POSY2 - Polymer Neutron Reflectometer
Posy II Website
Instrument Scientist -
| Beam Line |
C2 |
| Initial Flight Path |
6.2 m |
| Final Flight Path |
1.8 m |
| Beam Size |
(0-3 mm) x 50 mm |
| Detector |
Linear Position Sensitive Detector 20cm |
| Choppers |
None |
| Intensity |
100 neutrons/pulse |
| Wave-vector Range |
0-0.25 Å-1 |
| Wave-vector Resolution |
1 x 10-4 Å-1 |
Purpose
The primary purpose of reflectometers is to measure the chemical and/or magnetic depth profile in thin films and multilayers deposited over flat substrates. This is accomplished in the following way. First, the reflectivity is measured as a function of the neutron momentum transfer perpendicular to the surface q = 4p sinq / l , where q is the angle of the incident beam with the surface and l is the neutron wavelength. Second, the measured reflectivity is compared with that calculated for model depth profiles containing parameters (representing the thickness and "nature" of the layers) to be fitted. By "nature" is meant the nuclear scattering amplitude density, that can be calculated from available tables, and the magnetic induction B. The spatial resolution that can be reached is of the order of 10 Å. The maximum thickness of the layers over which a profile can be obtained is 5000 Å.
Operation
The primary pulsed beam contains all wavelengths in the spectrum moderated to 20 K by a solid methane moderator. The intensity reflected at one angle q is measured as a function of the wavelength. The reflectivity is calculated by normalizing the intensities reflected to those of the direct beam. This measurements covers a range of q's defined by the largest and the shortest wavelengths available. To extend the range of q measurements are taken at different angles. Reflectivity curves taken at different angles are spliced together by fitting the data in the partially overlapping q ranges. The data are put on an absolute basis by assuming that at the lowest q part of the q range covers the region of total reflection (R=1). The procedure assumes that the resolution function D q/q is constant.
Geometry
The layout of the instruments is presented in the top figure. The surfaces are vertical, and the scattering plane horizontal. The table below gives the characteristics of the beam.
| Instrument |
Beam size, cm |
Neutrons/Pulse |
Resolution, Dq/q |
q range |
l range |
| POSY I |
5 x 0.2 |
40 |
2.5% |
0 - 6° |
3 - 15 Å |
| POSY II |
5 x 0.3 |
100 |
3.0% |
0 - 3° |
2.5-16 Å |
POSY II has been used to measure the density depth profile of materials. Mostly these are polymers, either in solution or in a melt. The identification of one of the components of the sample is made by its selective deuterations. Problems studied include interdiffusion, segregation and ordering. For a review, see Ref. 1. Description of the working of the instrument is given in Ref. 2.
Sample Environment
The conventional sample is deposited on a round Si wafer, 5 cm. diameter and arbitrary thickness (normally 3 mm). This is mounted on a spring loaded sample holder to be placed into a three-point setting. If placed in this setting the sample is prealigned, and does not need adjustments. Other devices available are:
- heating stage up to 200 C in inert atmosphere.
- liquid cell between two silicon wafers of silicon of 10 cm diameter (the beam passes through the silicon)
- sample changer, accomodating up to four silicon wafer of 5 cm diameter.
- gas charging manifold for in-situ gas charging/annealing of samples up to 1 atmosphere and 200 C
Data handling and data analysis: see user guide. The data reduction is almost automatic. Screen shots of analyzed data are also available here.
References:
1. Of Butterflies and Terraces, G. P. Felcher, Neutron News 5, no. 4, 18 (1994).
2. An Automated Neutron Reflectometer (POSY II) at the Intense Pulsed Neutron Source, A. Karim, B. H. Arendt, R. Goyette, Y. Y. Huang, R. Kleb and G. P. Felcher, Physica B, 173, 17 (1991).
3. Neutron scattering for surface characterization, J. Penfold, Current Science, vol. 78, no 12, 1458 (June 2000).
4. Specular Reflectivity of Neutrons by Thin Polymer Films, T. P. Russell, A. Karim, A. Mansour, and G. P. Felcher, Macromolecules, Vol. 21, 1890-1893 (1988).
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