The ISIS_moderator McStas Component

14.2 The ISIS_moderator McStas Component

ISIS Moderators

Identiﬁcation

Author: S. Ansell and D. Champion
Origin: ISIS
Date: AUGUST 2005

Description

Produces a TS1 or TS2 ISIS moderator distribution. The Face argument determines which moderator is to be sampled. Each face uses a diﬀerent Etable ﬁle (the location of which is determined by the MCTABLES environment variable). Neutrons are created having a range of energies determined by the Emin and Emax arguments. Trajectories are produced such that they pass through the moderator face (deﬁned by modXsixe and yheight) and a focusing rectangle (deﬁned by xh,focus_yh and dist). Please download the documentation for precise instructions for use.

Example:   ISIS_moderator(Face ="water", Emin = 49.0,Emax = 51.0, dist = 1.0, focus_xw = 0.01,

focus_yh = 0.01, xwidth = 0.074, yheight = 0.074, CAngle = 0.0,SAC= 1)

Input parameters

Parameters in boldface are required; the others are optional.


Name	Unit	Description	Default



Face	word	Name of the face - TS2=groove,hydrogen,narrow,broad -	"hydrogen"
Emin	meV	Lower edge of energy distribution	49.0
Emax	meV	Upper edge of energy distribution	51.0
dist	m	Distance from source to the focusing rectangle	1.0
focus_xw	m	Width of focusing rectangle	0.01
focus_yh	m	Height of focusing rectangle	0.01
xwidth	m	Moderator vertical size	0.074
yheight	m	Moderator Horizontal size	0.074
CAngle	radians	Angle from the centre line	0.0
SAC	boolean	Apply solid angle correction or not (1/0)	1
Lmin	meV	Lower edge of wavelength distribution	0
Lmax	meV	Upper edge of wavelength distribution	0
target_index	1	relative index of component to focus at, e.g. next is +1	1
verbose	boolean	Echo status information at everu 1e5’th neutron	0

ISIS pulsed moderators

Introduction

The following document describes the functions obtained for models of TS2 as described in Table 14.2:


target	3.4cm diameter tantalum clad tungsten

reﬂector	Be + D\(_2\)O (80:20) at 300K

Composite Moderator	H\(_2\) + CH\(_4\)
Coupled	Groove: 3x8. cm 26K solid-CH\(_4\)
	Hydrogen: 12x11cm 22K liquid H\(_2\)

Poisoned Moderator	solid-CH\(_4\) 26K
Decoupled	Narrow: Gd poison at 2.4 cm - 8 vanes
	Broad: 3.3 cm – not fully decoupled

PreModerators	0.85 cm and 0.75 cm H\(_2\)O

Table 14.2.:

Description of Models

TS1 model is from the tungsten target as currently installed and positioned. The model also includes the MERLIN moderator, this makes no signiﬁcant diﬀerence to the other moderator faces.

Using the McStas Module

You MUST ﬁrst set the environment variable ‘MCTABLES’ to be the full path of the directory containing the table ﬁles:

1BASH: export MCTABLES=/usr/local/lib/mcstas/contrib/ISIS_tables/ 
2TCSH: setenv MCTABLES /usr/local/lib/mcstas/contrib/ISIS_tables/

In Windows this can be done using the ‘My Computer’ properties and selecting the ‘Advanced’ tab and the Environment variables button. This can of course be overridden by placing the appropriate moderator (h.face) ﬁles in the working directory.

The module requires a set of variables listed in Table 14.3 and described below.

The Face variable determines the moderator surface that will be viewed. There are two types of Face variable: i) Views from the centre of each moderator face deﬁned by the name of the moderator, for TS1: Water, H2, CH4, Merlin and TS2: Hydrogen, Groove, Narrow, Broad. ii) Views seen by each beamline, for TS1: Prisma, Maps, crisp etc. and for TS2: E1-E9 (East) and W1-W9 (West).

