[mcstas-users] Reflectivity and transmission files of Ge crystal

Thu May 17 15:51:38 CEST 2018

Hello again Saed,

I have now had the time to take a closer look at your instrument file - which to first order looks OK, but raises a few of questions:

1) You write in a previous email that
" although I get almost exact matching result at monochromator position. “
- which I take to be the number indicated in your instrument file, i.e.
"The Real (measured) flux at monochromator is 2.7e8 (n/cm2.s).” - correct?

Now, your instrument as it comes has a double definition of wavelength produced from the source, where the “narrow” one
lambda0 = Lambda, dlambda = 0.01*Lambda will take preference over the “wide” one between Wlmin=0.1 and Wlmax=10.

I have now defined an input parameter (Narrow = 0/1) that allows to switch between the “full range” and “narrow range” setting.
So I assume I should run with the wide interval to reproduce your “almost exact result”?
(Otherwise I only get Before_Mono_I=3.39633e+06)
With the wide interval I instead get: Before_Mono_I=2.54145e+08
Using the Monitor_nD feature “capture” I actually get a little less, namely 2.17419e+08 - this indicates to me that perhaps your source
intensity I=1.28e14 should be increased ever so slightly.

2) I think of lattice spacings as material constants, I have therefore defined a GeMode =0 /1 to switch between Ge 331 and 511. The wavelength is then calculated
   from DM and Theta_M.

3) Your monitor Before_Mono_I overlaps with the Monochromator component, and should therefore apply the restore_neutron=1 flag.
(Otherwise only the second half of your Monochromator will be illuminated, due to a propagation/shadowing effect: A monitor propagates to its geometrical plane).
- This brings a factor of 2 to all mono settings!

4) You have added a phenomenological size-change of your mono-sample collimator. I would correct this way, but rather adjust e.g. the RV of the monochromator slightly.
Optimising with a scan I found that RV=2.6 brings a slightly more focussed beam/smaller footprint.

5) With reference to both mosaicity, it is well known that these vary quite a lot over a batch of (wafer-stacked, deformed) Ge monochromator slabs. Do you have access to
     measurement data for all / some of the actual mounted crystals? Who was the original manufacturer or the monochromator?

6) I agree with Erik Iverson that 0.3 seems a little low - and it should certainly vary between the two reflections… Are there measurement data available from the actual,
    mounted mono slabs?

7) For which reflection / Bragg angle do you measure the quoted value of 1.3e6 n/cm.s at the instrument?
And what was the used collimator setting?
Was a Be filter or equivalent in place or were multiple orders in fact measured simultaneously?

Adjusting the reflectivity up by a factor of 2 and taking out your collimator (COLL_MOUNTED=0) brings the measured capture flux at the sample position closer to your
experimental value, I get:

Ge 331 @ 45 degrees: 8.5e5 with collimator, 1.4e6 without collimator
Ge 331 @ 59 degrees: 2.7e5 with collimator, 4.3e5 without collimator

Ge 511 @ 45 degrees: 1.0e6 with collimator, 2.0e6 without collimator
Ge 511 @ 59 degrees: 3.5e5 with collimator, 5.6e5 without collimator

And If I allow multiples (order=0) in the Ge 511 @ 45 degrees setting with collimator I get 1.1e6?

Bringing up the mosaicity another 10’ also seems to help.

The next steps require digging into experimental reports I don’t have access to, that work is for you. :-) Or maybe one could even repeat a couple of the measurements under known conditions? Varying some of the same parameters that I vary above in simulation?

Getting “complete” agreement is hard work, but it can be done. Requires both experimental and modelling-oriented considerations.

