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

Thu May 10 15:31:54 CEST 2018

Dear Saed,

At first glance, I think there are four things for you to check. You say that your value at the monochromator is what you expect, but the sample is down by a factor of eight - as you indicated earlier, this definitely suggests issues with the monochromator (germanium) modeling.

1) Your measured flux on sample is 1.3E6 n/cm2/s at 1.836 AA. This is a little bit incomplete for diagnostic purposes- what is the wavelength spread? What is the divergence? If you can compare those bounds to that produced by the calculation, you may find something informative. For example, it may be that your measured wavelength spread is larger than your modeled spread, and thus the calculation is missing something.

2) You use a reflectivity of 0.3 in the model (for 1.836 AA off of 331). While that will depend significantly on the thickness of the crystal as well as other factors, that seems low to me. Look at J Appl Crys 37 732+ (http://dx.doi.org/10.1107/S0021889804015390). There, they report peak reflectivity measurements of order 0.5 to 0.6, with a reflectivity FWHM half again larger than the intrinsic mosaic.

While not related to the monochromator per se, it is also worth asking:

3) Just to be paranoid - you quote a flux integral of 2.7E8 at the monochromator position - is that particle flux or capture flux? You're on a cold source (at least the Maxwellian temperature of 37 suggests that), so the capture flux is numerically significantly higher than the particle flux integral. You've almost certainly got the right value - but it's easy enough to check that I claim it's worth checking. (The same would technically apply to the beam tube flux of 1.28E14 - if that's a measured capture flux, it's a specific factor higher than the particle flux, and that same factor would apply at the monochromator position.)

4) Most complicated - hydrogen cold sources don't necessarily have a spectrum well fit by a Maxwellian, and your sample flux really only runs just around that one wavelength. It's easy for the integral to match with the value at one wavelength to be significantly different. Are there spectral measurements that support a 37-K Maxwellian?

Good luck,
Erik

On Thu, 2018-05-10 at 16:43 +0900, 새드하셈 wrote:

Dear Dr. Peter

First of all thank you so much for your quick response, I appreciate your help and effort

Regarding your request, you can find the attached file of my McStas code for the HANARO-HRPD with this email.

I would like to mention that I wrote almost all required descriptions and information as comments inside the code itself.
I'm sure that you can follow the design without any conflict. But in case you need more information I will response quickly.

Thank you in advance for your efforts and cooperation

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-----Original Message-----
보낸사람(From) : Peter Kjær Willendrup <pkwi at fysik.dtu.dk>
받는사람(To) : 새드하셈 <saed at kaeri.re.kr>
참조(Cc) : mcstas-users at mcstas.org <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

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Technical University of Denmark

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Erik B. Iverson, PhD.           Spallation Neutron Source
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