The McStas distribution includes some example moderator ﬁles for TS1 (water,h2,ch4) and TS2 (broad, narrow, hydrogen, groove), but others are available at
http://www.isis.rl.ac.uk/Computing/Software/MC/, including instrument speciﬁc models.

Variables E0 and E1 deﬁne an energy window for sampled neutrons. This can be used to increase the statistical accuracy of chopper and mirrored instruments. However, E0 and E1 cannot be equal (although they can be close). By default these arguments select energy in meV, if negative values are given, selection will be in terms of Angstroms.

Variables dist, xw and yh are the three component which will determine the directional acceptance window. They deﬁne a rectangle with centre at (0,0,dist) from the moderator position and with width xw meters and height yh meters. The initial direction of all the neutrons are chosen (randomly) to originate from a point on the moderator surface and along a vector, such that without obstruction (and gravitational eﬀects), they would pass through the rectangle. This should be used as a directional guide. All the neutrons start from the surface of the moderator and will be diverted/absorbed if they encountered other components. The guide system can be turned oﬀ by setting dist to zero.

The CAngle variable is used to rotate the viewed direction of the moderator and reduces the eﬀective solid angle of the moderator face. Currently it is only for the horizontal plane. This is redundant since there are beamline speciﬁc h.face ﬁles.

The two variables modYsize and modXsize allow the moderators to be eﬀectively reduced/increased. If these variables are given negative or zero values then they default to the actual visible surface size of the moderators.

The last variable SAC will correct for the diﬀerent solid angle seen by two focussing windows which are at diﬀerent distances from the moderator surface. The normal measurement of ﬂux is in neutrons/second/Å/cm\(^2\)/str, but in a detector it is measured in neutrons/second. Therefore if all other denominators in the ﬂux are multiplied out then the ﬂux at a point-sized focus window should follow an inverse square law. This solid angle correction is made if the SAC variable is set equal to 1, it will not be calculated if SAC is set to zero. It is advisable to select this variable at all times as it will give the most realistic results

Comparing TS1 and TS2

The Flux data provided in both sets of h.face ﬁles is for 60 \(\mu \)Amp sources. To compare TS1 and TS2, the TS1 data must be multiplied by three (current average strength of TS1 source 180 \(\mu \)Amps). When the 300 \(\mu \)Amp upgrade happens this factor should be revised accordingly.


Variable	Type	Options	Units	Description

Face (TS2)	char*	i) Hydrogen Groove Narrow Broad, ii) E1-E9 W1-W9	–	String which designates the name of the face

Face (TS1)	char*	i) H2 CH4 Merlin Water, ii) Maps Crisp Gem EVS HET HRPD Iris Mari Polaris Prisma Sandals Surf SXD Tosca	–	String which designates the name of the face

E0	ﬂoat	0\(<\)E0\(<\)E1	meV (Å)	Only neutrons above this energy are sampled
E1	ﬂoat	E0\(<\)E1\(<\)1e10	meV (Å)	Only neutrons below this energy are sampled

dist	ﬂoat	\(0< dist <\infty \)	m	Distance of focus window from face of moderator
xw	ﬂoat	\(0<xw<\infty \)	m	x width of the focus window
yh	ﬂoat	\(0<yh<\infty \)	m	y height of the focus window

CAngle	ﬂoat	-360 \(< CAngle <\) 360	\(^o\)	Horizontal angle from the normal to the moderator surface

modXsize	ﬂoat	\(0<modXsize<\infty \)	m	Horizontal size of the moderator (defaults to actual size)
modYsize	ﬂoat	\(0<modYsize<\infty \)	m	Vertical size of the moderator (defaults to actual size)

SAC	int	0,1	n/a	Solid Angle Correction

Table 14.3.:

Brief Description of Variables

Bugs

Sometimes if a particularly long wavelength ( \(> 20\) Å) is requested there may be problems with sampling the data. In general the data used for long wavelengths should only be taken as a guide and not used for accurate simulations. At 9 Åthere is a kink in the distribution which is also to do with the MCNPX model changing. If this energy is sampled over then the results should be considered carefully.

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