Best and hope this helps you further in your hunt for discrepancies,

Peter Willendrup

Peter Kjær Willendrup
Forskningsingeniør, Speciakonsulent
Næstformand for DTU Fysik LSU

DTU Physics

[cid:1c4f711e-6d4b-43f1-b332-d38856f8af6c at win.dtu.dk]

Technical University of Denmark

[cid:9c4626c0-9752-4080-8b3a-849e7c3f29cc at win.dtu.dk]

Department of Physics
Fysikvej

Building 307

DK-2800 Kongens Lyngby

Direct +45 2125 4612

Mobil +45 2125 4612
Fax +45 4593 2399

pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>

On 17 May 2018, at 08:08 , 새드하셈 <saed at kaeri.re.kr<mailto:saed at kaeri.re.kr>> wrote:

Dear Dr. Peter

I have sent you the source code of HANARO-HRPD as you requested before, so could you check it please to discover the problem in the difference between real measured flux (n/cm2.s) and the McStas flux at sample position

Thank you in advance

Best Regards,

Saed Almomani

Nuclear Engineer, HANARO

Research Assistant, Neutron Science Center

[http://mail.kaeri.re.kr/skina2/img/kaeri_footer_hybrid.jpg]

-----Original Message-----
보낸사람(From) : Peter Kjær Willendrup <pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>>
받는사람(To) : 새드하셈 <saed at kaeri.re.kr<mailto:saed at kaeri.re.kr>>
참조(Cc) : mcstas-users at mcstas.org<mailto:mcstas-users at mcstas.org> <mcstas-users at mcstas.org<mailto:mcstas-users at mcstas.org>>
보낸일자(Sent) : 2018-05-09 22:56:02
메일제목(Subject) : Re: [mcstas-users] Reflectivity and transmission files of Ge crystal

Dear Saed,

On 9 May 2018, at 11:30 , 새드하셈 <saed at kaeri.re.kr<mailto:saed at kaeri.re.kr>> wrote:

Actually I have one question and one inquiry:

Q: I'm trying to simulate the HANARO_HRPD by using McStas software.  But after I inserted all required components and their parameters as designed/as they are on ground now,  I found a kind of difference between the McStas_flux magnitude and the Real_flux magnitude both at sample position in (n/cm2.s) !! and the difference is high enough to say there is something wrong I can't figure it ! {Real_flux is around (1.3e6) n/cm2.s where McStas_flux is (1.9e5) n/cm2.s.} although I get almost exact matching result at monochromator position.

It is always easier to comment or correct if there is acces to the instrument file, as well as other necessary data.

I would therefor encourage you to send the instrument file, plus descriptive material about your beamline in response to this email.

And my concerns also increased when some experts (2 experts to be honest) told me that McStas doesn't show the flux magnitude as calculated or expected !! So is this True ?!  and if yes, what is the accepted difference between McStas and Real, i.e. the criteria or error margin to consider?

I my experience McStas is reasonably reliable, and to a large extent simulates exactly what one parametrises. This of course means that if the source emission or certain material- or geometrical data are put in with values that are off, the simulation will naturally also be off. :-)

A rule of thumb is that with modest effort, one can arrive within 10% of what one would measure. Better agreement than that typically requires lots of more thinking, coding, simulating and benchmarking.

But again, without access to what you are trying to simulate and how, it is hard to judge what is going on. We are "flying blind".

Inquiry: I tried to get the Reflectivity files of Ge crystal  to define a curved Monochromator with four wavelengths (1.54, 1.834, 1.866, 2.224 A), but unfortunately I couldn't find any source that provide a tabulated data of the wavelength and the absolute Ge crystal reflectivity. So is there anyone who could help me in this also?

I must admit I am not much of a monochromator-expert, but I would try looking among papers by Freund et. al from the 1970’s and 80’s.

Best regards,

Peter Willendrup

Peter Kjær Willendrup
Forskningsingeniør, Speciakonsulent
Næstformand for DTU Fysik LSU

DTU Physics

[http://mail.kaeri.re.kr/image/imgmime/kaeri.re.kr//s/saed/f0dcbf0385ee47218d64.gif]

Technical University of Denmark

[http://mail.kaeri.re.kr/image/imgmime/kaeri.re.kr//s/saed/0d4b357e94184263adb0.gif]

Department of Physics
Fysikvej

Building 307

DK-2800 Kongens Lyngby

Direct +45 2125 4612

Mobil +45 2125 4612
Fax +45 4593 2399

pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>

 <HANARO_TEST.instr>

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