Kristian Nielsen kristian.nielsen at
Wed Sep 1 14:41:08 CEST 1999

> Date: Thu, 26 Aug 1999 16:17:15 -0400
> From: John Copley <john.copley at>

> Kristian

> Regarding the optical filter calculation, many thanks! I would indeed
> appreciate more information, and I'm also wondering if you could do a
> few more runs? One of these days I'd like to get familiar with McStas

> You say the transmission for 0.5 AA neutrons is about 2e-5. What is it
> for 5 A neutrons?  How much work would it be to calculate the
> transmission in steps of 0.1 AA say from 0.1 to 5 AA, using (i) the
> reflectivity graphs that I sent you and (ii) idealized reflectivity
> graphs with R=1 for Q<=0.265 (sides), R=1 for Q<=0.43 (top/bottom), 0
> otherwise?

Ok, I found a little time to do a few more runs. I actually changed my
instrument definition somewhat. In your fax you mention three guide
sections, the first one immediately after the source. I realized that
the first 12 meters are just a 60mm channel with no mirror, right? Then
comes the focusing 6.9m guide and the straight 26m guide. This gives the
4.4AA leveling of the transmission, whereas with my first guess the
transmission does not level off until about 40AA. Well, the single most
difficult part in simulations is to get the details right, in my

I made the runs with lambda from 0.1AA to 5AA for two reflectivities:
either sharp cut-off at the critical angle, or the rough analytical
approximation to the measured curves you sent me. I also did the same
runs in the interval 0.1AA to 1.1AA (for detail at high wavelength). If
you want, you can send me the actual measured reflectivity data, and I
can put that into the simulation.

I put up some plots for you on our web site, at

Basically, they show that transmission vanishes at about 1AA, with small
traces of shorter wavelength. For the idealized reflectivity, the
transmission completely vanishes at about 0.6AA, whereas for my crude
approximations to your measured reflectivity, neutrons down to about
0.3AA are transmitted.

I wonder how you interpret these results? Of course, just ask if you
need any further information.

> In terms of running McStas here, Collin Broholm has a student who is
> running it. He could probably help me if needed.

I also put the files I used to produce the simulation on the server, in
case you want to look at it.

 - Kristian.

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