question regarding the user variables in Monitor_nD

[HaShuai123]

Dear experts,

I have some problems about how to use the user variables.

I copied an example in the user manual, but the compilation failed because it doesn't recognize the variable t.

COMPONENT UserMonitor = Monitor_nD(xwidth = 0.1, yheight = 0.1,
user1 = log(t),username1="Log(time)",
options ="auto user1" )
AT (0,0,2) RELATIVE Sample_rot

And, can I do some math operations on the variable t? for example, modulo operations and division by certain numbers?

Any help will be appreciated.

Best regards,
Ha Shuai.

[willend]

Dear @HaShuai123,

You have unfortunately stumbled upon an inconsistency between the manual and reality, in McStas 3.x Monitor_nD makes use of USERVARS defined in your instrument, e.g.:

USERVARS %{
  double log_of_t;
%}

At a given point in the instrument you can apply an EXTEND section to set the value:

COMPONENT Sample=Incoherent(…)
AT (0,0,0) RELATIVE somewhere
EXTEND %{
  log_of_t=log(t);
%}

These are then probed via a string-identifier, e.g.

COMPONENT Measure_t=Monitor_nD(xwidth = 0.1, yheight = 0.1,
user1 = “log_of_t",username1="Log(time)",
options ="auto user1" )
AT (0,0,2) RELATIVE Sample_rot

[HaShuai123]

Dear @willend ,

Thank you very much, this helps.

Best regards,
Ha Shuai.

[HaShuai123]

Dear @willend,

I am trying to use the Monitor_nD to model a detector. At every angle, the detector gives 2 other signals.
options = "banana, theta limits=[10,130], bins=240, user1 limits=[0, 1899], bins=1899, user2 limits=[0,800], bins=800"

But the Monitor_nD cannot output the 3D data. Is there a way to output 3D data? Or an easier way to slice the 3D data into 2D data at 240 angles?

Thanks in advance.
Best regards,
Ha Shuai.

[willend]

Dear @HaShuai123,

You are right, at least in ascii McStas output mode we have no direct way of saving 3D data, but there are workarounds:

1. You could try using NeXus output (compile your instrument with --format=NeXus - can be triggered to happen automatically by adding DEPENDENCY “ @NEXUSFLAGS@ “ in the instrument, see e.g. the Mantid category of the McStas examples. A 3D volume should be written to the relevant portion of the resulting hdf5 file.

2. You can add 240 Monitors (!) each writing one slice… (e.g. using WHEN statements)

[HaShuai123]

Dear @willend,

I tried to use --format=NeXus, but instead of a 3D volume, it gives me 3 datasets in the mccode.h5 file for theta, U1 and U2, respectively .

Below is the monitor description I used
Monitor_nD(user1 = "tof",username1="TOF(time)", user2 = "choppertime",username2="Chopper",
radius = 2.5, yheight = 2, options = "mantid banana, theta limits=[10,130], bins=240, user1 limits=[0, 1899], bins=1899, user2 limits=[0,800], bins=800", restore_neutron=1)

Best regards,
Ha Shuai