drawings):
- L_guide_end: length of H521 guide after the SuperADAM monochromator
- selector (alpha, length, radius) chosen to give dl/l ~ 16 %, not the
actual NeXT selector specification
- DCM modelled as two parallel flat PG002 crystals, mosaic 30', r0=0.9,
first order only by default (no lambda/2 filter modelled)
- pinhole placed 0.8 m after the guide exit / selection optics

Examples

(Test cases in bold)

• Test: lambda=5.5 dlambda=4.5 mode=0 D=0.03 L_coll=10 Detector: NeXT_flux_sample_I=3e8

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
lambda	AA	Centre of the simulated source wavelength band	5.5
dlambda	AA	Half width of the simulated wavelength band	4.5
mode	1	0=white beam, 1=velocity selector, 2=double crystal monochromator	0
sel_lambda	AA	Velocity selector wavelength (mode=1)	4
dcm_lambda	AA	DCM wavelength (mode=2), 2-6 AA	4
D	m	Pinhole diameter: 0.005, 0.010, 0.015, 0.023, 0.030, 0.040	0.03
L_coll	m	Pinhole - sample distance, 5.5-10.5	10
sample	1	1: put the cylinder sample in the beam, 0: open beam	1
sample_file	str	PowderN reflection list of the sample (e.g. Fe.laz, Al.laz)	"Fe.laz"
sample_radius	m	Radius of the cylindrical sample	0.005
sample_height	m	Height of the cylindrical sample	0.05
det_dist	m	Sample - detector (scintillator) distance	0.03
det_size	m	Detector field of view (square), up to 0.17	0.17
det_bins	1	Number of detector pixels along x and y	200
upstream_monos	1	1: remove the D16 and SuperADAM monochromator reflections	1
D16_lambda	AA	D16 monochromator wavelength	5.6
SADAM_lambda	AA	SuperADAM monochromator wavelength	4.4
L_guide_end	m	Length of H521 guide after the SuperADAM monochromator (assumed)	1.0
dcm_order	1	DCM Bragg orders: 1 = first order only, 0 = all orders (lambda/2, lambda/3 contamination, all with the same r0 -> overestimated)	1
sel_alpha	deg	Velocity selector twist angle (sets dl/l; assumed value)	26
dcm_mosaic	arcmin	DCM crystal mosaic (sets dl/l; assumed value)	60
direct_focus	1	1: source illuminates the H52 entrance directly (efficient),	1

Links

• Source code for ILL_H521_NeXT.instr.
• https://www.ill.eu/en/for-ill-users/instruments/instruments-list/next/
• https://www.ill.eu/en/for-ill-users/instruments/instruments-list/next/characteristics/
• ILL_H5_new.instr (McStas examples/ILL) for the source and H52/H521 guide

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

Generated for mcstas 3.9.2

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Last Modified: Tuesday, 06-Oct-2026 21:19:11 CEST