[mcstas-users] Fwd: Neutron Beam tracking - MCSTAS

[María Freiría López]

Hello everybody,

my name is María Freiría, Master Student in Nuclear Engineering at the
Technische Universität München. Right now I am working at my Master Thesis
at the research reactor FRM II, studying the viability of a future fuel
conversion.

The fact is that we did some experiments at the reactor in order to measure
the cold neutron spectrum for different reactor power levels.

Here is a brief description of the reactor:

[image: Imágenes integradas 1]
Several beam tubes supply various experimental stations with neutrons from
different parts of the reactor. In our experiment we considered the tube
SR-1 (connected to the cold source), which is subdivided into 6 smaller
tubes as shown in the figure above.

One of these 6 tubes (NL-4) is again divided into 2 tubes (named NL-4a and
NL-4b) which lead to the 2 stations (SANS-1 and PGAA) where the measures
were taken:

[image: Imágenes integradas 2]



The obtained results concerning neutron spectrum at the 2 stations differ
from each other. Eventhough the neutrons in both tubes come from the cold
source, based on the results, we think that the neutrons have a different
nature, due to the heterogeneous properties along the cold source. A point
to be taken into account is the fill level  of the cold source (D2 - liquid
deuterium aprox. 25K). Some experts suggest that the major part of the
neutron flux analyzed at SANS-1 comes from the lower part of the cold
source and in the case of PGAA the neutrons have their origin in the upper
part of the tank (near the fill level). This would explain the different
results at the 2 stations:

[image: Imágenes integradas 3]

In order to proof this theory, we started to work with MCSTAS.

I got the instruments and components defined in MCSTAS which describe the
geometry of the tube structure. But, as a beginner, I need to ask for some
help:

I need to know the origin of the neutrons which enter the tubes NL-4a and
b. In other words: from which part of the cold source do most of the
neutrons come in each case (NL-4a and NL-4b):

[image: Imágenes integradas 4]

Is it true that the neutron spectrum is different at the 2 experimental
stations because the beam has its origin in different parts (the lower and
the upper part) of the cold source? Or is the distribution almost
homogeneous so that the beam properties in both tubes should be the same?

In order to answer these questions, the suggestion made is to implement a
kind of "marker" at the beginning of SR-1 divided in "n" cells. This would
make it possible to know which of these neutrons "make their way" to SANS-1
and which others to PGAA.

Is it viable to create such a "marker"? Or do you have any other
suggestions for solving this problem?

As I said, as a beginner I am not conscious of the possibilities that
MCSTAS offers, that is why I was directed to you by Peter Link, Neutron
Optics department at the FRM II.

I hope I could explain my problem in a more or less understandable way :)

I would be really greatfull for your help!

Kind regards,

María
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